Tensile adhesion strength tests were carried out to evaluate interfacial strength between hydroxyapatite (HAp) coating layer and titanium (Ti) alloy substrate of HAp coated specimens, especially influence of substrate surface roughness and post heat treatment. In addition, residual stress evaluation was performed by curvature measurement during substrate cutting and chemical dissolution treatment based on the curvature method. Furthermore, in order to verify the modification of the HAp coating itself, the HAp surface hardness test was performed on the HAp free-standing specimens. Finally, the magnitude of thermal stress caused by cooling after thermal spraying was estimated from the heat transfer-static structure coupled analysis by a general-purpose finite element method program. As the results, a correlation between substrate surface roughness and interfacial strength was shown. Similarly, a correlation between post heat treatment temperature and interface strength was also shown. Moreover, residual stress in the HAp coating layer was significant decreased by applying 500℃ post heat treatment, in contrast to 300℃ post heat treatment. From the viewpoint of coating integrity, it is thought that reducing the residual stress by post heat treatment is useful method.
The bonding mechanism of thermal sprayed ceramic coating was not clear due to the complicated profile of blasted surface and microstructure of the coating. The objective of this study is to investigate the influence of residual blast-material on the adhesive strength of yttria-stabilized zirconia coatings sprayed by atmospheric plasma spraying. Stainless steel was used as a substrate, and blasted in three different pressures and five different blast angles by fused alumina abrasives with three different particle sizes. Three powders with different particle sizes were used as spray materials. Our observation revealed that roughness of blasted substrate were constant in each blasting material independent of blast angle, though mass of residual blast-material increased with increasing the blasting pressure. Interestingly, the tensile adhesive strength increased with increasing the mass of residual blast-material in each spray materials under the condition of the same roughness. Tensile residual stress was high at near the interface, and the residual stress of acute blast angle was higher than that of right angle. It was convinced that the residual stress reduced the adhesive strength in the case of acute angle. Therefore, it was concluded that residual blast-material did not almost affect the adhesive tensile strength.
As new materials for next-generation aircraft engines, SiC/SiC ceramic matrix composites (CMCs) are thought to be promising. The CMCs possess higher thermal resistance and lower density than those of conventional metallic materials. However, when exposed to oxidative steam environment, SiC/SiC CMCs will reduce the thickness resulting from oxidation reactions. Therefore, environmental barrier coatings (EBCs) are necessary to protect the surface of CMC components from water vapor. EBCs without cracks or pores are required so that water vapor will not reach the surface of CMC substrate. As a new deposition process, suspension plasma spraying (SPS) process is expected to produce much denser EBCs since much smaller powders can be supplied in this process. At the same time, however, SPSed powders are easily influenced by residence time in the plasma flame, resulting in evaporation of particular materials and phase transition of feedstock powders. In this study, SiO2-rich Yb silicate suspensions are applied to compensate for the evaporation of SiO2 during the process. Subsequently, the applicability of SiO2-rich SPSed EBCs was discussed.