Popular Carbon-based Thin Films and Their Lubrication Performances: Comparison
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To comply with the high demand for efficient and sustainable lubrications, in-situ carbon-based tribofilms and/or nanomaterials have emerged as a potential solution that can resolve the current major shortcomings of phosphorus- and sulphur-rich tribofilms and protective coatings. Although their employment is still in the early stages of realization and research, these tribofilms receive significant interest due to their capability to continuously and in situ repair/replenish themselves during sliding, which has been an ultimate goal of all moving mechanical systems. Structurally, these tribofilms are complex and predominantly amorphous or disordered with/without graphitic domains (e.g., graphene/graphite, onion-like carbon, etc.). Chemically, the compositions of these tribofilms can vary significantly with environments, conditions, and material precursors. 

  • carbon-based tribofilms
  • sustainable lubrications
  • tribocatalysis

1. Introduction

Carbon-based films are widely used in tribological systems to reduce friction and wear. These films can be polymeric, graphitic, or diamond-like, which are commonly prepared by chemical or physical methods. Publications about carbon-based films date back to the 1950s when a diamond-like carbon (DLC) coating comprising an amorphous network was first introduced by Schmellenmeier [43][1]. Since then, numerous kinds of DLC coatings with numerous characteristics have been applied in various technical fields, such as hard disks, space technology, biological applications, chemical pumps, and food processing [44][2]. The structural characteristic of the DLC coatings can be flexibly tuned to produce graphite-like coatings [45][3]. Apart from DLC and graphite-like coatings, some studies in the early 1960s also described the appearance of high-molecular-weight products, or carbon-based friction polymers, formed in sliding metal systems lubricated by hydrocarbons [46][4]. In very recent studies, some fundamental works have sought to explore the mechanisms of anti-friction polymers based on the context of mechanochemistry [47][5]. In general, carbon-based films can take several forms, and their tribological performance in terms of friction and wear is discussed and reviewed in the following parts.

2. Diamond-Like Carbon Films

In automotive applications, the use of DLC has been popular since the 1990s. It began with Volkswagen’s diesel injection system and later spread wide to various components like valve shafts, finger followers, piston skirts, tappets, piston rings, and piston pins [48,49][6][7]. Their properties are significantly attributed to their portions of carbon atoms’ sp2- (C=C) and sp3- bonds (C–C and C–H). The DLC coatings with a high sp3- bonding portion (highly diamond-like) structure will be harder and exhibit better wear resistance in the tribological system [50][8]. Meanwhile, high sp2- bond portion DLC films as graphitic films will be softer and result in better anti-friction performance compared to the high sp3- bond portion [51][9]. Moreover, the concentration of hydrogen content in these carbon-rich coatings can also exert a certain effect on their tribological performances [52][10]. Two important groups of DLC film structures comprise the nonhydrogenated networks with the amorphous (a-C) and tetrahedral amorphous carbon (ta-C), and the hydrogenated one with the significantly higher hydrogen content like a-C:H.
There are two mechanisms that are widely postulated as the main antifriction property of DLC tribofilm, including the “friction-induced graphitization” mechanism and the passivation mechanism. Particularly, graphitization is the transformation process of the carbon-based films from amorphous to graphitic ordered form, with a higher sp2 portion in bonding configurations [57][11]. Then, the “friction-induced graphitization” mechanism is defined as the process where the rise of the sp2-bond portion at the sliding interface due to sliding friction [55][12] would result in a graphitized tribofilm with stable and low frictional outcome corresponding to the lubricious nature of graphite. Meanwhile, the passivation mechanism is the procedure where the majority of carbon with free dangling σ-bonds are passivated or saturated by hydrogen terminations as passivating species to form a nonreactive surface [17][13]. The estimated content of hydrogen in DLC coating that can protect the interface areas from adhesive interactions due to the reformations of dangling carbon atoms is around 40 at% [52,58][10][14]. From the atomic-scale point of view, this mechanism is attributed to the charge density redistribution and the positive hydrogen protons’ repulsive forces, which suppress the adhesion interactions and produce low-friction outcomes [56][15]. It can be seen that the hydrogen atoms play a significant role in the DLC passivation mechanism. Its presence in the matrix of amorphous carbon would soften the structure by increasing the void density and/or releasing the intrinsic pressure [17][13].
Among the aforementioned film structures, the a-C:H coating has been highlighted as the only DLC film that can achieve a lower than 0.01 friction outcome [56,59,60][15][16][17]. This can be explained by its unique structure with sufficient sp2 and H contents, which can effectively process both “friction-induced graphitization” and passivation mechanisms during the sliding process [55][12]. In contrast, for a high sp3 portion and hydrogen-deficient coating in ta-C or nanocrystalline diamond, the long continuous graphitization process without a suitable saturation can easily result in a high-friction outcome and catastrophic adhesive wear. Furthermore, another major issue with the sp3-rich DLC films that have significantly restricted their wide employment is their low absorption strength to the base substrate [61][18]. This can be explained by the association of the sp3 bonding with the compressive stresses of the DLC film [61,62,63,64][18][19][20][21]. To reduce the residual stress and improve the films’ desired mechanical properties, the introduction of functional metallic atoms as doping elements and/or interlayers is recognized as one of the most effective approaches, as demonstrated by numerous studies [22,23,65][22][23][24]. It has been widely acknowledged that the graphitic degree of carbon-based tribofilms can be promoted by providing the carbon matrix with such catalytic metals as Cu, Re, Pt, Pd, and Ni during the manufacturing process [57,62][11][19]. Meanwhile, to increase the DLC coating’s sp3 bonding portion without weakening its adhesion, the use of Cr or Al interlayers could be considered [62,66][19][25].
According to Zhou et al. [65][24], many techniques to deposit the DLC coating on a target substrate have been introduced, with plasma-enhanced chemical vapour deposition (PECVD), physical vapour deposition (PVD), and chemical vapour deposition (CVD) categorized as the most common ones. Among these, magnetron sputtering as a PVD approach has been widely employed in many industrial fields. This approach utilizes energetic ions produced from the discharge plasma to bombard the target surface [22]. The popularity of the magnetron sputtering method can be explained by the diversity of power supplies like direct current, radiofrequency, etc., which can be monitored to tailor the desired carbon-based tribofilm structures [65][24]. Moreover, it is clear from Al Mahmud et al. [22] that such deposition techniques as plasma-activated chemical vapour deposition, plasma immersion ion implantation and deposition, or ion beam deposition have also received much interest due to their flexibility. In general, their working temperatures usually range between subzero and 400 °C. Furthermore, etching time, bias voltage, and carbon-precursor gases (like acetylene [65][24], methane [67][26], butane [68][27], etc.) are also important working parameters that can be controlled in order to achieve the target carbon-based coatings. Detailed advantages and disadvantages of each deposition approach can be found in the work performed by Hainsworth et al. [69][28].

3. Carbon-Based Tribofilms Produced by Catalytic Coatings

Although DLC coatings have already proven their lubricating functions in practice [48[6][7],49], their limitations come from the restricted deposition thicknesses and easy-delamination characteristic that will require redeposition after a long time of operation [26][29]. To overcome these disadvantages, attention has been paid to the in-situ formation of carbon-based films via the tribochemical and/or tribocatalytic reactions [70,71][30][31] that can replenish these carbon-rich layers from carbon-precursor sources [26][29]. In the pioneering work of Erdemir et al. [36][32], by coating the AISI52100 steel surface with copper-rich clusters through sputtering MoNx-Cu nanocomposite, the 10 h ball-on-disc tribo-experiment under 1.3 GPa normal pressure at room temperature results in a steady coefficient of friction (COF) of 0.08. This result is lower than the uncoated substrates in fully formulated oil (containing ZDDP) with a COF of about 0.1. In addition, the wear loss of the ball against the coated steel (with 7.35 × 10−15 m3) is also significantly lower than the ones against the uncoated steels (with 1.26 × 10−15 m3 and 1.34 × 10−12 m3 in the sliding tests with fully formulated oil and pure PAO 10 oil, respectively). Along with the mechanical enhancement of the MoNx-Cu coating [9][33], the outstanding tribological performance of this coating was explained by the formation of carbon-based tribofilm from the PAO10 oil base. Its Raman spectrum was significantly similar to the graphitic DLC coating with a high sp2 bonding portion. Moreover, this was also verified via the analysis of the resultant wear debris using electron energy loss spectroscopy (EELS), where graphite-like carbon nanocrystals (with 82% sp2-portion) were detected. This has paved a pathway to utilize active metallic atoms within hard coatings to promote the formation of carbon-based tribolayers and further advance their lubricating properties [72,73,74][34][35][36]. For example, using the same coating MoNx with Ag dopant, Xu et al. [73][35] demonstrated the ability to form onion-like carbon tribofilms from different hydrocarbon precursors.
Further works utilizing the same concepts of catalytic coatings were conducted with pure catalytically active alloy films. In the studies of Curry et al. [75][37], Pt-Au alloy film was introduced as a novel coating with an extraordinary antiwear property due to the thermally stable nanocrystalline of the alloying characteristics. The film Pt0.9Au0.1 of 2μm thickness deposited on 440PH stainless steel discs has a nanohardness of 7.1 ± 0.4 GPa. It slid against Si3N4 balls and sapphire balls as harder counterparts (with 15 and 25 GPa nanohardness, respectively) under 1.1 GPa maximum Hertzian pressure and 105 passes of 1 mm/s sliding speed at 20 °C. The maximum result wear rate of the disc was just 3 × 10−9 ± 10−9 mm3/Nm, which can be classified as atomic attrition [76][38]. The resulting friction was relatively high (ranging between 0.25 and 0.3), which can be explained by “third body” sliding. It is concluded that the wear property depends on the bulk characteristic, while the friction performance is attributed to the shear layer. To further investigate the tribochemical potential of this coating, the authors conducted similar tribotests with sapphire balls in a nitrogen-rich chamber containing ambient organics and adventitious carbon [77][39]. The friction was surprisingly decreased to just about 0.01, where the formation of 50–200 nm thick carbon-based tribofilms, which the authors claimed as hydrogenated-DLC-liked tribofilms, mixing with Pt-Au nanoparticles, was found on the worn surface. The formation of the resultant carbon-based tribolayer was attributed to the catalytic effect of both Pt and Au atoms on the organic species [78,79,80][40][41][42]. More importantly, its frictional outcome agrees well with the antifriction performance of a-C:H film in dry N2 sliding conditions [32][43]. In addition, the structural similarity of the resultant carbon-based tribolayer to the chemical vapour deposit (CVD) a-C:H coating (with 20% H content) was compared via Raman analysis. Combined with the work performed by Erdemir et al. [36][32], it is clear that the structure of the catalytic carbon-based tribolayer could be tailored based on the employment of different active metals, hydrocarbon precursors, and/or sliding conditions [81][44]. As a result, the later study by Shirani et al. [82][45] reported the promotion of highly graphitic carbon-based tribofilm when the tribotest with Pt-Au alloy was performed in ethanol ambient. Meanwhile, further research by DelRio et al. [83][46] optimized the performance of this tribolayer by altering the Pt and Au contents within the target coatings.

4. Carbon-Based Tribofilms Produced by Organic Additives

The employment of functional organic carbon-precursor additives, of which cycloalkane moieties are highly strained, has also been recognized as a distinctive way to form the in situ carbon-based tribofilm between two sliding surfaces [37][47]. Compared to the hard catalytic coating approach, the deposition of these carbon-based tribolayers is much more convenient, since it does not require any pretreatment before the sliding process. Noticeably, in 2018, Johnson et al. [34][48] introduced cyclopropanecarboxylic acid (CPCa) as a promising sulphur-free and phosphorus-free additive for automotive engine oil. By dissolving 2.5 wt% CPCa in PAO4 oil, remarkable tribological performances can be clearly observed with an average 93% wear rate reduction and 18% friction reduction at room temperature, 0.05–0.2 m/s sliding speeds, and 10–20 N normal loads. From the Raman analysis of the CPCa samples’ wear tracks, a similar configuration between the obtained carbon-based films and the commercial hydrogenated DLC coating was detected. Because the triboexperiments were carried out without any added catalysts, it is clear that CPCa performed as a carbon-precursor additive. Particularly, the cycloalkane fragmentation was confirmed to be triggered under high contact pressure and flash temperature that eventually promoted the carbon-rich film. Further computational outcomes detected the polymerization process from CPCa fragmentation to promote hydrocarbons with high molecular weight [40][49]. These processes were later found to be associated with the strong absorption of the carboxyl group (–COOH) on the steel substrates [37,39,84][47][50][51].
Besides the cyclopropane moiety, such carbon-precursor additives with the cyclobutane group as cyclobutanecarboxylic acid (CBCa) are also found to promote polymeric carbon-based tribolayers under severe sliding conditions. However, according to Ma et al. [37][47], this moiety exhibits a much more stable characteristic, which dissociates and polymerizes much more slowly, resulting in inferior tribological performances compared to the cyclopropane one’s tribotest under the same testing conditions. Also, cyclopropane-1,1-dicarboxylic acid (CPDCa) additive containing two carboxyl groups exhibited the best antiwear and antifriction properties in trimethylolpropane trioleate (TMPTO) base lubricant. By employing the molecular dynamics (MD) simulation, the superior performance of dicarboxylic acid additives was explained by their stronger binding strength on the sliding substrates than the monocarboxylic ones, resulting in negligible wear outcomes. Finally, apart from the aforementioned additives, the polymeric carbon-based tribofilms activated from other organic molecules like n-butyl acrylate [85][52], α-pinene [86[53][54],87], allyl alcohol [47][5], etc., are also investigated prior to the practical applications.
Along with the functional additives, the chemical nature of the base lubricants can also exert dominant influences on the structures of in situ carbon-based tribolayers [88,89][55][56]. This has been highlighted in the current study by Rouhani et al. [35][57]. In his work, the four-ball tribo-experiments were carried out to evaluate the boundary lubrication performance of in situ carbon-based tribofilms generated from pure palm oil (PO), palm oil formulation (POF), mineral oil (MO), and mineral oil formulation (MOF), in which PO is Triacylglyceride (TAG)-based, which has received a lot of interest due to its ecologically friendly characteristics [90][58]. According to the four-ball tests, PO performs worse than MO in terms of seizure resistance at higher loads due to its reactive allylic protons. POF and MOF performed better; however, POF shows higher resistance to seizures than MOF at severe loading conditions. The authors believe that the antioxidants in the POF gave sufficient chemical protection to the TAG molecules, while surfactants enhanced the wettability of the steel, and viscosity modifiers optimized fluid properties. Accordingly, at low experimental loadings, both POF and MOF produced carbon-based tribofilms that exhibited a resultant structure similar to a-C:H. Once the testing load increased, leading to an increase in seizure, the transformation from a-C:H to a-C, and eventually nanocrystalline-graphite-like structure (ncG), occurred [91][59] before the oil-based lubricants failed at the highest 400 kg load. The authors concluded from the Raman analysis that MOF oil is found to graphitize much easier than POF oil due to the appearance of aliphatic chains within the MOF oil base. These chains can promote kerogen formation [91[59][60][61],92,93], thus accelerating the graphitic transformation of resultant carbon-based tribofilms when the experimental load goes up. Combined with the corresponding tribological outcomes, the study concluded that the rapid graphitization of the polymeric carbon products can easily lead to early oil failure due to the failure in establishing the a-C tribofilms.

5. Carbon-Based Tribofilms Produced by Catalyst Additives

Beyond the novel hard coating developments, interest in the catalyst agents has also spread to the lubricating nanomaterial investigation. For instance, Hu et al. [41][62] demonstrated the carbon-based tribofilm structure, which was formed via the catalytic effect of Ni nanoparticles under extreme-pressure conditions. It should be noted that nanoparticles have also been considered as another modern extreme pressure additive. They enter the microroughness to polish the interfacing surfaces during the sliding process, of which the four tribological mechanisms that allow for “smooth” contact from nanoparticles in oil lubrication are rolling, protective film, mending, and polishing effects [94,95][63][64]. Among them, the nickel nanoparticle emerges as one of the softest nanoparticles, with the lowest melting temperature, which is more beneficial for promoting a compact protective film on the contacting area via the nanoparticles’ sintering by flash temperature [96][65]. In this example, cubic Ni nanoparticles with a 21.3 nm average size were synthesized from nickel formate and oleylamine in order to enhance their dispersion in the PAO6 oil base. Their test concentration in base oil was 1 wt%. The ball-on-disc tribotests were carried with 0.05 m/s sliding speed and 1800s duration under the 100 N normal load. The experimental disc was made of 304 stainless steel, while the ball’s material was GCr15 steel. From the experimental results, cubic Ni nanoparticles exhibited a remarkable tribological performance under extreme-pressure conditions. By employing the nanoadditive, the friction was dramatically reduced from 0.4 to around 0.1. Meanwhile, the average disc wear rate of the PAO oil containing Ni nanoparticles was just 0.89 × 10−6 mm3/Nm, which was ~92% lower than the pure oil test (11 × 10−6 mm3/Nm). Further analysis of the Ni test at 100 N demonstrated a compact film formation on the wear tracks that contained a high amount of nickel, carbon, and oxygen elements. This indicated that during the sliding process, Ni nanoparticles had entered the interfacing area. Then, these particles oxidized partially and created an iron–nickel oxide layer with high binding energy. After that, the embedded Ni nanocrystals further promoted the carbonizing process of based oil to create a DLC-based layer with a high graphitization degree, resulting in a low-friction outcome.
On the other hand, the concept of utilizing active elements in promoting and/or controlling in situ carbon-based material has also been applied in current research on 2D nanomaterials like layered double hydroxides (LDHs). It should be noted that LDH has already emerged as a “green” additive [97][66] that can achieve superlubricity performance in lubricant research [98][67] thanks to their manageable structures [99,100][68][69] as well as the outstanding shearing capability [101,102][70][71]. These nanomaterials are also famous for their catalytic applications, where many of which are well-known as functional catalytic agents to promote the formation of helical carbon nanotubes, carbon nanofibres, and CNT/graphene hybrids from carbon-precursor gases [103,104,105,106][72][73][74][75]. Therefore, by utilizing both lubrication and catalytic functions of NiAl-CO3 LDH [107,108][76][77], researchers have successfully advanced the tribological performances of CPCa under severe sliding conditions at different temperatures [109][78]. Extreme boundary rotation tribotests, of which the lambda value was just 0.09, have been carried out under 25 °C, 50 °C and 100 °C. It is clear from the tribotest outcomes that the combination of 2.5 wt% CPCa and 0.1 wt% NiAl-CO3 LDH in PAO4 ambient (CPCa + 0.1%LDH) results in the best tribological performances at all experimental temperatures. At room-temperature tests, the superior lubricating property of CPCa + 0.1%LDH was dominantly attributed to the easy exfoliation of the 2D nanomaterial’s layered structure [110][79]. Once the testing temperature increased, the formation of hierarchical tribofilms was observed, where the Ni-based phases within protective interlayers from LDH were found to exert the catalytic effect, replenishing the upper carbon-based tribolayers. These unique multilayer tribofilm promotions not only stabilized the performance of soft and low-absorption-strength carbon-based tribolayers from CPCa under high-temperature tribotests but also exhibited better antiwear and antifriction properties than the conventional ZDDP tribofilm under similar sliding conditions.

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