Abstract
The purpose of this study is to clarify the effect of CoCrAlY coating on biaxial thermo-mechanical fatigue (TMF) life property of lnconel 738LC. In-phase and out-of phase TMF tests at temperature between 450°C and 850°C were carried out by using a tension/torsion and compression TMF testing machine. In addition to these fundamental TMF tests, a complicated temperature and strain history tests, so called a blade waveform which simulates temperature and strain histories of actual gas turbine blade, were performed. Failure life of coated specimens under the in-phase condition is longer than that of the substrate and the main crack propagated from boundary between the coating and substrate to inside of the substrate. Therefore the coating plays a positive role for the substrate life due to suppression of crack initiation and propagation in the coating. On the other hand, failure life of the coated material under out-of-phase and blade waveform tests is approximately 1/2 of the substrate life indicating negative effect. It was suggested from failure surface observation that crack initiation period in the coating is shorten by the difference of deformation behavior between the substrate and coating, and that crack propagation rate in the substrate of the coated specimen is accelerated by the cracking of the coating compared with the crack propagation rate of substrate itself.
