Formation Of Lubricating Film Research Articles

Preparation and tribological properties of lanthanum-doped TiO2 nanoparticles in rapeseed oil

Oleic acid (denoted as OA) surface-capped lanthanum-doped titanium dioxide composite nanoparticles, abbreviated as OA/La-TiO2, were prepared via sol-gel method in association with surface modification by oleic acid. The microstructure of as-prepared OA/La-TiO2 was characterized by means of X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectrometry, and their thermal stability was evaluated by thermogravimetry and differential scanning calorimetry. Moreover, the tribological properties of OA/La-TiO2 as lubricant additives in rapeseed oil were evaluated with a four-ball friction and wear tester. The morphology as well as elemental composition and chemical characteristics of worn steel surfaces was investigated by scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray photoelectron spectroscopy. Results showed that as-prepared OA/La-TiO2 particles were of spherical shape and had an average diameter of 20 nm. In the meantime, OA/La-TiO2 particles doped with 1.5% (molar ratio; the same hereafter) and 2.5% of lanthanum could markedly improve the anti-wear and friction-reducing capacities of rapeseed oil. This is attributed to the formation of a complex boundary lubrication film mainly composed of the oxides of iron, titanium and lanthanum as well as the formation of an adsorption film of oleic acid on steel sliding surfaces.

Tribological properties of nano‐calcium borate as lithium grease additive

ABSTRACTNano‐calcium borate (NCB) with an average particle size of about 70 nm was synthesised via ethanol supercritical fluid drying technique, and the morphology and microstructures of as‐prepared particles were characterised by means of scanning electron microscope (SEM, JEOL LTD., Tokyo, Japan) and X‐ray powder diffraction. The friction and wear behaviour of the NCB as additive in lithium grease were evaluated with an Optimol‐SRV IV (Optimol Instruments Prüftechnik GmbH, Munich, Germany) oscillating friction and wear tester (SRV tester). The morphology and surface composition of the worn surfaces of lower discs after SRV test were analysed by SEM and X‐ray photoelectron spectroscopy (XPS, Physical Electronics, Inc., USA). The result demonstrated that the anti‐wear and load‐carrying capacities of the lithium grease were significantly improved, and the friction coefficient of the lithium grease decreased with the addition of NCB additive. The analytical results of XPS indicate that the good tribological performance of NCB is attributable to the formation of a boundary lubrication film composed of deposited NCB and the tribochemical reaction products such as B2O3, CaO and iron oxides on the rubbing surface. Copyright © 2013 John Wiley & Sons, Ltd.

Study of film formation in bovine serum lubricated contacts under rolling/sliding conditions

The aim of this study is to perform a detailed experimental analysis of lubricant film thickness of bovine serum within the contact between the artificial metal and ceramic heads (balls) and the glass disc to analyse the effect of proteins on film formation under various rolling/sliding conditions. Lubricant film observation of bovine serum solutions was carried out using an optical test rig. Chromatic interferograms were recorded with a high-speed CMOS digital camera and evaluated with thin film colorimetric interferometry. Film thickness was studied as a function of time. Under pure rolling conditions, film thickness increases with time as well as with rolling distance for all mean speeds and for both materials of the balls; however the metal ball always forms a thicker lubricating film in comparison to the ceramic ball. Under rolling/sliding conditions, when the disc is faster than the ball, the formation of lubricant film thickness is different compared to pure rolling conditions. At first, film thickness increases rapidly with a rolling/sliding distance for all mean speeds. When maximum film thickness is reached, then this effect is lost and film thickness starts to fall and finally, at the end of the measurement, film thickness drops down to a few nanometres. For the metal ball, maximum values of central film thicknesses are proportional to the mean speed; however this is not observed with the ceramic ball. An absolutely different formation of bovine serum film thickness is observed when the ball is faster than the disc. Under this condition, the protein layer is very thin for both materials of balls, and central film thickness reaches only about a few nanometres. Local protein spots are formed in a very small area of the contact zone and reach the thickness between 20 and 25 nm for the metal ball and 5 nm for the ceramic ball. From the performed experiments under rolling/sliding conditions, it is obvious that the formation of lubricant film thickness is markedly dependent on kinematic conditions acting in the contact, especially on the positive and negative slide-to-roll ratio and the mean speed. In addition, the material of the artificial head has a certain influence on the formation of bovine serum lubricating film.

Effects of hexagonal boron nitride and sintering temperature on mechanical and tribological properties of SS316L/h-BN composites

In this work, hexagonal boron nitride (h-BN)-embedded 316L stainless steel (SS316L/h-BN) composites were prepared using a conventional powder metallurgy process. In order to produce self-lubricating composites, various amounts of h-BN (10, 15 and 20vol%) were incorporated. Effects of h-BN content and sintering temperature on the mechanical and tribological properties were of primary interest. The results suggested that an increase in h-BN content reduced the hardness of the composites, but that the hardness could be improved by increasing the sintering temperature. Addition of h-BN up to 20vol% improved the friction coefficient of the composites. At a sintering temperature of 1250°C, h-BN transformed into a boride liquid phase, which formed a eutectic during cooling and exhibited a deterioration effect on lubricating film formation of the h-BN, resulting in an increase in the friction coefficient of the composites. The specific wear rate was greatly reduced when the composites were sintered at 1200°C. The lowest friction coefficient and specific wear rate in the composites could be found under the experimental conditions used in this work when using 20vol% of h-BN at a sintering temperature of 1200°C.

Tribological Properties and Wear Mechanisms of NiCr–Al2O3–SrSO4–Ag Self-Lubricating Composites at Elevated Temperatures

NiCr–Al2O3–SrSO4–Ag self-lubricating composites were prepared by powder metallurgy method and the tribological properties of composites were evaluated by a ball-on-disk tribometer against alumina ball at wide temperature range from the room temperature to 1,000 °C in air. The linear coefficient of thermal expansion was evaluated for investigation of thermal stability of composites. The tribo-chemical reaction films formed on the rubbing surfaces and their effects on the tribological properties of composites at different temperatures were addressed according to the surface characterization by SEM, XRD, and XPS. The results show that the NiCr–Al2O3 composite with addition of 10 wt% SrSO4 and 10 wt% Ag exhibits satisfying friction and wear properties over the entire temperature range from room temperature to 1,000 °C. The composition of the tribo-layers on the worn surfaces of the composites is varied at different temperatures. The synergistic lubricating effect of SrAl4O7, Ag, and NiCr2O4 lubricating films formed on worn surfaces were identified to reduce the friction coefficient and wear rate from room temperature to 800 °C. Meanwhile, at 1,000 °C, the SrCrO4 and NiAl2O4 was formed on the worn surfaces during sliding process, combining with the NiCr2O4, Al2O3, Cr2O3, Ag, and Ag2O, which play an important role in the formation of a continuous lubricating film on the sliding surface.

Nanoscale lubricating film formation by linear polymer in aqueous solution

Film-forming properties of polymer in aqueous solution flowing through a nanogap have been investigated by using a thin film interferometry. The film properties of linear polymer in aqueous solution flowing through a confined nanogap depend on the ratio of water film thickness to averaged radius of polymer chains H0/RPolymer. It was found that the lubrication film thickness of linear polymer in aqueous solution decreases as the polymer molecular weight increasing when H0/RPolymer < 2 ∼ 3. A new lubrication map was proposed, which includes the lubrication regime of weak confinement influence, the lubrication regime of strong confinement influence (LRSCI), and the transition regime of confinement influence. It is very difficult to increase the lubrication film thickness using the higher molecule weight in the LRSCI regime. The lubrication mechanism inferred from our experimental results may help to better understand the dynamic film properties of linear polymer in aqueous solution flowing through a nanogap.

Piezoelectric sensors to monitor lubricant film thickness at piston–cylinder contacts in a fired engine

The contact between the piston ring and cylinder liner is the most important sealing interface in an automotive engine. Understanding the contact interactions and lubricant film formation at this interface is crucial for the development of fuel-efficient and low emission engines. This article outlines the development of an ultrasonic approach to enable non-invasive measurement of the lubricant film thickness formed between piston and cylinder wall of a fired engine. The sensor system consisted of a series of small, low cost piezoelectric elements which were bonded to the external surface of a four-stroke, single-cylinder engine. Each element could be individually energised with a short duration voltage pulse and reflections from the cylinder inner bore recorded. By using high frequency pulsing and data capture it proved possible to image individual ring and skirt contacts at full engine speeds. These captured reflections were processed to give lubricant film thickness directly and without the need for independent calibration. The results show good repeatability between cycle sets at specific running conditions. The lubricant films at each of the ring contacts can be measured at sufficient resolution such that individual rails of the oil control ring can be monitored. In addition, the film generated at the skirt was measured, the results from which, suggest the occurrence of ‘piston slap’ and highlight the potential for this ultrasonic method to enable indirect measurements of piston secondary dynamics.

Effect of dry friction parameters on the tribosynthesis of secondary structures on composite antifriction iron-based material

The paper examines the tribological characteristics of a Fe–W–CaF2 composite antifriction material (CAM) in combination with 65G steel during dry friction in air at a high sliding velocity (15 m/sec) and insignificant (0.64–1.28 MPa) pressures. It is established that with twofold increase in pressure (from 0.64 to 1.28 MPa), CAM friction coefficient decreases from 0.25 to 0.20 (by 20%) and wear increases from 0.0158 to 0.03085 mg/km (by 49%) but remains insignificant. The factors acting in the friction process lead to the formation of secondary lubricating films. They prevent mechanical contact between the rubbing surfaces and provide necessary antifriction and operating properties. It is shown that the secondary lubricating films as thin layers with inclusions of solid lubricants differ from the starting material in chemical and phase composition, structural state, and better mechanical characteristics.

Friction and Wear Performances of Magnesium Alloy against Steel under Lubrication of Rapeseed Oil with N-Containing Additive

A N-containing additive, amide type modified rapeseed oil (named as NRO), was prepared by chemical modification of rapeseed oil and characterized by infrared spectrum The friction and wear performances of AZ91D magnesium alloy against GCr15 bearing steel under the lubrication of rapeseed oil formulated with NRO were evaluated on a SRV tribotester. The topographies and the chemical species of the worn surfaces of magnesium alloy were analyzed by a scanning electron microscope (SEM) and an X-ray photoelectron spectroscope (XPS), respectively. The results indicated that the friction and wear of the magnesium alloy—steel tribomates could be effectively reduced by formulating NRO into rapeseed oil lubricant. The friction coefficients and the wear volumes of magnesium alloy decreased with increasing contents of NRO. The surface lubricated with NRO-doped rapeseed oil was characterized by less wear as compared with that lubricated with neat rapeseed oil. The enhanced anti-wear and friction-reducing abilities of rapeseed oil by NRO in the lubrication of magnesium alloy against steel were ascribed to the formation of a composite boundary lubrication film due to the strong adsorption of NRO and rapeseed oil onto the lubricated surfaces and their tribochemical reactions with magnesium alloy.

Mechanical and tribological properties of acrylonitrile–butadiene rubber filled with graphite and carbon black

In this work, acrylonitrile–butadiene rubber (NBR)/expanded graphite (EG)/carbon black (CB) micro- and nanocomposites were prepared by two different methods, and the resulting mechanical and tribological properties were compared with those of NBR/CB composites. Meanwhile, the effects of graphite dispersion and loading content, as well as the applied load and sliding velocity on the tribological behavior of the above composites under dry friction condition were also evaluated. The worn surfaces were analyzed by scanning electron microscopy (SEM) to disclose wear mechanism. As expected, the better the dispersion of graphite, the more remarkable enhancement on tensile and dynamic mechanical properties, and the greater reduction in the coefficient of friction (COF) and specific wear rate (Ws). It was found that a small amount of EG could effectively decrease COF and Ws of NBR/CB composites because of the formation of graphite lubricant films. The COF and Ws of NBR/CB/EG composites show a decreasing trend with a rise in applied load and sliding velocity. NBR/CB/EG nanocomposite always shows a stable wearing process with relatively low COF and Ws. It is thought that well-dispersed graphite nano-sheets were beneficial to the formation of a fine and durable lubricant film.

Model and experiments to determine lubricant film formation and frictional torque in aircraft landing gear pin joints

Pin joints are found in many large articulating structures. They tend to be under high load and articulate slowly; so, the joints typically operate in the boundary or mixed lubrication regimes. This means that the operating torque depends on the respective proportions of liquid and solid contact between the joint mating faces. In this article, a mixed lubrication model of a grease-lubricated landing gear joint is established to determine a theoretical Stribeck curve, frictional torque and lubricant film thickness under different loads. Parameters describing pin joint working conditions, geometry, lubricant properties and pin/bush texture are used. The model can also predict the proportion of the load that is supported by contacting asperities and lubricant film. The changing proportions of these two parts indicate transformations between different lubrication regimes. Experiments on an instrumented pin joint have been carried out to compare with the predicted friction and torque performance. Theoretical calculation results show good consistency with experimental plots at high load. But under low load, the real friction between pin and bush is significantly lower than theoretical predictions.

Physio-chemical hydrodynamic mechanism underlying the formation of thin adsorbed boundary films

The formation of low shear strength surface-adhered thin films mitigates excessive friction in mixed or boundary regimes of lubrication. Tribo-films are formed as a consequence of molecular chemical reactions with the surfaces. The process is best viewed in the context of a lubricant-surface system. Therefore, it is usually surmised that the adsorption of lubricant molecular species to the contact surfaces is underlying to the formation of ultra-thin lubricant films. The paper considers contact between smooth surfaces at close separation. This may be regarded as the contact of a pair of asperity summits, whose dimensions, however small, are far larger than the size of fluid molecules within the conjunction. In such diminishing separations the constraining effect of relatively smooth solid barriers causes oscillatory solvation of fluid molecules. This effect accounts for the conjunctional load capacity but does not contribute to mitigating friction, except when molecular adsorption is taken into account with long chain molecules which tend to inhibit solvation. The paper presents an analytical predictive model based on the Ornstein-Zernike method with the Percus-Yevick approximation of a narrow interaction potential between conjunctional composition. The predictions confirm the above stated physical facts in a fundamental manner.

Study of tribo-chemical lubricant film formation during application of nanolubricants in minimum quantity lubrication (MQL) grinding

This paper presents the study of mechanism of nanolubricant impingement in minimum quantity lubrication grinding. To study the role of nanolubricants (a new class of advanced lubricants integrating multicomponent chemistries) during workpiece-tool interaction, surface-grinding tests were performed on ductile iron workpieces using an aluminium oxide wheel under varied infeed conditions. The process performance in terms of force ratio, specific energy, and G-ratio has shown substantial improvement when using nanolubricant. Formation of tribo-chemical films of Mo–S–P chemistry complex on the workpiece surface was identified as the mechanism responsible for these improvements.

Tribological Properties of Metal V-Belt Type CVT Lubricant

The priority for lubricant performance for metal V-belt-type CVT (B-CVTFs) should be the improvement of transmittable torque capacity between the belt and pulley plus excellent antishudder properties for lockup clutch used in B-CVTs. This study intends to investigate the effect of lubricant additives for improving these performances of B-CVTs. In addition, surface analysis techniques were utilized to gain a novel insight into the chemical composites and morphology of the tribofilms. As a result, it is vital for greater torque capacity to give higher boundary friction coefficient between the metal contacting interfaces, and the process of boundary lubricant film formation derived from antiwear additives used in B-CVTFs strongly impacts on the torque capacity. Moreover, it is found that a sort of lubricant formulation gave an excellent antishudder performance for wet clutch with keeping higher friction coefficient between the metals, which would result in improving the performance of B-CVTs.

Effect of expanded graphite (EG) dispersion on the mechanical and tribological properties of nitrile rubber/EG composites

In this work, in order to prepare expanded graphite (EG) and acrylonitrile-butadiene rubber (NBR) composites, two preparation methods were adopted, namely directly mechanical blending (micro-composites) and latex compounding technique (nanocomposites), and their resulting mechanical and tribological properties were compared. Meanwhile, the effects of the graphite dispersion state, volume fraction, load and sliding velocity on the friction and wear properties were also explored. As expected, nanocomposite showed better tensile properties, hardness, dynamic storage modulus and lower hysteresis loss peak. The coefficient of friction (COF) for both composites decreased as the load and sliding velocity increased, while the effect of the graphite volume fraction was subtle. The wear rate of nanocomposites decreased as the load and sliding velocity increased. As the load increased, the wear rate of micro-composites increased, but with increasing sliding velocity, the wear rate first increased, reached a maximum, and then decreased. The wear mechanism could be described as graphite sheets de-bonded from matrix, lubricant film formation and breaking, matrix wear, and lubricant film re-formation in cycles. Compared with micro dispersion, the nano-sized dispersion of graphite was beneficial to the formation of a continuous, uniform, and stable lubricant film under high sliding velocity and heavy load, contributing to reduce the COF and improve the wear resistance.

Friction and wear performances of magnesium alloy against steel under lubrication of rapeseed oil with S-containing additive

A S-containing additive, sulfuration modified rapeseed oil (named as SRO), was prepared by chemical modification of rapeseed oil with sulfur compounds. The results indicate that the friction and wear of the magnesium alloy–steel tribomates could be effectively reduced by formulating SRO into rapeseed oil lubricant. The friction coefficients and the wear volumes of magnesium alloy decrease with increasing contents of SRO. The surface lubricated with SRO-doped rapeseed oil was characterized by less wear as compared with that lubricated with neat rapeseed oil. The enhanced anti-wear and friction-reducing abilities of rapeseed oil by SRO in the lubrication of magnesium alloy against steel were ascribed to the formation of a composite boundary lubrication film due to the strong adsorption of SRO and rapeseed oil onto the lubricated surfaces and their tribochemical reactions with magnesium alloy.

Influence of surface velocity directions on lubricant film formation in EHL point contacts

Gear failures like wear, scuffing and micropitting strongly depend on lubrication conditions between gear tooth flanks. Many formulae have been developed to predict the EHL film thickness under various operating conditions. However, these methods are only valid if the surface velocities of the sliding partners are orientated in the same direction, like e.g. in spur gears. For bevel gears with hypoid offset the surface velocities of the contacting partners are orientated in different directions. For a reliable prediction of lubrication film thickness under conditions of different surface velocity directions film thickness measurements in a twin disc machine are performed.

The tribological behaviour of different clearance MOM hip joints with lubricants of physiological viscosities

Clearance is one of the most influential parameters on the tribological performance of metal-on-metal (MOM) hip joints and its selection is a subject of considerable debate. The objective of this paper is to study the lubrication behaviour of different clearances for MOM hip joints within the range of human physiological and pathological fluid viscosities. The frictional torques developed by MOM hip joints with a 50 mm diameter were measured for both virgin surfaces and during a wear simulator test. Joints were manufactured with three different diametral clearances: 20, 100, and 200 microm. The fluid used for the friction measurements which contained different ratios of 25 percent newborn calf serum and carboxymethyl cellulose (CMC) with the obtained viscosities values ranging from 0.001 to 0.71 Pa s. The obtained results indicate that the frictional torque for the 20 microm clearance joint remains high over the whole range of the viscosity values. The frictional torque of the 100 microm clearance joint was low for the very low viscosity (0.001 Pa s) lubricant, but increased with increasing viscosity value. The frictional torque of the 200 microm clearance joint was high at very low viscosity levels, however, it reduced with increasing viscosity. It is concluded that a smaller clearance level can enhance the formation of an elastohydrodynamic lubrication (EHL) film, but this is at the cost of preventing fluid recovery between the bearing surfaces during the unloaded phase of walking. Larger clearance bearings allow a better recovery of lubricant during the unloaded phase, which is necessary for higher viscosity lubricants. The selection of the clearance value should therefore consider both the formation of the EHL film and the fluid recovery as a function of the physiological viscosity in order to get an optimal tribological performance for MOM hip joints. The application of either 25 per cent bovine serum or water in existing in vitro tribological study should also be revised to consider the relevance of clinic synovial fluid viscosities and to avoid possible misleading results.

Plate-out modelling for cold-rolling systems lubricated with oil-in-water emulsions

Oil-in-water (O/W) emulsions are widely used for steel-rolling processes. An efficient lubrication is required to control the oil film deposition on the strip surface. This article deals with phase separation in lubrication film formation before roll bite (plate-out). A model has been developed to simulate the behaviour of an emulsion droplet that impacts onto a moving plate. The typical configuration of emulsion droplet impact is considered in a calculation which controls the initial formation of the film. To describe hydrodynamic phenomena, wetting and three-phase contact line models have been integrated in a Navier–Stokes-based computational fluid dynamic code. Both models have been validated against data from the literature. Numerical studies carried out with the model have highlighted the importance of the oil concentration in the droplet on the plate-out occurrence. A ballistic study has also been performed to evaluate the influence of the droplet velocity on the film formation.

Effect of surface texturing on elastohydrodynamically lubricated contact under transient speed conditions

Effect of surface topography modifications on lubrication film thickness within non-conformal lubricated contact operated under transient speed conditions is observed. Optical test rig is used to observe the lubricant film behaviour between the flat surface of a chromium coated glass disc and a steel ball under simplified operational conditions modelling the cam and tappet contact. Numerical simulation was used to be able to choose the operating conditions suitable for experiments. An array of micro-dents was produced on the ball surface to be able to demonstrate the effect of surface topography on lubrication film formation. Experiments were carried out under elastohydrodynamic lubrication conditions. Obtained results have shown that surface texturing could represent the way how to increase lubrication efficiency of rolling/sliding non-conformal contacts under transient operational conditions through the lubricant emitted from micro-dents. It was found that the lubricant emitted from the micro-dents helps to separate rubbing surfaces especially under thin film lubrication conditions where the rubbing surfaces moves in the opposite direction.