High-temperature oxidation of SiC-Ta-SiC sandwich cladding tubes in GFR atmosphere

Silicon carbide ceramic-matrix-composites (CMC) are considered as promising candidates for cladding tubes in GFRs and LWRs. In the framework of the EC funded project MatISSE SiC(-Ta)-SiC ceramic matrix composites (CMC) are investigated in impure helium prototypic for GFR cooling gas atmosphere.
Thermo-gravimetric experiments have been completed including SiC-SiC samples without Ta liner with three different surface qualities at 900-1500°C, sandwich samples at 1200-1400°C, and tantalum tube specimens at 900 and 1100°C.
All tests were performed in a TESTARAM TAG with He atmosphere with the following impurities: 210 vppm H2, 210 vppm CO, 140 vppm N2, and 70 vppm each of O2, H2O, CH4, and CO2. These should be the partial pressures of the impurity gases at operational pressure of 70 bar.
Passive oxidation, i.e. a slight mass gain due to the formation of a passivating silica scale was observed at temperatures up to 1200°C. Starting from 1300°C active oxidation, i.e. with mass loss due to the volatilization of SiC, took place as can be seen in Fig. KIT-1. The mass loss increased with raising temperature. The surface quality of the samples (as produced, ground, smooth) has minor effects on the oxidation especially at the lower temperatures. ... mehrPost-test examinations of the samples by SEM-EDX revealed the formation of a superficial SiO2 layer of all samples oxidized at temperatures up to 1200°C and a porous SiC surface without any oxide for the samples annealed at higher temperatures, see Fig. KIT-2.
The experiment with sandwich samples including an internal tantalum liner resulted generally in higher mass gains compared to the non-sandwich samples. Obviously, the tantalum liner was partially directly attacked by the oxidizing gases especially at the edges of the tube segments which lead to a non-prototypical oxidation kinetics. Hence, the sandwich samples cannot be used for analyzing oxidation kinetics of SiC-SiC based on in-situ mass changes. Anyway, valuable results of the post-test examination will help to understand the high-temperature stability of the tantalum liner. Fig. KIT-3 shows an increasing interaction with increasing temperatures. Post-test elemental and phase analyses of the interaction products are ongoing.
The oxidation of pure tantalum tube segments which are used for the liner in the sandwich samples has been investigated separately at 900 and 1100°C. Figure KIT-4 shows a very similar mass gain for both temperatures with linear kinetics. This is a strong indication for gas phase diffusion as the limiting mechanism for the oxidation, especially when compared to a mass gain of 50 mg/cm2 after only 2 hours at 900°C in 100 kPa steam atmosphere.