Fracture Strength of Single-Crystal Silicon Carbide Microspecimens at 24 $^{\circ}\hbox{C}$ and 1000 $^{\circ}\hbox{C}$

Three shapes of silicon carbide tensile specimens were tested - curved with a low stress-concentration factor and straight with a circular hole or an elliptical hole. The nominal thickness was 125 mum with a net section that is 100-mum wide; the overall length of these microspecimens was 3.1 mm. They were fabricated by an improved version of deep reactive ion etching, which produced specimens with smooth sidewalls and cross sections having a slightly trapezoidal shape that was exaggerated inside the holes. The novel test setup used a vertical load train extending into a resistance furnace. The specimens had wedge-shaped ends which fit into ceramic grips. The fixed grip was mounted on a ceramic post, and the movable grip was connected to a load cell and actuator outside the furnace with a ceramic-encased nichrome wire. The same arrangement was used for tests at 24degC and at 1000degC. The strengths of the curved specimens for two batches of material (made with slightly different processes) were 0.66plusmn0.12 and 0.45plusmn0.20 GPa, respectively, at 24degC with identical values at 1000degC. The fracture strengths of the circular-hole and elliptical-hole specimens (computed from the stress-concentration factors and measured loads at failure) were approximately 1.2 GPa with slight decreases at the higher temperature. Fractographic examinations showed failures initiating on the surface - primarily at corners. Weibull predictions of fracture strengths for the hole specimens based on the properties of the curved specimens were reasonably effective for the circular holes but not for the elliptical holes.