Sliding wear and fretting wear of diamondlike carbon-based, functionally graded nanocomposite coatings

Improving the tribological functionality of diamondlike carbon (DLC) films-developing good wear resistance, low friction, and high load-carrying capacity-was the aim of this investigation. Nanocomposite coatings consisting of an amorphous DLC (a-DLC) top layer and a functionally graded titanium–titanium carbide–diamondlike carbon (Ti–Ti x C y –DLC) underlayer were produced on AISI 440C stainless steel substrates by the hybrid technique of magnetron sputtering and pulsed-laser deposition. The resultant DLC films were characterized by Raman spectroscopy, scanning electron microscopy, and surface profilometry. Two types of wear experiment were conducted in this investigation: sliding friction experiments and fretting wear experiments. Unidirectional ball-on-disk sliding friction experiments were conducted to examine the wear behavior of an a-DLC/Ti–Ti x C y –DLC-coated AISI 440C stainless steel disk in sliding contact with a 6-mm diameter AISI 440C stainless steel ball in ultrahigh vacuum, in dry nitrogen, and in humid air. Although the wear rates for both the coating and ball were low in all three environments, the humid air and dry nitrogen caused mild wear with burnishing in the a-DLC top layer, and the ultrahigh vacuum caused relatively severe wear with brittle fracture in both the a-DLC top layer and the Ti–Ti x C y –DLC underlayer. For reference, amorphous hydrogenated carbon (H-DLC) films produced on a-DLC/Ti–Ti x C y –DLC nanocomposite coatings by using an ion beam were also examined in the same manner. The H-DLC films markedly reduced friction even in ultrahigh vacuum without sacrificing wear resistibility. The H-DLC films behaved much like the a-DLC/Ti–Ti x C y –DLC nanocomposite coating in dry nitrogen and humid air, presenting low friction and low wear. Fretting wear experiments were conducted in humid air (approximately 50% relative humidity) at a frequency of 80 Hz and an amplitude of 75 μm on an a-DLC/Ti–Ti x C y –DLC-coated AISI 440C disk and on a titanium–6 wt.% aluminum–4 wt.% vanadium (Ti–6Al–4V) flat, both in contact with a 9.4-mm diameter, hemispherical Ti–6Al–4V pin. The resistance to fretting wear and damage of the a-DLC/Ti–6Al–4V materials pair was superior to that of the Ti–6Al–4V/Ti–6Al–4V materials pair.