Tribology of diamond-like carbon sliding against itself, silicon nitride, and steel
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Diamond-like carbon (DLC) films were deposited on (100) silicon wafers and silicon nitride balls by RF plasma-assisted chemical vapor deposition at a pressure of 700 mTorr and a substrate temperature of 360 K. The friction coefficient and the wear rates were measured using a pin-on-disk tribometer in 40% humid and dry air. Friction coefficients are near 0.05 in all cases measured. In dry air, the wear of silicon nitride and steel against DLC is below measurement capability because of a protecting DLC transfer layer, and wear of DLC is 2.5 • 10" 8 mm 3 /Nm against silicon nitride and 6.5 • 10" 9 mm 3 /Nm against steel. In humid air, the DLC transfer layer does not adhere to the solids, and wear of both bodies is larger. Unmeasurable wear is obtained when DLC slides against itself in humid air; the wear rate is 5 • 10" 9 mm 3 /Nm in dry air. These results are interpreted in terms of the properties of a friction-induced transformation of the surface layer of DLC.
I. INTRODUCTION Diamond-like carbon (DLC) or amorphous carbon films can be readily prepared by various techniques such as plasma-assisted chemical vapor deposition, ion-beam sputtering, or cathodic arc evaporation. They exhibit high mechanical hardness, chemical inertness, high electrical resistivity, and abrasion resistance; their low friction coefficient and high wear resistance are attractive for application in magnetic recording devices. From earlier studies1"10 of the tribological behavior of DLC films the following picture emerges. Friction and wear of DLC films are low; they depend more on the environment than on the nature of the material against which they rub. The friction is extremely low (yu. = 0.02) in dry nitrogen, about 0.1 in humid environments, and quite high (/u = 0.6) in oxygen.1"5 The wear of DLC varies even more with environment, but is generally lower than that of other unlubricated systems.4-11 The surprising feature for such hard materials is that a transfer layer of DLC is often observed on the surface of the much softer steel.6 The formation of a friction-induced transformation of the surface of DLC has been proposed by Enke1 and Myioshi,5 a form of graphitization by the former and a tribochemical reaction by the latter. The formation of this layer is not instantaneous; it requires a certain amount of rubbing and its effects are reversed after friction ceases.2'5 This behavior of DLC raises a number of interesting questions: since a layer is transferred on steel, it is possible that materials rubbing against DLC wear less than the latter even though they are softer and their wear can depend on the environment as well. No information J. Mater. Res., Vol. 10, No. 6, Jun 1995 http://journals.cambridge.org
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has been published to date on this point. In addition, the tribological behavior of DLC sliding against itself is difficult to predict; it could be relatively high because of the loss of the transformed layer; generally also, friction is high when both bodies are made of the same material, but could be low in
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