Surface oxidation of Ni20Cr/Cr3O2 composite processed by ball milling and HVOF thermal spraying

A Ni20Cr/Cr2O3 composite powder was successfully processed by controlling different mechanical milling conditions. The powder was heat treated at various temperatures to evaluate the effects of fine Cr2O3 particles additions on the microstructure of the Ni20Cr matrix. Thermogravimetric analysis indicated an earlier oxidation temperature of the composite powder (250-270 °C) compared to the oxidation temperature of the Ni20Cr matrix (680-700 °C), the fine Cr2O3 particles may act as nucleation sites to promote matrix oxidation at lower temperatures. Ni20Cr/Cr2O3, commercial, and experimental Ni20Cr powders were used as feedstock to produce coatings by HVOF thermal spraying. Raman studies showed NiCr2O4 and Cr2O3 as main phases in the microstructure of Ni20Cr/Cr2O3 composite coating. A residual Ni20Cr phase, processed by ball milling, was preserved in the coatings microstructure; such a phase improved oxygen and chromium diffusion, then allowing the formation of a thick oxide protective layer in the coatings during oxidation tests. Oxide scales produced during sliding wear test were strongly anchored to the Ni20Cr/Cr2O3 coating surface, a wear rate of 20.2 × 10−6 mm3/N·m was measured when such a coating was tested employing a pin-on-disc configuration.