Comparative study on tribological and corrosion protection properties of plasma sprayed Cr₂O₃-YSZ-SiC ceramic coatings

Abstract

In this research, the tribological and corrosion protection properties of atmospheric plasma sprayed Cr₂O₃-20YSZ (CZ) and Cr₂O₃-20YSZ-10SiC (CZS) composite coatings on 304 L stainless steel substrates were evaluated and compared to pure Cr₂O₃ (C) coating. Microstructural evaluations were carried out using an optical microscopy, X-ray diffraction (XRD) analysis, and a Field Emission Scanning Electron Microscopy (FE-SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS). The ball-on-disk wear test was performed using an alumina counterpart at room temperature. The protective efficiency of the coatings with/without sealing treatment was evaluated by electrochemical potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in a 3.5 wt% NaCl solution. Evaluation of wear tracks showed that enhancement of the wear resistance of the composite coatings was ascribed to the zirconia phase transformation toughening and the presence of more ductile tribofilms participated in pore filling during the wear. The corrosion measurements revealed that the protection properties of the coatings were strongly dependent on the porosity content so that the as-sprayed coatings due to their porous nature could not protect the substrate. However, filling the porosity using an epoxy sealant prevented the penetration of the corrosive environment into the substrate and therefore increased the protective efficiency of the coatings. Moreover, the density of the unfilled defects such as micro-cracks determines the protection properties of the sealed coatings, so that the CZ coating due to its highest toughness had the lowest micro-crack density and thus the highest protective efficiency.

Introduction

Ceramic materials owing to their superior chemical stability as well as high hardness and wear/corrosion resistance have been widely used for structural applications; however, their relatively low fracture toughness and high manufacturing costs limited their applications. To overcome the mentioned problems, ceramics have been increasingly applied in the form of coatings on the metallic substrates [[1], [2], [3]]. Ceramic coatings have also been developed to provide enhanced corrosion and wear resistance for their substrates in most corrosive environments. Among various types of coating deposition processes, the Atmospheric Plasma Spray (APS) process has been further developed for manufacturing ceramic coatings, due to its elevated operating temperature and high deposition rate [4,5].

Over the past decade, many efforts have been made to investigate the mechanical, tribological, and corrosion properties of Al₂O₃ and Cr₂O₃ plasma-sprayed coatings [[6], [7], [8]]. Plasma-sprayed Cr₂O₃-based coatings have been broadly applied to improve the resistance of components against different kinds of wear and corrosion at room/high temperatures. In spite of such advantages, high brittleness and low fracture toughness of the Cr₂O₃ coating has limited its application; therefore, improving the mechanical properties of the Cr₂O₃ coating is still among the most promising areas of scientific research [[9], [10], [11], [12]].

One way to improve the fracture toughness and wear performance of Cr₂O₃ coating is to use the nano-agglomerated powders for producing nanostructured coatings [[13], [14], [15]]. Another approach is toughening the coating by addition of appropriate amounts of other ceramic reinforcements showing phase transformation toughening mechanism [16,17]. However, adding a ceramic reinforcement to enhance each mechanical property may negatively affect other properties of a material. To deal with this problem, other reinforcements can be added to the produced composite to compensate for the reduced mechanical properties [18]. Zirconium oxide (zirconia, ZrO₂) is a ceramic material showing outstanding fracture toughness [17]. However, owing to the high melting temperature of zirconia (2700 °C), the low melting degree of sprayed particles leads to a lot of semi-molten particles and therefore high porosity of the plasma-sprayed ZrO₂ coating [19]. Although considerable improvements in the mechanical properties and tribological behavior of zirconia-toughened alumina (ZTA) coating have been formerly reported [20,21], the effect of zirconia addition on the properties of Cr₂O₃ coating produced using plasma spraying still needs to be investigated. SiC reinforcement owing to its high hardness has already been added to many ceramics towards producing ceramic matrix composites (CMCs) with enhanced hardness, strength, and wear resistance [22,23].

Plasma-sprayed ceramic coatings generally contain interconnected pores and cracks caused by various factors, including imperfect melting of the particles, insufficient flow or fragmentation of molten droplets upon coming into contact with the substrate. Cracks can also arise from rapid solidification rate and poor interlayer bonding. These structural defects not only suppress the mechanical properties and hence the wear resistance of the coatings, but also increase the corrosion rate of the substrate. One way to modify such intrinsic defects in thermal spray coatings is to use a sealant in order to penetrate the pores and cracks which limits the exposure of the substrate surface to the corrosive environment [24,25]. Among the numerous organic/inorganic sealants previously applied for sealing the coatings, the most common organic ones are based on epoxy resin, phenolic compounds, furans, silicone, polyester, and waxes. The penetration depth of sealants into the coatings has already been reported to be less than 100 μm [[26], [27], [28]].

304 L stainless steel (SS304L) has been widely used in various industries for wear and corrosion applications [29]. Although stainless steels are generally well-known for their high corrosion resistance in a wide variety of corrosive environments, they are susceptible to localized corrosion in the presence of chloride ions. As of current, the higher degree of protection of stainless steels against corrosion has already been obtained by applying ceramic coatings onto the stainless steel substrates [30].

The main objective of this research is to enhance the tribological/corrosion properties of SS304L substrate by depositing Cr₂O₃-based coatings onto the substrate. For this purpose, 80Cr₂O₃-20YSZ and 70Cr₂O₃-20YSZ-10SiC composite coatings were sprayed using the APS process and afterward their tribological and corrosion performances were evaluated and compared to individual Cr₂O₃ coating. Corrosion protection properties of the coatings without and with the epoxy resin sealing treatment was investigated by immersion and electrochemical tests in a 3.5 wt% NaCl solution.