Short-Crack Fracture Toughness of Silicon Carbide

The fracture toughness of four different silicon carbides was measured using single-edge precracked beam (SEPB) and indentation/strength techniques. Two were development grades with similar microstructures and chemistries, and yet exhibited different fracture modes. The grade that exhibited a predominantly intergranular fracture had an SEPB fracture toughness (6.4 MPa root m) 88% higher than the one that showed primarily a transgranular fracture (3.4 MPa root m). The higher fracture toughness was associated with a modest increase in average strength (25%), although there was a significant increase in the Weibull modulus (11-32). Fracture toughness at short crack lengths was assessed by an indentation method that used fracture strengths, crack lengths at fracture, and a new method of estimating the constant delta that characterizes the residual driving force of the plastic zones based on the stable growth of the indentation cracks from the initial (c(0)) to the instability (c(*)) lengths. The results showed a rising crack-growth-resistance behavior for the grade exhibiting intergranular fracture, while the grade showing transgranular fracture had a flat crack-growth resistance. Tests on two commercial grades of silicon carbide showed similar behaviors associated with the respective fracture modes.