Fracture toughness behavior, stress–strain behavior, and flaw resistance of pressureless‐sintered Si3N4‐BN ceramics are investigated. The results are discussed with respect to the reported thermal shock behavior of these composites. Although the materials behave linear‐elastic and exhibit noR‐curve behavior, their flaw resistance is different from that of other linear‐elastic materials. Whereas the critical thermal shock temperature difference (ΔTc) is enhanced by adding BN, the content of BN has no influence on the strength loss during severe th