Microstructure formation in thermally-sprayed duplex and functionally graded NiCrAlY/Yttria-Stabilized Zirconia coatings

NiCrAlY bond coats were deposited on a number of Inconel-738LC specimens using HVOF spray technique. For duplex coating, a group of these specimens were coated with Yttria Stabilized Zirconia (YSZ) using plasma spray technique. Functionally graded NiCrAlY/YSZ coatings were fabricated by plasma spray using co-injection of the two different powders in a single plasma torch. The amount of Zirconia in functionally graded coatings were gradually increased from 30 to 100 vol.%. Duplex and functionally graded coatings were then characterized using optical microscope, Scanning Electron Microscopy (SEM), Energy Dispersive x-ray Spectrometry (EDS), map analysis and X-ray Diffraction (XRD). The strength of the Adhesive coatings of the substrate was also measured.The results show that microstructure, porosity and compositions are gradually varied in the functionally graded coatings. EDS analyses revealed that oxidation of aluminum, chromium and yttrium in the NiCrAlY alloy are occur in the high-temperature plasma-spray stream during deposition. The oxidized products, mixed with zirconia at high temperature in a wide composition range, produce ceramic composites and increase the cohesion strength between the layers. The results also show a better performance in as-sprayed functionally graded coatings comparing with duplex coatings, especially regard to the adhesion strength of the coatings.