The effect of Si content and substrate bias on structural and tribological properties of (Cr, Si)N coatings under water lubrication

Abstract

(Cr 1-x Si x )N coatings were deposited by a multi-cathode UBMS (unbalanced magnetron sputtering system) using Cr and Si targets on stainless steel (17-4 PH) substrates. Effect of Si content and substrate bias on the structural and tribological properties was investigated. X-ray diffraction analysis revealed that the coating showed a structural change from crystalline (rock-salt structure) to amorphous structure as the Si content was increased. The effect of the substrate bias was investigated for (Cr 1-x Si x )N coating with x=0.8. From the cross-sectional TEM analysis, it was confirmed that a compositional modulation was manifested as the substrate bias was increased. Tribological evaluations of the (Cr,1-xSix)N coatings were conducted in distilled water using a ballon-disk tribometer. The friction coefficient decreased drastically down to 0.01 as the Si content was increased more than approximately 0.6. The specific wear rate also decreased by a factor of ten from 4 x 10 -7 mm 3 N -1 m -1 to 5 x 10 -8 mm 3 N -1 . A thin (less than 10 nm) amorphous Si-Cr oxide or hydro-oxide layer was confirmed using TEM analysis after the tribological test that was concluded to be a tribo-chemically formed boundary lubrication film. In case of the coatings deposited under different substrate bias, lowest friction coefficient and specific wear rate was obtained in a bias range between 50to 100V.