Kinetics of isothermal oxidation of WC-20Co hot-pressed compacts in air

The present paper presents an isothermal analysis of the oxidation behavior in which hot-pressed compacts rather than powders are used over the temperature range 700-850 A degrees C. This was done to better simulate the extent of oxidation occurring on use. WC-Co powders were first subjected to non-isothermal kinetic analysis to follow the oxidation mechanism. In the isothermal runs, a thermobalance was used to follow up the mass with time at different constant temperatures. The diameter of compacts was measured as function of time at these temperatures, and a simple model was proposed to relate the diameters to extent of oxidation. Two reactions were found to take place that are controlled by chemical reaction at interface: Oxidation of cobalt and oxidation of WC with the formation of WO3 and CoWO4. The activation energies for the two steps of oxidation were calculated and found to equal 157 kJ mol(-1) and 205 kJ mol(-1), respectively. These values are in reasonable agreement with published data for WC-Co powders.