Calculations of temperature conditions in a contact spot of friction with seizure

Temperature conditions in a contact spot of friction with seizure are investigated using a macroscopic self-oscillation model. Both the flash temperature at the friction surface and the thickness of the surface material layer, where the temperature oscillates, are estimated at different loads, sliding velocities, and friction coefficients. The influence of the contact spot size on the flash temperature is studied. The calculated results for steel and copper samples are compared. Institute of Strength Physics and Materials Science. Translated from Izvestiya Vyshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 58–64, September 1999.     

Anti-sticking treatment for a nanoimprint stamp

Nanoimprint lithography (NIL), as a low-cost and mass production technique, has played an important role in micro/nano fabrication. However, the sticking problem between the stamp and resist blocks its further application. In order to modify the contact surface, a molecular dynamics (MD) method was used to choose the proper material to coat on the surface of the stamp. According to the MD analysis, CF₂ was found to be a good choice for this purpose. It was applied to the nanoimprint stamp by using the gases in reactive ion etching (RIE). A self-assembly monolayer (SAM) layer was also used in the experiment to release the contact surface energy. Both dry and wet methods were demonstrated as excellent anti-sticking approaches by measuring the contact angles and calculating the surface energy. Both the stamps after anti-sticking treatment can be used more times than the untreated stamps.     

浸润接触线的摩擦性质与固体表面张力的Wenzel行为

依据Adam和Jessop关于固-液-气三相接触线具有静摩擦性的观点，能从浸润接触角数据推算 出固体的表面张力系数，但结果显然会与摩擦条件的具体形式有关.以报道的不锈钢和聚丙 烯实验数据为例，通过对比Mises和Amonton两种摩擦定律给出的表面张力系数，后者给出的 张力才具有总是随表面粗糙性的增大而增加的Wenzel效应，以及当固-液，固-气界面的张力 系数之差等于液-气界面的张力系数时，退后角将变为零的全浸润条件.这似乎表明用Amonto n定律描写接触线的静摩擦要更为合理. 关键词： 浸润角 静摩擦 固体表面张力     

Fabrication of durable hydrophobic surfaces through surface texturing

Low surface energy polymer thin-films can be applied to surfaces to increase hydrophobicity and reduce friction for a variety of applications. However, wear of these thin films, resulting from repetitive rubbing against another surface, is of great concern. In this study, we show that highly hydrophobic surfaces with persistent abrasion resistance can be fabricated by depositing fluorinated carbon thin films on sandblasted glass surfaces. In our study, fluorinated carbon thin films were deposited on sandblasted and as-received smooth glass using deep reactive ion etching equipment by only activating the passivation step. The surfaces of the samples were then rubbed with FibrMet abrasive papers in a reciprocating motion using an automatic friction abrasion analyzer. During the rubbing, the static and kinetic friction forces were also measured. The surface wetting properties were then characterized using a video-based contact angle measuring system to determine the changes in water contact angle as a result of rubbing. Assessment of the wear properties of the thin films was based on the changes in the water contact angles of the coated surfaces after repetitive rubbing. It was found that, for sandblasted glass coated with fluorinated carbon film, the water contact angle remained constant throughout the entire rubbing process, contrary to the smooth glass coated with fluorinated carbon film which showed a drastic decrease in water contact angle with the increasing number of rubbing cycles. In addition, the static and kinetic friction coefficients of the sandblasted glass were also much lower than those of the smooth glass.     

Tribological properties of silicate materials on nano and microscale

We studied the friction properties of four model silicate materials at the nanoscale and microscale. From nanotribology, we characterized the tribological properties at single asperity contact scale and from microtribology, we characterized the tribological properties at multi asperity contact scale. First, for each material we measured chemical composition by XPS, Young's modulus by acoustical microscopy and roughness σ by atomic force microscopy (AFM). Second, we measured the nanofriction coefficients with an AFM and the microfriction coefficients with a ball probe tribometer, for three hardnesses of the ball probe. We identified one friction mechanism at the nanoscale (sliding friction) and two friction mechanisms at the microscale (sliding friction and yielding friction). Comparison of the nano and microfriction coefficients at the same sliding friction regime shown, that the tribological properties of these materials didn’t depend on roughness.     

Fretting wear behavior of laser-nitrided Ti–5Al–5Mo–5V–1Cr–1Fe alloy fabricated by laser melting deposition

Fretting wear behavior of laser-nitrided titanium alloy (Ti–5Al–5Mo–5V–1Cr–1Fe) fabricated by laser melting deposition (LMD) has been investigated to explore surface engineering for protection against wear damage of laser melting deposited titanium alloy. The morphology and volume of the wear scars of unmodified and laser-nitrided LMD Ti–5Al–5Mo–5V–1Cr–1Fe tested at different frequencies, 10 and 50 Hz, were studied using non-contact three-dimensional surface profilometer and scanning electron microscope. Friction coefficients measured at different frequencies or loading forces were compared for unmodified and laser-nitrided LMD specimens. Experimental results show that laser-nitrided LMD specimens have shown fretting resistance superior to unmodified LMD specimens due to the presence of hard TiN dendrites in the laser-nitrided layer. W-shaped wear scar caused by local rotation of fretting ball at the two ends of the scar was observed. Given a constant loading force of 50 N, unmodified and laser-nitrided LMD specimens exhibited similar friction coefficients and their friction coefficients increased with test frequency. The friction coefficients of both specimens increased with the reduction of normal load, which corresponds to the trend in Hertzian contact model.     

Complete boundary element formulation for normal and tangential contact problems

The boundary element method as a numerical tool in contact mechanics is widely used and allows for surface roughness to be investigated with very fine grids. However, for every two grid points, influence coefficients have to be employed for every force-displacement combination. In this paper, we derive the matrixes of influence coefficients for the deformation of an elastic half space, starting from the classical solutions of Boussinesq and Cerruti. We show how to overcome complexity problems by using FFT-based fast convolution. A comprehensive algorithm is given for solving the case of dry Coulomb friction with partial slip. The resulting computer program can be used effectively in iterative schemes also in similar problems, such as mixed lubrication and notably improves the applicability of the boundary element method in contact mechanics.     

Nonlinear shear wave interaction at a frictional interface: energy dissipation and generation of harmonics

Analytical and numerical modeling of the nonlinear interaction of shear wave with a frictional interface is presented. The system studied is composed of two homogeneous and isotropic elastic solids, brought into frictional contact by remote normal compression. A shear wave, either time harmonic or a narrow band pulse, is incident normal to the interface and propagates through the contact. Two friction laws are considered and the influence on interface behavior is investigated: Coulomb's law with a constant friction coefficient and a slip-weakening friction law which involves static and dynamic friction coefficients. The relationship between the nonlinear harmonics and the dissipated energy, and the dependence on the contact dynamics (friction law, sliding, and tangential stress) and on the normal contact stress are examined in detail. The analytical and numerical results indicate universal type laws for the amplitude of the higher harmonics and for the dissipated energy, properly non-dimensionalized in terms of the pre-stress, the friction coefficient and the incident amplitude. The results suggest that measurements of higher harmonics can be used to quantify friction and dissipation effects of a sliding interface.     

Comparative Study of Tribological Properties of Polyphenylene Sulfide (PPS), Polyethersulfone (PES), and Polysulfone (PSU)

The tribological properties of polyphenylene sulfide (PPS), polyethersulfone (PES) and polysulfone (PSU), which have similar molecular structures, were investigated using an end-face contact tribometer and a reciprocating tribometer. The thermomechanical behavior of the polymers was analyzed using dynamic mechanical analysis (DMA). PPS exhibited a maximum friction coefficient with increasing load and sliding speed, while the friction coefficients of PES and PSU decreased only slightly. The wear rate of PPS was much lower than that of PES and PSU under high loads and speeds. It is suggested that the main factors influencing the friction and wear properties of the neat polymers are their condensed state and heat resistance. Amorphous PES and PSU showed liquid-like behavior and very low friction when the frictional surface was in the molten-flow state. The macromolecular crystals of crystallizable PPS give it some solid-like behavior and load-carrying capacity; hence PPS exhibited lower wear than PES and PSU.     

Force indeterminacy in the jammed state of hard disks

Granular packings of hard disks are investigated by means of contact dynamics which is an appropriate technique to explore the allowed force realizations in the space of contact forces. Configurations are generated for given friction coefficients, and then an ensemble of equilibrium forces is found for fixed contacts. We study the force fluctuations within this ensemble. In the limit of zero friction, the fluctuations vanish in accordance with the isostaticity of the packing. The magnitude of the fluctuations has a nonmonotonous friction dependence. The increase for small friction can be attributed to the opening of the angle of the Coulomb cone, while the decrease as friction increases is due to the reduction of connectivity of the contact network, leading to local, independent clusters of indeterminacy. We discuss the relevance of indeterminacy to packings of deformable particles and to the mechanical response properties.     

Morphological modification of the surface of polymers by the replication of the structure of anodic aluminum oxide

Anodic aluminum oxide films have been used as stamps to modify polymer materials in order to create superhydrophobic surfaces. Using polyethylene terephthalate, it has been shown that a high replication degree leads to a strong increase in the water wetting contact angle (up to 174°). However, the inverse situation is observed for the case of low replication degrees: hydrophilization of the surface occurs, which can be explained by a change of the Cassie-Baxter wetting mechanism to the Wenzel mechanism.     

Melting Heat in Radiative Flow of Carbon Nanotubes with Homogeneous-Heterogeneous Reactions

The present article provides mathematical modeling for melting heat and thermal radiation in stagnationpoint flow of carbon nanotubes towards a nonlinear stretchable surface of variable thickness. The process of homogeneousheterogeneous reactions is considered. Diffusion coefficients are considered equal for both reactant and autocatalyst.Water and gasoline oil are taken as base fluids. The conversion of partial differential system to ordinary differential system is done by suitable transformations. Optimal homotopy technique is employed for the solutions development of velocity, temperature, concentration, skin friction and local Nusselt number. Graphical results for various values of pertinent parameters are displayed and discussed. Our results indicate that the skin friction coefficient and local Nusselt number are enhanced for larger values of nanoparticles volume fraction.     

Friction and Wear of Polyphenylene Sulfide (PPS), Polyethersulfone (PES) and Polysulfone (PSU) Under Different Cooling Conditions

The friction and wear properties of polyphenylene sulfide (PPS), polyethersulfone (PES) and polysulfone (PSU), which have similar molecular structure, were investigated using an end-face contact tribometer in three different cooling ways: sliding without air cooling, sliding with air cooling, and sliding in water. The worn surface and wear debris were observed using a scanning electron microscope (SEM). The effect of frictional heat on the tribological properties of the polymers was comparatively studied. When sliding in air, with increasing applied load, the wear rate of PPS decreased slightly initially then increased later while the wear rate of PES and PSU increased through out. The results suggested that the friction coefficient was mainly affected by the temperature of the worn polymer that was controlled by the balance of heat flow of the whole sliding contact system. When sliding in water, the friction coefficients of the three polymers decreased compared to that sliding in air and remained relatively steady through the whole process under different load. The wear rates of the three polymers had a close value and, remarkably, increased compared to that sliding in air. The water cooling and lubrication role decreased the tribological properties difference between the polymers.     

Observed transition from linear to non-linear friction-load behavior using a lateral force microscope

The most commonly observed friction behavior for sliding systems is that described by Amontons laws of friction. In this case, sliding friction is independent of the gross or apparent area of contact between the materials and a linear function of the applied normal load, where the constant of proportionality is called the friction coefficient. However, for dry sliding solids in contact via a single-asperity junction, Amontons (linear) friction-load behavior is not strictly relevant. In experiments measuring sliding friction between a silicon tip and a quartz surface using an atomic force microscope (AFM), a transition from linear to non-linear friction-load behavior has been observed. This is proposed to result from a nanoscale ‘conditioning’ of a multiple-contact tip-surface interface to form a single-asperity contact.     

Polymeric microstructure arrays consequence of Marangoni flow-induced water droplets

This paper describes a cost-effective approach to fabricate intricate arrays of polydimethylsiloxane (PDMS) and polymeric microstructures based on porous polystyrene (PS) films generated from arrays of water droplets. To start, a thin layer of ethanol film is exposed to a humid air flow. Upon the evaporation of ethanol and simultaneous condensation of water as the ethanol phase recedes, a Marangoni flow causes the flow of liquid from the ethanol phase into water fingers emerged along the receding contact line, which finally detach to form ordered water droplet arrays behind the receding contact line. The water droplet arrays are subsequently used as templates to generate porous PS films. The porous PS films are then used as sacrificial layers and masters to fabricate various arrays of PDMS dots and PDMS stamps with posts, respectively. The PDMS stamps containing various microstructures are further utilized to create polymer rings, PDMS dots, porous PDMS films, and PDMS aperture rings, and for contact printing of patterns of self-assembled monolayers.     

Ultrasonic excitation affects friction interactions between food materials and cutting tools

In the food industry, ultrasonic cutting is used to improve separation by a reduction of the cutting force. This reduction can be attributed to the modification of tool–workpiece interactions at the cutting edge and along the tool flanks because of the superposition of the cutting movement with ultrasonic vibration of the cutting tool. In this study, model experiments were used to analyze friction between the flanks of a cutting tool and the material to be cut. Friction force at a commercial cutting sonotrode was quantified using combined cutting–friction experiments, and sliding friction tests were carried out by adapting a standard draw-off assembly and using an ultrasonic welding sonotrode as sliding surface. The impact of material parameters, ultrasonic amplitude, and the texture of the contacting food surface on friction force was investigated. The results show that ultrasonic vibration significantly reduces the sliding friction force. While the amplitude showed no influence within the tested range, the texture of the contact surface of the food affects the intensity of ultrasonic transportation effects. These effects are a result of mechanical interactions and of changes in material properties of the contact layer, which are induced by the deformation of contact points, friction heating and absorption heating because of the dissipation of mechanical vibration energy.     

Friction Properties of Bio-mimetic Nano-fibrillar Arrays

Nano-fibrillar arrays are fabricated using polystyrene materials. The average diameter of each fiber is about 300 nm. Experiments show that such a fibrillar surface possesses a relatively hydrophobie feature with a water contact angle of 142°. Nanoscale friction properties are mainly focused on. It is found that the friction force of polystyrene nano-fibrillar surfaces is obviously enhanced in contrast to polystyrene smooth surfaces. The apparent coefficient of friction increases with the applied load, but is independent of the scanning speed. An interesting observation is that the friction force increases almost linearly with the real contact area, which abides by the fundamental Bowden-Tabor law of nano-seale friction.     

Influence of ultrasonic surface acoustic waves. on local friction studied by lateral force microscopy

We studied dynamic friction phenomena introduced by ultrasonic surface acoustic waves using a scanning force microscope in the lateral force mode and a scanning acoustic force microscope. An effect of friction reduction was found when applying surface acoustic waves to the micro-mechanical tip-sample contact. Employing standing acoustic wave fields, the wave amplitude dependent friction variation can be visualized within a microscopic area. At higher wave amplitudes, a regime was found where friction vanishes completely. This behavior is explained by the mechanical diode effect, where the tip's rest position is shifted away from the surface in response to ultrasonic waves.     

The effect of substrate roughness on the static friction of CuO nanowires

The dependence of static friction on surface roughness was measured for copper oxide nanowires on silicon wafers coated with amorphous silicon. The surface roughness of the substrate was varied to different extent by the chemical etching of the substrates. For friction measurements, the nanowires (NWs) were pushed by an atomic-force microscope (AFM) tip at one end of the NW until complete displacement of the NW was achieved. The elastic bending profile of a NW during this manipulation process was used to calculate the ultimate static friction force. A strong dependence of static friction on surface roughness was demonstrated. The real contact area and interfacial shear strength were estimated using a multiple elastic asperity model, which is based on the Derjaguin–Muller–Toporov (DMT) contact mechanics. The model included vertical elastic flexure of NW rested on high asperities due to van der Waals force.     

Interaction potential and hopping dynamics governing sliding friction

The friction force on a nanometer-sized tip sliding on a surface is related to the thermally activated hopping of the contact atoms on an effective atomic interaction potential. A general analytical expression relates the height of this potential and the hopping attempt frequency to measurements of the velocity dependence of the friction force performed with an atomic force microscope. While the height of the potential is roughly proportional to the normal load, the attempt frequency falls in the range of mechanical eigenfrequencies of the probing tip in contact with the surface.