Microstructural characterisation of carbon doped CrAlTiN nanoscale multilayer coatings

Carbon doped CrAlTiN coatings for reducing the coefficient of friction sliding against a WC-Co ball in air were developed using a closed field unbalanced magnetron sputtering system (CFUMS). The carbon content was controlled by increasing the current of the carbon target power from 0 A to 5 A. The surface morphology, cross-sectional microstructure, phase constituent, chemical bonding energy and mechanical properties of coatings were characterised by means of XRD, SEM, XTEM, XPS, and nanoindentation. The results showed that the hardness, Youngʹs modulus and coefficient of friction (CoF) of the coatings sliding against a WC-Co ball in air decreased with increasing carbon content. Microstructure characterisation revealed that the carbon doped coatings consisted of four sublayers from the substrate to the surface and the main phase formed is fcc B1-NaCl like (Cr, Al, Ti) N and (Cr, Al, Ti) (C, N) phase in sublayers III and IV respectively when the carbon content is below 3.96 at.%. When the carbon concentration exceeds this value, the excessive carbon will begin to form the amorphous carbon (or carbon-riched) phase which leads to a decrease in the coefficient of friction (CoF) of the coatings sliding against a WC-Co ball in air.