The influence of tungsten carbide contamination from the milling process on PCD materials oxidation

Abstract

The most popular polycrystalline diamond (PCD) is a compact with cobalt bonding phase. The thermal resistance of this material is rather low, up to 700°C. The best way to obtain a higher thermal resistance for diamond compacts (PCD materials) is to limit cobalt content or to sinter the diamond either without a bonding phase or with a ceramic bonding phase. The second group of commercial PCDs, used mainly in rock drilling, are materials with the Si bonding phase. Because of their residual porosity, these materials are not used for metalworking applications, where low roughness value is required. The main focus of the studies presented was the influence of methods of mixture preparation on the microstructures and selected properties of diamond compacts. The traditional method of preparing diamond powders with a bonding phase is to use WC-Co millers. This method causes tungsten carbide contamination of the material, which in these materials decreases their resistance to oxidation during machining. The second aim of these researches was lowering the level of residual porosity. The presence of nanoparticles between diamond microsize particles limits the residual porosity of diamond compacts and improves the roughness properties of cutting tools, but the addition of nanopowders requires intensive milling for good distribution of the ingredients, which increases the WC-Co contamination. In these studies, mixtures containing 90wt.% diamond with 10wt.% nanometric TiB2 powders were prepared with the use of the milling method and the ultrasonic method. The resulting mixtures were formed into discs (15mm in diameter) by pressing in a steel matrix under the pressure of 100MPa. The samples were heated using an assembly equipped with an internal graphite heater. The compacts were sintered at the pressure of 8.0±0.2GPa and the temperature of about 2000°C in a Bridgman-type ultra high pressure apparatus. The density of the materials was measured. Hardness measurements were carried out with a Vickers apparatus at 9.8N load. Young's modulus was measured using the ultrasonic method. Phase composition of the diamond compacts was identified by X-ray diffraction analysis. The method of the mixtures preparation has a strong influence on the microstructure and the properties of diamond compacts. The preparation of mixtures in a PULVERISETTE 6 Planetary mill with WC grinding balls results in tungsten contamination, and thus in the presence of WC, WB and W2C0,84 in the compacts. The ultrasonic method of mixing allows to receive diamond materials with nanometric bonding phases without tungsten contamination.