Stability of Mullite Protective Coatings for Silicon‐Based Ceramics

Abstract

Silicon‐based ceramics are prone to severe oxidation and volatilization problems in the presence of water vapor. To address these issues, mullite has been investigated in recent years as a possible protective coating material. To analyze the stability of such coatings, a multi‐species diffusion model for ionic species is developed. Onsager reciprocity is assumed for the mobility coefficients and the resulting moving boundary problem is solved numerically. The formulation also accounts for substantial Si volatilization away from the outer surface of the coating. This model is used to analyze an initial transient leading to the formation of an internal silica layer, a second transient period that follows this silica formation, and a steady‐state regime that can only be attained if Si diffusion is sufficiently fast.

Mullite and Mullite/ZrO2-7wt.%Y2O3 Powders for Thermal Spraying of Environmental Barrier Coatings

“…The functionality of this layer is to inhibit substrate corrosion by water vapor whereas taking advantage of its close thermal expansion coefficient to those of non-oxide ceramics (Ref 1,3). One of the main drawbacks of plasma-sprayed mullite coatings is that an important amount of amorphous phase develops, which may cause coating spall-off when thermal cycled (Ref 4,5).…”

Crystallization studies in mullite and mullite–YSZ beads

“…Moreover, even fully crystalline mullite coatings exhibited some volatilization of silica in water vapor rich atmospheres at high temperatures. [6][7][8] Different approaches have been attempted to solve this problem. Specifically, a ZrO 2 -7-8 wt.% Y 2 O 3 (YSZ) top coat was plasma sprayed over mullite 8,9 but the large mismatch in the coefficient of thermal expansion (CTE) between YSZ and mullite layers caused the fail of the system.…”

“…In this sense, mullite has been used as a transitional protective layer for Si-based components owing to its compatibility and close thermal expansion coefficient to these ceramics. [4][5][6] Mullite coatings processed by thermal spraying techniques normally become moderately amorphous, later crystallizing when heat-treated at T ≥ 1000 • C, thus producing volume changes and frequently through coating cracks 7-9 that allow further oxidation. Moreover, even fully crystalline mullite coatings exhibited some volatilization of silica in water vapor rich atmospheres at high temperatures.…”

Environmental durability of slurry based mullite–gadolinium silicate EBCs on silicon carbide

“…Mullite 2,11,12 or mullite-based ceramics 13,14 are the most studied EBCs for silicon (Si) based materials such as SiC or SiC/SiC ceramic matrix composites due to their close coefficient of thermal expansion (CTE) match (SiC or SiC/SiC ∼4.5-5.5 × 10 −6 / • C and mullite ∼5.1 × 10 −6 / • C).…”