A melt-infiltrated (MI) woven ceramic matrix composite consisting of a silicon carbide matrix reinforced by boron nitride coated Hi-Nicalon type STM SiC fiber, Hi-Nic-S/BN/SiC, was tested under tension-tension fatigue loading in combination with combustion conditions representative of those experienced by hot-section components such as turbine blades and vanes in modern gas turbine engines. The burner rig fatigue data and fracture surfaces were analyzed for the effects of oxidation on life, failure, and damage mechanisms. These test results were then compared with those obtained from similar fatigue tests performed in a standard furnace under laboratory air environment. Fatigue life in the combustion condition was lower by an order of magnitude in comparison to the isothermal furnace results across the range of applied stress, and so demonstrates the importance of representative combined environment testing in conjunction with fundamental load testing. The observed difference in fatigue performance is attributed to the thermal gradient stress and increased rate of oxidation due to a high moisture level in the combustion rig test condition. The former was verified using finite element analysis and the latter from microscopic analysis of the fracture surfaces.

The fatigue behavior of Hi-Nicalon Type-St/BN/SiC ceramic matrix composites (CMCs) was investigated in a combustion environment. Two versions of this CMC were tested. The first version was manufactured by a slurry casting process using woven fiber architecture and the matrix was densified by melt-infiltration (MI) of silicon. The other was a cross-ply laminate prepared by a traditional lamination process from unidirectional prepreg followed by the MI processing. Several tests were conducted at a frequency of 1 Hz and a stress ratio of 0.05 to develop the fatigue life diagrams for both material systems. The Prepreg MI CMC exhibited higher fatigue strength, especially for cycles-to-failure 410,000, in comparison with the slurry cast MI CMC. Micrographic analysis was conducted to document the oxidation and embrittlement, which was observed to be dependent upon the applied stress level and cycles-to-failure. Overall, the prepreg MI CMC was less prone to oxidation and had better fatigue properties, demonstrating the potential for long-term use as a structural high-temperature material for high-temperature oxidative service environments, such as those found in gas turbine engines.

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