This paper considers the investigations into adhesion, contact mechanics metal erosion effects, wear and tare as a result of the effects of frictional forces. Mechanical components rely on friction for the transformation and delivery of energy from point A to point B. This requires the knowledge of combined energies as well as their associated dynamic models and ancillary parameters. Adhesion, contact, friction and wear are major problems limiting both the fabrication yield and lifetime of any devices. Since it is the area of real contact, which determines the sliding friction, adhesion interaction may strongly affect the friction force even when no adhesion can be detected in a pull-off experiment. Therefore a good scientific dynamic modelling of friction forces is a prerequisite for the understanding and monitoring of friction adverse effect on mechanical systems for good maintenance purposes.

In this paper the influence of initial structure of alloys of system Cu-Al-Ni on wear rate under dry friction is shown. It is established that the wear rate of alloys of system Cu-Al-Ni with martensite structure (β 1 ′-phase) is in 2–38 times less than one of standard bronze.

We analyse the physical mechanisms taking place in unlubricated friction between surface layer of pavements and a piece of rubber at low sliding speeds < 200 mm/s. This analysis is based on extensive experimental investigations of the load and velocity dependences of dynamic friction forces. The dynamical behaviour caused by dry friction is studied for a rubber-block system pulled with constant velocity over a surface layer of pavements. The dynamical consequences of a general type of phenomenological friction law (stick-time dependent static friction, velocity-dependent kinetic friction) are investigated. Three types of motion are possible: stick-slip motion, continuous sliding, and oscillations without sticking events. In the last case, we propose values of friction coefficient of different surface layer of pavements.

This paper shows the effect of ultrathin lubricant films between sliding bodies on Acoustic Emission (AE) signals induced by the sliding friction. Experiments were conducted with a ball-on-disk friction tester to measure the friction coefficient, the raw AE signals and the root-mean-squarevalues of the AE signals (the AErms signals). The ball was a glass ball of 5mm diameter. The disk was a magnetic disk used for 2.5 inch HDD with a DLC protective layer on its surface, and was coated with PFPE Z-dol 4000 about 1.5nm thick. The AErms signals kept a low level for some time after the start of the test, and then increased. Its time variation was similar to that of friction coefficient. After the friction test, the circumferential distribution of the lubricant film thickness was measured with an ellipsometer. The distribution demonstrated the reduction of the lubricant film thickness at the circumferential position where the magnitude of AE signals became large. These facts showed that the AE signals correlated well with the lubricant film thickness.

The study of the effect of waiting time, loading and unloading on static and kinetic friction for real contact micro-systems was carried out. Three types of silicone fluid with the viscosity of 100, 450 and 5000 cSt at 25° C were used. Stop-start experiments allowed us to estimate the relaxation time after the static friction overshoot. For silicone liquid with viscosity of 100 cSt, relaxation of friction force ( F ) after the static friction overshoot occurred over one cycle of testing. Relaxation of F for fluids with the viscosity of 450 and 5000 cSt occurred during a long time and this effect was opposite for these silicone liquids. The analysis of loading-unloading cycles showed only a partial reversibility of F . The results were compared with static friction and stick-slip data obtained in other works, using SFA and FFM techniques. In order to explain the effect of viscosity and structure of the lubricant layers on stick-slip phenomenon, interior and wall slip of the lubricant film is discussed.

Electro-pulse spraying (EPS) is the coating technology of “electric explosion of conductive materials” when high-voltage and powerful impulse flows through a wire conductor. Object of our investigation — tribological properties of sliding pairs with copper micro-coats made by EPS after one time explosion. Small-grained dense structure coat with evaluated thickness about 4–6 ?m was obtained. Tribological tests, performed at marginal lubrication with multi-stage load, shows that using EPS-specimens the value of friction coefficient is lower as control version. At instantaneous setting of load for long-term running the copper films adopts well to the change of load. The wear of friction pairs according to worn mass show that EPS-specimens worn 79% less than CV-specimens. The investigations point out that copper micro-coats have better tribological properties comparing to control version of friction pairs.

In a typical tribological research vibration is only considered, when it is judged as excessive, otherwise it is not regarded as the key issue. Still even a small amplitude vibration can disturb the desired input load and sliding velocity in a test. Tribological tests carried out without the analysis of the accompanying vibration are always threatened by vibration related erroneous interpretation of results. It must be emphasised, that a test stand is a dynamic mechanical system in which friction is evoked for experimental reasons and that friction interacts with the entire system just as in real life machine.

The atomistic mechanisms are proposed for the origin of the static and the dynamic friction forces. The mechanism for the origin of the static friction force resembles the mechanical locking mechanism in a surface roughness model. The origin of the dynamic friction force is formulated as a problem of how the given translational kinetic energy dissipates into the internal relative motions of constituent atoms of bodies during sliding. From studying that the available phase space volume of the translational motion becomes negligible small for a large system size, compared with that of the internal motions, it is concluded that the energy dissipation occurs irreversibly from the translational motion to the internal motions. A phenomenon of superlubricity, where two solid bodies move relatively with no resistance, is discussed.

Understanding friction and wear of biomaterials when in contact with articular cartilage is vital within the development of future hemi-arthroplasty and cartilage substitution. This study aimed to compare the frictional properties of single phase and biphasic polymeric materials against articular cartilage. Continuous sliding friction was applied by means of a simple geometry wear simulator. The single-phase polymers produced peak frictional values of 0.37(±0.02). The biphasic hydrogel produced a peak frictional coefficient of 0.17(±0.05). It is postulated that this reduction in friction can be attributed to its biphasic properties, which instigates the fluid phase load carriage within the articular cartilage/hydrogel interface to be maintained for longer, reducing the frictional coefficient. This study illustrates the importance of biphasic properties within the tribology of future cartilage substitution materials.

Latest development in high-power lasers made possible a wide variety of laser surface modifications. Such surface modifications include: glazing, shock peening, alloying, cladding and texturing. The main reason behind applying these treatments is to improve the tribological performance of the modified surfaces. In addition to reducing friction and wear, it is favorable to improve the scuffing resistance. Scuffing can be defined as a sudden catastrophic failure of a lubricated sliding surface characterized by a sudden rise in friction; resulting in severe surface damage through localized plastic flow. This paper investigates friction and scuffing performance of laser glazed 1080 steel and laser textured H13 stainless steel. Results showed that laser glazed surfaces reduced sliding friction under dry conditions by approximately 35% and improved wear resistance. In addition, laser glazed surfaces showed high resistance to scuffing compared to unglazed surfaces. Also, laser surface texturing technique reduced sliding friction under lubricated conditions and improved scuffing resistance.

An experimental study is performed on micro textured surfaces using both elastic and Newtonian fluids in order to understand the effect of fluid rheology on the sliding friction of textured surfaces. Nickel micro textured surfaces containing 50 to 125 micron diameter pores and 1 to 20 percent area densities of pores are friction tested using a triborheometer, and the resulting data is examined using a Stribeck diagram. This research reveals that the Stribeck curve will experience a shift according to the lubricant elasticity, and that shift is independent of the texturing parameters. By modifying the Gumbel number to form a new dimensionless parameter, the effect of lubricant elasticity is normalized for all surfaces.

The behavior and life of a tilted flat thrust washer bearing is modeled by a comprehensive numerical code. The goal is to investigate the conditions that distress thrust washer bearings through numerical techniques. This work includes thermo-mechanical deformations (which have been neglected in previous studies). The thrust washer bearing supports non-axisymmetric loads within the planetary gear sets of automatic transmissions and consists of flat-faced washers placed between an idle helical gear and its contacting face. Various coupled numerical schemes model sliding friction, boundary lubrication, asperity contact, thermo-mechanical deformation, thermo-viscous effects, and full film lubrication. The model provides predictions of frictional torque, bearing temperature, hydrodynamic lift and other indicators of bearing performance. The experimental and numerical results show that the bearing is very susceptible to the mechanism of thermoelastic instability (TEI). Theoretical predictions indeed predict that the washer may operate in the range of TEI.

Studies of the fundamental origins of friction have undergone rapid progress in recent years with the development of new experimental and computational techniques for measuring and simulating friction at atomic length and time scales. The increased interest has sparked a variety of discussions and debates concerning the nature of the atomic-scale mechanisms, which dominate the dissipative process by which mechanical energy is transformed into heat. Measurements of the sliding friction of physisorbed monolayers and bilayers provides information on the relative contributions of electronic to phononic.

A micro-tribotester has been designed and fabricated to evaluate friction and wear on lateral contact surfaces of silicon materials. In the design of the tribotester, FEM analysis was used to find the resonant frequency and elastic displacement of the device under electrostatic excitations. The micro-tribotester was fabricated with a standard bulk silicon process. Static and dynamic friction tests were done on the fabricated device under various DC and AC stimulation conditions. From the measured maximum deflections of the detecting beams in the loading and sliding directions, static and dynamic friction coefficients of contacting sidewalls have been obtained. Characteristics of tribological behaviors in MEMS were discussed.

Low friction is an important and desirable attribute for a number of boundary lubricated systems. We ask the question what are the most promising strategies to attain low friction with nanometer thick lubricant films. Is it the structure or the dynamics of the boundary layers, or, maybe both? Using the extended surface forces apparatus we are investigating structural and dynamic effects on friction — in some cases at sub-Ångstrom resolution. Results obtained with different molecular systems exhibiting low friction are presented. A water-soluble, PEG containing co-polymer architecture was used to construct a water-based lubrication system. A high-resolution measurement of the compression isotherm revealed film-thickness transitions under confinement that are due to a water-induced restriction of the conformational space of PEG. These findings are in accord with known solution properties of this polymer. The friction is found to be vanishingly small in a dilute aqueous solution of this co-polymer. Using an alternative approach, the active control of static and dynamic friction was demonstrated using sub-nanometer mechanical oscillations across a nanometer thick liquid layer. The dynamic frustration of molecular arrangements is responsible for this effect. Oscillations of just a few Angstrom are sufficient to reduce friction to a small fraction of the unperturbed system. Molecular film-thickness changes were monitored throughout the contact zone and the effect of the oscillations on the molecular dynamics was measured.

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.

Superlubricity can only be achieved if the intra-bulk elastic interactions dominate interfacial shear forces at every single length-scale in a contact. Otherwise, there will be some type of plucking motion which will lead to friction-velocity relationships akin of Coulomb’s friction law. For nominally flat surfaces, it has been predicted theoretically and demonstrated experimentally that plucking motion and hence kinetic friction can be avoided under certain circumstances. We present theoretical arguments, why these findings may extend to fractal surfaces. The theories are checked by molecular dynamics simulations. It turns out that the roughness exponent and the absolute magnitude of the roughness both play a crucial role in determining whether there can be superlubricity.

A new method named as energy principle method is used to calculate and research the friction force and friction coefficient of two flat elastic surfaces. The studies reveal the relationships between friction coefficient and parameters of a tribo-system such as microstructure of interface, performance of material, operational parameters and geometry parameters. When the height of an asperity increases in the model, the friction coefficient increases rapidly to a critical value, and then hardly increases after that.

The objective of this work is to analyse the influence of interfacial properties on the sliding friction of polymers. In order to change the nature of interface, two types of smooth and rigid substrates are used: a hydrophilic silicon wafer (hydroxylated by a piranha treatment) and a hydrophobic silicon wafer, obtained by a chemical grafting with a CH 3 terminated silane. Both substrates exhibit identical stiffness and roughness and differ only by their surface chemical composition. Two different polymers are studied: crosslinked polydimethylsiloxane, PDMS (soft elastomer with a low glass transition temperature) and polystyrene (rigid thermoplastic polymer with a high glass transition temperature). Friction experiments are performed with a translation tribometer for different normal forces and friction speeds.

Dry sliding wear tests were performed using a ring-on-block configuration for copper sliding against steel. The wear mechanisms of this pair were investigated under different load ranges and sliding velocities. During the tests, the bulk temperature in the near-surface contact region was continuously recorded. The results of scanning electron microscope (SEM), energy dispersive X-ray spectrometry (EDS) and nanoindentation tests on the worn surfaces and the debris reveal a commanding role of the steel in the wear mechanisms of the pair. Additional tests of steel-steel pairs corroborate this assumption. It will be shown that the different wear mechanisms, which the steel experiences over the test range, markedly influence copper wear rates. Metallographic investigations on cross sections of copper samples show the formation of recrystallized layers on the worn surface at high sliding speeds. The onset of recrystallization is accompanied by extrusion of wear debris.