Phase transformation failure in YSZ TBCs induced by component-dependent CMAS corrosion

Generally, CMAS corrosion is a key factor leading to the spalling failure in thermal barrier coatings (TBCs). Figuring out the component-dependent failure mechanism is the premise to improve the service life of TBCs. Here, CMAS corrosion behaviors with different Ca/Si components have been investigated through analyzing microstructure evolution in 8-YSZ coatings by EB-PVD. Capillary force from the gap between columnar crystals could accelerate the infiltration speed of the molten CMAS, leading to a fully populated coating with lower strain tolerance in 4 h. Subsequently, CMAS infiltration in the coatings led to preferential dissolution of yttrium ions and precipitation of yttria lean YSZ, and secondary CMAS phases. The corrosiveness of CMAS could be further tailored by optical basicity and viscosity. TBC failure could be enhanced in high-Ca case, resulting in the morphology evolution from columnar crystals to coarsening grain structures. Raman spectra indicates CMAS corrosion with high-Ca content will facilitate the phase transformation of t-YSZ to harmful monoclinic phase. In addition, DFT calculation demonstrates Ca doping into 8-YSZ will make monoclinic phase more stable and generate a 4.1 % volume expansion from the view point of energy. Consequently, the combination of the above low strain tolerance, new phases and volume expansion will accelerate the harmful morphology evolution. This work provides direct insight into the failure mechanism of CMAS corrosion and paves the way to design of new TBCs.