Study Of Properties And Mechanism Of Tungsten Copper Composite Powders Prepared By Nitridation And Denitridation

The traditional preparation method for tungsten-copper composites is hydrogen reduction process combine with liquid-phase sintering. The disadvantages of these methods are abnormal grain growth of tungsten particles and grain coarsening, which are caused by volatilization of tungsten oxides during the hydrogen reduction process. Aiming at the question, we adopt the nitridation-denitridation process to prepare tungsten copper ultra or nano-composite powders. The mechanism of the process was discussed, and the effects of copper content on the properties of composite powders and alloys were studied. The main conclusions are as follows:(1) Nitridation and denitridation process can avoid abnormal grain growth of tungsten particles and grain coarsening, which are caused by volatilization of tungsten oxides during the hydrogen reduction process, effectively. During the nitridation process, the interface between W and Cu is semicoherent interface, which promotes the nucleation of W2 N phase. Meanwhile, the low interface energy reduces the Gibbs-Thomson effect which hinders the coarsening of particles. During the denitridation process, the interface between new and parent phase is semicoherent interface and free surface of Cu provides nucleation site for W. Both of that can reduce the nucleation resistance and lead to nucleation easier. The optimum nitridation temperature and nitrogen temperature are 650 oC and 875 oC, respectively, and the spherical tungsten copper composite powders with the mean particle size of 90.19 nm are obtained.(2) The relative density and hardness of W-15 Cu alloy prepared by nitridation-denitridation process can reach 98.2% and 258.7 HV, respectively. While the relative density and hardness of W-15 Cu alloy obtained by hydrogen reduction process are only 94.5% and 229 HV,respectively.(3) With rising sintering temperature, the densification degree increases first and then decreases. The hardness, electrical conductivity and thermal conductivity show the similar change trend because they have a close relationship with the densification degree.(4) With increasing the content, copper particles promote the heterogeneous nucleation of W particles, and the particle size of W-Cu composite powders decreases gradually. When the content is 40 wt.%, the heterogeneous nucleation effect weakens. The density and hardness of tungsten copper alloy increase first and then decrease, while the electrical conductivity and thermal conductivity increase gradually.(5) Properties of alloys have a close relationship with the content and characteristic of copper. With rising copper content, filling ability is rising and densification degree is increasing. However, when the content is 40%, the density decreases due to the exudation of liquid phase. With rising copper content, the hardness increases first and then decreases. Because of the hardness decreases caused by Cu content more than that increases caused by the densification degree. With rising copper content, the electrical conductivity and thermal conductivity increases because the movement and scattering of electrons and obstacle to heat flow by pores weak.