A review of the properties of hybrid ceramic nanocomposites

Abstract

Although monolithic ceramics have many desirable properties, such as high-temperature resistance, excellent hardness, and chemical inertness, they also have limitations that can restrict their use in various applications. The limitations include, but are not limited to, low fracture toughness, low ductility, high electrical resistance, and low thermal conductivity. Researchers have been able to address some of these limitations by developing ceramic composites, nanocomposites, and hybrid nanocomposites. The later are ceramic matrices that incorporate two or more reinforcements. They represent an advanced class of materials that combine the excellent characteristics of monolithic ceramics and the outstanding properties of the reinforcements, offering exceptional mechanical, thermal, and functional properties for advanced applications. There are many comprehensive review papers on ceramic composites and nanocomposites, however, only one review paper on hybrid ceramic nanocomposites has been published, and it was dedicated solely to alumina-based nanocomposites prepared by spark plasma sintering. The aim of this work is to review the properties of hybrid ceramic nanocomposites, highlight their potential applications in various industries, and formulate directions for future research endeavors.

Resumen

Aunque las cerámicas monolíticas tienen muchas propiedades deseables, como resistencia a altas temperaturas, excelente dureza e inercia química, también tienen limitaciones que pueden restringir su uso en diversas aplicaciones. Las limitaciones incluyen, pero no se limitan a, baja tenacidad a la fractura, baja ductilidad, alta resistencia eléctrica y baja conductividad térmica. Los investigadores han podido abordar algunas de estas limitaciones desarrollando compuestos cerámicos, nanocompuestos y nanocompuestos híbridos. Estos últimos son matrices cerámicas que incorporan dos o más refuerzos. Representan una clase avanzada de materiales que combinan las excelentes características de las cerámicas monolíticas y las propiedades sobresalientes de los refuerzos, ofreciendo propiedades mecánicas, térmicas y funcionales excepcionales para aplicaciones avanzadas. Hay muchos artículos de revisión exhaustivos sobre compuestos cerámicos y nanocompuestos; sin embargo, solo se ha publicado un artículo de revisión sobre nanocompuestos cerámicos híbridos, y este se dedicó exclusivamente a nanocompuestos a base de alúmina preparados por sinterización por plasma de chispa. El objetivo de este trabajo es revisar las propiedades de los nanocompuestos cerámicos híbridos, resaltar sus aplicaciones potenciales en diversas industrias y formular direcciones para futuros esfuerzos de investigación.

Introduction

Ceramics are materials used in various engineering applications because of their unique properties. Some of their important characteristics include: (i) the ability to withstand high temperatures, (ii) excellent wear and abrasion, (iii) chemical resistance, (iv) electrical insulation, (v) biocompatibility, and (vi) lightweight and low density [1–4]. These properties make ceramics suitable for: (i) use in harsh environments, (ii) manufacturing of cutting tools, bearings, and wear-resistant coatings, (iii) applications where corrosion resistance is critical, such as in the chemical industry, semiconductor manufacturing, and healthcare, (iv) electrical and electronic applications where high levels of insulation are required to prevent electrical leakage or short circuits, (v) medical implants and dental prosthetics, and (vi) applications where weight reduction is important, such as in the transportation, automotive, and aerospace industries [1–4]. Nevertheless, the utilization of ceramics in some applications has been restricted because of their low resistance to fracture, insufficient strength, high electrical resistance, and low thermal conductivity. As a result, composites [5], nanocomposites [6], and hybrid nanocomposites [7] were developed to overcome the limitations of unreinforced ceramic materials. Composites are materials made up of a matrix that incorporates particles, whiskers, fibers, or sheets, resulting in enhanced properties when compared to ceramics that are not reinforced. Additional enhancements to the properties have been achieved through the development of nanocomposites and hybrid nanocomposites. In the former, the matrix is reinforced with a single phase, while in the latter, the matrix is reinforced with two or more phases that have different dimensionalities, morphologies, and properties. Fig. 1[14] shows typical reinforcements and structures for ceramic nanocomposites while Fig. 2[33] displays the structure of Al₂O₃–SiC–CNTs nanocomposites. In the later, CNTs are present within the grain boundaries of Al₂O₃ while SiC particles are present within grains and at the grain boundaries. The properties of hybrid ceramic nanocomposites are influenced by: (i) the inherent characteristics, type, size, dimensionality (0D, 1D, 2D), and distribution of the reinforcements, (ii) nature and composition of the matrix, and (iii) process parameters. The matrix offers high strength, high hardness, and thermal stability, while the reinforcements further improve these properties and adds functionalities such as enhanced toughness, wear resistance, and electrical, magnetic, or optical properties. This enables the development of materials with tailored properties that are well-suited for a wide range of applications [8–13]. Review papers on ceramic composites and nanocomposites can be found in the literature [14–20]. However, only one review paper on spark plasma sintered hybrid ceramic nanocomposites [7] has been published so far. The objective of this work is to review the properties of hybrid ceramic nanocomposites, highlight their potential applications in various industries, and formulate directions for future research endeavors.

Fig. 1.

Some common nanoreinforcements and nanocomposite structures for ceramics. (a) zero-dimensional (0-D) round nanomaterials, (b) one-dimensional (1-D) needlelike-shaped nanomaterials, (c) two-dimensional (2-D) platelike shaped nanomaterials, (d) 0D nanomaterials embedded in the matrix grains, located at the grain boundaries or occupy both inter- and intra-granular positions, (e) 1D nanomaterials embedded in a micronic matrix, (f) 2D nanomaterials embedded in a micronic matrix, and (g) mixture of 0D and 1D nanoscale phases that are embedded in a micron-sized matrix [14].

Fig. 2.

A schematic of hybrid microstructure design [53].

Properties of some reinforcements

Carbonaceous reinforcements, such as carbon nanotubes (CNTs), graphite, and graphene, along with other reinforcements like SiC, exhibit outstanding mechanical and physical properties that make them highly suitable for enhancing the performance of ceramic composite materials. For example, CNTs have a high stiffness of around 1 TPa [21,22] and a tensile strength of up to 60 GPa [23]. As for thermal properties, at 300 K, CNTs have an electrical conductivity of around 10⁶ S/m for single walled carbon nanotubes (SWCNTs) and greater than 10⁵ S/m for multi-walled carbon nanotubes (MWCNTs) [24,25]. Moreover, they have very high thermal conductivity [26,27], with room temperature measured values of 3000 W/m K for MWCNTs [28] and 3500 W/m K for SWCNTs [29]. Graphene is a two-dimensional material consisting of sp2-hybridized carbon atoms and has exceptional mechanical properties. A perfect single-layer graphene has a stiffness of 1.0 TPa and a fracture strength of 130 GPa [30]. In contrast to monolayer graphene, graphene nanosheets (GNSs) or graphene nanoplatelets (GNPs) possess outstanding mechanical properties [31–33]. The stiffness of GNPs with a thickness of 2–8 nm is reported to be around 0.5 TPa [34]. The fracture toughness of graphene was found to be equal to 4 MPa m^1/2[35]. Graphene possesses outstanding in-plane thermal conductivity in the range of 1000–5300 W/m K and through-plane thermal conductivity in the range of 5–20 W/m K. The room temperature thermal conductivity of single-layer graphene was reported to be around 5300 W/m K [36]. Moreover, graphene has a negative coefficient of thermal expansion (CTE) of −1.28 × 10⁻⁶ to −8 × 10⁻⁶ K⁻¹[37,38]. SiC is known to have a hardness of approximately 30 GPa [39], and it possesses a thermal conductivity and CTE of 200–300 W/m K and 4.5 × 10⁻⁶ K⁻¹, respectively [40].

Hybrid ceramic nanocomposites

Al₂O₃ nanocomposites

Alumina is the most widely applied and investigated ceramic material because of its exceptional mechanical properties and excellent thermal stability. However, its inherent brittleness limits its use in a wider range of structural applications [41]. Improvements in alumina toughness were made possible through the addition of a second phase such as CNTs [41–44], SiC [41,45–48], graphene [49,50], TiC [51], WC [51], and Si₃N₄[52]. Additionally, it was found that reinforcing alumina with two phases, resulting in hybrid nanocomposites, can further improve its properties through the synergistic toughening mechanisms of reinforcements [53]. Below is a summary of published work on alumina-based hybrid nanocomposites.

Al₂O₃–SiC–CNTs nanocomposites

Hybrid microstructure design for Al₂O₃–SiC–CNTs nanocomposite materials was first introduced by Ahmad and Pan [53]. The authors prepared alumina and Al₂O₃–1 vol.%SiC–(5, 7, 10) vol.% CNTs hybrid nanocomposites through spark plasma sintering (SPS) at 1550 °C. The relative density of alumina was found to be 100%, but decreases with the increase in the reinforcement content and reached 95.1% for Al₂O₃–1 vol.%SiC–10 vol.%CNTs nanocomposite. Although no significant reduction in hardness was observed, the fracture toughness and bending strength of the nanocomposites increased by 117 and 44%, respectively, compared to alumina. In another study, Ahmad et al. [54] reported the effect of SiC on the microstructure of spark plasma sintered Al₂O₃–(1–3) vol.%SiC–5 vol.% CNTs nanocomposites as presented in Figs. 3 and 4, and on the mechanical properties as shown in Fig. 5[54]. A 104% increase in fracture toughness and 46% increase in bending strength were reported for the Al₂O₃–1 vol.% SiC–5 vol.%CNTs nanocomposite, compared to alumina. Al₂O₃ thermal conductivity decreased from 29.5 to 23 W/m K at 1 vol.% SiC and 5 vol.% CNTs and further decreased with an increase in reinforcement content. A very high electrical conductivity of 9 S/m was reported for the Al₂O₃–3 vol.% SiC–5 vol.%CNTs nanocomposite, compared to 10⁻¹² S/m for alumina.

Fig. 3.

Microstructure of Al₂O₃–SiC–CNT hybrid nanocomposites [54].

Fig. 4.

Toughening in Al₂O₃–SiC–CNT hybrid nanocomposites [14].

Fig. 5.

Properties Al₂O₃–(1–3) vol.% SiC–5 vol.% CNT hybrid nanocomposites [54].

Mohammad and Saheb [55] used molecular level mixing to prepare homogeneous Al₂O₃–5 wt.%SiC–1 wt.%CNTs nanocomposite powder with uniform distribution of SiC and CNTs, and sintered it using SPS at 1500 °C. They obtained full density alumina (100%), but the relative density of the nanocomposite decreased to 91.65%. The fracture toughness of the nanocomposite increased by 33% compared to alumina, while a slight decrease in hardness (4%) was observed. In another work, Saheb and Mohammad [56] investigated the influence of SiC and CNTs on mechanical and microstructural properties of Al₂O₃–SiC–CNTs hybrid nanocomposites as shown in Fig. 6[36]. Al₂O₃–(5, 10) wt.%SiC–(1, 2) wt.%CNTs nanocomposite powders were prepared using sonication and ball milling. Spark plasma sintering at 1500 °C resulted in densification higher than 98% for all samples. All nanocomposites showed improved hardness than alumina (18.56 HV) except Al₂O₃–10 wt.%SiC–2 wt.%CNTs nanocomposite (17.5 HV), which was attributed to low densification. An increase of a maximum of 12.12% in hardness was reported for Al₂O₃–10 wt.%SiC–1 wt.%CNTs nanocomposite. Similarly, all nanocomposites displayed enhanced fracture toughness values, with maximum increase of 93.95% for Al₂O₃–10 wt.%SiC–2 wt.%CNTs, compared. The effect of varying sintering temperature on microstructural and mechanical properties of Al₂O₃–5 wt.%SiC–1 wt.%CNTs nanocomposite was investigated by Saheb and Mohammad [57]. Nanocomposite powder, produced by molecular level mixing, was spark plasma sintered at 1500 °C, 1550 °C and 1600 °C. The relative density, hardness, and fracture toughness were found to increase with the increase in temperature, and reached maximum values of 98.91%, 23.32 GPa, and 7.10 MPa m^1/2, respectively, at 1600 °C. The increase in hardness (25.65%) and fracture toughness (96.67%) compared to monolithic alumina was attributed to higher densification, finer microstructure, change of fracture mode to transgranular along with other toughening mechanisms such as CNTs pull-out, CNTs breaking, crack deflection and crack bridging. In another work, Saheb and Hayat [58] reported thermal and electrical properties of spark plasma sintered Al₂O₃–SiC–CNT hybrid nanocomposites as shown in Fig. 7[58]. Thermal conductivity, thermal diffusivity, and specific heat capacity values of 34.44 W/m K, 7.62 mm²/s and 1.24 J/g K for alumina decreased to 21.2 W/m K, 6.64 mm²/s and 0.87 J/g K, respectively, for the Al₂O₃–5 wt.%SiC–1 wt.%CNTs nanocomposite. These properties continued to decrease with further addition of reinforcements. However, a drastic increase in electrical conductivity was observed with the addition of SiC and CNTs reinforcements, where the electrical conductivity of alumina increased from 6.87 × 10⁻¹⁰ S/m to 8.87 S/m for the Al₂O₃–5 wt.%SiC–2 wt.%CNTs nanocomposite. Siddique et al. [59] presented a model for the estimation of thermal conductivity of spark plasma sintered Al₂O₃–SiC–CNTs hybrid nanocomposites. In the proposed model, thermal conductivity was taken as a function of average matrix crystallite size compared to conventional approach of using constant thermal conductivity for the matrix. They found good agreement between the predicted and experimentally measured values, as shown in Fig. 8[59].

Fig. 6.

Properties of Al₂O₃–SiC–CNTs hybrid nanocomposites prepared by spark plasma sintering (a) density, (b) hardness, and (c) fracture toughness [56].

Fig. 7.

Physical properties of spark plasma sintered Al₂O₃–SiC–CNT hybrid nanocomposites (a) relative density, (b) electrical conductivity, (c) thermal conductivity, (d) thermal diffusivity, and (e) specific heat capacity [58].

Fig. 8.

Predicted and measured thermal conductivity of alumina as a function of its average crystallite size [59].

Ahmad and Islam [60] investigated the effect of SiC nanoparticles and CNTs on mechanical properties, bonding characteristics, and thermal stability of Al₂O₃–SiC–CNTs hybrid nanocomposites. The relative density of inductive hot-pressed Al₂O₃–6 wt.%SiC_np–4 wt.%CNTs nanocomposite, sintered at 1500 °C, was found to be 97.2% while that of alumina was 99.2%. Moreover, a 160% refinement in microstructure, a 81% increase in fracture toughness, and a 25% increase in hardness were reported for the Al₂O₃–SiC–CNTs nanocomposite, compared to alumina. However, 11% decrease in elastic modulus was recorded for the Al₂O₃–SiC–CNTs nanocomposite. Interfacial and thermal analysis suggested the formation of intermediate phase (Al₂OC) at Al₂O₃/CNTs interface in the nanocomposite. The Al₂O₃–SiC–CNT nanocomposite showed good thermal stability up to 760 °C in inert atmosphere and up to 425 °C in oxygen-rich environment. In another study, Ahmad et al. [61] used high frequency induction heat sintering (HFIHS) method to consolidate alumina nanocomposite reinforced with SiC nanoparticles (SiC_np) and CNTs (Al₂O₃–5 wt.%SiC_np–2 wt.%CNTs) at 1600 °C. Fourfold reduction in grain size along with 110% increase in fracture toughness and 30% increase in hardness were reported for the nanocomposite, compared to alumina. However, the elastic modulus of alumina decreased from 418 MPa to 350 MPa for the Al₂O₃–5SiC_np–2CNTs nanocomposite. Enhancements in mechanical properties and cutting performance of alumina–matrix nanocomposite reinforced with silicon carbide whiskers (SiC_w) and CNTs were reported by Lee and Yoon [62]. They prepared Al₂O₃–25 wt.%SiC_w–(0.1–1) wt.%CNTs nanocomposites using hot pressing at 1750 °C. All the Al₂O₃–SiC_w–CNTs nanocomposites showed relative density higher than 99%. Improvements in fracture toughness and flexural strength of more than 60% were reported for all Al₂O₃–SiC_w–CNTs nanocomposites, compared to Al₂O₃–CNTs. The cutting performance of Al₂O₃–SiC_w–CNTs nanocomposites was evaluated by finish turning of Inconel 718 super alloy. Fig. 9[62] shows the crater wear and flank wear performance of various Al₂O₃–SiC_w–CNTs nanocomposites. All Al₂O₃–SiC_w–CNTs nanocomposites showed lower wear rate compared to Al₂O₃–25 wt.%SiC_w, with Al₂O₃–25 wt.%SiC_w–0.5 wt.%CNTs nanocomposite showing the lowest wear rate.

Fig. 9.

(a) Crater wear and (b) flank wear performance for various Al₂O₃–SiC_w–CNT nanocomposites as a function of time [62]. (AS25C00 represents Al₂O₃–25 wt.%SiC_w, AS25C01:Al₂O₃–25 wt.%SiC_w–0.1 wt.%CNT, AS25C05: Al₂O₃–25 wt.%SiC_w–0.5 wt.%CNT, AS25C10: Al₂O₃–25 wt.%SiC_w–1 wt.%CNT nanocomposite).

Al₂O₃–ZrO₂–MWCNTs nanocomposites

Al₂O₃ ZrO₂ MWCNTs nanocomposites were prepared through colloidal processing and freeze drying, followed by hot pressing at 1500 °C [44]. Increase in fracture toughness was observed for all nanocomposites, with a maximum of 35% for Al₂O₃–5 vol.%ZrO₂–1 vol.%MWCNTs. However, around 21% decrease in hardness was reported for all Al₂O₃–5 vol.%ZrO₂–(0.5–2) vol.%MWCNTs nanocomposites. The electrical conductivity increased for all composites with a maximum value of 0.22 S/m for Al₂O₃–5%ZrO₂–2%MWCNTs as compared to 10⁻¹² S/m for alumina. In another work, a slight increase in thermal conductivity, from 29 W/m K for Al₂O₃ to 30 W/m K for Al₂O₃–5%ZrO₂–0.5%MWCNTs nanocomposite, was observed [63]. Further increase in CNTs content caused a decrease in thermal conductivity. Mechanical properties of Al₂O₃ hybrid nanocomposite were enhanced by addition of 8 vol.% ZrO₂ nanoparticles and 4 vol.% MWCNTs [64]. The materials were prepared using hot-pressing at 1600 °C for 60 min. High relative density values of 99.7% and 98.3% were obtained for Al₂O₃ and the nanocomposite, respectively, and the microstructure was refined by 10 folds. The fracture toughness and hardness of monolithic alumina were increased by 116% and 12%, respectively, as a result of the addition of reinforcements. The improvements in mechanical properties were attributed to the synergistic role of nano-ZrO₂ and CNTs reinforcements through grain refining and the introduction of various toughening mechanisms. Duntu et al. [65] studied the mechanical and wear properties of CNTs and graphene (GN) reinforced Al₂O₃–ZrO₂ hybrid nanocomposites. They prepared Al₂O₃–4 wt.%ZrO₂–2 wt.%CNTs and Al₂O₃–4 wt.%ZrO₂–0.5 wt.%GN nanocomposite powders and hot pressed them at 1600 °C and 60 MPa for 1 h. All samples showed a relative density of at least 99%. Grain size refinements of 77% and 37% were observed for the CNTs and GN reinforced hybrid nanocomposites. As a result, hardness and flexural strength were found to increase by 26 and 44% for CNTs-reinforced composites and by 14 and 23% for graphene-reinforced Al₂O₃–ZrO₂ nanocomposites. Moreover, 118 and 97% increases in fracture toughness were reported for Al₂O₃–4 wt.%ZrO₂–2 wt.%CNTs and Al₂O₃–4 wt.%ZrO₂–0.5 wt.%GN nanocomposites, respectively. The enhancements in mechanical properties were attributed to grain refinement, crack bridging, crack deflection, and strong interfacial bonding of reinforcements. Enhanced wear performance with a decrease in wear rate of up to 90% and 98% was also reported in CNTs and GN reinforced nanocomposites, respectively, compared to alumina. The effect of MWCNTs on zirconia-toughened-alumina (ZTA) reinforced with MgO composite was investigated by Biswas et al. [66]. The authors fabricated ZTA–0.6 wt.%MgO and ZTA–0.6 wt.%MgO–(0.05, 0.1 wt.%)MWCNTs nanocomposites by hot-pressing at 1500 °C for 30 min. All nanocomposites showed relative density of 99%. XRD analysis revealed the formation of MgAl₂O₄ secondary phase during sintering in all nanocomposites. The addition of MWCNTs resulted in grain refinement. Hardness and elastic modulus increased with increase in MWCNTs content, reaching values of 31.25 and 341.15 GPa for ZTA–MgO–0.1 wt.%MWCNTs compared to 24.57 and 302.88 GPa for ZTA–MgO. The fracture toughness was found to be 22% higher in the ZTA–MgO–0.05 wt.%MWCNTs nanocomposite. The ZTA–MgO–0.05 wt.%MWCNTs nanocomposite also showed superior wear properties, with 42% and 81% reduction in specific wear rate at room temperature and high-temperature, respectively. Moreover, 23% and 29% decrease in coefficient of friction was reported for the ZTA–MgO–0.05 wt.%MWCNTs nanocomposite at room temperature and high-temperature, respectively.

Al₂O₃–SiC–Graphene nanocomposites

Ahmad et al. [67] studied the mechanical, interfacial, and thermal performance of Al₂O₃ reinforced with SiC_np and GN hybrid reinforcements. The alumina and Al₂O₃–3 wt.%SiC–0.5 wt.%GN nanocomposite were inductively hot-pressed at 1300–1600 °C. Relative density values of 99% were obtained for all samples. Compared to alumina, the nanocomposite displayed 70% reduction in grain size, 97% increase in fracture toughness, 15% increase in hardness, and 78% higher ballistic performance. The superior ballistic performance was attributed to higher densification, finer microstructure, high hardness, and high fracture toughness. The authors performed an interfacial investigation and concluded that a carbo-thermal reduction reaction between Al₂O₃ and GN resulted in the formation of Al₂OC phase during sintering. In another investigation, Ahmad et al. [68] reported a 160% increase in fracture toughness and a 27% increase in hardness, when alumina was reinforced with 0.5 wt.%GN and 5 wt.%SiC to form a hybrid nanocomposite. Both Al₂O₃ and Al₂O₃–5 wt.%SiC–0.5 wt.%GN nanocomposite, were prepared through inductive sintering at 1300–1600 °C, and showed relative density values greater than 99%.

Improvements in mechanical and electrical properties of Al₂O₃–SiC_w reinforced with graphene oxide (GO) have been reported by Grigoriev et al. [69]. They prepared Al₂O₃–17 vol.%SiC_w–(0.2–5) vol.%GO nanocomposites using colloidal processing, followed by SPS at 1780 °C. Compared to Al₂O₃–SiC_w composite, the Al₂O₃–SiC_w–0.5 wt.%GO displayed increases of 4%, 29%, and 10% in hardness, strength, and fracture toughness, respectively. The electrical resistivity was found to decrease with the increase in GO content and reached 2.2 Ω cm for Al₂O₃–17 vol.%SiC_w–5 vol.%GO nanocomposite. In other work, spark plasma sintered Al₂O₃–(1–5) vol.%SiC–0.38 vol.%GPLs nanocomposites showed increased hardness, flexural strength, and fracture toughness by 36%, 40%, and 50%, compared to alumina [70]. Zhang et al. [71] prepared Al₂O₃–5 vol.%SiC–(0.5–1) vol.%GNS nanocomposites by SPS at 1450 °C. All nanocomposites had relative density values higher than 99%. The electrical conductivity of Al₂O₃–5 vol.%SiC–1 vol.%GNS nanocomposite was found to be 13 orders higher than that of alumina. The Nanocomposites showed moderate improvements in hardness and fracture toughness. The Al₂O₃–5 vol.%SiC–1 vol.%GNS nanocomposite displayed 29.4% decrease in coefficient of friction and 90.1% decrease in wear rate. The enhanced tribological properties of the hybrid nanocomposites were attributed to the formation of graphene tribofilm as sketched in Fig. 10[71], refined microstructure, and enhanced mechanical properties. Smirnov et al. [72] investigated the wear behavior of Al₂O₃–SiC_w–GO nanocomposites. They prepared nanocomposite powder of Al₂O₃, 17 vol.% SiC_w and 0.5 vol.% GO using colloidal processing and sintered it by SPS at 1780 °C. The authors also prepared Al₂O₃–17 vol.% SiC composite at same conditions for comparison. The Al₂O₃–SiC_w–GO nanocomposite displayed better tribological performance than Al₂O₃–SiC_w. At 10 N load, Al₂O₃–SiC_w–GO nanocomposite showed 20 and 45% decrease in the coefficient of friction and wear rate, respectively, compared to Al₂O₃–SiC_w. And at 40 N load, the coefficient of friction and wear rate further decreased to 30 and 55%, respectively. The decrease in the wear rate and coefficient of friction was attributed to the formation of a tribofilm in Al₂O₃–SiC_w–GO nanocomposite. Shah et al. [73] produced alumina reinforced with multilayer graphene (MLG) and SiC composites. Al₂O₃–0.4 wt.%MLG–(5, 10, 15 wt.%)SiC nanocomposites were prepared by SPS at 1500 °C and 21 MPa for 3 min. The authors investigated the effect of SiC content on the mechanical and thermal properties of the nanocomposites. They found that the densification decreased with the increase in the SiC content, however, all samples showed relative density values higher than 97%. The Al₂O₃–0.4 wt.%MLG–5 wt.%SiC nanocomposite displayed the highest mechanical properties where the bending strength and hardness increased by 57% and 19.22% compared to Al₂O₃. The enhancements were attributed to matrix grain refinement, homogeneous microstructure, toughening effects of the reinforcements, and change in fracture mode. The thermal conductivity and diffusivity of alumina were found to be 25.5W/m K and 8.1 mm²/s at 50 °C, respectively, which decreased with increase in the SiC content as well as with the increase in temperature, as shown in Fig. 11[73].

Fig. 10.

Schematic of the wear behaviour of alumina (a), and alumina–SiC–GNSs composites [71].

Fig. 11.

(a) Thermal conductivity and (b) thermal diffusivity of Al₂O₃–0.4 wt.%graphene–(0, 5, 10, 15 wt.%)SiC hybrid nanocomposites [73].

Al₂O₃–ZrO₂–graphene nanocomposites

Microstructural, mechanical, and ballistic properties of GNPs and ZrO₂ reinforced-alumina hybrid nanocomposites were investigated by Ahmad et al. [74]. The authors produced Al₂O₃ and Al₂O₃–4 wt.%ZrO₂–0.5 wt.%GNPs nanocomposite by hot-pressing at 1600 °C. Compared with alumina, the nanocomposite displayed: (i) slight decrease in relative density from 99.7 to 98.5%, (ii) 68% decrease in grain size, (iii) 155% increase in fracture toughness, (iv) 17% increase in hardness, and (v) 88% increase in ballistic properties. These enhancements were attributed to grain refinement as well as toughening mechanisms induced by the reinforcements. Microstructural analysis showed wrapping and anchoring of graphene around alumina grains, resulting in very good interfacial strength. Petrus et al. [75] reported mechanical and tribological properties of GML and GO reinforced Al₂O₃–ZrO₂ nanocomposites where 0, 0.2, 0.5, 0.7 and 1 wt.%MLG and nikel-coated-graphene were added to Al₂O₃–ZrO₂. The samples were consolidated by SPS at 1550 °C for 4 min. All nanocomposites had relative density values larger than 98%. The Al₂O₃–ZrO₂–0.2 wt.%GO nanocomposite displayed highest hardness and fracture toughness values of 1940 HV and 5.5 MPa m^1/2, compared to 1840 HV and 5.2 MPa m^1/2 for Al₂O₃–ZrO₂. Also, all graphene-reinforced nanocomposites showed higher coefficient of friction and higher wear rate (at a load of 10 N), compared to Al₂O₃–ZrO₂.

Al₂O₃–WC–TiC–graphene nanocomposite

Wang et al. [76] reported improvements in fracture toughness of Al₂O₃–WC–TiC ceramic tool material due to the addition of single layer graphene (SLG). Al₂O₃–18 vol.%WC–6 vol.%TiC was reinforced with 0.5–2 vol.%SLG and hot pressed at 1700 °C. Increase in densification and enhancements in mechanical properties were observed with the addition of 0.5 vol.% SLG, beyond which properties began to decrease. Densification, flexural strength, hardness, and fracture toughness values of 99.58%, 646 MPa, 24.64 GPa, and 9.42 MPa m^1/2, respectively, were obtained for Al₂O₃–WC–TiC–0.25 vol.%SLG nanocomposite, compared to values of 98.94%, 550 MPa, 23.54 GPa, and 8.76 MPa m^1/2 for Al₂O₃–WC–TiC. The enhancements in properties were attributed to rugged graphene surface, grain refinement during densification, and crack bridging and deflection. In another work, Wang et al. [77] reported superior cutting and wear performance of Al₂O₃–WC–TiC–0.5 vol.%Graphene nanocomposite while machining hardened 40Cr steel, in comparison with Al₂O₃–WC–TiC and other commercial tool materials including Al₂O₃–(W, Ti)C and Al₂O₃–TiC, as shown in Fig. 12[77]. Cutting force and cutting temperature were found to be lower in graphene reinforced Al₂O₃–WC–TiC tool. Moreover, reinforcement with graphene (thickness: ∼0.8 nm, lateral size: 0.5–5 μm) resulted in reduction in coefficient of friction from 0.33 for Al₂O₃–WC–TiC to 0.1 for Al₂O₃–WC–TiC–0.5 vol.% Graphene nanocomposite. The superior properties of graphene-reinforced nanocomposite were attributed to; increased mechanical properties due to graphene reinforcement, good lubrication properties and higher thermal conductivity which resulted in friction reduction and decrease in cutting temperatures. Wang et al. [78], in another work, studied the mechanical properties and interfacial structure of Al₂O₃–WC–TiC–Graphene nanocomposite. They used graphene with a thickness of ∼0.8 nm and lateral size of 0.5–5 μm. Al₂O₃–WC–TiC composite and Al₂O₃–WC–TiC–0.5 vol.% Graphene nanocomposite, with constant 18 vol.% WC and 6 vol.% TiC, were hot-pressed in two steps, first heated to 1750 °C and without dwell time cooled to 1600 °C for dwell period of 15 min. Apart from parent phases, (W, Ti)C solid solution was formed in sintered samples. Al₂O₃–WC–TiC composite and Al₂O₃–WC–TiC–0.5 vol.% Graphene showed relative density values of 98.41% and 99.52%, respectively. Hardness, flexural strength and fracture toughness were found to be 22.5 GPa, 860 MPa, and 9 MPa m^1/2 for Al₂O₃–WC–TiC–Graphene nanocomposite; compared to 19 GPa, 500 MPa, and 6 MPa m^1/2 for Al₂O₃–WC–TiC, respectively. A TEM analysis showed three type of interfaces; Al₂O₃–graphene, graphene–Al₂OC, and graphene–WC interface. On basis of calculated adhesion energy and interfacial strengths, the authors ranked the interfaces; Al₂O₃–graphene > graphene–WC > graphene–Al₂OC. Effects of different thickness and lateral sizes of graphene reinforcement on Al₂O₃–WC–TiC nanocomposites were reported by Wang et al. [79]. Al₂O₃–18 vol.%WC–6 vol.%TiC composite and Al₂O₃–18 vol.%WC–6 vol.%TiC–0.5 vol.%graphene nanocomposites, with different types of graphene, were hot-pressed at 1700 °C for 10 min. A relative density of 98.4% was reported for Al₂O₃–WC–TiC while other nanocomposites showed values greater than 99%. Compared to Al₂O₃–WC–TiC, the Al₂O₃–WC–TiC–graphene nanocomposite with SLG displayed increased hardness, flexural strength, and fracture toughness by 14.80%, 67.59%, and 54.10%, respectively. Moreover, the authors concluded that graphene thickness and lateral size has no effect on matrix grain size and hardness. However, thinner graphene with larger lateral area was found beneficial to improve densification, flexural strength, and fracture toughness. Cheng et al. [80] investigated the effect of varying GNPs content on mechanical and microstructural properties of Al₂O₃–TiC–GNPs nanocomposite ceramic tool material. 60 wt.%Al₂O₃, 30 wt.%TiC, 0.2–0.8 wt.%GNPs along with 10 wt.%sintering aids (Mo, Ni, MgO, Y₂O₃) were used for composite preparation. The samples were consolidated by microwave sintering at 1700 °C for 10 min. The densification of sintered samples decreased with the increase in GNPs content. As compared to Al₂O₃–TiC composite, the fracture toughness of Al₂O₃–30 wt.%TiC–0.2 wt.%GNPs increased by 67.3% while the hardness decreased by 12.7%. Similar investigation was made by Cui et al. [81], where (0.2–0.8 wt.%) GNP was used for toughness enhancement in Al₂O₃–(W, Ti)C ceramic tool material. Maximum improvements were reported for 0.2 wt.%GNP reinforced Al₂O₃–(W, Ti)C nanocomposite, with 35.3% increase in fracture strength and 49% increase in flexural strength, as compared to Al₂O₃–(W, Ti)C without graphene. The enhancements were attributed to break of GNPs and crack guiding along with other conventional toughening mechanisms. Improvements in fracture toughness and flexural strength of multi-layer graphene (MLG) reinforced Al₂O₃–TiC nanocomposite tool material were reported by Li et al. [82]. The authors hot pressed 65.4 wt.%Al₂O₃ and 33.2–33.7 wt.%TiC reinforced with 0.1–0.5 wt.%MLG nanocomposites at 1650 °C. Fracture toughness and flexural strength values of 6.14 MPa m^1/2 and 981.5 MPa were obtained for Al₂O₃–TiC–0.2 wt.%MLG nanocomposite, which are 23.2% and 30.9% higher, respectively, than those of the Al₂O₃–TiC composite without MLG reinforcement. Zhao et al. [83] investigated the effect of varying the content of WC microparticles and TiC nanoparticles on the properties of Al₂O₃–TiC–WC ceramic tool material. They prepared Al₂O₃–(21–36) vol.%TiC–(14–4) vol.%WC nanocomposites by hot pressing at 1700 °C for 10 min and found that the Al₂O₃–24 vol.%TiC–16 vol.%WC nanocomposite show optimal mechanical properties with flexural strength of 842 MPa, fracture toughness of 6.82 MPa m^1/2, and hardness of 22.19 GPa.

Fig. 12.

Wear behavior of different tool materials Al₂O₃–WC–TiC (AWTG0), Al₂O₃–WC–TiC–0.5 vol.%Graphene (AWTG0.5), Al₂O₃–(W, Ti)C (SG4) and Al₂O₃–TiC (LT55) in machining hardened 40Cr steel at; (a) 100m/min. (b) 150m/min [77].

Al₂O₃–CNTs–graphene nanocomposites

Synergistic effects of 1 wt.%CNTs and 0.5 wt.%graphene nanoplatelets (GNP) on alumina have been investigated by Rahman et al. [84]. The authors used spray drying for powder preparation and sintered the samples by SPS at 1500 °C. They reported an increase in relative density from 91% to 99.9%, a 44% decrease in grain size, and a 20% decrease in hardness for Al₂O₃–1 wt.%CNT–0.5 wt.%GNP nanocomposite compared to alumina. The interfacial shear stress for Al₂O₃–1 wt.%CNT–0.5 wt.%GNP nanocomposite was found to be 28 times higher than that of Al₂O₃–1 wt.%CNT. Improvements by 29% and 250% in the elastic modulus and fracture toughness, respectively, were reported for Al₂O₃–1 wt.%CNT–0.5 wt.%GNP nanocomposite compared to alumina. The significant enhancement in toughness was linked not just to conventional toughening mechanisms like CNT pullout, grain gluing, bridging, graphene pullout, bending, sliding, and grain wrapping, illustrated in Figs. 13 and 14[84], but also to other toughening mechanisms such as CNT yarning and CNT-embedded graphene, as depicted in Fig. 15[84]. Furthermore, the high interfacial shear stress could have played a role in the toughness improvement [84]. Damavandi et al. [85] examined the effect of different powder preparation processes and varying content of reduced graphene oxide (rGO) and CNTs on mechanical and microstructural properties of alumina nanocomposites. Al₂O₃–(0.5–2) wt.%rGO–(0.5–4) wt.% CNT nanocomposites were prepared using two different powder mixing techniques i.e. wet mixing and sol–gel mixing, followed by hot pressing at 1650 °C. All the nanocomposites showed relative density values higher than 98%. The nanocomposites prepared using sol–gel process displayed superior properties i.e. 13, 70, and 14% improvements in hardness, fracture toughness, and flexural strength for Al₂O₃–1 wt.%rGO–1 wt.%CNT compared to alumina. The Al₂O₃–1 wt.%rGO–1 wt.%CNT nanocomposite prepared by sol–gel showed 62% reduction in grain size, 13% increase in hardness, 14% increase in fracture toughness, and 13% increase in flexural strength, compared to the same nanocomposite prepared through wet-mixing. Effect of changing the sintering route on microstructural and mechanical properties of Al₂O₃–GNP–CNT nanocomposites was studied by Yazdani et al. et al. [86]. Al₂O₃–(0.5–1) wt.%GNP–(0.5–2) wt.%CNT nanocomposites were prepared at 1650 °C using hot pressing for 1 hour and SPSfor 10 min. All nanocomposites had relative density values greater than 98%. Samples prepared using HP showed finer microstructure with homogenous grain distribution, than those prepared through SPS. Consequently, hardness and flexural strength of hot-pressed samples were larger than those of samples prepared by SPS. Highest values of flexural strength and hardness, in both cases, were reported to be >400 MPa and 5.5 MPa m^1/2, respectively. The Al₂O₃–0.5 wt.%GNP–0.5 wt.%CNT nanocomposite, prepared by SPS, showed highest mechanical properties. While for the hot-pressed samples, the Al₂O₃–0.5 wt.%GNP–1 wt.%CNT nanocomposite displayed highest mechanical properties. A 63% increase in fracture toughness and a 12% increase in flexural strength were observed [87], compared to alumina. In another investigation, Yazdani et al. [88] reported tribological properties of hot pressed Al₂O₃–GNP–CNT hybrid nanocomposites, under various loading conditions, using ball-on-disk wear test configuration. The Al₂O₃–0.3 wt.%GNP–1 wt.%CNT nanocomposite showed 20% reduction in coefficient of friction and 74% reduction in wear rate, compared to alumina. The enhanced tribological properties of the nanocomposite were attributed to the formation of a tribofilm as well as superior mechanical properties due to CNT and GNP reinforcements.

Fig. 13.

(a) CNT-grain gluing and (b) CNT-grain bridging or anchoring in the composite [64].

Fig. 14.

(a) graphene sliding and bending between Al₂O₃ grains and (b) graphene-grain wrapping [84].

Fig. 15.

(a) CNT yarn in Al₂O₃–1 wt.% CNT and (b) CNT implanted graphene in Al₂O₃–1 wt.% CNT–0.5 wt% GNP sintered nanocomposites [84].

Shah et al. [89] prepared Al₂O₃ and Al₂O₃–1 wt.%CNT–(0.4, 0.8 wt.%) graphene nanocomposites by SPSat 1500 °C and 21 MPa for 3 min. All sintered samples had relative density values greater than 99%. The authors reported 49.5% increase in the fracture toughness of Al₂O₃–1 wt.%CNT–0.4 wt.%graphene nanocomposite, compared to alumina. The Al₂O₃–1 wt.%CNT–0.4 wt.%graphene nanocomposite and alumina had flexural strength values of 494 and 415 MPa, respectively. The thermal properties decreased with the increase in reinforcement content. At 50 °C, thermal conductivity and thermal diffusivity of Al₂O₃ were found to be 23.25 W/m K and 7.56 mm²/s, respectively, which decreased to values of 16 W/m K and 5.5 mm²/s for Al₂O₃–1 wt.%CNT–0.8 wt.%graphene nanocomposite. Duntu et al. [90] investigated the fracture behavior of graphene and/or CNT reinforced Al₂O₃–ZrO₂ hybrid nanocomposites. Al₂O₃–10 wt.%ZrO₂ was reinforced with 0.5 wt.%graphene and 2 wt.%CNTs, and the nanocomposite powders were hot pressed at 1600 °C for 1 h. Addition of 10 wt.%ZrO₂ and 0.5 wt.%graphene to Al₂O₃ resulted in 56% increase in fracture toughness. Similarly, Al₂O₃–10wt.%ZrO₂–2wt.%CNTs nanocomposite showed fracture toughness and bending strength values of 7.55 MPa m^1/2 and 460 MPa, compared to 3.4 MPa m^1/2 and 259 MPa for Al₂O₃. Highest increases of 159% in fracture toughness and 86% in bending strength were observed when 2 wt.%CNTs and 0.5 wt.%graphene were added to Al₂O₃–10wt.%ZrO₂. In another work, Duntu et al. [91] studied the wear behavior of Al₂O₃–ZrO₂–graphene/CNT hybrid nanocomposites. Al₂O₃–ZrO₂–0.5wt.%GN showed 2% increase in hardness and 91% increase in wear resistance compared to alumina. Similarly, the hardness and fracture toughness of Al₂O₃–ZrO₂–2wt.%CNT increased by 11 and 92%, while those of Al₂O₃–ZrO₂–0.5wt.%GN–2wt.%CNT increased by 48 and 93%.

Other Al₂O₃ nanocomposites

The effect TiC and Ni on alumina was investigated by Rodrigues Suarez et al. [92]. They produced Al₂O₃–25 vol.%TiC–1.9 vol.%Ni nanocomposite by SPS at 1375 °C. Improvements of 30% and 75% in hardness and flexural strength, respectively, were reported for Al₂O₃–TiC–Ni nanocomposite, compared to alumina, while no significant change in fracture toughness was observed. A very low value of electrical resistivity (very high electrical conductivity) of 3.15 × 10⁻⁵ Ωm was reported for the Al₂O₃–TiC–Ni nanocomposite. Moreover, the wear rate of Al₂O₃–TiC–Ni nanocomposite was found to be 25 times lower than alumina. In other work [73], no significant change in fracture toughness was observed when Al₂O₃ was reinforced with 10 vol.% niobium and 5 vol.% single wall carbon nanotubes (SWCNT) [93]. SPS sintering at 1200 °C resulted in 98.4% densification, fracture toughness of 3.3 MPa m^1/2, and hardness of 19.3 GPa for Al₂O₃–5 vol.%SWCNT–10 vol.%Nb nanocomposite. The authors proposed that the damage to SWCNTs during sintering and the non-optimal interfacial bond at the SWCNT–alumina interface were reasons for not observing improvements in fracture toughness. Effects of varying TiC content (2–6 vol.%) and particle size (40–500 nm) on mechanical and microstructural properties of Al₂O₃–20 vol.%SiC–TiC nanocomposites were investigated by Zhao and co-authors [94]. Increase in hardness, fracture toughness, and flexural strength were observed with the increase in TiC content up to 4 vol.%, beyond which the properties decreased. Varying particle size was found to affect only flexural strength, while hardness and fracture toughness remained unaffected. Optimized properties were obtained at 40 nm particle size and 4 vol.% TiC content for the Al₂O₃–20 vol.%SiC–4 vol.%TiC nanocomposite. In another work, Zhao and co-authors [95] used the same optimized Al₂O₃–20 vol.%SiC–4 vol.%TiC nanocomposite and studied its mechanical properties at elevated temperatures up to 1200 °C in comparison with Al₂O₃–20 vol.%SiC composite. Hardness and fracture toughness of the Al₂O₃–SiC–TiC nanocomposite at all temperatures were higher than those of the Al₂O₃–SiC composite. The Al₂O₃–SiC–TiC nanocomposite showed low high temperature flexural strength, in contrast with high flexural strength at room temperature, compared to Al₂O₃–SiC composite. The low high-temperature flexural strength was attributed to change in fracture mode to intergranular at elevated temperatures. Both hardness and flexural strength decreased with the increase in temperature. At 1000 °C, the Al₂O₃–SiC–TiC nanocomposite maintained 68.4% of its room temperature hardness. At 1100 °C, the flexural strength of the Al₂O₃–SiC–TiC nanocomposite was maintained at 60% of the room temperature value. The elastic modulus of both Al₂O₃–SiC and Al₂O₃–SiC–TiC nanocomposites remain unchanged up to 1000 °C and begin to decrease at 1100 °C. Sun et al. [96] studied the properties of Al₂O₃-based tool materials reinforced with nano-TiC and graphene. They prepared Al₂O₃–(0–30 vol.%)TiC–(0–0.75 vol.%)Graphene nanocomposites using hot pressing at 1650 °C for 10 min. They used 10 vol.% nano-Al₂O₃, 0.5 vol.% MgO, and Y₂O₃ as sintering aid and 0.5 vol.% Ni and Mo as metal binders. The optimum composition was found to be Al₂O₃–10 vol.%TiC–0.5 vol.%Graphene giving flexural strength of 705 MPa, fracture toughness of 7.4 MPa m^1/2, and hardness of 20.5 GPa. The enhancement in toughness was attributed to the toughening mechanisms induced by both reinforcements. Mechanical and tribological properties of graphene-reinforced Al₂O₃–Ti(C,N) nanocomposites have been reported by Petrus et al. [75]. They used 0, 0.2, 0.5, 0.7 and 1 wt.%graphene in form of multilayer graphene (MLG), graphene oxide (GO), and nikel-coated-graphene to produce Al₂O₃–Ti(C,N) hybrid nanocomposites by SPSat 1550 °C for 4 min. All nanocomposites showed relative density values between 95 and 98%. The Al₂O₃–Ti(C,N)–0.2 wt.%GO nanocomposite showed slightly higher hardness value of 2025 HV compared to 1950 HV for Al₂O₃–Ti(C,N) composite. A decrease in fracture toughness was reported for the graphene-reinforced Al₂O₃–Ti(C,N) nanocomposites. However, the Al₂O₃–Ti(C,N)–0.2 wt.%GO nanocomposite showed 800% lower wear rate, compared to Al₂O₃–Ti(C,N) composite. Sun et al. [97] reported the properties of Al₂O₃–TiC–GNP–MWCNTs prepared by addition of 0.2–0.8 wt.%MWCNT and 0.2–0.7 wt.%GNP to Al₂O₃–TiC. The composites were spark plasma sintered at 1600 °C for 5 min. The Al₂O₃–TiC reinforced with 0.8 wt.%MWCNTs and 0.2 wt.%GNP nanocomposite showed the best properties with relative density of 97.3%, hardness of 18.38%, and fracture toughness of 9.40 MPa m^1/2. Toughening mechanisms like crack bridging and branching, crack deflection and drawing by CNT and GNP reinforcements were mainly responsible for the enhanced toughness. Chen et al. [98] investigated the effect of sintering parameter and graphene content on mechanical properties of Al₂O₃–TiB₂ nanocomposites. Al₂O₃ was reinforced with 20 wt.%TiB₂ and varying GNP content (0.1–0.6 wt.%), and 0.5 wt.%MgO and 0.5 wt.%Y₂O₃ were used as sintering aids. The authors sintered Al₂O₃–TiB₂ samples by SPS at varying temperatures and times to find the optimum conditions. They found that 1525 °C and 5 min give the optimum mechanical properties and sintered the graphene-reinforced nanocomposites at the same conditions. All nanocomposites showed relative density greater than 98%. The Al₂O₃–TiB₂ reinforced with 0.3 wt.%GNP showed optimum mechanical properties with 6.4% increase in flexural strength and 46.1% increase in fracture toughness compared to Al₂O₃–TiB₂ composite. Nevertheless, a 5.2% decrease in hardness was observed.

ZrB₂ nanocomposites

Zirconium diboride (ZrB₂) belongs to the class of advanced materials known as ultrahigh temperature ceramics (UHTC) [99], which are applied in extreme environments as electrodes for electric discharge machining, crucibles in the metal industry, and structural and functional parts in the aerospace industry [99,100]. Among various UHTC materials, ZrB₂ is the most widely investigated and applied ceramic material because of its very high melting temperature, relatively low density and cost, chemical inertness, high temperature stability and strength, acceptable thermal and electrical conductivity [99]. Typical properties of ZrB₂ are listed in Table 1. However, despite such an excellent combination of properties, there are certain challenges that limit the use of ZrB₂ ceramic such as brittleness and difficulty to sinter. While the use of hot pressing (HP) and SPS improved the sinterability of ZrB₂, the improvement of its fracture toughness was possible by introducing a single reinforcement such as SiC [101–103], CNTs [104,105], and graphene [105,106], resulting in the formation of nanocomposites [99]. It was also reported that introducing reinforcements in ZrB₂ ceramic facilitated densification, and contributed to the improvement of fracture toughness and resistance to oxidation [107,108]. Moreover, it was found that further enhancements in fracture toughness were possible through the incorporation of two distinct reinforcements in the ZrB₂ matrix [109,110], forming hybrid nanocomposites.

Table 1.

Properties of some ultrahigh temperature ceramics.

	ZrB2[99,100]	ZrC[111–113]	TiN[113–115]	TiC[113,116]	TaC[113,117]	HfC[113,118]
Melting Point (°C)	3245	3420	2950	3160	3950	3900
Density (g/cm^3)	6.1	6.64	5.40	4.93	14.5	12.2
Fracture toughness (MPa m^1/2)	1.8–3.5	3.1	2.5	3.1–3.7	3.1	1.73
Hardness (GPa)	>20	25.5	20	>24	16.7	22–25
Elastic modulus (GPa)	489	350–440	251	320	285–560	300–500
Thermal conductivity (W/m K)	60–80	20.5	19.2	21	22.1	20
Coefficient of thermal expansion (×10^−6/°C)	6.6	6.6	9.3	7.4	6.3	6.6
Electrical conductivity (×10^6 S/m)	10	1.28	4	1.5	4	2.7

Graphite/graphene reinforced ZrB₂–SiC nanocomposites

Asl and co-authors [119] investigated the effect of SPS parameters and nano-graphite flakes (G_nf) content on the densification of ZrB₂–25 vol.%SiC–(5, 10 wt.%)G_nf nanocomposites. The authors used Taguchi design of experiments approach to set the experimental conditions. They observed that, among all parameters, sintering temperature and G_nf content had the most influential effect on the relative density. The ZrB₂–25 vol.%SiC–5 wt.%G_nf nanocomposite sintered at 1900 °C and 40 MPa for 4 min had a relative density of 99.5%. Microstructural analysis of the fully dense sample showed the formation of ZrC/B₄C clusters at ZrB₂/G_nf interface. The addition of G_nf helped in the removal of oxide film from ZrB₂ surface as well as formation of ZrC/B₄C within the grain boundaries, both of which have contributed toward increased sinterability of the nanocomposite. In another work, Asl et al. [120] investigated the interfacial behavior of spark plasma sintered ZrB₂–20 vol.%SiC–10 vol.%GNPs nanocomposite. Full densification was achieved using SPS conditions of 1800 °C and 6 min. Analysis of TEM nanographs of ZrB₂/SiC and ZrB₂/ZrB₂ interfaces did not reveal the presence of ZrC or B₄C compounds. Because of these clean interfaces, it was concluded that GNPs acted as sintering aids, by facilitating diffusion-induced joining of particles. Other researchers [107] reported the densification and mechanical behavior of GNSs reinforced ZrB₂–SiC nanocomposites, prepared by in situ thermal reduction of graphene oxide during hot-pressing. Formation of GNSs was confirmed by Raman spectroscopy. The relative density of the ZrB₂–SiC composite (98.2%) increased to 98.9% and 99.2% for the ZrB₂–20 vol.%SiC–2 vol.%GNSs and ZrB₂–20 vol.%SiC–5 vol.%GNSs nanocomposites, respectively. The fracture toughness and flexural strength increased with the increase in the content of GNSs, and reached 7.32 MPa m^1/2 and 1055 MPa for the ZrB₂–20 vol.%SiC–5 vol.%GNSs nanocomposite, which are 73.8% and 100% higher than those of ZrB₂–SiC. However, slight decrease in hardness was observed, from 23.07 GPa for ZrB₂–20 vol.%SiC composite to 22.76 GPa for ZrB₂–20 vol.%SiC–5 vol.%GNSs. Asl et al. [110] investigated the effect of the addition of G_nf (0–10 wt.%) on the properties of spark plasma sintered ZrB₂–25 vol.%SiC composites. A relative densification more than 98% was reported for all samples. The ZrB₂–25 vol.%SiC–5 wt.%G_nf nanocomposite showed uncommon relative density of 100.7% which was attributed to the in-situ formation of ZrC phase. The hardness of ZrB₂–25 vol.%SiC composite decreased from 19.5 to 12.1 GPa for ZrB₂–25 vol.%SiC–10 wt.%G_nf. However, fracture toughness increased with the increase in graphite content and reached a value of 8.2 MPa m^1/2 for ZrB₂–25 vol.%SiC–7.5 wt.%G_nf, which is twice the value of ZrB₂–25 vol.%SiC. The increase in fracture toughness was attributed to toughening effect of the reinforcements, microstructure refinement, and in-situ formation of ZrC and B₄C compounds. In another work, Asl et al. [121] reported an increase in relative density from 90.1% to 99.6%, with the addition of 20 vol.%SiC and 10 vol.% G_nf to ZrB₂. Also, the hardness and fracture toughness of ZrB₂–20 vol.%SiC–10 vol.%G_nf nanocomposite increased by 38 and 294%, respectively, compared to ZrB₂. In a similar work, Asl et al. [122] reported 30% increase in hardness and more than 250% increase in fracture toughness of hot-pressed ZrB₂–25 vol.%SiC–5 wt.%GNPs nanocomposite, compared to ZrB₂. The effect of varying the content of G_nf on microstructural and mechanical properties of ZrB₂–SiC_w–G_nf nanocomposites has been investigated by Asl et al. [123]. ZrB₂–SiC_w–G_nf nanocomposites were prepared by reinforcing ZrB₂ with 25 vol.%SiC_w and 0–7.5 wt.% G_nf. The samples were sintered using SPS at 1900 °C for 7 min. A relative densification of more than 99.9% was observed for all samples significant grain refinement was observed with the increase in graphite content. The obtained hardness and fracture toughness of ZrB₂–SiC_w–G_nf nanocomposites are shown in Fig. 16. A 10% decrease in hardness and 32% increase in fracture toughness were reported for ZrB₂–25 vol.%SiC_w–7.5 wt.%G_nf and ZrB₂–25 vol.%SiC_w–5 wt.%G_nf nanocomposites, respectively, compared to ZrB₂–25 vol.%SiC_w. Enhancements in fracture toughness of ZrB₂ because of the addition of 0–7.5 wt.% GNPs and 25 vol.% SiC_w were reported by Xia et al. [124]. The ZrB₂–GNPs–SiC_w hybrid nanocomposites were sintered by SPS at 1900 °C for 7 min. All nanocomposites showed relative density values higher than 99.75%. Microstructural analysis showed grain growth inhibition in the hybrid nanocomposites by GNPs. The average grain size of ZrB₂ in ZrB₂–25 vol.%SiC_w composite decreased from 5.2 to 3.1 μm for ZrB₂–25 vol.%SiCw–7.5 wt.%GNPs nanocomposite. A slight increase in hardness, from 22 to 22.2 GPa was observed because of the inclusion of 2.5 wt.%GNPs in ZrB₂–25 vol.%SiC_w nanocomposite. However, 30% increase in fracture toughness was reported for ZrB₂–25 vol.%SiC_w–7.5 wt.%GNPs nanocomposite. A ZrB₂–20 vol.%SiC–Graphite nanocomposite was prepared by sol gel process and spark plasma sintered at 1950 °C for 10 min [125]. Microstructural analysis of the sintered sample showed uniform distribution of SiC, along with the presence of graphite nanosheets. Flexural strength and fracture toughness of the ZrB₂–20 vol.%SiC–Graphite nanosheets composite was found to be 318 MPa and 3.09 MPa m^1/2, respectively. Parvizi et al. [126] studied the synergistic effect of graphite nano-flakes (G_nf) and SiC reinforcements on properties of spark plasma sintered ZrB₂. The relative density of ZrB₂ increased from 96.1 to 100.7% for ZrB₂–25 vol.%SiC–5 wt.%G_nf. Hardness, fracture toughness, and flexural strength of 13.1 GPa, 3.2 MPa m^1/2, and of 445 MPa for ZrB₂ increased to 16.6 GPa, 6.7 MPa m^1/2, and 631 MPa for ZrB₂–25 vol.%SiC–5 wt.%G_nf nanocomposite, respectively. The effect of varying diameter of graphite reinforcement on mechanical and thermal shock properties of ZrB₂–20 vol.%nano-SiC–20 vol.%Graphite nanocomposites was investigated by Hou et al. [127]. ZrB₂–20 vol.%SiC–20 vol.%Graphite nanocomposites, with different graphite diameters of 5, 10 and 20 μm, were hot pressed at 1900 °C. Relative density of more than 98% was observed for all samples. ZrB₂–20 vol.%SiC–20 vol.%Graphite nanocomposites with 10 μm diameter graphite showed the highest flexural strength of 523 MPa, fracture toughness of 4.54 MPa m^1/2, and Young's modulus of 356 GPa. The nanocomposite with smallest graphite diameter showed better thermal shock resistance. Nguyen et al. [128] investigated sintering behavior and microstructure of ZrB₂ reinforced with 25 vol.%SiC_w whiskers and 5 wt.%GNPs. Fully dense ZrB₂–SiC_w–GNPs nanocomposite was prepared using SPS at 1900 °C. XRD analysis showed no new phase formation during sintering. Microstructural analysis revealed ultra-thin amorphous layer at ZrB₂–GNPs interface and dislocation in ZrB₂.

Fig. 16.

(a) Hardness and (b) fracture toughness of ZrB₂–25 vol.%SiC_w–G_nf nanocomposites as a function of G_nf[123].

CNT reinforced ZrB₂–SiC nanocomposites

Tian and co-authors investigated the effect of CNTs on densification, mechanical and thermal properties of hot-pressed ZrB₂–SiC [129]. Slight decrease in densification, hardness (6%) and thermal conductivity (4%) were observed for ZrB₂–20 vol.%SiC–2 wt.%CNTs nanocomposite compared to ZrB₂–20 vol.%SiC. But, the same composite showed 15% increase in fracture toughness and 6% increase in flexural strength. In other work [130], hot-pressed ZrB₂ reinforced with 20 vol.%SiC and 10 vol.%MWCNTs displayed better properties compared to ZrB₂. The relative density of ZrB₂ increased from 90.1% to 93.9% for ZrB₂–20 vol.%SiC–10 vol.%CNTs nanocomposite. The reinforcements also contributed to inhibiting the grain growth during sintering. A 180% increase in fracture toughness while 30% decrease in hardness were reported for the ZrB₂–20 vol.%SiC–10 vol.%CNTs nanocomposite compared to ZrB₂. Nisar et al. [131] reported enhancements in the properties of ZrB₂ ceramic composite, produced by SPS, through the synergistic reinforcement of SiC and CNTs. The relative density increased from 93.1 (ZrB₂) to 99.7 (ZrB₂–20 vol.% SiC–10 vol.% CNTs), while the fracture toughness and flexural strength were 3 and 1.6 times higher, respectively. In other work, Nisar et al. [132] observed significant improvements in densification, mechanical properties and thermal performance of spark plasma sintered ZrB₂–20 vol.%SiC–10 vol.%CNTs nanocomposite. The relative density of ZrB₂ increased from 93.1 to 99.7% for ZrB₂–20 vol.%SiC–10 vol.%CNTs. To study the oxidation behavior, samples were exposed to plasma arc-jet with heat flux of 2.5 MW/m² for 30 seconds. After the oxidation test, 42% ZrB₂ phase retention along with lower oxidation rate of 0.44 μm/s were observed for ZrB₂–20 vol.%SiC–10 vol.%CNTs, compared to 30% and 0.77 μm/s for ZrB₂, respectively. The thickness of oxide layer was 10 μm for ZrB₂–20 vol.%SiC–10 vol.%CNTs, showing more oxidation resistance, compared to 23 μm for ZrB₂. Compared to ZrB₂, higher hardness was reported for ZrB₂–20 vol.%SiC–10 vol.%CNTs nanocomposite in both cases; 1.6 times higher in unexposed samples and 2.1 times higher in exposed samples. The thermal conductivity, thermal diffusivity, and specific heat of ZrB₂ increased from 48.9 W/m K, 0.236 mm²/s, and 340.4 J/kgK at 50 °C to 61.8 W/m K, 0.24 mm²/s, and 485.2 J/kgK for ZrB₂–20 vol.%SiC–10 vol.%CNTs, respectively. The TGA results showed that the oxidation temperature shifted from 679.3 °C for ZrB₂ to 406.3 °C for ZrB₂–20 vol.%SiC–10 vol.%CNTs, an indication of enhanced oxidation resistance. Popov and co-authors [133] prepared ZrB₂–31 wt.%SiC and ZrB₂–29 wt.%SiC–8 vol.%CNTs composites by reactive hot pressing ZrC, B₄C, Si, and CNTs powders in the temperature range 1100–1830 °C. The ZrB₂–29 wt.%SiC–8 vol.%CNTs nanocomposite showed 60% improvement in fracture toughness compared to ZrB₂–29 wt.%SiC.

Nano-carbon reinforced ZrB₂–SiC nanocomposites

Nano-sized carbon black (C_nb) was used to prepare a ZrB₂–20 vol.%SiC–10 vol.%C_nb nanocomposite by HP at 1850 °C for 60 min [134]. The sintered nanocomposite nearly reached fully density (99.8%). A pore-free microstructure and strong interfaces between ZrB₂, SiC and C_nb could be observed in the TEM image of ZrB₂–20 vol.%SiC–10 vol.%C_nb nanocomposite shown in Fig. 17. XRD analysis revealed partial conversion of carbon-black to crystalline graphite, which was also confirmed by TEM. In other work, Azizian Kalandaragh et al. [109] studied the effect of varying carbon nanoparticles (C_np) content on microstructural and mechanical properties of spark plasma sintered ZrB₂–25 vol.%SiC_w–C_np nanocomposites. They reported a relative density higher than 99.8% for all sintered samples and observed reduction in grain growth. The hardness was found to decrease linearly from 21.9 to 14.6 GPa for ZrB₂–25 vol.%SiC_w–7.5 wt.%C_np nanocomposite. However, the fracture toughness increased from 4.7 MPa m^1/2 for ZrB₂–25 vol.%SiC_w composites to 7.1 MPa m^1/2 for ZrB₂–25 vol.%SiC_w–5 wt.%C_np. Hu et al. [135] used carbon fibers (C_f) to produce ZrB₂–20 vol.%SiC–30 vol.%C_f by HP at relatively low temperature of 1450 °C. They reported a relative density of 95.8%, flexural strength of 341 MPa, and fracture toughness of 6.12 MPa m^1/2. The ZrB₂–20 vol.%SiC–30 vol.%C_f nanocomposite displayed high work of fracture compared to other ZrB₂-based composites without C_f. Nasiri et al. [136] studied the effect of varying the content of C_f (2.5–15 wt.%) and SiC nanoparticles (2.5–10 wt.%) on densification and mechanical properties of pressure-less sintered ZrB₂–SiC–C_f nanocomposites. The relative density, hardness, and fracture toughness increased with the increase in SiC content up to 10 wt.%, beyond which the properties began to decrease. However, the increase in C_f reinforcement content caused the density, hardness, and fracture toughness to decrease in ZrB₂–SiC–C_f nanocomposites. The ZrB₂–10 vol.%SiC–2.5 vol.%C_f had optimum properties with a relative density of 93%, hardness of 14.5 GPa, and fracture toughness of 5.9 MPa m^1/2. The effect of C_nb addition on microstructural and mechanical properties of ZrB₂–SiC composites were reported by Farahbakhs and co-authors [137]. They hot pressed ZrB₂, and ZrB₂–20 vol.%SiC and ZrB₂–20 vol.%SiC–10 vol.%C_nb composites at 1850 °C. Relative density values of 90 and 93% were reported for ZrB₂ and ZrB₂–20 vol.%SiC samples, which increased to 99.8% for ZrB₂–20 vol.%SiC–10 vol.%C_nb sample. The authors observed refinement of the microstructure and concluded that C_nb acted as a sintering aid. The SEM analysis of the ZrB₂–20 vol.% SiC–10 vol.% C_nb nanocomposite, presented in Fig. 18, showed the presence of some unreacted C_nb along with SiC and ZrB₂, while some carbon was believed to react with the surface oxides of ZrB₂, resulting in enhanced densification. Hardness and flexural strength values of 11.9 GPa and 355 MPa for ZrB₂ increased to 16.7 GPa and 687 MPa, respectively, for the ZrB₂–20 vol.%SiC–10 vol.%C_nb nanocomposite. Moreover, a 180% increase in fracture toughness was observed for ZrB₂–20 vol.%SiC–10 vol.%C_nb nanocomposite compared to ZrB₂ (2.1 MPa m^1/2). Zhou et al. [138] also observed improvements in fracture toughness and thermal properties of hot-pressed ZrB₂–20 vol.%SiC composite because of the addition of 5 vol.% C_nb. The ZrB₂–20 vol.%SiC–5 vol.%C_nb nanocomposite had a high fracture toughness of 6.6 MPa m^1/2. Also, the thermal shock resistance parameter of the hybrid nanocomposite was 15% higher than ZrB₂–20 vol.%SiC nanocomposite. Savari and co-authors [139] investigated the densification and toughness of ZrB₂ reinforced with SiC (15 and 30 vol.%) and C_nb(10 and 15 vol.%). The nanocomposites were spark plasma sintered at 1850 °C and 35 MPa for 8 min. The ZrB₂ showed low relative density of 81%, however, nearly full density was obtained when ZrB₂ was reinforced with SiC and C_nb. Significant improvements in fracture toughness were reported for single reinforcement with C_nb, but only a marginal increase of 26% was observed for hybrid reinforcement in ZrB₂–30 vol.%SiC–(10, 15 vol.%)C_nb nanocomposites.

Fig. 17.

TEM image of ZrB₂–20 vol.%SiC–10 vol.%C_nb nanocomposite [134].

Fig. 18.

FE-SEM image of fractured surface of ZrB₂–20 vol.%SiC–10 vol.%C_nb nanocomposite showing unreacted nano-carbon black [137].

Other ZrB₂–SiC nanocomposites

Li et al. [140] reported properties of hot-pressed ZrB₂–15 vol.%SiC_w–(10,15,20) vol.%MoSi₂ nanocomposites. A relative density of 99.9% was obtained for all samples at 1750 °C. ZrB₂–15 vol.%SiC_w–15 vol.%MoSi₂ nanocomposite showed the highest mechanical properties, with hardness of 17.64 GPa, fracture toughness of 5.52 MPa m^1/2, and flexural strength of 564 MPa. Yue et al. [141] used SPS to prepare ZrB₂–SiC reinforced with mixture of boron nitride nanotubes (BNNT) and boron nitride nanoplateles (BNNP). While the hardness and elastic modulus remained unchanged, a 24% increase in the fracture toughness was observed in the ZrB₂–20 vol.% SiC–1 wt.% (BNNT–BNNP) nanocomposite compared to ZrB₂–20 vol.% SiC. Fattahi et al. [142] studied the effect of 0–3 wt.% nano-diamond on microstructural and mechanical properties of ZrB₂–25 vol.% SiC nanocomposites. They prepared samples using SPS at 1900 °C for 7 min. All nanocomposites showed relative density higher than 98.8%. The authors analyzed the microstructure of the reinforced nanocomposites and observed the formation of ZrC and B₄C. They reported increased hardness and fracture toughness, and the highest values were 24.6 GPa for ZrB₂–25 vol.% SiC–2 wt.% nano-diamond nanocomposite and 5.8 MPa m^1/2 for the ZrB₂–25 vol.% SiC–3 wt.% nano-diamond, as compared to 19.5 GPa and 4.3 MPa m^1/2 for ZrB₂–25 vol.% SiC composite, respectively. They attributed the increase in hardness to the formed hard phases during sintering, while the toughness increase was attributed to the establishment of toughening mechanisms such as crack deflection, branching, and crack bridging due to the presence of nano-diamond.

ZrC nanocomposites

Zirconium carbide (ZrC) belonging to the UHTCs group and has low density, very good mechanical and electrical properties, and good chemical and thermal stability [111]. Typical properties of ZrC are listed in Table 1. Difficulty to sinter and small fracture toughness are some challenges in the application of ZrC. [143]. Using new sintering techniques along with reinforcing ZrC matrix with a second phase have resulted in significant improvements in both fracture toughness and sinterability [112,144–147]. However, limited work is available on ZrC reinforced with hybrid reinforcements.

Acicbe et al. [111] studied the effect of TiC and MWCNTs on densification and mechanical properties of ZrC–TiC and ZrC–TiC–CNTs nanocomposites. ZrC showed relative density of 95.5%, which increased to more than 98% for all ZrC–TiC and ZrC–TiC–CNTs nanocomposites. Microstructural analysis showed homogeneous distribution along with some micro-cracks in ZrC–TiC composite, which disappeared by introducing CNTs in case of ZrC–TiC–CNTs nanocomposite. The hardness of ZrC increased from 17.6 to 22 GPa for ZrC–TiC. However, introduction of CNTs caused the hardness to decrease, reaching a value of 20 GPa for ZrC–20 vol.%TiC–1 wt.%CNTs nanocomposite. Increase in fracture toughness of ZrC (3.30 MPa m^1/2) was observed with increasing the reinforcement content, reaching a value of 4.27 MPa m^1/2 for ZrC–20 vol.%TiC–1 wt.%CNTs nanocomposite. Similar enhancements in properties of ZrC, by addition of 20 vol.% Ti and 3 vol.%CNTs, have been reported by Sha et al. [143]. A relative density of 98.27%, a hardness of 18.11 GPa, a flexural modulus of 207 GPa, and a flexural strength of 415 MPa were measured for ZrC–20 vol.%Ti–3 vol.%CNTs nanocomposite. Moreover, a fracture toughness of 5.27 MPa m^1/2 was obtained for the nanocomposite, which is 32% higher than that of ZrC. Li et al. [147] investigated the effect of 20 vol.%Nb and 3 vol.%CNTs on properties of hot-pressed ZrC as shown in Fig. 19. Improved sinterability was observed for ZrC–20 vol.%Nb–3 vol.%CNTs nanocomposite, reaching 99.28%. Microstructural analysis showed the formation of (Zr, Nb)C solid solution and Zr_xC_yO_z compound during hot-pressing. An 80% increase in fracture toughness was reported for the nanocomposite, compared to ZrC. Furthermore, a hardness of 17.93 GPa, a flexural modulus of 281 GPa, and a flexural strength of 590 MPa were obtained for ZrC–Nb–CNT nanocomposite. The effect of GNPs addition on microstructural and mechanical properties of ZrC–TiC based nanocomposites have been reported by Ocak et al. [148]. ZrC–(20−x) vol.%TiC–x vol.%GNPs (where x = 0–9 vol.%) nanocomposites were spark plasma sintered at 1700 °C. Microstructural analysis showed the formation of (Zr, Ti)C solid solution by substitution of Zr⁺⁴ ions by Ti⁺⁴ during sintering. Densification of 99.49% and fracture toughness of 5.17 MPa m^1/2 for ZrC–17 vol.%TiC–3 vol.%GNPs nanocomposite was found to be highest. Further addition of GNPs beyond 3 vol % resulted in a decrease in densification and fracture toughness. The hardness was found to decrease with an increase in GNPs content in ZrC–TiC–GNPs nanocomposites.

Fig. 19.

Properties of ZrC-based composite doped with Nb and CNT (a) relative density, (b) flexural stress–strain, and (c) load–displacement [147].

TiN nanocomposites

Typical properties of titanium nitride (TiN) are listed in Table 1. TiN has very high melting temperature, high hardness, good thermal and electrical conductivity, and very high corrosion resistance. It is mostly used in fabricating cermets and as coatings on metals and cutting tools [114,115]. Its use in many industries is limited because of its inadequate fracture toughness and poor sinterability [114,149]. Some of these limitations have been countered by the addition of a single reinforcement such as TiC [149], Si₃N₄[150], AlN [151], HfC [152], and graphene [115]. Due to the fact that TiN is employed as reinforcement for ceramic matrices, very limited work is available on TiN hybrid nanocomposites.

The effect of SiC and MWCNTs on densification and microstructure of TiN was investigated by Delbari and co-authors [114]. TiN and TiN–20 vol.%SiC–5 wt.%MWCNTs nanocomposite were produced by SPS at 1900 °C and 40 MPa for 7 min. The relative density decreased from 81.5% for TiN to 79.9% for the TiN–20 vol.%SiC–5 wt.%MWCNTs nanocomposite as can be seen in Fig. 20. The authors analyzed the microstructure and observed the presence of SiC and MWCNTs at grain boundaries, which believed to hinder grain growth. They examined the fracture surface of the sintered nanocomposite, which showed several toughening mechanisms such as crack bridging, crack deflection, crack branching, and intergranular fracture mode [114].

Fig. 20.

Density of TiN-based ceramics [114].

TiC nanocomposites

Titanium carbide (TiC) has very high melting temperature, low density, high hardness, high thermal stability and conductivity, and high electrical conductivity as listed in Table 1. It is a suitable material for hypersonic space applications, cutting tools, refractory components, and in the nuclear industry [153–155]. To address issues associated with the sinterability and the inadequate fracture toughness of TiC, researchers developed TiC nanocomposites reinforced with a single phase such as graphene [154], SiC [155,156], ZrC [157], WC [158,159], nano-diamond [160], TiN [161] and CNTs [116], and hybrid nanocomposites that incorporates two phases [116,153,162–168].

TiC and hybrid TiC nanocomposite reinforced with 3.5 wt.% WC and 2 wt.% MWCNTs were prepared by SPS [116]. The relative density of TiC increased from 89 to 99% due to the addition of WC and CNTs. Furthermore, a reduction of 38% in grain size was observed for the TiC–3.5 wt.%WC–2 wt.%CNTs nanocomposite compared to TiC. The hardness, elastic modulus, and fracture toughness of the TiC–3.5 wt.%WC–2 wt.%CNTs nanocomposite increased by 41%, 42%, and 141%, respectively, compared to TiC [116]. In another study, Sribalaji et al. [153] investigated the thermal shock resistance of spark plasma sintered TiC and TiC–3.5 wt.%WC–2 wt.%CNTs nanocomposite. The samples were thermally shocked at 1700 °C in open air for 10 thermal cycles. Microstructural analysis of thermally shocked nanocomposite showed retention of most of CNTs, while some of them collapse to carbon. The nanocomposite showed 63% lower rate of mass change and 89% higher thermal shock resistance parameter (R_st), compared to the monolithic ceramic. The significant increase in thermal shock resistance was attributed to higher densification, higher fracture toughness, and homogenous distribution of CNTs. The thermally shocked TiC–3.5 wt.%WC–2 wt.%CNTs nanocomposite showed 74% higher hardness and 54% higher elastic modulus, compared to TiC. Fattahi et al. [162] studied the effect of 0–30 vol.% SiC_w whiskers and 3 wt.% nano-WC on properties of TiC nanocomposites. Samples were prepared by SPS at 1900 °C and 40 MPa for 7 min. The sintered nanocomposites showed high relative density and uniform dispersion of the reinforcements, and no significant grain growth was observed. The TiC–3 wt.%WC_n and TiC–3 wt.%WC_n–20 vol.%SiC_w samples had the highest hardness and flexural strength of 28.6 GPa and 694 MPa, respectively. In another work, Fattahi et al. [163] varied the amount of nano-sized WC (0, 1.5 and 3 wt.%) in TiC–10 vol.%SiC_w; and reported an increase in the relative density of the TiC–SiC_w composite from 98.73 to 99.43% with the addition of 1.5 wt.% nano-WC. However, the density decreased with further addition of WC. Non-stoichiometric TiC_x, in-situ TiC, SiC, and WSi₂ were found to form in sintered samples. The hardness and flexural strength decreased, with the increase in the content of nano-WC, and reached 12 GPa and 368 MPa for TiC–10 vol.%SiC_w–3 wt.%WC nanocomposite, compared to 24.54 GPa and 511 MPa for TiC–10 vol.%SiC_w, respectively. The authors attributed the decrease in the properties to the presence of the brittle WSi₂ at grain boundaries. They reported slight increase in thermal conductivity, from 19.8 to 24.3 W/m K when TiC–10 vol.%SiC_w was reinforced with 1.5 wt.% nano-WC. Nguyen et al. [164] reinforced TiC with 0–5 wt.% h-BN and 0–5 wt.% G_nf to form hybrid nanocomposites and characterized the microstructure, and mechanical and frictional properties. They sintered the samples using SPS at 1900 °C for 10 min, and found that the hybrid reinforcements had negative impact on densification, where the relative density decreased from 95.5% (TiC) to 93.2% (TiC–5 wt.%h-BN–5 wt.%G_nf). Microstructural analysis of sintered samples revealed the formation of TiB₂ and TiB in-situ phases, and the presence of carbon zones shown by dark areas in the scanning electron microscope (SEM) micrograph presented in Fig. 21 and confirmed by energy dispersive spectroscopy (EDS) results. The h-BN reinforcement and the in-situ phases appeared uniformly distributed while the G_nf were present in the form of zones. Vickers hardness and flexural strength of TiC decreased from 3128 HV and 504 MPa to 2477 HV and 457 MPa, respectively, with addition of 5 wt.%h-BN and 5 wt.%G_nf. However, the coefficient of friction increased from 0.299 for TiC to 0.315 for the TiC–5 wt.%h-BN–5 wt.%G_nf. In another work, Nguyen et al. [165] reported the mechanical and thermal properties of TiC reinforced with 5 wt.%TiN and 5 wt.%nano-diamond. The nanocomposite was sintered using SPS at 1900 °C for 10 min. The authors found that the relative density decreased from 95.5 to 87.4% as a result of the addition of TiN and nano-diamond, and attributed this decrease to the formation of N₂ gas due to the reaction between solid solution of Ti (C, N) formed due to TiN addition and graphitized carbon formed from the nano-diamond. The hybrid reinforcements were found to decrease the hardness and flexural strength. Similarly, the thermal conductivity decreased from 17.7 W/m K (TiC) to 15.4 W/m K (TiC–5 wt.%TiN–5 wt.% diamond). In another work, Nguyen and co-authors [166] reported the strengthening of TiC using 5 wt.%AlN and 5 wt.% GNPs. The nanocomposite samples were sintered using SPS at 1900 °C for 10 min. The relative density increased from 95.5% to 101.2% because of the addition of AlN and GNPs. Moreover, 35% increase in flexural strength was reported for TiC–5 wt.%AlN–5 wt.% GNPs nanocomposite, as compared to TiC. The increase in the flexural strength was attributed to grain refinement and uniform distribution of in-situ phases formed during sintering. Properties of MLG reinforced TiC–WC composites, prepared by two step sintering, have been reported by Sun et al. [167]. The authors used micro-TiC, nano-TiC, WC, and MLG powders to prepare TiC–10 wt.%nano–TiC–3 wt.%WC–(0–0.45 wt.%)MLG nanocomposites. They sintered the samples by inductive hot-pressing in two-steps; first step at 1850 °C for 5 min and second step at 1750 °C for 60 min. All nanocomposites showed densification higher than 97%, with TiC–WC reinforced with 0.45 wt.%MLG nanocomposite reaching 99.3%. Hardness and flexural strength were found to increase with the increase in MLG content, and reached 24.2 GPa and 718.6 MPa, respectively, for TiC–WC reinforced with 0.45 wt.%MLG. The enhancements in hardness and flexural strength were attributed to the high densification and grain refinement. A relatively higher fracture toughness of 7.2 MPa m^1/2 was also reported for TiC–10 wt.%nano-TiC–3 wt.%WC–0.45 wt.%MLG nanocomposite. In other work, Sun et al. [168] reinforced TiC matrix with SiC_nw and CNTs. TiC–2 wt.%HfC–3 wt.%ZrO₂ were synergistically reinforced with 1.5 wt.%CNTs and 0–6 wt.%SiC_nw. Two-step SPS was employed, 5 min at 1800 °C then cooling to 1650 °C and holding for 30 min. The relative density of all hybrid nanocomposite samples was found to be more than 98.5%. XRD analysis showed formation of solid solution with predominant phases of TiC, (Ti_0.8, Hf_0.2)C and ZrO₂ in all samples. The hardness decreased from 24.47 GPa (TiC–HfC–ZrO₂) to 23.97 GPa (TiC–HfC–ZrO₂–1.5 wt.%CNTs–3 wt.SiC_nw), but the flexural strength and fracture toughness increased by 48.1 and 56.9, respectively, as can be seen in Fig. 22. CNT/SiC_nw bending, pullout as well as crack deflection and bridging, as shown in Fig. 23, were believed to be the primary toughening mechanisms.

Fig. 21.

SEM image of polished TiC–5 wt.% h-BN–5 wt.% graphite nanocomposite with EDS analysis [184].

Fig. 22.

Properties of TiC-based nanocomposites with different CNT/SiCnw ratios (a) relative density, (b) hardness, (c) flexural strength, and (d) flexural toughness [168].

Fig. 23.

Primary toughening mechanism in TiC-based nanocomposites [168].

TaC nanocomposites

Besides its extraordinary mechanical properties, corrosion resistance, high thermal conductivity and stability, and high electrical conductivity, tantalum carbide (TaC) has the highest melting point among all the UHTCs [169]. Typical properties of TaC are listed in Table 1. However, like other UHTCs, TaC also has poor sinterability and inadequate fracture toughness [170]. Addition of reinforcements such as graphene [171], CNTs [172], SiC [173], B₄C [174], TaSi₂[175], MoSi₂[175] have resulted in improvements in sinterability and fracture toughness. Similarly, incorporating hybrid reinforcements in TaC have shown promising results [117,176–178].

Nisar et al. [117] studied the toughening of TaC by SiC and MWCNTs. They prepared TaC and TaC–15 vol.%SiC–15 vol.%CNTs nanocomposite by SPS (three-stage sintering) at 1850 °C. The authors analyzed the microstructure of the sintered nanocomposite and did not observe new phases, however, they reported partial CNTs transformation to graphite. The relative density of 93%, fracture toughness of 3.1 MPa m^1/2, and fracture strength of 23.4 MPa for TaC increased to 98%, 11.4 MPa m^1/2, and 182.6 MPa for the TaC–15 vol.%SiC–15 vol.%CNTs nanocomposite, respectively. However, a decrease in hardness from 15.5 GPa for TaC to 14.1 GPa for the TaC–15 vol.%SiC–15 vol.%CNTs nanocomposite was observed, which was attributed to partial CNTs to graphite transformation during sintering. In another study, Nisar et al. [176] reported enhanced fretting and scratch wear behavior of TaC–15 vol.%SiC–15 vol.%CNTs nanocomposite. For fretting wear, the wear rate of TaC–15 vol.%SiC–15 vol.%CNTs nanocomposite decreased by 61% (4.1 × 10⁻⁷ mm³/Nm) as compared to 10.5 × 10⁻⁷ mm³/Nm for TaC, while for scratch wear the rate decreased from 8 mm³/Nm to 2.7 mm³/Nm. In another work, Nisar and co-authors [177] investigated thermal performance of spark plasma sintered TaC and TaC–15 vol.%SiC–15 vol.%CNTs nanocomposite. They exposed the sintered samples to plasma-arc jet under heat-flux of 2.5 MW/m² for 30 seconds. After exposure, 25% and 22% retention of TaC phase was observed in TaC and TaC–SiC–CNTs nanocomposite, respectively. Moreover, the thickness of the oxide scale layer, having major phase of Ta₂O₅, was found to be 30.1 μm for TaC; which decreased to 20.1 μm for TaC–SiC–CNTs nanocomposite. Enhanced thermal conductivity, thermal diffusivity, and specific heat values of 40.8 W/m K, 1731 mm²/s, and 214.9 J/kg.K were reported for the TaC–SiC–CNTs nanocomposite, compared to 18.5 W/m K, 778 mm²/s, and 164 J/kgK for TaC at 100 °C, respectively. A shift in the oxide layer formation temperature, from 735 °C for TaC to 901 °C for TaC–SiC–CNTs nanocomposite, was confirmed by thermogravemetric analysis (TGA). TaC was completely oxidized at 800 °C during TGA analysis but not much change was observed in TaC–SiC–CNTs nanocomposite up to 1500 °C. The authors attributed the enhancement in the oxidation resistance of the TaC–SiC–CNTs nanocomposite to the formation of a protective SiO₂ along with grain sealing by CNTs. In other study, Al-Habib et al. [178] investigated the effect of graphene on sintering and mechanical properties of TaC–TiC–SiC composites. They obtained TaC–TiC–25 vol.%SiC composite and TaC–TiC–25 vol.%SiC–5 wt.%graphene nanocomposite using SPS at 2000 °C and 35 MPa for 8 min. Higher densification and hardness values of 95.1% and 19.8 GPa were reported for the graphene–reinforced TaC-based nanocomposite, as compared to 92.4% and 18.5 GPa for the TaC–TiC–SiC composite. Graphene was found to facilitate the densification process and improve hardness. XRD analysis of sintered samples showed the formation of many new phases during sintering such as Ti₅Si₃, TaC_0.47, Ti₃SiC₂, Ti₆C_3.75, TiSi₂ and Ta₄C₃.

HfC nanocomposites

Hafnium carbide (HfC) is a UHTC ceramic having potential applications in rocket nozzles, nuclear reactors, thermal field emitters, cutting tools, high temperature solar absorbers, and space air-craft components [179–181]. Typical properties of HfC are listed in Table 1. The high hardness, high stiffness, high thermal stability, and corrosion resistance made HfC potential candidate for various applications. Problems such difficulty to sinter and inadequate fracture toughness of HfC were countered using second phase reinforcements including SiCN [179], MoSi₂[182], SiC [173], and CNTs [118]. The properties of HfC-based nanocomposites were not fully explored because of its poor sinterability. Mukherjee et al. [118] prepared HfC–5 wt.%MoSi₂ and HfC–5 wt.%MoSi₂–2 wt.%CNTs nanocomposites by SPS at 1600 °C. The authors found that CNTs promoted grain refinement, and both MoSi₂ and CNTs acted as sintering aid, which enhanced the densification. The relative density increased from 96% for the HfC–5 wt.%MoSi₂ composite to 99% for the HfC–5 wt.%MoSi₂–2 wt.%CNTs hybrid nanocomposite. The fracture toughness of the HfC–5 wt.%MoSi₂–2 wt.%CNTs nanocomposite increased by 91%, while the hardness and stiffness decreased by 12% and 13%, respectively, compared with the HfC–5 wt.%MoSi₂.

Si₃N₄ nanocomposites

Silicon nitride (Si₃N₄) has good mechanical characteristics, high wear resistance, good thermal shock resistance, and self-lubricating attributes, which make it a promising candidate material for numerous applications, including high-temperature ball bearings, rollers, and cutting tools [183,184]. Overcoming the poor sinterability and inadequate fracture toughness of Si₃N₄ and improving its mechanical properties were possible through the addition of single-phase reinforcements like TiN [185], SiC [186], CNTs [187], WC [188], and graphene [189] or two phases [190–193].

The effect of TiN nanoparticles and CNTs on microstructural and mechanical properties of hot-pressed Si₃N₄ has been investigated by Tian et al. [190]. The authors produced Si₃N₄, Si₃N₄–2 wt.%CNTs, Si₃N₄–10 wt.%TiN, and Si₃N₄–2 wt.%CNTs–10 wt.%TiN samples by hot pressing at 1700 °C. They used 5 wt.% Al₂O₃ and Y₂O₃ as sintering aids and obtained a homogenous microstructure with uniform dispersion of reinforcements. They found that the fracture toughness of Si₃N₄–2 wt.%CNTs–10 wt.%TiN nanocomposite increased by 47% compared to Si₃N₄, which was attributed to crack deflection, intergranular fracture mode, crack bridging and CNTs pull-out. Puchy and co-authors [191] prepared Si₃N₄ and Si₃N₄–1.5 wt.%GNPs–(5, 10)wt.%Ag by SPS at 1700 °C for 10 min. The nanocomposites showed higher densification and uniform distribution of the reinforcements. However, a decrease in hardness and fracture toughness was observed for the Si₃N₄–Ag–GNPs nanocomposites compared Si₃N₄. The coefficient of friction slightly decreased for the Si₃N₄–5 wt.%Ag–1.5 wt.%GNPs nanocomposite, but then increased for the Si₃N₄–10 wt.%Ag–1.5 wt.%GNPs nanocomposite. The Si₃N₄–5 wt.%Ag–1.5 wt.%GNPs nanocomposite showed improved wear resistance compared to Si₃N₄. Zhang et al. [192] studied the effect of orientational angle of GNPs on tribological performance of Si₃N₄–TiC–GNPs nanocomposite tool materials. The authors used Si₃N₄, 15 wt.% TiC, and 5 wt.% GNPs for nanocomposite preparation and 2 wt.% Al₂O₃, 5 wt.% Y₂O₃, and 0.5 wt.% MgO as sintering aids. They sintered the samples by hot-pressing at 1700 °C for 60 min. They performed the tribological tests by keeping the angle between GNPs of tool material and rubbing surface at 0°, 15°, 30°, 45°, 60° and 90°. The Si₃N₄–TiC–GNPs nanocomposite with GNPs at 30° angle showed the optimum tribological properties, as shown in Fig. 24. The coefficient of friction of Si₃N₄–TiC–GNPs nanocomposite with GNPs at 30° was found to be 31.3 and 42.5% lower compared to the nanocomposite with GNPs oriented at 0° and 90°, respectively. The wear rates of the nanocomposite with 30° GNPs orientation were 59.1 and 50.8% lower compared to the nanocomposites with 0° and 90° orientation, respectively. At 30° orientation of GNPs, a continuous tribofilm with a maximum thickness of 2.57 μm was formed. The continuous, relatively thick, defect-free, and tightly bonded tribofilm to the base material was believed to be the cause of the relatively better performance of the nanocomposite with GNPs orientation at 30°. Chen and co-authors [193] predicted the optimum content of the hybrid reinforcements and mechanical properties of MLG and silicon nitride nano-whiskers (Si₃N_4w) reinforced Si₃N₄. The predicted results were then verified by testing samples produced by SPS at 1700 °C for 6 min. The simulation results showed the highest hardness of 23.19 GPa and the highest fracture toughness of 9.87 MPa m^1/2 at an optimum reinforcement content of 1 wt.% MLG and 3 wt.%Si₃N_4w. The predicted fracture toughness of the Si₃N₄–1 wt.%MLG–3 wt.%Si₃N_4w nanocomposite was found to be 11.85% lower while the predicted hardness was 14.28% higher than the experimental values, as shown in Fig. 25.

Fig. 24.

Friction coefficient and wear rate of Si₃N₄–15 wt.%TiC–5 wt.%GNP nanocomposites as a function of orientation angle of GNP reinforcement at 40N [192].

Fig. 25.

Modelling prediction and experimental results of mechanical properties for 1 wt.% MLG–(0–6 wt.%)Si₃N_4w–Si₃N₄ hybrid nanocomposites [193].

TiB₂ nanocomposites

Titanium diboride (TiB₂) has very high melting temperature of 3225 °C, low thermal expansion coefficient of 4.6 × 10⁻⁶/K, hardness of around 25 GPa, and very high elastic modulus of over 500 GPa [174]. TiB₂ also has good chemical stability, high oxidation resistance, and high electrical conductivity, which make it candidate material for many applications including hypersonic re-entry spacecraft, gas turbine blades, advanced rocket motors, wear-resistant coatings, impact-resistant armor, and cutting tools [194–198]. Researchers were able to enhance the properties TiB₂ by the inclusion of a single phase such as SiC [199,200], ZrO₂[201], boron [202], short-carbon fiber [203], CNT [204] Si₃N₄, graphene, CrB₂, TiC, AlN, TiN [198], or two or more phases [195–197,205–208].

Akin et al. [205] investigated the effect of SiC and GNPs on microstructural, mechanical and thermal properties of TiB₂. The prepared TiB₂ and TiB₂–(10–20)wt.%SiC–(0.5–10)wt.% GNPs nanocomposites by SPS at 1700 °C and 1800 °C, respectively, for 5 min. The nanocomposites showed relative density higher than 98%. Microstructural analysis revealed uniformly distributed reinforcements within TiB₂ matrix, with SiC present at intergranular triple points while GNPs wrapped around the matrix grains. The hardness of TiB₂ was found to decrease from 24.2 GPa with the increase in reinforcement content and reached 17 GPa for TiB₂–10 wt.%SiC–10 wt.% GNPs nanocomposite. TiB₂ and TiB₂–15 wt.%SiC–5 wt.% GNPs nanocomposite had fracture toughness values of 3.9 MPa m^1/2 and 6.4 MPa m^1/2, respectively, i.e. an increase of 64.1%. The nanocomposites had lower thermal conductivity than TiB₂, except for TiB₂–19.5 wt.%SiC–0.5 wt.% GNPs nanocomposite, which showed a slight increase in thermal conductivity. Other researchers [196] prepared TiB₂–25 vol.%SiC and TiB₂–25 vol.%SiC–2 wt.%G_nf nanocomposites at 1800 °C for 7 min using SPS and reported the effect of 2 wt.% G_nf on the microstructural and thermal properties of TiB₂–25 vol.%SiC composite. Microstructural analysis showed the formation of TiC, B₄C, and SiO₂. Relative density values of 95.5% and 96% were obtained for TiB₂–SiC and TiB₂–SiC–graphite, respectively. A slight decrease in the thermal conductivity was observed as a result of reinforcing TiB₂–SiC with G_nf. The properties of TiB₂–SiC–CNTs nanocomposites, prepared by reactive sintering, have been reported by Popov et al. [195]. The authors mixed TiB₂, SiC, TiC, B₄C and Si powders, hot-pressed the mixture at 1800 °C for 1 min, and obtained nearly fully dense nanocomposites. They reported slight increase in the hardness from 27.4 GPa (TiB₂–SiC) to 29.3 GPa (TiB₂–SiC–4.5 vol.%CNTs). All other nanocomposites showed a decrease in hardness. The fracture toughness values of all TiB₂–SiC–CNTs nanocomposites were lower than that of the TiB₂–SiC composite (5.9 MPa m^1/2), except TiB₂–SiC–8.6 vol.%CNTs, which had a fracture toughness value of 6.2 MPa m^1/2. The nanocomposites showed better thermal shock resistance. Kovalcikova et al. [197] produced TiB₂–20 wt.%SiC–(0–10 wt.%)GNPs nanocomposites by hot-pressing at 2100 °C for 7 min. The GNPs improved the sinterability of TiB₂–20 wt.%SiC composites. However, the hardness was found to decrease with the increase in GNPs content, and reached a value of 13.9 GPa for TiB₂–20 wt.%SiC–10 wt.%GNPs nanocomposite compared to 23.8 GPa for TiB₂–20 wt.%SiC. The fracture toughness slightly increased from 5 MPa m^1/2 to 6.2 MPa m^1/2 when TiB₂–20 wt.%SiC was reinforced with 10 wt.%GNPs. The coefficient of friction and wear rate were found to be less than those of the TiB₂–20 wt.%SiC composite when the content of GNPs exceeded 5 wt.%. The enhanced tribological performance of TiB₂–SiC–GNPs nanocomposites, with GNPs content higher than 5 wt.%, was attributed to the formation of a tribofilm. Asl and co-workers [206] prepared TiB₂–25 vol.%SiC–2 wt.%GNPs hybrid nanocomposite by SPSat 1800 °C for 7 min. The sintered sample had a relative density 96%. XRD and X-ray photoelectron spectroscopy analysis did not reveal the formation of new phases during sintering, while high-resolution transmission electron microscopy analysis showed the presence of dislocations. TiB₂–10 wt.%MgO–6 wt.%TiN hybrid nanocomposites reinforced with SiC whiskers and MLG were developed by Sun et al. [207] and their properties are shown in Fig. 26. TiB₂–10 wt.%MgO–6 wt.%TiN and TiB₂–10 wt.%MgO–6 wt.%TiN–2 wt.%SiC_w–(2,4,6 wt.%)MLG composites were produced by conventional sintering at 1750 °C for 45 min and two-step sintering (1800 °C for 5 min, 1700 °C for 45 min). The samples sintered in two-steps had a relative density of at least 99%. The hardness of TiB₂–MgO–TiN (33 GPa) was found to decrease with the addition of SiC/MLG and its maximum value was 21.2 GPa for TiB₂–MgO–TiN–SiC_w–4 wt.%MLG nanocomposite. In contrast, high flexural strength and fracture toughness values of 1006.3 MPa m^1/2 and 8.8 MPa m^1/2, respectively, were reported for TiB₂–MgO–TiN–SiC_w–4 wt.%MLG nanocomposite, compared to values of 700 MPa and 6 MPa m^1/2 for TiB₂–MgO–TiN. In other work, relative density values of 95.5% and 96% were achieved for TiB₂–SiC and TiB₂–SiC–GNPs samples prepared by SPS at 1800 °C for 7 min. The authors reported 8% decrease in thermal conductivity of TiB₂–25 vol.%SiC–2 wt.%GNPs nanocomposite compared to TiB₂–25 vol.%SiC [208].

Fig. 26.

Properties of various TiB₂ based nano-composites (a) hardness, (b) flexural strength, and (c) flexural toughness [207].

WC nanocomposites

Cui and co-authors [209] prepared 47.5 wt.%Al₂O₃–50.2 wt.%Ti(C,N) nanocomposite powders mixed with GNPs. The powders were pre-sintered at 1200 °C for 10 min, then hot-pressed at 1700 °C for 10 min. The authors reported 40.13% increase in fracture toughness and 40.78% increase in flexural strength when 47.5 wt.%Al₂O₃–50.2 wt.%Ti(C,N) was reinforced with 0.4 wt.%GNPs. However, the mechanical properties deteriorated at GNPs content higher than 0.4 wt.%. The nanocomposites did not show significant change in the thermal conductivity. The effect of varying the content of TiO₂ (0–6 wt.%) in WC–30 vol.%Al₂O₃–0.5 wt.%CNTs nanocomposite was investigated by Bai et al. [210]. The authors sintered the samples at 1540 °C for 90 min and found that the relative density increased with the increase in TiO₂ content, reaching a value of 99.5% for the WC–30 vol.%–Al₂O₃–0.5 wt.%CNTs–6 wt.%TiO₂ nanocomposite. A new phase of Al₂TiO₅ was found to form in WC–30 vol.%–Al₂O₃–0.5 wt.%CNTs samples containing more than 2 wt.% TiO₂. A hardness value of 1752 HV was reported for WC–30 vol.%Al₂O₃–0.5 wt.%CNTs–2 wt.%TiO₂ compared to 1623 HV for WC–30 vol.%Al₂O₃–0.5 wt.%CNTs. A slight increase in fracture toughness was observed when the WC–30 vol.%Al₂O₃–0.5 wt.%CNTs nanocomposite was reinforced with 4 wt.% TiO₂. The enhancement in mechanical properties, when the content of TiO₂ is equal to or greater than 2 wt.%, was attributed to the formation of Al₂TiO₅. Zhang et al. [211] investigated the effect of varying GPLs content (0–0.9 wt.%) on tribological behavior of WC–Al₂O₃ nanocomposites. The authors hot-pressed 85.4 wt.%WC–(13.7–14.6)wt.%–Al₂O₃–(0–0.9)wt.%GPLs nanocomposites at 1540 °C for 90 min. The relative density was found to decrease with the increase in GPLs content, reaching a value of 96.7% for WC–13.7Al₂O₃–0.9 wt.% GPLs nanocomposite. Enhancements in hardness and fracture toughness were reported, reaching values of 17.44 GPa and 9.47 MPa m^1/2, for WC–Al₂O₃ reinforced with 0.1 and 0.5 wt.% GPLs, respectively. The addition of 0.1 and 0.5 wt.% GPLs to WC–Al₂O₃ nanocomposites resulted in 7.8% and 41.2% decrease in the coefficient of friction, respectively, as well as 52% and 61.8% decrease in wear rate compared to WC–Al₂O₃ composites, as shown in Fig. 27. The superior tribological behavior of the WC–Al₂O₃–GPLs nanocomposites was attributed to GPLs which played lubrication role by forming tribofilms. In another work, Zhang et al. [212] hot-pressing WC–14.3 wt.%Al₂O₃ and WC–14.3 wt.%Al₂O₃–0.3 wt.%GPLs nanocomposites at 1540 °C for 90 min. The found that the addition of GPLs resulted in a decrease in density, from 99.1 to 98.3%, and in grain size from 3.49 to 2.61 μm, however, the hardness and fracture toughness increased from 16.18 GPa and 8.08 MPa m^1/2 to 18.34 GPa and 11.02 MPa m^1/2. The increase in hardness was attributed to grain refinement, while the increase in fracture toughness is believed to be due to toughening mechanisms induced by GPLs. Additionally, the coefficient of friction decreased by 40.4% and 33.3% at loads of 40N and 60N, respectively. Similarly, a decrease in the wear rate was observed. The decrease in coefficient of friction and wear rate of the WC–Al₂O₃–GPLs nanocomposite was attributed to the fine microstructure, high hardness, and formation of a tribofilm of graphene. Chen et al. [213] prepared WC–Co, WC–Co–0.06%SWCNTs–0.06%SLG, WC–Co–0.12% SWCNTs, WC–Co–0.12%SLG nanocomposites by SPS at 1200 °C for 10 min. The authors reported 49% and 68% increase in flexural strength for the WC–Co–0.06% SWCNTs–0.06%graphene and WC–Co–0.12%graphene nanocomposites, respectively; and 2.6% and 10% increase in the thermal conductivity for the WC–Co–0.06% SWCNTs–0.06%SLG and WC–Co–0.12%SLG nanocomposites compared to WC–Co. But no change in hardness was observed for WC–Co nanocomposites reinforced with SWCNTs and SLG. Sun et al. [214] reported enhancements in mechanical properties of WC when reinforced with 8 wt.%(ZrO₂–20 wt.%Al₂O₃) and (0–0.4 wt.%)GNPs. WC–ZrO₂–Al₂O₃ composite and WC–ZrO₂–Al₂O₃–GNPs nanocomposites with varying GNPs content were sintered using oscillatory pressure sintering. The 99.1% relative density of WC–ZrO₂–Al₂O₃ increased to 99.8% for WC–ZrO₂–Al₂O₃–0.2 wt.%GNPs. The hardness decreased with the increase in GNPs content. However, 12% increase in fracture toughness and 11% increases in flexural strength were reported for 0.2 wt.%GNPs reinforced WC–ZrO₂–Al₂O₃ nanocomposite as compared to the sample without GNPs. In another work, Sun et al. [215] reported enhanced mechanical and tribological properties of WC matrix reinforced with MLG and SiC_w. Two step sintering SPS (1950 °C for 5 min and then 1850 °C for 20 min) was used for sintering pure WC and WC–3 wt.%SiC_w–(0.1,0.2 wt.%)MLG nanocomposites. The addition of MLG to WC facilitated the densification and the relative density reached 99.5% for the WC–SiC_w–0.1 wt.%MLG nanocomposite. Also, a 81.9% increase in fracture toughness, a 61.2% reduction in friction coefficient, and a 81.3% reduction in wear rate were reported for WC–SiC_w–0.1 wt.%MLG nanocomposite compared to WC. In another study [216], binderless tungsten carbide samples were hot-pressed using two-step sintering; 1700 °C for 4 min and then 1600 °C for 60 min. The relative density of WC–Al₂O₃ improved from 97.8% to 99.8% because of the addition of TiC, TiN, SiC_w, and MLG. The improvement was attributed to the formation of solid solutions and minor-liquid phases. Increases up to 4.1%, 23.8%, and 54.9% in hardness, flexural strength, and fracture toughness were reported for WC–9 wt.%Al₂O₃ reinforced with 3 wt.%TiC, 3 wt.%TiN, 0.15 wt.%SiC_w, and 0.05 wt.%MLG [216]. The effect of ultrafine porous boron nitride nanofiber (BN_nf) on mechanical properties of WC–Si₃N₄ was investigated by Cao et al. [217]. The authors sintered WC–10 wt.%Si₃N₄ composites and WC–10 wt.%Si₃N₄–(0–0.15 wt.%)BN_nf hybrid composites at 1750 °C and found that all the BN_nf reinforced nanocomposites showed higher values of hardness, elastic modulus, and fracture toughness compared to WC–Si₃N₄. The WC–Si₃N₄–0.01 wt.%BN_nf nanocomposite showed the highest elastic modulus of 793 GPa and fracture toughness 10.99 MPa m^1/2 compared to 521 GPa and 7.70 MPa m^1/2 for WC–10 wt.% Si₃N₄. In other work, WC ceramic nanocomposites reinforced with graphene (G) and SiC nanowire (SiC_nw) were prepared through SPS [218]. The WC–0.15 wt.%–G–0.45 wt.%SiC_nw nanocomposite sintered at 1900 °C for 15 min under 60 MPa, had a hardness of 25.6 GPa, a flexural strength of 1499 MPa, and a fracture toughness of 11.6 MPa m^1/2. The high toughness value was mainly attributed to the combination of G and SiC_nw induced crack deflection, bridging and pullout.

Fig. 27.

Coefficient of friction and wear rates of WC–Al₂O₃–GNP nanocomposites. WA, WA1G, WA2G and WA9G corresponds to WC–Al₂O₃, WC–Al₂O₃–0.1 wt.%GNP, WC–Al₂O₃–0.5 wt.%GNP and WC–Al₂O₃–0.9 wt.%GNP nanocomposites. [111].

Mullite nanocomposites

A hybrid nanocomposite powder of mullite–TiB₂–CNTs was prepared by mixing 1 wt.%CNTs and 10 wt.% TiB₂ with nano-mullite powder using high-energy milling and, then, sintered by SPS at 1350 °C [219]. A relative density of 99.1% was reported for the mullite–TiB₂–CNTs nanocomposite. The authors performed XRD analysis and observed tiny peak of alumina, along with mullite and TiB₂, which might have been formed as in-situ phase during sintering. Uniform dispersion of TiB₂ and CNTs reinforcements was confirmed by EDS analysis. Vickers hardness, flexural strength, and fracture toughness of mullite–TiB₂–CNTs nanocomposite were found to be 18.31 GPa, 531 MPa and 5.46 MPa m^1/2, respectively. In another work, similar sintering conditions were used to sinter mullite reinforced with 10 wt.% ZrB₂ and 1 wt.% CNTs [220]. Hardness of 16.24 GPa, flexural strength of 488 MPa, and fracture toughness of 4.18 MPa m^1/2 were reported for the mullite–10 wt.%ZrB₂–1 wt.%CNTs nanocomposite. The influence of reinforcing mullite–TiN–CNTs nanocomposite with ZrB₂ and TiB₂, on the microstructure and mechanical properties, were investigated by Orooji et al. [221]. The authors prepared mullite–TiN–CNTs, mullite–TiN–TiB₂–CNTs, and mullite–TiN–TiB₂–ZrB₂–CNTs nanocomposites by SPS at 1350 °C for 5 min. The content of TiN, TiB₂ and ZrB₂ was 10 wt.% for each compound and the content of CNTs was 1 wt.% for the composites. All samples achieved a relative density higher than 97%. Microstructural analysis showed uniform distribution of the reinforcements with no new in-situ phase formation during sintering. The mullite-TiN–TiB₂–ZrB₂–CNTs nanocomposite showed the highest hardness and fracture toughness values of 17.15 GPa and 4.86 MPa m^1/2, as compared to 15.84 GPa and 3.98 MPa m^1/2 for the mullite–TiN–CNTs nanocomposite.

B₄C nanocomposites

Pure B₄C, B₄C–18 wt.%TiB₂ and B₄C–18 wt.%TiB₂–4 wt.%GNPs were hot-pressed at 1900 °C for 1 h at 30 MPa [222] and their mechanical properties were reported as shown in Fig. 28[222]. All samples had a relative density of more than 97%. B₄C–TiB₂–GNPs nanocomposite showed 169% and 68% increases in fracture toughness and flexural strength, respectively, compared to B₄C. However, marginal increase in hardness from 29.8 for B₄C to 31.8 was observed for B₄C–TiB₂–GNPs nanocomposite. The electrical conductivity of B₄C increased by three folds, from 1.6 × 10² S/m to 9.65 × 10⁵ S/m for B₄C–TiB₂–GNPs nanocomposite.

Fig. 28.

Densification and mechanical properties of hot-pressed B₄C ceramic, B₄C–TiB₂ composite and B₄C–TiB₂–GNP nanocomposite [222].

SiC nanocomposites

The effect of CNTs on mechanical properties of SiC–B₄C–Ni hybrid composites was investigated by Bahamirian and co-authors [223]. SiC–45 vol.%B₄C–10 vol.%Ni and SiC–45 vol.%B₄C–10 vol.%Ni–5 vol.%CNT samples were prepared by SPS at 1650 °C for 5 min. The relative density was found to be 98.3% and 97.8% for SiC–B₄C–Ni and SiC–B₄C–Ni–CNTs nanocomposite, respectively. XRD analysis confirmed the presence of Ni₂Si phase formed during sintering. FE-SEM analysis showed presence of cracks in the SiC matrix, which were attributed to the migration of Si from SiC to Ni to from Ni₂Si. Slight decrease in hardness from 16.7 to 15.63 GPa was observed when SiC–B₄C–Ni was reinforced with CNTs. Marginal increase in bending strength and fracture toughness from 362 MPa and 3.32 MPa m^1/2 to 369 MPa and 5.82 MPa m^1/2 was reported for SiC–B₄C–Ni and SiC–B₄C–Ni–CNTs composites, respectively. In other work, optimal flexural strength of 337.4 MPa and fracture toughness of 4.58 MPa m^1/2 were obtained for SiC hybrid nanocomposite reinforced with three–dimensional graphene oxide–carbon nanotubes (3D GO-CNTs) at a ratio of 1:2, i.e. enhancement by 40.9% and 34.7%, respectively, compared to that of SiC. The composites were prepared using direct ink writing and liquid silicon infiltration processes. The authors attributed the improvement in densification, hardness, strength, and toughness, as shown in Fig. 29[8], to the uniform distribution of GO and CNTs in the SiC matrix and the various toughening mechanisms including GO/CNTs pull-out, multiple microcracks, crack bridging, deflection, and branching [8]. In another study, SiC–TiB₂–Al₂O₃ nanocomposites, prepared using the powder metallurgy technique, followed by hot press sintering at a high temperature of 1400 °C, showed impressive reduction in linear ablation rate and in mass ablation rates [9].

Fig. 29.

Properties of SiC composites with different GO/CNT ratios (a) hardness and density, flexural strength (b), and (c) fracture toughness [8].

Yttria-stabilized zirconia nanocomposites

Yttria-stabilized zirconia hybrid nanocomposites reinforced with CNTs and BN were developed by Buyakov and co-authors [10]. Different compositions, with dual reinforcement content of 0.25–5 wt.%, were sintered using SPS between 1300–1400 °C for 30 min. The authors reported a relative density higher than 98.5% for all samples except YSZ–5 wt.%CNTs–5 wt.%BN, whose relative density was 97.3%. The samples displayed a homogenous microstructure and uniform distribution of the reinforcements. Analysis of sintered samples showed the presence of monoclinic and tetragonal zirconia phases. The elastic modulus of monolithic YSZ was reported to be 197 GPa and remained in the range of 190–198 GPa for all hybrid nanocomposites. The hardness of monolithic YSZ (12.2 GPa) decreased with the increase in reinforcement content and reached a value of 9.6 GPa for YSZ–5 wt.%CNTs–5 wt.%BN nanocomposite. However, the fracture toughness increased with the increase in reinforcement content and reached a value of 7.9 MPa m^1/2 for YSZ–5CNTs–5BN nanocomposite compared to 6.1 MPa m^1/2 for YSZ. The fracture toughness enhancement was attributed to zirconia phase transformation, CNTs toughening effects, and crack bifurcation due to BN [10].

A summary of composition, sintering methods and parameters, relative density, hardness, and fracture toughness of various hybrid ceramic nanocomposites is presented in Table 2.

Table 2.

Summary of properties of various hybrid ceramic nanocomposites.

Nanocomposite	Sintering method/parameters	Relative density (%)	Hardness (GPa)	Fracture toughness (MPa m^1/2)	Ref.
Al2O3	SPS. 1550 °C, 50 MPa	100	17	3.3	[53]
Al2O3–1 vol.%SiC–5 vol.%CNT		98.2	16.5	6.5
Al2O3–1 vol.%SiC–7 vol.%CNT		97.2	16	7
Al2O3–1 vol.%SiC–10 vol.%CNT		95.1	15	5.5
Al2O3–1 vol.%SiC–5 vol.%CNT	SPS. 1550 °C, 50 MPa	98	16.5	6.5	[54]
Al2O3–3 vol.%SiC–5 vol.%CNT		96.46	16	6.2
Al2O3–5 vol.%SiC–5 vol.%CNT		96.40	16	6
Al2O3–5 wt.%SiC–1 wt.%CNT	SPS. 1500 °C, 10 min, 50 MPa	91.65	17.81	5.38	[155]
Al2O3		99.3	18.56	3.61	[56]
Al2O3–5 wt.%SiC–1 wt.%CNT		99.36	19.77	3.89
Al2O3–5 wt.%SiC–2 wt.%CNT		98.28	19.11	4.2
Al2O3–10 wt.%SiC–1 wt.%CNT		98.63	20.81	4.58
Al2O3–10 wt.%SiC–2 wt.%CNT		98.02	17.50	6.98
Al2O3–5 wt.%SiC–1 wt.%CNT	SPS. 1600 °C	98.9	23.32	7.1	[57]
Al2O3	IHP. 1500 °C, 3 min, 50 MPa	99.2	16	3.2	[60]
Al2O3–6 wt.%SiCnp–4 wt.%CNT		97.2	20	5.9
Al2O3–5 wt.%SiCnp–2 wt.%CNT	HFIHS, 1600 °C, 3 min, 60 MPa	97.2	20.9	6.5	[61]
Al2O3–25 wt.%SiCw–0.1 wt.%CNT	HP. 1750 °C, 1 h	>99.5	20.4	4.5	[62]
Al2O3	HP. 1500 °C, 2 h, 30 MPa	99.8	17.5	5.5	[44]
Al2O3–5 vol.%ZrO2–0.5%MWCNT		99.4	14.5	6.9
Al2O3–5 vol.%ZrO2–1 vol.%MWCNT		99.1	14.5	7.5
Al2O3–5 vol.%ZrO2–2 vol.%MWCNT		99	14.5	6.9
Al2O3Al2O3–8 vol.%ZrO2–4 vol.%MWCNT	HP. 1600 °C, 50 MPa, 60 min	99.798.3	17.423.2	3.16.7	[64]
Al2O3Al2O3–4 wt.%ZrO2–2 wt.%CNTAl2O3–4 wt.%ZrO2–0.5 wt.%G	HP. 1600 °C, 60 MPa, 1 h	99.599.898.5	192421.6	3.37.26.2	[65]
ZTA–0.6 wt.%MgOZTA–0.6 wt.%MgO–0.05 wt.%CNTZTA–0.6 wt.%MgO–0.1 wt.%CNT	HP. 1500 °C, 30 min, 100 MPa	999999	24.5730.7031.25	5.747.026.28	[66]
Al2O3	IHP. 1300–1600 °C, 30–50 MPa, 3 min	99	16.5	3.5	[67]
Al2O3–3 wt.%SiCnp–0.5 wt.%G		99	19.3	6.9
Al2O3–5 wt.%SiCnp–0.5 wt.%GNP		99.5	21	8.3	[68]
Al2O3–17 vol.%SiCw–0.5 vol.%GO	SPS. 1780 °C, 3 min	–	22	10.6	[69]
Al2O3	SPS. 1500 °C, 3 min, 50 MPa	100	18.04	3.41	[70]
Al2O3–1 vol.%SiC–0.38 vol.%GPL		99.03	21.34	4.77
Al2O3–3 vol.%SiC–0.38 vol.%GPL		98.85	24.65	5.03
Al2O3–5 vol.%SiC–0.38 vol.%GPL		97.35	21.58	4.94
Al2O3–5 vol.%SiC–0.5 vol.%GNSAl2O3–5 vol.%SiC–1 vol.%GNS	SPS, 1450 °C, 5 min	>99>99	18.0717.07	3.513.92	[71]
Al2O3Al2O3–5 wt.%SiC–0.4 wt.%GAl2O3–10 wt.%SiC–0.4 wt.%GAl2O3–15 wt.%SiC–0.4 wt.%G	SPS, 1500 °C, 3 min, 21 MPa	99.799.698.797.4	16.4419.618.217.2	–	[73]
Al2O3Al2O3–4 wt.%ZrO2–0.5 wt.%GNP	HP. 1600 °C, 50 MPa	99.798.5	16.419.3	3.17.9	[74]
Al2O3–18 vol.%WC–6%TiC	HP. 1700 °C, 10 min, 35 MPa	98.94	23.54	8.76	[76]
Al2O3–18 vol.%WC–6%TiC–0.5%G		99.58	24.64	9.42
Al2O3–18 vol.%WC–6%TiC–0.75%G		98.88	23.2	10.35
Al2O3–18 vol.%WC–6 vol.%TiCAl2O3–18 vol.%WC–6 vol.%TiC–0.5 vol.%Graphene	HP two-step. 1600–1750 °C 15 min, 32 MPa	98.4199.52	1922.5	69	[78]
Al2O3–18 vol.%WC–6 vol.%TiCAl2O3–18 vol.%WC–6 vol.%TiC–0.5 vol.%graphene	HP. 1700 °C for 10 min	98.4199.68	18.6521.48	5.738.83	[79]
Al2O3–30 wt.%TiC	MS. 1700 °C, 10 min	99.2	21.2	5.2	[80]
Al2O3–30 wt.%TiC–0.1 wt.%GNP		97.9	19.1	7.2
Al2O3–30 wt.%TiC–0.2 wt.%GNP		97.7	18.5	8.7
Al2O3–30 wt.%TiC–0.4 wt.%GNP		96.8	18	8.6
Al2O3–69.2 wt.% (W, Ti)C–0.2%GNP	HP. 1700 °C, 15 min	99	24.22	7.78	[81]
Al2O3–33.7 wt.%TiC	HP. 1650 °C	–	19.5	4.98	[82]
Al2O3–33.5 wt.%TiC–0.2 wt.%MLG		–	20	6.14
Al2O3–24 vol.%TiC–16 vol.%WC	HP. 1700 °C, 10 min	98.9	22.19	6.82	[83]
Al2O3–21 vol.%TiC–14 vol.%WC		98.01	19.02	5.55
Al2O3–1 wt.%CNT–0.5 wt.%GNP	SPS. 1500 °C	99.9	16.66	8.33	[84]
Al2O3	HP. 1650 °C, 1 h	99	16.5	3.6	[85]
Al2O3–1 wt.%rGO–1 wt.%CNT		99.8	18.61	6.12
Al2O3–0.5 wt.%GNP–1 wt.%CNT	HP. 1650 °C, 1 h	99.9	17	5.7	[87]
Al2O3–0.3 wt.%GNP–1 wt.%CNT		99	16	5.8	[88]
Al2O3–1 wt.%GNP–1 wt.%CNT		99	11.2	3.5
Al2O3Al2O3–1 wt.%CNT–0.4 wt.%GAl2O3–1 wt.%CNT–0.8 wt.%G	SPS. 1500 °C, 3 min, 21 MPa	99.799.299.5	16.4415.811.6	4.97.3255.96	[89]
Al2O3Al2O3–10 wt.%ZrO2Al2O3–10 wt.%ZrO2–0.5 wt.%GAl2O3–10 wt.%ZrO2–2 wt.%CNTAl2O3–10 wt.%ZrO2–0.5 wt.%G–2 wt.%CNT	HP. 1600 °C, 1 h, 60 MPa	18.3913.818.7920.3127.21	1914192027.5	3.397.276.697.558.78	[90,91]
Al2O3–25 vol.%TiC–1.9 vol.%Ni	SPS. 1375 °C	>98	25.6	3.7	[92]
Al2O3–5 vol.%SWCNT–10 vol.%Nb	SPS. 1200 °C	98.4	19.3	3.3	[93]
Al2O3–20 vol.%SiC–4 vol.%TiC	HP. 1700 °C, 30 min	>99	21.18	6.04	[94]
Al2O3–5 vol.%TiCn–0.5 vol.%GAl2O3–10 vol.%TiCn–0.5 vol.%G	HP. 1650 °C, 30 MPa, 10 min	–	19.920.5	7.17.4	[96]
Al2O3–20 wt.%TiB2Al2O3–20 wt.%TiB2–0.1 wt.%GNPAl2O3–20 wt.%TiB2–0.2 wt.%GNPAl2O3–20 wt.%TiB2–0.3 wt.%GNPAl2O3–20 wt.%TiB2–0.4 wt.%GNPAl2O3–20 wt.%TiB2–0.6 wt.%GNP	SPS. 1520 °C, 30 MPa, 5 min	98.698.89999.398.598.3	19.2018.718.5518.2017.7016.56	5.196.747.157.356.756.10	[98]
ZrB2–25vol.SiC	SPS. 1900 °C, 40 MPa, 7 min	99.9	19.5	4.3	[110]
ZrB2–25vol.SiC–2.5 wt.%Gnf		100	18	4.9
ZrB2–25vol.SiC–5 wt.%Gnf		100.7	16	6.8
ZrB2–25vol.SiC–7.5 wt.%Gnf		99.7	14	8.2
ZrB2–25vol.SiC–10 wt.%Gnf		98.3	12.1	–
ZrB2	SPS. 1900 °C, 40 MPa, 7 min	96.1	13.1	3.2	[126]
ZrB2–25 vol.%SiC–5 wt.%Gnf		100.7	16.6	6.7
ZrB2	HP. 1850 °C, 60 min, 20 MPa	90.1	11.9	1.8	[121]
ZrB2–20 vol.%SiC–10 vol.%Gnf		99.6	16.5	7.1
ZrB2–25 vol.%SiC–5 wt.%GNP		99	15.7	6.4	[122]
ZrB2–25 vol.%SiCwZrB2–25 vol.%SiCw–2.5 wt.%GnfZrB2–25 vol.%SiCw–5 wt.%GnfZrB2–25 vol.%SiCw–7.5 wt.%Gnf	SPS. 1900 °C, 40 MPa, 7 min	99.999.910099.9	2220.619.819.5	4.75.56.26.2	[123]
ZrB2–25vol.SiCwZrB2–25vol.SiCw–2.5 wt.%GNPZrB2–25vol.SiCw–5 wt.%GNPZrB2–25vol.SiCw–7.5 wt.%GNP	SPS. 1900 °C, 40 MPa, 7 min	10099.9599.9599.75	2222.219.116.6	55.25.76	[124]
ZrB2–20 vol.%SiC–10 vol.%CNT	HP. 1850 °C, 60 min, 20 MPa	93.9	8.6	5.1	[130]
ZrB2–20 vol.%SiC–2 vol.%GNSZrB2–20 vol.%SiC–5 vol.%GNS	HP. 1950 °C, 1 h, 30 MPa	98.999.2	22.9322.76	6.077.32	[107]
ZrB2–20 vol.%SiC–2 wt.%CNT	HP. 1900 °C, 1 h	–	15.5	4.6	[129]
ZrB2–20 vol.%SiC–10 vol.%CNT	SPS. 1650 °C	99.7	–	9.5	[131]
ZrB2–25vol.SiCw	SPS. 1900 °C, 40 MPa, 7 min	99.9	21.9	4.7	[109]
ZrB2–25vol.SiCw–2.5 wt.%Cnp		100.1	18	5.9
ZrB2–25vol.SiCw–5 wt.%Cnp		100.2	17	7.1
ZrB2–25vol.SiCw–7.5 wt.%Cnp		99.8	14.6	7.2
ZrB2	HP. 1850 °C, 60 min, 20 MPa	90.1	11.9	2.1	[137]
ZrB2–20vol.SiC–10 vol.%Cnb		99.8	16.7	5.8
ZrB2–20vol.SiC–5 vol.%Cnb	HP. 1900 °C	99.2	12.3	6.6	[138]
ZrB2–20 vol.%SiCnp–20 vol.%Graphite	HP. 1900 °C, 1 h	100	–	4.54	[127]
ZrB2–20 vol.%SiC–30 vol.%Cf	HP. 1450 °C, 1 h	95.8	–	6.12	[135]
ZrB2–10 vol.%SiC–2.5 vol.%Cf	CS. 2150 °C	93	14.5	5.9	[136]
ZrB2ZrB2–30 vol.%SiC–10 vol.%CnbZrB2–30 vol.%SiC–15 vol.%Cnb	SPS. 1850 °C, 8 min, 35 MPa	81100100	–	2.93.63.6	[139]
ZrB2–15 vol.%SiCw–10 vol.%MoSi2	HP. 1750 °C, 30 min, 30 MPa	99.9	–	4.3	[140]
ZrB2–15 vol.%SiCw–15 vol.%MoSi2		99.9	17.64	5.52
ZrB2–15 vol.%SiCw–20 vol.%MoSi2		99.9	–	5.3
ZrB2–20 vol.%SiC–1 wt.% (BNNT–BNNP)	SPS. 1550 °C, 8 min	98.75	16.50	4.64	[141]
ZrB2–25 vol.% SiCZrB2–25 vol.% SiC–1 wt.% DnZrB2–25 vol.% SiC–2 wt.% DnZrB2–25 vol.% SiC–3 wt.% Dn	SPS, 1900 °C, 40 MPa, 7 min	99.910099.998.8	19.523.424.622.2	4.34.95.55.8	[142]
ZrC	SPS. 1750–1850 °C, 40 MPa, 300sec, 1.7 °C/s	95.5	17.60	3.30	[101]
ZrC–20 vol.%TiC–0.25 wt.%CNT		98.7	21.09	4.23
ZrC–20 vol.%TiC–0.5 wt.%CNT		98.5	20.86	4.99
ZrC–20 vol.%TiC–0.75 wt.%CNT		98.5	20.05	4.37
ZrC–20 vol.%TiC–1 wt.%CNT		98.5	20	4.27
ZrC–20 vol.%Ti–3 vol.%CNT	HP. 1600 °C	98.27	18.11	5.27	[143]
ZrC–20 vol.%Nb–3 vol.%CNT		99.28	17.93	7.23	[147]
ZrC–18 vol.%TiC–2 vol.%GNP	SPS. 1700 °C, 300s, 40 MPa	98.43	15.58	4.71	[148]
ZrC–17 vol.%TiC–3 vol.%GNP		99.49	15.16	5.17
TiC	SPS. 1600 °C, 5 min, 50 MPa	89	23.67	3.24	[116]
TiC–3.5 wt.%WC–2 wt.%CNT		99	33.56	7.81
TiC–3 wt.% WCTiC–3 wt.% WC–10 vol.%SiCwTiC–3 wt.% WC–20 vol.%SiCwTiC–3 wt.% WC–30 vol.%SiCw	SPS, 1900 °C, 40 MPa, 7 min	10094>100>100	28.6121627.3	–	[162]
TiC–10 vol.%SiCwTiC–10 vol.%SiCw–1.5 wt.%WCTiC–10 vol.%SiCw–3 wt.%WC	SPS, 1900 °C, 40 MPa, 7 min	98.7399.4394.13	24.5422.5412	–	[163]
TiCTiC–5 wt.% h-BN–5 wt.% Gnf	SPS. 1900 °C, 40 MPa, 10 min	95.593.2	30.6824.29	–	[164]
TiCTiC–5 wt.%TiN–5 wt.%Dn	SPS. 1900 °C, 40 MPa, 10 min	95.587.4	30.6824.93	–	[165]
TiC–5 wt.%AlN–5 wt.% GNP		>100	27.38	–	[166]
TiC–10 wt.%nano-TiC–3 wt.%WC–0.45 wt.%MLG	HP (Two-step sintering)	99.3	24.2	7.2	[167]
TiC–2 wt.%HfC–3 wt.%ZrO2TiC–HfC–ZrO2–1.5 wt.%CNT–3 wt.%SiCnwTiC–HfC–ZrO2–1.5 wt.%CNT–6 wt.%SiCnw	Two-step SPS. 1800 °C for 5 min. 1650 °C for 30 min	>98.5>98.5 >98.5	24.4723.9720	4.67.397.39	[168]
TaCTaC–15 vol.%SiC–15 vol.%CNT	SPS. 1850 °C, 5 min	9398	15.514.1	3.111.4	[117]
TaC–TiC–25 vol.%SiCTaC–TiC–25 vol.%SiC–5 wt.%G	SPS. 2000 °C, 8 min, 35 MPa	92.495.1	18.519.8	–	[178]
HfC–5 wt.%MoSi2	SPS. 1600 °C, 5 min, 50 MPa	96	24.1	3.08	[118]
HfC–5 wt.%MoSi2–2 wt.%CNT		99	21	5.86
Si3N4	HP. 1700 °C, 1 h, 30 MPa	–	15.2	5.6	[190]
Si3N4–2 wt.%CNT–10 wt.%TiN		–	15.4	8.26
Si3N4–1.5 wt.%GNPSi3N4–5 wt.%Ag–1.5 wt.%GNPSi3N4–5 wt.%Ag–1.5 wt.%GNP	SPS. 1700 °C, 25/50 MPa, 10 min	–	15.1610.2514.02	5.683.625.25	[191]
Si3N4–1 wt.% MLGSi3N4–1 wt.% MLG–1 wt.%Si3N4wSi3N4–1 wt.% MLG–2 wt.%Si3N4wSi3N4–1 wt.% MLG–3 wt.%Si3N4wSi3N4–1 wt.% MLG–4 wt.%Si3N4wSi3N4–1 wt.% MLG–5 wt.%Si3N4w	SPS. 1700 °C, 40 MPa, 6 min	–	1818.919.820.12019.8	7.481011.0410.27.64	[193]
TiB2TiB2–19.5 wt.%SiC–0.5 wt.%GNPTiB2–19 wt.%SiC–1 wt.%GNPTiB2–15 wt.%SiC–5 wt.%GNPTiB2–10 wt.%SiC–10 wt.%GNP	SPS, 1700/1800 °C, 40 MPa, 5 min	979899.499.998.6	24.220.120.618.717	3.95.75.96.45.4	[205]
TiB2–20 wt.%SiCTiB2–20 wt.%SiC–10 wt.%GNP	HP. 2100 °C, 7 min	98.299.9	23.814.5	56.2	[197]
TiB2–10 wt.%MgO–6 wt.%TiNTiB2–10 wt.%MgO–6 wt.%TiN–2 wt.%SiCw–4 wt.%MLG	Two step sinter.1800 °C, 5 min1700 °C, 45 min	99.6100	3321.2	8.86	[207]
Ti(C.N)–47.7 wt.%Al2O3Ti(C.N)–47.5 wt.%Al2O3–0.4 wt.%GNP	HP. 1700 °C. 32 MPa, 10 min	9899.5	2124	68.66	[209]
WC–30 vol.%Al2O3–0.5 wt.%CNTWC–30 vol.%Al2O3–0.5 wt.%CNT–2 wt.%TiO2	HP, 1540 °C, 40 MPa, 90 min	97.498.9	15.917.18	7.417.75	[210]
WC–14.6 wt.%Al2O3WC–14.5 wt.%Al2O3–0.1 wt.%GNPWC–14.1 wt.%Al2O3–0.5 wt.%GNPWC–13.7 wt.%Al2O3–0.9 wt.%GNP	HP. 1540 °C, 40 MPa, 90 min	99.298.597.396.7	15.8217.4414.6212.87	7.888.899.478.98	[211]
WC–8 wt.% (ZrO2–20 wt.%Al2O3)WC–8 wt.% (ZrO2–20 wt.%Al2O3)–0.2 wt.%GNP	Oscillatory pressure sintering.	99.199.8	2523.72	7.758.68	[21]
WCWC–3 wt.%SiCw–0.1 wt.%MLGWC–3 wt.%SiCw–0.2 wt.%MLG	Two step SPS.1950, 5 min.1850, 20 min.	96.599.599	26.325.623.7	7.213.113.2	[215]
WC–9 wt.%Al2O3WC–9 wt.%Al2O3–3 wt.%TiC–3 wt.%TiN–0.15 wt.%SiCw–0.05 wt.%MLG	HP two-step.1700 °C, 4 min.1600 °C, 60 min.	97.899.8	21.922.8	8.2512.7	[216]
WC–10 wt.%Si3N4WC–10 wt.%Si3N4–0.01 wt.%BNnfWC–10 wt.%Si3N4–0.05 wt.%BNnfWC–10 wt.%Si3N4–0.1 wt.%BNnfWC–10 wt.%Si3N4–0.15 wt.%BNnf	Sintering. 1750 °C	––	17.7618.3220.5619.0419.12	7.7010.9910.5710.5010.09	[217]
Mullite–10 wt.%TiB2–1 wt.%CNT	SPS. 1350 °C, 30 MPa, 5 min	99.1	18.31	5.46	[219]
Mullite–10 wt.%ZrB2–1 wt.%CNT		99.13	16.24	4.18	[220]
Mullite–10 wt.%TiN–1 wt.%CNTMullite–10 wt.%TiN–10 wt.%TiB2–1 wt.%CNTMullite–10 wt.%TiN–10 wt.%TiB2–10 wt.%ZrB2–1 wt.%CNT	SPS. 1350 °C, 30 MPa, 5 min	98.997.797.2	15.8416.4817.15	3.984.124.86	[221]
B4CB4C–18 wt.%TiB2B4C–18 wt.%TiB2–4 wt.%GNP	HP, 1900 °C, 1 h, 30 MPa	97.4598.8298.67	29.832.131.8	3.054.368.19	[222]
SiC–45vol.%B4C–10 vol.%NiSiC–45 vol.%B4C–10 vol.%Ni–5 vol.%CNT	SPS. 1650 °C, 30 MPa, 5 min	98.397.8	16.715.63	3.355.82	[223]
YSZYSZ–1 wt.%CNT–1 wt.%BNYSZ–3 wt.%CNT–3 wt.%BNYSZ–5 wt.%CNT–5 wt.%BN	SPS.1300–1400 °C, 40 MPa, 30 min	99.9898.698.797.3	12.211.29.49.6	6.177.27.9	[10]

G_nf: graphite nano-flakes, GNP: graphene nano-platelets, GNS: graphene nanosheets, BNNT: boron nitride nanotubes, BNNP: boron nitride nanoplatelets, C_np: carbon nano-particles, C_nb: nano-sized carbon black, D_n: nano-diamond, YSZ: yttria-stabilized zirconia.

G: graphene, rGO: reduced graphene oxide, MLG: multi-layer graphene, GNS: graphene nano-sheets.

SiC_w: silicon carbide whiskers, SiC_np: silicon carbide nano-particles, CS: conventional/pressureless sintering, IHP: inductive hot-pressing, HFIHS: high frequency induction heat sintering.

Potential applications

The process of developing quality materials still largely relies on extensive trial and error. It may take years, sometimes even decades, to find an appropriate material for a specific technological use, and even more time is needed to refine that material for commercial viability [11]. The enhancement in mechanical, wear, and tribological properties of ceramic materials brought about by reinforcing them with two or more phases is believed to improve their performance in structural applications. Moreover, hybrid ceramic nanocomposites that demonstrated enhanced physical properties will be candidate materials for many functional and novel applications. Enhancements in the properties of alumina hybrid nanocomposites may significantly improve their performance in a range of applications, including cutting tools, electrical insulators, igniters, heating elements, chemical insulators, and dental implants. Similarly, the improvement in sinterability, densification, and both mechanical and thermal properties of ZrB₂ hybrid nanocomposite materials could extend their use in extreme conditions and harsh environments, enhancing their performance as electrodes for electric discharge machining, crucibles in the metal industry, and structural and functional components in aerospace applications. Hybrid TiC nanocomposites are anticipated to outperform their conventional counterparts due to their high density, exceptional mechanical properties [96], and resistance to thermal shock [153], making them suitable for use in hypersonic space applications, cutting tools, refractory components, and the nuclear industry [133–155]. Despite limited research on HfC hybrid nanocomposites [118], their potential applications remain promising in fields such as rocket nozzles, nuclear reactors, thermal field emitters, cutting tools, high-temperature solar absorbers, and space structures [179–181]. Additionally, the advanced properties of Si₃N₄ hybrid nanocomposites are expected to expand their usage in high-temperature ball bearings, rollers, and cutting tools [183], while TiB₂ hybrid nanocomposites may find applicability in hypersonic reentry spacecraft, gas turbine blades, advanced rocket motors, wear-resistant coatings, impact-resistant armor, and cutting tools. WC hybrid nanocomposites exhibit outstanding mechanical properties, making them highly suitable for use as tool materials in cutting tools, drills, and inserts for the machining and metal fabrication industries. The substantial enhancements in the properties of SiC hybrid nanocomposites obtained through 3D printing could facilitate their future applications in the aerospace industry [8]. SiC–TiB₂–Al₂O₃ hybrid nanocomposites, demonstrating impressive properties [9], are also promising candidates for thermal barrier coatings in both gas turbines and the nuclear industry. Lastly, the high fracture toughness of the YSZ–5CNT–5BN nanocomposite, combined with its acceptable physical properties and thermal stability, paves the way for its application in membranes, oxygen sensors, solid oxide fuel cells, surgical instruments, and medical implants.

Future research directions

The ongoing research in hybrid ceramic nanocomposites is likely to continue capturing the attention of researchers, who aim to enhance their properties, optimize their performance, and expand their applications across different industries. As of today, the reported augmentation in the physical, mechanical, wear, friction, and lubrications properties of many hybrid ceramic nanocomposite materials is encouraging; however, developing hybrid nanocomposites with optimum properties and enhanced performance is a complex task that involves addressing several key challenges by researchers. Continuous research and advancements in processing methods are essential to obtain hybrid ceramic nanocomposites characterized by fully dense structures, minimal flaws, optimized contents of reinforcements, uniform distribution of reinforcements, and strong interfaces. Ultra-high-temperature ceramics present promising candidates for extreme environments; however, their brittleness and the challenges associated with sintering them limit their applicability in various scenarios. To address these limitations, further exploration of diverse matrices, processing routes, and various combinations of reinforcements, as well as their dimensionalities, morphologies, and inherent properties, is crucial. The layer-by-layer fabrication approach in additive manufacturing shows significant potential for developing hybrid ceramic nanocomposites, offering precise control over material deposition to create tailored structures along with a uniform distribution of nanoscale reinforcements, optimized reinforcement ratios, and improved interfacial bonding. Additionally, machine learning could revolutionize the development of these materials by predicting ideal combinations of ceramic matrices and reinforcements to achieve enhanced mechanical, thermal, or electrical properties without extensive physical trials, thereby reducing both the time and costs associated with material development. It can also optimize various parameters such as pressure, temperature, and sintering time, resulting in improved densification, reduced defects, and enhanced interfacial bonding.

Conclusion

This work presented the properties of hybrid ceramic nanocomposites and highlighted the role of the matrix in providing high strength, hardness, and thermal stability, as well as the role of the reinforcement in enhancing these properties and introducing additional functionalities such as improved toughness, wear resistance, and various physical properties. The combination of the matrix and reinforcement attributes, coupled with the use of innovative sintering methods and optimized process parameters, allows for the processing of hybrid ceramic nanocomposites with tailored properties suitable for a wide range of applications across different industries. Additionally, future research directions were formulated to develop nanocomposites with desired properties and enhanced performance. This requires obtaining dense and homogeneous nanocomposites with optimum composition that are free from defects and have a smooth and strong interface between the reinforcement and matrix.

Authors’ contributions

Nouari Saheb contributed to writing and editing the manuscript. Umer Hayat contributed to data collection and writing the manuscript. Syed Fida Hassan contributed to writing and editing the manuscript. All authors reviewed and approved the manuscript.

Funding

“The authors would like to acknowledge the support provided by the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum and Minerals (KFUPM) for funding this work through project No. DF181022.”