Gradient cemented carbide with a β-phase-free surface layer induced by a ZrN nitrogen source

Abstract

Using ZrN as a nitrogen source, a gradient cemented carbide with a β-phase-free surface layer (βFL) was fabricated in situ by a one-step vacuum sintering process. The gradient cemented carbide was explored by microstructure observation, phase-distribution analysis and mechanical property characterization. The results indicated that it was feasible to obtain a βFL when utilizing ZrN as the nitrogen source. ZrN decomposition fell appreciably to lower temperatures in the presence of a carbon source during vacuum sintering. The βFL thickness could be tailored by controlling the ZrN content, and the layer became thinner with increasing ZrN due to the inward Zr diffusion. The WC phase was found to be more prevalent in the βFL than in the bulk as a result of β-phase depletion. With ZrN increased, the lattice parameters of the β-phase in the bulk were increased, and the β-phase gathered together to form a strip when 0.8 wt% or 1 wt% ZrN was introduced, making the microstructure of the bulk less homogeneous. When more ZrN was added, the transverse rupture strength of the gradient cemented carbide declined owing to the thinner βFL, decreased wetting and clustering of the β-phase in the bulk.