Microstructure development during sintering and heat-treatment of cemented carbides and cermets

WC based cemented carbides and Ti(C, N) based cermets are used in cutting tool applications. They are produced by liquid phase sintering, but much of the microstructure is formed already during solid state sintering, before the eutectic temperature has been reached. Changes in the microstructure during the sintering process have been followed with microscopy and microanalysis, in particular SEM and TEM including energy filtered transmission electron microscopy (EFTEM). It was found that part of the hard phases are dissolved in the solid binder, transported by diffusion and re-precipitated onto undissolved hard grains with a composition given by the equilibrium conditions (“inner rim”). For vacuum sintering of nitrogen containing materials, this means a low nitrogen activity due to the open porosity at this stage. After the liquid has formed, further dissolution and re-precipitation occur, but now the porosity is closed so the “outer rim” is formed with the nitrogen activity of the material. The solid solution of the binder (often by tungsten) is determined primarily by the carbon activity in the liquid phase. During cooling after sintering, tungsten and other metals dissolved in the binder re-precipitate onto hard grains so that depleted zones around these are formed, whereas dissolved carbon and nitrogen have time to leave the binder almost completely. Sintering or post-sintering heat-treatment of nitrogen containing materials in an atmosphere with a lower or higher nitrogen activity than in the material results in the formation of surface zones with a composition different from the bulk (“gradient sintering”). In the former case, a tough zone enriched in binder and depleted of cubic carbides is created, which is beneficial if the material is going to be coated with a wear resistant layer. In the latter case a hard surface zone rich in cubic carbo-nitrides and depleted of WC and binder may be obtained, improving the wear resistance of the material.