The tendency of molten CMAS deposits to wet 7YSZ is sufficiently strong that columnar TBC structures can be infiltrated just above the onset of melting within times that are negligible compared with typical operation cycles of gas turbine engines. Infiltration is complete in isothermal exposures, but would be limited in a real operation to a depth dictated by the thermal gradient across the coating and the viscosity of the melt.
Thermochemical interactions between CMAS and the TBC occur at lower temperatures (12401C) and shorter times (o4 h ) than suggested by previous studies in the literature. In general, the mechanism involves dissolution of the metastablet 0 phase and re-precipitation with a composition and structure that depends on the local chemistry. Interactions in the bulk of the coating are minimized by the small volume of melt in relation to the amount of TBC material. Where larger volumes of CMAS are available, as in the near-surface region, the reprecipitated YSZ is sufficiently depleted in Y that it transforms to monoclinic upon cooling. The associated volume change could, in principle, contribute to the strains that motivate exfoliation of the coating upon thermal cycling. Near the substrate, the local chemistry is different due to the dissolution of the underlying alumina by the CMAS, inducing precipitation of a crystalline aluminosilicate and globules of a Y-enriched, non-transformable cubic YSZ.
The present study suggests a number of relevant research directions, ranging from understanding of the early stages of interaction, the effect of varying CMAS chemistry in the rate of dissolution and the nature of the crystalline products, the sequence in which these products evolve, and the potential implications for the mitigation of CMAS attack on thermal barrier coatings. These issues are presently under investigation and will be discussed in forthcoming publications.
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