Microstructure and phase composition of hard alloys produced from nanocrystalline powder mixture WC-6wt.%Co with C, Al and ZrC additives

A large specific surface area of WC nanopowder determines its high chemical activity and makes it very sensitive to various impurities, among which oxygen is most harmful and unavoidable. During heating, oxygen interacts with carbon of WC being removed in the form of CO/CO2, which finally leads to the appearance of embrittling η-phases in the hard alloy, abnormal growth of WC grains, and formation of a porous microstructure. To prevent heavy decarburization of WC during vacuum sintering of hard alloy from a nanocrystalline powder mixture WC-6wt.%Co, in this work we compared three methods: addition of extra carbon to compensate for carbon loss as a result of decarburization; addition of Al to bind impurity oxygen into Al2O3 before it interacts with carbon of WC; and addition of ZrC to compensate for carbon loss and bind impurity oxygen into ZrO2. Nanocrystalline powder mixtures based on WC-6 wt.%Co with and without additions of C, Al, and ZrC were prepared from microcrystalline powders of WC, Co, Al, ZrC, and carbon black by high-energy milling, then they were compacted in a cylindrical mold by uniaxial pressing at a pressure of ~460 MPa and sintered in graphite crucibles for 15 min at 1380 °C in vacuum of ~10-2 Pa. The heating rate to the temperature of sintering was 10 °C/min. The initial powders, powder mixtures prepared therefrom, and sintered hard alloys were certified using X-ray diffraction, chemical analysis, scanning electron microscopy, BET adsorption method, helium pycnometry, and Vickers method. The studies performed showed that the average particle size in all the prepared powder mixtures does not exceed 100 nm, and the content of impurity oxygen in them varies from 3.3 to 4.3 wt.% depending on the additives. It was established that only a part of oxygen contained in the powder mixtures is in the chemisorbed state and takes part in the decarburization of WC during vacuum sintering. The Al additive is completely oxidized during milling of the powder mixture and transforms into nanocrystalline Al2O3, which only aggravates carbon loss during sintering and results in the formation of a multiphase and relatively porous microstructure of the hard alloy. On the contrary, using carbon and ZrC additives we managed to prevent the decarburization of WC during sintering of the hard alloy and to form a less porous microstructure in it. It was shown that the presence of ZrO2 inclusions does not impede intensive growth of WC grains during sintering, but rather promotes it. Carbon deficit slightly suppresses intensive WC grain growth during sintering of hard alloy leading to the formation of η-phases and to an increase in the density and microhardness, but the presence of oxide inclusions Al2O3 and ZrO2 in the microstructure reduces the values of these properties.