Mechanical properties by instrumented indentation of solution precursor plasma sprayed hydroxyapatite coatings: Analysis of microstructural effect

doi:10.1016/j.surfcoat.2016.04.028

Surface and Coatings Technology

Volume 298, 25 July 2016, Pages 93-102

https://doi.org/10.1016/j.surfcoat.2016.04.028 Get rights and content

Highlights

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The paper studies the mechanical properties of SPPS-HA coatings
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The coatings sprayed using more concentrated solution had cauliflower-like surface and the other ones had dome like surface
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Coatings from concentrated solution had greater elastic modulus and hardness than coatings from low concentration solution
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The porous nature of coatings and their crystallinity influenced the hardness and elastic modulus measurements
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The hardness measurements are affected by the coating’s densification and by the presence of the substrate

Abstract

The mechanical properties of solution precursor plasma sprayed hydroxyapatite (SPPS-HA) coatings were investigated by instrumented indentation test in relation to their microstructural characteristics. The morphology of the coatings using solution that is more concentrated shows cauliflower-like surface with fine particles and agglomerated fragmented shells while dome-like formations with spherical particles around were observed for coatings from less concentrated solution. The coatings had elastic modulus and hardness values in the range of 5.0 to 22.0 GPa and 0.04 to 0.17 GPa, respectively. The strong disparities in elastic modulus and hardness values were attributed mainly to the morphology and crystallinity of the coatings. The coatings prepared from more concentrated solution precursors had average elastic modulus and hardness nearly two times greater than the coatings prepared from solution of low concentration. Variation of hardness values was due to the densification of the coating material upon indentation. Models employed enabled to describe the hardness and elastic modulus of coatings as a function of crystallinity volume fraction. Moreover, for the HA coatings to exhibit bone-like microstructure, it was found that the crystallinity volume fraction should be within 0.56–0.86.

Introduction

Hydroxyapatite (HA) is a very important bio-ceramic material employed in medical applications due to its exceptional biocompatibility and its close resemblance to the physico-chemical characteristics of the mineral component of human bone [1]. Synthetic HA has been greatly considered as an osteoconductive bone substitute and commonly used in research and development field for studying the nature of biomineralization of the human bone. However, properties of synthetic HA such as purity, composition and morphology among others, are very dependent and sensitive to experimental conditions [2]. The presence of impurity phases during synthesis is undesirable since it leads to inferior characteristics of the resulting HA.

It is well known that powder plasma spraying [3], [4] is the standard method in producing HA coatings for bone implant applications. The relatively coarse powder is directly injected to the plasma jet. The coatings sprayed in this process exhibit cracking and large splat boundaries that significantly affect the biomechanical properties of HA especially when submitted to physiological fluids. For obtaining finely structured HA coatings, innovation of this process resulted in the use of suspension plasma spraying [5] or high velocity suspension flame spraying [6]. The further progress in plasma spray technology led to the development of HA coatings using solution precursor plasma spraying (SPPS) [7]. This process enables to synthesize the coating directly from liquid precursors without using powder. The molecularly mixed calcium-phosphate solution precursors are injected radially into the free expanding plasma jet. The coatings were dense and had good deposition rate. On the other hand, the calcium carbonate was present in the coatings together with the phases of decomposition of HA what lowered the quality of the coatings. Consequently, the precursors were modified [8].

It is important to know the mechanical properties of HA coatings in order to predict their behavior in service and, in particular, compare them to that of the natural bones. The previous studies of the mechanical properties of HA coatings were focused on the conventional plasma spray process using powder feedstock [9], [10] and suspension plasma spraying [11] by means of nano-indentation method performed on a prepared surface of the coatings.

In the present work, the mechanical properties of solution precursor plasma sprayed hydroxyapatite (SPPS-HA) coatings are studied using instrumented indentation technique (IIT). The mechanical properties are determined by means of indentation experiments in the micro loads range in order to limit the influence of the heterogeneity of the microstructures as well as the roughness over the surface of the coating for which the effects are predominant in nano-indentation. Elastic modulus and hardness of the SPPS-HA coatings were compared to the mechanical characteristics of HA coatings plasma sprayed using powder and suspension feedstocks. The microstructural effect, specifically, the role of morphology and crystallinity of the SPPS-HA coatings on mechanical properties is illustrated and discussed regarding the application of some models related to this aspect. Furthermore, the results obtained from the applied models enabled us to estimate the elastic modulus and hardness of the SPPS-HA coatings as a function of crystallinity volume fraction.

Section snippets

Preparation of calcium-phosphate solution

The calcium-phosphate solution was prepared using calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) and triethyl phosphite (P(OEt)₃) as precursors. Triethyl phosphite was hydrolyzed initially using deionized water and appropriate amount of calcium nitrate solution was added dropwise to reach the 1.67 Ca/P stoichiometric ratio. Two different concentrations of solutions were used, namely high solution concentration having Ca/P ratio of [0.5 M/0.3 M] and low solution concentration of [0.167 M/0.1 M] Ca/P

Microstructural characterization

The micrographs of the SPPS-HA coatings show different morphological features that depend strongly on the initial concentration of used precursors. Fig. 2 shows the micrographs of a representative sample H-2 prepared using high solution concentration showing cauliflower-like morphology having visible fine particles and agglomerated fragmented shells distributed at the surface of the coatings (see in particular Fig. 2(a)). The image at the coating's cross-section shows a relatively dense coating

Discussion

The mechanical properties of solution precursor plasma sprayed hydroxyapatite coatings were investigated in relation to their microstructural characteristics. HA coatings prepared using high solution concentration were relatively dense and more crystalline compared to coatings sprayed using low solution concentration. The values of the elastic modulus of the coatings were between 5.0 and 22.0 GPa depending on the solution concentration used. The values obtained were much lower compared to that

Conclusions

The solution precursor plasma sprayed hydroxyapatite coatings (SPPS-HA) on stainless steel substrate were tested by multi-cycle micro-indentation analysis and the mechanical properties such as elastic modulus and coating hardness were found. The morphology of coatings prepared using solution of high concentration shows cauliflower-like structure with fine particles and agglomerated fragmented shells. On the other hand, the coatings obtained using low solution concentration had dome-like surface

Acknowledgments

The help of Paweł Sokołowski, MSc. Eng. during spraying experiments is highly acknowledged by the authors. Rolando T. Candidato, Jr. would also like to acknowledge Region Limousin of France for the scholarship grant.