Magnetron sputtered hard a-C coatings of very high toughness

Hydrogen-free amorphous carbon coatings of high hardness (≈30 GPa) and toughness (plasticity from 50 to 60%) were deposited on 440C steel substrates by DC magnetron sputtering at target power density of 10.5 W/cm2 in the bias range from −20 to −150 V. The surface topography, hardness and tribological behavior of the coatings were investigated. With the increase of bias voltage, coating hardness and surface smoothness increased at expense of some adhesion strength and an increase of coefficient of friction. All coatings showed low friction in humid air and graphitization was observed after a high number of rotation cycles. The graphitization adds more benefit aside from reducing friction: the graphite layer can considerably reduce the adhesive wear since it prevents the asperities of the two surfaces to be adhered to each other.