C/N ratios in TiCN-TiC-WC-Cr3C2-Ni cermets can be intentionally modified by changing the composition of the presintering atmosphere. This affects not only the sinterability but also the final microstructure of these materials. Melting occurs at lower temperatures as the nitrogen content of the presintering atmosphere rises. However, the final densities are lower due to the precipitation of free carbon. Grain growth of the carbonitride phase is accelerated in samples presintered in N2 due to the activation of solution reprecipitation kinetics. Finally, the precipitation of the Cr-rich M7C3 carbide can also be controlled by the appropriate selection of the presintering atmosphere up to 7.8 wt.% Cr contents.

Thermal shock damage has been investigated in a WC-6 wt.%Co hardmetal and in a TiMoCN-26 wt.%Ni cermet by measuring their loss of bending strength after repeated quenching in water. The behavior of the hardmetal is significantly better than that of the cermet, showing a less abrupt loss of strength as quenching temperature step increases. Experimental values are in good agreement with R parameter predictions, especially for the cermet. Biot numbers are low enough to neglect the effect of transient heat transfer phenomena. It is concluded that a low thermal expansion coefficient combined with high strength and toughness are critical parameters to ensure an optimum thermal shock resistance.

Microbeam testing is proposed as a new method for analysing the mechanical properties of individual microstructural features in WC-Co hardmetals; i.e. portions of WC grains or a single metallic ligament. Firstly, cantilever microbeams with dimensions below the microstructural scale of the material are machined by means of a focused ion beam (FIB). Afterwards, these beams are bended to fracture by means of an instrumented nanoindenter. In this way, both portions of WC grains and binder phase ligaments are broken while simultaneously recording the load and the vertical displacement of the nanoindenter tip. These cracking events are detected as sudden steps in the load vs. displacement curves. Afterwards, a scanning electron microscope is used to measure the distance from the main crack to the beam clamping. From these data, the stresses at which portions of cobalt ligaments and WC grains fail are estimated from linear elastic theory and FEM models.

In TiCN-W-Cr-Ni cermets produced by liquid phase sintering melting occurs at lower temperatures as their Cr content increases. For low Cr additions (up to 4 wt.%) eutectic temperatures are close to those found in the TiC-WC-Ni system. For 8 wt.% Cr and above, temperatures are similar to those found in the Cr-Ni-C system. The precipitation of M7C3 carbides is observed to start at 8 wt.% Cr in samples sintered at 1425 degrees C for 1 h. This sets a limit for the Cr solubility in the binder phase of these cermets around 18 wt.%. The dissolution of WC and Cr3C2 particles starts at temperatures as low as 1150 degrees C, but that the homogenization of the binder phase is only achieved after melting. The carbonitride phase exhibits the typical precipitation of inner and outer rims onto Ti(C,N) cores. However, a fine precipitation of Ni-rich particles is found inside Ti(C,N) cores, likely related to coalescence phenomena.

Thermogravimetric experiments confirm that the oxidation resistance of WC-Co and WC-Ni-Co-Cr alloys increases with their metallic content. This is due to the fact that, as the metallic content increases, the oxide scales present higher MWO(4) to WO(3) ratios and lower porosity. The good correlation found between the activation energies calculated by either the isothermal or the isoconversion method suggests that oxidation is controlled by surface chemical reaction. Activation energies increase with temperature between 650 and 800 degrees C for both WC-Co and WC-Ni-Co-Cr alloys. This increment is higher for WC-Co materials due to their tendency to form scales with higher tungstate contents.

Microstructural evolution of (Ti,Mo)(C,N)-Ni cermets consolidated by hot isostatic pressing (HIP) has been analysed. HIP processing allows full densification of these materials at lower temperatures than those normally employed in vacuum sintering cycles (VS). Solution and reprecipitation phenomena are limited and a nanometric fraction of (Ti,Mo)(C,N) grains is retained in the microstructure leading to a significant increase in hardness with respect to vacuum sintered materials (from 11 to 14 GPa). HIP-ed cermets show more tendency to uncombined carbon precipitation than those obtained by VS. In studied systems, carbon precipitation can be related either to an excess of carbon in the initial mixture of powders or to destabilization of carbonitride phase during HIP. Control of the C/N ratio has been carried out by the adequate selection of powder mixtures and the design of the thermal treatments. It has been proved that free carbon in these cermets can be avoided including presintering cycles under hydrogen before encapsulation.

Fully dense WC-Ni-Co-Cr alloys have been consolidated via sinter HIP processing. Dilatometric tests show that shrinkage undergoes several accelerations and decelerations during heating, a phenomenon likely associated to the heterogeneous distribution of Cr in the binder phase. WC grain growth follows trends similar to those described for WC-Co hardmetals, increasing with the C activity and the amount of liquid phase of the cermets. Finally, the oxidation resistance of WC-Ni-Co-Cr cemented carbides is observed to improve as the metal content increases and the C content decreases. In both cases, the oxide layers present a higher proportion of (Co, Ni)WO4 tungstates. The oxide scales formed on compositions with low metal content contain a higher amount of WO3 oxide.