Processing of functionally graded tungsten carbide-cobalt-diamond composites

Polycrystalline diamond compacts (PDCs) are widely used as drill bit cutters in rock drilling and as tool bits in machining non-ferrous materials. A typical PDC comprises a thin layer of sintered polycrystalline diamond bonded to a tungsten carbide-cobalt substrate. A well recognized failure mechanism is delamination at the interface between diamond and cemented carbide. High stresses at the diamond/carbide interface, due to thermal expansion and modulus mismatch, are the primary cause of in-service failure under impact loading conditions.; This work was undertaken to develop a tungsten carbide-cobalt-diamond composite, which has a continuously graded interface between the diamond and tungsten carbide. The process developed comprised the following steps: (i) generation of a pore size gradient by electrochemical etching of cobalt from the surface of a partially sintered tungsten carbide-cobalt preform; (ii) chemical vapor infiltration of the porous preform with carbon by catalytic decomposition of a methane/hydrogen mixture, resulting in a graded carbon concentration; and (iii) consolidation of the carbon infiltrated preforms at 8GPa/1500°C to complete densification and to transform the carbon into diamond. Thus, the final product consists of a functionally graded WC-Co-diamond composite, with controlled distribution of the constituent phases.; Tungsten carbide-cobalt powders with mean tungsten carbide particle size of 0.8μm(micro-grain) and 100 nm(nano-grain) were used as starting materials. Processing conditions were adjusted to obtain an optimal distribution of carbon in porous preforms. After high pressure/high temperature consolidation, both micro- and nano-composites showed a diffused interface between inner and outer regions of the fully dense materials.; A micro-composite showed columnar-like tungsten carbide grains and faceted diamond grains in the outer region of the sintered material. The grain size of the diamond in this region was ∼2μm, and the hardness was VHN = 3700 ± 60 kg/mm2. The inner region showed equiaxed tungsten carbide grains, with some grains having a coating of diamond-like carbon. A nano-composite showed equiaxed tungsten carbide grains and faceted diamond grains in the outer region of the sintered material. The grain size for both tungsten carbide and diamond was ∼200nm, and the hardness was VHN = 3186 ± 300 kg/mm2.