Oxidation Mechanism of YSZ/NiCr Coating Prepared by Hollow Cathode Glow Discharge Phenomenon and Multi-arc Ion Plating

Herein, NiCr was coated on γ-TiAl via the hollow cathode glow discharge, whereas yttria stabilized zirconia (YSZ) was coated on the NiCr by multi-arc ion plating. A strong bonding between the two coatings and substrate, i.e., NiCr, YSZ, and γ-TiAl occurred. However, the surface of YSZ coating displayed macroparticles of different sizes, which affected the oxidation resistance. High-temperature oxidation behavior of YSZ/NiCr coating was investigated at 850, 950, and 1050 °C. The results showed that the oxidation kinetics of YSZ/NiCr coating followed the parabolic law. Moreover, the YSZ coating did not showed a phase transition, consisting of t-ZrO2 and c-ZrO2. After 100 h of oxidation at 850 and 950 °C, macrocracks appeared on the surface of macroparticles. Yet, a thermally grown TGO oxide layer of Cr2O3 with a thickness of 2-4 μm effectively inhibited the oxidation of YSZ/NiCr coating. However, when the temperature increased to 1050 °C, the number and size of microcracks on the macroparticles surface increased and reached the YSZ coating surface from different directions, resulting in a high degree of oxidation. The oxidation kinetics and oxide scales formed during the test indicated that the YSZ/NiCr coating effectively improved the oxidation resistance of γ-TiAl at 850, 950, and 1050 °C. However, since the microcracks formed on the macroparticles caused a severe oxidation, the effect of the macroparticles on the oxidation of YSZ/NiCr coating at high temperature could not be ignored.