Wear Resistance Limited by Step Edge Failure: The Rise and Fall of Graphene as an Atomically Thin Lubricating Material

Owing to its intrinsically lubricious property, graphene has a high potential to be an atomically thin solid lubricant for sliding interfaces. Despite its ultrahigh breaking strength at the nanoscale, graphene often fails to maintain its integrity when subjected to macroscale tribological tests. To reveal the true wear characteristics of graphene, a nanoscale diamond tip was used to scratch monolayer graphene mechanically exfoliated to SiO₂ substrates. Our experimental results show that while graphene exhibited extraordinary wear resistance in the interior region, it could be easily damaged at the step edge under a much lower normal load (∼2 orders of magnitude smaller). Similar behavior with substantially reduced wear resistance at the edge was also observed for monatomic graphene layer on graphite surface. Using molecular dynamics simulations, we attributed this markedly weak wear resistance at the step edge to two primary mechanisms, i.e., atom-by-atom adhesive wear and peel induced rupture. Our findings shed light on the paradox that graphene is nanoscopically strong yet macroscopically weak. As step edge is ubiquitous for two-dimensional materials at the macroscale, our study also provides a guiding direction for maximizing the mechanical and tribological performance of these atomically thin materials

Revisiting the Critical Condition for the Cassie–Wenzel Transition on Micropillar-Structured Surfaces

Biological and engineering applications of superhydrophobic surfaces are limited by the stability of the wetting state determined by the transition from the Cassie–Baxter state to the Wenzel state (C–W transition). In this paper, we performed water droplet squeeze tests to investigate the critical conditions for the C–W transition for solid surfaces with periodic micropillar arrays. The experimental results indicate that the critical transition pressures for the samples with varying micropillar dimensions are all significantly higher than the theoretical predictions. Through independent measurements, we attributed the disparity to the incorrect assessment of the contact angle on the sidewall surfaces of the micropillars. We also showed that the theoretical models are still applicable when the correct contact angle of the sidewall surfaces is adopted. Our work directly validates and improves the theoretical models regarding the C–W transition and suggests a potential route of tuning superhydrophobicity using finer scale surface features

Friction of Droplets Sliding on Microstructured Superhydrophobic Surfaces

Liquid transport is a fundamental process relevant to a wide range of applications, for example, heat transfer, anti-icing, self-cleaning, drag reduction, and microfluidic systems. For these applications, a deeper understanding of the sliding behavior of water droplets on solid surfaces is of particular importance. In this study, the frictional behavior of water droplets sliding on superhydrophobic surfaces decorated with micropillar arrays was studied using a nanotribometer. Our experiments show that surfaces with a higher solid area fraction generally exhibited larger friction, although friction might drop when the solid area fraction was close to unity. More interestingly, we found that the sliding friction of droplets was enhanced when the dimension of the microstructures increased, showing a distinct size effect. The nonmonotonic dependence of friction force on solid area fraction and the apparent size effect can be qualitatively explained by the evolution of two governing factors, that is, the true length of the contact line and the coordination degree of the depinning events. The mechanisms are expected to be generally applicable for other liquid transport processes involving the dynamic motion of a three-phase contact line, which may provide a new means of tuning liquid-transfer behavior through surface microstructures

Water Resistance, Mechanical Properties and Water-Induced Shape Memory Properties of Poly(vinyl Alcohol) Materials Modified by Waste Polyester Depolymerization Monomer

Depolymerizing waste polyethylene terephthalate (PET) into monomers and subsequently processing and utilizing them is widely acknowledged as one of the most effective recycling methods for waste PET. The poor water stability of poly(vinyl alcohol) (PVA) necessitates modification to enhance its application scope. Thus, this study explores the use of recycled terephthalic acid (rTPA) obtained from waste PET depolymerization to modify PVA, aiming to improve its water resistance, functionalize it, and expand its potential applications. Initially, the k-rTPA modifier was synthesized by treating rTPA with a silane coupling agent (γ-aminopropyltriethoxysilane, KH550). Subsequently, the k-rTPA modifier was employed to enhance the mechanical properties and thermal stability of PVA. Specifically, the elongation at break of the PVA/k-rTPA mixture increased from 169.87 to 364.67%, representing a 114.6% improvement over pure PVA. Moreover, the water resistance was significantly enhanced, indicated by a reduction in the equilibrium swelling rate from 197.5 to 76.6%, marking a 157.8% increase, as well as an increase in the contact angle and extended water dissolution time. Furthermore, the PVA/k-rTPA material demonstrated remarkable water-induced shape memory properties. Consequently, the introduction of the k-rTPA modifier notably enhances the performance of PVA materials, suggesting significant potential applications and broadening its scope of utilization

Water Resistance, Mechanical Properties and Water-Induced Shape Memory Properties of Poly(vinyl Alcohol) Materials Modified by Waste Polyester Depolymerization Monomer

Depolymerizing waste polyethylene terephthalate (PET) into monomers and subsequently processing and utilizing them is widely acknowledged as one of the most effective recycling methods for waste PET. The poor water stability of poly(vinyl alcohol) (PVA) necessitates modification to enhance its application scope. Thus, this study explores the use of recycled terephthalic acid (rTPA) obtained from waste PET depolymerization to modify PVA, aiming to improve its water resistance, functionalize it, and expand its potential applications. Initially, the k-rTPA modifier was synthesized by treating rTPA with a silane coupling agent (γ-aminopropyltriethoxysilane, KH550). Subsequently, the k-rTPA modifier was employed to enhance the mechanical properties and thermal stability of PVA. Specifically, the elongation at break of the PVA/k-rTPA mixture increased from 169.87 to 364.67%, representing a 114.6% improvement over pure PVA. Moreover, the water resistance was significantly enhanced, indicated by a reduction in the equilibrium swelling rate from 197.5 to 76.6%, marking a 157.8% increase, as well as an increase in the contact angle and extended water dissolution time. Furthermore, the PVA/k-rTPA material demonstrated remarkable water-induced shape memory properties. Consequently, the introduction of the k-rTPA modifier notably enhances the performance of PVA materials, suggesting significant potential applications and broadening its scope of utilization

Revisiting Frictional Characteristics of Graphene: Effect of In-Plane Straining

Supreme mechanical performance and tribological properties render graphene a promising candidate as a surface friction modifier. Recently, it has been demonstrated that applying in-plane strain can effectively tune friction of suspended graphene in a reversible manner. However, since graphene is deposited on solid surfaces in most tribological applications, whether such operation will result in a similar modulation effect becomes a critical question to be answered. Herein, by depositing graphene onto a stretchable substrate, the frictional characteristics of supported graphene under a wide range of strain are examined with an in situ tensile loading platform. The experimental results show that friction of supported graphene decreases with increasing graphene strain, similar to the suspended system. However, depending on the adherence state of the graphene/substrate interface, the system exhibits two distinct friction regimes with significantly different strain dependences. Assisted by detailed atomic force microscopy imaging, we attribute the unique behavior to the transition between two friction modulation modes, i.e., contact-quality-dominated friction and puckering-dominated friction. This work provides a more comprehensive view of the influence of strain on surface friction of graphene, which is beneficial for active modulation of graphene friction through strain engineering

State-of-the-Art of Extreme Pressure Lubrication Realized with the High Thermal Diffusivity of Liquid Metal

Sliding between two objects under very high load generally involves direct solid–solid contact at molecular/atomic level, the mechanism of which is far from clearly disclosed yet. Those microscopic solid–solid contacts could easily lead to local melting of rough surfaces. At extreme conditions, this local melting could propagate to the seizure and welding of the entire interface. Traditionally, the microscopic solid–solid contact is alleviated by various lubricants and additives based on their improved mechanical properties. In this work, we realized the state-of-the-art of extreme pressure lubrication by utilizing the high thermal diffusivity of liquid metal, 2 orders of magnitude higher than general organic lubricants. The extreme pressure lubrication property of gallium based liquid metal (GBLM) was compared with gear oil and poly-α-olefin in a four-ball test. The liquid metal lubricates very well at an extremely high load (10 kN, the maximum capability of a four-ball tester) at a rotation speed of 1800 rpm for a duration of several minutes, much better than traditional organic lubricants which typically break down within seconds at a load of a few kN. Our comparative experiments and analysis showed that this superextreme pressure lubrication capability of GBLM was attributed to the synergetic effect of the ultrafast heat dissipation of GBLM and the low friction coefficient of FeGa₃ tribo-film. The present work demonstrated a novel way of improving lubrication capability by enhancing the lubricant thermal properties, which might lead to mechanical systems with much higher reliability

Superlubricity Enabled by Pressure-Induced Friction Collapse

From daily intuitions to sophisticated atomic-scale experiments, friction is usually found to increase with normal load. Using first-principle calculations, here we show that the sliding friction of a graphene/graphene system can decrease with increasing normal load and collapse to nearly zero at a critical point. The unusual collapse of friction is attributed to an abnormal transition of the sliding potential energy surface from corrugated, to substantially flattened, and eventually to counter-corrugated states. The energy dissipation during the mutual sliding is thus suppressed sufficiently under the critical pressure. The friction collapse behavior is reproducible for other sliding systems, such as Xe/Cu, Pd/graphite, and MoS₂/MoS₂, suggesting its universality. The proposed mechanism for diminishing energy corrugation under critical normal load, added to the traditional structural lubricity, enriches our fundamental understanding about superlubricity and isostructural phase transitions and offers a novel means of achieving nearly frictionless sliding interfaces

Fluorination of Graphene Enhances Friction Due to Increased Corrugation

The addition of a single sheet of carbon atoms in the form of graphene can drastically alter friction between a nanoscale probe tip and a surface. Here, for the first time we show that friction can be altered over a wide range by fluorination. Specifically, the friction force between silicon atomic force microscopy tips and monolayer fluorinated graphene can range from 5−9 times higher than for graphene. While consistent with previous reports, the combined interpretation from our experiments and molecular dynamics simulations allows us to propose a novel mechanism: that the dramatic friction enhancement results from increased corrugation of the interfacial potential due to the strong local charge concentrated at fluorine sites, consistent with the Prandtl-Tomlinson model. The monotonic increase of friction with fluorination in experiments also demonstrates that friction force measurements provide a sensitive local probe of the degree of fluorination. Additionally, we found a transition from ordered to disordered atomic stick–slip upon fluorination, suggesting that fluorination proceeds in a spatially random manner