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 capac ity-was the aim of this investigation. Nanocomposite coatings consisting of an amorphous DLC (a-DLC) top layer and a functionally graded titanium-tita nium carbide-diamondlike carbon (Ti-TixCy-DLC) underlayer were produced on AISI 440C stainless steel substrates by the hybrid technique of magnetron s puttering and pulsed-laser deposition. The resultant DLC films were charact erized by Raman spectroscopy, scanning electron microscopy, and surface pro filometry. Two types of wear experiment were conducted in this investigatio n: sliding friction experiments and fretting wear experiments. Unidirection al ball-on-disk sliding friction experiments were conducted to examine the wear behavior of an a-DLC/Ti-TixCy-DLC-coated AISI 440C stainless steel dis k 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 rat es for both the coating and ball were low in all three environments, the hu mid 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-TixCy-DLC underlayer. For r eference, amorphous hydrogenated carbon (H-DLC) films produced on a-DLC/Ti- TixCy-DLC nanocomposite coatings by using an ion beam were also examined in the same manner. The H-DLC films markedly reduced friction even in ultrahi gh vacuum without sacrificing wear resistibility. The H-DLC films behaved m uch like the a-DLC/Ti-TixCy-DLC nanocomposite coating in dry nitrogen and h umid air, presenting low friction and low wear. Fretting wear experiments w ere conducted in humid air (approximately 50% relative humidity) at a frequ ency of 80 Hz and an amplitude of 75 mu m on an a-DLC/Ti-TixCy-DLC-coated A ISI 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 materia ls pair was superior to that of the Ti-6Al-4V/Ti-6Al-4V materials pair. (C) 1999 Published by Elsevier Science S.A.