Development and Investigation on New Composite and Ceramic Coatings as Possible Abradable Seals

To improve gas turbine performance, it is possible to decrease back flow gases in the high-temperature combustion region of the turbo machine by reducing the shroud/rotor gap. Thick and porous thermal barrier coating (TBC) systems and composite CoNiCrAlY/Al₂O₃ coatings made by air plasma spray and composite NiCrAlY/graphite coatings made by laser cladding were studied as possible high-temperature abradable seal on shroud. Oxidation and thermal fatigue resistance of the coatings were assessed by means of isothermal and cyclic oxidation tests. Tested CoNiCrAlY/Al₂O₃ and NiCrAlY/graphite coatings after 1000 h at 1100 °C do not show noticeable microstructural modification. The oxidation resistance of the new composite coatings satisfied original equipment manufacturer (OEM) specifications. Thick and porous TBC systems passed the thermal fatigue test according to the considered OEM procedures. According to the OEM specifications for abradable coatings, the hardness evaluation suggests that these kinds of coatings must be used with abrasive-tipped blades. Thick and porous TBC coating has shown good abradability using tipped blades.