Degradation Mechanisms of SiC/BN/SiC after Low Temperature Humidity Exposure

Abstract

The environmental degradation of SiC/BN/SiC CMCs under low temperature water exposure is still an unexplored field. This work shows how the effect of low temperature humid environments can be detrimental for amorphous BN interfaces, leading to a drop in mechanical properties. Furthermore, initial low-temperature humid environments can induce a faster degradation during subsequent thermal exposure. In order to understand how low temperature water exposure affects the CMC and how these changes affect the material response to subsequent exposures, intermediate temperature (800°C) exposures have been studied before and after the low temperature humidity tests. The main challenge of this work consists of understanding how different constituents of the CMC structure (e.g. fibres and interfaces) are degrading and consequently affecting the overall bulk mechanical performance and failure modes of the material. For this, linking the change in morphology and chemistry of the interfaces with the micromechanical properties of the individual constituents has been crucial. STEM-EDS has shown a change in morphology of the BN interface after the low temperature water exposure, which has been linked to a decrease in interfacial shear strength and friction (measured via in-situ fibre push-out tests) of ≈ 90%. STEM-EELS has shown how, after the interfaces have been exposed to the humidity as well as to subsequent intermediate temperatures in the presence of the remaining BN, induces degradation of the SiC fibres which results in a tensile strength reduction of ≈ 25% (measured via in-situ 3-point bending of individual fibres). These results provide a new insight into how low temperature water exposures must be considered in future degradation studies of CMCs. Additionally, we propose a new methodology to fundamentally understand the complex failure modes of CMCs.