Determination of Triaxial Residual Stress in Plasma-Sprayed Hydroxyapatite (HAp) Deposited on Titanium Substrate by X-ray Diffraction

Measurement of residual stress in plasma-sprayed coating is of key importance to optimize their microstructure and mechanical properties. In this study, the x-ray diffraction analysis was carried out using the sin2ψ method to evaluate the residual stress distribution of hydroxyapatite (HAp) coatings produced on titanium substrate by atmospheric plasma spraying (APS) and vacuum plasma spraying (VPS). The sin2ψ method measured strains at different tilt ψ and rotating φ angles around the specimen surface normal. A non-uniform and inhomogeneous stress distribution was present in the both coatings. The measured strain εψφ is plotted versus sin2ψ, showing a nonlinear (elliptical) behavior, which indicates the presence of inhomogeneous triaxial stress distributions within coating, due to the crystalline anisotropy, inhomogeneous cooling rate or solidification of the molten particles. The normal stress values within both HAp coatings produced were found to be tensile in nature, but the values of tensile stresses are significantly higher in APS coating than those values obtained for VPS coating.