Chemical transformations of hardmetal (WC-Co) nanopowders

Abstract

The behaviour during heating of WC-6Co and WC-15Co hardmetal powders, of 30 nm average grain size (agglomerated to micrometre size particles), was investigated at temperatures from 600 to 1300°C. Powder samples were annealed in the furnace of a hot stage microscope in a N₂ atmosphere containing 20 ppm O₂. Melting of some nanoparticles has previously been observed below the equilibrium melting point of the WC-Co eutectic. Under reducing conditions (N₂-H₂ and N₂-CO) this phenomenon was not observed. The aim of the present work was to investigate the reactions between the component phases of the powder and between the powder and atmosphere during heating at 20 K min^-1. The reaction products were analysed by high temperature X-ray diffractometry and some annealing processes were videorecorded continuously. Compositional variation was also followed by chemical analysis of W, Co and C. Initially, the WC and Co phases disappear, and a transitional phase, CoWO₄, is observed. The upper temperature limits at which these phases and reactions occur are slightly influenced by powder Co content. At 1000°C the character of the reactions changes. The oxidation rate of carbon increases and a number of new phases (substoichiometric tungsten carbides, tungsten oxides and various intermetallic compounds) are formed, most of which persist to 1300°C.