The effect of vermiculite on the degradation and spallation of plasma sprayed thermal barrier coatings

A study has been made of the effect of vermiculite (VM) on the sintering-induced spallation of plasma sprayed zirconia thermal barrier coatings (TBCs). The glass transition (~ 1040 °C) and melting (~ 1360 °C) temperatures of VM are representative of a broad class of so-called CMAS (calcia–magnesia–alumina–silica) particulates likely to be ingested into gas turbines. Selected loadings of VM powder were introduced onto the surface of free-standing coatings, followed by heating (1500 °C) for periods of up to 80 h. The presence of VM induces various microstructural changes in the coatings and also accelerates the rise in their (in-plane) Young's modulus. Finally, results are presented concerning the effect of VM on spallation resistance, using coatings sprayed onto dense alumina substrates. Spallation lifetimes can be substantially reduced by VM, even at relatively low levels (~ 1 wt.%). This is related to acceleration of the sintering-induced increases in coating stiffness.

► The VM melts and penetrates rapidly into the coatings.
► This penetration causes an acceleration of sintering, partly involving the liquid phase.
► Sintering causes stiffening, which increases the driving force for debonding on cooling.