Influence of ion bombardment on structure and tribological performance of nanoscale multilayer C/Cr PVD coatings

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 U-b 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 (sp(2) 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 (L-c > 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 U-b was increased from -65 to -95 V. A strong correlation was found between the microstructure, the residual stress, the sp(2)/sp(3) content and the coating friction behaviour. Dry high speed milling trials on automotive aluminium alloy (Al-Si8Cu(3)Fe) 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