Crystallization behavior of CMAS + sea salt mixture and its effect on the mixture penetration into thermal barrier coatings

Abstract

When aero-engines are operated in dusty and near-sea environments, thermal barrier coatings (TBCs) are threatened by coupling corrosion of calcium-magnesium-alumina-silicate (CMAS) and sea salt. Investigating the crystallization behavior of CMAS + sea salt that affects the melt penetration into TBCs is the key to seeking protection methods against coupling corrosion. In this study, the crystallization temperature range and crystalline products of CMAS + sea salt were investigated. The results indicated that the crystallinity of CMAS + sea salt is affected by temperature. CMAS, CMAS+5SS (95 wt% CMAS + 5 wt% sea salt), and CMAS+10SS (90 wt% CMAS + 10 wt% sea salt) were crystalized initially at 1000 °C, but the crystallinity was low. At 1100 °C, more crystallization occurred, with diopside (CaMgSi2O6), akermanite (Ca2MgSi2O7), and anorthite (CaAl2Si2O8) being precipitated. With the temperature increasing to 1200 °C, wollastonite (CaSiO3) instead of akermanite was precipitated in both CMAS and CMAS+5SS samples, while the CMAS+10SS sample became almost amorphous glass with little crystallization indicating that higher sea salt content narrows the crystallization temperature range. Moreover, at the appropriate crystallization temperature (1100 °C), the addition of sea salt in CMAS played a depolymerization role and promoted discrete precipitation of dendritic akermanite, which reduced the crystalline layer compactness increasing the threat of mixture penetration into TBCs.