Microstructure of model cermets with high Mo or W content

The microstructure of (mol%) TiC-18TiN-24Ni-(10-29)WC and TiC-18TiN-24Ni-(5-14)Mo2C has been investigated using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and analytical electron microscopy (AEM). When the WC content in the raw materials was increased the W content in the outer rim of (Ti, W)(C, N) grains increased until it had a composition similar to that of the inner rim. If the WC content was high undissolved WC was present after sintering. When the Mo2C content in the raw materials was increased, the volume fraction of inner rim increased and the Mo content in both inner and outer rim increased. Thermodynamical calculations on the Ti-W-C-N system suggest that the inner rim is formed during solid state sintering when there is an open porosity and thus a low nitrogen activity. The composition of the outer rim can be explained by the equilibrium at the sintering temperature if the volume fraction of undissolved Ti(C, N) cores is subtracted. Calculations on the Ti-Mo-C-N system show that (Ti, Mo)(C, N) decomposes into two phases with different Mo content and that the Ti(C, N) cores might be regarded as a stable phase. (C) 2000 Elsevier Science Ltd. All rights reserved.