Synergetic evolution of the ordered nanopores and stabilizer-controlled phase-separations for the columnar structures of 8YSZ thermal-barrier coatings through high-temperature aging

The microstructures of yttria-stabilized-zirconia thermal-barrier-coatings (YSZ-TBC) are complex through their thermal cycles, while the metastable 8YSZ could be created as the durable TBC. With a combination of XRD, SEM, and TEM analyses, we observe a multiscale phase evolution process for columnar structures from EB-PVD aged at 1425 °C. In the initial aging stage, the columnar grains coarsened along the growth direction to maintain the highly textured columns in [110] twinning structures, while the feathery features created from the rotary deposition were all transformed into ordered nanopores. The metastable t’ phase decomposed steadily into t and c phases with prolonged aging, which is realized by a local phase separation of the 8YSZ matrix into coherent, alternating 4YSZ−15YSZ domain-layers. Such dual-phase layer-structures were created by a mutual migration of both cations through the tetragonal lattice until reaching a solubility gap by Y-stabilizers for the dual phases. Successive phase-separations of multilayers were initiated from different nanopores, which led to the hierarchical phase-decomposition to and within a confined pattern. The primary c-layers were nucleated along the primary set of nanopores with symmetric strain-gradients to drive yttria from either side, and zirconia in opposite directions. The phase-decomposition into the patterned multilayers re-distributes the residual stress from the as-deposited, highly porous microstructure to create the minimized yet elastic interfacial strains, which may further strengthen and improve the stability of columnar structures as suggested by the emerging of an intermediate, orthorhombic phase transformed only from surface with no or little expansion.