Oxidation Resistance and Evolution of Multi-layered Oxide Scale During Isothermal and Cyclic Exposure of ZrB2–SiC–LaB6 Composites at 1300 °C to 1500 °C

The effect of LaB 6 (7, 10, and 14 vol pct) addition on oxidation behavior of spark plasma-sintered ZrB 2 –20 vol pct SiC composites involving heating under non-isothermal condition till 1400 °C, and isothermal and cyclic exposures at 1300 °C to 1500 °C have been examined. The mass gain of the composite with 14 vol pct LaB 6 is found as the highest during non-isothermal oxidation, whereas it appears as the least on isothermal exposure for 24 hours at 1400 °C or 1500 °C. The mass gain after 24 cycles of 1-hour exposure at 1300 °C to 1500 °C is found to be lower than that recorded under isothermal condition for all the composites, with the difference decreasing with LaB 6 content. The oxidation exponent ( n ) is found to decrease to near parabolic rate law with the increase in cyclic exposure temperature. The parabolic rate constant ( k p ) is found to decrease with the increase in LaB 6 content after a few cycles of exposure at all the investigated temperatures. The oxide scales formed on isothermal exposure at 1400 °C or 1500 °C have shown a thin La 2 Si 2 O 7 layer with borosilicate glass (BSG) as the outer layer, followed by layers containing BSG along with coarse and fine ZrO 2 , and a SiC-depleted ZrB 2 layer. The oxide scales formed during cyclic exposure contain an outer compact layer comprising BSG, La 2 Si 2 O 7 , and ZrSiO 4 , followed by ZrO 2 + BSG. Additionally, a SiC-depleted layer is found at the oxide–composite interface on cyclic exposure at 1500 °C for 24 hours, and at 1400 °C for 100 hours. The outer compact layer appears to have a critical role in protection against oxidation, with k p decreasing with the increase in its thickness.