Superlubricity and Wearless Sliding in Diamondlike Carbon Films
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Superlubricity and Wearless Sliding in Diamondlike Carbon Films Ali Erdemir Energy Technology Division Argonne National Laboratory Argonne, IL 60439
ABSTRACT Diamondlike carbon (DLC) films have attracted great interest in recent years mainly because of their unusual optical, electrical, mechanical, and tribological properties. Such properties are currently being exploited for a wide range of engineering applications including magnetic hard disks, gears, sliding and roller bearings, scratch resistant glasses, biomedical implants, etc. Systematic studies on carbon-based materials in our laboratory have led to the development of a new class of amorphous DLC films that provide extremely low friction and wear coefficients of 0.001 to 0.005 and 10-11 to 10-10 mm3/N.m, respectively, when tested in inert-gas or high-vacuum environments. These films were produced in highly hydrogenated gas discharge plasmas by a plasma enhanced chemical vapor deposition process at room temperature. The carbon source gases used in the deposition of these films included methane, acetylene, and ethylene. Tribological studies in our laboratory have established a very close correlation between the composition of the plasmas and the friction and wear coefficients of the resultant DLC films. Specifically, the friction and wear coefficients of DLC films grown in plasmas with higher hydrogen-tocarbon ratios were much lower than films derived from source gases with lower hydrogento-carbon ratios. Fundamental tribological and surface analytical studies have led us to conclude that hydrogen (within the film, as well as on the sliding surfaces) is extremely important for the superlubricity and wearless sliding behavior of these films. Based on these studies, a mechanistic model is proposed to explain the superlow friction and wear properties of the new DLC films.
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INTRODUCTION Tribologically, carbon-based materials and coatings (such as diamond, diamondlike carbon [DLC], graphite, graphite fluoride, glassy carbon, carbon-carbon or carbon-graphite composites) have been playing very important roles in combating or controlling friction and wear between sliding, rolling, or rotating tribo-components of a wide range of moving mechanical assemblies. Some of them (such as graphite, diamond, and DLC) can provide very low friction and wear coefficients when applied as thin films on tribological surfaces, while others (i.e., certain carbon-carbon composites) can be tailored to attain very high friction and thus be used as brake materials [1,2]. Graphite has been available and used as a solid lubricant in numerous tribological applications for a long time [3]. Graphite fluoride is another carbon-based solid with excellent friction and wear properties [4]. Recently developed boron-containing carbon-carbon composites are proved to be extremely wear resistant and at the same time very lubricious over a wide temperature range (providing friction coefficients of 0.04-0.1 from room temperature to 500oC) [5]. Under certain conditions, even bucky-balls (C60) [6]
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