Tribochemical Surface Reaction and Lubricating Oil Film

A radioactive tracer technique was used to investigate a tribo-chemical surface reaction obtained by a thrust collar type friction Machine. Radioactive dibenzyl disulfide labeled with sulfur-35 was used as an additive. Steel and copper disks were used as friction specimens.

Radioactive copper sulfide on the friction surface was quantitatively measured with a G-M tube, and a kinetical analysis of the reaction was carried out.

The friction coefficient decreased linearly depending on the amount of surface produced on the friction surface. This dependency was accompanied by adsorption of dibenzyl disulfide, which made a more effective lubricating oil film and was enhanced by the sulfide on the friction surface.

The results of kinetical analysis were explained effectively by considering the oil film behavior related to the adsorptive action of the surface sulfide.     

Traction Characteristics of Graphite Lubricants at High Temperature

The high temperature (25°C to 538°C) traction characteristics of graphite lubricants containing additives of silver, phosphate glass, and zinc orthophosphate were evaluated in a specially designed ball-on-disk traction device. Bearing quality balls of hot-pressed silicon nitride burnished with the graphite lubricant were loaded against lubricated disks of hot-pressed silicon nitride. Characteristic behavior of the measured tractions was similar in nature to that of liquid lubricants, however, traction force magnitudes were often much higher. Traction coefficients ranged from us low us 0.07 to as high as 0.66. Based on the data collected, an empirical solid lubricant traction model was constructed and applied to an advanced bearing analysis code.     

Friction and wear response of nitride coating deposited through PVD magnetron sputtering

Surface engineering with applied coating plays a vital role in any industrial application. These coatings are meant for better mechanical and tribological characteristics when applied on to the materials. The major challenge in selecting a suitable coating strategy is their input process parameters. There are several parameters which influences the coating properties, but it is hard to choose one of them and ignoring others. Multilayers of tungsten nitride are attracting great interest to modulate their tribological and mechanical properties through physical vapour deposition process due to their wide application range. These multilayer nitride films were deposited through unbalanced reactive magnetron sputtering technique. The tribological tests were performed on a pin-on-disc tribometer at room temperature and it has been observed that friction and wear values reduce drastically while applying multilayer coatings. Later, artificial neural network (ANN) is employed to optimize the tribological properties of sputtered coatings.     

The Tribology of PS212 Coatings and PM212 Composites for the Lubrication of Titanium 6A1-4V Components of a Stirling Engine Space Power System

The Stirling space power machine incorporates a linear alternator to generate electrical power. The alternator is a reciprocating device that is driven by a solar or nuclear-powered Stirling engine. The power piston and cylinder are made of titanium 6A1–4V (Ti6–4) alloy, and are designed to be lubricated by a hydrodynamically-generated gas film. Rubbing occurs during starts and stops and there is the possibility of an occasional high speed rub. Since titanium is known to have a severe galling tendency in sliding contacts, a “backup,” self-lubricating coating on the cylinder and/or the piston is needed. This report describes the results of a research program to study the lubrication of Ti6–4 with the following chromium carbide based materials: plasma-sprayed PS212 coatings and sintered PM212 counterfaces. Program objectives are to achieve adherent coatings on Ti6–4 and to measure the friction and wear characteristics of the following sliding combinations under conditions simulative of the Stirling-driven space power linear alternator: Ti6–4/Ti6–4 baseline, Ti6–4/PS212-coated Ti6–4, and PS212-coated Ti6–4/PM212.     

Characteristics of the Thrust Bearing with Accelerating Slider

Thrust bearing characteristics are examined in the motion generated by the accelerating slider. The selected case is the situation where the fluid within the lubricating film initially is at rest and at time zero the infinitely long slider assumes a velocity which is a function of time. Numerical solutions to the governing differential equations are obtained for the case of constant but arbitrary acceleration and for the case of acceleration proportional to time. The corresponding previously published analytical solutions are compared with those numerical results.     

A tribocorrosion study of cobalt–tungsten electrodeposited coating with a mixed amorphous/nanocrystalline structure

Cobalt–tungsten alloy coating with a mixed amorphous/nanocrystalline structure was electrodeposited from a citrate-ammonia bath. The coating showed a nodular surface morphology which had a microcrack network. During wear rubbing at OCP, the potential continuously increased to nobler values and then remained at high values even after the end of rubbing. Under anodic polarisation, rubbing caused a sharp increase in current density. However, at the end of rubbing, it returned to the initial value. The mass transport, depassivation, coating cracking and structure variation towards higher nanocrystalline content occurring at the wear scar played the major roles in determining the tribocorrosion behaviour of the coating. The formation of the superficial microcracks in the vicinity of the wear scars indicated a surface fatigue wear mechanism.     

Tribology research in the twenty-first century

This paper attempts to predict and discuss some of the many challenges facing fundamental research in tribology over the first half of the new century. This is done in two stages, looking at likely developments over the first twelve years based on extrapolation of existing trends, and then, more speculatively, considering possible driving forces over subsequent decades.     

The significance and use of the friction coefficient

The quantity known as the friction coefficient (or ‘coefficient of friction’) has long been used in science and engineering. It is easy to define, but not easy to understand on a fundamental level. Conceptually defined as the ratio of two forces acting, respectively, perpendicular and parallel to an interface between two bodies under relative motion or impending relative motion, this dimensionless quantity turns out to be convenient for depicting the relative ease with which materials slide over one another under particular circumstances. Despite the fact that both static and kinetic friction coefficients can be measured with little difficulty under laboratory conditions, the time- and condition-dependent characteristics of friction coefficients associated with both clean and lubricated surfaces have proven exceedingly difficult to predict a priori from first principles.The shaky nature of friction's fundamental underpinnings, has not prevented investigators from compiling lists of friction coefficients and publishing them for general use. Problems often arise, however, when engineers attempt to use tabulated friction coefficients to solve specific problems in mechanical design or failure analysis. The systems-dependence of frictional behavior is sometimes ignored, leading to misapplication of published data. This is particularly true for applications in nano-technology and others that differ from typical laboratory size scales. This paper will review the measurement and use of static and kinetic friction coefficients, discuss their usefulness, and describe the sources of frictional resistances in terms of shear localization.     

Friction-induced structural modifications of Mg and Ti surfaces

High-purity magnesium and titanium have been examined in order to study modifications to the near-surface layer during friction. Upsetting tests were conducted under atmospheric and imposed hydrostatic pressure to assess the plasticity and deformability of these metals. It is shown that the deformability of magnesium and to a lesser extent that of titanium can be considerably enhanced by superimposed hydrostatic pressure. Both metals were subjected to uniaxial, constant load, dry friction, pin-on-plate tests in air at room temperature and atmospheric pressure and the near-surface layer examined destructively and non-destructively in order to characterize induced changes. High-resolution X-ray diffraction examination of the worn and unworn surfaces suggested that the deformability of the near-surface layer in magnesium is associated with an increase in 〈a〉 Burgers vector screw dislocations able to cross slip and which contribute to recovery and recrystallization in the deformed region. A similar effect may be present for titanium although no recrystallization was observed.     

An experimental study of the tribological behaviour of the brass/steel pair

This paper presents a study of the behaviour of the brass/steel pair in the presence of friction and wear. A plan of experiments, based on the techniques of Taguchi, was performed on a pin-on-disc machine. The objective was to establish a correlation between load, sliding velocity and temperature in contact with the wear and coefficient of friction. These correlations were obtained by multiple linear regression. Finally, confirmation tests were performed to make a comparison between the foreseen results from the mentioned correlations and the experimental results.