Optical spectra

Description of the technique

In the simplest form of optical emission spectrometer, an electrical discharge is produced between two electrodes, at least one of which consists of, or contains, the sample to be analysed. A high voltage accelerates electrons from the cathode (the negative electrode) onto the anode, where they produce heat and the release of positive ions. Hence, some portion of the sample is vaporized by the discharge, the vaporized species become dissociated, and the resultant atoms (or ions) are excited either thermally or through collisions with each other. An excited atom or ion emits light, which is dispersed into its component wavelengths (λ) by means of a diffraction grating. In Section 2.1.4 we saw that if we place two slits in front of the sample slit source, then maxima appeared at distances from the undeviated beam that were multiples of (λDss/cz), where the reader may recall that Dss was the distance from the slits to the screen and cz was the distance between the slits (see Figure 2.7). We can write this in terms of diffraction angle Ø, since in ΔGOM of that figure, we see tan Ø = (λ/cz) for the first maximum. If the light source behind the single slit is not monochromatic but contains several wavelengths, then we would obtain several overlapping fringe patterns from each of the wavelengths present.

Optical microscopy and optical interferometry

Introduction

The availability of powerful tools for viewing surfaces at very high magnifications through the medium of highly resolved images has often caused the optical microscope to be overlooked. As we shall see, this may be related to the fact that the depth of field of an optical micrograph varies inversely with increased magnification, whereas with electron microscopy, the depth of field is almost infinite. Nevertheless, in tribology, many of the processes occur on a scale for which the optical microscope is probably the best and easiest instrument to use, especially when used in the reflection mode. Furthermore, when the optical microscope is combined with an interferometer device, we have a surface examination tool which has a vertical resolution equal to the resolution of many electron microscopes (and at a fraction of the capital outlay). We shall see how optical interferometry has indeed been used to give information about lubricant film thickness contours within the contact between a ball and a flat. These thicknesses are of the order of micrometres. Interference techniques have been used (Tolansky, 1970) to deduce the height of 40 Å steps in cleared mica. The present author has used a home-made interferometer to deduce the thickness of vacuum-evaporated films of iron used by Halliday (1960) in his work on using the contrast of electron diffraction patterns to determine thickness.

Definitions of common tribological terms

Tribology

Tribology is a new word based on the Greek word ‘tribo’, which means ‘rubbing’. Hence tribology is the ‘study of rubbing’. The word was first used by a British Government committee (chaired by Dr Peter Jost and hence known as the ‘Jost Committee’) that produced a report, in 1966, calling for increased education and research into a subject that was estimated (at 1966 prices) to be costing the United Kingdom about £300 million per year.

The Jost Committee defined tribology as ‘the study of the science and technology of interacting surfaces in relative motion’. It was hoped that the new word might provide the basis of a more unified approach to subjects previously studied separately under titles such as ‘friction’, ‘adhesion’, ‘lubrication’ and ‘wear’. It is indeed unfortunate that, to date, the Jost Committee's awareness of the need for a more unified (that is, interdisciplinary) approach has not been shared by many tribologists. This resistance to the calls for a change in our partisan approaches to the subject is illustrated quite neatly by the fact that it has taken nearly 20 years for the Journal of Lubrication Technology (JOLT) to change its name to the Journal of Tribology (JOT), namely from 1966 to 1985.

Failure in rolling contact bearings in nominal point contact

Introduction

Czichos (1980) defines a tribo-system as one in which motion, energy and materials are transmitted, in various relative amounts, according to the required function of the system, from clearly prescribed inputs to desired outputs. Invariably, motion is a characteristic of any tribo-system. Sometimes the purpose of the system may be to change the rate of motion or to eliminate it altogether (e.g. in brakes). Such changes involve undesired outputs, such as frictional heating and the undesired removal of material from the surface through which the motion is transmitted. Gears are intended to transfer motion and power in rotating machinery, but sometimes unwanted transfer of material may also occur. The wheels of a railway engine are intended to transmit force to the rail and hence produce motion, but again we find that the unwanted transfer of material, and the loss of energy due to slip, will make the actual output somewhat different from that which was desired. Cams and tappets, valves and valve seats, piston-cylinder systems, hot and cold rolling mills, sheet-forming and wire-drawing dies, metal cutting tools, dry bearings and current collectors are further examples of tribosystems which involve some degree of transfer of motion, energy and materials. All such tribo-systems are said to have failed when the actual output deviates significantly from the intended output.

This book has been written with the aim of demonstrating the power of using modern techniques of physical analysis for studying the complex interactions that often occur between the contacting surfaces of tribo-systems, that is, systems involving relative motion between the various elements.

It is an interdisciplinary book, which should be of interest to tribologists whose major discipline is physics, chemistry, metallurgy or any branch of engineering involved with moving parts. Obviously, mechanical and production engineers will have more interest in tribology than (say) civil engineers or electrical engineers, but even with these disciplines, tribological problems can occur for example traction between asphalt and rubber and the wear of carbon brushes.

The book aims to be understandable by readers at all levels of technical competence with an interest in tribology. It should be of special interest to those final year undergraduate students intending to make a career in the research and development laboratories associated with the oil companies, the electricity generating industry, the aerospace companies, the steel-making and steel-forming enterprises, the automotive and diesel engine manufacturers, the railways, or any other industrial concern heavily dependent upon good tribological knowledge and practice.

Even those undergraduates destined for the production side of these industries, should find the introductory chapter on tribology useful background reading, especially if they are involved with design.

Basic crystallography

Introduction

Most solid substances are polycrystalline, that is, they consist of many small regions of near-perfect simple crystals (crystallites) which are generally oriented at random with respect to each other. Sometimes, however, these crystallites have preferred orientations with respect to each other. The facility for taking up preferred orientations can be a distinct advantage in metal-forming operations. It is also partly responsible for the successful operation of carbon brushes (as we shall see in Chapter 5). Separating the crystallites there are regions of very poor crystallinity, that is the atoms in these regions are not arranged in the regular arrays expected of a perfect single crystal. These regions are called ‘grain boundaries’ and they have a strong influence upon the mechanical strength of a polycrystalline solid. It is generally assumed that polycrystalline solids are isotropic as regards mechanical strength, electrical and thermal conductivity, and other physical properties relevant to the friction and wear of such solids. We shall see, however, that this assumption is not valid for materials which take up a preferred orientation upon interacting with another surface during motion. It is also not true in respect of materials that change the chemical composition of their interacting surfaces during contact, especially those materials that exhibit mild (oxidational) wear.