The tribological behavior of thiophosphates as additives in rapeseed oil

Two kinds of thiophosphate, tri-n-octyl thiophosphate and tri-n-octyl tetrathiophosphate, were synthesized. The tribological behavior of the synthetic compounds and tricresyl phosphate as additives in rapeseed oil for steel-steel frictional pair was investigated with a four-ball machine. The relationship between the additive structure and tribological properties was explored, while the lubrication mechanisms of the additives were investigated as well. Thus the worn surfaces of the steel ball lubricated with the additive-containing rapeseed oil were analyzed by means of X-ray photoelectron spectrometry and scanning electron microscopy, and the elemental compositions and distributions in the worn steel surfaces were observed and determined with an EPMA-810Q electron probe micro-analyzer. It has been found that the synthetic thiophosphates as additives in rapeseed oil at proper concentrations show better tribological properties than tricresyl phosphate. Tribochemical reactions were involved for steel–steel frictional pair lubricated with the rapeseed oil containing thiophosphates, with the formation of a boundary lubricating and protecting film composed of glyceride of rapeseed oil and tribochemical reaction products of the lubricants. This contributes to improve the tribological properties of the rapeseed oil base stock.     

The tribological behavior of diester-containing polysulfides as additives in mineral oil

The diester-containing polysulfides were synthesized and their anti-wear (AW) and friction-reducing behaviors as additives in HVIW H150 base oil were evaluated using a four-ball tester. The copper corrosion test was conducted to identify their anti-corrosion property. The nature of the film formed in the rubbed surface was investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Based on the experimental results, diester-containing polysulfides show good anti-corrosion properties and excellent AW abilities similar to that of ZDDP. The results of XPS analyses illustrate that an S-containing inorganic layer, generated by the reaction of diester-containing polysulfides with the metallic surfaces, mainly contains sulfate, sulfite and little sulfide, and there exists a polymer film protective layer over the inorganic layer surface.     

The tribological study of S-(1H-Benzotriazol-1-yl) methyl N,N-dialkyldithiocarbamates as additives in synthetic lubricants

The load-carrying capacity, wear, and friction properties of four S-(1 H-Benzotriazol-1-yl)methyl N,N-dialkyldithiocarbamates added to a synthetic lubricant (diester) were evaluated using a four-ball test machine and an Optimal SRV tester. The results indicate that these compounds added to the diester possess good load-carrying capacities and antiwear properties. The rubbed surfaces were investigated using X-ray photoelectron spectroscopy (XPS) analysis and the antiwear mechanism is discussed.     

Influence of fatty acid additives on the tribological performance of sunflower oil

Vegetable oils are potential substitutes for petroleum‐based lubricants because they are environmentally friendly, renewable, less toxic and readily biodegradable. The addition of free fatty acids has been shown to increase the lubrication performance of vegetable oils at elevated temperatures. The purpose of this study was to evaluate the relationship between the length of the carbon chain in the fatty acid and its effectiveness as an additive for a range of elevated temperatures. Stearic, arachidic and behenic fatty acid additives were added to commercial sunflower oil. All fatty acid additives were shown to be effective in lowering the wear rate and coefficient of friction in ball‐on‐disc tribological tests. The overall carbon chain length was not observed to have a consistent influence on the effectiveness of the additive. All additives were less effective at temperatures above 100°C. Copyright © 2010 John Wiley & Sons, Ltd.     

Study of PTFE composites tribological behavior

A comparative analysis of seven PTFE composites is presented showing how properties of PTFE can be improved even if the most attractive characteristic of low friction is lost due to the presence of hard particles in the polymer matrix. How the use of both soft and hard phases in a polymer matrix enhances the self-lubricating and the load-carrying properties of the matrix improving the tribological properties of the PTFE is presented.     

Tribological performance of pullulan additives in water-based lubricant

Water can be used as an ecological lubricant base if the water-based additives are properly developed to modify its tribological properties. Additionally, those additives should be friend both to human and nature. Pullulan is a naturally occurring polysaccharide, which is biodegradable and non-toxic and widely used in food and nonfood applications. We focus here on understanding the lubrication properties of pullulan, in aqueous solution in absence and presence of sodium salts. Lubrication is studied using a ball-on-disk tribometer with steel–steel surfaces and the friction coefficient and wear are measured in the boundary lubrication regimes. The adsorption of pullulan molecules onto steel surface is determined by measuring wettability of friction couples. It was found that the addition of pullulan improves the lubricating properties of water. The lubricating performance of Pullulan solution could be further improved if sodium chloride or sodium fluoride is added. The tribological results obtained were correlated with the adsorption capacity of pullulan molecules onto steel surface.     

The tribological behaviors of hydroxyl-containing dithiocarbamate-triazine derivatives as additives in rapeseed oil

Two ashless and non-phosphorus hydroxyl-containing dithiocarbamate-triazine compounds, 2,4,6-tri[N,N-di-n-butyldithiocarmate-(2′-hydroxyl)-propionylthio]-1,3,5-s-triazine (LDBA) and 2,4,6-tri[N,N-di-i-octyldithiocarmate-(2′-hydroxyl)-propionylthio]-1,3,5-s-triazine (LDIOA), were prepared and their tribological behaviors as additives in rapeseed oil (RSO) were evaluated using a four-ball tester. Thermal degradation tests were conducted to identify their thermal stabilities using a thermo-gravimetric analyzer. The worn surfaces of the steel balls were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). The results indicate that the additives possess high thermal stabilities and good load-carrying capacities. Moreover, they both have good anti-wear and friction-reducing property at relatively high concentration (>1.5 wt%) and under all test loads. The results of XPS analyses illustrate that the prepared compounds as additives in RSO forms a protective film containing inorganic sulfide, sulfate, oxidized compounds and organic nitrogen-containing compounds on the metal surface during the sliding process.     

A study of the tribological behaviour of dialkyldithiophosphate esters as additives in rape seed oil

Zinc dialkyldithiophosphates (ZDDP) cannot be used as additives in biodegradable lubricants because of their zinc content. In investigating substitutes for ZDDP, dialkyldithiophosphate esters have been synthesised, and their tribological behaviour as additives in rape seed oil has been evaluated using a four‐ball friction and wear tester and compared with that of ZDDP. The results show that these additives have better antiwear properties and load‐carrying capacity than rape seed oil alone. The morphologies and the elemental chemical states on the worn surfaces of the lubricated steel balls of the tester were examined using X‐ray photoelectron spectroscopy and scanning electron microscopy. The tribological mechanism is discussed on the basis of the experimental results.     

Influence of nanosized carbon additives on the structure formation and tribological behavior of galvanic chrome coatings

The paper discusses the features of the structure formation of electrochemical chrome coatings on the introduction into the electrolyte composition of a disperse phase in the form of nanosized carbon additives (ultradispersed diamonds of denotation synthesis and diamond-graphite charge). It is shown that nanosized carbon additives encourage the formation of more dense coatings having fine-crystalline structure. Chrome coatings with additives of nanosized components have higher tribological characteristics compared to those obtained from standard chroming electrolytes. Modification of the chrome coating decreases its coefficient of friction by 1.4–4.6 times and its wear rate by 8–12 times in unlubricated testing. Under boundary lubrication the coefficient of friction of the chrome-nanodiamond coating is 25–30% lower than that of the chrome coating and the wear rate decreases by 2.9 times.     

The friction and wear behavior of 2-(n-alkyldithio)-benzimidazole as additives in liquid paraffin

Three 2-(n-alkyldithio)-benzimidazoles were synthesized. The friction and wear behavior of the synthetic compounds as additives in liquid paraffin were examined with a four-ball machine, with emphasis on revealing the relationship between the chain length of the additive and the friction-reducing ability and lubricating mechanism of the additives. The film formed by the additive during the sliding process was investigated by using X-ray photoelectron spectroscopy. It was found that the synthetic compounds as additives in liquid paraffin had good antiwear performance. The longer the chain length of the synthetic compound, the more stable or less volatile is the compound and the more effective it is in improving wear resistance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Improvement in tribological performances of magnesium alloy using amide compounds as lubricating additives during sliding

The tribological characteristics of a magnesium alloy, AZ91D (die-casting), are investigated in a sliding lubricating system using various amide compounds as lubricating additives on a Timken type tester against a bearing steel (AISI52100) ring. Results indicate that a significant improvement in the tribological performance exists using the amide compounds as additives. The number of amido group (–CONH₂) in additive molecules and the molecular structure of amide compounds have significant effect on the tribological characteristics of magnesium alloy. Electromicroscopy reveals that the mild abrasive wear is a predominant wear mechanism of magnesium alloy using an amide additive while the dominated wear mechanism is a severe abrasive wear with severe material deformation using only base oil. Observation shows the formation of boundary film on the magnesium alloy. XPS analysis suggests the occurrence of tribo-chemical reactions between Mg and amide compounds with the formation of chemically stable compound (or complex) of magnesium and amide, as well as the formation of friction polymer.     

Influence of temperature on tribological behaviour of ionic liquids as lubricants and lubricant additives

Ionic liquids are low‐melting‐point salts composed entirely of ions, and many of them are liquids at room temperature. In recent years, studies have indicated that they might be good candidates for lubricants, either in neat or additive form. In this work, a sulfate‐based ionic liquid with a pyrollidinium cation was studied as neat lubricant and as additive for glycerol in lubrication of steel–steel contacts. Glycerol was chosen as the base oil because of its high polarity, which allows full miscibility with polar ionic liquids. Tests were performed on an oscillating friction and wear tribometer. The coefficients of friction and wear were measured. The tests were run at room temperature, 50 °C, 100 °C and 150 °C. By using profilometry, optical microscopy, scanning electron microscope and atomic force microscopy (AFM) analyses, it was shown that the ionic liquid plays an important role in the friction and wear reduction, as well as in the smoothening of the worn surface. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of Ag nanoparticles additive on the tribological behavior of multialkylated cyclopentanes (MACs)

Monodisperse Ag nanoparticles with a particle size of about 6–7 nm and low volatile multialkylated cyclopentanes (MACs) lubricant were prepared. The effect of Ag nanoparticles as additive in MACs base oil on the friction and wear behavior of MACs was investigated. The friction and wear test of a steel disc sliding against the same steel counterpart ball was carried out on an Optimal SRV oscillating friction and wear tester. The morphology and elemental distribution of the worn surface of both the steel ball and steel disc and the chemical feature of typical element thereof were examined using a JEM-1200EX scanning electron microscope (SEM) equipped with a Kevex energy dispersive X-ray analyzer attachment (EDS) and X-ray photoelectron spectroscope (XPS), respectively. Friction and wear test indicates that the wear resistance and load-carrying capacity of MACs base oil were markedly raised and its friction coefficient changed little when 2% Ag nanoparticles were added in it. Results of SEM/EDS and XPS show that Ag nanoparticles were deposited on the friction pair surfaces to form low shearing stress metal Ag protective film in rubbing process.     

Comparative study of the tribological properties of ionic liquids as additives of the attapulgite and bentone greases

Three kinds of ionic liquids (1‐butyl‐3‐methylimidazolium hexafluorophosphate (L‐P104), 1‐hexyl‐3‐methyl imidazolium hexafluorophosphate (L‐P106) and 1‐octyl‐3‐methylimidazolium tetrafluoroborate (LB108)) were added to the attapulgite base grease and the bentone base grease to investigate and compare the tribological behaviours of the ionic liquids with the two base greases at room temperature and 150°C. Tribological tests were performed using a ball‐on‐plate reciprocating tribometer. The attapulgite base grease showed better wear resistance properties than that of bentone base grease by adding ionic liquids as additives. At same time, the attapulgite base grease showed excellent friction‐reducing and wear resistance properties at high temperature (150°C). Also, we discussed the tribological mechanism of the attapulgite base grease at both room temperature and 150°C from the aspect of the structure of the grease thicker. Copyright © 2012 John Wiley & Sons, Ltd.     

A study of the tribological behaviour of heterocyclic derivatives as additives in rape seed oil and mineral oil

The tribological properties of two novel heterocyclic derivatives as additives in rape seed oil and in mineral oil were investigated using a four‐ball machine. The morphologies and chemical features of the worn steel surfaces were observed and examined by means of X‐ray photoelectron spectroscopy and scanning electron microscopy. Both heterocyclic compounds improved the load‐carrying and extreme‐pressure capacities and the friction‐reducing and antiwear properties of rape seed oil more than they improved those of the mineral oil. Surface analyses of the rubbed surfaces revealed the formation of a protective film containing FeSO₄, organosulphur compound, FeS₂, polymerised ester, and organonitrogen compound when the surfaces were lubricated by rape seed oil containing the additives. The protective film formed during sliding processes contributed to the increase in the wear resistance.     

Friction and wear behaviors of a (Ca, Mg)-sialon/SAE 52100 steel pair under the lubrication of various polyols as water-based lubricating additives

The friction and wear behaviors of (Ca, Mg)-sialon/SAE 52100 steel pair under the lubrication of water or various polyol aqueous solutions were investigated with an SRV friction and wear tester in a ball-on-disc configuration. This was conducted to simulate the effect of polyols as aqueous additive in machining sialon ceramic. The morphologies of and elemental distributions in the worn surfaces of the lubricated sialon ceramics and counterpart steel were observed and determined with scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). All solutions of the tested polyols decreased the friction coefficient of (Ca, Mg)-sialon/SAE 52100 steel effectively and increased the wear volume loss of (Ca, Mg)-sialon to some extent as compared with dry sliding. The friction coefficients under the lubrication of distilled water and various polyols aqueous solutions of polyols showed almost no difference, and propanetriol was found to be the most effective for machining (Ca, Mg)-sialon with the concentration of polyols in water fixed as 5 wt%. The friction coefficients under the lubrication of propanetriol aqueous solutions in varied concentrations are closely related with the concentration, which came to the lowest value of 0.04 at a concentration of 75%. The friction-reducing performance of the polyols as additives in water was roughly correlated with their wetting behaviors on the sialon ceramic surface. In other words, the higher the wetting ability is, the lower the friction coefficients will be. Moreover, the wear volume losses of (Ca, Mg)-sialon also varied with the variation in the concentration of propanetriol in water. Accounting for the friction-reduction and wear behavior, 20% concentration of propanetriol in water could be recommended for machining (Ca, Mg)-sialon. Electron microscopic analysis indicates that polyols as additives in water enhanced the corrosive wear of sialon ceramic, which could be beneficial for increasing the machining efficiency. There existed interactions among water, polyols and sialon surfaces, which were dependent on the compositions of the lubricant solution. This accounts for the variations in the friction and wear behaviors with the concentration of polyols in water.     

Effect of surface contamination on the UHV tribological behavior of the Cu(111)/Cu(111) interface

Friction and adhesion measurements have been made using two Cu(1 11) samples modified by the presence of atomic adsorbates. Copper surfaces with 10–15 Å thick contaminant films resulting from exposure to the atmosphere exhibited a static friction coefficient of _s = 0.75 ± 0.18. A sharp increase in the friction coefficient was observed when the contaminant layer was reduced to submonolayer thickness by sputtering and for the cleanest Cu(111) surfaces studied the static friction coefficient was _s = 4.4 ± 1.3. To further investigate the tribological effect of submonolayer coverages of surface contamination adhesion experiments were performed using surfaces modified with sulfur adsorbed at coverages in the range _s = 0.0–0.39 ML (saturation). The adhesion coefficient of the clean surfaces (_ad = 0.69 ± 0.20) was markedly reduced by the presence of 0.05 ML of sulfur and decreased monotonically with increasing sulfur coverage to a limiting value of _ad = 0.26 ± 0.07 at _s = 0.39.     

Modification of tribological properties of polyarylate by supercritical fluid impregnation of copper(II) hexafluoroacetylacetonate

Polyarylate has been used as a model polymer to study its tribological property alteration during impregnation of copper(II) hexafluoroacetylacetonate in a supercritical carbon dioxide environment followed by thermal decomposition of the copper compound in air. The initial, impregnated and thermally treated polymer samples were tested by various tribological techniques to compare their wear resistance, friction coefficient f_k and surface microhardness H. SEM, EDX and ESCA have been applied to determine surface and internal domain morphology, profiles of copper complex concentration, its types and influence on the tribological properties of the samples. Dramatic surface layer modification and significant enhancement of its wear resistance (more than 400%) due to this process have been observed and discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Comparative tribological properties of magnesium hexasilicate and serpentine powder as lubricating oil additives under High temperature

Friction and wear experiments were carried out with nanoscale serpentine (magnesium silicate mineral) and magnesium hexasilicate powder as lubricating oil additives at 400 °C. The tribological test results showed that the self-repairing protective layers could be well formed on the contact surfaces, whether nanoscale serpentine or magnesium hexasilicate powder was added into lubricating oil. The SEM and EDAX analysis demonstrate that the morphology and elements of both self-repairing layers are in accordance with each other. The research results indicate that the essence of self-repairing of serpentine power is isomorphic replacement and tribochemical reaction between magnesium silicate and metal matrix.     

New unsymmetrical complex diesters of adipic acid considered as tribological fluids

The paper presents results concerning the synthesis and characterisation of some unsymmetrical diesters of adipic acid with a mixed, complex structure in order to produce synthetic ester oils with specific tribological properties. These complex diesters have been synthesised by taking into account medium and superior aliphatic alcohols, such as n‐butanol, n‐hexanol, 2‐ethyl‐hexanol, isodecanol and isotridecanol along with special alcohols of a complex alkyl–aryl nature, namely 2‐phenoxy‐ethanol, 2‐([o‐sec butyl] phenoxy) ethanol and 2‐([p‐nonyl] phenoxy) ethanol, respectively. By varying the length of the aliphatic alcohols and also by modifying the length and position of the alkyl pendant group on the aromatic ring of the complex alkyl–aryl alcohols, three series of products have been realised. On the basis of this programme, it has been possible to obtain fluid products that show valuable tribological features with adequate and versatile characteristics. Copyright © 2010 John Wiley & Sons, Ltd.