The Effect of Adhesion on the Static Friction Properties of Sidewall Contact Interfaces of Microelectromechanical Devices

Static friction between sidewall contact surfaces of polycrystalline silicon micromachines was investigated under different contact pressures, vacuum conditions, relative humidity levels, and temperatures. The static coefficient of friction exhibited a nonlinear dependence on the external contact pressure. A difference between in-contact and pull-out adhesion forces was observed due to the elastic recovery of the deformed asperities at the contact interface. The true static coefficient of friction was determined by considering the effects of the dominant adhesion forces (i.e., van der Waals and capillary forces) on the normal force applied at the sidewall contact interface. The roles of van der Waals and capillary forces in the sidewall friction behavior were analyzed in light of results for the interfacial shear strength and the adhesion force. The major benefits of the present friction micromachine and the developed experimental scheme are discussed in the context of static coefficient of friction and adhesion force results obtained under different environmental and loading conditions     

An on-chip micro-friction tester for tribology research of silicon based MEMS devices

An on-chip micro-tribotester has been developed to investigate the friction and wear properties on side contacting surfaces of single crystal silicon that is most widely used in usual microelectromechanical systems actuators. The device is fabricated with standard bulk silicon process and bonding technology based on parameters that have been theoretically calculated to get the stiffness and friction forces. In this device, two comb shuttles are used. One comb shuttle is used to contact the friction surfaces under a certain normal load. The other comb shuttle moves back and forth to provide relative motion between the two friction surfaces. The tested two surfaces are the top surface of an anchor with rounded end and the lateral surface of a beam that has been connected to the two comb shuttles. Tribology experiments on the etched silicon surfaces that are side contacted have been carried out. Friction coefficients testing results suggest that dynamic friction coefficient is about 0.31–0.33 and the obtained static friction coefficient increases with the decrease of normal force. Wear experiment indicates that oxidation happens between the rubbing surfaces during the course of the testing. Wear debris is collected as agglomerates because of the function of adhesion force and water vapor and the agglomerates that are collected on top and lateral surfaces are of different shapes.     

Static Friction in Polysilicon Surface Micromachines

Surface micromachines of polycrystalline silicon were used to investigate the dependence of static friction in microelectromechanical systems on the external load, apparent contact area, and environmental conditions. An analytical model of the micromachine at the inception of sliding was used to determine the normal load consisting of the restoring and levitation forces exerted by the micromachine's comb-drive actuators. The apparent shear strength at the contact interface(s) exhibited a nonlinear dependence on the apparent contact pressure. Relatively higher static coefficient of friction and interfacial shear strength were obtained in room air than vacuum ambient. The static coefficient of friction was found to depend on the normal load, apparent contact area, and ambient conditions (i.e., relative humidity). Electrical contact resistance measurements indicated that sliding in room air promoted thickening of the native oxide film at asperity contacts. The experimental evidence suggests that modification of the surface topography occurred at the asperity level. However, these submicroscopic changes in the surface topography did not affect the overall static friction behavior, for the test cycles simulated in the friction experiments.$hfillhbox[1225]$     

The role of friction in the measurement of slipperiness,Part 1: Friction mechanisms and definition of test conditions

Friction has been widely used as a measure of slipperiness. However, controversies around friction measurements remain. The purposes of this paper are to summarize understanding about friction measurement related to slipperiness assessment of shoe and floor interface and to define test conditions based on biomechanical observations. In addition, friction mechanisms at shoe and floor interface on dry, liquid and solid contaminated, and on icy surfaces are discussed. It is concluded that static friction measurement, by the traditional use of a drag-type device, is only suitable for dry and clean surfaces, and dynamic and transition friction methods are needed to properly estimate the potential risk on contaminated surfaces. Furthermore, at least some of the conditions at the shoe/floor interface during actual slip accidents should be replicated as test conditions for friction measurements, such as sliding speed, contact pressure and normal force build-up rate.     

The role of friction in the measurement of slipperiness, Part 1: friction mechanisms and definition of test conditions.

Friction has been widely used as a measure of slipperiness. However, controversies around friction measurements remain. The purposes of this paper are to summarize understanding about friction measurement related to slipperiness assessment of shoe and floor interface and to define test conditions based on biomechanical observations. In addition, friction mechanisms at shoe and floor interface on dry, liquid and solid contaminated, and on icy surfaces are discussed. It is concluded that static friction measurement, by the traditional use of a drag-type device, is only suitable for dry and clean surfaces, and dynamic and transition friction methods are needed to properly estimate the potential risk on contaminated surfaces. Furthermore, at least some of the conditions at the shoe/floor interface during actual slip accidents should be replicated as test conditions for friction measurements, such as sliding speed, contact pressure and normal force build-up rate.     

Friction model of fingertip sliding over wavy surface for friction-variable tactile feedback panel

A friction-variable touch panel is capable of presenting virtual bumps and holes on its flat surface through the control of the surface friction when a fingertip slides over it. To improve the presentation, we developed a friction model of a fingertip sliding over a sinusoidal surface with an amplitude of 0.5–2.5 mm and a spatial wavelength of 20–50 mm. When a metal ball rolls over a wavy surface with a low friction and contact area, the ratio of the horizontal force to the normal force is equal to the gradient of the surface (this is referred to as the ball bearing model) and is hardly affected by the normal load and rolling speed. In contrast, the profile of the force ratio of a sliding finger is substantially skewed and affected by the sliding direction and normal force exerted by the finger. To model this skewed force ratio, we formulated the asymmetric pressure distribution in the finger-surface contact area and used the effects of the adhesion friction to model the dependency of the force ratio on the normal force and sliding direction. The developed model of a bare finger with these features was found to sufficiently simulate the experimentally observed force ratios. The model can be easily applied to friction-variable touch panels and enables the achievement of a wide variety of haptic contents with macroscopically concave or convex surfaces.     

Operational experience with a portable friction testing device in university buildings

Following a high incidence of falls on the level recorded in a university refectory building, a more detailed examination of accident reports suggested that slippery floors in the main kitchen, serveries, dining area and corridors should be subjected to a quantitative investigation. Using a specially designed lightweight portable friction-testing device, dynamic and static coefficients of friction were measured. This enabled an assessment to be made of the ease of use and effectiveness of this portable device, in responding to accidental falls, undertaking surveys of the slip resistance of floor areas and in checking the coefficients of friction of new materials being considered as alternative floor coverings. From the results presented floor surfaces can be compared. Information was also obtained on the effect of surface projections in increasing coefficients of friction. A detailed series of measurements taken on polished woodblock floors indicated the changes during the drying of the polish, and the importance of buffing in maximizing slip resistance. The replacement of two major floor surfaces, ceramic tiles and woodblock, is examined and the friction-testing device together with recent accident statistics are used to assess the effectiveness of the new flooring materials used.     

Fabrication of metallic micromirror using electroplating technology

A nickel micromirror array was designed and successfully fabricated using a thick photoresist as a sacrificial layer and as a mold for nickel electroplating. It was composed of two address electrodes, two support posts and a nickel mirror plate. The mirror plate, which is supported by two nickel posts, is overhung about 10???m from the silicon substrate. The nickel mirror plate is actuated by an electrostatic force generated by electrostatic potential difference applied between the mirror plate and the address electrode. Optimized fabrication processes have been developed to reduce residual stress in mirror plate and prevent contact between the mirror plate and the substrate, which ensure a reasonable flat and smooth micromirror for operation at low actuation voltage.     

Fabrication of metallic micromirror using electroplating technology

A nickel micromirror array was designed and successfully fabricated using a thick photoresist as a sacrificial layer and as a mold for nickel electroplating. It was composed of two address electrodes, two support posts and a nickel mirror plate. The mirror plate, which is supported by two nickel posts, is overhung about 10 μm from the silicon substrate. The nickel mirror plate is actuated by an electrostatic force generated by electrostatic potential difference applied between the mirror plate and the address electrode. Optimized fabrication processes have been developed to reduce residual stress in mirror plate and prevent contact between the mirror plate and the substrate, which ensure a reasonable flat and smooth micromirror for operation at low actuation voltage.

     

Progress in the prevention of falls caused by slipping

Initial research by the INRS showed the importance of biomechanical factors in the causes of accidents by slipping. Dynamic friction was shown to be far more significant than static fraction. The measuring procedure which has been developed does not provide a model of walking or slipping but a physical measurement giving the same ranking as subject evaluation methods.

Systematic measurements show the effect on slip resistance of the material used and the configuration of the sole. Following publication of these results, manufacturers have improved the slip resistance of shoes. An internationally acceptable method (ISO) should, however, be developed so that slip resistance, which is such an important factor in accident prevention, can be given as much attention as other features of the sole.

Scientific research on the psychophysiology of equilibrium on slippery surfaces is needed, as is applied research on friction with lubricated elastomers and on-site research into floor-surface friction.     

Grip-force control of an elastic object by vision-based slip-margin feedback during the incipient slip

In this paper, a grip-force control of an elastic object is proposed based on a visual slip-margin feedback. When an elastic object is pressed and slid slightly on a rigid plate, a partial slip, called "incipient slip," occurs on the contact surface. The slip margin between an elastic object and a rigid plate is estimated based on the analytic solution of a Hertzian contact model. A one-degree-of-freedom gripper consisting of a camera and a force sensor is developed. The slip margin can be estimated from the tangential force measured by a force sensor, the deformation of the elastic object and the radius on the contact area both measured by a camera. In the proposed method, the friction coefficient is not explicitly needed. The "eccentricity" is used to estimate the displacement of the elastic object at the contact area with high accuracy. The grip force is controlled by a direct feedback of the estimated slip margin. The proof of the contact stability by the proposed control is analytically given. As a result, the slip margin is maintained at a desired value, without occurring the gross slip against a disturbance traction force to the object. The validity of the proposed method is confirmed by experiments.     

Assessment of the slip-resistance of floors in the laboratory and in the field: Two complementary methods for two applications

There are numerous methods available to measure the slip-resistance of different floor-coverings. The INRS has developed two distinct methods for the evaluation of the slip resistance of a given surface within the framework of its studies on the prevention of slips:

• - One method that can be used to compare new surfaces. It uses a static device developed at the INRS and it is based on the evaluation of a coefficient of dynamic friction between a sample of a new oiled surface and an elastomer. This method is well-adapted to the needs of standardisation work;

• - Another method that can be used to evaluate slippage in the field where the surfaces are often worn and polluted with a specific product. It uses a portable device developed in Sweden and it is based on the continuous evaluation of a coefficient of dynamic friction over a variable distance between the surface to be tested and an elastomer.

These two methods which present well-correlated results are described in this publication, and their distinctly different uses will be underlined.     

Available friction of ladder shoes and slip potential for climbing on a straight ladder

Straight ladder accidents are a major safety problem. As a leading cause of injuries involving straight ladders, slips at the ladder base occur when the required friction exceeds the available friction at the ladder shoe and floor interface. The objectives of this experiment were to measure the available friction at the base of a portable straight ladder in contact with a floor and to estimate the slip potential of the ladder. The results of friction measurements indicated that the measured friction coefficient on the oily surfaces differed among the six commercially available ladder shoes evaluated. A statistical model was used to compare the available friction results from the current study with the friction requirements under different climbing conditions from a previous study based on their stochastic distributions to estimate the slip potential at the base of the ladder. The results showed that different climbing conditions used in the previous study could be supported by available friction on dry surfaces. However, when the ladder was put onto oily surfaces, resulting in a significant reduction in the available friction due to contamination, slip potential was significantly increased.     

Available friction of ladder shoes and slip potential for climbing on a straight ladder

Straight ladder accidents are a major safety problem. As a leading cause of injuries involving straight ladders, slips at the ladder base occur when the required friction exceeds the available friction at the ladder shoe and floor interface. The objectives of this experiment were to measure the available friction at the base of a portable straight ladder in contact with a floor and to estimate the slip potential of the ladder. The results of friction measurements indicated that the measured friction coefficient on the oily surfaces differed among the six commercially available ladder shoes evaluated. A statistical model was used to compare the available friction results from the current study with the friction requirements under different climbing conditions from a previous study based on their stochastic distributions to estimate the slip potential at the base of the ladder. The results showed that different climbing conditions used in the previous study could be supported by available friction on dry surfaces. However, when the ladder was put onto oily surfaces, resulting in a significant reduction in the available friction due to contamination, slip potential was significantly increased.     

Sidewall morphology of electroformed LIGA parts-implications for friction, adhesion, and wear control

LIGA fabricated parts are finding application in a wide variety of micro-mechanical systems. For these systems to operate reliably, friction between contacting sidewall surfaces must be understood and controlled. The roughness of the as-plated sidewall is an important determinate of friction forces at such contacts. LIGA sidewalls were characterized in order to provide a basis for predicting the friction, adhesion, and wear behavior of LIGA micromachines. A variety of unexpected sidewall morphologies were observed during this investigation. Three morphologies were identified: a fine scale roughness, a linear through thickness feature, and a group of larger high aspect ratio features. Each morphology has been associated with a specific aspect of the LIGA manufacturing process. Potential friction, adhesion, and wear management strategies suggested by these features have been discussed. In addition, the asperity behavior in a LIGA sidewall contact has been predicted based on the finest roughness observed.     

Dichlorodimethylsilane as an anti-stiction monolayer for MEMS: acomparison to the octadecyltrichlorosilane self-assembled monolayer

This paper presents a quantitative comparison of the dichlorodimethylsilane (DDMS) monolayer to the octade-cyltrichlorosilane (OTS) self-assembled monolayer (SAM) with respect to the film properties and their effectiveness as anti-stiction coatings for micromechanical structures. Both coatings have been evaluated in several ways, including atomic force microscopy (AFM), contact angle analysis (CAA), work of adhesion by cantilever beam array (CBA) technique and coefficient of static friction using a sidewall testing device. While water and hexadecane contact angles are comparable, the DDMS coated microstructures exhibit higher adhesion than OTS coated ones. Furthermore, coefficient of static friction data indicate that the DDMS films are not as effective at lubrication as the OTS SAMs are, although both exhibit much improvement over chemical oxide. However, AFM data show that the samples which receive DDMS treatment accumulate fewer particles during processing than those which receive the OTS SAM treatment. The thermal stability of the DDMS film in air far exceeds the OTS SAM, as the DDMS remains very hydrophobic to temperatures upwards of 400°C     

A method for analyzing three-dimensional dynamic contact problems in visco-elastic media with kinetic and static friction

A solution method is presented for analyzing three-dimensional dynamic contact problems in visco-elastic media with kinetic and static friction considered. The approach is an extension to three dimensions of our previous work where a two-dimensional case was considered. The time integration scheme adopted in the algorithm is explicit and has second-order accuracy. No iteration is performed when dynamic contact is determined and dynamic contact conditions are exactly satisfied. Determining friction in a three-dimensional problem is very different from that in a two-dimensional case. Formulations integrating static and kinetic friction are given. In addition a sub-procedure for calculating the direction of kinetic friction is employed. Finally several numerical examples including one benchmark test and four general problems are given to illustrate the precision and effectiveness of the algorithm.     

Robust Interaction Control of a Mobile Manipulator – Dynamic Model Based Coordination

This paper discusses the modeling and control of a spatial mobile manipulator which consists of a robotic manipulator mounted upon a wheeled mobile platform. The nonholonomic model, which assumes perfect contact between the wheels and the ground, is obtained using the Lagrange–d'Alembert formulation. Also, the dynamic model, which considers slip of the platform's tires, is developed using the Newton–Euler method and incorporates Dugoff's tire friction model. The complexity of the model is increased by introducing kinematic redundancy which is created when a multi-linked manipulator is used. The kinematic redundancy is resolved by decomposing the mobile manipulator into two subsystems; the mobile platform and the manipulator. Based on the coordination scheme used to resolve the kinematic redundancy, a robust interaction control algorithm, in which suitable controllers are designed for the two subsystems, is developed and applied. The adverse effect of the wheel slip on the tracking of commanded motion is discussed in the simulation. For the dynamic model, a robust control approach is employed to minimize the harmful effect of the wheel slip on the tracking performance. Simulation results show the promise of the developed algorithm.     

Biomechanical characteristics of slipping during unconstrained walking,turning, gait initiation and termination

Slipping biomechanics was investigated on both non-contaminated and oil-contaminated surfaces during unconstrained straight-line walking (‘walking’), turning, gait initiation and termination. In walking, backward slipping was more frequent, whereas forward slipping was more frequent when turning. Stopping and gait initiation engendered only forward and backward slipping, respectively. Based on slip distance and sliding velocity, severity of forward slipping was least in walking than for the other gait tasks, whereas the tasks had similar effects on backward slipping. Relative to the dry surface, heel and foot contact angles reduced and heel contact (HC) velocity increased for all gait tasks on the contaminated surface. Ground reaction forces were generally lower on the contaminated surface, suggesting kinetic adaptation immediately following HC. Required coefficient of friction (RCoF) did not correlate with slip distance suggesting that RCoF may not be a useful kinetic parameter for assessing slipping risk on contaminated surfaces.

Practitioner Summary: Slipping is hazardous in everyday locomotion and occupational settings. This study investigated foot control kinematics and kinetics across various gait tasks on both a non-contaminated and an oil-contaminated walking surface. Turning, gait termination and gait initiation were associated with a greater risk of slip-related falls than unconstrained walking.     

Modeling of the incudo-malleolar joint within a biomechanical model of the human ear

Under large quasi-static loads, the incudo-malleolar joint (IM joint), connecting the malleus and the incus, is highly mobile. It can be classified as a mechanical filter decoupling large quasi-static motions while transferring small dynamic excitations. To investigate the influence of the behavior of the IM joint, a detailed simulation model of the IM-complex is created. Mathematical modeling of the IM joint behavior under quasi-static excitation requires adequate modeling of the mechanics of the diarthrodial joint. Therefore, the geometry of the articular surfaces, the ligaments, as well as their viscoelastic properties have to be included in the model. The contact of the articular surfaces is implemented using a penalty based contact formulation utilizing the geometric information obtained from micro computer tomography (micro-CT) scans. The ligaments of the joint capsule are modeled by distributing force elements along the joint capsule, with the position and orientation derived from the micro-CT scans. It is shown that the effects which were observed in measurements on human temporal bones are described adequately by the model, if the contact of the articular surfaces and the preload of the viscoelastic fibers are taken into account in the simulation model. In the following, the detailed model is implemented in an elastic multibody system of the entire ear. The model allows the study of different quasi-static load cases of the ossicles, such as it occurs in the reconstruction of the middle ear and form the basis for future simulative studies of sound transmission in natural or reconstructed ears.