Molten silicate interactions with thermal barrier coatings

The strain tolerance of thermal barrier coatings (TBCs) used in gas turbine engines is enhanced by incorporation of through-thickness pores within the coating. However, molten calcium–magnesium–alumino–silicate (CMAS) deposits are then able to permeate through the interconnected pore network resulting in dissolution of the coating in the silicate melt, precipitation of new phases, and the elimination of the pores needed for strain tolerance when silicate melt solidification occurs upon cooling. Here we used an electron beam directed vapor deposition method to deposit strain tolerant coatings containing a high volume fraction of inter-columnar pores to assess the effect of increasing the pore volume fraction upon silicate melt infiltration in a 7 wt.% yttria stabilized zirconia (7YSZ) TBC. We then explore two potential mitigation approaches. One investigated the infiltration of a samarium zirconate (SZO) coating deposited on a 7 wt.% yttria stabilized zirconia (7YSZ) buffer layer since recent work has indicated the related gadolinium zirconate (GZO) system was much more CMAS resistant than 7YSZ. The second explored the effects of an embedded platinum layer located near the outer surface of the SZO layer. The response of both systems to attack by a model 33CaO–9MgO–13AlO1.5–45SiO2 CMAS melt at 1250 °C has been compared to that of the 7YSZ composition coating deposited by the same process. The 7YSZ specimens were fully infiltrated by CMAS in less than 1 min and significantly dissolved and re-precipitated a globular phase after 4 h of exposure. The penetration rate in SZO coatings was reduced by about a factor of five, but the coatings were still completely penetrated after a 4 hour exposure. Significant SZO dissolution and precipitation of both globular fluorite and faceted apatite phases were observed after the 4 h of exposure, with concomitant debonding at the SZO/YSZ interface. The insertion of a 5 μm thick Pt layer about 30 μm below the SZO surface arrested permeation of CMAS for about 16 h and might provide a promising strategy for controlling CMAS degradation in TBCs without reducing the thermo-cyclic delamination resistance.