Macroscale Superlubricity with Ultralow Wear and Ultrashort Running-In Period (∼1 s) through Phytic Acid-Based Complex Green Liquid Lubricants

Breaking the Tradeoff between Oil Film Thickness and Viscous Friction: n-Alcohol-Containing Lubricants in High-Pressure Contacts

Low-friction lubricant formulations are urgently needed to improve the energy efficiency of machines. Here, we show that blending 1-dodecanol with a hydrocarbon oil improves lubrication in nonconformal sliding/rolling contacts by simultaneously increasing hydrodynamic film thickness and reducing viscous friction. This is due to pressure-induced polymorphic phase transformations in the 1-dodecanol molecules after they flow through the film-thickness-determining inlet and reach the load-supporting zone. At relatively low pressures, 1-dodecanol forms a lamellar hexagonal solid polymorph that gives durable superlubricity and then, at higher pressures, it forms an orthorhombic polymorph. Both polymorphs cause anomalously low friction when blended into various hydrocarbon base oils over a wide range of speed, pressure, and shear rate conditions representative of rolling bearing and gear contacts. By breaking the ubiquitous tradeoff between friction and film thickness and enabling superlubricity, these blends pave the way for considerable energy efficiency improvements in widespread lubricated contacts.

Macro-Superlubricity Induced by Tribocatalysis of High-Entropy Ceramics

Macroscale superlubricity has attracted considerable attention as a promising strategy to minimize frictional energy dissipation and achieve near-zero wear. However, realizing macroscale superlubricity with prolonged durability remains an immense challenge, particularly on engineering steels. Current superlubricants render steel surfaces susceptible to corrosion, causing severe wear and superlubrication failure. Herein, high-entropy ceramics (HEC) with catalytic properties are innovatively introduced to prevent corrosion of engineering steels and achieve macro-superlubricity through tribo-catalytic effect. Furthermore, this catalytically induced superlubricity system exhibits an ultra-low friction coefficient of 0.0037 under contact pressure up to 1.47 GPa, an ultra-long cycle lifetime of 1.25 × 106 cycles (corresponding sliding distance up to 5 km), and an extremely low wear rate of 3.032 × 10-10 mm3·N-1·m-1 on the HEC surface. Based on the experimental analysis and theoretical simulation, the in situ formed HEC nanocrystals reduce the Gibbs free energy of hydrolysis of PA molecules into inositol and phosphoric acid molecules in the lubricant. Notably, the hydrolysis products favorably contributed to the reduction of shear force in the lubrication system, which is essential for achieving macroscale superlubricity over a long time. This study provides a new perspective for designing robust superlubricity systems by harnessing the tribocatalytic effect of high-entropy materials.

Improved Friction Reduction and Wear Resistance of Steel Using a Subnanometer Nanowires-Poly-α-Olefin Gel Lubricant

Lubricating oil is commonly utilized due to its excellent lubricating properties in mechanical motion systems. However, high fluidity in lubricating oil often leads to leakage during machine operation, causing mechanical components to fail. Herein, a gel lubricant of SNWs-PAO6 was devised by combining subnanometer nanowires (SNWs) with poly α-olefin 6 (PAO6) at room temperature, effectively confining PAO6 and preventing PAO6's creeping and leakage while enhancing its load-bearing capacity. SNWs-PAO6 outperforms PAO6 in reducing friction and wear for steel-on-steel interfaces. The friction coefficient is markedly diminished by 57.53%, from 0.223 to 0.095, while the wear rate is significantly curtailed by 84.98%. Furthermore, SNWs-PAO6 remains stable even after 180 000 cycles at 200 N and 25 Hz, withstanding high-speed centrifugation without releasing PAO6. It remained stable over 6 months of resting and can well withstand low temperatures. Surface analysis of the wear scar and the formed tribochemical film after friction has demonstrated that PW12O403- is more likely to adsorb onto the steel surface, forming a lubricating medium film through tribochemical reactions and thus reducing interfacial friction and wear. This method facilitates the development of domain-limited nanomaterials-based gels for mass production and their applications in engineering moving parts.

Macroscale Superlubrication Achieved with Shear-Thinning Semisolid Lubricants

Macrosuperlubric materials are pivotal for reducing friction and wear in engineering applications. However, current solid superlubricants require intricate fabrication and specific conditions (e.g., vacuum or inert atmospheres), while liquid superlubricants are prone to creep, leakage, and corrosion. Here, a novel semisolid subnanometer nanowire (SNW) superlubrication material based on the shear-thinning effect is introduced to overcome these challenges. The SNWs achieve an exceptionally low friction coefficient (0.008-0.009) with silicon nitride (Si3N4) and polytetrafluoroethylene (PTFE) tribo-pairs, demonstrating a brief running-in period (≈39 s) and stable superlubrication over extended friction (12 h, >120 000 cycles). The combination of the shear-thinning network structure mechanism, the adsorption membrane mechanism, and hydrodynamic effects provides a synergistic effect, playing a crucial role in achieving superlubricity. Additionally, SNWs can be combined with various base oils to create semisolid gel lubricants with superlubricating properties. This innovative approach addresses the limitations of current superlubrication systems and introduces a new category of semisolid gel lubricants, significantly expanding the applications of superlubrication materials.

Competition-Induced Macroscopic Superlubricity of Ionic Liquid Analogues by Hydroxyl Ligands Revealed by in Situ Raman

High load-bearing capacity is one of the crucial indicators for liquid superlubricants to move toward practicality. However, some of the current emerging systems not only have low contact pressures but also are highly susceptible to further degradation due to water adsorption and even superlubricity failure. Herein, a novel choline chloride-based ionic liquid analogues (ILAs) of a superlubricant with triethanolamine (TEOA) as the H-bond donor is reported for the first time; it obtains an ultralow coefficient of friction (0.005) and high load-bearing capacity (360 MPa, more than 2 times that of similar systems) due to adsorption of a small amount of water (<5 wt %) from the air. In situ Raman combined with 1H NMR and FTIR techniques reveals that adsorbed water competes with the hydroxyl group of TEOA for coordination with Cl-, leading to the conversion of some strong H-bonds to weak H-bonds in ILAs; the localized strong H-bonds and weak H-bonds endow the ILAs with high load-bearing capacity and the formation of ultralow shear-resistance sliding interfaces, respectively, under the shear motion. This study proposes a strategy to modulate the interactions between liquid species using adsorbed water from air as a competing ligand, which provides new insights into the design of ILA-based macroscopic liquid superlubricants with a high load-bearing capacity.