Nitridation sintering of WC-Ti(C,N)-(Ta,Nb)C-Co hardmetals

Abstract

Gradient sintering ofWC-Ti(C,N)-(Ta,Nb)C-Co hardmetals in vacuumand nitrogen atmospherewas investigated via interrupted sintering experiments and evolving gas analysis. Reduction of surface oxides with corresponding gas evolution was followed by spreading of the binder, shown with coercivity measurements. With low carbon balance, η phases were formed during the intermediate stages of sintering. By introducing nitrogen gas at a later stage all of the η phases could be decomposed again and a graded near-surface microstructure could be formed. If nitrogen is introduced at an early stage when porosity is still open, the TiC powder particles are nitrided, leading to a considerable uptake of nitrogen. Carbon released by this reaction must be balanced by addition ofW metal.Wand Co binder promote the reaction of TiC with N, whereas TaC and NbC do not. The concept of reaction sintering hardmetal by bulk nitridation was applied on two hardmetal grades and was found an effective way of introducing the nitrogen into such hardmetals.

Gradient sintering ofWC-Ti(C,N)-(Ta,Nb)C-Co hardmetals in vacuumand nitrogen atmospherewas investigated via interrupted sintering experiments and evolving gas analysis. Reduction of surface oxides with corresponding gas evolution was followed by spreading of the binder, shown with coercivity measurements. With low carbon balance, η phases were formed during the intermediate stages of sintering. By introducing nitrogen gas at a later stage all of the η phases could be decomposed again and a graded near-surface microstructure could be formed. If nitrogen is introduced at an early stage when porosity is still open, the TiC powder particles are nitrided, leading to a considerable uptake of nitrogen. Carbon released by this reaction must be balanced by addition ofW metal.Wand Co binder promote the reaction of TiC with N, whereas TaC and NbC do not. The concept of reaction sintering hardmetal by bulk nitridation was applied on two hardmetal grades and was found an effective way of introducing the nitrogen into such hardmetals.