Fatigue of a SiC/SiC ceramic composite with an ytterbium‐disilicate environmental barrier coating at elevated temperature

Tension‐tension fatigue performance of a SiC/SiC composite with an ytterbium‐disilicate environmental barrier coating (EBC) was investigated at 1200°C in air and steam. The composite is reinforced with Hi‐Nicalon™ SiC fibers and has a melt‐infiltrated matrix processed by chemical vapor infiltration of SiC with subsequent infiltration with SiC particulate slurry and molten silicon. The EBC consists of a Si bond coat and an Yb2Si2O7 top coat applied via air plasma spraying. Tensile properties were evaluated at 1200°C. Tension‐tension fatigue was examined for maximum stresses of 110‐140 MPa. To assess the efficacy of EBC, experimental results obtained for the coated composite are compared to those for a control uncoated composite. 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 steam. Fatigue runout set to 200 000 cycles (55.6 hours at a frequency of 1.0 Hz) was achieved at 110 MPa in air and steam. Retained properties of pre‐fatigued specimens were characterized. Composite microstructure, along with damage and failure mechanisms were investigated. Damage and failure of the composite are attributed to the growth of cracks originating from numerous processing defects in the composite interior.