Slurry based multilayer environmental barrier coatings for silicon carbide and silicon nitride ceramics - II. Oxidation resistance

摘要

In part I of this study, the dip-coat processing of mullite/gadolinium silicate (Gd_2SiO_5) environmental barrier coatings (EBCs) applied on α-SiC and SN282? Si_3N_4 through alcohol based and sol based slurries was presented. Here, the performance of selected EBCs by evaluating their oxidation resistances during thermal cycling in simulated combustion (90% H_2O-balance O_2) environment between 1350°C and RT for up to 400cycles is being reported. Oxidation of un-coated α-SiC was severe, leading to aligned and layered porous silica scale formation (~17μm thick) on its surface with frequent scale spallation when exposed to 100cycles. Mullite/Gd_2SiO_5/B_2O_3 (83.5/11.5/5wt.%) EBCs remained adherent to α-SiC substrate with an underlying porous silica layer formed at substrate/coating interface, which was ~12μm after 100cycles, ~16μm after 200cycles, and ~25μm after 400cycles. In contrast, α-SiC substrate coated with mullite/Gd_2SiO_5 (88/12wt.%) EBC had only limited oxidation of ~10μm even after 1350°C/400cycles. The sol based mullite/Gd_2SiO_5 (88/12wt.%) EBC on α-SiC substrate after 400cycles was adherent, but showed more interfacial damages (~20μm after 400cycles) though it had increased coating density. However, the mullite/Gd_2SiO_5 (88/12wt.%) EBC (alcohol based) delaminated from the SN282? Si_3N_4 substrate after 1350°C/100cycles, because of the formation of interconnected interfacial voids and hairline cracks. Parabolic growth kinetics for the underlying silica was observed for both the alcohol and sol based coated samples.