Friction and tribochemical reactions occurring at shearing interfaces of nanothin silver films on various substrates

Geocolloidal interactions and relaxation dynamics under nanoconfinement: Effects of salinity and particle concentration

HYPOTHESIS

Energy-related contaminants are frequently associated with geocolloids that translocate in underground fissures with dimensions comparable with geocolloids. To assess the transport and impact of energy-related contaminants in geological systems, fundamental understandings of interfacial behaviors of nanoparticles under confinement is imperative. We hypothesize that the dynamic properties of geocolloids, as well as their dependence on aqueous medium conditions would deviate from bulk behaviors under nanoconfinement.

EXPERIMENTS

Force profiles and rheological properties of 50 nm silica nanoparticles in aqueous media confined between mica surfaces as a function of surface separation, particle concentrations, and salinity were measured utilizing the surface forces apparatus.

FINDINGS

Force profiles revealed the critical surface separation for nonlinear rheological behaviors coincides with the onset of exponential repulsion between mica surfaces. When salts were absent, the normal forces and viscosity values of colloidal suspensions resembled pure water. In contrast, with salts, the force profiles and corresponding critical length scales were found to be highly sensitive to the particle concentration and the degree of confinement. A Newtonian to shear-thinning transition was captured with increasing degrees of confinement. Our results show that the interplay among confinement, particle, and ionic concentrations can alter the interparticle forces and rheological responses of true nanosized-colloidal suspensions and thus their transport behaviors under nanoconfinement for the first time.

Microstructure and Tribological Properties of Spark-Plasma-Sintered Ti3SiC2-Pb-Ag Composites at Elevated Temperatures

Ti₃SiC₂-PbO-Ag composites (TSC-PA) were successfully prepared using the spark plasma sintering (SPS) technique. The ingredient and morphology of the as-synthesized composites were elaborately investigated. The tribological properties of the TSC-PA pin sliding against Inconel 718 alloys disk at room temperature (RT) to 800 °C were examined in air. The wear mechanisms were argued elaborately. The results showed that the TSC-PA was mainly composed of Ti₃SiC₂, Pb, and Ag. The average friction coefficient of TSC-PA gradually decreased from 0.72 (RT) to 0.3 (800 °C), with the temperature increasing from RT to 800 °C. The wear rate of TSC-PA showed a decreasing trend, with the temperature rising from RT to 800 °C. The wear rate of Inconel 718 exhibited positive wear at RT and negative wear at elevated temperatures. The tribological property of TSC-PA was related to the tribo-chemistry, and the abrasive and adhesive wear.

Phase-dependent friction of nanoconfined water meniscus

A water meniscus naturally forms under ambient conditions at the point of contact between a nanoscale tip and an atomically flat substrate. Here, we study the effect of the phase state of this nanoscale meniscus-consisting of coexisting monolayer, bilayer and trilayer phase domains-on the frictional behavior during tip sliding by means of molecular dynamics simulations. While the meniscus experiences a domain-by-domain liquid-to-solid phase transition induced by lateral compression, we observe an evident transition in measured friction curves from continuous sliding to stick-slip and meanwhile a gradual increase in friction forces. Moreover, the stick-slip friction can be modulated by varying lattice orientation of the monolayer ice domain in the meniscus, choosing the sliding direction or applying in-plane strains to the substrate. Our results shed light on the rational design of high-performance micro- and nano-electromechanical systems relying on hydration lubrication.

Mechanochemical Reactions of Adsorbates at Tribological Interfaces: Tribopolymerizations of Allyl Alcohol Coadsorbed with Water on Silicon Oxide

Mechanochemical reactions of adsorbed molecules at tribological interfaces can benefit or impede lubrication, depending on the type of reactions induced by the interfacial shear or friction. Shear-induced polymerization of oxidatively chemisorbed organic species can occur at tribological interfaces, and their products can mitigate the wear of the surface in the case of the intermittent cessation of the lubricant supply. In contrast, tribochemical reactions involving water molecules impinging from the ambient air could facilitate surface wear. In this study, we investigated how such processes are affected when a silicon oxide surface is exposed to the environment containing both water and polymerizable organic molecules. For the polymerizable organic moiety, allyl alcohol was chosen because it is known to have a good tribopolymerization activity and can compete with water for surface adsorption sites. The adsorbate composition can be divided into two regimes: water-rich and alcohol-rich. The tribopolymerization yield was found to be significantly enhanced, compared to the alcohol-only case, in both water-rich and alcohol-rich regimes. The coadsorbed water molecules appeared to be incorporated into the tribopolymerization product of allyl alcohol. The friction coefficient qualitatively correlated with the tribopolymerization yield. Surprisingly, a small degree of surface wear was observed in the alcohol-rich regime, although wear was completely suppressed in the water-rich regime and the alcohol-only condition. These results suggested that the wear prevention effect does not necessarily correlate with the tribopolymerization effects.

Ultralow friction induced by tribochemical reactions: a novel mechanism of lubrication on steel surfaces

The tribological properties of two steel surfaces rubbing against each other are measured while they are in contact with 1,3-diketones of varying structure. Such systems show after a short running-in period ultralow friction properties with a coefficient of friction of as low as μ = 0.005. It is suggested that the extremely favorable friction properties are caused by a tribochemical reaction between the 1,3-diketones and the steel surfaces, leading to formation of a chelated iron-diketo complex. The influence of temperature and the molecular structure of the 1,3 diketo-lubricants onto the friction properties of the system is elucidated under both static and dynamic conditions. With progression of the tribochemical reaction, the sliding surfaces become very conformal and smooth, so that the pressure is greatly reduced and further wear is strongly reduced. All iron particles potentially generated by wear during the initial running-in period are completely dissolved through complex formation. It is proposed that the tribochemical polishing reaction causes a transition from boundary lubrication to fluid lubrication.