Carbon dots as an additive for improving performance in water-based lubricants for amorphous carbon (a-C) coatings

Abstract

As a new type of carbon material, carbon dots (CDs) with exceptional properties are regarded as a type of potential water-based lubricant additive. In this study, it is first demonstrated that CDs perform excellently when used as lubricant additives for water-lubricated amorphous carbon (a-C) contacts. The results show that the introduction of CDs into water at a concentration of 0.1 wt% could reduce friction and wear in a-C contacts by 33% and 80%, respectively. Additionally, the lubricating performance of CDs is highly sensitive to its adding concentration. The excellent tribological performance of CDs results from the minimized CDs sediment particles (deposited in situ on worn a-C surfaces) and the reduced friction and wear via nano-bearing and nano-filling mechanisms. With high CDs concentrations, however, micron and submicron-sized sediment particles form when friction occurs and cause undesirable abrasive wear. For water-lubricated a-C contacts, therefore, the CDs concentration is the crucial determinant of lubrication performance that has to be appropriately controlled.

Introduction

Compared with conventional oil and grease lubricants, water-based lubricants afford the inimitable advantages of being non-polluting and energy resource-saving. This is because water is environmentally friendly, abundant, and a renewable resource. Water-based lubricants have been applied as cutting, machining, or drilling fluids in metal-forming operations and oil extraction. In view of the low viscosity, and inadequate lubricity of water as well as the corrosion problem that it creates, however, the further application of water-based lubricants in the industry is considerably limited [1].

Lubricant additives are widely used to improve the tribological performances of base fluids [[2], [3], [4], [5], [6], [7], [8]]. The most commonly used additives for water are polymers, water soluble polyols, and nanomaterials. Compared with conventional organic lubricant additives, nanomaterials exhibit superior chemical stability with substantially lower toxicity and less harmful emissions, which conform with the idea of using environmentally friendly and energy-saving materials [[9], [10], [11]]. Nanomaterials have been extensively explored as novel lubricant additives for aqueous environments. Among the variety of nanoparticles, two-dimensional graphene and graphene oxide could provide excellent lubricating effects because of their superior properties, such as high elastic modulus, self-lubricating behavior, and good thermal conductivity. For example, Kinoshita et al. [9] reported that graphene oxide (GO) performed a positive function in remarkably reducing friction between stainless steel and tungsten carbide contact. Elomaa et al. [12] evaluated the tribological properties of GO as a water additive to lubricate the contact between stainless steel and diamond-like carbon. It was reported that the GO nanosheets significantly reduced the wear volume loss. The aforementioned carbon nanomaterials, however, may experience the embedded stability problem between micro-bulges of rubbing interfaces [13,14]. The morphology and size of particles also affect the lubricating performance. In general, the spherical morphology is favorable for rolling mechanisms and small particles could more easily enter into the frictional interfaces and consequently improve tribological performance.

As a new type of carbon material, carbon dots (CDs) have attracted considerable interest in various fields of research because of their unique properties, such as superior biocompatibility, high specific surface area, and robust chemical inertness [[15], [16], [17], [18], [19]]. Recently, Wang et al. [17] synthesized ionic liquid-capped carbon quantum dots with anion responsiveness. They also found that N-doped CDs-based fluorescent probes exhibited good selectivity and high sensitivity toward Hg²⁺, Cu²⁺, S₂O₃²⁻, and Cr (VI) ions [18,19]. For tribological application, positive results have been reported regarding CDs in oil-based lubricants [[20], [21], [22], [23]]. Wang et al. [22] synthesized branched polyelectrolyte-grafted CDs, which exhibited excellent friction-reduction and anti-wear performances when added to polyethylene glycol.

Carbon dots are potential water-based lubricating additives because of their good solubility in water. Research investigations that are relevant to the water lubrication behavior of CDs, however, remain limited. Liu et al. [24] successfully synthesized ionic liquid-modified CDs by the one-pot pyrolysis method and evaluated its lubrication properties in water via a four-ball tester. They found that CDs–IL (ionic liquid) exhibited exceptional effectiveness in reducing friction and wear in steel contacts; they attributed the improved performance to the boundary tribofilm formed by the absorption and deposition of CDs–IL. Tang et al. [25] studied the tribological properties of CDs and CDs–IL as water-based lubricant additives for steel contacts and found that both additives reduced friction and wear. They observed that the latter exhibited a better performance than the former. They further suggested that the IL group is conducive to the formation of a good adhesion film of CDs–IL onto the rubbing surfaces. Xiao et al. [26] employed sulfur-doped CDs to enhance the lubricity of water for Si₃N₄-vs-steel and Si₃N₄-vs-Si₃N₄ contacts. The results showed that CDs effectively reduced friction and wear in the two types of tribopairs. These enhancements could be attributed to the rolling effect, interlayer shear sliding effect, and tribofilm formation. More recently, Wang et al. [27] found that CDs, as eco-friendly nanoadditives in water-based lubricants, exhibited good lubricating property and inhibition effect for 316 stainless steel. They suggested that the CDs-absorbed protective film formation, nano-filling effects, and nano-bearing effects are the main mechanisms of improved performances. In summary, CDs are considered as promising additives for water-based lubricants with lubrication effects and mechanisms that vary in different tribopairs. Further studies are therefore necessary particularly for tribopairs that possess excellent tribological properties in a water environment.

Amorphous carbon (a-C) coatings have low coefficients of friction and wear rates in water, and their deposition has been recognized as a highly effective approach to improve the tribological performance of tribopairs under water-lubricated conditions [[28], [29], [30], [31]]. The a-C, however, exhibits low reactivity with chemically based additives because of their chemical inertness [[32], [33], [34]]. It has been found that graphene and its derivatives could significantly improve the tribological performances of a-C contacts [35,36]. Theoretically, because of their small size and high water solubility, CDs should provide better lubricating effects for a-C contacts. To the best of our knowledge, however, there is no literature that reports on this subject.

In this study, the tribological performance of CDs as additives for water-lubricated a-C/a-C contacts is investigated. The possible changes in the boundary lubricating mechanisms of CDs at different concentrations are examined. In particular, the remarkable synergetic effects between a-C and CDs at an exceedingly low concentration of 0.1 wt% are elucidated based on the observation of the morphology, distribution, and chemical composition of CDs on the worn a-C surface after tribological tests.