Measurements of residual stresses in plasma-sprayed hydroxyapatite coatings on titanium alloy

In an attempt to investigate the stress state and stress distribution in hydroxyapatite coatings (HAC), the residual stresses in thick HACs on titanium alloy were studied by the materials removal method, as a function of cooling media during spraying. In addition, the X-ray diffraction sin2ψ method was adopted as a comparison. The Young's moduli of hydroxyapatite coatings were measured on separated free coating by a three-point bending test. The results show that the measured Young's modulus of the HACs with an average of 22.8 GPa was found to be much lower than the theoretical value of bulk HA. During measurements by the materials removal method, on the three types of HACs using different cooling media, the interface between the HAC and Ti substrate displays higher residual stress than the top surface of HAC. The residual stresses in all the HACs measured by both methods were in a compressive mode, and the residual stresses on the top surface obtained from the two methods are consistent. It was also found that the compressive residual stresses on the top surface of HAC and at the interface between the coating and the substrate both increased with increasing temperature of the HAC. Therefore, the coating temperature, and the effect of varying the cooling media during plasma spraying, had a significant effect on the residual stress states of the HACs.