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 (Gd(2)SiO(5)) environmental barrier coatings (EBCs) applied on alpha-SiC and SN282 (TM) Si(3)N(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(2)O-balance O(2)) environment between 1350 degrees C and RT for up to 400 cycles is being reported Oxidation of un-coated alpha-SiC was severe, leading to aligned and layered porous silica scale formation (similar to 17 mu m thick) on its surface with frequent scale spallation when exposed to 100 cycles. Mullite/Gd(2)SiO(5)/B(2)O(3) (83.5/11.5/5 wt.%) EBCs remained adherent to alpha-SiC substrate with an underlying porous silica layer formed at substrate/coating Interface, which was similar to 12 mu m after 100 cycles, similar to 16 mu m after 200 cycles, and similar to 25 mu m after 400 cycles. In contrast, alpha-SiC substrate coated with mullite/Gd(2)SiO(5) (88/12 wt.%) EBC had only limited oxidation of similar to 10 mu m even after 1350 degrees C/400 cycles. The sol based mullite/Gd(2)SiO(5) (88/12 wt%) EBC on alpha-SiC substrate after 400 cycles was adherent, but showed more interfacial damages (similar to 20 mu m after 400 cycles) though it had increased coating density. However, the mullite/Gd(2)SiO(5) (88/12 wt.%) EBC (alcohol based) delaminated from the SN282 (TM) Si(3)N(4) substrate after 1350 degrees 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. (C) 2010 Elsevier B.V. All rights reserved.