Crack initiation and microstructure in tungsten carbide-cobalt composites

Tungsten carbide makes up between 75 and 90% of the volume of tungsten carbide-carbide tools used in machining and rock drilling. Some of the abrasive wear conditions in which these tools operate may be simulated in the laboratory by indentation. The carbide is known to deform plastically on (1010) <1213> systems under such conditions. This work examines the consequences of such a combination of slip systems for polycrystalline deformation in sintered tungsten carbide-cobalt. Also studied are microstructural features associated with thin intergranular cobalt films and a chemical reaction that follows the annealing of deformed material. Commercially sintered compositions were deformed by Vickers indentation and by compression with a superimposed constrained pressure. Slices from these samples were thinned and examined in the transmission electron microscope. Two cases of strain discontinuity are considered: those in the plane of, and those normal to the boundary between two grains. It is shown that the form can lead to intergranular cracking from a triple point and transgranular failure. Experimental observations of cracks and uncracked boundaries confirm the validity of such a crystallographic analysis of incompatible deformation. The implications for fracture initiation in other brittle materials are considered