Reduced wear and friction enabled by graphene layers on sliding steel surfaces in dry nitrogen

Abstract

We report on the friction and wear behavior of graphene-lubricated 440C steel test pairs in dry nitrogen under different loads. Tribological test results have revealed that a few-layer graphene is able to drastically reduce the wear and the coefficient of friction (COF) of 440C steel during the initial sliding regime and under low load conditions. Specifically, the COF has been reduced from ≈1 for bare steel to 0.15 for steel covered by a low concentration of graphene flakes. Such low COFs have persisted for thousands of sliding passes, even though the graphene layers formed on sliding surfaces have not been continuous or continuously replenished; they were made of a few sheets of graphene. The wear rates of the steel test pairs have been also reduced (by as much as two orders of magnitude), again despite the very sporadic and thin nature of the graphene layers. A possible explanation for the low friction and wear reduction is that graphene as a two-dimensional material shears easily at the sliding contact interface and, hence, provides low friction.

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.