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

This article addresses the problem of gas permeability of thermal sprayed y ttria-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 t echnique was used, beginning with aluminum isopropoxide as sol precursor, s ubsequently hydrated to aluminum hydroxide sol, which decomposed at relativ ely low temperatures to extra-fine, readily sinterable aluminum oxide. In s ome experiments, the sol-gel (SG) precursor was combined with fine grains o f calcined alumina, constituting high solid-yield composite sol-gel (CSG) d eposits within the pores of TBCs, Sinterability in the model systems, inclu ding aluminum hydroxide sol-calcined alumina and aluminum hydroxide sol-cal cined alumina-zirconia, has been studied. A number of TBC specimens were im pregnated with suspensions of alumina sols and CSG, It is shown that these ceramics effectively penetrated into the pores and cracks of TBCs and reduc ed the coating permeability to gases. The overall reduction of porosity was however small (from similar to 12 to similar to 11%), preserving the strai n and thermal shock tolerance of the coatings, Burner rig tests showed an i ncrease in sealed coating lifetime under thermomechanical fatigue condition s.