ABSTRACT
Cracking of protective coatings is one of the most common types of aircraft engine blade surface damage. Investigations and calculations have shown that crack initition in the coating is caused by thermal stresses in the system of ‘coating-superalloy’.1 These thermal stresses are caused by the difference in the temperature expansion coefficients and modules of elasticity of the coating and the superalloy.
Coated turbine blade design techniques1–4 allow the calculation of thermal stresses in the blade’s surface layers and the coating service life under the thermocyclic loading. However, to predict the coated blade’s service life it is necessary to compare the results of the calculations and the coating thermal fatigue test data.
This paper presents the results of thermo-mechanical fatigue tests of the specimens made with a CMSX-4 superalloy substrate and LCO-22 coating under conditions of elastic deformation of the substrate. Special load-adjusted thermo-mechanical fatigue-test techniques were developed for this purpose.
The suggested test method has shown that the coating service life is considerably shorter than that of the uncoated alloy under conditions which result in high temperature oxidation.
