Thermal Treatment of c-BN compounds

Cubic boron nitride (c-BN) possesses excellent hardness, being the second hardest material after diamond. Thanks to its high abrasion resistance with ferrous alloys and oxidation resistance, c-BN is used for machining elements where diamond is weak. When sintering c-BN powder with other binders, some swelling and shrinkage occur at the pre-sintering step in the production of grinding and cutting tools at the company Element Six (E6), but the complicated physicochemical reactions taking place are not fully understood. Its study is then of great interest from the technological point of view, as the production of such tools could be improved.Two different c-BN powders provided by E6 were studied in the current project. Heat treatments of both batches (called B and C) were carried out by means of a dilatometer at different temperatures up to 1100 ºC at different heating rates, in two different furnaces, graphite and alumina furnace, and in argon atmosphere and vacuum in order to study the influence of the heating rate, temperature, furnace and atmosphere on both batches. Dilatometry was first used to determine length changes and temperatures where reactions occur, and afterwards X-ray diffractometry (XRD) was used to find out the compounds formed at different temperatures.On the one hand, the study performed on batch B, initially containing c-BN, tungsten (W), tungsten carbide (WC), cobalt (Co), aluminium (Al) and organic additives, revealed that ditungsten carbide (W2C), cobalt tungsten boride (CoWB), ditungsten boride (W2B) and dicobalt boride (Co2B) are formed when heated between 800 ºC and 1100 ºC, thus indicating that c-BN reacts with the hard compounds. On the other hand, the study carried out on batch C, initially containing c-BN, titanium nitride (TiN), titanium carbide (TiC), Al, another unidentified compound, and organic additives, showed the formation of new compounds at temperatures between 800 ºC and 1100 ºC. Such compounds are: aluminium nitride (AlN) and titanium boride (TiB2). Moreover, some unidentified compounds present in the raw powder react at high temperatures and therefore no traces of them are found at 1100 ºC.The study also showed that burnout and pyrolysis of the organic binders present in both batches occur between 250 ºC and 450 ºC. Thus, carbonaceous materials are left in the sample and they further react with the other compounds.Further investigation is needed in order to confirm the reactions suggested in the current project.Validerat; 20110713 (anonymous