Precipitation phenomenon in nanostructured Cr3C2-NiCr coatings

Precipitation in the nanostructured Cr3C2/NiCr coatings was investigated. U ltrafine 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 oxid e particles. the phase transformations in the original NiCr amorphous phase . which was always observed in the as-sprayed nanostructured Cr3C2/NiCr coa tings, were also identified. Internal oxidation was thought to be responsib le for the precipitation of the dispersed oxide particles. The results of m icrohardness and scratch-resistance tests showed that microhardness of the conventional coating slightly increased only with an increase in the exposu re temperature, while that of the nanostructured coating increased signific antly from 1020 to 1240 HV300. Compared with the conventional Cr3C2/NiCr co atings, the scratch-resistance and coefficient of friction were found to be increased and reduced respectively in the nanostructured coatings. Heat tr eatment 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 fricti on of the nanostructured coatings were attributed to a high density of oxid e nanoparticles precipitating within the coating as the exposure temperatur e increased. (C) 2001 Elsevier Science B.V. All rights reserved.