Sintering, characterisation, and analysis of functional gradient hardmetals

Four WC–carbonitride–Co-based hardmetal green bodies of different carbonitride and N content were sintered in vacuum and in nitrogen at two different pressures. Phase formation upon heating was investigated by interrupting the sintering process at various stages and was studied by means of standard metallographic methods. Shrinkage behaviour and magnetic properties of the partly sintered samples were measured as a function of temperature and atmosphere. The surface and the bulk of the samples were analysed by XRD, showing the homogenisation of the cubic hard phases as a function of temperature. During vacuum sintering all samples showed formation of η phases M12C and M6C, especially at the surface. M12C was converted to M6C between 1100 and 1300 °C. By the introduction of N2 the η phases decomposed and the samples ended up with either a carbonitride-depleted or a carbonitride-enriched zone, depending on which sintering cycle was performed.