Superlubricity [the coefficient of friction (COF) is <0.01], or the sliding regime with friction approaching almost zero values, is one of the research hotspots in the tribology field (Berman et al., 2018a). It was first proposed by Motohisa Hirano (Hirano and Shinjo, 1993) to describe a theoretical state: the friction force is extremely small or vanished between the tribopairs. So far, the superlubricity was mostly demonstrated for the laboratory-scale experiments, while use of the superlubricity in the large-scale industrial set-ups remains challenging. Previous studies demonstrated superlubricity mostly with 2D materials (Berman et al., 2018a), including graphite (Dienwiebel et al., 2004; Liu et al., 2012), graphene (Berman et al., 2015, 2019; Kawai et al., 2016), molybdenum disulfide (MoS₂) (Martin et al., 1993; Li et al., 2017; Berman et al., 2018b), boron nitride (BN) (Zeng et al., 2013), black phosphorus (BP) (Wang et al., 2018a,b). The mechanism of the superblurcitiy that uses the easy shearing properties of 2D materials is also called structural superlubricity. In addition to two-dimensional materials, there are other ways to achieve superlubricity. For example, Li et al. (2014) tried to mix acids with glycerol or polyhydroxy alcohols that contained large amounts of hydroxyl groups indicating that the possible superlubricity mechanism was associated with the formation of a fluid-hydrated water layer between glycerol and water or polyhydroxy alcohol and water on the positively charged surfaces.

Meanwhile, most of the previous studies demonstrating superlubricity regime with solid lubricants focused on material systems that work at room temperature conditions. As a result, the justified cases of the superlubricity became very limited in terms of their applications. In efforts to solve the tribological challenges related to high temperature applications, focus was rather directed toward ternary oxides and carbides that are in general incapable of near-zero friction sliding (Gao et al., 2015; Aouadi et al., 2020). A possible solution is to enable adaptive behavior of materials under changes in the testing conditions which was successfully shown in case of graphite-MoS₂ chameleon coatings (Shirani et al., 2020). The COF values were significantly reduced (down to 0.02 values) when testing the materials at elevated temperature conditions.

Here, we demonstrate that the superlubricity regime can be achieved at elevated temperature conditions in an open-air system by activating the formation of the low-friction tribofilm during sliding when using magnesium silicate hydroxide coated with a carbon layer (MSH/C). Magnesium silicate hydroxide is a novel lubricant additive and the main component of serpentine mineral with its tribological properties mainly being dependent on the release of Si-O, Mg-O, and -OH active groups that form a tribochemical layer during the friction process (Yu et al., 2010, 2013). To improve adhesion of the MSH/C film to the substrate material, we used antimony oxide (Sb₂O₃) powders (melting point: 656°C, boiling point: 1,425°C) (Jha et al., 2009). Though Sb₂O₃ is rarely used as an anti-wear or friction reduction additive, it is beneficial as a dopant to play a synergistic role with MoS₂ in the solid lubricant coating (Zabinski et al., 1993, 1995; Hu et al., 2006). It was previously reported that the MoS₂-Sb₂O₃-C film can change its surface with the environment changes to maintain good tribological performance by adapting to the testing conditions (Shirani et al., 2020). In another study it was found the COF of the sputter-deposited amorphous MoS₂-Sb₂O₃-Au films decreases during sliding in a dry nitrogen environment as a result of MoS₂ changing from amorphous to crystalline phase during this process (Scharf et al., 2010). Here, we also observe the synergistic effect of Sb₂O₃ and MSH/C as a result of tribochemically-activated material transformation into an amorphous carbon film which could support the easy sliding performance of the system. Our results propose a new solution to high temperature tribological issues.

The coatings used in this study were designed using MSH/C powders (Figure 1). The final crystal structure (Figure 1A) of the powders indicates that they are composed of Si-O tetrahedral layer, Mg-O octahedral layer, and -OH group (Dixon, 1989; Zhao et al., 2012). Scanning electron microscopy (SEM) and TEM images indicate the the MSH/C poswders are the mixtures of granular and tubular shape particulates with diameter of ~30–50 nm and the tube length reaching hundreds of nanometers (Figures 1B,C). Transmission electron microscopy images also confirm that the particles are coated with an amorphous carbon layer with a thickness of several nanometers as indicated in Figure 1D.

The powders were used for coating the inconel substrates using the burnishing process. The schematic of the process and the final surface morphology are shown in Figures 2a–d. The average surface roughness of the coatings after burnishing was about 0.27 μm (Supplementary Figure 2). And the thickness of the prepared film was ~1 μm (Supplementary Figure 3). The hardness values for the inconel substrates and Sb₂O₃ and Sb₂O₃-MSH/C coatings were about 6.39, 2.517, and 2.196 GPa, respectively (tested by a nanoindentation tester, Supplementary Figure 4).

The morphology and element distribution of as-prepared surfaces are shown in Figure 3. It can be seen that all the elements on the surface can be divided into three groups: Ni, Cr, and Fe belonging to the substrate material; Sb and O elements belonging to the Sb₂O₃ layer; and Mg, Si, O, and C belonging to the MSH/C composite nanoparticles. Note that Sb₂O₃ film prepared by the burnishing process is not uniformly distributed. Except for the layer on the surface, there are a large number of small Sb₂O₃ peaks in the form of aggregates, making the surface uneven. The presence of MSH/C composite nanoparticles is higher in the depression area composed of rough peaks, which is confirmed by the distribution of Mg and Si elements: except for the layer of MSH/C present on the surface, a significant increase in the content of Mg and Si elements was observed in the area surrounding the Sb₂O₃ peaks.

Designed coatings, after burnishing with Sb₂O₃ (labeled as Sb₂O₃) and after burnishing with Sb₂O₃ and MSH/C (labeled as Sb₂O₃-MSH/C) powders, were tested for their tribological performance. The COF results for the coatings tested at different temperatures are shown in Figures 4A,B. For the Sb₂O₃ coating, room temperature and 200°C tests resulted in considerably high COF values, 0.9 and 0.6 correspondingly. Meanwhile, further increase in the temperature to 300°C led to the COF reduction to 0.15. Such a decrease is attributed to elimination of water vapors adsorbed on the sample from the humid environment during burnishing process. After adding the MSH/C, the COF value was still high at room temperature and 200°C, reaching 0.95 and 0.8, respectively. However, as the temperature further increased to 300°C, the COF demonstrated rapid decrease after starting the test. After ~12 revolutions, the COF had dropped to 0.065. Further testing resulted in the COF showing a slow downward trend, and after 135 revolutions, the friction coefficient dropped to 0.008, thus entering the superlubricity region.

To further test the temperature sensitivity of the friction behavior of the coating, we continuously monitored the friction behavior while increasing the temperature from RT up to 300°C. The results indicated similar trend in the COF decrease under the elevated temperature conditions with friction approaching superlubricity values once the system reaches the higher temperature regime (Figure 4C). In the first 50 revolutions performed at RT, the COF continued to increase; ramping the temperature up to 100°C during additional 50 revolutions resulted in a more sporadic COF behavior around 0.4–0.5. Once the temperature reached 200°C, the COF dropped rapidly, changing from 0.63 to 0.25 within 38 revolutions. As the temperature increased further to 300°C over 150 revolutions, the COF continued to decrease steadily, and finally decreased to 0.008 at 400 revolutions, entering the superlubricity region.

To understand the origin of such behavior, we analyzed the wear tracks produced on the samples. Optical images of the wear scars and wear marks clearly show a difference when transitioning from the RT to the elevated temperature (Figure 5 and Supplementary Figure 5). While the high temperature tests indicate very mild wear of the materials, exposure of the samples to the lower temperature tests results in significant wear of the coatings. The micrographs suggest that under room temperature conditions the coating has been removed completely from the wear track during sliding resulting in the formation of a blackish film materials. The samples were further subjected to the SEM analysis suggesting that the blackish film is higher in the oxygen concentration, perhaps related to the metal oxide formed during sliding, rather than the coating materials (Figure 6 and Supplementary Figure 6).

As a result of lacking protective layer, large amounts of scratches appeared on the corresponding ball surface. As the temperature increased to 200°C, although more Sb, Mg, Si elements could be detected on the Sb₂O₃-MSH/C wear track area, the major part of the film has been worn out from the contact area, exposing the substrate surface and generating large amounts of oxidation products and scratches. However, when the tests were performed at 300°C, the wear track area was still covered by a Sb₂O₃-MSH/C layer even after the whole duration of the test, as shown in Figure 6. These results suggest that uniform coverage of the MSH/C film prevented exposure of the substrate to the sliding, thus inhibiting formation of the scratches and oxidation of the substrate materials. Evenmore, with the temperature increase, presence of the oxide products on the surface of the wear mark substantially decreased. This suggests that there is a synergy of protective properties of the Sb₂O₃ and MSH/C layers that do not only isolate the direct contact of friction pairs but also protect the substrate material from the oxidation. Analysis of the Si₃N₄ ball surfaces after the Sb₂O₃ tests indicated no obvious signs of wear but rather transfer of the protective coating to promote beneficial tribological characteristics of the films (Figure 7). Note that in case of Sb₂O₃-MSH/C, no obvious material transfer, such as Sb₂O₃ or carbon layer, is observed (Figure 7b and Supplementary Figure 7). This suggests that the composite film shows overall better integrity and the generated transfer film is weakly bonded to the silicon nitride surface and is easily wiped of during the post-processing. Therefore, the low-friction sliding at 300°C mostly occurs at the interfaces formed between MSH-generated carbon film and the silicon nitride ball surface. Similar wear-free behavior of the silicon nitride ball in contact with the diamond-like carbon (DLC) surface has been observed before by Jia et al. (1995).

Our results indicate that temperature plays an important role in the realization of superlubricity. Antimony trioxide (Sb₂O₃) itself still shows relatively high COF values suggesting that the Sb₂O₃ surface prepared by the burnishing process is rough with large number of Sb₂O₃ peaks being formed. Consequently, prolonged duration of the tests may lead to the complete removal of the top Sb₂O₃ layer and exposure of the metal substrate, which could cause high COF values and severe wear of the tribopairs. Observed decrease in the COF value at 300°C proved high temperature could be beneficial to the friction reduction.

We further probed the origin of temperature effect by in-situ Raman analysis of the coatings while heating (Figure 8). For Raman characterization of Sb₂O₃ film (Figure 8a), the measurement parameters and measurement positions were kept the same for the tested different temperatures. Resulting spectra indicate presence of six Raman peaks in the range of 150–950 cm⁻¹: the peaks at 190 and 451 cm⁻¹ come from the bending mode of Sb-O-Sb while the peaks at 255, 356, 373, and 714 cm⁻¹ correspond to the stretching mode of Sb-O-Sb (Zeng et al., 2004). It can be seen that with the temperature increase, the intensity of each Raman peak gradually decreases, suggesting decrease in the corresponding vibrational groups. These results are in agreement with previously reported studies (Naidu et al., 2009) reporting that with the increase of temperature from room temperature and up to 400°C, the intensities of diffraction peak corresponding to Sb₂O₃ (110) crystal plane and (200) crystal plane gradually decrease, indicating that the crystal is gradually destroyed. The results suggest that such structural changes affect the adhesion of Sb₂O₃ to the metal substrate, thus reducing the direct contact of the tribopairs. Because of the Sb₂O₃ being used to improve the adhesion of the MSH/C layer to the substrate material, more MSH/C could be involved the friction reduction process. Thus, Sb₂O₃ provides support for the lubricant to remain at the sliding interface. Considering the combined storage and support role of Sb₂O₃ demonstrated by the presence of the rough peaks on the MSH/C powder shown in Figures 8c,d, the worn surface could still be covered by a layer of lubricant. Meanwhile, the addition of top MSH/C also protects the bottom Sb₂O₃ layer, which, as reflected by the mapping results at 200°C, results in more Sb being detected on the Sb₂O₃-MSH/C than on the Sb₂O₃ surface thus supporting the findings in Figure 6 and Supplementary Figure 6.

We evaluated the necessity of the MSH/C presence for achieving superlubricity under high temperature conditions. The Raman results of MSH/C under different temperatures reflected the changes in the MSH/C crystal structure upon temperature increase, as shown in Figure 8b. The -O-H vibration peak located at 229 cm⁻¹ and the Si-O-Si peak located at 691 cm⁻¹ disappeared after temperature reaching 300°C. Besides, the intensity of the SiO₄ symmetrical stretching vibration peak at 386 cm⁻¹ also decreased significantly. This indicates that the -O-H and Si-O bonds in the MSH/C powders are destroyed with the temperature increase, thus leading to the release of a large amount of Si-O and -OH groups. The TG-DSC results in our pervious work also confirmed the release of -OH group (also called as the stuctural water) with the increase of temperature (Gao et al., 2018b). These active groups released from the MSH/C powders are the key reason for their excellent tribological performance (Chang et al., 2017, 2019). Local heating induced by friction is expected to further promote the decomposition of the MSH/C and the release of various active groups. Considering the mild tribological test condition (0.59 N−10 rpm) used in our study, the MSH/C could present the best performance under 300°C since the heat conduction energy will make up for the lack of friction energy.

In addition to the analysis of the properties of the powders at different temperatures, we have carried out a more in-depth analysis of the coating surfaces after the tests. The Raman results of worn surfaces under different temperatures are shown in Figure 9. It can be noticed that under RT and 200°C, due to the large removal of the Sb₂O₃-MSH/C layer in the friction process, the metal substrate is exposed to sliding causing formation of oxides as it is confirmed by the presence of various metal oxide Raman peaks, including iron oxide (323 cm⁻¹) (de Faria et al., 1997), nickel oxides (476, 560 cm⁻¹) (Nan et al., 2006; Kumar et al., 2020), and chromium oxides (680 cm⁻¹) (Guinneton et al., 2005). Besides, the Sb₂O₃ peak located at 190 cm⁻¹, a new peak at 255 cm⁻¹ also appeared, though its intensity is weak. No carbon or MSH related Raman peaks could be found on the worn surfaces. However, in case of 300°C test, strong Sb-O vibration peaks located at 190 and 255 cm⁻¹ appeared, which further proved better integrity of the coating at higher temperature and better survivability of Sb₂O₃ on the metal surface. Meanwhile, during sliding at the lower temperatures, Sb₂O₃ does not have strong enough adhesion to the substrate and the coating does not remain intact during sliding. Importantly, high temperature test at 300°C also indicated presence of the carbon G peak and D peak located at 1,583 and 1,340 cm⁻¹ respectively, that were not observed for the coatings heated but without being subjected to the shear stresses. It is worth to mention that the D peak shifts toward the low wavenumber with the influence of the Si element (Veres et al., 2006).

The structure and composition of the tribofilm formed on the Sb₂O₃-MSH/C coating at 300°C were analyzed using the TEM analysis. For this, we use the Focused Ion Beam (FIB) technology to prepare a samples from the worn surface of Sb₂O₃-MSH/C after the test (Figure 10). The cross-sectional TEM image in Figure 10a and mapping results in Figures 10b–h indicate the existence of a Sb₂O₃ layer and an amorphous carbon layer with thicknesses reaching 52.2 and 40.1 nm, respectively. In contrast to the layered structure of the carbon layer observed in Figure 1D, the formed tribofilm is mostly of the amorphous nature. The High-resolution TEM image of Section A in Figure 10i shows the borderline between alloy base and Sb₂O₃ layer. Nanocrystalline structure and amorphous structure both existed in this Sb₂O₃ layer. Interestingly, the border section is occupied by the amorphous Sb₂O₃, not only in Section A but also in Section B shown in Figure 10j, which is the transition region between the Sb₂O₃ layer and amorphous carbon layer.

The TEM results of the wear track were complemented by the XPS characterization of the tribofilm surface (Figure 11). There was no obvious signal of Fe, Ni and other elements belonging to the metal substrate in Figure 11, which indicates the metal matrix is not exposed during the friction process. The worn surface is mainly composed of C, O, and Sb elements with the elemental concentration (at.%) of 24.7, 62.2, and 7.6%, which is in agreement with the TEM results: an outer carbon layer, an inner Sb₂O₃ layer. The amorphous carbon layer in Figure 10j is made up of C-C, C-O, and COO⁻ as the C 1s peak can be fitted by three peaks at 284.8, 286.1, and 288.6 eV (Grzybek et al., 2004; Morent et al., 2008; Cai and Zhang, 2013; Samanta et al., 2019). The C-C bond only accounted for 66.1%, which indicates the oxidation reaction takes place in the process of high temperature friction resulting in about third of C atoms being bonded with O atoms to form C-O and O-C=O groups. The O 1s peak at 531.5 eV is covered by Sb 3d_5/2 at 530.6 eV, but it can be separated by the rules, such as the equal half peak width, the peak distance (9.3 eV), and the relative area ratio (1.5) between Sb 3d_5/2 peak and Sb3d_3/2 peak that is centered at 539.9 eV (Honma et al., 2000; Huang and Ruiz, 2005). In addition, Mg and Si are also detected on the surface of the tribofilm, though at much lower concentration: Mg 2p 2.1%, Si 2p 3.2%, existing in the form of Si-O-C and Mg-O (Ardizzone et al., 1997; Jantschner et al., 2015; Zhang et al., 2020), which indicates the decomposition of in the friction process.

Overall, the characterization results suggest that during sliding the carbon initially covering the MSH surface has been tribochemically converted to a more disordered amorphous carbon layer protecting the underlying substrate. At the same time, the structural transformation has been accompanied by the oxidation induced by the open air conditions and MSH chemistry and accelerated by high temperature. Previously it has been shown that during sliding the MSH crystals tend to decompose, releasing Si-O, Mg-O, and -OH active groups which could promote formation of the oxide layer as the lubricating oil addtive (Wang et al., 2020). Here, in dry sliding conditions, these MSH-based active groups could cause the formation of the Si-O-C bonds (Jantschner et al., 2015). The tribochemically-activated formation of the composite amorphous carbon layer doped with Si and O on the surface of Sb₂O₃ is the key to reducing of the COF and achieving the superlubricity at 300°C, which has been confirmed earlier (Jantschner et al., 2014, 2015; Evaristo et al., 2016). The thermomechanical process also promotes healing of the creacks and defects in the film and improves the coating resistance to fracture and fatigue in the high-temperature contact. M. Evaristo et al. found that the incorporation of Si and O in the amorphous carbon layer can reduce the friction coefficient from 0.17 to 0.034 (Evaristo et al., 2016). O. Jantschner et al. pointed out that the addition of a small amount of Si (<5 at.%) to the amorphous carbon coating at 240–450°C also beneficially affects the tribological performance of the film [the COF was reduced from 0.3 (amorphous carbon) to <0.02] (Jantschner et al., 2014, 2015). Although the mechanism of friction reduction with Si and O doping is not clear, it is assumed the formation of Si-O-C compounds in the sliding surface could help protect the surface near C=O, thus inducing easier sliding at the interfaces (Jantschner et al., 2015).

In conclusion, we designed a new MSH/C-based coating using burnishing process. While the friction remained relatively high during the room temperature tests, our analysis indicated excellent tribological performance of the coating when tested at elevated temperature of 300°C. The wear track analysis suggests that the origin of such high temperature behavior of the coating can be attributed to the formation of low-friction tribofilm that is enabled by tribochemically-induced formation of carbon layer on the surface of the coating. This film provides better protection of the surface during sliding and prevents material wear or transfer to the counterpart surface.

High temperature in-situ Raman analysis of the films further supported the findings of the study suggesting that the superlibric nature of the coating is a synergistic effect of the antimony oxide adhesion layer, tribochemically-active MSH powders, and tribochemically-generated surface carbon film.

Our results provide new insights into high temperature lubrication systems that can be easily self-repaired by supplying the materials to the sliding contacts through the burnishing process.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

KG: investigation, data curation, and writing. BW and AS: data curation. QC: resources. DB: supervision, investigation, conceptualization, and writing. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.