Thermodynamic calculation of hfB ₂ volatility diagram

The thermodynamics of the oxidation of HfB ₂ at temperatures of 1000, 1500, 2000, and 2500 K have been studied using volatility diagrams. Both the equilibrium oxygen partial pressure ( PO ₂) for the HfB ₂(s) to HfO ₂(s) plus B ₂O ₃(l) and the partial pressures of B-O vapor species formed due to B ₂O ₃(l) volatilization increase with increasing temperature. Vapor pressures of the predominant gaseous species also increase with PO ₂. At 1000 K, the predominant vapor transition sequence is predicted be BO(g) → B ₂O ₂(g) → B ₂O ₃(g) → BO ₂(g) with increasing PO ₂, and the predominant gas is BO ₂(g) with a pressure of 1.27 × 10 ^-6 Pa under the condition of PO ₂ = 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) → B ₂O ₂(g) → B ₂O ₃(g) → BO ₂(g). Under the same condition of PO ₂ = 20 kPa, the predominant vapor species is B ₂O ₃(g) with pressures of 2.38, 4.49 × 10 ³, and 3.55 × 10 ⁵ Pa, respectively. Volatilization of B ₂O ₃(l) may produce porous HfO ₂ scale, which is consistent with the experimental observations of HfB ₂ oxidation in air. The present volatility diagram of HfB ₂ shows that HfB ₂ exhibits oxidation behavior similar to ZrB ₂, and factors other than volatility of gaseous species affect the oxidation rate.