Abstract
Low friction C/Cr coatings have been successfully deposited by the combined steered cathodic arc/unbalanced magnetron sputtering technique. The present paper focuses on the properties, tribological performance and microstructure evolution of C/Cr coatings as a function of the bias voltage Ub ranging from −65 to −350 V. As the bias voltage increases from −65 to −95 V, the structure changes from columnar to dense structure which comprises of randomly distributed onionlike carbon clusters. Further increase in the bias voltage to −350 V led to segregation and self-organisation of the carbon atoms induced by the high energy ion bombardment and finally to the formation of a new type self-organised multilayer structure. On the other hand, the phase composition transforms from graphitic (sp2 C–C bonded) to Me carbon (Cr–C bonded), where the content of the carbide phase increases with increasing the bias voltage. The C/Cr coatings showed an excellent adhesion (Lc>70 N), with hardness ranging from 8·23 to 25 GPa depending on the bias voltage. Pin on disc tests showed that the friction coefficient was reduced from 0·22 to 0·16 when the Ub was increased from −65 to −95 V. A strong correlation was found between the microstructure, the residual stress, the sp2/sp3 content and the coating friction behaviour. Dry high speed milling trials on automotive aluminium alloy (Al–Si8Cu3Fe) showed that C/Cr coated cemented carbide end mills enhance the tool performance and the tool life compared with the uncoated tools by a factor of 2, suggesting the potential for dry high speed machining of ‘sticky’ alloys.
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