Y. C. Yang, National Taipei University of Technology, Taipei, Taiwan; E. Chang, National Cheng Kung University, Tainan, Taiwan
The compressive residual stress would promote the tendency of plasma-sprayed coating to de-bond. In-plane compressive residual stress would induce through-thickness tensile stress acting in the direction normal to the interface of the HA coating and the Ti-substrate. This normal tensile stress would weak the adhesive strength of HAC to substrate. In this study, post vacuum heating treatment was employed to improve the residual stress of HACs. We investigated the influence of post vacuum heating on the residual stress and bonding strength of plasma-sprayed HACs. According to the experimental results, the HAC with 500ºC vacuum heating shows the highest bonding strength value of all (44.6 MPa compare to the as-sprayed HAC 28.7 MPa). However, with the heating at the temperature above 500ºC, the bonding strength decreased gradually, even lower than 28.7 MPa after heating at 800 and 900 ºC. The index of crystallinity (IOC) of heat-treated HACs increase and impurity phase content decreases with the increasing heating temperature. The residual strain of the HACs changed from compressive to tensile state after 400ºC, 500ºC and 600ºC post heating. However, with the heating temperature above 700ºC, the residual strain revealed the compressive state again. We can deduce that the highest bonding strength at 500ºC is contributed to the reducing of compressive strain. 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.
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