In this study, the mechanisms responsible for enhancing the adhesion strength of thermally sprayed metallic coatings subjected to vacuum heat treatment were investigated using atmospheric plasma sprayed (APS) CoNiCrAlY coatings as an example. The formation of metallurgical bonding between the coating and the substrate, which determined the increase in the adhesion strength of the coatings, was studied by analyzing the effect of morphological changes of the oxide film in the coating. The results showed that during the vacuum heat treatment process, the oxide film formed during the coating deposition gradually broke down and subsequently shrank into round oxides. After vacuum heat treatment, the adhesion strength of the coating improved significantly, and there was a positive nonlinear relationship between the treatment time and the adhesion strength. The increase in the adhesion strength was caused by the formation of metallurgical bonding between the coating and the substrate. However, the prerequisite for the formation of metallurgical bonding was that the oxide film had to break during the vacuum heat treatment process. A thermodynamic 2D model based on the thermal grooving theory was proposed to explore the essential conditions for the breaking and shrinking of the oxide film.

The thermally grown oxide (TGO), formed on the bond coat (BC) surface of thermal barrier coatings (TBCs), has a significant impact on the TBCs life time. In order to reduce growth kinetics of TGO, vacuum pre-treatment was taken for some bond coats before pre-oxidation and isothermal oxidation in air. The other bond coats were directly subjected to pre-oxidation and isothermal oxidation. The grain size and grain boundary distribution of the TGO were analyzed by electron backscattered diffraction (EBSD). Element diffusion was taken into account to analyze growth kinetics of the TGO. Results indicate that the change of Al 2 O 3 TGO thickness with isothermal oxidation duration doesn’t fit well to conventional oxidation theory, i.e. exponential function. It is attributed to the change of the grain structure during the pre-treatment. This mechanism would benefit the development of long-life and highly reliable TBCs.

Pre-diffusion treatment (high temperature vacuum treatment) plays an important role in the growth of the thermally grown oxide (TGO) on the low pressure plasma spray (LPPS) bond coat surface. Results show that the initial thin oxide scale, formed during deposition process, on the as-sprayed bond coating surface has broken and shrunk to discontinuous oxide particles through the elements diffusion during the pre-diffusion. Two kinds of pre-diffused bond coats deposited in different oxygen partial pressure atmosphere show different results of the average distance between the individual oxide particles on the pre-diffused bond coating surface. In this study, a three dimensional model with thermal grooving theory was developed to explore the essential condition for the scale breaking and explain the differences of these results. This research can provide reference for the preparation optimization and pre-treatment optimization of bond coat towards high performance TBCs.