The nature of engineering surfaces

No real engineering surface, no matter how carefully, or indeed expensively, prepared can possess perfect geometry. As well as errors in the form or shape of the component there will always be a roughness on the surface which is apparent when this is examined at a sufficiently high magnification. When two such surfaces are loaded together it is the tips of the surface roughnesses or asperities that must first carry the applied load: the geometry of individual contact spots and the way in which these islands of real contact are distributed throughout the nominal or apparent contact area is clearly of interest to tribologists in attempting to predict the overall performance, or likely life history, of the contact.

The geometric texture of an engineering surface reflects both its production route and the nature of the underlying material. It is possible to produce a truly smooth surface (for example, cleaving specimens of mica can produce a surface with roughness only on the atomic scale) and if two such surfaces are loaded together real and apparent areas are very nearly equal. The asperities on the surface of a very compliant surface, such as a soft rubber, may, if sufficiently small, be squeezed flat by quite modest contact loads, and in this way there can again be equality between real and apparent areas of contact. However, these are special cases; in general, useful metal surfaces exhibit a range of surface fluctuations which, although large compared to molecular dimensions, are small compared to the dimensions of most engineering components.

The term tribology is scarcely twenty-five years old and yet there can be few university or college courses in mechanical engineering which do not now include material under this heading. Of course, the problem of producing bearings, slides, seals, and other tribological systems to give smooth machine running and long component lives is one which has faced practitioners for generations, and consideration of their design has always played a part in the education of mechanical engineers. What has become increasingly obvious in recent years is the inherently interdisciplinary nature of the tribologist's task; as well as involving practising and academic engineers, advances in the subject have drawn upon the ingenuity and expertise of physicists, chemists, metallurgists, and material scientists. Consequently, although envisaged principally for use by final year undergraduates and post-graduate students in mechanical engineering, I hope that this volume may be of interest to students and specialists in these other related areas. Tribology is still very much an area of active research and the published literature in the fields of lubrication, friction and wear–already dauntingly voluminous–continues to grow at an alarming rate. I have made no attempt to produce a research monograph but rather to provide a framework of fundamental analytical tools which can be used in a wide variety of different physical situations. Each chapter is concluded with a short list of suggestions for further reading which provide access to the more specialised literature.

Introduction

The satisfactory operation of both hydrostatic and hydrodynamic bearings requires that the solid surfaces which constitute the bearing faces are completely separated by the intervening fluid film. Since the bearing surfaces are then not physically touching, the resistance to their tangential motion, that is, the force of friction, is directly attributable to viscous losses in the lubricant. If the lubricant (whether liquid or gas) exhibits Newtonian rheological behaviour with constant viscosity then the value of this frictional force, and the associated coefficient of friction, will increase with the value of the tangential sliding velocity. We have seen (eqn (7.27)) that the coefficient of friction within a hydrodynamically lubricated bearing is generally dependent on the square root of the group ULη/W where U is the relative sliding speed of the surfaces, W/L the normal load supported per unit length, and η the Newtonian viscosity. A reduction in speed, or an increase in the specific load on the bearing, leads to a fall in the friction coefficient. However, there is a limit to this process: when the specific load is very high, or the relative sliding speed small, it is difficult to build up a sufficiently thick film to entirely separate the bearing faces, and so there will be some mechanical interaction between opposing surface asperities. This is inevitable, even allowing for the large increase in effective lubricant viscosity and the elastic flattening of the surface profiles that can occur in the elasto-hydrodynamic regime.

Introduction

The definition and measurement of wear

Wear is the progressive damage, involving material loss, which occurs on the surface of a component as a result of its motion relative to the adjacent working parts; it is the almost inevitable companion of friction. Most tribological pairs are supplied with a lubricant as much to avoid the excessive wear and damage which would be present if the two surfaces were allowed to rub together dry as it is to reduce their frictional resistance to motion. The economic consequences of wear are widespread and pervasive; they involve not only the costs of replacement parts, but also the expenses involved in machine downtime, lost production, and the consequent loss of business opportunites. A further significant factor can be the decreased efficiency of worn plant and equipment which can lead to both inferior performance and increased energy consumption.

The wear rate w of a rolling or sliding contact is conventionally defined as the volume lost from the wearing surface per unit sliding distance; its dimensions are thus those of [length]. For a particular dry or unlubricated sliding situation the wear rate depends on the normal load, the relative sliding speed, the initial temperature, and the thermal, mechanical, and chemical properties of the materials in contact. There are many physical mechanisms that can contribute to wear and certainly no simple and universal model is applicable to all situations.

Tribology

The word tribology was coined only just over twenty years ago and appears in only the most up to date of dictionaries; however, the topics with which tribologists are concerned have been of vital interest to scientists, engineers, and those who design or operate machinery, for as long as mechanical devices have existed. Formally, tribology is defined as the science and technology of interacting surfaces in relative motion and of related subjects and practices; it deals with every aspect of friction, lubrication, and wear. The word is derived from the Greek τριβοσ (TRIBOS) meaning rubbing, although the subject embraces a great deal more than just the study of rubbing surfaces.

Perhaps as much as one third of our global energy consumption is consumed wastefully in friction: at a time when energy resources are at a premium the contribution that can be made to their efficient utilization, as well as to the reduction of pollution, by making use of the best tribological practices is obvious. In addition to this primary saving of energy there are very significant additional economies to be made by reductions in the cost involved in the manufacture and replacement of prematurely worn components. An important landmark in the development of the subject was the publication in 1966 in Great Britain of the report of the government committee, chaired by Mr Peter Jost, which had been formed to report on the position of industrial lubrication in the United Kingdom: it was asked specifically to identify those areas of industrial practice where significant improvements could be made.

Introduction

Not all sliding tribological pairs are designed to operate in the presence of a generous supply of lubricant. The nature of the working environment may make it impossible, or impracticable, to arrange for the contact to be lubricated by a full hydrostatic or hydrodynamic fluid film, for example, in deep space or satellite applications where any liquid lubricant would be lost or degraded by evaporation, or in food processing or chemical plant where contamination of either the product or the environment by any escape of a lubricating fluid would be unacceptable. In other, simpler applications the design constraint may simply be the cost of the lubricants supply and handling equipment. Generally, the aim in the design of machinery is to minimize friction; however, in some devices (clutches, brakes, friction drives, and so on) friction is beneficial, indeed essential, and often components of this sort are operated unlubricated. In this chapter, as well as the tribology of dry sliding (taking as examples both brakes and clutches as well as dry rubbing bearings) we shall also consider the tribological aspects of some ‘marginally’ lubricated contacts. These are only intermittently lubricated by a less than complete fluid film and so rely for their success on a combination of hydrodynamic, elasto-hydrodynamic, and boundary lubrication. Bearings and bushes designed to run completely dry are often manufactured as monolithic solid components and involve at least one nonmetallic material, while marginally lubricated bearings often make use of porous, sintered metals (usually bronze) impregnated by an appropriate mineral oil, grease, or solid boundary lubricant.