Fatigue of an Advanced SiC/SiC Ceramic Matrix Composite With an Environmental Barrier Coating at Elevated Temperature in Air and in Steam

Abstract

Uniaxial tension-tension fatigue performance of a SiC/SiC composite with an ytterbium-disilicate environmental barrier coating (EBC) was investigated at 1200°C in laboratory air and in steam. The composite is reinforced with Hi-Nicalon™ SiC fibers and has a melt-infiltrated matrix processed by chemical vapor infiltration of SiC followed by infiltration with SiC particulate slurry and molten Si. The EBC consists of a Si bond coat and an ytterbium disilicate (Yb2Si2O7) top coat applied via air plasma spraying (APS). Basic tensile properties of the EBC/SiC/SiC composite were evaluated at 1200°C. Tension-tension fatigue was examined for maximum stresses of 110–140 MPa in air and in steam. To assess the efficacy of the EBC, experimental results obtained for the coated composite are compared to those obtained for a control composite without the EBC. Surface grit-blasting inherent in the EBC application process degrades tensile strength of the composite. However, the EBC effectively protects the composite from oxidation embrittlement during cyclic loading in air or in steam. Fatigue runout defined as survival of 200,000 cycles (55.6 h at a frequency of 1.0 Hz) was achieved at 110 MPa in air and in steam. The retained properties of pre-fatigued specimens were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated. Damage and failure of the EBC/SiC/SiC composite are attributed to growth of cracks originating from numerous processing defects (voids) present in the composite interior.