Thin Lubrication Research Articles

Laser-based fluorescence techniques for measuring thin liquid films

Laser-based systems for measuring the thickness of thin lubricant films by fluorescence assay are described. The laser illumination systems are simpler in design than earlier versions based on a mercury lamp; they also have a longer working distance and enable the technique to be applied to a wider range of practical situations. A variant using blue laser light had many advantages over an equivalent UV system. The behaviour of oil films in the laser light was studied, and blended fluids were developed to enable the fluorescence activity to be tailored to requirements.

Numerical solution of the point contact problem using the finite element method

AbstractThe elastohydrodynamic lubrication problem, in which the lubricant pressure and film thickness are sensitive to surface deformation, is solved by using a finite element procedure and the Newton method. The numerical procedure is applied to the point contact problem, in which a thin lubricant film is maintained between two balls loaded together by a high load under conditions of pure rolling. The present analysis shows that pressure spikes are formed near the outlet region, a result which has been found in the line contact problem and which has been conjectured in the present problem.

A unified treatment of thick and thin film elastohydrodynamic problems by using higher order element methods

A unified mathematical treatment of thick and thin film elastohydrodynamic lubrication problems is presented. The construction of approximate solutions is discussed by using Ritz-Galerkin methods with piecewise polynomial basis functions. In particular for the line contact problem, smooth Hermite and cubic spline spaces are used to obtain solutions over a wide load-speed range. Pressure ‘spikes’ are obtained.

Electrical conduction in thin lubrication oil films—an explanation of the coherer?

Electrical conduction in thin lubrication oil films—an explanation of the coherer?

Electrical conduction in thin lubricating oil films

Under conditions of hydrodynamic lubrication oil forms a thin film between the lubricated surfaces. An investigation of the electrical properties of such films is described, the measurements being made under dynamic conditions. It is shown that the electrical conduction is consistent with space-charge limited current in the presence of a uniform distribution of traps. The oil film is considered to form a pressure-frozen quasi-solid of glass-like material. A rise in current corresponding to the trap-filled limit is observed and imparts a conductivity switching type of behaviour to the film, reminiscent of amorphous glass switching of the Ovshinsky type.

Reply to ``Comment on the coefficient of friction of high polymers as a function of pressure''

In a recent paper, Towle has criticized our studies in which we ralated the coefficient of friction for thin solid lubricant films to a pressure effect on the shear strength of the film material. This a rebuttal to that paper.

The shear strength of thin lubricant films

This paper describes a study of the effects of pressure and temperature on the shear strength of very thin layers of a number of lubricants. The shear strength is deduced from measurements of the tangential (frictional) force required to slide glass spheres over glass plates coated with the lubricant. It is assumed that no glass-glass contact occurs through the lubricant film and that the area of lubricant sheared is the geometrical contact area determined by the elastic constants of the sliding surfaces. Consequently by varying the radius of the sphere and the normal load the contact pressure can be varied from 10 7 to 5 x 10 9 Pa (1 Pa = 1 Nm -2 ). The results show that the shear strength of Langmuir-Blodgett films of calcium stearate is constant at low pressures, but at pressures greater than 5 x 10 7 Pa it increases approximately in proportion to the pressure. At higher temperatures the behaviour is similar but the shear strength is reduced at all pressures. Similar experiments have been carried out on retracted stearic acid films, on vapour deposited sebacic acid films and on smeared films of copper stearate and anthracene. Thin films of high density polythene and PTFE have also been studied. For these materials both the absolute value of the shear strength and its variation with pressure resemble the behaviour of calcium stearate. Experiments on the effect of temperature on the shear strength show that calcium stearate, stearic acid and high density polythene exhibit similar ‘activation energies’ for the shear process. This suggests that these materials all shear by a similar process involving the sliding of linearly orientated methylene groups over one another. PTFE behaves in a similar way although its ‘activation energy’ is a little higher. By contrast anthracene and sebacic acid have much lower activation energies suggesting that a different shear process is involved.

Comment on ``The coefficient of friction of high polymers as a function of pressure''

Bowers and Zisman have postulated a simple expression relating the friction coefficient of thin solid lubricant films to the shear strength of the film material and the contact pressure involved. They have attempted to demonstrate the validity of this expression by comparing shear strength and friction measurements made using two types of experimental configurations. It is argued here that the measurements compared are intrinsically equivalent and will yield equivalent results even when the friction mechanism is such that the postulated equation is not valid. Hence, the comparisons made do not in any way test the validity of the postulated equation.

Measurement of Wet cornering Traction of Tires

Abstract We have shown that the cornering wet traction performance of tires, as measured with a special cornering trailer, is influenced by a number of factors and their interaction with each other. Unlike conventional low speed “spin-out” wet cornering traction testing, we have evaluated tire traction over the range 30–60 mph. Over this range there is a marked speed dependence in the rating of various tread rubbers and tread patterns. In general, tread rubbers show a wide range of performance ratings at the lower speeds (30–35 mph) and a narrower range at high speeds (55–60 mph). Various tread patterns on the contrary show similar behavior at low speeds but a wide divergence in traction level at high speeds. Higher durometer tread compounds show improved high speed traction for any given rubber. Tread hardness cannot be used as an omnibus indicator of wet traction performance, however, as each rubber has its own separate correlation line. Low coefficient pavement can have either low or high degrees of macrotexture, but the lack of microtexture or harshness (asperities in the fraction of a millimeter range) produces this type of pavement. Tires must perform safely on such pavement sections of public highways and the testing reported here was done on such test surfaces. Evaluations of four types of tread rubber show that they rank from high to low traction level in the order: SBR, Butyl, NR and BR (solution type) on smooth, low microtexture surfaces. Although BR gives low traction when used alone it is not so used in commercial tread compounds. When properly blended with SBR or NR, tread compounds containing BR give satisfactory traction performance and improved wear performance. The overall behavior of tires can be explained in terms of the concepts of hydrodynamic and boundary layer lubrication. At low speeds boundary layer lubrication predominates on all but the smoothest pavements. This accounts for the marked influence of tread rubber at low speeds. At high speeds both thick and thin film elastohydrodynamic lubrication predominate. In this speed range tread materials play a lesser role and tread pattern or geometry plays a larger role. The relative softness and deformability of tread compound, compared to pavement aggregate, accounts for the importance of elastohydrodynamic lubrication. Drawing on the work of many previous investigators and the data of this work it is postulated that the fraction of the tire contact area of a cornering tire that is in the elastohydrodynamic mode of lubrication is a linear function of speed. This accounts for the good linearity of the plots of traction as a function of speed. Test variability is discussed and steps taken to measure and control such relevant factors as water depth are outlined. The use of statistically designed testing programs with their inherent averaging character are advocated for those doing this work. In addition to their power at averaging test results, such designs uncover the strong interaction between tire and test variables that underlie all wet traction testing.

Lubrication of nylon by polysiloxane fluids

A fully comprehensive study of the tribological behavior of palladium nanoparticles (Pd NPs) capped by tetrabutylammonium chains using a ball-on-disk tribometer under different conditions of applied load, concentration, tribometer motion, linear speed and nature of the counterface is revised. A low concentration of NPs (2 wt%) in tetrabutylammonium acetate was found sufficient to improve the tribological properties due to the formation of a protective transfer film (TF) comprised of metallic Pd. The increase of the applied load (up to 20 N, 1.82 GPa of contact pressure) confirmed the excellent extreme-pressure behavior avoiding the counterfaces from severe wear. After a running-in period whose duration depends on the operating conditions, the TF build-up allows to maintain a low contact electrical resistance through the contact (<0.1 kΩ) during the entire test. When the Pd NPs are used with ceramic counterfaces, the nanoparticles increase the load-bearing capabilities and performance of the base without forming TF, likely by mixed or boundary lubrication and healing effects. Finally, the Pd NPs are demonstrated to be useful as a thin solid lubricant film in reciprocating motion yielding a comparable tribological behavior. Hence, the presented surface Pd NPs can be very helpful to extend life of sliding components due to their high strength resistance providing a gateway to electrical conduction as well.

An improved magnetic reluctance transducer for the measurement of thin lubricant film thickness

An improved magnetic reluctance transducer of high sensitivity is developed for use with laboratory disk-machine which simulates conditions associated with elastohydrodynamic lubrication. It measures the change in the magnetic flux across the contacts of a double rimmed disk in contact with a solid disk when a film is introduced. Details of the design principles and procedures of optimization are discussed. The transducer is particularly suited for the study of the film thickness of lubricating grease because, unlike their dielectric constants, the permeabilities of greases, being nonferromagnetic, are close to unity. This simplifies the calibration.

The Effects of Load on the Frictional Properties of Molybdenum Disulfide

The kinetic coefficient of friction was determined for several grades of commercially available molybdenum disulfide powder. These powders were individually applied to separate steel-supporting substrates to form a thin lubricant film. Friction measurements were made at loads ranging from 0.1 to 10 kg and at a slow speed of sliding. The results of Ike investigation showed that the coefficient of friction decreased with increasing load, contrary to Amontons' Second Law. A theory was postulated to explain the observed variation in the friction coefficient with load, It was concluded that the variation in the friction coefficient could be explained solely on the basis of the macroelastic and/or macroelastic and plastic deformation characteristics of the supporting substrates.

An X-ray method for measuring thin lubricant films between rollers : L. B. Sibley and A. E. Austin, ISA trans., 7 (3) (1963) 237–243; Battelle tech. rev., 11 (12) (1962)

An X-ray method for measuring thin lubricant films between rollers : L. B. Sibley and A. E. Austin, ISA trans., 7 (3) (1963) 237–243; Battelle tech. rev., 11 (12) (1962)

Wear of Lubricant Films : Part 3, Effect of Temperature

Effect of temperature on the wear property of thin lubricant film is studied as continuation of the studies reported before. When the temperature of lubricant film is raised the wear property of the film is in general decreased in proportion to the rise of the temperature, reducing to nil at the transition temperature of the boundary lubricating film. This point is known as the desorption temperature of the absorbed lubricant molecule. When the load is varied the wear property of the film also is decreased in the same way as reported before in Part 2 of this study. Discussions of the correspondency between the effects of load and temperature on the wear property of thin lubricant film are also given.

Wear of Lubricant Films : Part 2, Effects of Load and Speed

In this paper is described how the wear properties of lubricant films are affected by load and speed. The wear property of lubricant film is studied by stick-slip analyser, which allows to find the critical number of runs with no abrupt rise of friction coefficient Nc during repetition of sliding on the same track of lubricated surface. From the experiment it is found that Nc changes in proportion to P-0.55 (P is load), which is explained to be deduced reasonably when analysing the above relation taking the frictional stress into consideration. Wearing process of thin lubricant film is also analysed and by taking the relation between frictional coefficient and the thickness of lubricant film into consideration, the following relation is obtained ; [numerical formula] where μ=friction coefficient ; μ0=friction coefficient for abundant quantity of lubricant ; y0=given thickness of lubricant film ; N=number of runs on the same track ; c1, c2=constants.