ABSTRACT
Titanium alloys are very important light weight construction materials. They have broad applications in aviation, and aerospace; and always have been a research focus in the field of materials. However, the high temperature performance of titanium alloy is relatively poor;therefore, researches are carried out on high temperature protection technique on titanium alloy actively.
Thermal barrier coatings (TBCs) are one of the advanced high temperature protective coatings, and the state-of-art thermal barrier coating material 8YSZ has been successfully applied to the protection of aircraft engine combustion chamber and other high temperature parts. However, little research has focused on the oxidation resistance, mechanical properties and thermal property of 8YSZ coatings on titanium alloy. The main purpose of this study is to fabricate 8YSZ TBCs on titanium alloy surface, study the oxidation resistance, thermal insulation and thermal shock resistance performance of TBCs on titanium alloy.
CoNiCrAlY bond coat and 8YSZ ceramic top coat are deposited on titanium alloy surface by atmospheric plasma spraying (APS). In order to research the influence of bond coat on the performance of coating, TBCs without bond coat on titanium alloy surface were also deposited. The performance of oxidation resistance, thermal insulation and thermal shock resistance of TBCs at different temperatures and their failure behavior were studied and analyzed. Mechanical properties of coatings such as surface roughness, microhardness and adhesion strength were also tested in this paper.
The performance of microhardness has improved greatly after spraying TBCs on titanium alloy, and the adhesion strength between titanium alloy and coatings is well. The thermal insulation effect of sample with bond coat or without bond coat is almost the same.The surface temperature is higher, the greater the temperature difference between coating and substrate. Without doubt, the 8YSZ ceramic top coat has advantage of providing thermal protection for titanium alloy.
8YSZ ceramic top coat and CoNiCrAlY bond coat have a layered structure on titanium alloy surface. It is obvious that the microhardness of 8YSZ ceramic layer is much higher than that of substrate. The adhesion strength between titanium alloy and coatings is well. The results of thermal insulation test show that thermal insulation effect of sample with bond coat or without bond coat is almost the same. It is worth noting that thermal insulation of coating has different values at different testing temperatures; and it increases with the increasing surface temperature. The higher the surface temperature of the coating, the better the thermal insulation effect is. Without doubt, the 8YSZ ceramic top coat has advantage of providing thermal protection for titanium alloy.
Oxidation resistance and thermal shock life of samples with bond coat is better than that without bond coat. The failure reason of samples with or without bond coat is mainly caused by the mismatch of thermal expansion, while for samples with bond coat on titanium alloy besides the mismatch of thermal expansion stress; bond coat oxidation and elemental
diffusion also affect coatings performance significantly. But on the other hand, because of the bond coat could provide oxidation protection for substrate at high temperatures, the TBCs with bond coat has better composite performance. Therefore, TBCs with bond coat has better application prospects on titanium alloy surface, which could improve the oxidation resistance of titanium alloys, and has good thermal shock resistance and longer thermal cycling life.
Keywords: Titanium alloy; Thermal barrier coatings; Oxidation resistance; Thermal insulation; Thermal shock
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