Tribological properties and wear mechanism of sputtered C/Cr coating

Abstract

Carbon/chromium (C/Cr) coatings were co-deposited using unbalanced magnetron sputtering deposition technique. The tribological properties including wear mechanism and microstructures of the coatings were investigated using pin-on-disc tribometer, X-ray diffraction (XRD) and transmission electron microscope (TEM). The coatings were characterised by a high hardness (∼2200 HV), a low coefficient of friction (<0.1) and a very low wear rate under very high loads. The pin-on-disc tests of C/Cr coated tool steel against WC ball (5 mm of diameter) revealed low friction coefficient 0.06–0.1 depending on load and low wear rate ∼10−17 m3/Nm with little dependence on load and sliding speed for the test using load up to 80 N and siding speed up to 400 mm/s for 1 h. Critical sliding speed was found above which, the wear rate was higher. The pin-on-disc tests of C/Cr coated austenitic 316 stainless steel against WC ball under a load of 10 N at siding speed of 400 mm/s for 1 h revealed a low friction coefficient ∼0.1 and a very low wear beyond the limitation of the as used wear measurement technique could detect. Microstructure of the coated stainless steel was investigated. XRD profiles of the coating exhibited typical patterns for amorphous-like material, with no crystal-like peaks identified. Cross-sectional TEM analysis of the rubbed tracks of the coated stainless steel revealed some deformation in the substrate near the interface. However, no sign of interfacial spallation was observed, indicative of excellent adhesion of the coating to the substrate. The selected area electron diffraction patterns taken from the rubbed and unrubbed regions of the coating were interpreted and discussed. The tribological performance of this novel C/Cr coating is believed to be closely related to its graphite nature. The high resolution TEM (HRTEM) cross-section analysis of the coating on stainless steel substrate revealed that the outer-most surface of the rubbed track was reoriented due to the rubbing in a 1-h pin-on-disc test under a load of 10 N. Finally, the relationship between deposition parameters and tribological performance of the coating is also discussed in this paper.