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

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

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.