Short‐Crack Fracture Toughness of Silicon Carbide

Abstract

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√m) 88% higher than the one that showed primarily a transgranular fracture (3.4 MPa√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 δ that characterizes the residual driving force of the plastic zones based on the stable growth of the indentation cracks from the initial (c0) 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.