Formation Of Lubricating Film Research Articles

Friction behaviour of aluminium composites mixed with carbon fibers with different orientations

The primary goal of this study work it was to distinguish a mixture of materials with enhanced friction and wearing behaviour. The composite materials may be differentiated from alloys; which can contain two more components but are formed naturally through different processes such as casting. The load applied on the specimen during the tests, is playing a very important role regarding friction coefficient and also the wearing speed. Sintered composites are gaining importance because the reinforcement serves to reduce the coefficient of thermal expansion and increase the strength and modulus. The friction tests are carried out, at the room temperature in dry condition, on a pin-on-disc machine. The exponentially decreasing areas form graphs, represented to the curves coefficient of friction, are attributed to the formation of lubricant transfer film and initial polishing surface samples. The influence of the orientation of the carbon fibers on the friction properties in the sintered polymer composites may be studied by the use of both mechanical wear tests by microscopy and through the use of phenomenological models.

Microstructure evolution and tribological behavior of the solid lubricant based surface composite of cast nickel aluminum bronze developed by friction stir processing

In the present work, friction stir processing (FSP) of the cast nickel aluminum bronze (NAB) alloy (C95500) and fabrication of graphite reinforced surface composite by means of FSP route have been carried out. Optical microscope (OM) and scanning electron microscope (SEM) were used for the microstructural study. Microstructural observation suggested the presence of a sound processed surface with a fine equiaxed grain in the stir zone (SZ). Microhardness measurement revealed that FSP increases the surface hardness as compared to as-cast surface, as it constitutes of refined grain structure. Wear behavior of the surfaces was assessed by performing sliding wear test on pin-on-disk tribometer. It was found that upon addition of the graphite particles, wear properties were improved by 43.87% and 51.93% in comparison to both FSPed and as-cast surface respectively. The enhanced wear resistance is due to the formation of protective lubricant film between sliding surfaces in case of surface composite. The measurement of stacking fault energy (SFE) by X-ray line profile analysis was carried out and its effect on sliding wear behavior was also studied. The X-ray line profile analysis suggested that FSP results in the decrease of SFE as compared to the as-cast surface.

Tribological performance of rice husk ceramic particles as a solid additive in liquid paraffin

Rice husk ceramic (RHC) particles were prepared and its effect on the lubrication performance of liquid paraffin (LP) was investigated using a four-ball tribometer to expand the comprehensive utilization of rice husk. The wear and friction mechanisms of RHC particles were also investigated. Results showed that RHC particles can strengthen the antiwear and friction reduction properties of LP in the presence of 2wt% polyisobutylene succinimide (T154A) at 75 or 100°C. The friction and wear mechanisms of RHC particles were ascribed to high temperature, which ensures the involvement of RHC particles in the formation of boundary lubrication film.

Effects of lubricant viscoelasticity on film thickness in elastohydrodynamic line contacts during start-up

By means of numerical simulation the elastohydrodynamic lubricating film formation during start-up of motion of a line contact from rest is studied. The transition from solid contact to lubricated contact is of importance in investigating the start-up process and its effects on bearing performance. The numerical results of Holmes et al. (2002) and others, concerning point contact and Newtonian lubricant, are not in good agreement with the known experimental data. The numerical film thickness is significantly smaller than the experimental values and the experimental velocity at which lubricant travels through the conjunction does not equal the entrainment velocity of numerical results. In the current paper, a lubricant with viscoelastic properties is considered. The Maxwell model of a lubricant at pure rolling of elastic cylinders is used. Also, as well as in work of Usov (2008) the model assumes the division of the whole area of contact into three zones: the dry contact of surfaces, the transient zone and the zone in which the surfaces are separated by a liquid lubricant layer. The rigid body model is assumed for the lubricant in the transient zone. The effects of viscoelasticity and size of transient zone on film thickness and the velocity at which the lubricant moves through the contact are studied. It is shown that viscoelasticity raises film thickness in a start of the motion and lowers the velocity at which the lubricant moves through the contact. This velocity is lower than the entrainment velocity in the first half of the contact, as in the experimental results with point contact. In the second half of the contact, this velocity may be lower or higher than the entrainment velocity depending on the size of the transient zone. The film thickness may increase considerably at the start of motion when the size of the transient zone increases.

The effect of lubricant constituents on lubrication mechanisms in hip joint replacements

The aim of the present paper is to provide a novel experimental approach enabling to assess the thickness of lubricant film within hip prostheses in meaning of the contribution of particular proteins. Thin film colorimetric interferometry was combined with fluorescent microscopy finding that a combination of optical methods can help to better understand the interfacial lubrication processes in hip replacements. The contact of metal femoral head against a glass disc was investigated under various operating conditions. As a test lubricant, the saline solution containing the albumin and γ-globulin in a concentration 2:1 was employed. Two different mean speeds were applied, 5.7 and 22mm/s, respectively. The measurements were carried out under pure rolling, partial negative and partial positive sliding conditions showing that kinematic conditions substantially affects the formation of protein film. Under pure rolling conditions, an increasing tendency of lubricant film independently on rolling speed was detected, while the total thickness of lubricant film can be attributed mainly to albumin. When the ball was faster than the disc (negative sliding), a very thin lubricant film was observed for lower speed with no significant effect of particular proteins. The increase in sliding speed led to the increase of film thickness mainly caused due to the presence of γ-globulin. On the contrary, when the disc was faster than the ball (positive sliding), the film formation was very complex and time dependent while both of the studied proteins have shown any qualitative change during the test, however the effect of albumin seems to be much more important. Since a very good agreement of the results was obtained, it can be concluded that the approach consisting of two optical methods can provide the fundamental information about the lubricant film formation in meaning of particular proteins while the simultaneous presence of other constituents in model synovial fluid.

Prediction of micropitting life in spur gears operating under mixed-lubrication regime using load-sharing concept

Micropitting is a surface fatigue failure of the gear tooth that occurs due to repeated loading and unloading of the tooth surface in a manner that the contact stresses exceed the surface fatigue strength of the material. The micropitting process in essence is a fatigue problem and thus it occurs during millions of cycles of operation. Micropitting can lead to problems such as wearing of the gear tooth, vibrations, noise, and misalignment. Therefore, micropitting life prediction is necessary for design, performance, and reliability improvements. The present study predicts the coefficient of friction for each point along the line of action of the pinion and gear using the load-sharing concept. The subsurface stresses are then calculated and the Von Mises stresses at each point are obtained. Then, the pitting life is predicted by employing a model which was developed by Zaretsky. The predicted pitting lives are compared with the experimental data from the literature. A parametric study on the effect of applied load, speed, and surface roughness on the micropitting life of a pair of spur gears is conducted. It is shown that increasing the applied load results in the decrease in the micropitting life of the gear. Increase in the speed, on the other hand, results in formation of a better lubricant film and therefore the micropitting life increases. An increase in the surface roughness leads to a decrease in the micropitting life.

Tribological Characteristics of NiAl Matrix Composites with 1.5 wt% Graphene at Elevated Temperatures: An Experimental and Theoretical Study

In this study, the tribological properties of NiAl matrix composites with 1.5 wt% graphene nanoplatelets (NAG) at elevated temperatures were simulated and tested. NAG exhibits excellent characteristics at 100 and 200°C due to the formation of metal oxides and graphene nanoplatelets lubrication film. Furthermore, the removed layer thickness, the stress distributions, and the high stresses of the affected layer have been estimated theoretically. At 400°C, the friction coefficient increased due to the absence of the lubrication layer. In addition, the wear rate increased due to the excessive stresses, the increased layer thickness removal, and the propagation of subsurface cracks.

Preparation and action mechanism of inclusion complex of β-cyclodextrin and sulfurized isobutylene as additives in solution of polyethylene glycol-600

The inclusion complex of β-cyclodextrin (β-CD) and sulfurized isobutylene (T321) was prepared with a co-precipitation method. The tribological properties of the complex with different concentrations were investigated by a four-ball tester in the solution of polyethylene glycol-600 (PEG-600). The experimental results suggest that the complex exhibits better anti-friction and anti-wear properties than β-CD under different load conditions. The tribo-system shows the least friction coefficient when the concentration of the complex is 0.8%. During the friction process, the complex was decomposed into various molecular fragments and the T321 molecules were released onto the friction interface to provide effective lubrication. The XPS analytical results on the worn surfaces reveal that sulfide film formed by the released T321 plays a major role, and the iron alkoxide and carbon deposition films formed by the β-CD fragments have better anti-friction effect on the sulfide film surface. The interactions of different films result in the formation of a mixed boundary lubrication film.

Tribological characteristics of monodispersed cerium borate nanospheres in biodegradable rapeseed oil lubricant

Stearic acid-capped cerium borate composite nanoparticles, abbreviated as SA/CeBO3, were prepared by hydrothermal method. The morphologies, element compositions, size distributions, crystal and chemical structures, hydrophobic characteristics, of SA/CeBO3 were characterized by scanning electron microscope, energy dispersive X-ray spectrometer, dynamic laser particle size analyzer, X-ray diffraction, and Fourier transform infrared spectrometer, respectively. The friction and wear performances of SA/CeBO3 as a lubricating additive in a rapeseed oil were evaluated on a four-ball tribo-tester. The tribochemical characteristics of the worn surfaces were investigated by X-ray photoelectron spectroscopy. The results showed that the hydrophobic SA/CeBO3 were monodispersed nanospheres with an average diameter of 8nm, and exhibited excellent dispersing stability in rapeseed oil. Meanwhile, SA/CeBO3 nanospheres were outstanding in enhancing friction-reducing and anti-wear capacities of rapeseed oil. The prominent tribological performances of SA/CeBO3 in rapeseed oil were attributed to the formation of a composite boundary lubrication film mainly composed of lubricous tribochemical species of B2O3, CeO2 and Fe2O3, and the adsorbates of SA/CeBO3 and rapeseed oil, on the tribo-surfaces.

Study on the Wettability and Tribological Behavior of Different Polymers as Bearing Materials for Cervical Prosthesis

Tribological behaviors of four polymers (conventional and cross-linked UHMWPE, conventional and glass fiber-reinforced PEEK) articulated with Ti6Al4V ball were studied under both dry sliding and 25% bovine serum lubrication. The hardness, static contact angle, surface damage topography, and wear parameter of wear scar were tested. Both cross-linked process of UHMWPE and glass fiber-reinforced treatment of PEEK improved wettability while they did not increase hardness. PEEK revealed higher surface hardness and better wettability than UHMWPE. The dominant wear mechanisms for UHMWPE were plastic deformation and fatigue wear while the failure mechanisms were severe adhesive and abrasive wear for PEEK. Cross-linked process of UHMWPE could form multi-molecular arrangement and reduce stratification, also decreasing friction coefficient and wear rate in both dry sliding and lubrication conditions. However, glass fiber-reinforced treatment of PEEK only decreased its friction coefficient and wear rate in dry condition, which was closely related to the function and wear mechanism of glass fiber. Cross-linked UHMWPE revealed the lowest friction coefficient and wear rate under lubrication condition, which was attributed to the cross-linked treatment and the formation of both protein adsorption film and lubrication film. Hence, cross-linked UHMWPE may be an alternative polymer for use as artificial cervical disc bearing material when it articulated with Ti6Al4V.

Preparation and Action Mechanism of Inclusion Complex of β-Cyclodextrin and Dibutyl Phosphite as Additives in Solution of Polyethylene Glycol-600

The inclusion complex of β-cyclodextrin (β-CD) and dibutyl phosphite (T304) was prepared by trituration. The tribological properties of the complex with different concentrations were investigated by a four-ball tester in the solution of polyethylene glycol-600 (PEG-600). The tribological test results showed that the complex exhibited better anti-friction and anti-wear properties than β-CD under different loads, and it was noted that the tribo-system showed the least friction coefficient when the concentration of the complex was 0.9%. It was proposed that the complex was decomposed into various molecular fragments and the T304 molecules were released under the friction condition. The XPS analytical results on the worn surfaces revealed that phosphide film formed by the released T304 played a major role and the iron alkoxide and carbon deposition films formed by the β-CD fragments had better anti-friction effect on the phosphide film surface. The interactions of different films resulted in the formation of a mixed boundary lubrication film.

Effects of Skewed Surface Textures on Lubricant Film Thickness and Traction

The effects of surface texture skewness (Ssk) on lubricant film thickness and traction were investigated for elastohydrodynamic lubrication (EHL) contacts. This was accomplished by analyzing three-dimensional simulated surface textures with EHL computational modeling and then verifying the results with rotating ball-on-disk traction experiments. EHL modeling was used to identify critical values for surface roughness average (Sa) and skewness (Ssk) corresponding to a stable lubricant film formation. The critical values were then used to specify test specimen textures that were fabricated for traction experiments. The results from EHL modeling and traction experiments suggest that surfaces with individual skewness Ssk values ranging from −0.4 to −2.2 at constant roughness Sa do not yield significant differences in lubricant film thickness or measured traction. Surfaces with extreme negative skewness Ssk values (approximately −4.7 or lower) may yield significant differences in contact pressure and lubricant film thickness.

Wear Behavior and Self Tribofilm Formation of Infiltration-Type TiC/FeCrWMoV Metal Ceramics Under Dry Sliding Conditions

A new type high temperature self-lubrication TiC/FeCrWMoV metal ceramic was fabricated successfully by applying an innovating technology which molten solid lubricant (60Pb40Sn-15Ag-0.5RE) was infiltrated into metal ceramic preforms with an interpenetrating network using a vacuum high pressure infiltration furnace. The friction and wear behaviors of the composites were investigated using a pin-on-disk high temperature wear testing machine at different temperature (up to 800°C). The compositions, images and structures of worn surfaces were analyzed by means of scanning electron microscope (SEM), energy dispersive X-ray analysis (EDXA) and X-ray diffraction (XRD). The self-lubrication mechanisms of the composites were discussed. The experimental results indicated that during elevated temperature sliding, the solid lubricants were squeezed out of the micropores to the frictional surfaces to form PbMoO4, PbO, SnWO4, Ag2WO4 and Ag3Sn. The formation of lubrication film containing of these oxides and of intermetallic compounds was the main reason that the composites had good self-lubrication properties at high temperature. It was considered that the micro-pores on friction surface would be the crucial factors determining the self-lubricating properties of the self-lubrication composites.

Calculation of Spur Gear Dynamic Transmission Error in Consideration of the Progressive Engagement of Compliant Profile-modified Teeth

A simple yet accurate model is developed for the dynamical simulation of profile-modified gears, considering the effects of progressive tooth engagement, stiffness, elastohydrodynamic lubricant film formation and hysteresis. The real path of contact, stiffness and elastohydrodynamic lubricant film thickness are calculated for various operating conditions and the results are input to the dynamical simulation, resulting in a prediction of the dynamic transmission error.

Lubrication of soft and hard interfaces with thermo-responsive F127 hydrogel

In this study, we have investigated the lubricating properties of an aqueous fluid prepared with polyoxamer triblock copolymer in water, namely “F127-20” (F127 at the concentration of 20% wt./vol.). In coherence with its well-known thermo-responsive rheological properties, lubricating properties of F127-20 also displayed varying lubricating properties, both in the lubricating mechanism and efficacy, as a function of temperature, speed and tribopairs. F127-20 was most effective in lubricating a soft interface (PDMS–PDMS) based on its gel-forming properties in 22.5–60 °C and feasible formation of hydrodynamic lubricating films at all speeds. More importantly, enhanced shear thinning of F127-20 and an optimum pressure opposed from PDMS–PDMS tribological contact led to a substantial reduction in viscosity of the lubricant and smooth gliding of the interface while maintaining fluidic lubricating films. At temperatures lower or higher than temperature range 22.5–60 °C, F127-20 behaved as a liquid, and boundary lubrication became the dominant lubrication mechanism.

Transient mixed non-Newtonian thermo-elastohydrodynamics of vehicle differential hypoid gears with starved partial counter-flow inlet boundary

The paper presents solutions for transient mixed thermo-elastohydrodynamics of meshing differential hypoid gears of a vehicle under low speed urban driving and high speed cruising. Realistic gear meshing conditions, such as contact load including inertial effects are used, in line with engine power torque and wheel traction. This constitutes simultaneous solution of gear pair dynamics, non-Newtonian elastohydrodynamics as well as vehicle longitudinal inertial dynamics, an approach not hitherto reported in the literature. The important link between contact tribology and vehicle gearing dynamics is highlighted. It is also shown that gear teeth pairs are subjected to a starved inlet boundary condition, represented by realistic inlet flow analysis. These conditions lead to the formation of a thin lubricant film with non-Newtonian shear and with modest boundary interactions.

Preparation, Friction, and Wear Behaviors of Cerium-Doped Anatase Nanophases in Rapeseed Oil

Composite cerium-doped anatase nanophases with an average diameter of 20 nm were prepared via sol–gel method followed by surface modification with stearic acid, abbreviated SA/Ce-TiO2. The microstructure of materials was characterized by XRD, FTIR, and SEM. Furthermore, the antiwear and friction-reducing capacities of SA/Ce-TiO2 as lubrication additives in rapeseed oil were evaluated on a four-ball tribotester. The chemical characteristics, elemental composition, and morphology of worn surfaces of steel balls were investigated by XPS and SEM. The results show that SA/Ce-TiO2 could obviously improve the antiwear and friction-reducing capacities of rapeseed oil, especially for the nanophases doped with 2.0 and 3.0% cerium in molar ratio, respectively. The excellent tribological performance of SA/Ce-TiO2 in rapeseed oil can be attributed to the formation of a complex boundary lubrication film mainly consisting of Fe2O3, TiO2, and CeO2 as well as the formation of an adsorption film of SA and base oil on the f...

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

A S-containing additive, sulfuration modified soybean oil (named as SSO), was prepared by chemical modification of soybean oil with sulfur compounds. The friction and wear performances of AZ91D magnesium alloy against GCr15 bearing steel under the lubrication of rapeseed oil formulated with SSO 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 SSO into rapeseed oil lubricant. The friction coefficients and the wear volumes of magnesium alloy decreased with increasing contents of SSO. The surface lubricated with SSO-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 SSO 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 SSO and rapeseed oil onto the lubricated surfaces and their tribochemical reactions with magnesium alloy.

The effect of transient conditions on synovial fluid protein aggregation lubrication

Little is known about the prevailing lubrication mechanisms in artificial articular joints and the way in which these mechanisms determine implant performance. The authors propose that interfacial film formation is determined by rheological changes local to the contact and is driven by aggregation of synovial fluid proteins within the contact inlet region. A direct relationship between contact film thickness and size of the protein aggregation within the inlet region has been observed.In this paper the latest experimental observations of the protein aggregation mechanism are presented for conditions which more closely mimic joint kinematics and loading. Lubricant films were measured for a series of bovine calf serum solutions for CoCrMo femoral component sliding against a glass disc. An optical interferometric apparatus was employed to study the effects of transient motion on lubricant film formation. Central film thickness was measured as a function of time for a series of transient entrainment conditions; start-up motion, steady-state and non-steady-state uni-directional sliding, and bi-directional sliding. The size of the inlet aggregations was found to be dependent upon the type of transient condition. Thick protective protein films were observed to build up within the main contact region for all uni-directional tests. In contrast the inlet aggregation was not observed for bi-directional tests. Contact film thickness and wear was found to be directly proportional to the presence of the inlet protein phase. The inlet phase and contact films were found to be fragile when disrupted by surface scratches or subjected to reversal of the sliding direction.

In situ measurements of thin films in bovine serum lubricated contacts using optical interferometry

The aim of this study is to consider the relevance of in situ measurements of bovine serum film thickness in the optical test device that could be related to the function of the artificial hip joint. It is mainly focussed on the effect of the hydrophobicity or hydrophilicity of the transparent surface and the effect of its geometry. Film thickness measurements were performed using ball-on-disc and lens-on-disc configurations of optical test device as a function of time. Chromatic interferograms were recorded with a high-speed complementary metal-oxide semiconductor digital camera and evaluated with thin film colorimetric interferometry. It was clarified that a chromium layer covering the glass disc has a hydrophobic behaviour which supports the adsorption of proteins contained in the bovine serum solution, thereby a thicker lubricating film is formed. On the contrary, the protein film formation was not observed when the disc was covered with a silica layer having a hydrophilic behaviour. In this case, a very thin lubricating film was formed only due to the hydrodynamic effect. Metal and ceramic balls have no substantial effect on lubricant film formation although their contact surfaces have relatively different wettability. It was confirmed that conformity of contacting surfaces and kinematic conditions has fundamental effect on bovine serum film formation. In the ball-on-disc configuration, the lubricant film is formed predominantly due to protein aggregations, which pass through the contact zone and increase the film thickness. In the more conformal ball-on-lens configuration, the lubricant film is formed predominantly due to hydrodynamic effect, thereby the film thickness is kept constant during measurement.