Thermodynamic Analysis of Sintering Processes for the Production of Cemented Carbides

Abstract

A thermodynamic data set for the 11 component system C–Co–O–Ti–W–Y–Zr–Ca–Si–Al–S has been assembled to study the reactions and equilibria occurring during the sintering of Co–Ti–W–C cemented carbides on yttria stabilised zirconia (YSZ) in the presence of calcium alumino silicate and calcium sulphide impurities. Results are presented for the following equilibria at 1450°C: YSZ + graphite + (Co,W,Ti,C); CaO–SiO₂–Al₂O₃ + graphite + CaS; and YSZ+(Co,W,Ti,C)+CaO–SiO₂–Al₂O₃+CaS+ graphite. In the absence of impurities, CO(g) and Co(g) are the only gas species present with pressures greater than 1 × 10⁻⁶ bar. In the presence of impurities a liquid oxide phase can be formed, and appreciable pressures (∼3 × 10⁻³ bar) of SiO(g) and SiS(g) occur. These could form deposits on the cemented carbides on cooling, leading to possible contamination with Si and S. PM/0588