Thermo-mechanical Degradation of NiCr Thin Film Subjected to Thermal Cycling

The thermo-mechanical degradation of NiCr films during thermal cycling was investigated in terms of microstructural and chemical changes to the surface. Thermal cycling was performed between room temperature and 200 degrees C in thermally evaporated NiCr films on an Mn-Ni-Co-O substrate. Isothermal exposures were also performed as a control experiment. The sheet resistance of the NiCr films increased linearly (from 35 to 52 Omega/sq.) with a temperature difference of the thermal cycling, whereas the films subjected to isothermal aging increased only slightly, from 35 to 38 Omega/sq. After 300 thermal cycles at Delta T=175 degrees C, cracks formed and proliferated on the coating surface, and local delamination occurred at the interfaces. XPS results showed that the intensity of a metallic peak at 574.0 eV decreased and that of the Cr oxide peak at similar to 577 eV increased with an increase of the thermal cycles. Consequently, the degradation of the NiCr films by accelerated thermal cycling stress was attributed to a proliferation of lateral surface cracks from thermally induced tensile stress compounded by surface oxidation, causing an increase in sheet resistance.