Tribochemistry of alcohols and their tribological properties: a review

Recently,alcohols have attracted more attention due to their excellent tribological performance,especially superlubricity under low loads.Alcohol solution,as a liquid lubricant,can easily reach the superlubricity state under low loads because of the formed low shear hydroxylation interfaces induced by the tribochemical reactions.A general picture and its influencing factors have been elucidated,not only at the macroscopic scale but also at the nanoscale,which is sufficient to provide effective guidance for lubrication design and tribology research in engineering.Herein,we provide a review on the recent applications of alcohols in lubrication.In addition,the material transformation caused by alcohols in friction is a key factor affecting the tribological properties.As an important two-dimensional material,the growth mechanisms of graphene are variable,and the most famous is the formation of carbon radicals under the action of metal catalysts.Thus,based on the formation mechanism of carbon friction film(such as amorphous carbon and graphene),the main content of this review also includes the transformation of graphene in alcohol solution friction process.

Differences in nano-topography and tribochemistry of ZDDP tribofilms from variations in contact configuration with steel and DLC surfaces

In this work,we evaluated the effect of the counter-body material(the same or dissimilar)and contact configuration(moving or stationary body),at similar contact tribological conditions,on the tribochemical and nanotopography characteristics of adsorbed surface films.Zinc dialkyldithiophosphate(ZDDP),the best performing anti-wear additive,was used in self-mated steel/steel and DLC/DLC contacts,which were compared with mixed steel/DLC and DLC/steel contacts in 1-h and 6-h sliding tests.The macroscale(tribometer)and nanoscale(atomic force microscopy)friction,thickness,topography,and chemical(attenuated total reflection-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy)properties of the tribofilms were studied.The results revealed unexpectedly large differences in all the studied tribofilm parameters;this is because all the tribofilms are completely different;this includes the chemical composition,which is known to have a crucial effect on the nano-and macro-scale tribological properties.These results clearly demonstrate that the surface material,additives,and common contact operating parameters,that is,pressure,velocity,and temperature,crucially affect the ZDDP tribofilm as well as the position of the moving or stationary surface within the contact,and the material of the moving/stationary bodies.

Tribological behaviour of oil-soluble organo-molybdenum compound as lubricating additives

A four-ball machine was employed to investigate the tribological performances and the synergetic function of organo-molybdenum compounds (molybdenum dialkyl-dithiophosphate and molybdenum dithiophosphate, coded as MoDDP and MoDTP, respectively) as additives in liquid paraffin. The morphologies of the worn surfaces and elemental distributions were observed and determined by means of scanning electron microscopy and energy dispersive spectrometry. The chemical states of some typical elements on the worn surface were analyzed on X-ray photoelectron spectroscope. The results show that MoDDP and MoDTP additives in liquid paraffin have excellent antiwear and friction-reduction properties, which are attributed to the reaction film composed of MoS2, FeS and MoO2, and the deposited film composed of phosphate on the worn surface. Moreover, these two additives have synergetic friction-reducing and antiwear effect, which is closely related to the chemical states of S on the worn surface.

Effect of amide type modified rapeseed oil as lubricating additive on friction and wear behavior of steel-steel and steel-aluminum alloy systems

A new type of environmentally friendly lube additive-amide type modified rapeseed oil was synthesized and characterized by infrared spectrum. Its effect on the friction and wear behavior of steel-steel and steel-aluminum alloy systems were investigated with a four-ball machine and an Optimol SRV friction and wear tester respectively.The morphographies of the worn surfaces were analyzed by means of scanning electron microscopy(SEM). The worn surfaces of the 2024Al alloy block were analyzed by means of X-ray photoelectron spectroscopy(XPS). The results show that the modified rapeseed oil as additives can obviously decrease the wear rate and friction coefficient of steel pair and steel-aluminum frictional pair. Its lubrication mechanism is inferred that a high strength complex protection films form on the worn surface of the Al alloy due to the adsorption or tribochemistry reaction of a long chain additive molecule and high reaction activity of N element.

Mechanical Activation of Chemical Process

Recently, some specificities of hydrothermal synthesis have been discussed by Avvakumov, Senna and Kosova, emphasizing the ability of water to dissolve selected materials at increased temperature and pressure, and suggest that the process is based on self-ionization. The present author proposes an alternative approach based on low-energy electrons generated by processes of mechanical action (mechanical activation). These types of reactions encompass both mechanochemistry and tribochemistry. Mention wise is that tribochemical reactions are distinct from those of thermochemical reactions. G. Kaupp considers mechanochemistry as mechanical breakage of intramolecular bonds by external force. Sakaguchi et al. introduced the concept of mechano-anion-radicals (MARs) generation process in the polymer mechanical degradation via two types of carbon-carbon (C-C) bond cleavage, homogeneous and heterogeneous. Based on the negative-ion-radical-action-mechanism (NIRAM) approach, this paper proposes a novel mechanism which eliminates the heterogeneous C-C bond splitting. It is evidenced that both the proposed alternative mechanism and the substitution of the heterogeneous C-C bond splitting are possible.

Mechanisms behind the environmental sensitivity of carbon fiber reinforced polytetrafluoroethylene(PTFE)

Carbon fiber reinforced polytetrafluoroethylene (CF/PTFE) composites are known for their exceptional tribological performance when sliding against steel or cast iron in inert gas environments. Compared to experiments in humid air, about an order of magnitude lower wear rate and several times lower coefficient of friction have been reported for tests conducted in dry nitrogen and hydrogen. Moreover, trace moisture has been shown to affect the friction and wear significantly of this tribosystem, although a possible effect of oxygen cannot be ruled out due to uncertainties regarding the oxygen concentrations. While several studies have pointed out the environmental sensitivity of CF/PTFE, the understanding of the underlying mechanisms are very limited. The objective of this research is to investigate the individual and combined effect of oxygen and moisture on the tribological behavior of CF/PTFE sliding against steel. Additionally, this study aims to elucidate the underlying mechanisms that govern the environmental sensitivity of the system. Climate-controlled three-pin-on-disc experiments were conducted in nitrogen atmospheres at various concentrations of oxygen and moisture. The tribological results clearly demonstrate that both moisture and oxygen contribute to increased friction and wear. However, the adverse effect was much more pronounced for oxygen than moisture. A qualitative method was developed to estimate the tribofilm coverage on the CF/PTFE surface. Results showed strong correlation between high coverage of strongly adhered tribofilm and low wear rate. Moreover, a loosely adhered tribofilm was observed on top of the CF/PTFE surface in presence of moisture. FTIR analysis indicated that the loosely adhered tribofilm found in the moisture-enriched environment contained a significant amount of adsorbed water, which may explain the lower coefficient of friction in presence of moisture compared to oxygen. The adsorbed water in the loosely adhered tribofilm could be an indication of moisture-driven lubrication by the non-graphitic carbon in the tribofilm.

Tribological behaviour of Ti_(3)C_(2)T_(x) nano-sheets: Substrate-dependent tribo-chemical reactions

MXenes,a newly emerging class of layered two dimensional(2D)materials,are promising solid lubricants due to their 2D structure consisting of weakly-bonded layers with a low shear strength and ability to form beneficial tribo-layers.This work aims at evaluating for the first time MXenes lubrication performance and tribofilm formation ability on different metallic substrates(mirror-lapped Fe and Cu discs).After depositing MXenes via ethanol(1 wt%)on the substrates,pronounced differences in the resulting substrate-dependent frictional evolution are observed.While MXenes are capable to reduce friction for both substrates after the full evaporation of ethanol,MXenes lubricating effect on Cu is long-lasting,with a 35-fold increased lifetime compared to Fe.Raman spectra acquired in the wear-tracks of the substrates and counter-bodies reveal notable differences in the friction-induced chemical changes depending on the substrate material.In case of Fe,the progressive failure of MXenes lubrication generates different Fe oxides on both the substrate and the ball,resulting in continuously increasing friction and a poor lubrication effect.For Cu,sliding induces the formation of a Ti_(3)C_(2)-based tribofilm on both rubbing surfaces,enabling a long-lasting lubricating effect.This work boosts further experimental and theoretical work on MXenes involved tribo-chemical processes.

The friction of diamond-like carbon coatings in a water environment

Diamond-like carbon(DLC)coatings are known to provide beneficial mechanical and tribological properties in harsh environments.Their combination of high wear resistance and low friction has led to their extensive use in any number of industries.The tribological performance of a DLC coating is varied however,and the frictional response is known to be strongly dependent on the surrounding environment,as well as the material composition and bonding structure of the DLC coating.This paper presents an up-to-date review on the friction of DLC coatings in a water environment,with a special focus on transfer layer formation and tribochemistry.

Mechanism of friction reduction of unsaturated fatty acids as additives in diesel fuels

This paper investigates the effect of unsaturated fatty acid additives on the lubricating properties of a biodiesel,composed of a blend of fatty acid methyl esters(FAMEs)and a conventional diesel fuel.The presence of fatty acids as additives for biodiesel improves their lubricating properties at elevated temperature conditions.The formation of a protective lubricating tribochemical film on the steel contact surfaces has been evidenced.Elevated temperature conditions and possibly oxygen are needed to form this film that reduces friction and limits wear.Several analytical tools have revealed the insulating nature of the formed tribofilm and its chemical properties.Friction-induced reticulation is thought to play a major role in the growth of this insulating film.

Galvanically induced potentials to enable minimal tribochemical wear of stainless steel lubricated with sodium chloride and ionic liquid aqueous solution

The effect of galvanically induced potentials on the friction and wear behavior of a 1 RK91 stainless steel regarding to tribocorrosion was investigated using an oscillating ball-on-disk tribometer equipped with an electrochemical cell. The aim of this investigation is to develop a water-based lubricant. Therefore 1 molar sodium chloride(NaCl) and 1% 1-ethyl-3-methylimidazolium chloride [C_2 mim][Cl] water solutions were used. Tribological performance at two galvanically induced potentials was compared with the non-polarized state: cathodic potential-coupling with pure aluminum- and anodic potential-coupling with pure copper. Frictional and electrochemical response was recorded during the tests. In addition, wear morphology and chemical composition of the steel were analyzed using scanning electron microscopy(SEM) and X-ray photoelectron spectroscopy(XPS), respectively. The galvanically induced cathodic polarization of the stainless steel surface results in electrochemical corrosion protection and the formation of a tribolayer. Cations from the electrolyte(sodium Na^+ and 1-ethyl- 3-methylimidazolium [C_2 mim]^+) interact and adhere on the surface. These chemical interactions lead to considerably reduced wear using 1 NaC l(86%) and 1% 1-ethyl-3-methylimidazolium chloride [C_2 mim][Cl](74%) compared to the nonpolarized system. In addition, mechanical and corrosive part of wear was identified using this electrochemical technique. Therefore this method describes a promising method to develop water-based lubricants for technical applications.

Quantitative analysis of the tribological properties of phosphate glass at the nano- and macro-scales

Processing(grinding,polishing)of phosphate laser(PL)glass involves material removal at two vastly different(spatial)scales.In this study,the nano‐and macro‐tribological properties of PL glass are investigated by rubbing the glass against a SiO_(2) counter‐surface in both dry and humid conditions.The results indicate that the friction of the PL glass/SiO_(2) pair has opposing trends at the nano‐and macroscales.At the nanoscale,the friction coefficient(COF)in humid air is much higher than in dry air,which is attributed to the capillary effect of the absorbed water‐film at the interface.At the macroscale,on the other hand,the COF in humid air is lower than in dry air,because the water‐related mechanochemical wear makes the worn surface less susceptible to cracking.Material removal for PL glass is better facilitated by humid air than by dry air at both scales,because the stress‐enhanced hydrolysis accelerates the material‐removal process in glass.Moreover,the material‐removal is more sensitive to contact pressure at the macroscale,because stronger mechanical‐interaction occurs during material removal at the macroscale with the multi asperity contact mode.At the macroscale,the material removal is more sensitive to contact pressure in humid air compared to dry air.Because almost all mechanical energy is used to remove material in humid air,and most of the mechanical energy is used to produce cracks in PL glass in dry air.The results of this study can help optimize the multi‐scale surface processing of optical glasses.

Ga-based liquid metal as an extreme pressure lubricant for steel-ceramic pairs

Extreme pressure lubrication is a crucial engineering problem for many mechanical assemblies.Herein,an extraordinary and novel functional material,Ga-based liquid metal(GLM),is found to be a promising extreme pressure lubricant for steel-ceramic sliding pairs and bears applied load above 1500 N.A Ga-rich metallic tribofilm forms on the frictional interface during sliding,providing good lubricity and bearing capability.In addition,the lubrication performance of GLM strongly depends on its composition and the contacting materials.Ternary Ga-In-Sn alloy has superior lubrication to binary Ga-In alloy.GLM lubrication is more effective on the interfaces of AISI52100/Si_(3)N_(4)than on those of AISI52100/SiC sliding pairs.

Ionic liquids as boundary additives in water-based and PAO lubricants

Ionic liquids have been widely discussed as potential lubricants,however,their properties make them also very good potential candidates as lubricant additives(e.g.,friction modifiers and anti-wear).In this work,the tribological study of two ionic liquids(tributylmethylphosphonium dimethylphosphate(PP),and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate(BMP))as lubricant additives has been performed on stainless steel(AISI 316L)exposed to polar(water-glycol)and non-polar(polyalphaolefin)based lubricants under boundary lubricating conditions.The performance of these ionic liquids as lubricant additives has been compared to a classical organic friction modifier(dodecanoic acid(C12)).The water–glycol lubricant formulated with the two ionic liquids showed friction values higher than the same base lubricant formulated with dodecanoic acid,however,opposite results were observed for polyalphaolefin(PAO).A detailed surface chemical analysis using X-ray photoelectron spectroscopy(XPS)revealed differences in the passive/tribofilm thickness and chemical composition of the stainless steel surface tested in all lubricants.In the case of the polar lubricant additivated with ionic liquids,the tribochemical reaction accompanied by a tribocorrosion process led to the formation of an unstable passive/tribofilm resulting in high friction and wear.However,in the absence of tribocorrosion process(polyalphaolefin base lubricant),the tribochemical reaction led to the formation of a stable passive/tribofilm resulting in low friction and wear.A detailed surface and subsurface investigation of the microstructure using scanning electron microscopy equipped with a focused ion beam(SEM-FIB)showed that high wear rates resulted in thicker recrystallization region under the wear track surface.Among all lubricant additives tested in this work,BMP in non-polar lubricant media showed the best tribological performance.