Post-deposition treatment of zirconia thermal barrier coatings using sol-gel alumina

This article addresses the problem of gas permeability of thermal sprayed yttria-stabilized zirconia thermal barrier coatings (TBC)s. The objective of this study was to decrease the open porosity of TBCs through deposition of dense alumina ceramic on the surface of the pores. A simple infiltration technique was used, beginning with aluminum isopropoxide as sol precursor, subsequently hydrated to aluminum hydroxide sol, which decomposed at relatively low temperatures to extra-fine, readily sinterable aluminum oxide. In some experiments, the sol-gel (SG) precursor was combined with fine grains of calcined alumina, constituting high solid-yield composite sol-gel (CSG) deposits within the pores of TBCs. Sinterability in the model systems, including aluminum hydroxide sol-calcined alumina and aluminum hydroxide sol-calcined alumina-zirconia, has been studied. A number of TBC specimens were impregnated with suspensions of alumina sols and CSG. It is shown that these ceramics effectively penetrated into the pores and cracks of TBCs and reduced the coating permeability to gases. The overall reduction of porosity was however small (from approximately 12 to approximately 11%), preserving the strain and thermal shock tolerance of the coatings. Burner rig tests showed an increase in sealed coating lifetime under thermomechanical fatigue conditions.