Precipitation phenomenon in nanostructured Cr3C2-NiCr coatings

Precipitation in the nanostructured Cr3C2/NiCr coatings was investigated. Ultrafine Cr2O3 particles with an average size of 8.3 nm were observed using transmission electron microscopy in the nanostructured Cr3C2/NiCr coatings exposed to elevated temperatures. In addition to the precipitation of oxide particles. the phase transformations in the original NiCr amorphous phase. which was always observed in the as-sprayed nanostructured Cr3C2/NiCr coatings, were also identified. Internal oxidation was thought to be responsible for the precipitation of the dispersed oxide particles. The results of microhardness and scratch-resistance tests showed that microhardness of the conventional coating slightly increased only with an increase in the exposure temperature, while that of the nanostructured coating increased significantly from 1020 to 1240 HV300. Compared with the conventional Cr3C2/NiCr coatings, the scratch-resistance and coefficient of friction were found to be increased and reduced respectively in the nanostructured coatings. Heat treatment led to further increase in scratch-resistance and further decrease in coefficient of friction of the nanostructured coatings. The increases in microhardness and scratch-resistance and decrease in coefficient of friction of the nanostructured coatings were attributed to a high density of oxide nanoparticles precipitating within the coating as the exposure temperature increased. (C) 2001 Elsevier Science B.V. All rights reserved.