Gradient cemented carbide with beta-phase free surface layer formed by nitrogen source AlN

摘要

Using AlN as nitrogen source, the gradient cemented carbide with beta-phase free surface layer was fabricated in situ by one-step vacuum sintering. The beta-phase free layer was explored by phase characterization, elemental-distribution analysis, microstructure and fracture observation. The results indicated that it was feasible to obtain beta-phase free layer when utilizing AlN as the nitrogen source. The AlN decomposition fell appreciably to lower temperature in the presence of binder Co and Ni under vacuum sintering. The thickness of beta-phase free layer could be tailored by controlling AlN contents, and the nitride former Al remained in the 13-phase free layer. When applying nitrogen source AlN, the relative Ti content in the subsurface layer was lower than that with the conventional nitrogen source Ti(C,N) or TiN. By contrast, Co enrichment in the beta-phase free layer is less significant than Ni as a result of higher solidification temperature. WC phases were much coarser averagely in the beta-phase free layer than in the bulk, which was considered to be favorable for resisting against fracture. The beta-phase free layer containing Al played a substantially improved role on the transverse rupture strength when AlN addition was 0.6 wt% and 1.2 wt%, while it is detrimental to the transverse rupture strength due to the formation of the intermetallic phase of Al and binder when AlN addition was 1.8 wt%.