Crystal structure identification results using XRD, TEM and Raman
spectroscopy clearly indicate that the TC affects the crystal structure
and the wear behavior. The crystal structure of Cr–C–N coatings can
be clearly separated into two categories. The Cr–C–N coatings with a
TC less than 2 A exhibit an amorphous DLC structure due to the higher
target poising rate, while the Cr–C–N coatings with a TC greater than 3
A exhibit a crystalline structure. The Cr–C–N coatings with an amorphous DLC structure exhibit higher C content, lower friction coefficient
and higher adhesion strength, while the Cr–C–N coatings with a crystalline structure exhibit lower C content, a higher friction coefficient and
lower adhesion strength.
The low friction mechanisms of DLC films was attributed to the
graphitization of the top layer during wear testing and led to the formation of a thin graphite-like interface layer or transfer film which acts as a
solid lubricant layer [18]. With lower friction and higher adhesion
strength, the Cr–C–N coatings with an amorphous DLC structure exhibit
a lower wear rate.
The Cr–C–N coatings with a crystalline structure exhibit a higher friction coefficient during wear testing and also exhibit a lower adhesion
strength of 50–56 N. Although the Cr–C–N coatings with a TC greater
than 3 A possess higher hardness, they exhibit a higher wear rate due
to poor adhesion and a higher friction coefficient.