Papers by Author: Francis Rebillat

Abstract: To be used as environmental barrier coatings, yttrium silicates must be thermally and chemically stable at high temperatures, under high pressures in a moist environment. This work proposes a full method to quantify their thermo chemical and mechanical stabilities: (i) in a corrosive environment at high temperature and (ii) against the covered material to protect. These stabilities were first estimated by thermodynamic calculations and further confirmed with corrosion tests. This first step needed: (i) to measure the partial pressures of yttrium hydroxides through corrosion tests on the simple oxide, Y2O3 and (ii) to extract the free energies of formation of gaseous yttrium-hydroxides formed. In a second step, the measured values of coefficients of thermal expansion (CTE) on these materials allowed identifying what compositions should be preferentially used, to get CTE close to that of the substrate material to prevent delaminating or cracking due to CTE mismatch stress. Finally, these materials are deposited on SiC/SiC composites by plasma spraying. The impact of the morphology, crystallinity, porosity and composition of elaborated coatings on their corrosion behaviour is highlighted.

Abstract: The solar furnace is a heating system based on concentrated sunrays on the material surface. It is an original method for testing ultra-high-temperature ceramics (UHTC) at very high temperature (above 2200°C) in air with an exposure time of several minutes. In this study, the solar flux is 15.5 MW.m-2 with a homogeneous exposed surface of 10 mm2. A large temperature-time composition parameters space is covered producing a large set of oxidized samples. Massive cylindrical specimens of UHTC materials are prepared by spark plasma sintering at 1900°C under a pressure of 100 MPa for 5 minutes. Then, samples are tested in air from 1750°C up to 2400°C with dwell times varied from 1 to 5 min. During oxidation of ZrB2-SiC (20%vol) material, the formed and known complex oxide scale identified from literature is easily reproduced using this method. It consists of a thin outer silica layer and zirconia columnar layer with a region of SiC depleted zone in ZrB2 phase. The impact of the reduction of Si content is quantified and the coating ZrB2-20%vol SiC is tested as protection on C-C composite.

Abstract: Silicon-based ceramics are among the main candidates for high temperature structural components in aeronautic applications. One key drawback of silicon-based ceramics for these applications is the volatilization of the protective silica scale, in moisture and the resulting ceramic recession. Therefore, the further use of these ceramics components depends on the development of external protection against water vapour attack. Some of the most promising materials seem to be rare earth silicates. Based on much richer data in the bibliography, the purpose of this work is to investigate the influence of two yttrium silicates elaboration processes on both the capability to crystallize and the corrosion resistance in an oxidative moist atmosphere at high temperature. Taking into account the material’s morphology, composition and degree of crystallization, the composition (Y2SiO5 or Y2Si2O7) and the preferable synthesis process are discussed.