High temperature oxidation of SiC under helium with low-pressure oxygen. Part 3: β-SiC–SiC/PyC/SiC

In the frame of generation IV gas-cooled fast reactor (GFR), the cladding materials currently considered is a SiC/SiC-based composite with a pyrocarbon interphase and a β-SiC coating on the surface to close the porosity (noted β-SiC–SiC/PyC/SiC). These elements are subjected to temperatures going from 1300 to 1500 K in nominal operating conditions to 1900–2300 K in accidental conditions. The coolant gas considered is helium pressurized at 7 MPa.After a thermodynamic study carried out on the oxidation of β-SiC under helium and low oxygen partial pressures, an experimental approach was made on β-SiC–SiC/PyC/SiC composites under active oxidation conditions (1400 ≤ T ≤ 2300 K; 0.2 ≤ pO2 ≤ 2 Pa). This study follows two preceding studies carried out on two polytypes of SiC: α (Part 1) and β (Part 2) under the same conditions. In these studies, the influence of the crystalline structure on the transition temperature between passive and active oxidation and on the mass loss rate was discussed.The experimental study allows to determine the oxidation rates in incidental and accidental conditions under pO2 = 0.2 and 2 Pa. The variation of the mass loss rates according to the temperature for β-SiC–SiC/PyC/SiC oxidized under pO2 = 0.2 and 2 Pa shows the existence of three domains in the zone of active oxidation. These tests also show the weak impact of the oxygen partial pressure on the mass loss rate of the material in this range of pressure for temperatures lower than 2070 K. On the other hand, beyond 2070 K, an increase of the mass loss rate leading to important damage of the material has been observed, at lower temperature under pO2 = 0.2 Pa than under pO2 = 2 Pa. This variation was associated to the effect of the oxygen partial pressure on the sublimation temperature of SiC. Similar experiments were performed on pre-oxidized samples and on the face without CVD β-SiC coating and both the results are close to the ones obtained for the face with the CVD β-SiC layer.