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

In part I of this study, the dip-coat processing of mullite/gadolinium silicate (Gd2SiO5) environmental barrier coatings (EBCs) applied on α-SiC and SN282™ Si3N4 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% H2O–balance O2) environment between 1350 °C and RT for up to 400 cycles is being reported. Oxidation of un-coated α-SiC was severe, leading to aligned and layered porous silica scale formation (~ 17 μm thick) on its surface with frequent scale spallation when exposed to 100 cycles. Mullite/Gd2SiO5/B2O3 (83.5/11.5/5 wt.%) EBCs remained adherent to α-SiC substrate with an underlying porous silica layer formed at substrate/coating interface, which was ~ 12 μm after 100 cycles, ~ 16 μm after 200 cycles, and ~ 25 μm after 400 cycles. In contrast, α-SiC substrate coated with mullite/Gd2SiO5 (88/12 wt.%) EBC had only limited oxidation of ~ 10 μm even after 1350 °C/400 cycles. The sol based mullite/Gd2SiO5 (88/12 wt.%) EBC on α-SiC substrate after 400 cycles was adherent, but showed more interfacial damages (~ 20 μm after 400 cycles) though it had increased coating density. However, the mullite/Gd2SiO5 (88/12 wt.%) EBC (alcohol based) delaminated from the SN282™ Si3N4 substrate after 1350 °C/100 cycles, 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.