Biaxial residual stress states of plasma-sprayed hydroxyapatite coatings on titanium alloy substrate

Biaxial residual stress states of plasma-sprayed hydroxyapatite coatings (HACs) on titanium alloy substrate as a function of plasma power, powder feed rate and coating thickness were studied by X-ray ‘sin2 ψ’ method. The Youngʹs modulus of hydroxyapatite (HA), required for the stress analysis, was measured from the separated free coating by three-point bending test method. It was found that the directions of principal stresses were in proximity to and perpendicular to the spraying direction. The measured Youngʹs moduli of HACs were much lower than the theoretical value reported. The denser, well-melted HAC exhibited a higher residual stress, as compared with the less dense, poor-melting HAC. The denser coatings could be effected by higher plasma power and lower powder feed rate. Significantly, the thicker 200 μm HAC exhibited higher residual stress than that of the thinner 50 μm HAC. The implications of residual stress in HAC for biomaterials are discussed in detail.