Showing papers in "Tribology Online in 2015"

Experimental study of starvation and flow behavior in grease-lubricated EHD contact

Abstract: This paper describes an experimental study of starvation and flow behavior in grease-lubricated elastohydrodynamic contact. Rolling tests were conducted with four grease samples with different thickeners and base oils in a conventional ball-on-disk test rig. The EHL central film thickness was determined with SLIM, the Spacer Layer Imaging Method, and grease flow around the conjunction and the flow pattern on the track of the disk specimen were observed with CCD cameras. The grease track included some thickener deposited on the track and a corrugated fingerlike flow pattern outward from the center of the track. It was found that the flow pattern varied with grease type and test conditions, and that the average interval between fingers decreased with the entrainment speed. At higher speeds, inlet starvation occurred and the finger pattern was deformed and gradually ruptured. The speed of the finger-loss was higher than the speed at which starvation started. It was also found that the starvation speed was greater with greases having lower apparent viscosity at lower shear rate and higher apparent viscosity at high shear rate. This implied that the replenishment and entrainment of the greases depended on the viscosity characteristics of the greases.

Friction and Wear Characteristics of PEEK and PEEK Composites in Water Lubricated Slow Speed Sliding

Abstract: Friction and wear performances of natural grade poly-ether-ether ketone (PEEK) and three PEEK composites e.g. 30 mass% carbon fibre reinforced (PC-1), 30 mass% glass fibre reinforced (PC-2), and 10 mass% each PTFE/graphite/carbon fibre reinforced (PC-3) were studied under water lubricated, slow speed sliding conditions. Tribological tests were carried out on Pin-on-Disc setup and 17-4 PH stainless steel was used as counterface material. Test conditions were; 3, 6, 12 MPa contact pressures (P) and 0.05, 0.005 m/s sliding speeds (V). Natural grade and glass-fibres reinforced composite (PC-2) exhibited poor friction and wear characteristics at all PV conditions. Composite reinforced with high percentage of carbon (PC-1) showed good tribological performances at all PV conditions. However PTFE/graphite/carbon fibre reinforced grade (PC-3) showed improved performance at very low sliding speed (0.005 m/s). Increased sliding speed resulted into dramatically increase in wear rate of PC-3, may be due to non-retention of transfer film of PTFE and graphite on the counterface material. Good tribological behavior of PC-1 attributed to addition of 30 mass% carbon fibres, which was effective to protect the matrix material from shear and abrasion. Wear mechanisms were discussed based on scanning electron microscopy (SEM) of the worn surfaces. These results were also compared with conventional material pair.

Characterization of Tribofilm with the Remaining Lubricating Oil by Raman Spectroscopy

Abstract: As a first step to develop a method of in situ analysis of tribological behavior under oil lubrication, surfaces after the tribo-test with the remaining lubricating oil were analyzed with Raman spectroscopy. A ball-on-disk type tribo-test using a S45C disk and a SUJ2 ball was performed, and then its friction track of the disk surface was analyzed by Raman spectroscopy. Polysulfide and mineral oil were used as the lubricating oil without dilution. In addition to each lubricant peak, iron disulfide (FeS2) tribofilm peaks were detected on the friction track in the case of polysulfide, whereas the magnetite (Fe3O4) peak was detected on the friction track in the case of mineral oil. FeS2 intensity increased overtime in the case of the test at 200°C. In this way, tribofilm and oxide were detected as the remaining lubricating oil by Raman spectroscopy, which therefore suggests that Raman spectroscopy is an effective analysis method to investigate the formation process of tribofilm and oxide under oil lubrication.

Study on Tribology Performance of Diesel Engine Oil Using SRV4 Tribometer

Abstract: This paper investigates the friction and wear properties of the fresh and soot-contaminated API CF-4 and CI-4 diesel engine oil using the SRV4 tribometer. It was found that the comprehensive wear properties of CI-4 oil was better than those of the CF-4 oil. Tests of the fresh and contaminated CF-4 oil in combination with two types of rings of different surface treatment showed discriminations, and these discriminations also reflected on the CF-4 oil tests with test rings of different surface coatings.

The Effect of Temperature on the Tribological Properties of Palm Kernel Activated Carbon-Epoxy Composite

Abstract: The aim of this study was to investigate the effect of temperature on the tribological properties of Palm Kernel Activated Carbon-Epoxy (PKAC-E) composite. All specimens were formed into 10mm diameter pins of 30mm length, using a hot compaction technique. Tribological testing was carried out using a pin-on-disc tribometer in dry sliding conditions by applying temperatures in the range of 27oC to 150oC, at constant sliding speed, applied load, and sliding distance. The results showed that both coefficients of friction (COF) and wear rate of the composite increased with operating temperatures. Abrasive wear and crack formation that would induce delamination wear were identified as the predominant wear mechanisms.

Flexible Control and Coupling of Adhesion and Friction of Gecko Setal Array During Sliding

Abstract: Gecko has remarkable ability to control its adhesion and friction during running to realize a swift climbing or running on vertical walls or upside down ceilings. This ability has received considerable interest of researchers. During the recent decade, significant progress has been made in understanding the mechanism and the biomimetic fabrication of gecko-inspired dry adhesives. While people generally desire a strong and reversible adhesion property, this article described the progress in the investigation of the properties of gecko hierarchical structures, and the corresponding theoretical understandings, and then presented our recent progresses on the flexible controlled adhesion and friction of gecko surface and biomimetic surfaces and a demonstration of a hybrid clamp to ascertain the mechanical property of control principles is also involved. Gecko-inspired adhesives with flexibly controlled adhesion and friction performances have been developed. Macroscopic sliding of those adhesive surfaces with anisotropic hierarchical microscopic structures in a desired direction could be carried out and engage or disengage a flexible control of strong adhesion and friction, and acquire an “intelligent” adhesion.

Friction Modification by Shifting of Phonon Energy Dissipation in Solid Atoms

Abstract: Toyota Central R&D Labs., Inc. 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan Correlated Electronics Group, Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba Central 4, 1-1-1 Higashi Tsukuba, Ibaraki 305-8562, Japan Diamond Research Group, Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST) 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Corresponding author: fine-controller@mosk.tytlabs.co.jp

Formation Processes of Metal-Rich Tribofilm on the Counterface During Sliding Against Metal/Diamondlike-Carbon Nanocomposite Coatings

Abstract: 1) Department of Mechanical Engineering, Ube National College of Technology, 755-8555 Ube, Japan. 2) College of Engineering, Nihon University, 963-8642 Koriyama, Japan. 3) Laboratoire de Tribologie et Dynamique des Systèmes, Ecole Centrale de Lyon, 69134 Ecully, France. 4) Graduate School of Engineering, Tohoku University, 980-8579 Sendai, Japan. 5) Center for Interdisciplinary Research, Tohoku University, 980-8577 Sendai, Japan. Corresponding author for mi-goto@ube-k.ac.jp

Friction and wear behavior of polyamide 66 composites filled with rice bran ceramics under a wide range of Pv values

Abstract: Hard particulate fillers are used to improve the mechanical and tribological properties of thermoplastic resins. In this study, we investigated the fundamental tribological behavior of polyamide 66 (PA66) resin composites containing rice bran ceramics (RBC) particles at a wide range of pressure-velocity (Pv) values under dry conditions. Irrespective of Pv value, PA66/RBC composite exhibited relatively low friction coefficients (0.4), whereas the friction coefficients for pure PA66 increased. In addition, the PA66/RBC composite better prevented an increase in the surface temperature compared with pure PA66 at high Pv values. To distinguish the effect of the flowability of the resin, surface temperatures normalized by the glass transition temperature were defined. The specific wear rates of pure PA66 changed with the normalized surface temperature because the wear mode transited. Low specific wear rates (< 0.3 × 10 mm/N) were obtained for the PA66/RBC composite without the transition of the wear mode against the normalized surface temperature. These wear rates were reduced by 55-86% compared with those of pure PA66.

Effects of Hydrogen on Frictional Properties of DLC Films

Abstract: The tribological behavior of diamond-like carbon films (DLC) is strongly dependent on the hydrogen content, sp/sp ratio, and sliding environment. Some hydrogenated amorphous carbon films (a-C:H) exhibit superlow friction in hydrogen conditions. However, previous works have not clarified the dominant factors of the superlow friction phenomena of DLC films. In this research, we focused on the effects of hydrogen derived from the surrounding atmosphere and the hydrogen within the DLC films on superlow friction phenomena. To investigate these effects, friction tests were conducted on three DLC films having different hydrogen contents (0, and 18, 30 at%) in the air and in low-pressure-hydrogen conditions at various hydrogen pressures. After the friction tests, the wear tracks were examined by confocal laser scanning microscopy, Raman spectroscopy, elastic recoil detection (ERDA) analysis, and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The hydrogen derived from the surrounding atmosphere and the formation of the hydrogen-rich tribofilm were key factors for the superlow friction phenomena.

Main Chain Effects in Tetraol-Functionalized Perfluoropolyethers

Abstract: The interfacial properties of perfluoropolyether (PFPE) films having the [(CF2)nO] main chain with n = 1.5, 3 and 4, were systematically compared on carbon surfaces. The monolayer thickness decreases with increasing n, indicating that stiffer chains (larger n) lie flatter on the surface. The surface energy-derived disjoining pressure also increased with increasing n. Spin-stand tribological experiments indicated that clearance (slider-disk spacing) increased and PFPE lubricant pick-up decreased with increasing disjoining pressure and main chain stiffness. Slider wear rate increased with decreasing PFPE coverage and increasing main chain stiffness. Structure-property correlations are developed and discussed.

Static Analysis of Finite Hydrodynamic Journal Bearing in Turbulent Regime with Non-Newtonian Lubricant

Abstract: The aim of the present paper is to investigate and predict the static performance of finite journal bearing in turbulent flow condition considering non-Newtonian lubrication. The Navier-Stokes equation has been modified considering turbulent as well as non-Newtonian lubrication and is solved for steady state parameters. Ng and Pan’s linear turbulence model is utilised in the analysis. The momentum and continuity equations in cylindrical coordinates representing the flow field in the clearance space of a finite circular journal bearing using a Newtonian lubricant were solved by the finite element method. The non-Newtonian lubrication effect is introduced by modifying the viscosity term using the cubic shear stress law in each iteration. In this paper, the effect of turbulence and non-Newtonian lubrication has been determined by solving the modified Navier-Stokes equations obtained by the application of linear theory proposed by Ng and Pan and using the cubic shear stress law model simultaneously. The modified Navier-Stokes equations, in cylindrical coordinates, have been solved by finite element method using Galerkin's technique and a suitable iteration procedure. Steady-state performance characteristics of a finite circular journal bearing have been analysed in terms of Sommerfeld number, load carrying capacity, friction coefficient variable, temperature rise parameter and total flow at various eccentricities for different Reynolds numbers up to 13300 and various values of non-linear factor of cubic shear stress law of non-Newtonian model. Computed results have been compared with the published results obtained by linearized theory of Ng and Pan and cubic shear stress law of non-Newtonian lubrication. The results obtained revealed better performance in turbulent regime as compared to laminar condition for cubic shear stress non-Newtonian model.

Effects of Microscale Texture on the Tribological Behavior of Paper-Based Friction Materials for a Wet Clutch

Abstract: Paper-based friction materials used for wet clutches in automatic transmissions require various tribological functions such as a high and stable friction coefficient, good μ-V (friction coefficient-sliding velocity) characteristics, heat resistance, and wear resistance. In this study, microscale texture was formed on the surface of a paper-based friction material using a picosecond pulse laser, and the friction characteristics were investigated using the SAE#2 test system. Stripe-shaped microtexture (microgrooves) with several patterns of depth and pitch were formed on the friction material surface and controlled by the laser processing conditions. The dynamic friction coefficient was particularly improved by the microgrooves under low temperature and low surface pressure conditions. The coefficient changed according to the depth and pitch of the microgrooves, which is related to the volume ratio (microgroove volume per unit volume of paper-based friction material). These results indicate that the dynamic friction coefficient for the wet clutch can be controlled by changing the groove volume ratio according to the laser processing conditions. A new paper-based friction material with a high static friction coefficient (μs) and good μ-V characteristics could be developed by control of the dynamic friction coefficient without changing the pore characteristics of the paper.

Experimental and Theoretical Verification of Impact Response on Air Thrust Bearing with Topological Optimized Groove

Abstract: This paper describes experimental and theoretical verification in impact response of optimized grooved bearing designed by topological optimization. Firstly, in order to measure the impact response characteristics accurately temperature compensation method for air film thickness of bearing is proposed and confirmed by experiment of bearing reaction force with air film thickness. Then, in this study, impact experiments were conducted with the optimized bearing and spiral grooved bearing and compared with theories of linear and non-linear analyses. As a result, the optimized bearing showed the high impact resistance compared with spiral grooved bearing. In addition, it was confirmed that the results of non-linear analyses are good agreement with those of experiment. Finally, based on above results, the impact response calculations of both bearings by non-liner theory under more high impact force condition were conducted. As a result, it is found that the response of spiral grooved bearing generates the self excited vibration. On the other hand, the response of optimized bearing showed damping waveform with time even under the high impact force conditions.

Tribological Properties of Copper Molybdate Powder Solid Lubricants under High Temperature Conditions

Abstract: In our previous research, it was suggested that a sort of copper molybdate Cu3Mo2O9 generated on the friction track of aluminum bronze coated with MoO3 powder reduced friction under high temperature conditions. In order to study the lubricity of copper molybdate as high temperature lubricant, the lubrication properties of two kinds of copper molybdate powders, CuMoO4 and Cu3Mo2O9, were studied by comparing with the lubricity of CuO and MoO3 powders. These powders were supplied to the sliding surface of the stainless steel specimens as solid lubricant and the friction test was conducted at various ambient temperatures ranging from room temperature to 700°C. Cu3Mo2O9 and CuMoO4 powders were synthesized by heating the mixture of CuO and MoO3 powders in their own specific heating conditions and identified by XRD. Both of Cu3Mo2O9 and CuMoO4 powders showed lower friction coefficient and smaller wear amount of specimen with increasing ambient temperature. On the other hand, CuO and MoO3 powders showed poor lubricating ability and high wear amount of specimens comparing with two kinds of copper molybdate powders. It was suggested that the enhanced maintaining abilities of copper molybdate on the stainless steel substrate at high temperature and generation of unoxidized copper under high temperature conditions could enhance lubricating abilities.

Influences of Journal Rotating Direction on Cryogenic Hybrid Journal Bearings

Abstract: In the present study, two types of nine-recess hybrid journal bearings (HJBs) with different recess geometries, i.e., the Young Leaf Mark and the square, were operated at a rotational speed of up to 67,000 rpm using the cryogenic journal bearing test apparatus at JAXA to clarify influences of the journal rotating direction on the cryogenic HJB. Liquid nitrogen was supplied to the test bearings with a pressure of up to 4 MPa. The test bearings were 60 mm in diameter and 25 mm in width, with a 0.055 mm clearance and a recess area ratio of 0.186. Based on the experimental results the dependence of the rotor vibration on the rotational speed was confirmed to be affected by the journal rotating direction. The test HJBs with the positive sweepback angle of the recess leading edge showed the smallest rotor vibration in the high-rotational-speed operation where the hydrodynamic effect was predominant.

Synthesis and Characterization of Nano-Particles Based Magnetorheological Fluids for Brake

Abstract: Magnetorheological (MR) fluids, to be used as brake friction materials, must have high heat transfer rate to dissipate the heat generated during the braking action. The aim of this manuscript is to synthesize MR fluids with nano-silver and nano-copper particles to increase the heat transfer rate and characterize the demagnetizing effect of those particles on the shear stress of MR fluids. Five different MR fluids, containing a different percentage of silver and copper nano particles, were synthesized. Shear stresses of all five MR fluids were measured using magnetorheometer and the results have been plotted. A flywheel based MR brake experimental setup was developed to analyze the performance of synthesized MR fluids. “T” type thermocouples were used to measure the temperature distribution of the fabricated MR brake. The results of the temperature distribution of brakes containing five synthesized MR fluids have been presented and compared.

Thin Surface Layers of Iron-Based Alloys Deposited by TIG Hardfacing

Abstract: Hardfacing layers developed by tungsten inert gas (TIG) surface melting on commercial purity titanium placing with a mixture of Fe, C and Si powders under two different traverse speeds (1 mm/s and 2 mm/s) and energy input of 1080 J/mm in an argon gas environment were investigated in terms of surface condition, microstructure, hardness and wear of the processed tracks. The surface appearance of treated layers was found to be free from any obvious defect. The TIG hardfacing layer produced dendritic structure due to dissolution of preplaced powder in the Ti melt. A maximum microhardness value of 630 HV 0.5 kgf was found on the surface layer processed with lower speed which was 2.5 to 3.5 times higher than the base material. Ball-on-plate wear tests exhibited better performance of the hardfacing layer than the untreated CP-Ti which is attributed to the presence of carbides and silicides in the Ti melt.

Friction and Wear Behaviour of Graphite Filled Polymer Composites in Hydrogen Environment

Abstract: High performance polymer composites have been intensively investigated for tribological applications in air, but rarely in hydrogen environment. Author’s previous benchmark of composites in liquid hydrogen (LH2) showed that graphite filled polymers have beneficial friction behaviour in this extreme condition. Therefore, further investigations have been undertaken in hydrogen environment. This paper presents first results obtained with polyimide composites filled with different types (natural, synthetic) and amounts of graphite in air, vacuum and hydrogen environments. A particular attention is taken to the influence of hydrogen on graphite as well as on the polymer matrix.