High temperature oxidation of SiC under helium with low-pressure oxygen. Part 2: CVD β-SiC

Abstract

In the frame of

Measurement of thermal residual stresses in ZrB2–SiC composites

“…The temperatures of the experiment combined with the atmospheric pressure fall into a regime where SiC is known to actively oxidize. [34][35][36] As a result, SiC particles near the specimen surface underwent active oxidation leaving a surface layer of ∼50 m that was depleted of SiC (Fig. 2).…”

Initial assessment of environmental effects on SiC/SiC composites in helium-cooled nuclear systems

“…The conclusion of these studies 3,24,58,59 is that in order to withstand high temperatures for prolonged durations that may occur in severe accident scenarios, the SiC/SiC composites must be protected by thicker layers of β-SiC. Substitution of He (with 2 Pa O 2 ) by N 2 (with 0.5 Pa O 2 ) did not cause important differences in oxidation behavior of β-SiC (Fig.…”

“…Charpentier et al measured oxidation kinetics of sintered α-SiC and CVD β-SiC at oxygen pressures between 0.2 and 100 Pa in He (total pressure 100 kPa) and a broad temperature range. The β−SiC was obtained from Rohm & Haas (USA) and was processed by CVD at 1200-1300 o C. 24 The authors cited above found that the transition between passive and active oxidation regime occurs at higher temperatures for β-SiC than for α-SiC, and the mas loss rate of β-SiC is lower than the one measured for α-SiC on the common temperature range investigated (up to 1800 o C). Figure 2 show theoretical and experimental transitions observed for the two SiC allotropes.…”

“…Figures 3 and 4 show selected SEM images 3,24 of oxidized surfaces of sintered α-and crystalline β-SiC exposed to 0.2 and 100 Pa oxygen in helium. The temperatures where active to passive oxidation occurs vary, depending on the crystalline form and the oxygen pressure.…”

“…24,58 Unfortunately the SiC fibers used were Hi-Nicalon of the second generation; no information was found about the behavior in similar conditions of composites obtained from third generation, stoichiometric SiC fibers. Even so, the few existing data demonstrate good oxidation stability at low oxygen concentrations in the temperature range of normal operation conditions of VHTR.…”