HIGH-TEMPERATURE TENSILE BEHAVIOR OF A BORON NITRIDE-COATED SILICON CARBIDE-FIBER GLASS-CERAMIC COMPOSITE

Tensile properties of a cross-ply glass-ceramic composite were investi gated by conducting fracture, creep, and fatigue experiments at both r oom temperature and high temperatures in air. The composite consisted of a barium magnesium aluminosilicate (BMAS) glass-ceramic matrix rein forced with SiC fibers with a SiC/BN coating, The material exhibited r etention of most tensile properties up to 1200 degrees C, Monotonic te nsile fracture tests produced ultimate strengths of 230-300 MPa with f ailure strains of similar to 1%, and no degradation in ultimate streng th was observed at 1100 degrees and 1200 degrees C. In creep experimen ts at 1100 degrees C, nominal steady-state creep rates in the 10(-9) s (-1) range were established after a period of transient creep. Tensile stress rupture experiments at 1100 degrees and 1200 degrees C lasted longer than one year at stress levels above the corresponding proporti onal limit stresses for those temperatures. Tensile fatigue experiment s were conducted in which the maximum applied stress was slightly grea ter than the proportional limit stress of the matrix, and, in these ex periments, the composite survived 10(5) cycles without fracture at tem peratures up to 1200 degrees C. Microscopic damage mechanisms were inv estigated by TEM, and microstructural observations of tested samples w ere correlated with the mechanical response, The SiC/BN fiber coatings effectively inhibited diffusion and reaction at the interface during high-temperature testing, The BN layer also provided a weak interfacia l bond that resulted in damage-tolerant fracture behavior, However, ox idation of near-surface SiC fibers occurred during prolonged exposure at high temperatures, and limited oxidation at fiber interfaces was ob served when samples were dynamically loaded above the proportional lim it stress, creating micro-cracks along which oxygen could diffuse into the interior of the composite.