Assessment of Fatigue Interaction in Coated Superalloys

Abstract Coatings are commonly used in industrial gas turbines to protect materials from oxidation when operating at high temperatures. However, coatings can be ignored when assessing the lives of critical components. This study explores the influence of coatings on the fatigue lives of CMSX-4 single-crystal superalloy material via experiments and analysis. Experimental results show that coatings significantly reduce the fatigue life of the superalloy base material. Scanning electron microscopy of the failed specimens indicates that cracks initiate in the coating and propagate into the superalloy base material, leading to premature fracture. Analysis results show that coating stresses are maximized at high temperatures and under compressive loading due to the dissimilar thermal expansion of the coating and the superalloy base material. Therefore, operation under these conditions leads to a higher likelihood of coating cracks. Analysis indicates notable stress concentrations occur in the base material in the region of coating cracks. The Langer fatigue model is modified to consider the influence of coating cracks on fatigue life. The modified Langer model shows a general improvement in accuracy when predicting the fatigue lives of coated CMSX-4 material. These findings indicate that the influence of coatings should be considered when lifing critical gas turbine industrial components.