The influence of molecular structure on the frictional behaviour of lubricating fluids

In order to establish correlations between the chemical structure or constitution of fluid lubricants and their friction characteristics under elastohydro dynamic conditions, a number of model test fluids were tested in a twin-disk machine. As test fluids, special saturated mono-, bi-, and tricyclic compounds, alkanes and polyethers were selected. The traction coefficients were shown to be strongly dependent on the chemical structure of the lubricants and on operational parameters. High coefficient values mostly arise from lubricant molecules that are shaped unregularly, with indentations and teeth as seen with Stuart-Briegleb models, and allowing only heavily hampered rotational intramolecular motions along single bond axes. Low traction coefficients derive from lubricant molecules with a thread-like shape and a minimum of structural subunits or functional groups involved in significant intermolecular interactions. In order to account for the results obtained, several paradigms and hypotheses are established and discussed in detail.     

The influence of lubricating fluid type on the properties of biodegradable greases

The lubricating ability of a grease depends on both the base oil and the thickener. As a result of their intrinsic properties and/or because of their com‐patibility with thickeners and specific additives, base fluids have different influences upon the properties of grease formulations. It is well known that mineral oils are the most widely used lubricant bases due to their inherent lubricity and lower cost, but recent environmental concern has led to consideration of the use of vegetable oils and readily biodegradable synthetic fluids as raw materials in lubricating grease formulations. As well as the base materials, the additives for biodegradable greases should also be biodegradable. This requirement limits the kind of products that may be used in environmentally friendly greases. This paper presents comparative data concerning the tribological and physico‐chemical properties of biodegradable greases formulated with certain vegetable oils, such as rape seed oil, castor oil, and soybean oil or their mixtures, and synthetic esters. The improvement of the load‐carrying properties of biodegradable greases and the antioxidative effect of some suitable additives have also been studied, and the results are presented here.     

The effect of ethoxylated esters on the lubricating properties of their aqueous solutions

In many applications strict ecological criteria are crucial. Water is often used in these cases as a lubricant base. Its disadvantageous properties, in particular lubricity, can be modified by introduction of additives. Ethoxylated methyl esters of fatty acids from rapeseed oil were chosen for that purpose. In order to verify tribological properties of aqueous solutions of these compounds, anti-seizure properties under linearly increasing load, friction and wear under 2 kN (four-ball machine) were assessed. Not only radical change of tribological properties as compared to water was observed, but also the results are comparable to the ones obtained for mineral oils, which contained typical, commercial additives. Significant changes in scuffing load (P _t), seizure load (P _oz), limiting pressure of seizure (p _oz) were recorded even at 0.1% wt concentration of the additive. At optimal concentration P _t, P _oz, p _oz, increased even almost 6-, 2- and 3-fold (respectively). The quantities measured increased with ethoxylation degree. Also at constant load, friction (6-fold) and wear (almost 2-fold) were reduced. In this case, however, no influence of concentration and ethoxylation degree on the quantities measured, was observed.The tribological properties can be discussed in terms of results of physical–chemical tests. Ethoxylates are surface active compounds. Within the range of concentrations analyzed, they form micelles in a bulk phase and liquid-crystalline structures in a surface phase. High surface activity of these additives and creation of specific structures in solutions were proved by measuring surface tension, wetting angle and mesophases observation in polarized light. On the basis of the results of the physical–chemical and tribological tests one can conjecture that profitable tribological properties of the lubricating compositions are the result of strong affinity to the surface and their ability to form ordered structures. One can expect that over the cooperating surfaces a deposit is formed. Presence of the deposit increases real contact area and prevents seizure. This hypothesis gives a simple explanation of the unexpected improvement of anti-seizure properties, which accompanies the increase of ethoxylation degree. Ethoxylates with higher number of ethylene oxide attached, reveal smaller surface activity and better solubility in water. On the other hand, they can be characterized by higher dehydratation temperature. This means that at higher temperatures they reveal surface activity. That is why, during seizure tests, at high temperature, under high loads the esters of higher ethoxylation degree protect friction couples against seizure much more efficiently.     

The influence of some additives on the lubricating properties of semi‐synthetic ester oils

This article presents research on the lubricating properties of semi‐synthetic oils, made as compositions of mineral oil and esters. The esters were synthesized from acid substrates from the oxidation of paraffins. The last non‐seizure load (P_n), the weld point (P_z), the load wear index (I_h), and the limiting wear load (G_oz) were measured, and the effects of additives or mixtures of additives were investigated by their effects on these parameters. It was found that the efficiency of the additives depended on their chemical characteristics and concentration, and also on the chemical nature of the ester component of the semi‐synthetic oils.     

The influence of lubricating greases on rolling bearing failure

Significant savings can be made by the correct use of lubricants in terms of energy consumption, replacement parts, maintenance costs and the reduction of machinery downtime, because the service life of machines is one of the most important aspects of engineering. Planned preventive maintenance means the minimisation of failures and of maintenance costs; the technology should allow identification of the tribomechanical systems in all operating conditions and of the tribological regime and duration of operation. Despite the enormous advances that have been made over the last thirty years in the theory of rolling bearing lubrication, the successful operation of rolling bearings remains highly dependent on empirical knowledge. The aim of the present study is to investigate the influence of lubricating grease on rolling bearing failure; the experimental work was carried out at a metalworking factory.     

The influence of external effects on the structurization of lubricating materials

The methods of drop spreading over the solid body surface and rotational viscosimetry have been used to explore the processes of structurization of lubricating material. It is demonstrated how the surfactants and superdispersed oil particles (artificial structurization nuclei) influence the evolution of these processes.     

The influence of molecular structure on the frictional behaviour of lubricating fluids 2: Low coefficients of traction

In order to establish correlations between the chemical structure or constitution of fluid lubricants and their friction characteristics under elastohydrodynamic conditions, a number of model test fluids were tested in a twin-disk machine. As test fluids, special saturated monobi-, and tricyclic compounds, alkanes and polyethers were selected. The traction coefficients were shown to be strongly dependent on the chemical structure of the lubricants and on operational parameters. High coefficient values mostly arise from lubricant molecules that are shaped irregularly, with indentations and teeth as seen with Stuart-Briegleb models, and allowing only heavily hampered rotational intramolecular motions along single bond axes. Low traction coefficients derive from lubricant molecules with a thread-like shape and a minimum of structural subunits or functional groups involved in significant intermolecular interactions. In order to account for the results obtained, several paradigms and hypotheses are established and discussed in detail.     

The influence of chemical composition on the pour-point depressant properties of methacrylate copolymers used as additives for lubricating oils

Methacrylate polymers are much used as additives in lubricating oils, as pour-point depressants and viscosity index improvers. Although the mechanism of such pour-point depression is still controversial, it is thought to be related to the length of the alkyl side chains of the polymethacrylates, and to the nature of the base oil. In this paper, work is reported on: (a) the depressant activity of polymethacrylate copolymers in relation to the content of short alkyl methacrylates and styrene, and the average side-chain length of the basic methacrylate constituents; (b) the pour point of the base stock; and (c) the additive concentration.     

The lubricating properties of used engine oil

The lubricating properties of oil samples from four cars using either leaded or unleaded gasolene were examined using a cross-pin-type lubricant tester and a JIS four-ball tester. The zinc dialkyldithiophosphate (ZnDTP) content in the oil samples was determined by thin layer chromatography (TLC). The wear scar diameter increased with running distance up to 2000 km. At running distances above 2000 km, the wear scar diameter decreased for oil from cars using unleaded gasolene but increased for oil from cars using leaded gasolene. The load-carrying capacity also varied depending on the type of fuel used. The TLC spot characteristic of ZnDTP disappeared after running distances of 3000 km with both fuels. However, another spot, characteristic of lead dialkyldithiophosphate (PbDTP), appeared below the ZnDTP spot for oil from cars using leaded gasolene. These results indicate that the differences in the lubricating properties of oils from cars using leaded and unleaded gasolene are due to the formation of PbDTP.     

Fe3O4纳米粒子添加量对润滑油润滑特性的影响

主要研究了润滑油中纳米粒子添加剂的含量对冷挤压过程中润滑特性的影响规律。采用粒径为20～30nm的Fe3O4纳米粒子分散于52#汽缸油中配置成具有不同质量分数的纳米粒子改性润滑油,并将该润滑油应用于钛合金（TA2）棒材的冷挤压实验,论文系统研究了润滑油中Fe3O4纳米粒子的添加量对钛合全冷挤压成形的最大成形力、成形功、表面质量（Ra）及HV的影响规律并对其润滑机理进行了分析,结果表明：当润滑油中Fe3O4纳米粒子质量分数为8％时。纳米改性润滑油的润滑效果最佳,挤压成形力和成形功最小,成形件表面质量最好。纳米粒子的填充与滚动作用及其对52#汽缸油膜的支承作用是改善润滑油润滑性能的主要原因。     

The filler content of the dental composite resins and their influence on different properties

The purpose of this study was to compare the inorganic content and morphology of one nanofilled and one nanohybrid composite with one universal microhybrid composite. The Vickers hardness, degree of conversion and scanning electron microscope of the materials light‐cured using LED unit were also investigated. One nanofilled (Filtek? Supreme XT), one nanohybrid (TPH®₃) and one universal microhybrid (Filtek? Z‐250) composite resins at color A₂ were used in this study. The samples were made in a metallic mould (4 mm in diameter and 2 mm in thickness). Their filler weight content was measured by thermogravimetric analysis (TG). The morphology of the filler particles was determined using scanning electron microscope equipped with a field emission gun (SEM‐FEG). Vickers hardness and degree of conversion using FT‐IR spectroscopy were measured. Filtek? Z‐250 (microhybrid) composite resin shows higher degree of conversion and hardness than those of Filtek? Supreme XT (nanofilled) and TPH®₃ (nanohybrid) composites, respectively. The TPH₃® (nanohybrid) composite exhibits by far the lowest mechanical property. Nanofilled composite resins show mechanical properties at least as good as those of universal hybrids and could thus be used for the same clinical indications as well as for anterior restorations due to their high aesthetic properties. Microsc. Res. Tech. 75:758–765, 2012. © 2011 Wiley Periodicals, Inc     

The adsorption of zinc dialkyldithiophosphates on partially stabilized zirconia from hydrocarbon solution

Kinetics, isotherms and enthalpy of adsorption of zinc di-n-alkyl(di-n-alkylphenyl, di-n-dialkylphenyl)dithiophosphates (ZDTPs) lubricating oil additives on zirconia partially stabilized by yttria (PSZ) powder from hydrocarbon solution at temperatures 25 °C and 35 °C were done. The adsorption enthalpy has been determined using a Montcal calorimeter. ZDTP adsorption isotherms are Langmuir-like. The adsorption quantities and differential molar enthalpies of ZDTPs adsorption on PSZ are low for n-alkyl derivatives, the adsorption decreases with the increasing length of the hydrocarbon chain of the additive ZDTP, the unexpectedly for di-n-propyl ZDTP the differential molar enthalpies of adsorption at 25 °C are very low, near zero and for di-n-dialkylphenyl ZDTP only at lower coverage ratios are endothermic, at higher surface coverage for this and remaining additives are exothermic. At 35 °C all n-alkyl ZDTPs adsorb on PSZ endothermically endothermic, while both aryl ZDTPs exhibit exothermic effects. All ZDTPs are reversibly physisorbed at 25 °C and at 35 °C except for both mono- and dialkylphenyl ZDTPs, which are weakly chemisorbed.     

The effect of emulsifier concentration on the lubricating properties of oil-in-water emulsions

Although the use of oil-in-water (O/W) emulsions as metalworking fluids is widespread, the mechanisms of emulsion lubrication are not yet well understood. Several theories have been proposed but there is not a clear agreement about the effect of different operating conditions and emulsion properties on the lubricating performance of O/W emulsions. In the present study, the film forming ability of O/W emulsions as a function of emulsifier concentration is studied. The emulsifier content exerts a strong influence on all the emulsion properties, such as stability, droplet size distribution, surface and interfacial tension, wetting ability, etc., as well as on the lubricating behaviour, so it has been used to ascertain the relationship between all the properties involved. Three different emulsifiers—anionic, nonionic and cationic—were used at different concentrations in the design of lubricant O/W emulsions. Experimental results show that the work of adhesion of oil droplets on the metal surface is a valuable parameter to predict the ability of emulsions to form thick films in elastohydrodynamic (EHD) contacts. The influence of pH value of O/W emulsions on their lubricating behaviour is also verified. The overall conclusion is that the interactions between metal and oil droplets rule the mechanism of lubrication and that this interaction is primarily controlled by emulsifier concentration.     

The relationship between the ferrogram entry deposit and the iron content of used lubricating oils

The volume of the ferrogram entry deposit is shown to be proportional to the product of its height and cross-sectional area by approximating it to a segment of a sphere. The volume of the entry deposit of a number of ferrograms prepared from worn oils is shown to correlate with the amount of iron in the oil, determined spectrometrically, provided the entry deposit is mainly composed of free metal particles.     

The determination of iron in lubricating oils by X-ray fluorescence spectrometry

The measurement of iron in lubricating oils taken from an experimental gear rig and various truck assemblies has been examined by X-ray fluorescence (XRF) spectrometry. This simple and rapid technique is shown to give accurate and reproducible results for these systems. Particle size limitations were found not to be as severe for XRF as is evidently the case for spectrometric oil analysis methods, and XRF appears to be more suitable for the detection of the relatively large particles which are indicative of the onset of severe wear.Iron contents of from 1 to 1200 wt.ppm in oils were measured with results in agreement with those obtained by atomic absorption spectrometry after a wet-ashing procedure, but they were higher than those obtained by direct atomization. To obtain reliable XRF results the reference solutions used were prepared from the same oil as that in the assembly under test.     

Tribological thermodynamic model of the viscosity,flow properties and lubricating performance of lubricating oils

Lubricating phenomena are described thermodynamically. Tribological parameters, in particular pressure and relative sliding speed, are related to thermodynamic properties of the tribological system such as thermodynamic potential, enthalpy and entropy. On this basis tribological thermodynamic models of the viscosity, flow properties and lubricating performance of materials are deduced. Changes of lubricating performance in a tribological system are determined by considering such tribological thermodynamic parameters. A tribological thermodynamic model of lubricating phenomena is therefore obtained which with the use of suitable thermodynamic data for optimum tribological parameters allows the prediction of the lubricating performance of tribological systems.     

Effect of structure and composition of solid carbon coatings of steel parts on the lubricating properties of synthetic oil

The authors studied the friction and wear properties of PAOM-4 synthetic oil under the friction of steel samples, the working surfaces of which are coated by a thin layer of hard carbon coatings of different composition and structure. It was found that some of them result in improved antiwear and antifriction behavior of the test oil. The polyalphaolefin oil shows the highest lubricity when steel rubs against carbon diamond-like coatings, namely, monocrystalline tungsten-alloyed coatings and amorphous coatings with increased sp ³ phase.     

An investigation of the tribological properties of thin KCl films on iron in ultrahigh vacuum: modeling the extreme-pressure lubricating interface

The frictional properties of thin KCl films deposited onto clean iron are measured in ultrahigh vacuum using a tungsten carbide tribotip, where the observed initial rapid decrease in friction coefficient with film thickness is proposed to be due to the formation of a complete KCl monolayer where the friction coefficient of this film is ∼0.27. A 1800 Å thick KCl film shows a hardness and friction coefficient similar to those for bulk KCl when the width of the surface height distribution of the tribotip measured by atomic force microscopy (AFM) is 2000–3000 Å. This implies that the KCl film behaves like the bulk material when the film thickness exceeds the roughness of the interfaces.     

The effect of water on the acid-base properties of new and used IC engine lubricating oils

The absolute values of total base number (TBN) were determined for several sets of both fresh and used engine oil samples. Basic and acidic compounds from these oils have been extracted into water, 7% sea water and ethanol-water (1:1, v/v) mixtures. The extracts have been determined by potentiometric titration as pH extracts, TBN (water extract) and alkalinity. The oil formulations are not resistant to the presence of water but basic additives in fresh oil are more resistant than those for used oils. The percentage of TBN extractable into water significantly increases with service life. The presence of significant amounts of water in lubricating oil is serious and should form part of any lubricant condition monitoring system. The case is argued for both the use of dehydration canisters in oil systems and for the development of a new method of water content determination for used formulated lubricating oils which is quick, accurate and suitable for automated condition monitoring systems. The pH of aqueous extracts for condition monitoring of lubricating oils is shown to be only meaningful for TBN values of 2 or less.