Surface roughness affects metastable non-wetting behavior of silicate melts on thermal barrier coatings

Airborne silicate pollutants in flight corridors pose a serious threat to aviation safety whose severity is directly linked to the wettability of molten silicates on thermal barrier coatings (TBCs) at high temperatures (1200-2000 degrees C). Despite its importance, the wettability of silicate melt on TBCs has not been well investigated. In particular, the surface morphology characteristics of TBCs can be expected to have a first-order effect on the wettability of silicate melt on such TBCs. Here, a series of atmospheric plasma spray (APS) yttria-stabilized zirconia (YSZ) TBCs with varying surface roughness were generated through the application of mechanical polishing. The metastable non-wetting behavior of three representative types of airborne silicate ash (volcanic ash, fly ash and a synthetic calcium-magnesium-aluminum-silicates (CMAS) powder) on these TBCs with varying surface roughness was investigated. It was observed that the smoother the surface of TBCs was, the larger the contact angle was with the molten silicate melts, and consequently, the smaller the area of damage was on the TBCs. Thus, the reduction in TBCs surface roughness (here via mechanical polishing) led to an improvement in the wetting and spreading resistance of TBCs to silicate melts at high temperature. In support of these observations and conclusions, the surface morphology of the TBC (both before and after polishing) had been characterized, and the mechanism of the surface roughness-dependence of wettability had been discussed. These results should contribute to reducing the deposition rate of silicate melt on TBCs, thus extending the lifetime of turbine blades and reducing maintenance costs.