Ultra-Wear-Resistant MXenes-Based Composite Coating via In-Situ

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Surfaces, Interfaces, and Applications

Ultra-Wear-Resistant MXenes-Based Composite Coating via In-Situ Formed Nanostructured Tribofilm Xuan Yin, Jie Jin, Xinchun Chen, Andreas Rosenkranz, and Jianbin Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11449 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 18, 2019

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ACS Applied Materials & Interfaces

Ultra-Wear-Resistant MXenes-Based Composite Coating via In-Situ Formed Nanostructured Tribofilm Xuan Yin,† Jie Jin,‡ Xinchun Chen,*,† Andreas Rosenkranz,§ Jianbin Luo*,† †State

Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua

University, Beijing 100084, China ‡School

of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing

100044, China §Department

of Chemical Engineering, Biotechnology and Materials, FCFM, Universidad de

Chile, Santiago, Chile ABSTRACTS: The newly-emerging two-dimensional (2D) material of MXenes possesses lots of merits, which provide potential solutions for the lubrication issues in harsh conditions. Here, preliminary efforts were devoted to developing MXenes-based 2D composite coating and its anti-wear interfacial performance in ambient environments. Macro-scale and atomic-scale characterizations were utilized to explore the lubrication behaviors of the composite coating to clarify the influence of the coating composition and tribo-test parameters in the establishment of ultra-wear-resistant sliding interfaces. The results highlighted a unique lubrication mechanism for 2D MXenes composite coating. They suggested that the MXenes/nanodiamond coating exhibited almost no wear when rubbing against polytetrafluoroethylene (PTFE) ball. A nanostructured tribofilm with unprecedented bonding features was in-situ formed along the sliding interface. The ultra-wear-

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resistance highly depended on the combined effects of shielding and self-lubrication of PTFE, layer shearing of MXenes and self-rolling of nanodiamond. These discoveries clearly enrich the 2D material-based lubrication theories and offer technical guidance for designing and exploiting high-performance ultra-wear-resistant materials. KEYWORDS: MXenes, 2D composite coating, nanodiamond, PTFE, ultra-wear-resistant, tribofilm 1. INTRODUCTION With the development of advanced technical equipment such as aviation, aerospace and nuclear power, many mechanical parts are faced with harsh service environments like heavy load, high vacuum, cryogenic temperature and high speed. Lubricants endowing the sliding interface with ultra-low wear are highly desirable for their potential huge impact on materials and energy saving. Conventional two-dimensional (2D) lubricants like graphene14

and MoS25-7 are definitely on the candidate list owing to their exceptional anti-friction

capacities. However, a super-low friction state is usually achieved by them in specific environments. Some inherent performance defects like moisture-induced structural degradation seriously hinder practical applications in ambient environments. Recently, a new family of 2D early transition metal carbides and carbonitrides, named MXenes8-12, have emerged as a promising material used for surface and interface areas. A single MXenes sheet is composed of three, five or seven atomic layers, between which larger interlamellar spacing than graphene is observed.11 Theoretically, MXenes like 2D Ti3C2 has properties of self-lubrication, high wear-resistance, conductivity, high-temperature structural stability and chemical inertness.


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Although MXenes has been widely investigated in electrochemical energy storage recently, it is still rarely reported in tribology.13-14 Yang15 and Liu16 as pioneers found a proper addition of exfoliated 2D Ti3C2 in base oils could form uniform and continuous tribofilms to induce anti-wear performance on the contact surface during sliding. Nevertheless, morein-depth insights into underlying mechanisms have not been mentioned. Therefore, it is significant to disclose the mysterious veil of this new kind of 2D lubricant to the public, and to explore the possibility of constructing a robust lubricative interface with ultra-low wear rate. Additionally, as common functional-reinforced materials, nanodiamonds are spherical inorganic nanoparticles, which have been widely applied in lubricating oils, metal deposition, magnetic recording system and medicine, and are still continuously expanding their application range.17-20 For instance, lubricating oil combined with nanodiamond could dramatically reduce friction and extend working life of the as-protected objects.21 But there are still few studies that combine nanodiamond with layered 2D materials. When combining MXenes with nanodiamond, the composite material might have integrated characters of both, especially the reinforced tribological properties. The underlying lubrication mechanism is therefore worth particular attention. Besides, polymers as rubbing counterparts are drawing a lot of attention due to their capacities to adapt harsh conditions. General-purpose plastics like polypropylene (PP) and polymethyl methacrylate (PMMA), engineering

plastics

like

polyamide-66

(PA66),

polyformaldehyde

(POM),

polyetheretherketone (PEEK) and polytetrafluoroethylene (PTFE), are widely utilized in mechanical

engineering

area,

which

have

special

and

excellent

tribological

performances.22-23 Especially, as a representative self-lubricating polymer, PTFE exhibits


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a synergetic lubrication effect during rubbing against 2D material, which fits well with the application demands in extreme conditions like cryogenic joint bearing.24-25 Above all, we designed MXenes coating and MXenes/nanodiamond composite coating, respectively, by a new synthesis technique named coating method. This novel method can straightway combine different lubricants and yield a better adhesion for the as-synthesized coating so that it will adapt to various working environments. The aim of this paper is to provide reference evidence for design criterion and lubricity characteristics of MXenes-based 2D composite coating, and to illuminate the unprecedented ultra-wear-resistant mechanism. Significantly, not only lubrication theory is enriched upon these achievements, but also theoretical and technical guidance are developed for designing and exploiting high-performance solid lubricants. 2. RESULTS AND DISCUSSION 2.1. MXenes Coating 2.1.1. Structure Figure 1 shows the characterization of atomic microstructure and tribological properties for MXenes coating tested under 1.0 N. From Figure 1a, the surface and interlayer structures of 2D Ti3C2 are clearly observed. Its atom arrangement is very regular and the interlamellar spacing is close to 1.0 nm. Besides, the crystal lattice arranges in a regular hexagonal form (Figure 1b). Figure 1c and 1d show the atomic-scale microstructural details of 2D Ti3C2 through dual-aberration-corrected scanning transmission electron microscope and electron energy-loss microscopy (STEM-EELS). The triatomic layer structure of 2D Ti3C2 is clearly resolved in the false-colour displayed high angle annular dark field (HAADF) STEM image. Meanwhile, the EELS data of C-K and Ti-L edges recorded from the inner- and interlayer positions confirms the subtle layer-by-layer


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microstructure of 2D Ti3C2 lubricant.26 Figure 1e further shows structural details of the multi-layered 2D Ti3C2 nano-sheets via paired bright field (BF) and HAADF-STEM images. The well-ordered triatomic layer structure of 2D Ti3C2 is confirmed. 2.1.2. Tribological Properties Furthermore, the tribological properties of MXenes coating tested under 1.0 N vs various counterfacing balls are shown in Figure 2. The average and final coefficients of friction (COF) are listed in Table 1. As seen from the friction curves, the maximum average COF is generated from the bearing steel ball and the minimum average COF is produced by the PTFE ball. PTFE is a self-lubricating material, which exhibits a synergetic lubrication effect when rubbing against MXenes coating.22, 25 MXenes is a family of 2D metal carbide materials. Low flexible strength counterpart ball can quickly break the regular layered-structure of MXenes to produce chippings and amorphous carbon. Therefore, PMMA as a poor surface hardness polymer material cannot provide good wear resistance for MXenes coating. In addition, as tribological polymer materials, the low modulus is considered to be desirable since both tribo-noise and frictional forces were reduced during sliding motion.27-28 The low modulus of materials can significantly alter the pathway of contact between asperities, promoting premature and extended contact. The tensile strength and flexural modulus of six counterfacing polymer balls are listed in Table 2.29 Among the six types of polymers, the hardness of PTFE and PP is relatively low, especially PTFE. Therefore, the average COFs of PP and PTFE are relatively low. Besides, PP and PTFE are long straight chain polymers, which have only two types of chemical bonds (PP consists of C-H and C-C bonds, and PTFE consists of C-F and C-C bonds).30,31 The surface energies of them are low enough to form hydrophobic interfaces to yield effective lubrication. According to these friction results, the counterpart PTFE ball can improve the lubrication of MXenes coating obviously.


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Therefore, the tribo-couple of PTFE ball vs MXenes coating is chosen for the characterization of the wear properties (Figure 2b and 2c). The two-dimensional (2D) white-light interference morphology indicates the ultra-smoothness of the wear track and the corresponding near-zero wear depth, which clearly confirms the exceptional wear-resistance of MXenes 2D Ti3C2. The contact areas are further characterized by Raman spectrum. Figure 2d displays some interesting findings: the Raman signal from the wear scar resembles the MXenes coating and the wear track resembles the Si disc. It suggests that the MXenes coating is transferred from the disc side and thus covers the PTFE ball’s surface, as peaks of PTFE are not found in Raman spectrum of the wear scar. Besides, stronger D-peak (1344 cm-1) and G-peak (1570 cm-1) are detected in the wear scar as compared to the MXenes coating, which implies the formation of more amorphous carbon materials during sliding. Moreover, only a very strong Si peak (521 cm-1) along with two nearlyindistinguishable MXenes characteristic peaks (216 cm-1 and 373 cm-1) are detected for the wear track surface.32 This also verifies the tribo-induced transfer of 2D Ti3C2. Therefore, the formation of 2D Ti3C2-rich tribofilm is speculated as the gradual accumulation of 2D Ti3C2 nanoflakes during sliding, which can retain the 2D structure of MXenes to produce low COFs. However, the durability of low COFs does not last long, probably due to the fact that the as-formed tribofilm cannot provide a robust protection and lubrication effect on the interface. 2.2. MXenes/Nanodiamond Composite Coating 2.2.1. Basic Anti-Wear Test Figure 3 displays tribological properties for nanodiamond coating tested under 1.0 N vs various counterfacing balls. Interestingly, the friction test results are quite different between MXenes coating and nanodiamond coating. When rubbing against PEEK and PP, the average COFs are the minimum. PEEK and PP have brilliant fatigue durability and relatively high hardness.33-35 Due to


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its excellent elastic modulus and strength, PEEK is normally used as a replacement for machined metals in a wide variety of high-performance applications. However, considering the friction test results of MXenes coating and nanodiamond coating, the PTFE ball exhibits the most superior friction reducing performance. Besides, nanodiamond coating has a better lubrication effect than MXenes coating as revealed by the friction test. Therefore, a composite coating is designed for further improving the tribological properties of MXenes coating. For the sake of comparison with MXenes coating, the PTFE ball is still chosen to further characterize the wear behaviors of MXenes/nanodiamond coating. Figure 4 shows the characterization of tribological properties and the as-formed nanostructured tribofilm for MXenes/nanodiamond coatings tested under 1.0 N against the PTFE ball. From Figure 4a, although the as-achieved COF of 0.16 is still much larger than 0.01 (definition level for superlubricity materials), it is almost half of that in MXenes coating. Moreover, the wear track is extremely smooth, as there is no wear depth detectable (Figure 4b). Rather, partial regions of the wear track surface are covered by some brown and black tribo-products. This implies that MXenes/nanodiamond coating is a type of ultra-wear-resistant material. In Figure 4c, the Raman peaks of the wear scar are found to resemble that of PTFE. Interestingly, a weak Raman peak of nanodiamond also appears at 1473 cm-1. Meanwhile, nanodiamond overlaps with some signals (1582 cm-1 and 1622 cm-1) of PTFE. For the wear track, besides the above peaks, another peak of nanodiamond appears at 1143 cm-1.21 This indicates that nanodiamond abundantly exits on the wear track surface. Additionally, three peaks of 2D Ti3C2 from 120 to 600 cm-1 are also found in the wear track, even though these peak positions are affected by the structure defects of PTFE. Moreover, obvious D-peak (1348 cm-1) and G-peak (1570 cm-1) are also detected in the wear track. The G-peak generally originates from sp2 planar packing in graphitic networks. Meanwhile, the


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increasing intensity of G-peak is significant as the sliding proceeds, which is speculated to come from the enhanced self-lubrication of PTFE upon contact.36-37 It suggests that both 2D Ti3C2 and PTFE are involved in forming anti-wear tribofilms. These results also verify the interfacial transfer of nanodiamond and MXenes during sliding. Hence, to further confirm our deductions, the wear track’s cross-section is in-situ lifted out by focused ion beam-scanning electron microscope (SEM/FIB) and then observed by high-resolution transmission electron microscope (HRTEM). 2.2.2. Durability Test To investigate the influence of applied load and sliding durability, a higher load of 2.0 N and a longer testing period of 30 min are used for the composite coating. Figure 5 shows the characterization of tribo-induced interfacial microstructure for MXenes/nanodiamond coating under 2.0 N sliding against the PTFE ball. In Figure 5a, the COF (1.0 N) is initially 0.14 and quickly increases to 0.26 (the average COF is 0.24). While, the COF under 2.0 N decreases from 0.16 to 0.15 (the average COF is 0.15) and can maintain this lower COF throughout the 30 min sliding period even though very mild wear occurs. PTFE is inclined to suffer plastic deformations during long-term loading, which leads to the polymer’s compaction.38-39 This is beneficial to form dense tribofilms to reduce COF. Although the wear depth appears to be visible (Figure 5b), the wear rate is still immeasurable by the instrument. From SEM images (Figure 5c), material accumulation is found on the wear track surface. 2.2.3. Nanostructures of the As-Formed Tribofilms The as-formed tribofilms are then investigated for details. For the case of 1 N, Figure 6a shows the presence of obvious material accumulations in the wear track by SEM. As shown by the Supporting Information (Movie S1), the wear track surface continuously fluctuates during electron beam scanning. This implies that the tribofilm is rich in polymer-related materials from PTFE. To


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verify it, an area containing key features is lifted out by FIB. Figure 6b shows the whole crosssection of tribofilm, divided into three regions. For Area I (Figure 6c), the tribofim is very thin (

Ultra-Wear-Resistant MXenes-Based Composite Coating via In-Situ

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