Reduced wear and friction enabled by graphene layers on sliding steel surfaces in dry nitrogen☆
Introduction
Reducing energy and material losses in moving mechanical systems due to friction and wear still remains one of the greatest challenges of our time [1], [2]. Worldwide concerted efforts continue in developing new materials, coatings, and lubricants that can potentially provide low friction and wear even under severe operating conditions. Many dedicated studies have identified numerous types of tribological mechanisms for different kinds of materials operating under different environments, temperatures, or lubricated test conditions [3], [4]. Most traditional methods of controlling friction and wear involve the uses of solid and liquid lubricants at sliding contact interfaces [5], [6], [7]. However, finding the most effective lubricant that is also environmental friendly and cost-competitive [8] remains difficult. One of the emerging self-lubricating materials is graphene, which has been widely studied for its unusual thermal, electrical, mechanical, and recently, tribological properties [9], [10], [11], [12], [13], [14], [15]. However, previous studies on graphene tribology have mostly focused on nano- to microscale friction and wear behavior [16], [17], [18], [19], [20] and these studies have greatly increased our understanding of the fundamental lubrication mechanisms involved. However, macro/meso-scale tribological studies of graphene have remained relatively unexplored but urgently needed to realize graphene’s full potential for diverse tribological applications. At present, using various forms of carbon-based nanomaterials (i.e., nanotube, nano-onions, nano-diamond, etc.) to reduce the wear in general is gaining high momentum [21], [22], [23], [24]. Graphene is also being tried as an oil additive [25], and in the development of polymer-matrix composites [26] providing impressive lubrication and mechanical properties, thus resulting in excellent wear resistance under a wide range of test conditions [27]. Other solid lubricants such as MoS2, Pb–Mo–S or graphite can also provide low friction and wear [1], [28]. However, the majority of them require large amount and, often times, full coverage of lubricant material [29]. Also, specific deposition processes (such as high temperature processing [30] or magnetron sputtering [31], [32]) limit the application of these lubricants. In contrast, in this study, we aim to demonstrate that using very little or few layers of graphene on sliding steel surfaces can lead to remarkable reductions in their friction and wear, thus further corroborating its potential to improve efficiency and durability of many moving mechanical systems. In a previous study, we reported dramatic reductions in friction and wear for steel tribo-pairs that were continuously lubricated with graphene layers under humid air conditions [33]. Also, graphene’s great ability to inhibit tribo-corrosion under humid conditions was confirmed. In this study, we concentrated our attention on the friction and wear behavior of graphene layers (2–3 sheets) on steel plates in dry nitrogen as a function of load. It is well known that bulk graphite (a source of graphene) works the best in humid environments but fails to provide low friction and wear in inert, dry, or vacuum environments [34], [35]. One of the major objectives of this study was to ascertain whether graphene behaves similarly to bulk graphite under dry and inert test conditions. In our study, graphene flakes were applied on sliding surfaces by spreading graphene-containing ethanol solution on the surface and then evaporating ethanol to leave the solution processed graphene flakes (SPGF) behind as a non-continuous film that is only a few layers thick.
Section snippets
Experimental procedure
Tribological studies were performed in dry nitrogen environment (900 mbar) at room temperature using a CSM high vacuum tribometer with a ball on disk contact geometry. The stainless steel flat samples (AISI 440C grade) were initially cleaned by sonication in acetone and then in isopropanol in an ultrasonic bath to remove any contamination that may have been left from the sample preparation steps. As a counterpart, the stainless steel ball (440C grade) of 9.5 mm diameter was used. Both the flat
Effect of SPGF on friction and wear during initial cycling
To investigate the effect of graphene on tribological performance of 440C steel, a baseline test for 600 cycles was carried out with bare steel in dry nitrogen environment under a 2 N load (average Hertz contact pressure is 0.41 GPa). As it is shown in Fig. 2a, the frictional behavior of bare steel is rather unsteady and fluctuates substantially due to adhesive wear and perhaps third-body wear debris accumulation within the sliding contact interface. In the absence of oxygen and/or humidity, this
Conclusions
A series of experiments were conducted to compare the effect of graphene on friction and wear behavior of 440C steel in dry nitrogen environment.
The results indicate that small amounts of graphene on the sliding surface are able to afford reasonably low friction coefficients (i.e., 0.15–0.3) compared to very high and unsteady friction of steel without graphene. Moreover, the wear volume of graphene lubricated test pair is 2 orders of magnitude smaller than that of bare 440C for 600 cycle tests.
Acknowledgments
Use of the Center for Nanoscale Materials was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
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