An oxide/oxide ceramic fiber‐matrix composite (CMC) has been extensively characterized for high‐temperature aerospace structural applications. This CMC is called GEN‐IV™, and it has a porous and cracked aluminosilicate matrix reinforced by 3M Nextel 610™ alumina fibers woven in a balanced eight harness weave (8HSW). This CMC has been specifically designed without an interphase between the fiber and matrix, and it relies on the porous matrix for flaw tolerance. Stress‐strain response is nearly linear to failure and without a well‐defined proportional limit in tension and compression. In‐plane shear and interlaminar strength increases with increasing temperature. The 1000°C fatigue limit in air at 105 cycles is 160 MPa, and the residual tensile strength of run‐out specimens is not affected by the fatigue loading. The creep‐rupture resistance above 1000°C is relatively poor, but it can be improved with a more‐creep‐resistant fiber.

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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 environmental stability of uncoated and boron nitride‐coated (BN‐coated) Nicalon fiber has been investigated by studying the effect of annealing in air at 1000°C (2 h) on the strength of the fibers. The results imply that uncoated and BN‐coated fibers both degrade in strength, with the BN‐coated fiber suffering a higher strength loss. The degradation is significantly enhanced if the fibers are exposed to salt (NaCl) water prior to the air anneal, if the concentration of salt is >0.5 wt%. The BN‐coated Nicalon fibers also have been studied at 800° and 900°C; the degradation in strength due to salt water exposure is greater at 800°C than that at 900° or 1000°C.