On the mechanism of WC coarsening in WC-Co hardmetals with various carbon contents

Abstract

It is well known that the WC coarsening in WC-Co is suppressed in hardmetals having low carbon contents and consequently high concentrations of tungsten dissolved in the binder. However, the mechanism of this phenomenon is not completely understood yet. Heavily milled ultra-coarse WC powders in mixture with 10 wt.% Co and either W metal or carbon black were employed to examine the WC coarsening of hardmetals with various carbon contents. The rate of WC coarsening is found to be strongly suppressed when decreasing the carbon content and consequently increasing the concentration of W dissolved in the binder. The apparent activation energy of WC coarsening in the alloys containing neither η-phase nor free carbon is found to be similar and vary from 163 to 177 kcal/mol. These values are close to those obtained in the literature for dissolution of WC in liquid Co and WC coarsening, which is evidence that the limiting stage of WC coarsening in this case is related to interfacial reactions between WC and liquid binders. It can be supposed that the slowest reaction determining the process of WC dissolution as a whole is the carbon dissolution from WC grains to the liquid binder. The apparent activation energy of WC coarsening in the hardmetal with the lowest carbon content, the microstructure of which comprises η-phase, is found to be equal to 98 kcal/mol, which is close to the activation energy of carbon self-diffusion in WC (88 kcal/mol). Thus the mechanism of suppression of WC coarsening in WC-Co hardmetals with medium carbon contents when decreasing the carbon content is thought to be related to the carbon dissolution in the liquid binder. The limiting stage of WC coarsening in hardmetals with very low carbon contents corresponding to the formation of η-phase can be the carbon self-diffusion in WC grains towards the interface WC-liquid Co. It cannot be excluded also that the limiting stage of WC coarsening in this case might be related to the processes of W and C re-precipitation onto WC grains from the extremely W-rich and C-poor binder. © 2008 Elsevier Ltd. All rights reserved.