From the past studies of the residual stress occurred during the plasma spraying, the different results were obtained; the arguments of these previous works were not only in the value of residual stress but the stress state existed in the HACs. The residual stress generation theory stated in these literatures appear to be oversimplified, hence a more basic study is needed to explore the contention on the residual stress state in HA coating during plasma spraying. In this study, in order to obtain the whole and accurate residual stress value and state, the residual stress was measured by using the material removal method. For the purpose of inferring the generation mechanism of residual stress, the particular forms of the substrate holder and various cooling system were employed. The results show that the difference between the HAC and substrate temperature is the main factor that dominates the residual stress state. HAC/Ti-substrate on the hollow rotational holder with the back cooling system would lead to the higher temperature in coating than substrate during plasma spraying and result in the tensile residual stress in the HAC. On the contrary, HAC with the front cooling system displayed much lower temperature than the Ti-substrate that fixed on non-hollow holder and hence the compressive residual stress occurred. These could be attributed to the differences of volume shrinkage between the HAC and Ti-substrate that from different initial temperature to room temperature. In another word, the residual stress state is depended on the spraying program not always tensile or compressive state.

Through the measurement results of the residual strains of Hydroxyapatite coatings (HACs) and the values of Young’s moduli, the initial vacuum heating treatment (<600°C) can release the compressive residual strain of HAC after plasma spraying, so enabling HACs to obtain the optimal bond strength at the heating temperature of 500~600°C. Nonetheless, the post-vacuum heating temperature higher than 600°C led to the deterioration of the bond strength of HA/Ti-substrate. With the increasing temperature, the increased compressive residual strain will weaken the interfacial adhesive force of coating to substrate, resulting in the lower bond strength with higher temperature. The results of this paper demonstrated that the proper temperature of post-vacuum heating treatment improved effectively the bonding strength between HAC and Ti-alloy substrate.

Hydroxyapatite (HA) has recently been used as a bone substitute in orthopedic and dental surgery applications, owing to its excellent biocompatibility. However, the poor mechanical properties of HA limit the material’s application in the loading condition. In this study, HA was coated onto the surface of alumina substrate, with a view to overcome the poor mechanical properties of HA and the biocompatibility of alumina. Improvement of the bonding strength of HA coatings to alumina substrate was attempted by adding a bond coat of porcelain via plasma-spraying and by post heat-treatment. HA-50wt% porcelain and pure porcelain were used to manufacture two kinds of bond coat before HA coating, and then all the specimens were heat-treated at 750°C, 800°C, 850°C, 900°C for 0.5 hours. The results shows that the bond strength between HA coating and Al 2 O 3 substrate could be improved by employing the porcelain as the bond coat after the heat treatment. The strengthening mechanisms of the two systems are discussed.