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 LaB6 (7, 10, and 14 vol pct) addition on oxidation behavior of spark plasma-sintered ZrB2-20 vol pct SiC composites involving heating under non-isothermal condition till 1400 degrees C, and isothermal and cyclic exposures at 1300 degrees C to 1500 degrees C have been examined. The mass gain of the composite with 14 vol pct LaB6 is found as the highest during non-isothermal oxidation, whereas it appears as the least on isothermal exposure for 24 hours at 1400 degrees C or 1500 degrees C. The mass gain after 24 cycles of 1-hour exposure at 1300 degrees C to 1500 degrees C is found to be lower than that recorded under isothermal condition for all the composites, with the difference decreasing with LaB6 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 LaB6 content after a few cycles of exposure at all the investigated temperatures. The oxide scales formed on isothermal exposure at 1400 degrees C or 1500 degrees C have shown a thin La2Si2O7 layer with borosilicate glass (BSG) as the outer layer, followed by layers containing BSG along with coarse and fine ZrO2, and a SiC-depleted ZrB2 layer. The oxide scales formed during cyclic exposure contain an outer compact layer comprising BSG, La2Si2O7, and ZrSiO4, followed by ZrO2 + BSG. Additionally, a SiC-depleted layer is found at the oxide-composite interface on cyclic exposure at 1500 degrees C for 24 hours, and at 1400 degrees 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.

Automated summary

The addition of LaB6 has complex effects on the oxidation behavior of ZrB2-SiC composites, with the composite containing 14% LaB6 showing the highest mass gain during non-isothermal oxidation and the lowest during isothermal exposure. The oxidation exponent decreases to near parabolic rate law with increasing cyclic exposure temperature, while the parabolic rate constant decreases with increasing LaB6 content. The outer compact layer seems to play a critical role in protection against oxidation, with the parabolic rate constant decreasing as its thickness increases.