Wear of hydrogen free C/Cr PVD coating against Al2O3 at room temperature

Nanoscale H-free C/Cr coatings have been produced by unbalanced magnetron (UBM) sputtering with high ion-irradiation. The coating was a nanocomposite composed of nanoscale C clusters in a chromium carbide crystalline matrix. Reciprocating wear testing of the coating on stainless steel substrates was undertaken against an Al2O3 ball counterface (4 mm) at 5 Hz and 1 N load. The friction coefficient went through three stages from a low friction of 0.10 to a relatively high friction of 0.24. Plan-view worn surfaces were investigated using Raman spectroscopy and scanning electron microscopy. Roll-shaped wear debris were present perpendicular to the sliding direction. The rolls contained graphitic carbon and Cr2O3. Cross-sections of the worn surfaces were examined using transmission electron microscopy (TEM) of focused ion beam derived specimens, which showed that oxidational wear was important. The composition of the rolls and tribofilm is correlated with the Raman spectroscopy and SEM investigations. A tentative mechanism of the origin and evolution of the wear debris is proposed.