STRESS-RELAXATION OF LOW-PRESSURE PLASMA-SPRAYED NICRALY ALLOYS

The stress relaxation behavior of three NiCrAlY alloys that are commonly used as bond coats for thermal barrier coatings (TBCs) has been directly measured. The relaxation study was conducted at temperatures of 800-1000-degrees-C and over a wide range of stresses. It was established that all three bond coat alloys relaxed quite rapidly at temperatures of 900-degrees-C and above. Since the upper use temperatures for bond coats in gas turbine engines are between 900 and 1000-degrees-C, bond coat relaxation is expected to occur in service. Therefore, relaxation of the bond coat has the potential to affect TBC life. Furthermore, the relaxation differences observed between the three alloys offers a possible explanation for the differences in TBC life observed for these bond coats. While bond coat relaxation is expected to occur for a TBC in service, the mechanism for a relaxation effect on TBC life, if it exists, is yet to be determined.