Effect of MCrAlY coating on crack formation during thermal fatigue testing of a directionally solidified Nickle based superalloy

Nickle based superalloys are widely used in gas turbine components such as blades due to their excellent mechanical properties at high temperatures. These blades are subjected to extensive temperature fluctuations which induce thermal fatigue damage. In this research, thermal fatigue properties and behavior of directionally solidified Nickel based superalloy was investigated.

Thermal fatigue specimens were coated with CoNiCrAlY powder using the HVOF method. Test temperature fluctuated between minimum of 400 and maximum of 800°C such that each cycle lasted for 1h. All the samples were weighted before and after the test with the precision of 10-4g. X-ray diffraction method was used to identify oxide phases. Scanning electron microscopy was also used to study microstructure of all the samples. Elemental analysis of the microstructure was carried out by energy dispersive spectroscopy.

It was found out that oxidation was the main reason of crack formation while thermal fatigue testing. The weight change of the coated sample is 100 times less than the bare. The cracks in the bare alloy formed from the surface mainly at the interface of Carbide particles and the γ matrix, while crack formation at coating/substrate interface was found to be the preferential location in the coated samples.