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

Abstract A Ni20Cr/Cr 2 O 3 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 Cr 2 O 3 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 Cr 2 O 3 particles may act as nucleation sites to promote matrix oxidation at lower temperatures. Ni20Cr/Cr 2 O 3 , commercial, and experimental Ni20Cr powders were used as feedstock to produce coatings by HVOF thermal spraying. Raman studies showed NiCr 2 O 4 and Cr 2 O 3 as main phases in the microstructure of Ni20Cr/Cr 2 O 3 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/Cr 2 O 3 coating surface, a wear rate of 20.2 × 10 −6  mm 3 /N·m was measured when such a coating was tested employing a pin-on-disc configuration.