Objective
BACKGROUND
Tribology: The subject area is primarily concerned with the lubrication, friction and wear of moving parts in machinery. It is strongly interdisciplinary in nature, embracing contributions from mechanical engineers, materials technologists, physicists, chemists and others. It is generic technology with relevance to all industrial sectors.
Economic significance: The establishment of this subject area in a coherent fashion some twenty-five years ago was due to its remarkable potential to effect financial savings in the industrial and commercial sectors. The breakdown of machinery in a technical society is extremely expensive, the main factors being lost production and the increased capital investment required. Several countries (e.g. China, Germany, Japan, United Kingdom, United States of America) have carried out detailed financial audits and potential savings of the order of 1% of the gross domestic product have been identified. Particular industries which could benefit from reduced energy consumption, minimized wear failure and scheduled maintenance, which can be optimized by good tribological practice, are the transportation, power generation and process sectors.
Competition: There is fierce competition on an international scale to improve the tribological performance of elements and components such as bearings, gears, cutting tools, cams, piston rings, dies etc. Such developments have to be effected whilst ensuring durability. Many national communities have developed co-ordinated policies but the advanced status of Japan and the United States of America represents a considerable threat to the relatively loose connection between the European countries in the area of tribology.
Advantages: In addition to the savings which can be achieved through better tribological practices, it is important to note other advantages which can occur due to a concerted European programme:
(i)Within the Eastern European countries, a considerable human resource with a strong tribological background exists. Up until now, this community has been starved of financial support and equipment and the opportunity for mutually advantageous co-operation with tribologists in Western Europe exists.
(ii)Environmental issues are of paramount concern around the world. The proper control of friction and wear through satisfactory lubrication has a vital role to play with regard to the emission of greenhouse gases, the control of noxious emissions, the lowering of noise levels, the reduction of scrapped components, the specification of acceptable lubricant additive packages and reduction in the use of scarce resources.
(iii)It is usually accepted that improved communication between academics and industry would be desirable for both parties, and this is particularly true in the field of tribology. Industry now makes a considerable contribution to fundamental tribological studies, of which academics should be aware, whilst academics educate future scientists and engineers who need tribological information. Industrial input on curricula is thus important.
(iv)The role of tribology for perceived product quality is now clearly recognized. The ability to use the science and practice of tribology to achieve surface quality, texture and other factors of attractiveness to customers, represents an important development.
(v)The pace of technological development can be significantly influenced by developments in standardization and databases. Whilst the kernel of such activities is in place in a number of European countries, accelerated activity is important.
RECOMMENDATIONS
The interdisciplinary nature of tribology and its generic nature have led to the establishment of small, active groups in most European countries. These groups have to connect knowledge and unstructured research and development relating to many scientific areas. The technology transfer process regarding the characterization and validation of design data in the field of tribology is vital to the European economy. It is important to mould the European tribological activity into a cohesive scheme in order to compare effectively with the other major world economic markets. The special circumstances demand action to enable a synergism of European effort in the area such that the total output will be considerably greater than the composite parts. Greatly improved capability to generate the economic savings possible and to realize the other advantages available could result.
Whilst all tribological projects might be considered within the framework of such an activity, in order to focus attention the group has identified three important areas which it would recommend for immediate consideration:
1)tribology resource utilization in Europe (TRUE);
2)coatings and surface treatments (CAST);
3)grease lubrication in tribology (GRIT).
The main justification for these activities will now be set out.
1.TRIBOLOGY RESOURCE UTILIZATION IN EUROPE (TRUE)
*The major aim of this project is to facilitate the effective use of human resources and available equipment in Europe to generate a scale of activity capable of competing with that existing, in particular, in Japan and the United States of America.
*Throughout the COST community there exists in academic laboratories, research organizations and industrial companies, an extensive range of scientific equipment, apparatus and hardware dedicated to tribological studies. However, the human resource is more limited and there is to some extent a mismatch in these resources (e.g. in Eastern Europe).
*Bearing in mind the special nature of the field of tribology, its highly inter-disciplinary nature and generic aspects, a number of research groups of limited size have resulted in the COST countries.
*A highly effective approach, both scientifically and economically, to the development of a European synergism in tribology will be to encourage mobility, on various scales, of tribologists between laboratories. The diffusion of the distinct knowledge profiles around Europe would result in an overall cohesion and facilitate co-operation in the longer term of well-defined specific projects.
*Within the COST countries there already exist activities and institutions which would aid the TRUE project:
-in France and the United Kingdom: a European Tribology Institute (IET) formed between the Institut National des Sciences Appliquées de Lyon and Ecole Central de Lyon, and Imperial College of Science, Technology and Medicine in London and the University of Leeds;
-in Germany, the Federal Institue for Materials Research and Testing (BAM) and the Fraunhofer Institut für Schicht-und Oberflächentechnik;
-in Sweden, the Co-ordinating Committee on Tribology;
and many others too numerous to list.
2.COATINGS AND SURFACE TREATMENTS (CAST)
The performance (i.e. friction or wear resistance) of many tribological systems can be improved by applying one of several existing surface modification techniques (e.g. coating deposition, by PVD or CVD, plasma spraying, ion implantation etc.). An illustrative example of such an effect is metal cutting tools, the productivity of which has increased significantly in the last decades. This increase can partly be attributed to the development of new bulk materials, but the main cause is definitely the introduction of (PVD or CVD) coated tools.
Wear protection by CVD and PVD coating is well established technology. It is an expanding field which calls for new methodology in coating evaluation in tribological applications. Coated machine elements can fail for both surface and bulk reasons and both bulk and surface characterization is needed. Coating to substrate adhesion must also be considered.
Bulk characterization
Thermomechanical aspects
The thermomechanical properties of both coating and substrate must be compatible to avoid creating high stress fields in the composite material under either thermal or mechanical loading or both. Indeed, even modest differences in thermal dilation coefficients or in Young's modulus can, under load, generate stresses that are much higher than those found in uncoated systems. Such events can be predicted theoretically if:
-the thermomechanical properties of coatings and substrates are known,
-adequate theories are developed,
-data concerning limiting stresses for coatings and substrates are identified.
Residual stresses
Residual stresses are generated during coating deposition. These stresses must be known and added to those calculated above. Different residual stress evaluation techniques must be defined and recommendations made for their use in different configurations.
Adhesion
Coating to substrate adhesion is usually characterized empirically using different techniques such as scratch tests etc. Recent theoretical work has shown that adhesion can be characterized using energy restitution concepts. Test devices based on this concept should therefore be developed.
An ambitious programme of characterization of thermomechanical properties and limits of coatings and substrates is needed. It must be completed by data on residual stresses and by the development of new tools to characterize, on a satisfactory scientific basis, coating to substrate adhesion.
Surface characterization
Friction and wear are not intrinsic material properties and testing of materials (coated or not) can only be performed for given applied conditions (load, speed, temperature and environment). It is therefore necessary to identify classes of operating conditions and define contact simulators for these conditions. Luckily, the work performed in the determination of running conditions of most mechanisms can be transposed directly to the case of coated systems.
Generalities
A field that has been somewhat neglected is the chemical interaction between coating and lubricant (or cutting fluid). In many lubricated systems it should be possible to avoid using environment polluting additives by applying a proper coating or surface modification. Here, much work remains to be done, both on a fundamental and on a more applied level.
Finally, it should be noted that many new, potential coating materials are studied at laboratory level. However, the implementation of these new coatings in actual applications often remains to be carried out because it requires extensive tests under real conditions. In addition, the possibility of depositing the coatings onto complicated or large parts must be assessed.
3.GREASE LUBRICATION IN TRIBOLOGY (GRIT)
One area of tribology identified for a concerted research action is that of grease lubrication. Greases have been used successfully for many years and are perceived as an established technology: one consequence of this is that fundamental research has been neglected in recent years. This is not true of machine engineering where continuing development in design and materials has resulted in a significant increase both in the life of components and the severity of their operating conditions. Because this has not been matched by a concomitant improvement in grease performance, grease lubrication is seen as a limiting design factor for a wide variety of applications, particularly bearings and gears.
In recent years, fundamental research into the mechanism of liquid lubrication has successfully answered many of the basic questions concerning their operation so that for many applications performance can now be accurately predicted from simple lubricant parameters. This is not possible with greases, mainly due to the complexity of their two-phase structure. As a consequence, most grease lubrication research is application driven rather than generic, being undertaken for a specific application. As such it requires expensive and complex experimental simulation of the problem. Such research does not contribute to our fundamental understanding of lubrication mechanisms. It is therefore difficult currently to predict grease performance, in any given application, from simple tests or even from the standard grease classification system.
This last point is an important one as the customer has little real information on the selection of the correct grease for their application. There is considerable confusion both in the meaning and relevance of the grease characterization parameters in use today. The only standardized classification system is the NLGI cone penetration number which gives very basic information about the consistency of a grease. One of the problems in developing new classification tests is the identification of the relevant parameters for grease lubrication, and this is one area which must be addressed. The ultimate aim must be a classification system which will accurately describe the lubricating performance of a grease for any application yet be intelligible to the customer.
The fundamental problems associated with grease usage, both in application and manufacture, and the benefits derived from improved grease technology have been recognized in the recent founding of the European Lubricating Grease Institution (ELGI), a sister organization to the National Lubricating Grease Institution (NLGI) in the United States. Two areas currently being investigated by committees set up under the auspices of the ELGI are the development of new classification systems and fundamental grease research.
The committee on fundamental research of the ELGI has canvassed opinion within Europe and considerable interest has been expressed by both industrial and research organizations about participation in such a research effort. Three specific areas have been identified:
(i)identification and development of simple tests to characterize greases so that accurate prediction of lubricating performances is possible. This is only really possible if the underlying relationships between grease structure, composition, rheology and performance are understood;
(ii)fundamental studies of the mechanism of grease lubrication;
(iii)the effect of chemical additives, included to improve lubrication performance under severe conditions, on the fatigue life of rolling element bearings.
The research areas identified consolidate our knowledge for the present and represent a minimum level of understanding required to solve current problems. Such research should also promote the development of new grease technology with benefits for manufacturer, customer and the environment.
In conclusion, the need for concerted action within Europe on fundamental research into grease formulation and lubricating performance is widely recognized within the industrial and research communities. However, action has been hampered by lack of funding for generic research into grease lubrication, and the possibility of support via COST would be both valuable and timely.
Current status
The Action started on 30 June 1994. The research programme itself started after Summer 1995. Management Committee decided to divide the accepted project into several packages : GRIT (grease lubrication), REAL (renewable environmentally adapted lubricants), and CAST (coatings and surface treatments). The CAST is divided into six subpackages. At present 65 projects are active.
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Belgium