High temperature environmental attack and mechanical degradation of coatings in gas turbine blades

Abstract

This paper examines how in-service and thermal environmental attack influence the mechanical properties (22–950°C) of CoNiCrAlY coatings over René 80 substrates in gas turbine blades using a small punch (SP) testing technique in conjunction with scanning Auger microprobe analysis. SP tests have clearly demonstrated strong dependence of mechanical degradation of near surface coatings on the elevated temperature environmental condition. The room temperature (RT) ductility in blade coatings decreased with increasing operating time under combined fuels of kerosene and liquefied natural gas (LNG) despite softening in used coatings. All the coatings depicted lower ductility at 825°C in air than at RT but not in vacuum so that the oxidizing environment would produce deleterious effects. In-service operation under the combined fuels led to a two-fold increase in the ductile—brittle transition temperature (DBTT) over coatings observed under mainly LNG because of more extensive oxidation and grain boundary sulfidation. However, the DBTT of coating did not change during thermal ageing at 870°C in air that produced only oxidation. These findings imply that the grain boundary sulfidation would exert a stronger embrittling effect on the CoNiCiAlY coatings than the oxidation.