ABSTRACT
In the present work, coated IN738 LC was tested in thermo-mechanical fatigue (TMF) in-phase (IP) between 400 and 850°C as well as low cycle fatigue (LCF) at 850°C. The coating was of a plasma sprayed NiCoCrAlY type, PWA 1365–2. In addition LCF testing was performed on polished and heat treated specimens. When comparing the results for the two testing types it was found that the LCF tests had a longer fatigue life with a factor of two or more compared to the TMF tests. Polishing of the coating resulted in a minor increase in fatigue strength compared to as sprayed tests and the heat treated material had a lifetime similar to the polished specimens. Metallography after testing revealed distinct differences in crack initiation and propagation behaviour between LCF and TMF specimens. In the LCF specimens all cracks initiated in the coating surface and, probably because of the slow propagation rate, many cracks could initiate. TMF specimens exhibited fewer cracks in the coating, instead there was evidence of cracks initiating between the coating and the base metal in one case. These cracks initiated at defects in the interdiffusion zone and could propagate into the substrate alloy with the coating intact. In the same specimen there were internal intergranular cracks in the substrate indicating some creep damage as well. When comparing the results with data on uncoated IN738 LC from earlier work, it was found that the influence from the different cycle types was much larger for the coated material.
