Residual stress and microstructural evolution in environmental barrier coatings of tantalum oxide alloyed with aluminum oxide and lanthanum oxide

Abstract

Pure Ta_2O_5 has been investigated as a possible environmental barrier coating (EBC) material for silicon nitride-based ceramics, but was found to be inadequate because of localized buckling and the formation of microcracks. To address these issues, alloying additions were explored to improve some of the properties of Ta_2O_5 for this application. Aluminum oxide (Al_2O_3) was investigated to stabilize the low-temperature β-Ta_2O_5 phase and reduce grain growth in the coating through the formation of a solid solution. Lanthanum oxide (La_2O_3) was investigated as a second phase former designed to pin grain-boundaries and reduce grain growth. High-brilliance X-rays at the Advanced Photon Source at Argonne National Laboratory were used to evaluate residual stress and phase evolution in the alloyed coatings. Scanning electron microscopy was used to study microstructural changes and grain growth. Al_2O_3 was successful at stabilizing β-Ta_2O_5 and the Al_2O_3 and La_2O_3 significantly reduced grain growth in the coatings. Significant microcracking was observed in the coatings alloyed with Al_2O_3 after thermal cycling in air. Coatings alloyed with Al_2O_3 and La_2O_3 showed no evidence of microcracking. However, some of the alloyed coatings which were first thermally cycled in air and subsequently exposed to high-temperature steam completely delaminated from the AS800 silicon nitride substrates, a highly undesirable result for an EBC.

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