Reactively sintered B4C-TiB2 composites: Effects of nanolayer films and secondary phase size on mechanical and fracture properties

High theoretical density ( > 99%) B4C-TiB2 composites containing either well dispersed, fine TiB2 or coarser ag-glomerated TiB2 were processed by spark plasma sintering in argon or reactive nitrogen environments. Two types of nanoscopic grain boundary films during reactive sintering were determined with STEM, EFTEM and EELS, namely amorphous titanium oxynitride and hexagonal boron nitride that provide weak interphase bound-aries as preferential paths for crack propagation resulting in increased fracture toughness up to 9.4 MPa root m for the coarser agglomerated TiB2 composites. Conversely, the fine TiB2 composite is beneficial for increasing the flexural strength up to 460 MPa. The strengthening and toughening mechanisms responsible for the tradeoff in properties were determined with microscopy of the fracture surfaces.