Superlubrication is a sliding state in which friction almost disappears under strict material conditions. This remarkable phenomenon first appeared between surfaces with disproportionate lattice structures, and later extended to materials with weak shear forces, such as many two-dimensional layered materials. However, these materials face significant challenges that limit their application, including the difficulty of scaling up to meet the needs of many real-world applications, as well as low durability due to inherent structural weaknesses. Overcoming these limitations depends on the new design and manufacture of advanced superlubricating materials.
Scholars from Jilin University and the University of Nevada, Las Vegas, USA, report on the achievement of macroscopic super-durable superlubrication on passivated transition metal diboride (TMB2), which employs an unconventional design principle of first constructing a material with strong wear resistance and then creating superlubricity through effective surface passivation. TMB2 not only offers excellent mechanical strength and durability, but also activates an in-situ load-induced passivation layer through surface tribochemistry. The resulting passivated TMB2 has an ultra-lubricated, ultra-durable coefficient of friction (0.002) and wear rate (10-19 m3/N-m). These findings introduce an entirely new approach to the rational design and application of superlubricating materials, opening up a new avenue for large-scale (even industrial-scale) multifunctional applications. The work was published in Acta Materialia in a research article titled "Macroscale ultradurable superlubricity on passivated transition-metal diborides".
Paper Links:
Hattaps://doi.org/10.1016/j.aktamat.2023.119439
Figure 1. a) Representative load-unload curves of WB2 films. b) Representative hardness of WB2 films, TMB2 films, and conventional lubricating materials as reported in the literature. c) 3D view of the WB2 crystal structure. d) Comparison of the bonding mode of the WB2 structural element under wear shear strain in the direction of (001)[1-10] with a non-strain equilibrium structure at a strain of 0.15. e, f) Stress responses to pure shear strain and biaxial strain (additional normal stress σzz, can be set to multiple values) in the (001)[110] and (001)[1-10] shear-slip directions as shown in the figure. g) The peak shear stress as a function of normal stress, confirming the compressive strain stiffness of WB2.
Figure 2. Friction and wear behavior of WB2 films in NPGD. a) Coefficient of dynamic friction of WB2 films measured in ball-to-disc mode. The sliding speed and normal load are 3 cm/s and 5 N, respectively. Illustration I shows the structural formula of NPGD. Illustrations II and III show the coefficient of friction of the WB2 film in NPGD at different loads and velocities. Yellow shaded areas indicate a state of superlubrication. b) 3D cross-sectional and cross-sectional view of the wear trajectories of the WB2 film taken after 1 × 105 laps. To measure the wear state, the film is sonicated with acetone before the 3D profile and cross-section are taken. By fitting the cross-sectional data, the wear rate of the WB2 film was calculated to be 8.8 × 10-19m3/N-m. c) Diagram of the coefficient of friction and wear rate of the SL system, including the sliding results of WB2 vs. Al2O3 currently in NPGD
Figure 3. a) HRSEM micrograph of the worn surface of the WB2 film after 1 × 105 turns of sliding in NPGD. b-d) Determine and cut the location of the wear trajectory. e) Cut image. f) HRTEM and IFFT images of WB2 film after 1 × 105 slides. h) HAADF-STEM image of the cross-section of the wear trajectory after 1 × 105 slips and the element EDS diagram.
Figure 4. Post-test characterization of three membranes on WB2 thin film wear tracks. a) Raman spectra of the initial NPGD and WB2 thin film wear track surfaces b) Fourier transform infrared spectra of the initial NPGD and WB2 thin film wear track surfaces c) TOF-SIMS images showing the distribution of chemical fragments within the wear orbit d) Simulation of tribocatalysis on WB2 (001) surfaces by AIMD. An NPGD molecule is randomly placed on the surface. At 30 and 80 fs, the NPGD molecule splits
Figure 5. Verify the feasibility of overlubrication with MD to verify the lubrication model. a) Schematic diagram of a disk-on-ball loading device for tribobology testing of WB2 films in NPGD. b) A tribological model of WB2 thin films in NPGD for molecular dynamics simulations based on the knowledge obtained from the experiments. WB2, represented by C3H7O- and CH3COO-, adsorbed organic self-assembled passivation layers on the surface of Al2O3 and WB2 films under the tribocatalysis of NPGD. c) Typical simulation models. d) Change in coefficient of friction at 0.6 GPa pressure. The light yellow area represents the SL datum
Figure 6. The versatility of the ultra-lubricated and ultra-wear-resistant TMB2 system. a) Triboride tests of a series of transition metals from groups IV.B to VI.B were selected as anti-wear films to verify the versatility of the super-lubricated and super-anti-wear systems. The parameters of the friction test were consistent with those of WB2+NPGD, and the test was performed for 50,000 turns. b) Steady-state friction coefficients of TiB2, ZrB2, HfB2, NbB2, and TaB2 in NPGD.
In this study, a new lubricating material design strategy was designed and implemented, and the macroscopic super-durability and super-lubrication were achieved through unconventional approaches. This study skillfully leverages and combines the physical and chemical properties of transition metal diborides and unsaturated reactive lubricants. Analysis based on extensive and in-depth experimental characterization and computational simulation reveals its intrinsic mechanism. From the results of the study, the following conclusions can be drawn:1. TMB2 compounds exhibit superior mechanical strength and durability and are resistant to structural deformation and degradation, which contributes to ultra-low wear rates under frictional shear loads. When TMB2 film and unsaturated reactive lubricant form a solid-liquid composite lubrication system, the tribochemical reaction of TMB2 film under tribothermal and mechanical stress will promote the formation of organic self-assembled passivation layer at the frictional interface. Boundary lubrication and elastohydrodynamic lubrication at the contact interface coexist under the appropriate sliding speed and normal load conditions, and in this special sliding state, superlubrication is activated. (Text: SSC)
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