Spark plasma sintering of a multilayer thermal barrier coating on Inconel 738 superalloy: Microstructural development and hot corrosion behavior

In the present work, spark plasma sintering (SPS) process was employed to prepare a nanostructured yttria-stabilized zirconia (8YSZ) coating on a nickel-based superalloy (INCONEL 738) with functionally graded structure. A stack layer of INCONEL 738/NiCrAlY powder/Al foil/NiCrAlY+YSZ powder/YSZ powder was SPSed in a graphite die at an applied pressure of 40 MPa under an vacuum atmosphere (8 Pa). The sintering temperature was ~1040 °C. For comparison purpose, the air plasma spray (APS) technique was employed to prepare the thermal barrier coating (TBC). Microstructural studies by scanning electron microscopy showed that the SPSed coating was sound and free of interfacial cracks and large pores. Energy dispersive X-ray spectroscopy determined a limited inter-diffusion between the layers. A significant improvement (~26%) in the Vickers micro-hardness was also measured. In order to examine the functionality of the coatings, hot corrosion test was performed at 1050 °C in a molten bath of Na2SO4 and V2O5 (45 to 55 weight ratio). The results showed that spallation occurred after 12 and 20 h for the SPSed and APS coatings, respectively.