High temperature oxidation of LaB₆-ZrB₂ eutectic in situ composite

LaB₆-ZrB₂ eutectic in situ composites are interesting candidates for manufacture of turbine blades because of their excellent mechanical properties. The oxidation behavior of LaB₆-ZrB₂ eutectic in situ composites prepared by electric are melting was studied in a temperature range of 916-1223°C and in a pressure range of 1.5 × 10⁴-7.2 × 10⁴ Pa. The specific weight change of the LaB₆-ZrB₂ eutectic obeys the Arrhenius equation below the oxidation temperature of 1094°C and appears to obey a paralinear equation above this temperature. The rate of weight change is described by k_p = 9.71 × 10^-3 exp(-31000/RT) mg²/cm⁴ min in the range 916-1094°C. Above 1094°C, rapid oxidation kinetics were observed, La₂O₃·B₂O₃ and La₂O₃·3B₂O₃ being the primary scale constituent with ZrO₂ deficiency in the outer scale. The oxidation increases with P_O(2) up to 3.9 × 10⁴ Pa, above which the trend is reversed due to the active oxidation of ZrB₂. The oxidation appears to be controlled by oxygen diffusion through the scale and the oxidized layers consist mainly of La₂O₃·B₂O₃, La₂O₃·3B₂O₃ and trace of ZrO₂.