Coating Pre-Cracking Effect in Combined Cycle Fatigue Tests of Superalloys For Gas Turbine Blades

In gas turbine engines for aerospace propulsion, the application of coatings on HP and LP stage blading where the highest temperatures are experienced is a common practice to prevent environmental degradation. However, since the strength of the coating is lower than that of the substrate material, upon loading the static strength of the coating may be exceeded and coating cracking may occur. In order to assess the effect of cracking in the coating on polycrystalline nickel superalloy MAR-M002, a number of combined cycle fatigue (CCF) and low cycle fatigue (LCF) tests with and without dwell have been carried out, at temperatures up to 870 °C. In order to experimentally assess the potential detrimental effect of coating cracking, controlled cracking in the coating prior to fatigue testing has been generated by using a special procedure. CCF tests have carried out by superimposing to strain controlled zero to maximum LCF cycles with dwell time stress controlled smaller HCF cycles, simulating the high loading ratio vibrations occurring in the blades. The loading mode applied in the CCF tests, even if much simpler than effective service conditions, is sufficiently representative of the loading experienced by the materials in correspondence of critical geometrical features of the turbine blades, where HCF amplitudes due to blade vibrations are superimposed to major (ground-air-ground) LCF cycles occurring during the regular service of the gas turbine engines. Comparison of the CCF and of the LCF tests with dwell with conventional LCF tests is presented herein, with special consideration of the effect of coating cracking.