Diamond-derived carbon onions as lubricant additives

Among all the new carbon forms like fullerenes or nanotubes, carbon onions are of particular interest for tribological applications because of their spherical and tiny shape, and their chemical inertness. Used as additives blended to a synthetic base oil, they present excellent anti-wear and friction-reducing properties. In this paper, we compare tribological performances of two kinds of carbon onions with different structural features. Due to the synthesis method used, carbon onions present or not a residual diamond core inside the graphitic shells. Carbon onions without the diamond core present better anti-wear properties.     

Borate esters used as lubricant additives

Borate esters possess friction‐reducing, antiwear, and anti‐oxidant characteristics when blended in lubricating oils. However, borate esters are susceptible to hydrolysis. The formation of a stable five‐member ring structure in the ester molecules, involving coordination of nitrogen with boron, contributes substantially to the resistance to hydrolysis of borate esters. The susceptibility of borates to hydrolysis can be reduced by introducing N,N‐dialkylaminoethyl groups with alkyl radicals containing more than three carbon atoms. X‐ray photo‐electron spectroscopy and X‐ray diffraction reveal that the borate ester can be adsorbed on the rubbing surface, and some of the adsorbed borate film degrades and forms boron nitride. Four‐ball wear tests indicate that the combination of oil‐soluble copper, tin, and cadmium compounds with organoborates gives better antiwear properties than the components separately. An antiwear synergistic mechanism is postulated in which borates with electron‐deficient boron p orbitals catalyse the tribo‐reduction of the metal compounds on the rubbing surfaces, producing elemental metals.     

Oil-soluble MO-S compounds as lubricant additives

The use of soluble Mo-S compounds as lubricant additives for reducing friction and wear, increasing load-carrying capacity, promoting fuel economy and as antioxidants is related to their structures and chemical properties. The compounds discussed include tetrathiomolybdates and complexes of molybdenum with xanthates, dithiocarbamates, dithiophosphates, diketones, dithiols and dimercaptothiadiazole.

Structures of the compounds are described. Features of their UVvisible, IR and nuclear magnetic resonance spectra which are useful for characterization are listed.

The preparative chemistry of the molybdenum dithiophosphates is discussed in relation to the composition of additive packages.

Antiwear and extreme pressure mechanisms of the compounds are discussed with reference to their decomposition in bulk oil and at surfaces, sorption processes, formation of surface films and layers and decomposition to molybdenum disulphide. The importance of the Mo-S combination is emphasized.

The tribological performance of Mo-S complexes, organosulphur compounds, molybdenum disulphide and zinc compounds is compared.     

The assessment of dithiocarbamates as extreme pressure lubricant additives

A number of aryl/alkyl-dithiocarbamates have been assessed as extreme pressure additives using a Seta-Shell four-ball lubricant testing machine. p-Tolyl dithiocarbamate and o-tolyl dithiocarbamate were found to be effective.     

Preparation and tribology of aerogel silica and alumina-silica as lubricant oil additives

Aerogel silica and an aerogel mixture of aluminium and silica were prepared using the ethanol supercritical fluid drying technique. Their physical properties were characterised using XRD, transmission electron microscopy, (TEM), and N₂ adsorption. The tribological properties of a 500 SN oil containing the aerogel and a dispersing agent were evaluated using a four-ball machine and a block-on-ring tribotester. Results indicated a greater wear resistance and load-carrying capacity and a lower friction coefficient than for the basic oil. In the oil, the aerogel worked by its deposition on the rubbing surface. Copper stearate was superior to dodecyl alcohol as a dispersing additive, and ultrasonic treatment helped promote the dispersion of the aerogel in oil. There existed an optimum ratio of aluminium isopropyl alcoholate to tetraethanol silicate. The corresponding aerogel mixture gave a maximum value of P_E.     

Tribochemistry and antiwear mechanism of organic–inorganic nanoparticles as lubricant additives

Lubricant-dispersible nanoparticles are prepared via a surface modification synthetic way and characterized by state of the art tools such as TEM. The tribological properties of the nanoparticles as lubricant additives were studied and the results show that the addition of nanoparticles as additives reduced wear and increased load-carrying capacity of base oil remarkably, indicating nanoparticles can be used as anti-wear and extreme-pressure additives with excellent performances. The boundary lubrication mechanism of nanoparticles was discussed that probably an ultra thin film, which is formed by the melting and elongation of nanoparticles under tribological heat and shear force, attributed to good tribological performance of nanoparticulate as additives.     

Tribological properties of few-layer graphene oxide sheets as oil-based lubricant additives

The performance of a lubricant largely depends on the additives it involves. However, currently used additives cause severe pollution if they are burned and exhausted. Therefore, it is necessary to develop a new generation of green additives. Graphene oxide (GO) consists of only C, H and O and thus is considered to be environmentally friendly. So the tribological properties of the few-layer GO sheet as an additive in hydrocarbon base oil are investigated systematically. It is found that, with the addition of GO sheets, both the coefficient of friction (COF) and wear are decreased and the working temperature range of the lubricant is expanded in the positive direction. Moreover, GO sheets has better performance under higher sliding speed and the optimized concentration of GO sheets is determined to be 0.5wt%. After rubbing, GO is detected on the wear scars through Raman spectroscopy. And it is believed that, during the rubbing, GO sheets adhere to the sliding surfaces, behaving like protective films and preventing the sliding surfaces from contacting with each other directly. This paper proves that the GO sheet is an effective lubricant additive, illuminates the lubrication mechanism, and provides some critical parameters for the practical application of GO sheets in lubrication.     

Effect of viscosity variation due to lubricant additives in journal bearings

Micropolar fluid theory, which is a possible non-newtonian model of a suspension of rigid particle additives, is applied to the study of the lubrication of an infinitely long and an infinitesimally short journal bearing. The variation in viscosity along the film thickness is taken into consideration. The prominent feature of the presence of solid particles in the lubricant is an increased effective viscosity which increases the load capacity and decreases the coefficient of friction. Whereas viscosity variation tends to decrease both the load and the coefficient of friction for the non-micropolar case, the combined effect is to increase the load and to decrease the coefficient of friction.     

Highly reactive polyisobutene as a component of a new generation of lubricant and fuel additives

This paper describes the synthesis of highly reactive polyisobutene (PIB), a special type of PIB used as a component in the production of lubricant and fuel additives. The reactivity and the chemical structure are described in detail, in relation to the nature of the product and its uses as an additive component.     

Succinimide phosphoric acid esters as multipurpose additives

Succinimide phosphoric acid esters have been synthesised and evaluated as antioxidants and ashless dispersant additives for crankcase lubricating oils. The synthesis is based on reacting succinimides (as prepared by the reaction of polyisobutylene, maleic anhydride and tetraethylene pentamine) with phosphorus pentoxide in the presence of alcohols with different carbon numbers Results showed that the magnitude of the oils' oxidation stability, as well as their dispersive power, are both highly related to the chemical structure of the alcohol used.     

Coking and microcoking: Tools for evaluating and developing lubricant additives

Two tests for evaluating an engine lubricant's tendency to form deposits at high temperature have been developed by a working group of the GFC,

1 Groupement Français de Coordination (French Coordination Group).

set up to establish laboratory pre-selection tests. The first is a new static microcoking test where the oil is deposited on an aluminium alloy plate and submitted to thermal conditions similar to those found in the piston ring zone of an IC engine. The results are analysed to determine the temperature at the oil's thermal stability limit and to evaluate the quantity and nature of the deposits in the form of varnish, according to a grading system based on the CEC M-02-A-78 method of gauging engine cleanliness. The second is an improved panel coking test in which a thin film of oil of a given volume is fed on to a stainless steel panel, at high temperature, and the deposits formed at the end of the test are evaluated, again using the CEC rating method, in terms of quantity and type (carbon or varnish). In addition, a video system has been designed to grade the micro-coking panels through colour recognition. This not only allows visual grading to be faithfully reproduced but also eliminates the subjective element of colour grading. The results obtained with the ‘Video-Rater’ constitute a first step towards computer-assisted grading of engine parts.     

Tribological behaviour of two imidazolium ionic liquids as lubricant additives for steel/steel contacts

In this paper two room-temperature ionic liquids (ILs), 1-hexyl-3-methylimidazolium tetrafluroborate [HMIM][BF₄] and 1-hexyl-3-methylimidazolium hexafluorophosphate [HMIM][PF₆], have been studied as 1%wt. additives of a mineral hydrocracking oil for steel–steel contacts. Rheological properties of the mixtures and base oil were determined over shear rates and temperatures ranging 1–1000 s^?1 and 40–100 °C, respectively. Friction and wear testing was made using a block-on-ring tribometer set for pure sliding contact and XPS was used to analyze wear surfaces. [HMIM][PF₆] and [HMIM][BF₄] increased the viscosity of the base oil and decreased friction and wear. Friction and wear reduction are related to reactivity of the anion of the ionic liquids with surfaces forming FeF₃, B₂O₃, and species such as P₂O₅ or PO₄^3?.     

Tribological investigation of layered sodium silicate as lubricant additives prepared by freezing titration ion exchange

Two layered sodium silicate products were prepared using a new ion exchange method called the freezing titration ion exchange. The tribological properties of the ion exchange products as additives in mineral oil were evaluated using a four-ball tester. The products exhibited excellent friction behavior when used as additives in mineral oil. When 1 wt% was added, the tribological properties of the oils improved: the maximum non-seizure loads increased by over 27% and the wear scar diameter and friction coefficient decreased by over 50% and 25%, respectively. The friction and wear behaviors of the silicates were better than those of the typical lubricant additives.     

Hydrogen influence on material interaction with ZDDP and MoDTC lubricant additives

In this paper the interaction of lubricant additives with hydrogen at the frictional interface has been investigated. Three different states of the base AISI 52100 alloy steel have been tested: untreated, nitrided and sulphonitrided, with different combinations of PAO6 base oil and ZDDP/MoDTC (Zinc DialkylDithioPhosphate and Molybdenum Dialkyldithiocarbamate) additives. Experiments have been carried out on pin-on-plate reciprocating tester, immersed in the lubricant heated to 100 °C. In the boundary lubricated regime the results showed the best friction behaviour for treated surfaces tested in presence of PAO6 with additives suggesting some interaction at the frictional interface of nitrided and sulphonitrided surfaces with lubricant additives. The minimum recorded value of coefficient of friction was as low as 0.05 for the sulphonitrided sample with PAO6+MoDTC oil. In the case of the treated surfaces a characteristic “low friction phase” has been observed when tested with PAO6 with additives. After a given time, the coefficient of friction was increasing to a higher steady-state value and the duration of this low friction phase varied from sample to sample. This can be explained by the mechanism of hydrogen interaction in the boundary lubrication regime, which was postulated for the base oil case by some of the authors in their previous papers. To validate the hypothesis, an experiment has been carried out where the test was stopped at the end of the “low friction phase” and during the hold period the sample was re-saturated with hydrogen. After resuming the experiment the low friction regime was again observed. The effect of a potential synergistic mechanism between hydrogen and ZDDP or MoDTC lubricant additives on frictional behaviour of nitrided and sulphonitrided surfaces is discussed in this paper.     

Wear reduction using some extreme pressure lubricant additives and their tribological studies

Certain S-containing organic compounds were synthesized and their extreme pressure (EP) properties assessed. The series of compounds studied were: S-alkylisodithiobiurets, alkyl-S-alkyl xanthates, S-alkylisothioamides, S-benzylisothiocarbamides and 1-formamidino-2-benzyliso-3-arylthiocarbamides. These additives were compared with a commercial additive. The tribological end effects were studied using a Scanning Electron Microscope. Some of the compounds tested were found to be effective EP additives. Wherever possible, a plausible explanation for the enhanced activity has been given, and we have attempted to correlate the effect of substituents on efficiency. Also, the effect of additives on balls of different compositions is discussed. The possibility of replacing the costly chromium steel alloy balls by less costly mild steel balls is suggested. Some of the additives developed are found to be effective on balls of certain compositions while they are a total failure on the chromium steel balls. The studies reveal that some additives are effective on mild steel balls as well as chromium alloy steel balls. They can be very good substitutes for commercial additives with bearing balls of standard composition. As chromium alloy steel balls are much costlier there is a need to replace expensive alloy steel balls with cheaper low carbon steel balls, for certain specific purposes. This can be achieved by using suitable indigenous additives that are found to be effective on low carbon steel balls.     

Changes in friction and wear performance caused by interactions among lubricant additives

Engine oil additives generally fall into one of two general categories: those that affect bulk oil properties and those that affect the surface properties of lubricated parts. Interactions among surface-active additives can lead to less-than-expected lubricant performance. Laboratory friction and wear test results are presented to illustrate the changes in lubricant performance that interactions among additives can cause. Some possible explanations are proposed.     

Tribological properties of lubricant additives of Fe,Cu and Co nanoparticles

Tribological investigations were performed on mineral oil containing Fe, Cu and Co nanoparticles and their combinations. The tribological tests showed that each set of nanoparticles significantly reduced the friction coefficient and wear (up to 1.5 times) of friction pairs. The use of Cu nanoparticles provides the most effective reduction of friction and wear in each combination of nanoparticles. Surface analysis shows that the constituent elements of nanoparticles precipitated on the contact surface during the use of the oils with nano-additives. Different structures formed on the friction surface are observed in the contact zone and over the remainder of the ball surface. The SEM micrographs and EDX chemical analysis confirm the formation of a tribo-layer composed of the elements from the nanoparticles.     

The performance and antiwear mechanism of (2-sulfurone-benzothiazole)-3-methyl esters as additives in synthetic lubricant

The load-carrying capacity, friction and wear properties of three (2-sulfurone-benzothiazole)-3-methyl esters added to a synthetic lubricant (triester) were evaluated using a four-ball tester and a ring-on-block machine. The results indicate that these compounds added to the triester possess good load-carrying capacities and antiwear properties. The rubbed surface was investigated by X-ray photoelectron spectroscopy (XPS). The analysis indicates that a lubricating film is formed on the metal surface by adsorption and chemical reaction.     

Synthesis and tribological behaviour of phosphorus—nitrogen‐modified rape seed oils as biodegradable lubricant additives

Two types of phosphorus—nitrogen‐modified rape seed oils as biodegradable lubricant additives were synthesised and characterised by infrared spectroscopy. Their tribological properties in rape seed oil and in mineral oil were evaluated in a four‐ball tester. The morphologies of the worn surfaces were analysed by scanning electron microscopy. The results show that the modified rape seed oil additives improve the load‐carrying capacity and the antiwear and friction‐reducing properties of rape seed oil more than they do those of mineral oil. The inferred mechanism of lubrication is that a high‐strength adsorption film and/or tribochemical reaction film forms on the rubbing surfaces, due to the carrier effect of long‐chain rape seed oil molecules and to the high reaction activities of phosphorus and nitrogen and their synergism.