High temperature oxidation resistance and thermal growth oxides formation and growth mechanism of double-layer thermal barrier coatings

Abstract

In order to reveal the formation and growth mechanism of thermal growth oxides (TGO) in double-layer thermal barrier coatings (TBCs) during high temperature oxidation, and to further clarify the mechanism of lanthanum zirconate (LZ) as the outermost ceramic coating to improve the high temperature oxidation resistance of double-layer TBCs, two kinds of TBCs were prepared by air plasma spraying, whose ceramic coating were single-layer YSZ and double-layer LZ/8YSZ, respectively. Subsequently, the two TBCs were oxidized at 1100 °C, whereupon the microstructure and growth behavior of TGO during high temperature oxidation were studied comparatively. The results show that the weight gain of the two TBCs were faster in the early stage of oxidation, and obviously slowed down when the oxidation time exceeded 100 h. But for the whole process of high temperature oxidation, the oxidation weight gain of double-layer TBCs was obviously slower than that of single-layer TBCs. In the process of high temperature oxidation, TGO growth in double-layer TBCs was slower. After oxidation for the same time, not only the morphology of TGO in double-layer TBCs was thinner and more compact, but also the formation time of holes was delayed than in single-layer TBCs, besides, no obvious cracks appeared. With oxidation time extending, Al2O3, spinel oxides and complex compounds formed in TGO successively. Spinel oxides and complex compounds were collectively called SC, which was critical for cracks nucleation and propagation during high temperature oxidation of TBCs. Due to better ability to isolate external oxygen of LZ ceramic coating, the diffusion kinetics of elements in bonding coating of double-layer TBCs was changed. In the early stage of oxidation, the oxidation of Al3+, Ni3+, Co3+, Cr3+ metal cations was delayed. Accordingly, the formation of SC was postponed. In later stage of oxidation, with the gradual growth and thickening of Al2O3, it is difficult for O2− to diffuse to the interface between TGO and bonding coating through Al2O3 protective barrier, which also prevents the growth of SC. Consequently, the high temperature oxidation resistance of double-layer TBCs is superior to single-layer TBCs.