Structural analysis of (Cr1−xSix)N coatings and tribological property in water environment

A series of (Cr1−xSix)N coatings were deposited by a multicathode unbalanced magnetron sputtering system (UBMS) using Cr and Si targets under Ar/N2 atmosphere on stainless steel (AISI630). Si content x was varied from 0 to approximately 0.8 by changing the input power to Si target. X-ray diffraction (XRD) analysis revealed that the coating underwent a continuous structural change from crystalline (NaCl structure) to amorphous structure as x was increased. Average grain size, which was derived from the XRD measurements using the Scherrer's equation, decreased from 17 to 2 nm by increasing the Si amount x from 0 to 0.45. Hardness of (Cr1−xSix)N coating increased slightly from 15 to 17 GPa by increasing the Si content x from 0 to 0.3. Tribological evaluations using a ball-on-disk tribometer of (Cr,Si)N-coated substrates were conducted in distilled water against the (Cr,Si)N coating itself. The friction coefficient decreased drastically nearly down to zero as x was increased more than ca. 0.6. The specific wear rate was also decreased by a factor of 10 from 4×10−7 to 5×10−8 mm3 N−1 m−1. A thin (approximately 10 nm) amorphous Si-Cr oxide layer was formed on the surface of worn section that was estimated to be formed by the tribochemical reaction during the tribological test. It was concluded that this tribochemically formed amorphous oxide layer was responsible for the ultralow friction behavior.