Experimental and Analytical Study of Ceramic-Coated Turbine-Tip Shroud Seals for Small Turbine Engines.

An experimental investigation was conducted to validate component rig results and the analytical model of the behavior of a ceramic material in the operating environment of a small turbine engine. The ceramic shroud has the potential of increasing small turbine-engine efficiency through use of higher gas-path temperatures or less shroud cooling air, of extending component life through reduced metal temperatures, or of reducing component weight. The ceramic shrouds were subjected to 1001 cycles between idle and high power and steady-state conditions for a total of 57.8 engine hr. Posttest engine inspection revealed mud-flat surface cracking, which the authors attributed to microcracking under tension with crack penetration to the ceramic-and bond-coat interface. Sections and micrographs tend to corroborate the thesis. The engine test data provided input to a thermomechanical analysis to predict temperature and stress profiles throughtout the ceramic gas-path seal. The analysis predicts cyclic thermal stresses large enough to cause the seal to fail. These stresses are, however, mitigated by inelastic behavior of the shroud materials and by microfracturing that tensile stresses produce. Microfracturing enhances shroud longevity during early life but provides the failure mechanism during extended life when coupled with time-dependent inelastic materials effects. Keywords Turboshaft engines, Ground tests, and Ceramic bond.