The influence of sintering in nitrogen gas on the microstructure of a WC-VC-TiC-Co cemented carbide

This paper investigates the effects of nitrogen-sintering on the V-rich cubic phase grain size in a material with large additions of VC and TiC. Nitrogen transport is made the rate determining factor for the retardation of grain coarsening during sintering. A compacted sample comprising TiC, VC, WC and Co powders was sintered in 1 bar nitrogen gas. The results show that the cubic (Ti, V, W)(C, N) formed during sintering separates into two phases, one with high Ti content and the other with high (W, V) content. This phase separation is in agreement with results from thermodynamic modelling. The grain size of the cubic carbonitrides in the nitrogen-sintered material was found to be approximately equal to the size of the cubic carbide grains produced in vacuum-sintered material, showing that nitrogen-sintering does not play a significant role in the reduction of the mean cubic carbonitride grain size. However, the grain size distribution of the V-rich cubic phase is narrowed from (d₉₀ - d₁₀)/d₅₀ = 1.9 (vacuum-sintered) to 0.78 for material sintered in nitrogen gas. The surface of the nitrogen-sintered material was covered by a Ti-V-rich cubic carbonitride phase. The formation of this surface layer is attributed to the higher N-activity in the sintering furnace compared to the bulk sample and the low solubility of N in the liquid binder.