AbstractFracture toughness K 1C and subcritical crack extension were measured as functions of temperature, loading rate, environment (air, vacuum 2.10−5 mbar) and microstructure. The materials were hot-pressed SiC with a brittle B4C phase and Si infiltrated SiC, denoted SiC(HP) and SiC(IF) respectively. K 1C and the subcritical crack extension parameter n were determined in a three-point-bend test. Subcritical crack extension does not playa dominant role before fast fracture takes place. Consequently, fracture toughness K 1C could be determined as functions of loading rate and environment without any correction of the crack length. K 1C of SiC(IF) is increased at low loading rates due to a viscous reaction of the free silicon phase Si at higher temperatures. For materials with a brittle second phase, SiC (HP), loading-rate effects could not be observed in the measured temperature range. Also K 1C is nearly independent of environment and temperature.