Effect of Corrosion-Resistant Coatings on Fatigue Properties of Gas Turbine Blade Materials

Isothermal low-cycle fatigue tests and thermal-mechanical fatigue tests with linear out-of phase cycles were carried out using cylindrical specimens made of Ni-base superalloy and coated with several kinds of corrosion-resistant coatings. In some tests, fatigue loading was interrupted several times and coating cracks were observed by means of replica technique. As a result, three types of coating cracks were observed, namely, multiple parallel long cracks, multiple parallel short cracks and non-parallel cracks. Multiple parallel long cracks are caused from lack of ductility of coating materials. This type of coating cracks initiate in an early stage of fatigue loading and has a strong effect to reduce fatigue life. Multiple parallel short cracks are caused by fatigue of coating layer. These cracks propagate into the substrate before cracks initiate in the substrate and reduce fatigue life of the coated specimen. Non-parallel coating cracks are caused by propagation of cracks initiated in the substrate and have little effect to reduce fatigue life.