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Residual Stress and Microstructural Evolution in Environmental Barrier Coatings of Tantalum Oxide Alloyed with Aluminum Oxide and Lanthanum Oxide
Author(s) -
Weyant C. M.,
Faber K. T.,
Almer J. D.,
Guiheen J. V.
Publication year - 2006
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2005.00830.x
Subject(s) - materials science , oxide , nitride , lanthanum , metallurgy , residual stress , thermal barrier coating , grain boundary , grain growth , coating , tantalum , silicon nitride , lanthanum oxide , ceramic , temperature cycling , silicon , grain size , microstructure , composite material , layer (electronics) , inorganic chemistry , thermal , chemistry , physics , meteorology
Pure Ta 2 O 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 2 O 5 for this application. Aluminum oxide (Al 2 O 3 ) was investigated to stabilize the low‐temperature β‐Ta 2 O 5 phase and reduce grain growth in the coating through the formation of a solid solution. Lanthanum oxide (La 2 O 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 2 O 3 was successful at stabilizing β‐Ta 2 O 5 and the Al 2 O 3 and La 2 O 3 significantly reduced grain growth in the coatings. Significant microcracking was observed in the coatings alloyed with Al 2 O 3 after thermal cycling in air. Coatings alloyed with Al 2 O 3 and La 2 O 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.