Crack propagation characteristics and fracture surface energy of boride cermets

The present study was carried out to clarify the relationship between the microstructures and mechanical properties of Mo₂NiB₂-Zvol%Ni (Z = 0, 12, 18, 24 and 50). Fracture surface energy, γ_i, for crack initiation and work of fracture surface energy, γ_wof, for crack propagation were selected as suitable parameters for evaluating the mechanical properties. γ_i and γ_wof were measured by the Chevron notch three-point bend methods. Both γ_i and γ_wof increased with the increase in amount of metal phase in cermets, and γ_wof were about two or three times higher than γ_i for all specimens. Both γ_i and γ_wof did not increase proportionally to the volume fraction of cermets. In order to clarify that microstructure affected crack propagation, crack propagation patterns were classified to four patterns; transgranular fracture in ceramics (c), intergranular fracture in ceramics (c/c), transgranular fracture in metal (m) and intergranular fracture between metal and ceramics (m/c). The intergranular fracture (c/c) ratio to total crack propagation patterns was large in 0, 12, 18 and 24 vol% cermets, and intergranular fracture (m/c) ratio was large in 50 vol% cermet. Increase in the volume fraction of metal was rearranged as the frequency of cracks to touch metal phase; m + 0.5 (m/c). Linear relationship was obtained between m + 0.5 (m/c) and fracture surface energies. The result of fracture surface analysis showed that the large increase in γ_wof compared with γ_i with the increase in the volume fraction of metal resulted from the difference of the roughness of fracture surfaces. The fracture surfaces were rougher for the specimens with larger amount of metal phase. γ_wof was estimated to be about equal to γ_i when correction was carried out based on the roughness value. It was concluded that fracture surface energies γ_i and γ_wof increased proportionally to the fraction of the metal phase which was contiguous to crack.