Development of Composition Gradients in Cemented Carbides

Doktorsavhandling, 2001

This thesis concerns the formation of tough surface zones, depleted of cubic carbo-nitrides, in cemented carbides. Several materials with varied phase compositions, binder phase volume fraction, and carbon content have been investigated. The main experimental techniques that have been used are scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and atom probe field ion microscopy (APFIM). Some of the experimental results have been compared to computer simulations based on thermodynamics and diffusion. Good agreement between computer simulations and experimental results of the distribution of elements and phases was found. It was therefore concluded that the formation of surface zones is a process controlled by thermodynamics and diffusion. It was investigated how the gradient surface zone formation depends on cubic phase composition. Addition of Ta to the cubic phase was found to decrease the elemental variations in the gradient profile and to give a thinner surface zone. Addition of Nb gave the opposite effect, with larger elemental variations and a wider surface zone. The effect of the binder phase volume fraction on gradient zone formation was studied. It was found that the zone width is directly proportional to the binder content. It was also found that the diffusivity is reduced with a factor equal to the binder volume fraction due to the presence of dispersed phases, i.e. carbides and carbo-nitrides. Study of materials with varied carbon content showed that with a high C content the driving force for gradient formation becomes stronger, and wider surface zones are formed. The effect of high C content was found to be reduced if Ta or Nb was added to the cubic phase. Methods to prepare APFIM specimens of near surface regions were developed. The methods are based on dimple grinding, electropolishing, and focused ion beam milling (FIB). The methods allow for positioning of APFIM specimens with high accuracy in the material, and to re-sharpen analysed specimens. Analysis was performed of the cubic phase just inside the zone border and the results could be related to the sintering process.