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.