Nanoindentation reveals mechanical properties within thermally sprayed hydroxyapatite coatings

Nanoindentation offers a unique capability to assess the mechanical properties of polished cross-sections of thermal spray coatings. This study set out to investigate the suitability of nanoindentation to extend the analysis of the cross-section to multiple points for a more detailed insight into variations through the thickness of the coating. Hydroxyapatite (HAp) was classified to three different particle sizes (20-40, 40-60 and 60-80 μm) and then thermally sprayed to produce a structurally homogeneous coating. Micro-Raman Spectroscopy (MRS) revealed dehydroxylation of the powder during the traverse in the thermal zone. Multiple indentations with a separation of 4 μm showed a transition within the thickness of the coating. Use of smaller particles led to higher hardness and elastic modulus compared to powder containing larger particles. The mechanical property profile provides valuable information for optimizing the processing of such coatings and feedback for the design of coating properties.