Search

Pure Ta2O5 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 Ta2O5 for this application. Aluminum oxide (Al2O3) was investigated to stabilize the low-temperature β-Ta2O5 phase and reduce grain growth in the coating through the formation of a solid solution. Lanthanum oxide (La2O3) 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. Al2O3 was successful at stabilizing β-Ta2O5 and the Al2O3 and La2O3 significantly reduced grain growth in the coatings. Significant microcracking was observed in the coatings alloyed with Al2O3 after thermal cycling in air. Coatings alloyed with Al2O3 and La2O3 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. [ABSTRACT FROM AUTHOR]

Due to its coefficient of thermal expansion (CTE) and phase stability up to 1360°C, tantalum oxide (Ta2O5) was identified and investigated as a candidate environmental barrier coating for silicon nitride-based ceramics. Ta2O5 coatings were plasma sprayed onto AS800, a silicon nitride ceramic from Honeywell International, and subjected to static and cyclic heat treatments up to 1200°C in air. Cross-sections from coated and uncoated substrates were polished and etched to reveal the effect of heat treatments on microstructure and grain size. As-sprayed coatings contained vertical cracks that healed after thermal exposure. Significant grain growth that was observed in the coatings led to microcracking due to the anisotropic CTE of Ta2O5. High-energy X-ray diffraction was used to determine the effect of heat treatment on residual stress and phases. The uncoated substrates were found to have a surface compressive layer before and after thermal cycling. Coating stresses in the as-sprayed state were found to be tensile, but became compressive after heat treatment. The microcracking and buckling that occurred in the heat-treated coatings led to stress relaxation after long heat treatments, but ultimately would be detrimental to the function of the coating as an environmental barrier by affording open pathways for volatile species to reach the underlying ceramic. [ABSTRACT FROM AUTHOR]