Contact-induced off-track vibrations of air bearing-slider-suspension system in hard disk drives

Contact-induced vibration of air bearing-slider-suspension system is a crucial issue for slider flying stability and head positioning precision of 1 Tbit/in² hard disk drives. In this paper, the contact-induced off-track vibrations of air bearing-slider-suspension system are investigated by simulation. A dynamic simulator is developed to calculate the interactions between the air bearing dynamics and vibrations of slider-suspension assembly. The simulation model consists of a finite element model of suspension assembly, an air bearing model based on the generalized lubrication equation, and a slider–disk contact model based on the probability distributions of surface roughness. A sequential method is used to couple all these models and analyses. The time history of the slider and suspension motions, together with the time-varying forces including air bearing force, air shear forces, contact force and friction force can be obtained. The effects of different contact conditions, such as the contact intensity, friction coefficient, and disk surface waviness on off-track vibrations are investigated numerically in details. The results reveal some mechanisms on how these factors contribute to the off-track vibrations of suspension assembly.     

Modelling of frictional and adhesive contacts at silicon/polymer interface for nanotechnological applications

Abstract

The minimisation of friction and adhesion during sliding contacts is crucial for the industrial fabrication of many micro/nanodevices (e.g. MEMS/NEMS), as well as in nanotechnological processes, e.g. in nanoimprint lithography where a silicon mould is used to fabricate polymeric nanostructures by imprinting. We have conducted intensive research on the contact between the mould and PMMA polymeric resist film via advanced modelling and computer simulations. The properties of the contacting surfaces have been identified with the atomic force microscope and nanoindentation, as well as wettability tester applied for the identification of the surface free energy. A model of contact has been elaborated and adequate original software was used to calculate the frictional and adhesive forces in particular at the silicon mould/polymeric resist interface.     

Surface characterization and adhesion and friction properties of hydrophobic leaf surfaces

Super-hydrophobic surfaces as well as low adhesion and friction are desirable for various industrial applications. Certain plant leaves are known to be hydrophobic in nature. These leaves are hydrophobic due to the presence of microbumps and a thin wax film on the surface of the leaf. The purpose of this study is to fully characterize the leaf surface and to separate out the effects of the microbumps and the wax on the hydrophobicity. Furthermore, the adhesion and friction properties of the leaves, with and without wax, are studied. Using an optical profiler and an atomic/friction force microscope (AFM/FFM), measurements on the hydrophobic leaves, both with and without wax, were made to fully characterize the leaf surface. Using a model that predicts contact angle as a function of roughness, the roughness factor for the hydrophobic leaves has been calculated, which is used to calculate the contact angle for a flat leaf surface. It is shown that both the microbumps and the wax play an equally important role in the hydrophobic nature as well as adhesion and friction of the leaf. This study will be useful in developing super-hydrophobic surfaces.     

Tribological study on the surface modification of metal-on-polymer bioimplants

The tribological performance of artificial joints is regarded as the main factor of the lifespan of implanted prostheses. The relationship between surface roughness and coefficient of friction (COF) under dry and lubricated conditions is studied. Results show that under dry test, friction coefficient is not reduced all the time with a decrease in surface roughness. On the contrary, a threshold of roughness value is observed, and frictional force increases again below this value. This critical value lies between 40 and 100 nm in S_a (roughness). This phenomenon is due to the transfer of friction mechanisms from abrasion to adhesion. Under wet test, COF always decreases with reduction in surface roughness. This result is mainly attributed to the existence of a thin layer of lubricant film that prevents the intimate contact of two articulating surfaces, thus greatly alleviating adhesion friction. Furthermore, surface texturing technology is successful in improving the corresponding tribological performance by decreasing friction force and mitigating surface deterioration. The even-distribution mode of texturing patterns is most suitable for artificial joints. By obtaining the optimal surface roughness and applying texturing technology, the tribological performance of polymer-based bioimplants can be greatly enhanced.     

轮轨接触界面摩擦管理研究进展

列车向着高速与重载方向迅速发展,显著加剧了轮轨接触界面间的损伤。通过在轮轨接触界面进行摩擦管理能够有效地降低轮轨之间的磨损、显著提高列车的运行安全性以及降低运营成本。对轮轨接触界面摩擦管理研究现状进行综述,并介绍轮轨界面摩擦控制对轮轨作用力、黏着、磨耗、滚动接触疲劳以及振动与噪声影响的研究进展;展望了轮轨接触界面摩擦管理未来研究方向,即应针对不同应用环境和接触部位,研发合理的摩擦控制材料,以克服摩擦管理过程中对轮轨损伤及使用局限性等问题;应探究车轮踏面/轨顶面和轮缘/轨距面摩擦控制方式,严格控制摩擦材料喷涂量使两接触面不相互干扰,优化改进轮轨接触界面摩擦管理的最佳应用参数;应研发环境友好型的轮缘/轨距面润滑剂与车轮踏面/轨顶面摩擦控制剂,稳定调控轮轨接触界面的黏着特性。     

Experimental and theoretical evaluation of friction at contacting magnetic storage slider–disk interfaces

To understand better the friction force and wear processes at contacting slider–disk interfaces, we have developed an experimental method for measuring and a theoretical method for calculating the friction force. For this study, a slider with a 1500 μm² contact pad located at the recording head is burnished against a relatively rough disk (～12 Å rms), which ensures smooth sliding. In the experimental method, the friction force is measured as the disk is spun-down to bring the slider–disk interface into an increasing degree of contact. A modified air bearing code is used to determine the experimental normal contact force for each friction measurement. In the theoretical method, the friction force and other relevant interfacial forces are calculated using an improved sub-boundary lubrication (ISBL) rough surface model. The friction force calculation in this model is based on the force needed to induce yielding of the individual disk asperities contacting the flat surface of the contact pad without any assumption of the coefficient of friction. Good agreement is found between the measured and theoretical friction vs. normal contact force curves, indicating that the model is capturing the essential origins of friction at this interface. The model also provides valuable insights into how wear particles may be generated at this contacting slider–disk interface.     

The frictional behaviour of materials

A review of friction is made and a general equation of friction, based on dimensional analysis, is presented. The equation takes into account the environment, material properties, surface roughness, the area of real contact, rubbing velocity, deformation and surface energy, and suggests friction to be system dependent. By imposing certain conditions, it was shown that the equation satisfies equations based on the adhesion and agrees with deformation and surface energy theories. A model of friction is presented which suggests that the friction force results from adhesion, plowing and interlocking of asperities, the magnitude of which is governed by surface energy and the nature and magnitude of surface deformation.The experimental results presented suggest the dependence of friction on load and velocity to be heavily influenced by the oxidation characteristics of the materials in rubbing contact.     

Dynamics in the Bridged State of a Magnetic Recording Slider

A novel region of tribological interaction is explored by inducing near contact between the magnetic recording slider and disk. In this study, we performed frictional measurements over a wide range of subambient air pressure and disk rotation rate. Since the slider is supported over the disk by an air bearing, it has been found that cycling from ambient to subambient and then back up to ambient pressure over several minutes of time forms a frictional hysteresis loop. The high-friction branch of the loop, referred to as the bridged state, is characterized by an average frictional displacement and resonant vibration of the suspension mount assembly. The bridged state is currently employed for accelerated wear testing of magnetic slider/disk/lubricant systems. Future magnetic recording systems designed to operate at increasingly lower physical spacing will need to take into account these frictional forces which accompany the incipient contact between the lubricated disk and slider with finite surface roughness. A single degree of freedom model is solved to determine the equivalent dynamic friction force on the slider as an impulse series with random impulse frequency and amplitude from the measured frictional displacement in the bridged state. The mean slider-disk spacing in the bridged state is derived from the experimental friction force, the spacing probability density function, and the adhesion stress from the Lifshitz model for dispersion interaction energy.     

Adhesion and friction studies of silicon surfaces processed using a microparticle-based method

A surface processing method that combines electrostatic deposition of microparticles and dry etching is utilized to modify the surface topography of silicon surfaces to reduce adhesion and friction force. Microscale adhesion and friction tests were conducted on flat (smooth) and processed silicon surfaces with a low elastic modulus thermoplastic rubber (Santoprene) probe that allowed a large enough contact area to observe the feature size effect. Both adhesion and friction force of the processed surfaces were reduced comparing to that of the flat surfaces.     

Friction and Adhesion Properties of Clean and Oxidized Al-Ni-Co Decagonal Quasicrystals: A UHV Atomic Force Microscopy/Scanning Tunneling Microscopy Study

The tribological properties of adhesion and friction between 10-fold Al-Ni-Co decagonal quasicrystals and conductive W₂C and TiN coated tips were studied in ultrahigh vacuum (UHV) with an atomic force microscope. Contacts between the tip and clean quasicrystals are dominated by strong adhesion forces, which result in irreversible deformations and material transfer. The irreversible adhesion was suppressed following an oxygen exposure of 10 Langmuir, which also reduced the adhesion force by a factor of two. An additional 2/3 reduction in adhesion force occurred after several 100 Langmuir exposure. A much larger decrease (by a factor 10) occurred by air-oxidation. The friction force decreased also with oxygen exposure although not as dramatically. A linear decrease by a factor two, was observed between the clean surface and the surface exposed to 200 Langmuir of oxygen. After that the friction force remained constant. Air-oxidation reduced friction by roughly another factor of two. In contrast with the clean surface, contacts with the air oxide substrate are well described by the Derjaguin-Müller-Toporov (DMT) model, while contacts with oxygen chemisorbed substrates are in the transition regime between Johnson-Kendall-Roberts (JKR) and DMT models.     

Pre-sliding Behaviour of Single Asperity Contact

The pre-sliding and static friction force behaviour at asperity level between a smooth ball and a smooth flat surface at different normal loads, as well as friction behaviour during full slip has been studied. The normal load dependence of the friction force and the preliminary displacement is discussed when the mean contact pressure is kept under 100 MPa. The theoretical model to calculate the shear stress and the preliminary displacement in the contact is discussed and the experimental data were used to verify the model. The results show that for low applied normal loads the adhesion force has an influence on the friction force measurements. Furthermore, the results for the friction force and preliminary displacement show good agreement with the theoretical trends. The experiments along with the model can be used to analyse the tangential traction in the contact and the behaviour of the stick–slip area. The measurement results along with the model were used to calculate the maximum shear stress at the point of sliding for different applied normal loads. It is also shown that at low applied normal loads the shear stress is not constant as compared to relatively high applied normal loads due to the presence of adhesion force.     

Stick-slip algorithm in a tangential contact force model for multi-body system dynamics

Contact force of Multi-body dynamics (MBD) system can be classified two parts. First is a normal force and the other is a tangential force called friction force. And the friction force can be represented by two states such as stick and slip. The stick-slip phenomenon is simply described as a simple contact model which is a rigid body contacted on a sloped surface. If the calculated friction coefficient between the body and sloped surface is less than the static friction coefficient, the body should be stuck. If the calculated friction coefficient is greater than the static friction coefficient, the body will be sliding along the surface. The phenomenon is called as stick and slip state of friction, respectively. Usually many researchers and commercial MBD software used a coulomb friction force model which is defined with an only function of relative velocity. This kind of friction force model will be called a conventional friction force model in this paper. A big problem of the conventional model can not describe a stick state of friction phenomenon. In the case of conventional friction force model, the body will be sliding even though friction state is stick. Because, the relative velocity must have a non-zero value in order to generate the friction force. To solve this kind of problem, we propose a stick-slip friction force model including a spring like force. In the case of stick-slip friction force model, the body can be stuck on the sloped surface because the friction force will be a non-zero value, even though the relative velocity approaches zero. We defined a relative displacement variable called stiction deformation. In this paper, the stick-slip friction model is proposed and applied in the contact algorithm of MBD system. And then two friction models are compared with numerical examples. With the proposed stick-slip friction model, more realistic results are achieved.     

Microdevice for measuring friction and adhesion properties of sidewall contact interfaces of microelectromechanical systems

A microdevice was specifically designed to characterize the static and dynamic friction and adhesion characteristics of sidewall contact interfaces of microelectromechanical systems (MEMS). The microdevice was fabricated by surface micromachining and tested under conditions that accurately mimic those of typical MEMS contacts. The developed experimental scheme enables the direct measurement of the critical normal force at the instant of surface separation and the friction force at the onset of sliding. Additional capabilities include evaluation of the dynamic friction behavior, measurement of the electrical characteristics across the contact interface, and characterization of the tribological response under impact contact loading. The microdevice can operate over a wide range of normal forces and different environmental conditions. Because the design is independent of process environment, the microdevice can be used to study the effects of different surface treatments and variations in fabrication process steps on the tribological properties of MEMS contact interfaces. Characteristic results of static and dynamic friction behaviors, electrical contact resistance, and response to dynamic impact loading illustrate the experimental capabilities and versatility of the designed microdevice.     

Micro–Macro-Shear-Displacement Behavior of Contacting Rough Solids

Interlocking asperities are shown to have a fundamental effect on the friction behavior of contacting solids through theoretically derived shear force–displacement relationship. The key aspect of this relationship is the asperity contact orientation probability distribution obtained using the random process theory in terms of measurable surface roughness parameters. Thus, the obliquity of surface asperity contact is included in the contact shear analysis in a fundamental manner. The interlocking asperities are found to result in a normal load-dependent friction coefficient for a contact. The interlocking also affects contact partial slip and the shear displacements that precede sliding. The derived relationship can be used to evaluate factors, such as asperity adhesion, plasticity, damage, normal-shear coupling and scale dependency, which are difficult to separate in experiments and atomistic simulations.     

高载荷条件下石墨-石墨摩擦副的摩擦学特性研究

利用研制的高载荷条件下摩擦因数测试装置,研究了石墨/石墨摩擦副在空气、水和油介质中的摩擦学特性。结果表明在4~15MPa范围内,随着载荷的增加,摩擦副在空气、水和油介质中的摩擦因数都逐渐降低;在油介质中摩擦副的摩擦因数最小,在水介质中摩擦因数变化最平稳,在空气中摩擦因数最大,且随载荷的增加变化幅度最大。磨损表面原始形貌对比分析表明,在空气中,摩擦副表面处于边界润滑状态,主要磨损机制是粘着磨损和犁削;水润滑条件下为轻微犁削;油润滑条件下,摩擦副表面处于为边界润滑和流体润滑状态,油中的减摩剂对试样表面有抛光作用。     

Note: An adhesion measurement setup for bioinspired fibrillar surfaces using flat probes

Current adhesion measurement setups designed for experiments on bioinspired fibrillar surfaces, either commercial or constructed in-house, do not allow adhesion measurements with in situ visualization, high resolution, high force range, and controlled alignment at the same time. In this paper a new adhesion tester is presented, which enables contact experiments with controlled tilt angle (accuracy of ±0.02°). This allows the use of flat probes and thus greatly simplifies the determination of experimental parameters such as pull-off strength or Young's modulus. The deflection of a double-clamped glass beam is measured by laser interferometry with an accuracy of ±60 nm, which yields a precise force measurement over three orders of magnitude force range without changing the glass beam. Contact formation and detachment events can be visualized in situ. The current adhesion tester is designed for force measurements in the range of 1 μN to 1 N and fills the gap between macroscopic tests and atomic force microscopy measurements.     

Macro- and nanotribological properties of organosilane monolayers prepared by a chemical vapor adsorption method on silicon substrates

Organosilane monolayers are novel ultrathin films used to control the physicochemical properties, such as friction and wear, of solid surfaces. In this study, the authors prepared alkylsilane and fluoroalkylsilane monolayers with a series of chain lengths by a chemical vapor adsorption method. The monolayers tribological properties were investigated by lateral force microscope (LFM) and friction tester. LFM nanoscale measurements of tribological properties showed that alkylsilane monolayer gave lower lateral force than the Si substrate surface. The lateral force decreased as the length of the alkyl chain increased. On the macroscale, friction test revealed that the organosilane monolayers gave lower dynamic friction coefficients than the Si substrate surface in air at room temperature. The longer the alkyl chain, the greater the wear resistance of the organosilane monolayers. Friction experiments using tetradecane as a lubricant showed better tribological properties than were obtained in air. Furthermore, microscopically line-patterned two-component organosilane monolayers were prepared and their macroscopic friction behavior was investigated. Even though the height difference between the two-components was less than 1 nm, friction force anisotropy between the parallel and perpendicular directions against the line pattern was observed.     

Adhesion and Friction Evaluation of Textured Slider Surfaces in Ultra-Low Flying Head-Disk Interfaces

To achieve extremely high-density magnetic recording of 1Tbit per square inch using conventional technologies, the distance between the recording slider and the rotating disk needs to be less than 5nm. For successful operation, disk and slider surfaces must also be extremely smooth with root-mean-square roughness values of few angstroms. However, ultra-low flying super smooth head-disk interfaces may be exposed to a significant amount of intermittent contact, adhesion, stiction and friction that can cause the interface to collapse. In order to circumvent such problems, many novel techniques have been proposed, such as laser zone texturing, contact pads and surface microtexturing. A reliable method to reduce adhesion and friction in ultra-low flying head-disk interfaces is to control the area of contact and roughen the interface, which allows the slider to fly at sub-5nm with minimal contact. A technique known as preferential texturing provides a unique roughening of the air-bearing surface, where parts of the surface are removed, i.e., subtractive texturing process. In this paper, the effect of preferential texturing (roughening) of slider air-bearing surfaces on the adhesion and friction forces are investigated using quasi-dynamic models. The simulation results show that surface texturing reduces adhesion and friction by reducing the effective area of contact between the slider and media surfaces and by preferentially roughening the interface. The simulation results of friction compare favorably with experimental data.     

Atomic force microscopy characterization of the chemical contrast of nanoscale patterns fabricated by electron beam lithography on polyethylene glycol oxide thin films

The present paper shows that atomic force microscopy (AFM) imaging of friction force and phase lag in ambient air can be used to characterize the chemical contrast induced by electron beam (EB) irradiation on polyethylene glycol oxide (PEO) surface. Time-of-flight secondary emission mass spectroscopy measurements showed that the EB irradiation generates chemical contrast on PEO surface by decreasing the ether bond density. The AFM measurements showed smaller phase lag and lower friction and adhesive forces on the EB irradiated PEO surface, as compared to the non-irradiated PEO surface. While the chemical contrast in friction force had a linear dependence on the EB irradiation dose, the dependence of the chemical contrast in the phase lag was strongly non-linear. As the friction and adhesive forces depended on the AFM probe hydrophilicity and air humidity, the contrast in friction and adhesive forces is ascribed to different capillary condensation of ambient water vapour at the AFM tip contact with the EB irradiated and non-irradiated PEO surfaces, respectively.     

Tribological Performance of Room-Temperature Ionic Liquids as Lubricant

The tribological performance of room-temperature ionic liquid of alkylimidazolium tetrafluoroborate was evaluated using an Optimol SRV oscillating friction and wear tester in air and a CZM vacuum friction tester in vacuum (1×10^-3 Pa) using a steel/steel (SAE52100) contact. From the results, the ionic liquid exhibits excellent friction-reduction, antiwear proprieties, both in air and vacuum, which are superior to phosphazene (X-1P) and perfluoropolyether (PFPE). During friction, the ionic liquid forms a surface protective film mainly composed of FeF₂ and B₂O₃, which contributes to low friction and wear.