Influence of residual stress on bonding strength of the plasma-sprayed hydroxyapatite coating after the vacuum heat treatment

Plasma-sprayed HACs with a fair amount of impurity phases and amorphous calcium phosphate will induce the dissolution and the dissociation for a period of implantation. Besides, the compressive residual stress in the coating would promote the tendency of plasma-sprayed coating to de-bond. For the biological stability, a significant crystallization and stress release of plasma-sprayed HACs with an appropriate post vacuum heating treatment is an effective method. According to the experimental results, post vacuum heating treatment could improve the cohesive strength of HACs by sintering and re-crystallizing. Meanwhile, the heat treatment also releases the compressive residual stress and replaced by the tensile stress. Hence, the HAC shows the highest bonding strength value when heating up to 600 °C. Then, the heating temperature higher than 600 °C led to the deterioration of the bonding strength of HA/Ti-substrate. This could be contributed to the reverse compressive residual stress in HAC. With regard to the variation of stress state in HACs might due to the various thermal expansion coefficients, which as a result of the crystallization during vacuum heating and the volume contraction of HACs. These will be discussed in detail.