Thermodynamic Calculation of HfB2 Volatility Diagram

The thermodynamics of the oxidation of HfB2 at temperatures of 1000, 1500, 2000, and 2500 K have been studied using volatility diagrams. Both the equilibrium oxygen partial pressure (P-O2) for the HfB2(s) to HfO2(s) lus B2O3(l) and the partial pressures of B-O vapor species formed due to B2O3(l) volatilization increase with increasing temperature. Vapor pressures of the predominant gaseous species also increase with P-O2. At 1000 K, the predominant vapor transition sequence is predicted be BO(g) -> B2O2(g) -> B2O3(g) -> BO2(g) with increasing P-O2, and the predominant gas is BO2(g) with a pressure of 1.27 x 10(-6) Pa under the condition of P-O2 = 20 kPa. At higher temperatures of 1500, 2000, and 2500 K, the system undergoes vapor transitions in the same sequence of B(g) -> BO(g) -> B2O2(g) -> B2O3(g) -> BO2(g). Under the same condition of = 20 kPa, the predominant vapor species is B2O3(g) with pressures of 2.38, 4.49 x 10(3), and 3.55 x 10(5) Pa, respectively. Volatilization of B2O3(l) may produce porous HfO2 scale, which is consistent with the experimental observations of HfB2 oxidation in air. The present volatility diagram of HfB2 shows that HfB2 exhibits oxidation behavior similar to ZrB2, and factors other than volatility of gaseous species affect the oxidation rate.