Variation with load of the coefficient of friction and metallic transfer under conditions of boundary lubrication

1952 ◽  
Vol 212 (1111) ◽  
pp. 516-519 ◽  
Keyword(s):  
Coefficient Of Friction ◽  
Boundary Lubrication ◽  
General Pattern ◽  
Copper Plate ◽  
Experimental Conditions ◽  
Average Hardness ◽  
Boundary Lubricant ◽  
The Coefficient Of Friction ◽  
Fixed Load ◽  
Molecular Layers

Several papers have been published recently which show for a number of metals that, under certain experimental conditions, several molecular layers of boundary lubricant are necessary to give effective lubrication, i.e. a coefficient of friction of 0.1 or less. Recent experiments by the authors suggest that these results represent parts of a more general pattern. In the previous work, the experiments were carried out at a fixed load; in the present, the load has been varied. The apparatus used was a copy of that described by Bowden & Leben (1939) for studying friction at low speeds of sliding. A hemispherical copper slider of radius 0-45 cm was caused to slide upon a flat copper plate at a speed of approximately 1 cm/min. Several specimens were used; their average hardness was 100 v .p .h .

The friction of clean crystal surfaces

1966 ◽  
Vol 295 (1442) ◽  
pp. 233-243 ◽  
Keyword(s):  
Coefficient Of Friction ◽  
High Surface ◽  
High Vacuum ◽  
Surface Films ◽  
Surface Adhesion ◽  
Crystal Surfaces ◽  
Deformation And Fracture ◽  
The Coefficient Of Friction ◽  
Molecular Layers ◽  
Frictional Behaviour

A study is made of the frictional behaviour of crystals (diamond, magnesium oxide, sapphire) sliding on themselves in high vacuum (10 -10 torr). The surface films normally present on these crystals are very tenacious but they may be worn away by repeated sliding in the same track. Under these conditions the friction of the clean crystals may increase by a factor of ten so that the coefficient of friction may rise to μ ≈ 1. The frictional rise is limited because of the elastic and brittle behaviour of the contact regions. Under these conditions subsurface deformation and fracture of the crystal occurs and this, combined with the high surface adhesion, causes pronounced wear. Adsorption of a few molecular layers of gas can again reduce the friction to a low value. The results are relevant to the operation of bearings and to the wear of surfaces in space.

Analysis of the Contribution of Adhesion and Ploughing to Shoe-Floor Lubricated Friction in the Boundary Lubrication Regime

2011 ◽  
Author(s):  
Caitlin Moore ◽  
Kurt Beschorner ◽  
Pradeep L. Menezes ◽  
Michael R. Lovell
Keyword(s):  
Coefficient Of Friction ◽  
Boundary Lubrication ◽  
Ambient Conditions ◽  
Lubrication Regimes ◽  
Lubrication Regime ◽  
Material Hardness ◽  
Lubrication Layer ◽  
The Coefficient Of Friction ◽  
Dry Conditions ◽  
Pin On Disk

Slip and fall accidents cost billions of dollars each year. Shoe-floor-lubricant friction has been shown to follow the Stribeck effect, operating primarily in the boundary and mixed-lubrication regimes. Two of the most important factors believed to significantly contribute to shoe-floor-lubricant friction in the boundary lubrication regime are adhesion and ploughing. Experiments were conducted using a pin-on-disk tribometer to quantify adhesion and ploughing contributions to shoe-floor friction in dry and lubricated conditions. The coefficient of friction between three shoe materials and two floor materials of different hardness and roughness were considered. Experiments were conducted under six lubricants for a sliding speed of 0.01 m/sec at ambient conditions. It was found that the contribution of adhesion and ploughing to shoe-floor-lubricant friction was significantly affected by material hardness, roughness, and lubricant properties. Material hardness and roughness are known to affect adhesion, with increased hardness or increased roughness typically resulting in decreased adhesion. The smoothest shoe material, while also being the hardest, resulted in the greatest adhesional contribution to friction. The roughest material, while also being the softest, resulted in the lowest adhesional contributions under dry conditions. Canola oil consistently resulted in the lowest percent of full adhesion and water consistently resulted in the highest percent of full adhesion, presumably due to the thickness, of the boundary lubrication layer. Ploughing contribution was dependent upon the hardness of the shoe and floor materials. A positive correlation was found between the shoe and floor hardness ratio and ploughing coefficient of friction.

Metallic transfer between sliding metals: an autoradiographic study

1951 ◽  
Vol 208 (1095) ◽  
pp. 455-475 ◽  
Keyword(s):  
Coefficient Of Friction ◽  
Photographic Plate ◽  
Frictional Resistance ◽  
Autoradiographic Study ◽  
Lubricant Film ◽  
Small Range ◽  
Main Function ◽  
Boundary Lubricant ◽  
The Coefficient Of Friction ◽  
Higher Temperature

This paper describes a study of the friction and metallic transfer between sliding metal surfaces in the absence and in the presence of boundary lubricant films. One surface is made radioactive and is slid over the surface of a second, non-radioactive metal, the amount of metal transferred being detected by the blackening of a photographic plate placed in contact with the second surface. The results show that, in general, the metallic transfer or ‘pick-u p ’, consists of a relatively small number of discrete particles. For unlubricated surfaces the pick-up is about 40 times larger for similar than for dissimilar metals, although the coefficient of friction covers a relatively small range (μ ≈ 0.4 to μ ≈ 1). With well-lubricated surfaces the friction is reduced by a factor of not more than 20 (μ ≈0.05), whilst the ‘pick-u p ’ may be diminished by a factor of 20,000 or more. A simple analysis suggests that under these conditions the welded metallic junctions formed through the lubricant film play a very small part in determining the frictional resistance to motion. Consequently two lubricants possessing widely differing abilities to protect the surfaces may give essentially the same coefficient of friction. The lowest friction and ‘pick-up ’ are observed when the lubricant film is solid. As the temperature is raised a marked increase in friction and ‘pick -u p ’ occurs a t a temperature close to the melting-point of the film. A new observation is that at a somewhat higher temperature a further deterioration in lubricating properties occurs; although the surfaces are visibly covered with lubricant, the frictional behaviour and the metallic transfer are similar to those observed with unlubricated surfaces. These changes are reversible on cooling, and it is suggested that they correspond to changes in state of the lubricant film. The results provide direct support for the view that the friction between metals is due largely to the formation and shearing of metallic junctions, and that the main function of a boundary lubricant is to reduce the amount of metallic interaction. The investigation also shows that the metallic transfer is immensely more sensitive to changes in surface conditions than is the coefficient of friction.

scholarly journals Tribo-electricity and friction

1926 ◽  
Vol 111 (758) ◽  
pp. 339-355 ◽  
Keyword(s):  
Molecular Weight ◽  
Organic Compound ◽  
Coefficient Of Friction ◽  
Solid Surface ◽  
Boundary Lubrication ◽  
Exposed Surface ◽  
Flexible Materials ◽  
The Coefficient Of Friction ◽  
Glass Surfaces ◽  
Frictional Effects

This paper deals with the changes in the condition of glass surfaces when rubbed with flexible materials, chiefly cotton, linen, and silk, of varying purity. The condition is ascertained by two different, but associated, properties :— ( a ) The electric charges displayed by the glass surfaces when rubbed on textiles or on one another. ( b ) The coefficient of friction found between two of them after being subjected to identical rubbing. We have derived great help in the friction work both as regards theory and technique from the papers on “Boundary Lubrication,” by Sir William Hardy and his colleague (1, 2, 3 4). A solid surface may be modified in two distinct ways :— (1) By Addition of Material .—A solid surface exposed to an atmosphere con­taining water (or other) vapours, condenses and retains these; or if solid or liquid matter be rubbed on the surface, this will in general acquire and retain a covering film. The actual exposed surface is thus a complex of various materials, so that tribo-electric and frictional effects are complicated. We know that the friction of the surface is dependent on the character of the condensed film and on that of the solid underlying the film, according to the expression given by Sir William Hardy and Miss I. Doubleday (2), μ = b — a M, where M is the molecular weight of a condensed pure organic compound, a and b being parameters.

scholarly journals The influence of temperature on boundary lubrication

1942 ◽  
Vol 181 (984) ◽  
pp. 23-42 ◽  
Keyword(s):  
Oleic Acid ◽  
Coefficient Of Friction ◽  
Boundary Lubrication ◽  
Surface Film ◽  
Adsorbed Layer ◽  
Surface Measurement ◽  
Polar Group ◽  
The Coefficient Of Friction ◽  
Low Speeds ◽  
Frictional Behaviour

The frictional behaviour between mild steel surfaces lubricated with excess of pure hydrocarbons, ketones, alcohols, amides, acids and esters has been investigated at low speeds and under high loads. In all cases a transition from smooth sliding to irregular stick and slip motion takes place at a temperature characteristic of the lubricant employed. Experiments in which lubricant films one or more molecules thick were built up by the Langmuir-Blodgett technique have shown that the transition from smooth sliding to stickslips occurs when the adsorbed surface film of lubricant breaks down and becomes disoriented. Acids and esters are shown to be strongly adsorbed, while hydrocarbons, ketones, alcohols and amides are not appreciably adsorbed. It is shown that adsorption of acids and esters occurs by the interaction of the dipoles in their polar group with the metal atoms in the surface. The results also suggest that molecules of long-chain compounds are oriented on a metal surface in the same way as they have been shown to be arranged on an aqueous surface. Measurement of the coefficient of friction between surfaces lubricated with films one and many molecules thick has shown that under conditions of ‘ boundary lubrication ’ prevailing at high loads and low speeds, excess of lubricant is squeezed out, and lubrication is effected by a unimolecular film adsorbed on each surface. The variation of the coefficient of friction with load in the case of oleic acid shows that orientation with this lubricant extends beyond the primary adsorbed layer. This result accounts for the low values of the coefficient of friction obtained by previous workers, and explains the good lubricating properties of oleic acid. These experiments show that a study of the frictional behaviour provides a m ethod of investigating the properties of surface films on metals.

Experimental Investigation of Porous Bronze Bearings

1973 ◽  
Vol 95 (2) ◽  
pp. 173-179 ◽  
Author(s):  
C. Cusano ◽  
R. M. Phelan
Keyword(s):  
Experimental Study ◽  
Experimental Investigation ◽  
Coefficient Of Friction ◽  
Boundary Lubrication ◽  
Operating Conditions ◽  
The Coefficient Of Friction ◽  
Theoretical Analyses ◽  
Lubrication Conditions

An experimental study was made of the performance of porous bronze bearings under different operating conditions. A PV value of 50,000 psi ft/min was found to be too high for the assembly used when the bearings were lubricated only by the oil initially provided within their structure. Tests at a PV value of 33,000 psi ft/min gave satisfactory results. The coefficient of friction was found to vary with load and to be almost independent of speed for the bearings tested under boundary lubrication conditions. Except for relatively light loads and moderate and higher speeds, the bearings operate under boundary lubrication conditions. When pressurized oil was supplied to the bearings, it was found that, for the same operating conditions, porous bearings run at higher eccentricity ratios than solid bearings, as predicted by theoretical analyses.

Lubricity Synergism in Boundary Lubrication

2005 ◽  
Author(s):  
Ion M. Sivebaek ◽  
Nadja L. Hansen ◽  
Kenneth H. Johansen
Keyword(s):  
Molecular Structure ◽  
Diesel Engine ◽  
Recent Work ◽  
Boundary Lubrication ◽  
Boiling Point ◽  
Injection Equipment ◽  
Significant Challenge ◽  
Low Viscosity ◽  
Boundary Lubricant ◽  
Molecular Layers

Dimethyl Ether (DME) has been known as a ultra clean smokeless fuel for diesel engines for over a decade now. A significant challenge is to ensure full lifetime for the diesel engine injection equipment when handling the low boiling point DME as this fuel has very low viscosity and lubricity. Pressurised lubricity evaluators show that boundary lubrication additives increase DME lubricity significantly but recent work indicate that adequate performance in quasi-boundary lubrication is necessary for reducing the wear effectively in real pumps. In this regime where the pump surfaces are only separated by a few molecular layers fuel molecular structure and viscosity seem to be important properties. Earlier work showed that addition of low lubricity alkanes raised the high lubricity of a boundary lubricant to an even higher level. In the present work the long linear alkane n-hexadecane was added to DME in different proportions. The lubricities of the blends do not show any minimum but reflect the lubricity of pure n-hexadecane. It is not conclusive whether DME is a boundary or a quasi-boundary lubricant but clarifying tests are currently in progress.

Experimental investigation on the tribological property of functionalized graphene lubricant against steel

2020 ◽  
Vol 72 (3) ◽  
pp. 307-314
Author(s):  
Yong Qian ◽  
Hongying Gong ◽  
Xiaoyun Zhao ◽  
Lei Cao ◽  
Weizhong Shi ◽  
...  
Keyword(s):  
Peer Review ◽  
Coefficient Of Friction ◽  
Tribological Properties ◽  
Substrate Surface ◽  
Lubricant Additive ◽  
Functionalized Graphene ◽  
Experimental Conditions ◽  
Content Type ◽  
The Coefficient Of Friction ◽  
Modified Graphene

Purpose The purpose of this study is to corroborate the advanced tribological properties of graphene as a lubricant additive. Design/methodology/approach Different concentrations of functionalized graphene were coated on the substrate surface. Tribological properties of the graphene lubricants were carried out by ball-on-disk tribology tests. Wear mechanism of functionalized graphene was studied by observing wear scars on the substrate surface. Finally, the wear resistance of modified graphene was calculated by calculating and analyzing the applied experimental conditions and the obtained experimental data. Findings The best concentration of graphene lubricant is 0.5 wt.% which shows the best tribological performance. And the coefficient of friction is 0.08. Compared with the dry friction condition, the coefficient of friction and wear rate of best graphene lubricant decreased by 80% and 82%. Originality/value The formula of graphene lubricant is independently developed and works very well. Graphene lubricant can prevent the substrate from oxidation. The thickness of the graphene lubricant is about 4-7µm. The concept of anti-wear strength was introduced in this paper. When 0.5 Vol.% graphene was added, the anti-wear strength was greatly improved from 115.3 kg·mm-2 to 657.6 kg·mm-2. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-08-2019-0344

Sliding Friction Analysis of Phosphatidylcholine as a Boundary Lubricant for Articular Cartilage

1993 ◽  
Vol 207 (1) ◽  
pp. 59-66 ◽  
Author(s):  
P F Williams ◽  
G L Powell ◽  
M LaBerge
Keyword(s):  
Articular Cartilage ◽  
Coefficient Of Friction ◽  
Relative Velocity ◽  
Sliding Friction ◽  
Weight Bearing ◽  
Contact Geometry ◽  
Synovial Joint ◽  
Dipalmitoyl Phosphatidylcholine ◽  
Boundary Lubricant ◽  
The Coefficient Of Friction

Dipalmitoyl phosphatidylcholine (DPPC), the major lipidic component of the synovial fluid (45 per cent), has been implicated in previous studies in synovial joint lubrication as a potential boundary lubricant for articular cartilage. The purpose of this study was to evaluate the effectiveness of DPPC as a boundary lubricant at physiological stresses experienced by weight-bearing joints (up to 7.5 MPa). The sliding coefficients of static and kinetic friction for glass surfaces coated with DPPC layers of physiological thickness (70 nm) were measured as a function of average contact stress, contact geometry (point and line), applied load and relative velocity (from 25 to 0 mm/s) and compared to the coefficient of friction for clean glass in the same conditions. The coefficient of friction for DPPC-lubricated surfaces was dependent on contact geometry, obeyed Amonton's law (not dependent on axial load or contact area), was dependent on relative velocity within the range stated and was an effective lubricant at physiological stresses. This study showed that dipalmitoyl phosphatidylcholine can be an effective boundary lubricant at stresses observed in load-bearing joints. Because of their surface-active nature, these adsorbed molecules might also act as a protective layer for the articular surfaces.

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