bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic Contact Between a Rigid Indenter and a Kelvin-Voigt Half-Space
Authors: Ciornei Florina Carmen , Diaconescu Emanuel
Published: November 2008

Following the theory developed by Ting, analytical expression of contact radius for an advancing contact is derived. For receding phase, contact radius is numerically obtained. Contact pressures for increasing and decreasing contacts are derived as a sum of two terms: the first is a Hertz-type pressure and the second term gives infinite pressure at the edge of the contact. Theoretical results are compared with the experimental data.
The Role of Electron Emission in Tribochemistry: A Review
Authors: Furey Michael J. , Kajdas Czeslaw , Molina Gustavo J. , Vick Brian
Published: November 2008

This paper discusses the phenomena of electron emission from sliding contacts with a focus on the possible role of charged-particles in tribochemical processes. A review is presented of (i) experimental work on triboemission of low-energy electrons from scratching of ceramics at low sliding speeds, (ii) the importance of emitted electrons for initiating tribochemical reactions, and (iii) research on the conditions for thermionic electron emission from metals during sliding contact, Specific connections are made between the Negative-Ion Radical Action Mechanism (NIRAM) and recent experimental results in the fields of triboemission and tribochemistry, including tribopolymerization.
On the Finite Volume Upwind Solution of the Reynolds Equation for a Compressible Fluid
Authors: Arghir Mihai , Le Lez Se´bastien , Frene Jean
Published: November 2008

The present work deals with the numerical solution of the compressible Reynolds equation in the frame of the finite volumes method of discretization. The first original aspect of the numerical solution is a new upwind procedure borrowed from aerodynamics, namely from the transonic potential flow solutions. The second original aspect of the numerical solution is the finite volume discretization for unstructured grids. The method was developed in an effort to obtain a tool for analyzing hybrid bearings so its first validations consists of comparisons between calculated static and dynamic characteristics and values found in the literature.
Relationship Between Wear Performance and Solid Lubricants in Sintered Friction Materials
Authors: Miyauchi T. , Day A. J. , Wright C. S.
Published: November 2008

Cast iron (FC and NCM) and forged steel (ST) discs, and four types of sintered friction material pads have been employed in the braking systems of Japanese high-speed trains. These sintered friction materials have complex microstructures. This paper reports result of an investigation to establish correlations between friction, wear and temperature characateristics of the disc–pad friction pairs and pad microstructure. A scale friction rig was used to examine friction, wear and temperature performance of the selected brake friction pairs. Area fraction of lubricants in the sintered friction materials was, depending on formulation, in the range 30–47%. It was found that increasing the lubricant area fraction up to 40%, was effective in reducing pad wear. Above 40% lubricant, wear rate either remained constant or increased slightly to decrease pad wear. For FC discs wear was a minimum at 40% lubricant whilst wear drecreased with increasing overall lubricant content for NCM discs and wear was unaffected by variation in lubricant content for the ST discs. The harder ST forged steel discs exhibited less wear than the softer FC and NCM cast irons.
A Geometrically Defined All-Diamond Pad Conditioner
Authors: Andersson Joakim , Forsberg Markus , Hollman Patrik , Jacobson Staffan
Published: November 2008

This work presents a geometrically defined pad conditioner fabricated by anisotropic etching of silicon to produce a well-defined mold and subsequently CVD diamond layer is deposited on this mold. Finally, the silicon wafer is etched away, exhibiting the all-diamond tool surface. The new type of pad conditioner offers very sharp pyramidal tips, all equal in size. The excellent sharpness leads to high roughening capability while keeping the pad removal low. The precise tip position and height results in a unique uniformity of scratches produced, hence offering a new level of process control.
Effect of Processing Parameters on the Wear Behaviour of Sintered Iron and Steam Oxidized Sintered Iron in Reciprocating Sliding Wear Tests
Authors: Bozzi A. C. , De Mello J. D. B.
Published: November 2008

In spite of the extensive use of sintered iron-based components in sliding situations, there are very few studies about their wear behaviour. In this paper, the effect of processing parameters on the wear behaviour of sintered iron and steam oxidized sintered iron were analyzed. Three levels of density were used, ranging from 7.10 to 6.55 g/cm3. In addition, two different steam oxidation processes were used with the objective of obtaining different types of surface oxides. Reciprocating sliding wear tests in a ball on flat geometry were performed under a constant stroke and frequency. Three levels of normal load were used. Wear was evaluated in terms of the volume loss by using data of surface topography provided by a 3D laser interferometer. The wear mechanisms were analyzed by scanning electron microscopy. The wear behaviour of the specimens was compared and analyzed in terms of the wear volume and wear mechanisms. The results show that wear behaviour is influenced mainly by the load level and steam oxidation. Steam oxidation greatly decreased the wear volume of these materials under low and medium loads.
Shear Properties of Polymer Films Within Confined Contacts
Authors: Gacoin E. , Fretigny C. , Chateauminois A.
Published: November 2008

Approximate contact models for film/substrate systems have been developed and validated in order to determine the shear moduli of mechanically confined films using lateral contact stiffness measurements. This approach allowed to discuss the effects of material compressibility, hydrostatic pressure and plastic deformation on the mechanical response of thin polymers films within macroscopic contacts.
Laser Surface Processing of “Chameleon” Coatings for Aerospace Tribology
Authors: Voevodin A. A. , Zabinski J. S.
Published: November 2008

Adaptive solid lubricants were developed to provide friction and wear reduction in aerospace environments such as ambient humid air, vacuum, or dry nitrogen (storage and on ground testing). Achieving high durability and long life requires solid lubricant reservoirs near the tribological surface. Reservoirs were fabricated using precision laser processing to generate arrays of micrometer-sized holes with well-controlled size, location, and density on the surface of hard ceramic coatings applied to steel substrates. Solid lubricants based on dichalocogenides and carbon were applied by burnishing and sputtering to the laser treated surfaces. Sliding friction tests were performed against steel balls in environments with different humidity. The result indicates that there is an optimum reservoir surface density, where the life of the solid lubricant is improved by an order of magnitude. The laser-processed reservoirs enable coating operation in variable environments.
Micro/Nano-Tribological Behavior of Octadecyltrichlorosilane on Silicon as a Coating for MEMS
Authors: Barriga J. , Ferna´ndez B. , Abad E. , Coto B.
Published: November 2008

Despite progresses achieved in the technology of MEMS, the tribological problem continues being an unresolved matter. Wear and stick-slip phenomena are many times the origin of failure of these devices. The application of self-assembled monolayers (SAMs) in liquid phase seems to be a solution to this problems. SAMs of octadecyltrichlorosilane (CH3(CH2)17SiCl3, OTS) were attached to Si(100) oxidized in liquid phase. Contact angle measurements were used for characterizing the grade of hydrophobicity. The topography of the coating was obtained with an Atomic Force Microscopy (AFM) in semicontact mode. The images showed the presence of particles related to the polymerization of the precursor molecule during the formation process of the SAMs. Creating the film of lubricant in vapour phase would avoid this undesirable effect. Tribological tests were carried out with a microtribometer in linear reciprocating movement with a ball of 2 mm of diameter (100Cr6 and Si3N4) and load of some milinewtons. Results were compared with those obtained for silicon oxidized without any coating. The coefficient of friction (COF) and wear (substrate and ball) were studied under different test conditions.
Fragmentation Processes in Polystyrene Films Under Sliding Conditions
Authors: Chateauminois A. , Baietto Dubourg M. C. , Gauthier C. , Schirrer R.
Published: November 2008

Fracture processes of thin (10–100 μm) polystyrene films on PMMA substrates have been investigated within macroscopic single-asperity sliding contacts with rigid spherical counterfaces. Using the resources of in situ contact visualisation, the development of cracks has been analysed for various values of the ratio of the contact radius to the film thickness. Damage mechanisms were dominated by the formation of a network of regularly spaced cracks at the leading edge of the contact. These processes were analysed in the light of a fragmentation model based on contact mechanics simulations of the stress field induced within cracked films. It emerged from the simulations that the mean spacing between adjacent cracks can be correlated to the strength of the polymer coating.
Modeling of the Generation of UHMWPE Particles With Microfabricated Surface Textures
Authors: Fang Hsu-Wei , Hsu Stephen M. , Sengers Jan V.
Published: November 2008

Microfabricated surface textures have been utilized to generate narrowly distributed UHMWPE wear particles with different sizes and shapes. The ultimate purpose of the research is to study bioactivity effects induced by UHMWPE particles that lead to the failure of total joint implants. Previously we developed a method of surface-texture design to control the particle size and shape [1]. The objective of the present paper is to model the generation UHMWPE particles with surface textures containing wedge-shaped cutting edges.
Ultrasound as a Tool to Measure the Wear of Human Tooth Enamel
Authors: Dwyer-Joyce R. S. , Lewis R. , Goodman M.
Published: November 2008

A reliable tool for assessing the extent of human enamel wear would be useful to dental practitioners. Current in-vivo methods for determining tooth wear are largely qualitative in nature or depend on measurements taken from tooth impressions, which is very time consuming. The aim of this work was to investigate the feasibility of using ultrasound to measure enamel thickness with a view to developing an in-vivo tool for enamel wear assessment. Three different ultrasonic techniques were used in-vitro to take measurements of enamel on extracted teeth. The first used a focusing immersion transducer (25 MHz) and a time of flight approach to obtain enamel thickness. The other two techniques used planar contact probes (10 MHz), the first with a time of flight approach and the second with a resonance method to determine enamel thickness. The results were compared with direct measurements of sectioned teeth. All three methods showed good correlation with these measurements. The contact probe technique was the easiest measurement to carry out, which would also be the simplest to implement in a measurement tool. While the resonance measurements obtained were good, the time of flight approach was thought to be most likely to obtain accurate repeatable measurements.
Effects of Biological Lubrication on the Morphology of UHMWPE Wear Particles Generated With Surface Textures
Authors: Fang Hsu-Wei , Su Yu-Chih
Published: November 2008

Osteolysis induced by ultra-high molecular weight polyethylene (UHMWPE) wear particles has emerged as a critical failure mechanism of artificial joints. Many researchers have performed cell culture studies or animal tests to investigate the UHMWPE particles induced immunological responses. However, the effect of size and shape remains elusive. This is partly because of the difficulty in obtaining UHMWPE wear particles with specific sizes and shapes for bioactivity tests. Previously, we have shown the feasibility of producing narrowly distributed UHMWPE particle sizes and shapes by rubbing UHMWPE pins against textured surfaces [1]. The correlations between the surface texture dimensions and the size and shape of wear particles generated in water have been obtained [2]. In addition, this novel technique of UHMWPE particle generation enables the investigation of the effects of UHMWPE particle size and shape on bioactivity. It is concluded that the elongated particles and the particles which can be engulfed by macrophage cells induced stronger immunological responses [3]. Thus, it is possible to enhance the life of total joint replacements by reducing the production of the most toxic particle populations in terms of size and shape. Our idea is to apply surface textures on the articulating surface of joint implant in order to control the size and shape of UHMWPE wear particles. While maintaining a low wear rate of UHMWPE parts, further reduction of the most “toxic” particles released into human body shall prevent particle-induced osteolysis.
Temperature Dependence of Friction Coefficient in Ultrahigh Vacuum for Hydrogenated Amorphous Carbon Coating
Authors: Iwaki Masanori , Le Mogne Thierry , Fontaine Julien , Martin Jean-Michel
Published: November 2008

Among diamond-like carbon (DLC) coatings, hydrogenated amorphous carbon (a-C:H) coatings are of great interest since some of them may exhibit coefficients of friction in the millirange, so-called “superlow friction” in ultrahigh vacuum. However, there are still many points to be clarified and improved to employ them as solid lubricant for actual vacuum applications. For example, in space environment solid lubricants are required to function at both low and high temperature ranging from −150 to 100°C. To apply them as solid lubricant in such an extreme environment, it is necessary to know the evolution of the tribological behavior in temperature, leading to their application limit. Furthermore, tribological behavior of a-C:H coatings is known to depend on tribochemistry and on mechanical properties like viscoplasticity. Since both could be affected by temperature, a better understanding of superlow friction mechanisms is expected from experiments at various temperatures. In this present work, pin-on-disk reciprocating friction tests were conducted at various temperature conditions ranging from −130 to 300°C under ultrahigh vacuum (10−7Pa) to study the effect of temperature on the coefficient of friction of an a C:H coated flat mated against steel (AISI 52100) pins. For all temperatures, superlow friction regime could be reached, as it was observed usually at room temperature for this sample. However, an effect of temperature is evidenced on the duration of “running-in” phase, i.e. the number of cycles required to reach a superlow friction regime. The duration becomes shorter at higher temperatures and longer at lower temperatures. Also, the application limit in temperature is found between 200 and 300°C, at which the friction coefficient slowly increases after running-in, to reach values above 0.01. In light of these results, the mechanisms of superlow friction are discussed in terms of tribochemistry and mechanical properties of the coating.
Characterisation of Pre-Sliding and Pre-Rolling Friction
Authors: Al-Bender Farid , Symens Wim
Published: November 2008

This paper gives an overview of pre-sliding and pre-rolling friction, showing that it is characterized by rate-independent hysteresis with nonlocal memory. The dynamic behavior in this region can be described by a ‘damping ratio’. The scaling law in the normal force is discussed.
Investigation of Influence of Inclination on Friction and Wear Mechanisms in Piston Ring Cylinder Liner Contact With the New SRV® 4 Test Rig
Authors: Pauschitz A. , Jech M. , Ebrecht J. , Lebersorger T.
Published: November 2008

Due to the high demand for wear and friction tests in industry, caused by high costs of field tests, preceding experiments with a model test rig like the SRV®3 test rig is common practice. With earlier SRV test rigs only horizontal movement of the test sample can be realized, thus effects of oil adhesion and wear particle removal can be significantly different to real applications. Using the new SRV®4 [1] up to 90° inclination of the tribological interacting surfaces is possible. To isolate the effects of inclination on lubrication conditions and wear particle concentration, experiments with different parameters and samples have been made (e.g. piston ring cylinder liner contact). Diverse sample material, fresh oil and artificially aged oil were used. To investigate different oil conditions in the tribocontact selected experiments with and without oil circuit were made. The oil alterations during the model tests were determined by infrared spectroscopy (IR) and by inductive coupled plasma atomic emission spectroscopy (ICP-AES). The friction coefficient and the wear rate measured during the experiments and the surface topography of the samples measured by confocal microscopy after the test run showed strong dependence on inclination.
Dynamic Modeling of a Dual Clearance Squeeze Film Damper: Part III
Authors: Moraru L. , Dimofte F. , Cioc S. , Keith T. G. , Fleming D. P.
Published: November 2008

Squeeze film dampers (SFD) are devices utilized to control vibrations of the shafts of high-speed rotating machinery. A dual squeeze film damper (DSFD) consists of two squeeze film bearings that are separated by a sleeve, which is released when the rotor experiences abnormal operating conditions. In this part of our study of DSFD we analyze the case when both the inner and the outer oil films are active and the separating sleeve is supported by a squirrel cage. Numerical results are compared with the experimental data.
Tribological Investigation of a Low Friction, Low Wear Polymer/Polymer Composite
Authors: Burris David L. , Sawyer W. Gregory
Published: November 2008

Wear resistant and low friction solid lubricants offer the design engineer attractive options for extreme environment lubrication, particularly in situations where the use of fluid lubricants are precluded. Past work with wear resistant PTFE nanocomposites made with sub 100nm particles of alumina showed that the nanoparticles decorate the PTFE surface during jet-milling process. Subsequent compression molding of these powders is believed to result in a compartmentalize PTFE composite. It is hypothesized that these reinforcing nanoparticles arrest crack propagation, although there is qualitative evidence that they weaken strength of the composite. The hypothesis for this investigation was to use small particles of PEEK as the filler, which can act to compartmentalize damage and form a structural filler network that would not require matrix/filler strength for mechanical integrity. The tribological properties of composite samples ranging from 0–100 weight percent PEEK filled PTFE were evaluated in this study.
Determination of a Critical Forming Parameter for the Cross-Wedge Rolling of Tubes
Authors: Morrow Clint A. , Lovell Michael R. , Li Qiang
Published: November 2008

An explicit dynamic finite element model is used to analyze the tribological behavior of the cross-wedge rolling (CWR) of tubular structures. Cross-wedge rolling is a manufacturing process used to form axisymmetric parts by the relative motion of two opposing hardened dies. To date, the CWR process has been used to form only solid parts. In the present investigation, results are presented for the counteraction of flat dies that are used to deform tubular structures. By combining, numerical and experimental evidence, a critical forming parameter is developed which predicts the critical rolling conditions for the CWR forming of tubular structures. This parameter combines the effects of friction, wall thickness to radius reduction, and tube hardness.
A Study on the Efficacy of Airflow Mitigation Devices in Hard Disk Drives
Authors: Kirpekar Sujit , Bogy David B.
Published: November 2008

The turbulent flow of air generated by spinning disks in hard disk drives (HDD) is known to be major contributor to undesirable vibrations of the head stack assembly (HSA). The continuing demand for higher capacities in HDD has resulted in a continual increase in areal densities. Densities in excess of 100 Gb/ in2 have been demonstrated and it is widely projected that conventional technology will ultimately achieve 1 Tb/in2. It is foreseeable that at such areal densities, a track density of 0.5 Million tracks/inch will be required, with each recorded bit being roughly 13 × 50 nm. Under such conditions, the tracking accuracy required is approximately 1.5 nm RMS (root mean square). These goals place stringent requirements on the positioning accuracy of the actuator while accentuating the need to mitigate the various sources of track misregistration (TMR).
Thermal Influence on Torque Transfer of Wet Clutches in Limited Slip Differential Applications
Authors: Marklund Pa¨r , Ma¨ki Rikard , Larsson Roland , Ho¨glund Erik , Khonsari Michael M. , Jang Joonyoung , Ganemi Bager , Olsson Richard
Published: November 2008

Wet clutches operating under low velocity and high load are studied with the aim of obtaining reliable models for the torque transfer during boundary lubrication conditions.
Wear Evaluation of Crosslinked UHMWPE for Knee Prostheses Using a Multi-Station Rolling-Sliding Tribotester
Authors: Van Citters D. W. , Kennedy F. E. , Currier J. H. , Collier J. P.
Published: November 2008

Tribology-related failure of total knee replacement prostheses can lead to risky and expensive revision surgery. Recent changes in polyethylene processing may reduce the incidence of, or even eliminate contact fatigue failure related to bearing oxidation, but devices are still subject to failure modes relating to wear or non-oxidation related fatigue. Contemporary materials claim wear reduction and improved oxidation resistance through various crosslinking and annealing steps, but clinical wear performance of these materials in the knee is difficult to determine. A device incorporating clinically relevant motions and stress states has been used in this work to compare the wear properties of crosslinked polyethylene materials subjected to different irradiation and sterilization conditions. It can potentially be used to analyze the wear behavior of polyethylene materials from different manufacturers, allowing for better prediction of relative performance in vivo.
Effect of Crystallographic Texture on Deformation Fields in Fretting Contacts
Authors: Mayeur J. R. , McDowell D. L. , Neu R. W.
Published: November 2008

Fretting contacts in the partial slip regime are simulated by a finite element model of a rigid cylinder on an elastic-crystal viscoplastic half-space. The half-space is modeled as duplex Ti-6Al-4V, a polycrystalline metal alloy consisting of equiaxed primary alpha grains and secondary lamellar alpha+beta grains. Various realistic 3-D crystallographic textures are considered. The deformation fields generated by fretting are quantified in terms of cumulative effective plastic strain distributions and plastic strain maps. The results clearly demonstrate the importance of the various sources of microstructural heterogeneity in the surface layers. The main sources of microstructural heterogeneity include the distribution of phases, slip system strength anisotropy, and crystallographic texture. In basal textured materials with fretting on the edge, the plastic strain is more evenly distributed in the subsurface regions than in other textured cases. This is explained by the greater number of grains able to deform by soft slip modes and the symmetry of this type of texture relative to the fretting orientation. Transverse and basal/transverse textures result in more heterogeneously-distributed plastic strain with strain often concentrated in narrow vein-like structures with maximum accumulation near alpha/alpha+beta grain boundaries. Elastic shakedown is more difficult to achieve in the later case. Ratcheting is the primary mechanism for cyclic plastic strain accumulation.
Mechanics Modeling of Blade/Disk Contacts
Authors: Gean Matthew C. , Farris Thomas N.
Published: November 2008

A predictive procedure capable of calculating dovetail surface contact loads in gas turbine engines is developed. The procedure determines contact normal and tangential loads, for a complete mission involving radial blade and thermal loads. Once Finite Element calculations to determine empirical constants for a specific blade/disk geometry are completed, the predictive procedure handles complicated load histories in near real-time for both single and double tang dovetails. The resulting load histories can be combined with Singular Integral Equations to calculate edge-of-contact stresses. The resulting contact stresses have been used to successfully predict fretting fatigue lives under controlled laboratory conditions.
Simulation of the Tribological System in Sheet Metal Processes on the Micro Scale
Authors: Gru¨ebler R. , Hora P.
Published: November 2008

The topography of the sheet metal and tool surface influences the contact behavior between the tool and the sheet metal. With a simulation on the micrometer scale the topographical deformation of the sheet metal is elasto-plastically modeled using an explicit finite element method. The model of the lubricant in between is based on a simplified method calculating the pressure and velocity of the liquid with Darcy’s law. A flat rigid tool is pressed on the sheet metal surface simultaneously with a constant lateral motion. Simulations were performed with real and modeled surfaces. The topographies show different tribological behavior depending on their texture.
Frictional and Electrical Properties of Multiwalled Carbon Nanotubes
Authors: Dickrell P. L. , Raravikar N. R. , Pal S. K. , Schadler L. S. , Ajayan P. M. , Sawyer W. G.
Published: November 2008

This presentation examines the tribological properties and contact resistance of oriented capped carbon multiwalled nanotube (MWNT) films. Highly anisotropic tribological behavior of MWNT films oriented in mutually orthogonal directions is observed. The average values of coefficient of friction varied from high values (μ = 0.795) for vertically aligned nanotubes grown on rigid substrates to low values (μ = 0.090) for the same nanotubes dispersed flat on the same substrates. The results were insensitive to humidity, which is in contrast to graphite materials. The multiwalled nanotube layers also had a monotonic decrease in friction coefficient with increased surface temperature in both orientations, having a 32% drop in friction coefficient over a 73°C temperature rise. Preliminary results from contact resistance measurements of nanotube films grown through a porous alumina are investigated as a function of applied static load.
Applications of Carbon Based Nano-Particles to Space and Aeronautics Lubrication
Authors: Street Kenneth W. , Vander Wal Randy L. , Andrews Rodney , Jacques David , Khabashesku Valery N.
Published: November 2008

At The National Aeronautics and Space Administration Glenn Research Center we have active tribology programs for aeronautics and space applications involving carbon based nano-particles. The conditions for application are considerably different for aeronautics and space applications ranging from high temperature atmospheric to ambient vacuum. Some of the lubricants and surface coatings of tribological significance that we have evaluated include neat carbon based nano-particles, both grown in situ and as bulk material deposited on the substrate, and dispersed in oils. These applications prove to be highly substrate dependent. The substrates we have considered for tribological couples include 440C stainless steel (SS), specialty alloys such as Inconel 718, quartz, silicon carbide and silicon nitride coupled to similar material, one of the other substrates listed or eutectic ceramics. In this presentation we will discuss results of our testing these systems in spiral orbit tribometers (SOT), in unidirectional pin-on-disc (PoD) tribometers and in pin-on-flat fretting tribometers.
Effects of the Fraction of PTFE and Film Thickness on Wear and Friction in an ePTFE and Epoxy Composite Solid Lubricant Coating
Authors: McCook N. L. , Burris D. L. , Hanrahan J. R. , Sawyer W. G.
Published: November 2008

Wear tests were performed on various expanded PTFE / epoxy composite films, using a 304 stainless steel pin, in a pin on disk configuration. The density and thickness of the expanded PTFE films were varied, and the effects on friction and wear were examined. It was found that there are trends for increased wear resistance with increasing density, and increasing film thickness. Wear rates less than 10−8 mm3/Nm were calculated on some of the composite films. The film thickness range from 75–510μm and the density ranged from 0.304 to 0.904 g/cm3. The tests were run at a 5N load and 1m/s sliding speed with varying sliding distances.
Elasto-Hydrodynamic Lubrication Modelling of Spherical Metal-on-Metal Artificial Hip Joints
Authors: Wang F. C. , Jin Z. M. , Udofia I. J.
Published: November 2008

A full numerical methodology was developed for the elasto-hydrodynamic lubrication analysis of hip joint implants for the lubrication problem in spherical and conformal contacts. Typical results of a metal-on-metal hip implant were obtained to illustrate the applicability of the numerical methodology developed in the present study.
An Integrated Experimental and Theoretical Contact Mechanics Study of UHMWPE Hip Implants Tested in a Hip Simulator
Authors: Wang F. C. , Galvin A. L. , Jin Z. M. , Ingham E. , Fisher J.
Published: November 2008

A contact mechanics model was developed to investigate the effect of macroscopic and microscopic surface features of the bearing surface of both conventional and cross-linked polyethylene acetabular cups tested in hip simulators. Wear of the bearing surface was found to increase the conformity and to decrease the contact pressure for both materials. The surface roughness on the cup surface was found to increase the contact pressure perturbation, particularly for the conventional polyethylene.
Thermoelastic Instability in an Automatic Transmission Clutch With a “Finger” Piston
Authors: Zagrodzki Przemyslaw , Zhao Wenping
Published: November 2008

An important practical case of frictionally excited thermoelastic instability (TEI) in a wet multidisk friction clutch of an automotive transmission that includes so-called finger piston is investigated. Advanced finite element model of thermoelastic multi-surface contact problem is used. Theoretical results are verified experimentally. The study shows that the finger piston used in transmission clutches can trigger unstable thermo elastic behavior which leads to severe hot spots.
Design and Hydrodynamic Performance of Hybrid Flexural Pivot Gas Bearings for High Speed Oil-Free Micro Turbomachinery
Authors: Sim Kyuho , Kim Daejong
Published: November 2008

This paper introduces new flexural pivot tilting pad gas bearings for high speed oil-free micro turbomachinery. The new flexural pivot tilting pad gas bearings have a special web geometry that provides a radial stiffness to accommodate rotor growths and high vibrations at critical speed, a pitching stiffness to accommodate rotor-bearing misalignments or rotor bending vibrations, and a very small tilting stiffness for rotor stability. Comprehensive numerical simulations involving orbit simulations and coast-down simulations were performed to investigate the effects of preloads and pivot offsets on the critical speeds and onset speeds of instability. Higher preload and pivot offset increased both critical speeds of the rotor-bearing system and onset speeds of instability due to the increased wedge effect. Design procedures of radial stiffness were presented considering both rotor centrifugal and thermal growths. From simple adiabatic solution of temperature distribution of gas film under pure hydrodynamic mode, enough bearing clearance at pivot was found to be a very important design aspect for high speed hydrodynamic gas bearings. Asymmetric radial stiffness was chosen as a final design to meet the target design speed of 180 krpm for bearing diameter of 28.52mm. Suggested tilting pad gas bearing with asymmetric radial stiffness was predicted to be very stable even under high external destabilizing forces.
Experimental Measurement of Thrust Foil Bearing Temperature Profiles
Authors: Dykas Brian , Radil Kevin
Published: November 2008

Gas foil thrust bearings provide axial support for modern Oil-Free turbomachine rotors, reducing maintenance costs and overall system mass. Large temperature gradients can arise in these bearings due to the viscous dissipation in the gas film and the high thermal conduction resistance of the support structure, which consists of thin superalloy foils. Relatively little experimental temperature data exists for this type of bearing, due in large part to the difficulty in installation of instrumentation. In the current work, temperature gradients in the uncooled bearing are measured across a speed range from 25,000 rpm to 65,000 rpm at low to moderate loads. Bearing torque is measured to correlate temperature gradient magnitude with bearing power loss.
Finite Element Modeling of Sliding Contact Between Rough Surfaces
Authors: Pei L. , Molinari J. F. , Robbins M. O.
Published: November 2008

Friction and wear are critical issues in mechanical design and manufacturing. As these mechanisms originate at the asperity level, research models increasingly attempt to capture the influence of surface roughness. In this paper, we study the sliding contact between two rough surfaces. We choose silicon as a model material for its wide use in MEMS applications. We implement an update master/slave contact algorithm accounting for periodic boundary conditions. The obtained results shine new light on the link between small-scale contact clusters and frictional forces and wear mechanisms at the large scale.
Comparison of WC/DLC/WS2 and YSZ/Au/DLC/MoS2 “Chameleon” Coatings for Tribological Applications
Authors: Baker C. C. , Voevodin A. A. , Zabinski J. S.
Published: November 2008

Chameleon coatings are nanocomposite systems that adapt their tribological performance to changes in environmental conditions such as humidity and temperature. In this research we have investigated the tribological properties of two nanocomposite “chameleon” coatings and compared their properties. The two material systems of interest included: (i) Diamond like carbon (DLC) matrix with nanocrystalline WC and WS2 inclusions, and (ii) Yttrium Stabilized Zirconia (YSZ) in an Au matrix with DLC and MoS2 nanoparticle inclusions. The coating design approach included formation of nanocrystalline hard carbide or oxide particles for wear resistance, embedding them into an amorphous matrix for toughness enhancement, and inclusion of nanocrystalline and/or amorphous solid lubricants for friction adaptation to different environments. The coatings were produced using a combination of laser ablation and magnetron sputtering. Chemical and structural analysis of the coatings included x-ray photoelectron spectroscopy, x-ray diffraction, transmission electron microscopy, and micro-Raman spectroscopy. Mechanical properties such as coating hardness and toughness were investigated using nanoindentation, scratch, and indentation adhesion tests. It was observed that both YSZ and WC are valuable in enhancing film toughness. The chemical analysis was used to ascertain a correlation between chemical bonding of species and frictional properties. Friction measurements were studied by cycling between humid air and dry nitrogen conditions. The graphitic carbon component imparted low friction in humid air, MoS2 and WS2 were excellent for dry N2 conditions, and Au was valuable for low friction at elevated temperatures. The direct comparison among coatings demonstrates that similar “chameleon” behavior can be achieved with different material systems, validating the universal nature of the design approach.
Contact and Adhesion of Curved Rough Surfaces With Application to MEMS
Authors: Majumdar B. S. , Lavan D. A. , Ye Bing , Prasad S. V.
Published: November 2008

Contact, adhesion and stick-slip are major detrimental factors in the performance of micro electromechanical systems (MEMS). One method to reduce adhesion may be the incorporation of tailored rough surfaces, as suggested by the classical experiments of Fuller and Tabor some 3 decades ago. However, before embarking on a costly experimental approach, it is important to quantitatively assess the possible reduction in adhesion through such tailored surfaces. Past analysis has generally been confined to the adhesion of flat surfaces with a random roughness profile, or to Hertzian contact (without adhesion) of a cylindrical surface with a sinusoidal surface profile. In this work, we extend the analysis to the adhesion of cylindrical surfaces with any arbitrary roughness profile. The application may, for example, be in the contact of MEMS gears. The load-displacement and contact area profiles provide both qualitative and quantitative insight on how rough curved surfaces interact under adhesion conditions.
Load Scenarios Influence Fluid Absorption of Polyethylene
Authors: Schwenke T. , Rieker C. , Wimmer M. A.
Published: November 2008

Wear of total joint replacements is determined gravimetrically in simulator studies. A mix of bovine serum, distilled water, and additives is intended to replicate the lubrication conditions in-vivo. Weight gain due to fluid absorption during testing of UHMWPE components is corrected using a load soak station. In this study six sets of UHMWPE pins were tested for their fluid soak behavior. The samples were subjected to three different loading scenarios while being submersed in two types of commonly used lubricants. After two million cycles or 23.1 days, respectively, the different fluids lead to significantly different soaking results. Test groups that were dynamically loaded gained more weight than unloaded or statically loaded samples. The results suggest that dynamically loaded soak control stations are required during wear testing of UHMWPE components. Otherwise the fluid uptake masks the wear measurement, especially for new polyethylene materials with low wear rates. Furthermore, an agreement on detailed lubricant specifications is desirable.
Disk-Like Compounds Showing Low Traction Coefficient Under EHL Contacts
Authors: Kawata Ken
Published: November 2008

The temperature dependence of traction coefficient was evaluated for some kinds of disk-shaped liquid crystal compounds by using the Reciprocating type (SRV) friction/wear tester under EHL contacts. Regardless of disk-shaped frame structure, all of these compounds were found to show traction coefficients of less than 0.04 within each liquid crystal temperature range, and moreover under the extreme pressure condition of about several hundred MPa.
Thermal EHL Analyses of Point Contacts With Surface Roughness
Authors: Wang J. , Yang P. , Kaneta M.
Published: November 2008

The Newtonian thermal EHL analyses of point contacts with two-sided surface roughness have been performed under pure rolling, sliding/rolling and simple sliding conditions. Thermal results are compared with isothermal ones. It has been found that the sliding motion produces high temperature rise and reduces the film thickness greatly. The influence of amplitude and wavelength of the surface roughness on variation of tribo-characteristics of oil film is also discussed.
Experimental Observation of Fractal-Regular Surfaces and a Transformation Scheme for Extracting Fractal Parameters
Authors: Wang Shao , Chan Wai Kin
Published: November 2008

To account for the effects of asperity contacts at various length scales, it is appropriate to characterize an engineering surface as a fractal-regular surface. In spite of significant theoretical advancement, there is a desperate need for experimental verification of the theory of fractal-regular surfaces and a consistent scheme of obtaining the fractal parameters. In the present study, the existence of a fractal region and a regular-shape region in the power spectral density function for fractal-regular surfaces was confirmed experimentally, for the first time, with data obtained from magnetic hard disk and silicon wafer surfaces. A novel scheme involving a variable transformation was developed to extract fractal parameters. This scheme was validated by accurate recovery of fractal parameters from simulated surfaces. The fractal dimension, the fractal roughness parameter and the fractal domain length were found for magnetic hard disk and silicon wafer surfaces.
Analysis of the Damage of Uncoated Polymeric Surfaces During Scratching
Authors: Demirci Ibrahim , Gauthier Christian , Fond Christophe , Schirrer Robert
Published: November 2008

Attempts have been made to correlate the scratch behaviour and basic material properties of polymers and a correlation has been established between the scratch damage and friction coefficient. However, no clear relationship has yet been established between the bulk response and scratch damage. The scratch behaviour of three solid polymers (PMMA, PC and CR39) was investigated to determine how the behaviour of the bulk material affects the scratch resistance. The bulk damage responses of these materials are well known: crazing, shearing and cracking, respectively. The surfaces were scratched under progressive scratch loading and an imaging system was used to record real time photographs of the in-situ contact area and scratch damage. PMMA exhibited crazing under the apex of the scratching tip while the shearing of PC started under the front part of the contact area and cracking appeared in CR39 at the rear edge of the contact. Numerical simulations confirmed these damage mechanisms.
Investigation of Copper CMP Mechanisms Using an Atomic Force Microscope
Authors: Kulkarni Milind , Ng Dedy , Liang Hong
Published: November 2008

In order to understand the mechanisms of Chemical Mechanical Planarization (CMP), an atomic force microscope (AFM) is used to characterize polished copper surfaces after a set of polishing experiments. Surface analysis using the AFM revealed detailed surface characteristics of CMP. It was found that removal mechanisms depend on the slurry chemistry, potential, per cent of oxidizer, and the applied load. This presentation discusses these findings.
Tribological Properties of Magnetron Sputtered MoS2/Metal (Ti, Mo) Nanocomposite Coatings
Authors: Xia Z. Z. , Tu J. P. , Lai D. M. , Zhang L. L. , Wang Q. , Chen L. M. , He D. N.
Published: November 2008

The MoS2/metal (Ti, Mo) nanocomposite coatings were deposited on aluminum alloy substrate by magnetron sputtering. The morphology and microstructure of the coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM). The tribological properties of MoS2/metal (Ti, Mo) nanocomposite coatings were investigated using a ball-on-disk tribometer in ambient air. The MoS2/Ti nanocomposite coating showed lower friction coefficient and higher wear resistance as compared to the pure MoS2 coating, which were caused by the microstructure of the composite coating that serve as perfect intermediate lubricants between the contact faces. The effects of metal inclusion in the composite coatings were further discussed combined with the former reports.
On the Touchdown-Takeoff Hysteresis Observed at the Head-Disk Interface in Hard Disk Drives
Authors: Ambekar Rohit P. , Bogy David B.
Published: November 2008

The touchdown-takeoff velocity hysteresis observed in hard disk drives during CSS or L/UL tests is analyzed using an experimental approach. Tests similar to L/UL were conducted for different slider-disk combinations at different humidities. Factors affecting the touchdown and takeoff velocity were identified on the basis of their domain of operation. It is concluded that the intermolecular forces and meniscus forces are contributing factors to hysteresis, which is also influenced by disk topography and slider dynamics.
A Study on Adhesion Between Mold and Resist in Nanoimprint Lithography
Authors: Kim Kwang-Seop , Kang Ji-Hoon , Kim Kyung-Woong
Published: November 2008

Nanoimprint lithography (NIL) is one of the promising technologies to fabricate integrated fine patterns due to high-throughput and large processing latitude at low cost. However, adhesion between the mold and the resist occurs inevitably in NIL because the mold physically contacts with the resist. The adhesive force and friction force acting on the interface between the mold and the resist can cause the distortion and fracture of the mold and the patterns transferred on the resist when the mold is separated from the resist after imprinting process. In this study, the adhesion properties between quartz mold and mr-I PMMA and mr-I 7020 as resist usually used in thermal NIL is investigated. As the results of the comparison of the adhesion properties of two resists, it is found that mr-I PMMA has good release characteristics.
Studies on Wear of Inlet Valve and Cylinder Bore in Diesel Engines
Authors: Nayak Nagaraj , Dani A. D. , Lakshminarayanan P. A. , Gajendra Babu M. K.
Published: November 2008

Wear is a characteristic of the engine system and a wide variety of conditions causes wear and damage of engine parts. In the current work, wear of the engine components are classified on the basis of fretting and adhesive phenomenon. In this regard, we formalized the knowledge of wear of inlet valve and liner quantitatively on the basis of operational and design aspects. Since there are many design and operational variables, the most important variables are selected and tuned by adding new features to the empirical models. Therefore, in the present work case studies on critical engine parts like inlet valve and liner are made to improve the wear behavior of the special components.
Wear Characteristics of the Sprayed Coatings With Amorphous Material
Authors: Han Myoung-Seoup , Lee Sang-Ek , Kim Dae-Young , Youn Joong-Geun
Published: November 2008

Characteristics of the sprayed amorphous material’s coating have been investigated in order to improve the life of liners used in a blasting system. The amorphous materials of interest were commercial Fe-Cr-B alloys and the processes employed were arc spray and HVOF spray. Erosion wear property of the coatings were evaluated using a steel grit blasting machine and actual blasting system. The coating produced by HVOF spraying method showed a very dense structure so that porosity content of the coating was less than 1% and split was not formed. Due to these features with its high hardness, the wear resistance of the HVOF sprayed coating was superior to that of the arc sprayed coating. Microstructural characteristic of the amorphous material’s coating were analyzed using SEM, EDX and XRD.
Innovative Laser Sources in the Micro- and Nanostructuring of Coated Functional Surfaces (Keynote)
Authors: Dumitru Gabriel
Published: November 2008

Research works and industrial applications demonstrated that laser structured surfaces can exhibit enhanced tribological behaviors and increased lifetimes, as compared with similar unstructured surfaces. Laser structuring techniques based on IR beams and micro- or nanosecond pulses are nowadays already available, but they show certain limitations in the processing of coated surfaces and in microtribology. Physical features yielding these technological limits are discussed in the beginning. Through alternative laser techniques (e.g., femtosecond pulses, UV laser beams), the aforementioned drawbacks can be overcome. An overview of these novel laser systems and of their first applications in micro- and nanostructuring of surfaces is subsequently given. Two laser structuring approaches are discussed in the experimental part of this contribution: direct and indirect processing. Laser structured surfaces were tribologically tested and results of these tests are here exposed.