Identification of a new oxidation/ dissolution mechanism for boria‐accelerated SiC oxidation

Abstract

Boria effects on accelerated SiC oxidation kinetics were investigated by conducting thermogravimetric analysis on SiC substrates coated with sol‐gel derived borosilicate glass isothermally exposed to dry O2 and argon at 800°C and 1200°C for 100 hours. Boria concentrations in the glass coatings were 0, 14‐38, and 92‐94 mol%, balance silica. Accelerated weight gain was observed for SiC exposures in dry O2 at 800°C when boria concentrations were ≥ 92 mol%, corroborated by oxide thickness ranging from 3.5 to 10 µm. The oxide thickness predicted for pure SiC exposed to these conditions in the absence of boria is 0.15 µm. Microstructural analysis of SiC surfaces after oxide removal revealed that boria etched the underlying SiC substrate. Oxidation exposures at 1200°C in dry O2 suppressed boria effects on accelerating SiC oxidation kinetics due to rapid boria volatilization coupled with the formation of a protective thermally grown silica scale. Accelerated weight gain or oxide growth did not occur with argon exposures at either temperature. A new mechanism for boria‐accelerated SiC surface‐reaction kinetics is presented based on evidence for boria etching of SiC.

Investigations into the thermal stability of sol‐gel‐derived glasses as models for thermally grown oxides

“…The thermal stability of retained carbon in the sol-gel-derived silica and borosilicate glasses in the high-temperature dry O 2 environments is also expected to occur with the solgel-derived glass coatings applied onto SiC substrates, investigated in a companion paper. 39 These coatings, applied at thicknesses ranging from <1-15 µm, are significantly thinner than the 2.2-3.1 mm (2200-3100 µm) thick glass disks investigated here. Retained carbon, shown in this present study to oxidize at the glass surface at depths of at least 0.3 µm, is postulated to be predominantly oxidized from the sol-gelderived glass coatings during exposure of the glass-coated SiC coupons to the isothermal TGA environment (100 hours isothermal exposures in dry O 2 at 800 or 1200°C).…”

“…This study is in support of a companion study which used the approaches by Schlichting 13 and Hatta et al 14 to coat SiC substrates with sol-gel-derived glass layers to investigate boria effects on accelerated SiC oxidation that cause degradation of CMC components. 39 In this present study, deficiencies in the approaches by Schlichting 13 and Hatta et al 14 are addressed. Freestanding sol-gel-derived glasses were characterized for composition and structure after pyrolysis.…”

“…10,42 Results from the characterization of the sol-gel-derived glasses for weight change, composition, structure, and morphology of this present study will be shown to directly relate to understanding the thermal behavior of the sol-gel-derived glass coatings applied onto SiC substrates investigated in a companion study. 39…”

“…Isothermal exposures were conducted at 800 and 1200°C for 100 hours to coincide with oxidation studies conducted in the companion study on SiC substrates coated with sol-gel-derived glass layers. 39 These exposure temperatures are relevant to those that would be experienced by CMC components in the aircraft engine hot section. The glass thermal stabilities were also investigated by conducting non-isothermal exposures at temperatures ranging from 25 to 1200°C.…”

“…Structural properties of the sol‐gel‐derived silica and borosilicate glasses are compared with those of pure silica and binary borosilicate glasses synthesized from the melt, 40 vapor deposition, 41 or thermally grown 10,42 . Results from the characterization of the sol‐gel‐derived glasses for weight change, composition, structure, and morphology of this present study will be shown to directly relate to understanding the thermal behavior of the sol‐gel‐derived glass coatings applied onto SiC substrates investigated in a companion study 39 …”