The Effect of Volcanic Ash on the Thermomechanical Stability of Plasma- Sprayed Thermal Barrier Coatings (TBCs)

Sintering- driven changes in the microstructure of TBCs in gas turbines, due to prolonged exposure to high temperatures in service, can impair their thermomechanical stability. In particular, sintering can cause substantial increases in stiffness and therefore reduce its strain tolerance. These changes can be accelerated by the presence of impurities that segregate to the grain boundaries, where they enhance the solid state diffusivity or, at sufficiently high concentrations, by liquid phase sintering.

In this study, vermiculite (VM) powder (~100 μm) was selected as having a composition (42%O-36%Si-8%Mg-5%K-3%Al-3% Na-2.5%Fe-0.5%Ti) representative of volcanic ash. Its glass transition was observed to be ~ 1100°C and its melting temperature is ~1390˚C.  Selected loadings of this powder were introduced onto the surface of plasma- sprayed yttria stabilized zirconia (YSZ) coatings, detached from their substrates. The mass of these additions, relative to coating mass, was in the range 0.1 - 5.5 wt.%. A short heat treatment was initially given, designed to promote adhesion of the vermiculite particles, after which samples were exposed for periods (up to 50 hours) at elevated temperature (up to 1500˚C). The penetration of VM species was monitored by EDX mapping of transverse sections. In order to monitor the thermochemical reaction between VM and YSZ, XRD and Raman spectra of free surfaces was measured as a function of depth into the coating. Changes in the in- plane Young’s modulus was monitored by four point bending. Some results are also presented on the effect of VM- assisted sintering on the spallation resistance of TBCs, using specimen sprayed onto dense alumina and YSZ substrates and a custom- built periodic quenching furnace.

These measurements were used to provide information about the effect of environmental deposits on the acceleration of sintering.  It is shown that there is potentially a significant effect, with associated impairment of TBC lifetime.