Tribological interactions account for almost 23% of total energy consumption, with 20% going towards overcoming friction and 3% towards remanufacturing worn parts and spare equipment as a result of wear-related failures, according to a recent study titled Influence of tribology on global energy consumption, prices, and emissions [1]. Vehicles, machines, and other equipment can benefit greatly from lubrication methods, which greatly reduce friction and provide wear prevention [[2], [3], [4]].

Typical lubricants, such as synthetic oils or minerals, are typically used in machine parts (bearings and rolling) to minimize wear and friction. However, bearings cannot always be greased due to a variety of physical, governmental, and environmental constraints as well as application-specific requirements [5]. Examples include applications in the medical, paper, textile, or food industries as well as restrictions caused by ultra-clean environments, intense heat, vacuum, or radiation. Solid lubricants are attracting an increasing attention due to the potential to preserve vital lubrication qualities, which helps to prevent associated early failures. In the past, solid lubricant materials have been applied as thin films or coatings in four different ways: as soft metals, polymers, transition metal dichalcogenide (TMD) compounds, and carbon-based materials [6,7].

Due to weak adhesion, hexagonal lattice and low surface energy, graphite has incredibly simple basal planes and requires adsorbed gases or moisture to passivate the covalent bonds and edges of the basal planes in order to retain low friction [8,9]. Its application is therefore restricted in dry conditions, especially in a vacuum [10]. This limitation can be partially alleviated by taking on a particular shape, like graphene [11]. Numerous researches have been conducted on low loaded sliding contacts at the micro- and nano-scale, confirming their outstanding frictional performance under optimum environmental instances [12,13]. However, there isn't much research about its application in macro-scale systems that are similar to machine parts like rolling element bearings (higher-loaded rolling-sliding contacts) [14]. Therefore, its most commonly employed feature is as a self-lubricating agent in polymer- or metal-matrix or as partially/loose bonded resin coatings [6].

TMD, such as molybdenum disulfide (MoS2), is another material used as a solid lubricant. The initial investigations related to MoS2 were primarily motivated by space travel [15], while recent reports are more environmentally driven [16,17]. There have been several analyses of the tribological characteristics of MoS2 as a solid lubricant for sliding contacts; however, there has been less research when employed in rolling contact situations [18]. MoS2 has good tribological properties due to its hexagonal crystalline architecture and low coefficient of friction (COF) because its basal plane shears easily, especially in a vacuum [[19], [20], [21]]. However, oxidation makes MoS2 vulnerable to ambient air with humidity.

Polytetrafluoroethylene (PTFE) is a polymeric material generally acknowledged for its low friction properties because of its lower intermolecular cohesiveness [22]. Due to its minimal chemical reactivity, this is accurate both under pressure (atmosphere) and vacuum [10]. Using suitable fillers can somewhat enhance the poor wear resistance and restricted load-carrying capability of pure PTFE [23]. Additionally, PTFE's low thermal inertia prevents heat from dissipating, which leads to early failure from melting and restricts usage to low-speed sliding applications [10].

Similarly, metals such as lead, silver, tin, indium or gold and their corresponding alloys own low friction on comparatively hard substrates when applied as a coating. As opposed to other solid lubricants, shear within the coating is typically used to lubricate soft metal films, which increases friction [6]. It is, therefore, most pertinent in a vacuum and higher temperatures, and is extremely advantageous in non-sliding and rolling applications [24]. For instance, barium and silver films have been successfully employed in lightly loaded ball bearings in high X-ray tubes, whereas gold and silver films have been employed in spacecrafts [25].

This succinct overview of well-known solid lubricants in use shows that they have undergone in-depth research in basic, model tribological test rigs under a range of working situations. It is still uncommon to find their use in machine components that are actually put to use in more practical working environments. Furthermore, it becomes clear that the aforementioned solid lubricants have all of their drawbacks and shortcomings, which compel further study in this area as well as the development of new, more sophisticated solid lubricant systems.

Due to their exceptional performance, MXenes an emergent class of 2D nanomaterials comprising early transition metal nitrides, carbides, and carbonitrides, have attracted significant consideration in the scientific realm. The Mn+1XnTx (n = 1 to 4) is the general representation of MXenes [26]. MXenes share many similarities with graphene and its derivatives, including their 2D nature, potent in-plane bonding properties, and high surface-to-volume ratios [27]. MXenes provide improved interlayer contacts compared to other cutting-edge 2D nanomaterials because they have functional surface groups that allow for intermolecular and electrostatic and interactions in addition to van der Waal forces [28]. Additionally, MXenes have a naturally diverse composition because they provide property tuning by the use of various early transition metals or by employing and/or modifying the ratio between nitrogen and carbon [29]. MXenes are attractive 2D nanomaterials for tribological and mechanical applications due to their compositional variety, chemical, structural, and strong intralayer primary bonding, and relatively weaker interlayer secondary bonding features [30].

Based on these features, MXenes are extensively employed in various research areas, such as energy, catalysis, sensing, and biomedical [[31], [32], [33], [34]]. In addition, they have dragged much interest in the field of tribology because of their self-lubricating ability, low shear strength and graphite-like architecture [35] that has already been exploited by density functional theory and molecular dynamics simulations [36]. Thus, MXene nano-sheets can be seen as a promising replacement for conventional solid lubricants [37,38]. They have the potential to be used in machine components under a variety of environmental conditions and real-world working conditions, which could help them overcome some of the limitations mentioned above.

A number of reports on tribological applications of MXenes have been published to date [27,[39], [40], [41]]. On the other hand the number of review articles on MXenes as solid lubricants are limited [42,43]. Therefore, this review discusses the current state-of-the-art surface functionalization of MXenes, mechanical behavior and lubrication potential of MXenes and composite at micro- and macro-scale, and opportunities and challenges of the research area with possible future research directions. This article would be a valuable resource for MXenes and open the door to improving their chemical, physical and mechanical properties in various applications as a solid lubricant.