Kinetics and evolution of oxide scale during various stages of isothermal oxidation at 1300 °C in spark plasma sintered ZrB2 – SiC – LaB6 composites

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

“…[32][33][34] The possible reactions involved in the formation of oxidation products along with the values of DG f at 1400 °C or 1500 °C are shown in Table III, whereas those at 1300 °C have been already published in an earlier report. [29] The thermodynamic stability of the oxides at the investigated temperatures is found to increase in the following sequence:…”

“…The bar charts depicting the change in total mass of the composites subjected to exposure at 1300 °C to 1500 °C for 24 hours in isothermal and cyclic conditions are shown in Figures 3(a) and (b), respectively. Here, the total mass changes recorded after isothermal exposure at 1300 °C for 24 hours, which has been published in an earlier study, [29] are shown only for the purpose of comparison. Comparison of the results depicted in Figures 3(a) and (b) leads to the following inferences: (i) after exposure for 24 hours in both isothermal and cyclic conditions, the total mass gain has increased sharply on increasing the final hold temperature from 1300 °C to 1400 °C; (ii) the net mass gain after exposure at 1500 °C for 24 hours is less than that at 1400 °C, with the difference being marginal for cyclic exposure; (iii) the net mass gain recorded after isothermal exposure for 24 hours is reduced with the increase in the LaB 6 content, whereas for cyclic exposure, the least mass gain has occurred in the composite with 10 vol pct LaB 6 ; (iv) the total mass gain on isothermal exposure is in general greater than that after cyclic exposure, except for the ZSBCL-14 composite subjected to cyclic oxidation at 1500 °C; and (v) the difference between total mass gain after isothermal and cyclic exposures is reduced with the increase in the LaB 6 content of the composites.…”

“…Further, the oxide scale thicknesses recorded on isothermal exposure of ZSBCL-7, ZSBCL-10, and ZSBCL-14 at 1300 °C for 24 hours have been found as 129 ± 5.8 lm, 113 ± 3.6 lm, and 103 ± 6.8 lm, respectively. [29] The results shown in Tables I and II lead to the following inferences: (i) the total thickness of the oxide scales formed after either isothermal or cyclic exposure increases with the increase in temperature as well as exposure duration at 1400 °C; (ii) on isothermal exposure at 1300 °C or 1400 °C for 24 hours, the net thickness of the oxide scale decreases with the increase in LaB 6 content, whereas on cyclic exposure for similar duration the net oxide scale thickness follows the trend: ZSBCL-10 < ZSBCL-14 < ZSBCL-7; (iii) the thickness (as percentage of total thickness) of the coarse ZrO 2 + BSG layer is increased with the increase in the LaB 6 content at the expense of fine ZrO 2 + BSG on isothermal exposure at 1400 °C; (iv) the ZrO 2 + BSG layer thickness percentage decreases by 4 to 5 pct with the increase in LaB 6 content from 7 to 10 vol pct on cyclic exposure for 24 hours at 1300 °C to 1500 °C, whereas the change is insignificant on further increase in the LaB 6 content to 14 vol pct, and an opposite (decreasing) trend is observed on cyclic exposure at 1400 °C for 100 hours; (v) the net thickness of the oxide scale formed after isothermal as well as cyclic exposure at 1500 °C decreases with the increase in LaB 6 content; (vi) the thickness of outer compact layer (OCL) (La 2 Si 2 O 7 + BSG + ZrSiO 4 ) increases with the increase in LaB 6 content after cyclic exposure for 24 hours at all Bold indicates the total thickness of the oxide scale (sum of the thicknesses of sub-layers of the oxide scale).…”

“…The decrease in the value of k p after a period of exposure as mentioned above indicates that the rate of oxidation is reduced significantly after a critical value of the oxide scale thickness is reached, due to the increase in the diffusion distance for oxygen through the protective layer, as has been observed for isothermal exposure at 1300 °C. [29] C. Oxidation Mechanism…”

“…In an earlier study on isothermal oxidation behavior of the presently investigated composites at 1300 °C for durations up to 24 hours, it was found that the rate of mass gain was significantly reduced after 8 to 10 hours of exposure, because of the formation of a viscous and stable BSG layer with optimum thickness and viscosity within the oxide scale. [29] Keeping the positive results of the earlier study in mind, a hold duration of 10 hours has been chosen in the present investigation for carrying out cyclic oxidation experiments at 1400 °C.…”