Cross-Sectional Imaging of Boundary Lubrication Layer Formed by Fatty Acid by Means of Frequency-Modulation Atomic Force Microscopy

Adsorption Behavior of TEMPO-Based Organic Friction Modifiers during Sliding between Iron Oxide Surfaces: A Molecular Dynamics Study

Organic friction modifiers (OFMs) added to lubricating oils to reduce friction and wear are crucial for reducing energy loss and CO₂ emissions. In our previous studies, we have developed N-(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)dodecaneamide, referred to as C₁₂TEMPO, as a new type of OFM and experimentally demonstrated its superior performance to conventional OFMs of stearic acid and glycerol monooleate. However, the behavior of C₁₂TEMPO adsorbing onto solid surfaces from base oil during sliding, which largely dictates the lubrication performance, is yet to be elucidated. Here, we performed molecular dynamics simulations for confined shear of a C₁₂TEMPO solution in poly-α-olefin between hematite surfaces. Unlike conventional OFMs, which typically have one functional group or multiple functional groups of the same type, C₁₂TEMPO features two functional groups of different types: one amide and one terminal free oxygen radical. The results showed that adsorbed boundary films with a double-layer structure form stably during sliding, owing to double- or single-site surface adsorption and interlayer hydrogen bonding via the two functional groups. Additionally, some molecules in each of the first and second layers also form intralayer hydrogen bonding. Such multitype adsorption is unique and favorable for enhancing the strength of boundary films to withstand heavily loaded and prolonged sliding. The velocity distribution indicates that the first and second layers are solid- and liquid-like, respectively. The second layer could act as a buffer for the first layer, which is the last barrier to prevent solid-solid contact, against shear. We also found that the second layer can act as a reservoir to rapidly repair the once depleted region in the first layer because of the interlayer hydrogen bonding. The combination of the high strength and self-repair ability of the C₁₂TEMPO boundary films can rationally explain the experimentally observed properties of high load-carrying capacity, excellent antiwear effect, and high stability of friction over time.

Comparison of atomic force microscopy force curve and solvation structure studied by integral equation theory

Atomic force microscopy can observe structures of liquids (solvents) on solid surfaces as oscillating force curves. The oscillation originates from the solvation force, which is affected by the interaction between the probe, substrate, and solvents. To investigate the effects of the interactions on the force curve, we calculated the force curves by integral equation theory with various probe and substrate conditions. The probe solvophilicity affected the force curves more than the substrate solvophilicity in our calculation, and its reason is qualitatively explained by the amount of the desolvated solvents. We evaluated the probes and parameters in terms of the qualitative estimation of the number density distribution of the solvent on the wall. The negative of the force curve's derivative with respect to the surface separation reflected the number density distribution better than the force curve. This parameter is based on the method that is proposed previously by Amano et al. [Phys. Chem. Chem. Phys. 18, 15534 (2016)]. The normalized frequency shift can also be used for the qualitative estimation of the number density distribution if the cantilever amplitude is small. Solvophobic probes reflected the number density distribution better than the solvophilic probes. Solvophilic probes resulted in larger oscillation amplitudes than solvophobic probes and are suitable for measurements with a high S/N ratio.

Improvements in fundamental performance of in-liquid frequency modulation atomic force microscopy

In-liquid frequency modulation atomic force microscopy (FM-AFM) has been used for visualizing subnanometer-scale surface structures of minerals, organic thin films and biological systems. In addition, three-dimensional atomic force microscopy (3D-AFM) has been developed by combining it with a three-dimensional (3D) tip scanning method. This method enabled the visualization of 3D distributions of water (i.e. hydration structures) and flexible molecular chains at subnanometer-scale resolution. While these applications highlighted the unique capabilities of FM-AFM, its force resolution, speed and stability are not necessarily at a satisfactory level for practical applications. Recently, there have been significant advancements in these fundamental performances. The force resolution was dramatically improved by using a small cantilever, which enabled the imaging of a 3D hydration structure even in pure water and made it possible to directly compare experimental results with simulated ones. In addition, the improved force resolution allowed the enhancement of imaging speed without compromising spatial resolution. To achieve this goal, efforts have been made for improving bandwidth, resonance frequency and/or latency of various components, including a high-speed phase-locked loop (PLL) circuit. With these improvements, now atomic-resolution in-liquid FM-AFM imaging can be performed at ∼1 s/frame. Furthermore, a Si-coating method was found to improve stability and reproducibility of atomic-resolution imaging owing to formation of a stable hydration structure on a tip apex. These improvements have opened up new possibilities of atomic-scale studies on solid-liquid interfacial phenomena by in-liquid FM-AFM.

Adsorption of Organic Friction Modifier Additives

Organic friction modifier additives (OFMs) are surfactant molecules added to engine oils to reduce friction in the boundary lubrication regime. They are thought to work by forming an absorbed layer, which provides low friction. This paper studied the relationship between the adsorption of OFMs and their friction and wear-reducing properties in a rubbing contact formed by a stationary glass ball and a rotating silicon disk under the boundary lubrication regime. The effect of molecular structure was investigated by using OFMs of various tail saturation and head group chemistry. OFMs tested were oleic acid, octadecylamine, oleylamine, and glycerol monooleate. The thickness of an OFM-adsorbed layer in hexadecane, examined in situ by spectroscopic ellipsometry and quartz crystal microbalance (QCM), depends on the molecular structure and the concentration of the OFM. As expected, the saturated, linear chain gives the thickest film. A critical OFM layer thickness of about 0.6 nm is necessary to achieve low initial and maximum friction. The thicker OFM layers are accompanied by narrower wear tracks, which are rougher than the wider, smoother wear tracks formed with thinner OFM layers. The interplay between the thickness of the OFM layer and wear track surface roughness results in all OFM layers having similar steady state friction. This shows that the apparent effect of OFM depends on the stage of the rubbing test: initially on friction, and then subsequently on surface damage. Despite OFMs and the base oil having similar refractive indices, ellipsometry was found to be a suitable technique for examining the adsorption of OFM additives from an oil-based solution and showed reasonable correlation with QCM results.

Three-dimensional solvation structure of ethanol on carbonate minerals

Calcite and magnesite are important mineral constituents of the earth's crust. In aqueous environments, these carbonates typically expose their most stable cleavage plane, the (10.4) surface. It is known that these surfaces interact with a large variety of organic molecules, which can result in surface restructuring. This process is decisive for the formation of biominerals. With the development of 3D atomic force microscopy (AFM) it is now possible to image solid-liquid interfaces with unprecedented molecular resolution. However, the majority of 3D AFM studies have been focused on the arrangement of water at carbonate surfaces. Here, we present an analysis of the assembly of ethanol - an organic molecule with a single hydroxy group - at the calcite and magnesite (10.4) surfaces by using high-resolution 3D AFM and molecular dynamics (MD) simulations. Within a single AFM data set we are able to resolve both the first laterally ordered solvation layer of ethanol on the calcite surface as well as the following solvation layers that show no lateral order. Our experimental results are in excellent agreement with MD simulations. The qualitative difference in the lateral order can be understood by the differing chemical environment: While the first layer adopts specific binding positions on the ionic carbonate surface, the second layer resides on top of the organic ethyl layer. A comparison of calcite and magnesite reveals a qualitatively similar ethanol arrangement on both carbonates, indicating the general nature of this finding.

Fatty Acid-Derived Ionic Liquid Lubricant. Protic Ionic Liquid Crystals as Protic Ionic Liquid Additives

Fatty acids are natural products which have been studied as green lubricants. Ionic liquids are considered efficient friction reducing and wear preventing lubricants and lubricant additives. Fatty acid-derived ionic liquids have shown potential as neat lubricant and additives. Protic ionic liquid crystals (PILCs) are protic ionic liquids (PILs) where cations and anions form ordered mesophases that show liquid crystalline behavior. The adsorption of carboxylate units on sliding surfaces can enhance the lubricant performance. Ionic liquid crystal lubricants with longer alkyl chains can separate sliding surfaces more efficiently. However, they are usually solid at room temperature and, when used as additives in water, transitions to high friction coefficients and wear rates, with tribocorrosion processes occur when water evaporation takes place at the interface. In order to avoid these inconveniences, in the present work, a protic ammonium palmitate (DPA) ionic liquid crystal has been added in 1 wt.% proportion to a short chain citrate ionic liquid (DCi) with the same protic ammonium cation. A spin coated layer of (DCi + DPA) was deposited on AISI316L steel surface before the sliding test against sapphire ball. Synergy between DCi PIL and DPA PILC additive reduces friction coefficient and wear rate, without tribocorrosion processes, as shown by scanning electron microscopy (SEM)/energy dispersive X-ray microanalysis (EDX) and X-ray photoelectron spectroscopy (XPS) results.

Nanometer-Scale Distribution of a Lubricant Modifier on Iron Films: A Frequency-Modulation Atomic Force Microscopy Study Combined with a Friction Test

Liquid lubricants used in mechanical applications are low-vapor-pressure hydrocarbons modified with a small quantity of polar compounds. The polar modifiers adsorbed on the surface of sliding solids dominate the friction properties when the sliding surfaces are in close proximity. However, a few methods are available for the characterization of the adsorbed modifiers of a nanometer-scale thickness. In this study, we applied frequency-modulation atomic force microscopy to evaluate the vertical and lateral density distributions of the adsorbed modifier in a real lubricant, namely, poly-α-olefin (PAO) modified with an orthophosphoric acid oleyl ester. The liquid-induced force on the probing tip was mapped on a plane that was perpendicular to the lubricant-iron interface with a force sensitivity on the order of 10 pN. The PAO in the absence of the ester modifier was directly exposed to the film, which produced a few liquid layers parallel to the film surface with layer-to-layer distances of 0.6-0.7 nm. A monomolecular layer of the modifier was intermittently adsorbed with increasing ester concentration in the bulk lubricant, with complete coverage seen at 20 ppm. The C₁₈H₃₅ chains of the oleyl esters fluctuating in the lubricant produced a repulsive force on the tip, which monotonically decayed with the tip-to-surface distance. The dynamic friction coefficient of sliding steel-lubricant-steel interfaces, which was separately determined using a friction tester, was compared with the force map determined on the iron film immersed in the corresponding lubricant. The complete monomolecular layer of the ester modifier on the static lubricant-iron boundary is a requirement for achieving smooth and stable friction at the sliding interface.