The Friction Reducing Properties of Molybdenum Dialkyldithiocarbamate Additives: Part I — Factors Influencing Friction Reduction

Abstract

Calcium sulfonate complex greases have excellent extreme pressure, antiwear, and anticorrosion properties and are widely applied in rolling bearings, particularly in humid environments. In this article, the shear stability of dry and water-contaminated lubricating calcium sulfonate complex greases is described using a novel aging method. Unlike lithium and polyurea greases, no shear softening is observed for the dry greases due to the good mechanical stability of the particle-like thickener structure. For water-contaminated greases, no water separation was found during the prolonged aging. Instead, a homogeneous water–calcium sulfonate thickener micellar structure is generated. These micelles function as apparent thickeners and effectively increase the thickener concentration, which thickens the grease. This may explain why calcium sulfonate complex grease has excellent water absorption properties.     

Observations of Stick-Slip Friction in Velcro®

Friction, and in particular stick-slip friction, occurs on every length scale, from the movement of atomic force microscope tips at the nanoscale to the movement of tectonic plates of the Earth’s crust. Even with this ubiquity, there still appears to be outstanding fundamental questions, especially on the way that frictional motion varies generally with the mechanical parameters of a system. In this study, the frictional dynamics of the hook-and-loop system of Velcro^® in shear is explored by varying the typical parameters of driving velocity, applied load, and apparent contact area. It is demonstrated that in Velcro^® both the maximum static frictional force and the average kinetic frictional force vary linearly with apparent contact area (hook number), and moreover, in the kinetic regime, stick-slip dynamics are evident. Surprisingly, the average kinetic friction force is independent of velocity over nearly two-and-a-half orders of magnitude (~2 × 10^?4 to ~6 × 10^?2 m/s). The frictional force varies as a power law on the applied load with an exponent of 0.28 and 0.24 for the maximum static and kinetic frictional forces, respectively. Furthermore, the evolution of stick-slip friction to more smooth sliding, as controlled by contact area, is demonstrated by both a decrease in the spread of the kinetic friction and the spread of the fluctuations of the average kinetic friction when normalized to the average kinetic friction; these decreases follow power-law behaviors with respect to the increasing contact area with exponents of approximately ?0.3 and ?0.8, respectively. Lastly, we note that the coefficients of friction μ _s and μ _k are not constant with applied load but rather decrease monotonically with power-law behavior with an exponent of nearly ?0.8. Phenomenologically, this system exhibits interesting physics whereby in some instances it follows classical Amontons–Coulomb (AC) behavior and in others lies in stark contrast and hopefully will assist in the understanding of the friction behavior in dry surfaces.     

Friction in metal machining—Mechanical aspects

A review is presented concerning the mechanical aspects of friction at the land and rake faces of a cutting tool when machining under orthogonal conditions. Methods are described for determining the mean frictional and normal forces on these faces and calculating the mean coefficients of friction.Considerable confusion exists concerning the nature of the frictional conditions in the region of the tool land to the point where definitive conclusions cannot be drawn. However, it is suggested that sliding and subsurface plastic deformation may occur simultaneously.When machining under dry, unlubricated conditions regions of sticking friction where the frictional stress is constant and sliding friction where the coefficient of friction is constant can exist simultaneously on the tool rake face. Theories which predict a linear relationship of the form (φ) = A ? B (β?α) between the angle parameters used to describe the geometry of the cutting process cannot be used to interpret much of the available experimental data. The apparent linear relationships observed in some previous experimental work are thought to be due partly to the method of presentation of the results. It is suggested that a more significant combination of the angle parameters is the difference between the shear angle φ and rake angle a, that is (φ?α). Two parameters which can be used to describe the frictional conditions at the tool rake face are the mean frictional stress and mean normal stress which can vary independently of each other. It is concluded that the mean angle of friction β is insufficient in itself to describe the frictional conditions on the tool rake face.The effects of changes in cutting conditions on the mean coefficient of friction at the tool rake face are examined and the results interpreted in terms of the associated changes in the mean normal and mean frictional stresses in that region. The lubricating action of cutting fluids is discussed briefly.     

WEC Formation in Rolling Bearings under Mixed Friction: Influences and “Friction Energy Accumulation” as Indicator

White etching cracks (WEC) can lead to premature rolling contact fatigue. Possible drivers of WEC can be mixed friction, sliding between rolling elements and raceways, electrical current, critical additives, and water-contaminated lubricant. With respect to WEC failures induced by sliding between rolling elements and raceways under mixed friction, an approach is presented that can explain the experimentally observed failure characteristics of cylindrical roller thrust bearings. Variants of the bearing were tested using a WEC-critical lubricant. The tests showed that not only the contact pressure and sliding between rolling elements and raceways but also the lubrication conditions (specific film thickness) and the frequency of the contact load cycles have an influence on WEC life. These influences are reflected best by a newly introduced characteristic parameter termed friction energy accumulation. As far as WEC failures induced by sliding under mixed friction are concerned the friction energy accumulation could be used for a comparative assessment of the WEC risk of arbitrary rolling bearing applications. A link between the friction energy accumulation and the absorption of hydrogen is discussed and can provide further explanations for the susceptibility of bearing components to WEC formation.     

Prandtl–Tomlinson-Type Models for Coupled Molecular Sliding Friction: Chain-Length Dependence of Friction of Self-assembled Monolayers

Tribology Letters - Most asteroids with a diameter larger than $$sim 300 {mathrm{m}}$$ are rubble piles, i.e., consisting of more than one solid object. All asteroids are rotating but almost...     

Nanoscale Friction Measurements Up to 750 °C

A new experimental capability for elevated temperature nanoscale friction measurement is described. Its stability and resolution were demonstrated in two case studies up to 750 °C. A stainless steel probe was used to study friction between steel and glass at 25, 200 and 400 °C. Friction forces were calibrated at temperature. The friction coefficient increased between 25 and 200 °C, but stick–slip was dominant at 400 °C due to chemical interaction between the stainless steel probe and the glass. This was verified by scanning Energy Dispersive X-ray Spectroscopy analysis. A WC–Co probe was used to study friction on a range of TiN-based and Cr₅₄Al₂₀C₂₆ (so named MAX-phase composition) coatings at 25, 400 and 750 °C. A maximum in friction coefficient was observed at 400 °C. The decrease in friction at 750 °C was associated with the formation of lubricating surface oxides and oxidation-associated surface roughening.     

Multiscale Surfaces and Amontons’ Law of Friction

The theories of Archard, Greenwood, and more recently Persson all predict that the area of actual contact between two rough surfaces will be approximately proportional to the applied normal force, though these authors make different assumptions and approach the problem from different points of view. Here we discuss the nature of these approximations and show that the common conclusion follows from the multiscale nature of the surface profile. In particular, it is shown that whatever assumption is made about the nature of friction on the microscale, the macroscale frictional behaviour will always approximate Amontons’ law if the surface has sufficient multiscale content.     

Friction behaviour evaluation in beryllium–copper threaded connections

Beryllium–copper alloys present a combination of high tensile and compressive strength, high hardness, good thermal conductivity and high resistance to seizing and galling (conditions where metals begin to cold weld or bond under the joint influence of compressive stress and relative motion at temperatures far below the melting temperature). These properties make beryllium–copper an excellent choice for bearing and bushing applications. Objective of this paper is to report the results of experimental tests carried out in order to evaluate the friction behaviour of threaded joints with at least one element made of Cu–Be; the other element is made of Cu–Be, steel or aluminium. The analysis of the results enabled us to establish that a threaded connection where an element is made of Cu–Be and the other one of steel, dry or at most lubricated by mineral oil, shows very encouraging frictional properties and can be considered as a good substitute for the usual joints made of steel and lubricated with environmental dangerous compounds.     

Fast Fourier Transform Analysis of Computer-Acquired Friction Data©

A commercial computer program for data acquisition and analysis has been used to facilitate the evaluation of the frequency content of friction force data obtained from a sliding wear test. The program uses a fast Fourier transform (FFT) algorithm to calculate the frequency spectrum of the friction force. The validity of the FFT program has been demonstrated by applying it to a recently published example that used a detailed mathematical treatment to remove an oscillatory component from friction data. When applied to friction force data from a pin-on-disk sliding wear test of NBD 200 silicon nitride sliding on NBD 100 silicon nitride, filtering of the dominant frequencies revealed by the FFT method reduces both the general dispersion of the friction values and almost completely eliminates localized signal perturbations. Successive filtering steps permit the association of particular frequencies with observed signal disturbances. Although the mean friction value in this example is not greatly altered by the frequency filtering, knowledge of the existing vibration frequency components in the tribosystem is important for a complete interpretation of wear mechanisms and for characterization of the machine/specimen interaction.     

Advanced Tribometer for In Situ Studies of Friction, Wear, and Contact Condition—Advanced Tribometer for Friction and Wear Studies

An advanced ball-on-disk tribometer was developed for in situ studies of friction, wear, and contact condition during sliding. Kinetic friction force, contact resistance (R _c), acoustic emission (AE), ball position perpendicular () to the plane of the disk (ball and disk wear), and disk surface reflectance (disk wear) were all measured simultaneously during sliding experiments. Metal (440C steel) balls were slid against ceramic (n-doped polysilicon) wafers at light load (10g) and short test duration (2.5min). Significant changes in measured parameters were observed as sliding progressed. These changes are discussed, and when considered together provide new insights into friction and wear mechanisms not readily obtainable from more standard tribometers. The effects of disk run-out (effective surface waviness) on and R _c were also investigated. Friction and R _c were periodic with a period equal to the period of disk rotation. The behavior was complex, but generally going up a hill increased and decreased R _c, with the opposite behavior going down a hill. We established a critical link between low-frequency friction oscillations (LFFO) and the nature of the contact between sliding surfaces (R _c measurement). The geometric ratchet mechanism was ruled out as a cause of LFFO, as the surface slope was too small to explain the large friction oscillations. Coating the balls and wafers with lower friction materials resulted in negligible LFFO, which makes it unlikely that LFFO were simply a result of an oscillating normal force created by dynamic effects. LFFO likely have their origins in the complex nature of the contact between rubbing surfaces.     

Viscoseal—Pressure Generation and Friction Loss under Turbulent Conditions

A new theory is developed that predicts the sealing coefficient and the friction factor of a viscoseal under turbulent flow conditions. This theory, being a synthesis of Boon and Tal's laminar viscoseal theory and Ng, Pan, and Elrod's turbulent lubrication theory, shows very satisfactory correlation with available experimental data. The results have been given in simplified formulas such that they are readily usable for design purposes. At high Reynolds numbers the seal geometry has been optimized, although performance near the optimum is rather constant for rather different values of the geometrical parameters. Existing theories on the turbulent behavior of the viscoseal are briefly discussed.     

The Mathematical Simulation and Study of the Electrical Resistance of the Friction Zone of the Hip Joint Endoprosthesis with a Metal–Metal Friction Pair

The paper has described a model of the static load on the hip joint that takes into account the anthropological parameters and a mathematical model of the change in the resistance of the endoprosthesis under the influence of an external load at different rotation angles of the cup component. The theoretical studies have revealed the character of the changes and assessed the possible ranges of variations in the diagnostic parameter that are required to develop diagnostic equipment and methods for testing and interpreting diagnostic information during the tribotesting of individual types of implants.     

Hydrodynamic Friction Reduction in a MAC–Hexadecane Lubricated MEMS Contact

Recent research has shown that hydrodynamic lubrication is an effective means of reducing friction in high sliding micro-electromechanical systems (MEMS). At high speeds, however, such lubrication can lead to increased friction due to viscous drag. This article describes a series of hydrodynamic tests on a silicon MEMS contact lubricated with a blend of hexadecane and a multiply-alkylated cyclopentane (MAC). Results show that the presence of the MAC reduces hydrodynamic friction compared with neat hexadecane. Such behaviour is contrary to conventional hydrodynamic theory, since the viscosity of the MAC—a mixture of di- and tri-(2-octyldodecyl)-cyclopentane—is significantly higher than that of neat hexadecane. This effect increases with MAC concentration up to an optimum value of 3 wt%, where the hydrodynamic friction coefficient at 15,000 rpm is reduced from 0.5 to 0.3. Above this concentration, friction begins to rise due to the overriding effect of increasing viscosity. The viscosity of the blended lubricant increased monotonically with MAC concentration, when measured using both a Stabinger and an ultra-high shear viscometer. In addition to this, no reduction in friction was observed when a squalane–hexadecane blend of equal viscosity was tested. This suggests that some property of the MAC–hexadecane lubricant, other than its viscosity, is influencing hydrodynamic lubrication. A tentative explanation for this behaviour is that the MAC induces the liquid to slip, rather than shear, close to the silicon surfaces. This hypothesis is supported by the fact that the friction reducing ability of the MAC blend was inhibited by the inclusion of octadecylamine—a substance known to form films on silicon surfaces. Furthermore, the MAC reduces friction in the mixed regime, in a manner suggesting that the formation of a viscous boundary layer. This unusual behaviour may have useful implications for reducing hydrodynamic friction in liquid-lubricated MEMS devices.     

Friction of molybdenum disulfide–titanium films under cryogenic vacuum conditions

The tribological behavior of steel and sapphire sliding on a sputtered MoS₂+Ti coating was studied in ultra-high vacuum as a function of temperature over the range of 4–300 K. The coefficient of kinetic friction for the steel/moly interface was determined to be approximately 0.05 from room temperature to 240 K, and increased monotonically to 0.125 at 4 K. The sapphire/moly friction coefficient was measured to be 0.15±0.05 at room temperature and increased monotonically to 0.25 at 4 K. We also analyze in detail the flash temperature due to frictional heating at the sliding contacts. Flash heating is a particularly strong effect at cryogenic temperatures.     

表面织构(surface texture)

surface texture,在产品几何量技术规范（GPS）表面结构系列标准中有这词.ISO 4287：1997《产品几何量技术规范（GPS）表面结构：轮廓法 表面结构的术语、定义及参数》中没有给出surface texture的定义.     

Analysis of Physical Friction Characteristics about Handwrite Feeling—Rough and Smooth

Writing was an important factor in human society since long time ago. And previous studies indicate many consumers evaluate pens in handwriting feeling above all other factors when they select recently pens. The fact they work always grabbed in hands considered, the emotional design of writing tools is essential. However, the feeling such as handwriting feelings is gradually disappearing due to the development of mobile devices. So we thought that the need for an engineering approach. The technology of the sense of touch belongs to the technology of the sensibility field, but it is positively necessary to objectify the sense of touch in order to develop new technologies or products by expressing this numerically and applying it to engineering. In order to objectify the sense of touch, the variables (the pressure, temperature, texture of material, etc.) of the friction when the stylus pen first touches the surface of the object should be digitized. And static and dynamic friction models should be established and the variables that affect the friction behavior should be organized. This study investigated the handwriting feeling of various writing tools and attempted to use it for the development of new mobile devices in the future through physical friction test. This study investigated the handwriting feeling of various writing tools and attempted to use it for the development of future mobile devices through physical friction test. Using developed model which can explain how the experimental parameters and emotional user experience, developer can design their product which can give user expected emotional sensibility.     

Predicting surface quality of γ-TiAl produced by additive manufacturing process using response surface method

Electron beam melting (EBM) has been found to be a promising technology for producing complex shaped parts from gamma titanium aluminide alloys (γ-TiAl). The parts produced by this process are projected to have dimensions very close to the desired final shapes. However, the surface roughness of the parts produced by EBM is excessively rough. In many applications, it is necessary to improve the quality of manufactured parts using a convenient post process. This paper determines process parameters of end milling when it is used as a post process for the parts produced by EBM. Design of experiments has been used to study the effect of the selected input parameters of end milling (spindle speed, feed rate, depth of cut and coolant type) on the surface roughness of γ-TiAl parts. Response surface methodology is used to develop a predictive model for surface roughness. Effects of the selected milling process are investigated. This paper also optimizes the selected process parameters to minimize the value of the obtained surface roughness.     

Non‐Contact Evaluation of Osmosis‐Induced Shrinkage and Swelling Behavior of Articular Cartilage In‐Situ Using High‐Frequency Ultrasound

Abstract

The aim of the present study was to use high‐frequency ultrasound for the investigation of the transient osmosis‐induced free shrinkage‐swelling behaviors of normal articular cartilage in situ. Full‐thickness cartilage‐bone specimens were prepared from normal bovine patellae. The transient shrinkage and swelling strains of the cartilage induced by changing the bathing solution between physiological saline (0.15 M) and hypertonic saline (2 M) were monitored using a 50 MHz focused ultrasound beam. Both shrinkage and swelling strains showed temporary overshoots, followed by relaxation phases. The absolute peak value of the shrinkage strain (1.01%±0.62%) was significantly larger (p<0.05) than that of the swelling strain (0.40%±0.33%). It was found that the change of the mean ultrasound speed in cartilage could be approximately represented by an exponential function of time after the change of saline concentration.

This study successfully demonstrated that a high‐frequency focused ultrasound beam could be used to monitor the transient osmosis‐induced deformation of articular cartilage in a non‐contact way. Since the osmosis‐swelling behavior of cartilage relates to its compositional and structural characteristics and degeneration status, the reported ultrasound method may have potential for the characterization of cartilage degeneration, such as osteoarthritis.     

Hysteresis Behavior in the Stick–Slip Mode at the Boundary Friction

A tribological system is considered that consists of two atomically smooth solid surfaces separated by an ultrathin lubricant film. A thermodynamic model based on the Landau theory of phase transitions is built that describes the behavior of this system in the boundary friction mode. The free energy density for an ultrathin lubricant film is given in the form of expansion in series in terms of the powers of order parameter that is reduced to the shear modulus of the lubricant. The kinetics of the system is studied on the basis of a model describing first-order phase transitions between kinetic modes of friction. It is shown that in the presence of spring between the external drive and block the width of temperature hysteresis increases versus fixed coupling.