Lubricated Contact Conditions Research Articles

Investigation of lubricant viscosity and third-particle contribution to contact behavior in dry and lubricated three-body contact conditions

The generation of third particles and change in viscosity lead to the gradual degradation of the performance of the machine interface. The generation of third particles may come from wear debris or environmental particles, which form a three-body contact system at the contact interface. The viscosity of the lubricant will also change with the long-term operation of the components. This paper uses a three-body lubrication model to study the influence and interaction of lubricant viscosity change and the presence of third particles on the contact characteristics, including the real contact area, the particle contact area ratio, the solid load percentage, the film thickness, and the evolution of the lubrication regime. The results show that when the interface is in a three-body mixed lubrication regime, the dimensionless total real contact area increases with the increase in particle size and density at the same lubricant viscosity, while the trend is the opposite in dry contact and boundary lubrication interfaces. When viscosity decreases, a three-body contact interface is more prone to entering boundary lubrication than a two-body contact interface, resulting in surface damage. Regardless of surface roughness, particle size, and dry or lubricated contact conditions, the turning point of the contact area (TPCA) phenomenon is usually when the ratio of particle size to surface roughness is 0.8–1.3. Under the same ratio of particle size to surface roughness, the critical load of the TPCA phenomenon increases with the increase in third-particle size and surface roughness, but decreases with the increase in lubricant viscosity and particle density.

Effect of Shot Peening Process on Rolling Contact Fatigue Performance of EN 31 Alloy Steel

This paper aims to study the effect of shot peening on the Rolling Contact Fatigue (RCF) life of EN 31 Steel subjected to a pure rolling condition. Tests were carried out under lubricated contact conditions using a two-disc on-cylinder test rig. The orthogonal stress distributions and depth of deformation zones under static loading conditions were calculated using an elastic model by Finite Element Analysis (FEA) method. Shot peening increased the surface hardness by 20 % and imparted residual stress, which resulted in a 2-fold improvement of life. The depth of origin of the crack from the sub-surface was found to be in good agreement with the depth of maximum shear region obtained from the finite element model.

Tribological Performance of Bamboo Fabric Reinforced Epoxy Composites

AbstractBamboo fiber is one of the strongest natural fibers with high strength‐to‐weight and stiffness‐to‐weight ratios and can be used economically for manufacturing fiber‐reinforced composites. In this paper, bamboo fabric‐reinforced epoxy composite is manufactured and its tribological properties for load‐bearing applications are investigated. Sliding wear tests are conducted using a linear reciprocating tribometer and the effect of dry and lubricated contact conditions, applied load, sliding speed, temperature, and woven fabric direction on the coefficient of friction and wear rate are investigated. A scanning electron microscope is used to define the wear mechanisms at room and elevated temperatures. It is observed that the fabric orientation influences the mechanical and tribological performances of the composite material. Wear rate increases at higher loads and working temperatures; however, the effect of sliding speed is not remarkable, especially under lubricated contact conditions. The results present in this paper can be used for designing bamboo‐reinforced epoxy composites for load‐bearing applications, under different working conditions.

Coated Piston Ring Pack and Cylinder Liner Elastodynamics in Correlation to Piston Subsystem Elastohydrodynamic: Through FEA Modelling

A finite element model is developed to assess the effects of the TiSiCN thin film coating of the piston ring on the structural strength of the piston subsystem. The complex, cyclically variable forces are considered in load and boundary conditions. The model included combustion dynamics, contact kinetics, piston subsystem primary and secondary motions, and lubricated contact conditions to evaluate the applied forces. A comparative analysis is performed for coated and uncoated cases. Four different crown geometries are tried for selecting the best case of crown design for coated piston subsystem components. The analysis predicts better strength in coated cases compared to uncoated ones. The type-A crown design develops less stress, while the compression ring suffers the most due to elastic deformation and is more prone to fatigue failure.

Electrification effects on dry and lubricated sliding wear of bearing steel interfaces

In electric vehicles' powertrains, bearings can be susceptible to failure more rapidly than some of the other components mainly because of accelerated fatigue, corrosion and wear. Specifically, due to much severe operating conditions (including high speeds, torques, temperatures) of such vehicles combined with the presence of an electrical environment in which uncontrollable or stray electrical currents discharge through the contact interfaces can further exacerbate the situation and reduce their service lives by triggering much severe adhesive, abrasive and oxidative wear. In this work, we explore the effects of DC current discharges through the contact interfaces of AISI 52100 bearing steel using a novel two-electrode cell-based micro-abrasion tester under electrified dry and boundary lubricated contact conditions. A mineral base oil (as a reference), a gear oil and an automatic transmission fluid (ATF) with different dielectric strengths were tested under unelectrified and DC electrified (1.5 A) conditions. Overall, our test results suggest that passing DC current increases coefficient of friction (CoF) and wear of dry sliding contacts while it slightly reduces CoF but increases wear significantly for lubricated contacts due to accelerated oxidation by electrification. SEM, Raman spectroscopy and non-contact optical profilometry were used for analyzing the physical and chemical nature of worn surfaces and understanding of the specific wear modes and mechanisms.

Friction Properties of the Heat-Treated Electroless Ni Coatings Embedded with c-BN Nanoparticles

The nickel (Ni) coatings without and with embedded (5–7 vol. %) cubic boron nitride (c-BN) nanoparticles (10 nm in diameter) were deposited on carbon steel substrate by an electroless plating process. Coatings were tested in as-deposited and heat-treated (heating at 300 °C for 6 h) conditions. Coating structure characterisation was performed, as well as hardness and roughness measurements. Friction properties were tested in dry and in water (seawater) lubricated contact conditions, with bronze as a counter-body material. Both static and kinetic coefficients of friction were measured for two different surface texture preparations (initial and working). The first surface texture simulated the running-in condition, and the second surface texture represented the steady-state conditions. The enhancement of the abrasive and erosive wear resistance of heat-treated electroless Ni coatings with embedded c-BN nanoparticles was already proved in our previous studies. This study aims to investigate those influences on friction properties of electroless Ni coatings in different sliding conditions. The results show that the coefficients of friction did not differ too much between the coatings and that the surface roughness and presence of seawater had a much stronger influence.

Fast laser surface texturing of spherical samples to improve the frictional performance of elasto-hydrodynamic lubricated contacts

Textured surfaces offer the potential to promote friction and wear reduction by increasing the hydrodynamic pressure, fluid uptake, or acting as oil or debris reservoirs. However, texturing techniques often require additional manufacturing steps and costs, thus frequently being not economically feasible for real engineering applications. This experimental study aims at applying a fast laser texturing technique on curved surfaces for obtaining superior tribological performances. A femtosecond pulsed laser (Ti:Sapphire) and direct laser interference patterning (with a solid-state Nd:YAG laser) were used for manufacturing dimple and groove patterns on curved steel surfaces (ball samples). Tribological tests were carried out under elasto-hydrodynamic lubricated contact conditions varying slide-roll ratio using a ball-on-disk configuration. Furthermore, a specific interferometry technique for rough surfaces was used to measure the film thickness of smooth and textured surfaces. Smooth steel samples were used to obtain data for the reference surface. The results showed that dimples promoted friction reduction (up to 20%) compared to the reference smooth specimens, whereas grooves generally caused less beneficial or detrimental effects. In addition, dimples promoted the formation of full film lubrication conditions at lower speeds. This study demonstrates how fast texturing techniques could potentially be used for improving the tribological performance of bearings as well as other mechanical components utilised in several engineering applications.

Vibration modeling of wind turbine shaft as rigid shaft supported by EHL contact ball bearings with overhung disc system

An investigation into the dynamic behavior of Elastohydrodynamic Lubricated (EHL) angular contact ball bearings employed in wind turbines is given in this paper considering non-linear contact characteristics due to combined dynamic loads, different rotation speeds, hydrodynamic effects and load-deformation relation. In order to investigate the dynamic behavior of shaft-bearing system, a rigid shaft with overhung disk which is supported by a couple of EHL angular contact ball bearings is modeled as a 5 DoF nonlinear system.The contact between the rolling element and running surfaces in the bearing is modeled as an equivalent spring and damper. Hertz contact theory and EHL theory are used to obtain EHL contact characteristics and contact forces. Simplified rolling contact theory is used and pure rolling is assumed in the kinematic analysis. The contact angle and mutual approach of rolling elements are obtained analytically. The contact forces and film thickness are calculated iteratively for the quasi-static case using the Newton–Raphson method. In order to express the dynamic behavior of the system, nonlinear equations of motion are solved by a numerical method using Runge Kutta algorithm. The results indicate that the stiffness in EHL contact increases with the increase in load, the damping decreases with the increase in speed, the vibration behavior of the system is more complicated than the dry contact as the natural frequencies are now dependent on system running speed. It has also been observed that there is a significant decrease in vibration amplitude of the system under lubricated contact conditions as well as the frequencies.

An experimental characterization of the friction coefficient of a wind turbine gearbox lubricant

AbstractIn this study, a method to predict the contact friction coefficients at the rolling‐sliding contacts of isotropic superfinished and axially ground gear and bearing surfaces in wind turbine gearboxes is proposed. A two‐disc test rig was used to measure friction coefficient values within a slide‐to‐roll ratio range of −0.8 to 0.8, rolling velocities of 2 to 12 m/s, at oil temperatures of 50°C, 70°C, and 100°C and Hertzian contact pressures of 1.2, 1.6, and 2.0 GPa. A polyalphaolefin (PAO) International Standards Organization (ISO) viscosity grade (VG) 320 was the lubricant. Data from 90 individual friction coefficient tests were combined to develop two closed‐form friction coefficient formulae. A multivariable linear regression analysis was used to derive the regressed formulae. Using the developed regressed formulae, the predicted friction values follow the trend of measured values at all of the operating conditions. The results show that the regressed formulae are close to experimental friction coefficient values. The friction coefficient predictions confirm that the regressed formulae are capable of simulating contact friction values for the lubricated contact conditions considered.

THE EFFECT OF OIL POCKET ARRAYS ON FRICTION COEFFICIENT

Experiments were carried out using pin-on-disc tester in conformal lubricated contact conditions for different normal loads. Surface texturing was done using abrasive jet machining with the application of laser cut mask. In order to eliminate the effect of different input variables in all the experiments, pit-area ratio and sizes of oil pockets were very similar. Five types of oil pocket arrays were tested: radial, concentric, spiral, of a square arrangement, and of a random arrangement for 5% and 17% of pit-area ratio. Different dimple diameters caused various oil pocket densities. The experiments were also made for untextured polished discs. During tests, the friction force was monitored as a function of time. Before and after tests, disc surface topography was measured using a white light interferometer Talysurf CCI Lite. The beneficial effect of surface texturing was obtained for spiral arrays of dimples on disc surface. The presence of a radial array of oil pockets resulted in the worst tribological properties of tribological assemblies containing textured discs.

Transient softening in lubricated sliding contacts due to strain path changes

The present work investigates microstructure evolution in pure aluminium under lubricated contact conditions. The initial wear scars were obtained in unidirectional sliding up to a pre-defined number of cycles N. After this pre-imparted severe plastic deformation, the sliding direction was reversed and the resulting microstructure was characterized using hardness measurements and microscopy techniques. The results show that immediately after reversing the strain path, the material significantly softens, as known in uniaxial tension-compression experiments. After the transient softening stage, the new microstructure becomes established and the polycrystal work-hardens again. The results obtained proof the presence of strain path change effects in metals undergoing sliding contact and highlight the importance to take these effects into account in component design in order to avoid undesired material softening.

Tribo-mechanical characterization of reinforced epoxy resin under dry and lubricated contact conditions

The aim of the present work is to investigate the influence of the reinforcing material and architecture on the voids content, mechanical properties and tribological behavior of fiber reinforced epoxy composite laminates manufactured by VARTM under different processing conditions. Two different textile architectures, namely unidirectional non-crimp fabrics (UD) and 0/90 plain wave (PW), were considered, reinforcing an EPIKOTE RIMR 135 epoxy matrix with glass (GF) as well as carbon (CF) continuous fibers. Optical observations revealed an unexpected trend relatively to the intra- and inter-bundle voids concentration with respect to the impregnation velocity, especially using UD-CF and UD-GF reinforcements and low impregnation rate. Tensile and three points bending tests highlighted the dominant role of fiber material and architecture on mechanical properties, whereas the presence of voids played a minor role with respect to the analyzed features. Tribological outcomes evidenced a reduction of the friction coefficient (μ) when the resin is reinforced by carbon or glass fibers. The lowest values were detected when the sliding direction of the counterbody is oriented parallel to the fiber direction for UD samples. Further reduction of μ, for both UD and PW specimens, was obtained by interposing a lubricant at the interface.

Rapid surface hardening and enhanced tribological performance of 4140 steel by friction stir processing

Tribological performance of steel materials can be substantially enhanced by various thermal surface hardening processes. For relatively low-carbon steel alloys, case carburization is often used to improve surface performance and durability. If the carbon content of steel is high enough (>0.4%), thermal treatments such as induction, flame, laser, etc. can produce adequate surface hardening without the need for surface compositional change. This paper presents an experimental study of the use of friction stir processing (FSP) as a means to hardened surface layer in AISI 4140 steel. The impacts of this surface hardening process on the friction and wear performance were evaluated under both dry and lubricated contact conditions in reciprocating sliding. FSP produced the same level of hardening and superior tribological performance when compared to conventional thermal treatment, using only 10% of the energy and without the need for quenching treatments. With FSP surface hardness of about 7.8GPa (62 Rc) was achieved while water quenching conventional heat treatment produced about 7.5GPa (61 Rc) hardness. Microstructural analysis showed that both FSP and conventional heat treatment produced martensite. Although the friction behavior for FSP treated surfaces and the conventional heat treatment were about the same, the wear in FSP processed surfaces was reduced by almost 2× that of conventional heat treated surfaces. The superior performance is attributed to the observed grain refinement accompanying the FSP treatment in addition to the formation of martensite. As it relates to tribological performance, this study shows FSP to be an effective, highly energy efficient, and environmental friendly (green) alternative to conventional heat treatment for steel.

Fast thermo-elasto-hydrodynamic modeling approach for mixed lubricated journal bearings in internal combustion engines

Journal bearings are key components in internal combustion engines. Their reliability, durability, and economy are of highest importance. Especially, the accurate prediction of friction loss power and wear are essential for the development of an engine. For an appropriate representation of the hydrodynamic load-carrying capacity and also the friction behavior, both the dynamics of the contacting components, the shape of the contacting component surfaces, the amount of the available lubricant, and the properties of the lubricant itself are of importance. A Reynolds-averaged equation with laminar flow conditions in combination with an asperity contact model is a typical modeling approach for that purpose. The lubricant properties are in particular influenced by its thermal conditions on one hand. On the other hand, the thermal conditions are influenced by the mixed lubricated contact conditions as well. These interactions require a coupled modeling approach, which combines the component flexibility and its interaction with load-carrying capacity as well as the thermal behavior of the lubricant and the component surfaces. In this work, a thermo-elasto-hydrodynamic contact model is presented, which computes the oil film temperature using a 2D energy equation. The 2D equation is derived from the equivalent 3D energy equation by integration over the clearance gap height. Besides component material properties such as specific heat capacity, density, heat conductivity for lubricant and structures, also heat transfer through mixed lubricated regimes and partly filled clearance gaps, as implied in cavitation regions, are being considered. The presented method is applied for a typical engineering task of a sensitivity analysis for oils with different viscosity index (VI) improvers in a main bearing of a four-cylinder inline diesel engine. The influence of the oil film temperature on the oil film viscosity and therefore on the load-carrying capacity is shown. Furthermore, the simplified 2D approach is compared with a 3D approach both in terms of obtained result data and in terms of elapsed calculation times. The presented results show similar accuracy of the 2D approach with significantly reduced simulation time compared to the equivalent 3D case.

Characterization of friction behaviour of AZ80 and ZE10 magnesium alloys under lubricated contact condition by strip draw and bend test

In this paper, a sheet metal forming simulator (SMFS) is used for evaluation of the frictional behaviour of AZ80 and ZE10 magnesium alloys under lubricated contact conditions. The results showed that the friction coefficient increases by increasing the contact pressure and decreasing the sliding velocity. A friction model is further developed for lubricated contact taking into account the surface roughness characteristics and the viscosity of lubricant. The proposed model showed very good agreement with the results of experiments. Finite element (FE) simulations were also carried out to investigate the effect of key process parameters on the results of SMFS. Based on the results of the FE model, the coefficient of friction increases by increasing the bending angle and pin diameter; however, these increases are not significant.

A model for single asperity perturbation on lubricated sliding contact with DLC-coated solids

In lubricated sliding contact systems with DLC coated solids, recent studies have shown that DLC coatings are highly sensitive to asperities breaking through the lubricant film within the contact area. Those asperities produce damages similar to those obtained from scratch tests, from where coating delamination can initiate and propagate. To better understand the link between scratches and coating delamination, endurance tests can then be performed on coating with controlled initial scratches. To ensure the scratches’ representativity with respect to the actual asperities’ contact condition, one has to estimate the load transmitted by such asperities as well as the induced perturbation on surface and subsurface stresses. In this paper, a simplified numerical model corresponding to such problems is presented. It is consistent with elasto-hydrodynamic (EHD) lubrication approximations and can be used on coated or uncoated systems. As a simplified model, it can be run quickly on multiple configurations, enabling the creation of representative scratch maps for the given lubricated contact conditions.

Tribological behaviours of textured surfaces under conformal and non-conformal starved lubricated contact conditions

Wear tests were carried out using a pin-on-disc tester. The sliding friction experiments were made using textured and untextured discs of 50 HRC hardness, under starved lubricated conditions in unidirectional sliding. Two batches of tests were done. The special construction was made to obtain conformal contact in the first type of tests. In the second test type in non-conformal initial point contact condition, a steel disc was put in contact with a ball from bearing steel of 3.175 mm radius. Two kinds of textured discs were tested; the oil pockets were positioned in spiral and radial arrays the number of oil pockets within friction track also varied. It was found that in conformal contact conditions the presence of dimples resulted in considerable decrease of the friction force compared to untextured samples. Surface texturing caused transition from point to conformal contact during test, which resulted in decrease of friction force but increase of wear of balls. The tribological behaviour of assembly with spiral dimples layout was better than that with radial array of oil pockets.

Frictional Properties of AZ80 and ZE10 Magnesium Alloys Under Dry and Lubricated Contact Conditions

The frictional properties of two types of magnesium alloys, i.e. AZ80 and ZE10 were investigated. A purpose-developed sheet metal forming simulator was used to conduct the experiments under constant plastic deformation. Both lubricated and dry sliding contact conditions were simulated and the effect of key process parameters such as contact pressure and sliding velocity on the frictional properties of these alloys were investigated. Due to the different sliding velocities, the contact pressure rose during each experiment which enables the measurement of the coefficient of friction for a wide range of contact pressures. The results showed an increase in the friction coefficients of both alloys with increasing contact pressure. Furthermore, a decrease of the friction coefficient was observed for higher sliding velocities.

Friction and wear behavior of shot peened surfaces of 36CrNiMo4 and 36NiCrMo16 alloyed steels under dry and lubricated contact conditions

Tribological behavior of two alloyed steels subjected to shot peening was investigated, under dry and lubricated sliding. 36CrNiMo4 and 36NiCrMo16 alloy steels were investigated at block-on-disc sliding testing machine, and three different applied loads (10N, 30N, 50N) and sliding speeds (0.25m/s, 0.5m/s, 1m/s) were observed. Shot peened surfaces were compared to ground ones, from aspects of their micro hardness and friction and wear properties. Microhardness tests were realized for both steel materials, ground and shot peened samples.Shot peening increased hardness of the near surface layers with approximately 10% increase near the top surface, in comparison to ground surfaces. Both friction and wear properties of the investigated steels showed improvement after shot peening, especially under high loads and speeds for dry sliding and low speeds and high loads for lubricated sliding. For all test conditions, 36NiCrMo16 steel showed better wear resistance and frictional characteristics than 36CrNiMo4 steel. Shot peening produced lowering of the friction coefficient, as well as wear rate, in comparison with ground surfaces, in both dry and lubricated sliding and for both materials. Wear tracks exhibited severe abrasive wear, accompanied with adhesive wear, in all test conditions, but in case of lubricated sliding of shot peened surfaces, at the lowest speed and the highest load, wear tracks showed regions with mild wear also. Important result was significantly lower number of cracks, with smaller sizes within wear tracks of shot peened surfaces, in comparison with ground surfaces.

Surface Texturing with Laser Micro Cladding to Improve Tribological Properties

In this work, laser microcadding with continuous wave laser is proposed as a productive, flexible and simple method for surface modification of 3D metallic components, to generate microfeatures which influence the surface interaction of tribological pairs and surfaces under lubricated contact conditions. In this paper, texturing additive process by means of laser micro-cladding [LµC] is presented. LµC texturing has been developed for generation of functional and tribological surfaces on metallic components. On one hand, cladding allows the engineering of surface properties, as a wide range of clad materials can be chosen for increasing properties and performance of the components. On the other hand, microtexturing adds the possibility of generating controlled geometrical features in the micron scale, which influence the interactions with the lubrication fluids and the wearing mechanisms. The proposed LµC technique combines these two advantages in a single step for a novel surface treatment.