PHASE STABILITY ISSUES IN EMERGING TBC SYSTEMS

摘要

The high temperature stability of metastable single-phase zirconias co-doped with Y plus trivalent rare earth cations is discussed. The motivation arises from strategies to enhance the insulating efficiency and/or higher temperature capability of thermal barrier systems by co-doping the conventional ZrO_2-7.6%YO_(1.5) composition. The fundamental issue is the resistance to partitioning of the metastable tetragonal (t') or cubic (c') phases into the equilibrium assemblage dictated by the phase diagram, whereupon the stabilizer-depleted tetragonal phase becomes susceptible to the disruptive monoclinic transformation. The experiments rely on compositions synthesized by precursor pyrolysis, all of which yield initially supersaturated single-phase solid solutions. Clear trends are noted in the phase stability of single and co-doped compositions, which increases with decreasing the size of the rare earth cation. Thermodynamic arguments are offered to rationalize these trends, with additional comments on the potential roles of diffusion and clustering phenomena.