Molten silicate reactions with plasma sprayed ytterbium silicate coatings

The reactions between molten calcium aluminum magnesium silicates (CMAS) at 1300 °C and atmospheric plasma spray (APS) deposited environmental barrier coatings on SiC substrates have been investigated. The tri-layer coatings comprised a silicon bond coat protected by a layer of mullite and either Yb2SiO5 (ytterbium monosilicate, YbMS) or Yb2Si2O7 (ytterbium disilicate, YbDS) as the topcoat. The APS deposition process resulted in two-phase top coats; the YbMS coating contained Yb2O3 regions in a matrix of Yb2SiO5 while the YbDS coating contained Yb2SiO5 in a matrix of Yb2Si2O7. Exposure of both coatings to a model CMAS resulted in dissolution of the topcoat accompanied by a rapid rise in the concentration of Yb in the melt, and formation of the same Ca2Yb8(SiO4)6O2 apatite reaction product phase. The thickness of the apatite layer initially varied with (time)1/4, but transitioned to approximately parabolic kinetics after 5-10 h of CMAS exposure. The reaction mechanism on the YbMS layer was consistent with recent observations on Y2SiO5, wherein molten CMAS transport to the undissolved silicate was controlled by diffusion through thin amorphous films at the apatite grain boundaries. The reaction mechanism for the YbDS layer was more complex, and involved preferential reaction with the YbSiO5 rich regions, resulting in a reaction zone that contained CMAS, the apatite reaction compound and undissolved Yb2Si2O7. The coating composition and microstructure significantly influenced the mechanism and rate at which the YbDS top coat was consumed by the reaction.