Thermal barrier coating made of porous zirconium oxide on a nickel-based single crystal superalloy formed by plasma electrolytic oxidation

A thermal barrier coating (TBC) made of porous ZrO2 ceramic was formed on a nickel-based single crystal superalloy (NSA) by plasma electrolytic oxidation (PEO). An oxidation barrier coating (OBC) made of Al2O3 based ceramics was created between the TBC and NSA substrate by coating the NSA with aluminum before the PEO treatment that oxidized it into Al2O3. The key finding of this study is that ZrO2-based TBC can be formed by PEO of an OBC-coated NSA in a K2[Zr(CO3)2(OH)2] solution. The TBC contained monoclinic-, tetragonal- and cubic-ZrO2 crystals and had a unique gradient porous texture. Relatively large pores existed near the interface between OBC and TBC, and the number and size of pores in the TBC decreased with increasing distance from the interface. The TBC had ZrO2 grains with a diameter of about 300 nm. The TBC shows a low thermal conductivity equivalent to that of conventional TBCs owing to this porous texture with fine grains. The bond strength between the PEO coating and substrate was 26.8 ± 6.6 MPa, which is close to that of conventional TBCs.