The excellent protection effects of silicon/ytterbium silicate bilayer environmental barrier coatings on SiC _f /SiC composites

Abstract The protection effects of an environmental barrier coating (EBC) consisting of silicon bond coat and mixed ytterbium monosilicate and ytterbium disilicate composite topcoat are directly evaluated by measuring the strength retention rate of SiC _f /SiC composites completely wrapped up by the previous EBCs after soaking in a mixed oxygen and water vapor environment at 1300°C for up to 200 h. The results show that the mixed topcoat exhibits not only extremely excellent phase stability but also fantastic protection effects toward composites. After 200 h of corrosion, the fully protected composites are unveiled to present not only dramatically reduced weight gain ratios, less than .6%, compared to ∼10% for those unprotected ones, but also extremely higher strength retention rates, more than 90%, compared to only 10%–15% for those unprotected ones. Further, the fully protected composites show a quasi‐ductile load versus displacement curve, suggesting the retention of the oxidation‐prone pyrolytic carbon interphase of current composites.