Effect of AlN Substitutions on the Oxidation Behavior of ZrB₂–SiC Composites at 1600°C

Abstract

The oxidation behavior of ZrB2–SiC composites, with varying amounts of AlN substituting for ZrB2, was studied isothermally under static ambient air at 1600°C for up to 5 h. Small amounts of AlN substitutions (≤10 vol%) were found to result in marginal improvement in the oxidation resistance, whereas larger amounts resulted in a significant deterioration. The size of ZrO2 clusters formed on the oxidized surface was found to be a function of the AlN content. This effect was more pronounced after longer oxidation times (~1 h) as opposed to shorter durations (~5 min). It was postulated that presence of AlN results in the formation of Al2O3 during the oxidation process, subsequently resulting in a lowering of viscosity of the glassy silica scale, which facilitates the coarsening of ZrO2 clusters. This also increases oxygen permeation through the scale which adversely affects the oxidation resistance of the high AlN content composites.

Evaluation of rare‐earth element dopants (Sm and Er) on ablation resistance of ZrB2/SiC‐sintered billets

“…After ablation for 300 seconds, the c 1 -(Sm/Er) 0.2 Zr 0.8 O 1.9 islands grow, coalesce, and inhibit the formation of petals by forming the crystalline grains shown in Figure 6G, H, and I. 19,22 Overall, the convection cell mechanism occurring in the present study is similar to both Karlsdottir et al 18,19 and Brenner et al 17 studies. However, significant differences are shown due to the rare-earth element dopants used in the present study which cause the formation of different oxide scales during ablation testing.…”

“…Changes in the emittance caused by using Sm 3+ dopant have been shown in previous studies which demonstrated that the emittance of ZrO 2 can be increased via doping with rare‐earth oxides which intentionally introduce defects into a pure material . By comparing the ionic conductivity of the c 1 ‐(Sm/Er) 0.2 Zr 0.8 O 1.9 phase formed in the present study with the m ‐ZrO 2, it is important to note that the ionic conductivity will increase for the c 1 ‐(Sm/Er) 0.2 Zr 0.8 O 1.9 as the incorporated Sm 3+ and Er 3+ ions creates oxygen vacancies . These oxygen vacancies provide transitions within the material band gap which modify the emittance .…”

“…For example, a “flower‐like” structure is noted in Figure A for the 1Sm:0Er sample. After ablation for 300 seconds, the c 1 ‐(Sm/Er) 0.2 Zr 0.8 O 1.9 islands grow, coalesce, and inhibit the formation of petals by forming the crystalline grains shown in Figure G, H, and I …”

“…9,23 By comparing the ionic conductivity of the c 1 -(Sm/Er) 0.2 Zr 0.8 O 1.9 phase formed in the present study with the m-ZrO 2, it is important to note that the ionic conductivity will increase for the c 1 -(Sm/Er) 0.2 Zr 0.8 O 1.9 as the incorporated Sm 3+ and Er 3+ ions creates oxygen vacancies. 9,22 These oxygen vacancies provide transitions within the material band gap which modify the emittance. 9,22 Figure 8 shows that the amount of m-ZrO 2 increases as the distance from the ablated surface is further increased.…”

“…9,22 These oxygen vacancies provide transitions within the material band gap which modify the emittance. 9,22 Figure 8 shows that the amount of m-ZrO 2 increases as the distance from the ablated surface is further increased. Based on that, the ionic conductivity should be higher in the ablated surface than in deeper regions of the billets.…”