Residual stress evaluation within hydroxyapatite coatings of different micrometer thicknesses

Abstract

Bioceramic coatings, including calcium phosphate coatings composed of hydroxyapatite (HA), represent a common surface modification for metallic bone implants. In view of the mismatch in material properties between the ceramic coating material and the metallic substrate material, this study aimed to evaluate the residual stress within an HA-coating deposited using RF magnetron sputtering via two different analyses, based on either the Stoney formula (i.e. based on curvature measurements of the substrate [Stoney, 1909]) or the sin2ψ method (i.e. based on X-ray diffraction patterns of the coating). Additionally, the effect of HA-coating thickness (i.e. 1 or 4μm) on residual stress and surface topography was addressed. We hypothesized that: (i) both methods to determine residual stress would show similar results, and (ii) residual stress would vary between coating thicknesses. The curvature measurements showed low residual stress values (ranging from −60 to 80MPa) and indicated relaxation upon post-deposition heat treatment at 650°C. Similarly, lattice-based measurements of residual stress showed relaxation of HA coatings upon post-deposition heat treatment. Coating thickness variation in the micrometer scale showed no major effects on residual stress magnitude. For reasons of accuracy and similarity to the actual conditions, determination of residual stress via lattice-based techniques is preferable.