Development and characterization of tungsten carbide based composites

The mining industry currently uses tri-cone, or roller cone bits to drill blast holes. Bit inserts used in these drill heads are predominantly manufactured from either cemented carbides or steels. The cemented carbides afford high hardness along with good wear resistance; however they have a low fracture toughness which can result in catastrophic failure. Steels, while exhibiting a much higher fracture toughness, have a low hardness which results in poor wear resistance. A material which could combine high hardness, good wear resistance, and high fracture toughness would represent a leap forward over today’s technology. Research being performed at the University of Missouri Rolla (UMR) has been focused on meeting this need through the development of an engineered architecture designed to deflect cracks. This material has a cellular structure with the cells being composed of a low cobalt content cemented carbide; whereas the cell boundary is composed of a higher cobalt content cemented carbide. These structures are formed through a co-extrusion process using a thermoplastic, fugitive binder system. In addition, 2D-layered samples have also been manufactured through a sheet lamination process in order to study crack propagation behavior. Studies have been performed in order to develop powder/binder compositions not only suitable for processing, but which also allow for easy binder removal. Characterization of these materials will include density, macrostructure, microstructure, hardness, and preliminary mechanical property results --Abstract, page 32. Laminate materials were manufactured by alternating WC-6%Co layers with equal thickness layers of one of the following compositions: WC-16%Co, WC-16%Ni, Co, a W-Ni-Fe alloy, and Ni. One composition used WC-6%Ni in place of WC-6%Co, layered with the W-Ni-Fe alloy. Flexure specimens, consisting of nine alternating layers, were fabricated with either WC-6%Co or WC-6%Ni on the tensile surface of the specimen. The ability of these composites to deflect or arrest cracks was investigated in four point bending. Two of the compositions, WC-6%Co/W-Ni-Fe and WC-6%Co/Ni, exhibited the ability to fail non-catastrophically, showing inelastic work of fracture as high as 34,400 J/m2 --Abstract, page 47. Cemented tungsten carbide composites were manufactured via a coextrusion process to produce a functionally designed cellular (FDC) architecture. The FDC architecture contained 100-150/xm WC-6%Co cells and an interpenetrating cell boundary phase consisting of either a W-Ni-Fe alloy or Ni. Sample billets containing 17.5, 30, and 50% by volume of the cell boundary phase were engineered and pressurelessly sintered. Flexure specimens were fabricated consisting of cell/cell boundary coextruded filaments uniaxially aligned in the long axis of the specimen. Indentation behavior as well as flexural response was investigated in order to determine the ability of these composites to deflect cracks. While samples did not exhibit graceful failure under flexure testing, compositions with 17.5% W-Ni-Fe, 30% W-Ni-Fe, 30%Ni, and 50% Ni displayed the ability to deflect cracks generated by Vickers indents produced using a load of 50 kg --Abstract, page 68