Effect of the composition profile and density of LPPS sprayed functionally graded coating on the thermal shock resistance

Low Pressure Plasma Spraying (LPPS) is a promising coating method for Functionally Graded Material (FGM) expected to be able to reduce the thermal stress in high temperature environments such as a gas turbine. In this paper, we report the effect of the composition profile and coating density of LPPS sprayed FGM, consisting of ZrO2–8 wt%Y2O3 (YSZ) top coating, YSZ–Ni–20 wt%Cr (NiCr) FGM coating, NiCr under coating and copper substrate, on the thermal shock resistance evaluated by a modified temperature difference test. The density of YSZ and NiCr coating was successfully controlled by the chamber pressure and initial particle size in the range from 5.43 to 5.79 g/cm3 and from 7.89 to 8.09 g/cm3, respectively. For an YSZ composition profile from NiCr under coating to YSZ top coating (in FGM), the highest thermal shock resistance was obtained when the fraction of YSZ increased with gentle slope just over NiCr coating and acute slope just under YSZ coating. Also, the higher density coatings tended to perform the higher thermal shock resistance. Initial cracks formed in the YSZ top coating propagated into YSZ parts in FGM coating through the grain boundary of YSZ and/or the interface between flattened NiCr and YSZ particles. After the cracks connected, the coupled cracks caused the coating spallation.