Triboscience and Tribotechnology: Superior Friction and Wear Control in Engines and Transmissions

Obiettivo

A. BACKGROUND

Tribology is the science and technology of interacting surfaces in relative motion, the word tribology being derived from the Greek word tribos, meaning rubbing. Tribology encompasses a wide range of problems related to friction, wear and lubrication. Overcoming such problems has played a central role in the advancement of mankind, from moving massive ancient stone blocks and constructions and the invention of the wheel through to the development of important machinery components in the industrialised society such as bearings, gears, seals, liners, cylinders, tools, etc. Today the movement of the piston in the cylinder of a car engine is enabled by the shear of a well controlled thin lubricating oil film, the digital data transmission in computers depends crucially on the tribological air-bearing solution in the magnetic storage system and the successful function of replacement hip joints is dependent upon on advanced wear and lubrication control.

The level of tribological knowledge and its application influences society in several ways. Optimised friction in industry and transportation results in energy savings, controlled wear and lubrication improves the efficiency and quality of production and results in increased reliability and safety, reduced costs due to down-time and reduced environmental risks and impact. It has been estimated that about half of all the energy produced is used to overcome friction and that almost half of all the products produced in the industrialised countries are to replace worn out products. The calculated direct cost for friction and wear in an industrialised nation has been estimated at 7% of GNP and detailed studies carried out in several countries (e.g. USA, UK, Germany, Finland) show that 1% of the GNP can be saved by the use of more optimised tribological solutions.

Many problems related to tribology were long neglected, principally because of their multidisciplinary character, which meant that they did not fit well into any of the traditional scientific disciplines. It is not possible to find optimised solutions for tribological problems without expertise in material science, physics, chemistry and mechanical engineering. These disciplines were in the 1970's brought together under the name Tribology. This initiative quickly triggered increasing worldwide research activity in universities, research institutes, as well as in industry. As a direct result of such efforts there have been several examples of new products and solutions in transportation and production that have had a remarkable economical and safety impact. For example, it is today possible to deposit, by vacuum techniques, thin nanolayers on the surfaces of sliding contacts that reduce friction and wear by several orders of magnitude.

However, even if recent development has been rapid, our knowledge today about the mechanisms and interactions taking place between two moving surfaces is still poor in comparison with the detailed knowledge that exists about atomic structures, molecular reactions and biological interactions. On the scientific level we are still only at the very beginning of understanding friction on the atomic scale, there is very little generic understanding of the chemomechanical mechanisms occurring in boundary lubrication and the role of lubricant rheology in the sliding of real rough contacts is not well understood. A better understanding of these phenomena will form the basis for finding better and optimised solutions to important industrial problems related to the functionality, reliability and lifetime of machinery, engines and transmissions and the control of manufacturing processes.

Due to industrial needs, higher power output is required from smaller and more lightweight machinery, i.e. the power density increases. This places higher demands on the sustainability of the contacting surfaces. Other factors important to industry are prolonged life, weight reduction, reduced noise, environmental acceptability, recyclability and robustness. These problems require, to a great extent, better tribological knowledge in order to be solved.

Engines and transmissions play a central role in the function of all kinds of vehicles and production machinery. From a tribological point of view they are both very challenging, the engines because of the requirement to create lubricated low friction and low wear contacts at high temperature in chemically reactive environments and the transmissions because of the requirement to transmit a huge force through a very thin lubricant film, typically of the order of one micrometre, in a reliable way for 10-20 years.

There is a fairly good knowledge and vast experience of how engines and transmissions perform with the lubricants and fuels that are in use today. But to solve future environmental and other sustainable performance requirements we need to introduce completely new design solutions such as engines where the oil derived fuels are replaced with hydrogen, where the lubricant supply is reduced to microamounts of about 3 orders of magnitude less than in use today or completely replaced by dry constructions. Such constructions will completely change the friction and wear conditions and the understanding and methods we use today will not be applicable. We will need new fundamental knowledge about the prevailing tribological mechanisms and interactions occurring to enable proper operation.

Significant research on friction, wear and lubrication was carried out early in the last century, especially in Europe, the USA and Russia. The concept of tribology as a scientific discipline in itself originates from Great Britain (the Jost ministerial report of 1966) and is now accepted and supported worldwide. Today the forefront research in tribology is carried out in the USA, Europe, Japan, Russia and Australia whilst high level contributions in increasing number are appearing from China, India and other Asian countries.

The International Tribology Council organises World Tribology Congresses every four years, regional tribological conferences are held regularly in Scandinavia (NORDTRIB), the Balkans (BALKANTRIB), the Mediterranean area, UK-France (Leeds-Lyon) and there are national conferences in most European countries.

The European Commission has announced a Virtual Tribology Institute open for applications. The framework programmes of the European Commission have not directly addressed tribology but some separate projects dealing with surface coatings and the wear of ceramics have been carried out.

COST 516 Tribology was the first European joint research action dedicated directly to tribology. It focused on three areas of tribology: surface coatings and surface treatment, grease lubrication and lubrication by environmentally acceptable lubricants. The action was successfully carried out in the period from 1995-2000 involving 64 projects from 23 countries and was evaluated in June 2000. Several European research organisations and industrial companies have expressed a large interest in starting a new more focused action in this field. They feel that it would fit well into the research strategy of the European Commission, emphasising the importance of improvements in transportation, sustainable growth and material research.

COST 516 was a broad Action investigating largely tribological mechanisms and interactions in a large variety of industrial applications. Differing from this new tribology related Action has a focused approach on four specific scientific topics to achieve improved understanding and new technical solutions and it will be implemented on two important applications, that is engines and transmissions.

B. OBJECTIVES AND BENEFITS

The main objective of the Action is to generate new scientific knowledge about the fundamental physical, chemical and mechanical phenomena governing friction, wear and lubrication. This knowledge will be used to develop novel low friction, wear control and environmentally adaptable lubrication solutions to solve the functionality of future engines and transmissions such as engines working with hydrogen fuels, micro-lubricated and dry lubricant free transmission applications.

The benefit of the action is that the engine and transmission producing industry in Europe, that is especially the automotive industry but also other sectors such as ship and rail industry, power plants and automated production, will be better prepared to meet the new challenges presented by changes taking place in our society and will be competitive on a global scale in the future. The aim is that European related industry will have a leading role when developing more sustainable solutions in transportation and industrial production.

The role of this research on sustainable development and its economical impact on the whole of society is well illustrated by the following statistics on oil consumption. The oil production in Europe was 325 million tons in 1998 which is 9,3 % of the world production and the European consumption was 760 million tons which is 22,4 % of the world consumption. Transportation stands for 57 % of the oil consumption and industry for 20 %. The crude oil resources available worldwide today is only the quantity required for some 50-100 years.

Automotive fuel consumption in West Europe is about 250 million tons per annum. An efficiency improvement of only 0,5 % would save some 1,2 million tons of fuel annually. For this reason the energy consumption originating from friction in the engine is of crucial importance and has a large impact on sustainable development.

The use of oils as lubricants is today a large waste of valuable natural raw material. Europe stands for 34 % of the world lubricant consumption and of that about 50 % is recycled. This means that some 2,5 million tons disappears into the environment every year. Even though some of the base oils are biodegradable, the presence of chemically active and toxic additives in the finished lubricants lowers their total compatibility with the ecosystem.

The user sees a well functioning tribological system as a high quality system, as it is predictable and efficient. The possibility to market and sell products is a very steep function of the perceived quality - price relationship. Well-designed tribological systems in a product can easily double its perceived quality (e.g. usable life) without increasing its manufacturing cost.

New innovations are expected as spin-off effects from the Action. They will come both as results of small stepwise improvements and in the form of a technical break-through made possible by the scientific research. This has already been shown during earlier developments, e.g. the use of diamond-like carbon coatings with and without metal-containing dopants.

The objective of this COST Action is to bring together researchers and industrial experts from different parts of Europe to solve scientific problems relevant to future engine and transmission applications and thus to make the European industry better prepared for future challenges and competitive in the future global market. The purpose is to produce new scientific knowledge related to tribological problems in engines and transmissions and to make sure that it is quickly applied in the related industry.

The dissemination of the technical and scientific achievements will target direct transfer of the results to the companies participating in the projects and to widespread publication of the results to all potential exploitation organisations in Europe through scientific and professional articles as well as by arranging conferences, seminars and courses. A further aim is to strengthen role and image of European research organisations on the international scientific and technological stage and to ensure their position in the forefront of both fundamental and applied tribology research.

The COST 516 Action that was completed in June 2000 built up a very good structure for contacts and knowledge exchange in the field of tribology largely in Europe and this can be used in COST Action 572. In this new Action there is a need to build up a new and effective network around the topics to focus on in the Action and to bring in the needed new groups of competence. The co-operation structure within this COST Action will also help to produce good applications for the EU VI framework programme.

C. SCIENTIFIC PROGRAMME

The programme has a scientific part, 1. Triboscience, which focuses on Lubricant chemistry, Scale aspects of micro- and nanomechanisms, Rheology, Materials and coatings and Evaluation testing. The implementation part of the programme, 2. Tribotechnology, includes the implementation of the scientific knowledge in two selected application areas Engine tribology and Transmission tribology. The application areas have been selected because of their great importance in the future development of competitive logistics in Europe and on the development of new sustainable industrial solutions.

1. TRIBOSCIENCE

1.1. Lubricant chemistry and chemomechanical effects

To understand the influence of chemically active lubricant additives or contaminants like water in oil, it is necessary to have the chemo-mechanical interactions developed and understood. The chemical interactions not only take place at the surface, but also deep under the surface by diffusion or other processes. When lead-containing oil additives were banned about fifteen years ago, sulphur and phosphorus containing additives were developed to make it possible to run and run-in heavily loaded gear transmissions. The additives made it possible to have a well controlled running in process for the surfaces, which became smoother and achieved a low wear rate and long life. Later it was discovered that those additives had an extremely detrimental effect on the sub-surface fatigue life of bearings, especially if they were running at high temperatures. The additives, which were supposed to increase the machine life, decreased the bearing life by about one order of magnitude when the bearing temperature reached 150Ã¸C. The chemical activity on the steel surface made the material deep below the surface crack.

From normal steel fatigue investigations it is well known that the air humidity around the tested machine element has a major influence on the endurance life. The life to failure is much shorter at a given stress level when the humidity is higher. It is also known that the endurance stress level for infinite life becomes zero when water is present at the steel surface. This is a very common and important situation in for example paper mills where water and steam are surrounding heavily loaded mechanical parts.

Also for oils with low laboratory levels of humidity, the molecular structure is known to have an influence on the endurance life and crack propagation. In rotating beam experiments it has been shown that a beam surface wetted with a naphthenic oil gave twice as long endurance life as if the surface was wetted with a paraffinic oil. The combined mechanisms of chemical activity and mechanical stresses are not at all understood today, and need to be investigated.

As an example, it is currently not understood, nor possible to model, how water in a lubricant will influence the hydrogen content in the steel of a rolling bearing. When the bearing is running loaded and lubricated with oils containing a few hundred ppm of water, the bearing steel will absorb hydrogen as long as the bearing rotates. As soon as it is stopped, any free water outside the bearing surface will not only stop diffusing hydrogen into the steel, but also help to remove the hydrogen from the steel at a higher speed than in a dry environment. The bearing rotation under load changes the direction of hydrogen transport from going out from the steel to going into the steel. High hydrogen content in the steel makes it brittle and susceptible to spalling failure and gives a short endurance life.

1.2. Scale aspects, micro- and nanomechanisms

The science of tribology has developed from simple mean value predictions towards atomic level understanding. The phenomena seen at dry contacts and thin film lubrication contacts have now reached the micro-to-nanometre level. Wear and lubrication cannot be predicted using continuum mechanics when the phenomena studied take place within a few atom layers. The miniaturisation of electro-mechanic devices, like magnetic storage heads, as well as the hot spots in wearing devices and grease lubrication films all need basic tribological knowledge on the micrometre, nanometre and atomic scale.

The fractal properties of mechanically manufactured surfaces, indicates that the surface roughness slopes are strongly dependent on the length scale they are measured at. That means that manufacturing processes, which work for millimetre size surface features probably not will work for atomic scale features. The measured surface slopes will increase when the surface features decrease in size, and thus the features will not function in thin film lubrication conditions.

The nano-sciences open up new perspectives on both observing and modifying spatially localised single molecules and small atomic clusters. From a traditional point of view this is, like tribology as a whole, a truly interdisciplinary endeavour, making use of new methods like molecular dynamic computer simulations and atomic force microscopy applicable to numerous problems in condensed matter physics, in many areas of chemistry and in cellular and sub-cellular biology.

Recent advances in atomic manipulation and self-organised growth make it possible to use a surface as a two-dimensional nano-scale quantum laboratory in which the built nano-systems are subject to lateral boundary conditions on a length scale where quantum coherence prevails. Work is also underway into further development of nano-structuring techniques, fabrication of novel metal quantum constructions and on exploration of their physical and chemical properties. The physical and chemical properties of nano-structures are unique functions of their size and shape and differ largely from the behaviour of bulk matter, e.g. magnetic nano-structures can be made out of traditionally non-magnetic elements.

1.3. Rheology

It is well known that high pressure lubricant rheology has a major influence on power loss in lubricated contacts. The higher the shear strength of the oil is, the higher the power loss will be for a given sliding speed. It is not, however, known how the high-pressure shear stress - shear strain rate relationship will influence the lubrication of rough engineering surfaces in pure rolling and different degrees of sliding. When the lubricant is compressed into the glassy solid state in the high pressure region of the elastohydrodynamic contact, the ability to separate elastically deformed asperities with an oil film is very strongly dependent on the total stress tensor and thus on the high pressure
rheology of the lubricant. In experiments with lubricated rather smooth steel surfaces (Ra =0.18

-m) a sliding distance during the contact time of 29 m made the lubricated surfaces break through the oil film and cause metallic contact. This happened even when the lubricant viscosity was 100 times higher than the viscosity needed to separate the surfaces when no sliding was present. A sliding distance of one surface roughness wavelength was thus enough to break through the oil film.

It is now possible to manufacture surfaces to very high quality, low surface roughness and surface slope, such that elastic deformation on two such surfaces can become totally conforming. This means that they are extremely easy to lubricate in pure rolling, but when sliding is introduced, the lubricant rheology will determine if it is possible to keep the elastically deformed asperities compressed into the surface or if they will break through the oil film. It is thus extremely important to measure the lubricant rheology and contact behaviour at the boundary between oil and solid to be able to predict oil film collapse and metal to metal contact.

1.4. Materials and coatings

Materials used in industry have until recently mainly been produced and developed to have specific bulk properties, but have not been optimised for their surface properties. That is changing quickly, and the bulk material properties can now be changed locally at the surface to give the required surface properties. The most vital areas of materials development for tribological applications is confined to surface engineering that offers several new possibilities: to implant alloying atoms to different depths, to deposit surface layers, both thin and thick, which change the surface properties regarding friction, wear, elasticity and contact strength and to make new surfaces using powder technology, yielding material mixtures not possible to manufacture in bulk processes.

The combination of all these possibilities has great potential for implementation in engines and transmissions to decrease power and material losses, and to make it possible to increase the power density far beyond the levels presently possible. New break-through solutions are the DLC (diamond- like carbon) and MoS2 (molybdenum disulphide) based doped and layered coatings. With such thin coatings of about 1 (m thickness it has recently been possible to reduce both friction and wear with two orders of magnitude in dry sliding contacts.

One main trend today is actually not to invent new coating materials, but to optimise the existing materials through a better understanding of their function when applied to mechanical components.

The influence of the parameters to be investigated in detail are e.g. hardness and thickness of coating, hardness of substrate and counter face, initial topography of both partners, generated debris, reaction and transfer layers, and composition of lubricant and its additives. To be prepared for reduced, minimised, or even excluded need of lubrication is a better understanding of running-in effect on the ability of ceramic thin coatings on components to endure oil starvation needed.

1.5. Evaluation testing

The chemical composition or the manufacturing process of new lubricants, additive systems, coatings and materials is specified in articles and often protected by patent. Even when it is public, it is still difficult to specify their properties for the application from their chemical or physical structure. Their mechanical and technological properties relevant for the application can be evaluated in test methods. A generalised way to describe the frictional properties of fluids, e.g. for different design solutions of continuously variable traction drives with or without spin conditions could be in a twin disk machine under defined conditions of pressure, speed and temperature. Mathematical simulation systems have to be established to adjust such neutral data for lubricants from tests to any of the existing design solutions. Similar considerations apply for the definition of properties of coatings for applications in bearings, gears, synchromesh systems or wet clutches.

The benefit of such empirical testing lies in the fact that completely different approaches to one and the same problem can be compared on the same technically relevant basis. Solutions for one specific application "as good as needed" can easily be found and then a solution "as good as possible" does not have to be applied with too high costs.

Mathematical simulation systems help to transfer test results at test conditions into specific machinery at operating conditions. They often help to save substantive cost by a reduced amount of necessary field testing.

2. TRIBOTECHNOLOGY

2.1. Engine tribology

There was relentless pressure in the second half of the 20th century to develop ever more fuel efficient and compact engines with reduced environmental impact, especially in the automotive sector. In the 1980's and 1990's research was directed towards the main tribological systems of the engine, the piston assembly, the engine bearings and the valve train. The drivers for engine tribology research shifted cyclically during this period between the three major concerns. The concern to better control friction for improved fuel economy and maximising power; the concern to control wear for increased durability and improved reliability; and the concern to minimising harmful exhaust emissions by reducing oil consumption, combustion products and carbon dioxide.

There were specific studies responding to changes in fuel composition such as the removal of lead from gasoline, which resulted in inlet valve seat recession problems, and the reduction of sulphur in diesel, which raised concerns about the boundary lubrication of high pressure fuel pumps.

Much progress has been made in experimental and theoretical research but, however, there are still major shortfalls in our knowledge in relation to crucial tribological understanding of contact mechanisms of components in engines.

Engine lubricants are complex mixtures of chemicals, which, in essence, are optimised on the basis of empirical testing in laboratories and test engines. The mechanisms by which the additives in the lubricant react with each other and the surfaces of the components in the chemically reactive environment of the engine, resulting in the observed improvements in friction and wear, are far from established.

The mild wear of components such as piston rings, cylinder walls, cams and followers is an essential part of the running-in process of the engine and crucial to its long term performance. This takes place over a significant time period when the physical and chemical properties of the lubricant are constantly evolving due to degradation, principally thermal oxidation, and contamination, mainly by fuel, soot and the reactive products in exhaust gas recirculation. The interaction over time between the evolving surfaces and lubricant is not well understood. Furthermore, the growing tendency to use more hard wearing coatings for such components, with less opportunity for running-in, and to leave the same lubricant fill in the engine for greatly extended periods demands that we gain a much better understanding of this lifecycle.

Many of the interacting surfaces in an engine are poorly controlled dimensionally with respect to geometric tolerances and relatively rough in tribological terms such that the surface texture, principally waviness and roughness, can significantly influence hydrodynamic performance and cyclic stressing of the surfaces. In addition, the rheology of the lubricant is complex with high likelihood of piezoviscous and shear thinning behaviour and the far more uncertain flow properties arising form degradation and contamination, for example the influence of soot in suspension. The amalgamation of complex rheology with real surface texture is a major challenge for the mathematical modellers.

Engine tribology has implications for overall engine performance beyond friction and wear and relies also on other aspects of system response. In the former category, poor tribological performance can manifest itself as unwanted engine noise, for example piston slap and crankshaft bearing rumble. Latterly, tribology is highly dependent on the detailed thermal behaviour of the engine. Such linkages have received relatively little attention despite their undoubted importance.

Today the above challenges are becoming ever more acute as a result of Original Equipment Manufacturer (OEM) and legislative pressure in relation to:

-step changes to completely new fuels, e.g. hydrogen, and lubricants responding to environmental concerns and the depletion of world reserves of mineral oil,
-more compact, smaller displacement, turbocharged, high speed engines,

-more thermally efficient and thereby higher temperature engines,
-improvements in fuel economy through friction reduction and reductions in oil consumption,
-extended oil drain intervals aiming at "fill for life",
-changing engine combustion strategies to minimise unwanted exhaust emissions and improve efficiency, e.g. exhaust gas recirculation and gasoline direct injection, and
-growing constraints on the composition of lubricants and fuels to minimise unwanted exhaust emissions and avoid the poisoning/inhibition of exhaust after-treatment devices.

Tribological interfaces in the engine will be subjected to higher temperatures and loads and will be supplied with a less effective but environmentally adaptable lubricant in much reduced quantities. To maintain good engine functionality in these new conditions controlling friction and wear will be a huge challenge and demands distinct improvements in the analysis and design of the components and lubricants to maximise lubricant film thickness between the surfaces throughout the engine cycle and the introduction of improved materials, surface coatings and surface engineering.

2.2. Transmission tribology

The design of new transmissions and gearboxes has to offer increased transmitted torque, improved efficiency, reduced emission of noise and vibrations, increased life expectancy, prolonged service intervals, good shift quality not to mention low cost. New challenges in the design of transmissions are also to be found in the demand for automatic transmissions. The current percentage of automatic transmissions in Europe is some 40%, whereas in Japan more than 70% and in the US close to 100% are automatic transmissions in automotive applications. The future trend involves not only standard automatic transmissions with planetary gear sets and clutches and brakes but increasingly continuously variable transmissions with belts and chains as power transmitting elements or toroidal traction drives. These new designs demand completely new lubricant properties like high friction in the contact under elastohydrodynamic as well as mixed lubrication conditions to transmit high shear forces and high scuffing performance to prevent damages under sliding conditions.

All these demands are limited by dynamic phenomena in the transmission and by surface related phenomena such as friction, traction, formation of tribological layers and surface failures such as wear, scuffing, micropitting and pitting fatigue as well as noise emission.

In reducing the churning losses in gearboxes oil quantities are constantly reduced. This leads to higher stressing of the available oil volume. At the same time the oil change intervals are prolonged with again high demands on ageing properties of the lubricant. Lubrication and failure phenomena under almost lubrication starvation conditions are not well understood. In some applications even lubricant free operation - dry gearboxes - are under consideration. This puts emphasis on the development of materials or coatings with superior frictional properties and high strength values under high temperature conditions. Also lubrication with solids not only as coatings but also as powders comes into consideration. Chances of dry lubrication are not only to reduce churning losses but also to save natural resources and waste material and make recycling of oil-free machinery easier.

Prolongation of oil change intervals, reducing service costs, demands for superior oil ageing properties of the lubricant and challenges the development of filter and sealing components.

As for surface fatigue and micro-cracks, the use of coating technology has proven itself rewarding concerning prolongation of component life. By optimising the choice of coating material, thickness and deposition mechanism, benefits can be achieved in areas such as strength, wear and corrosion resistance. Extensive studies have to be performed since different applications call for different coating properties. Recent advancement in coating technology show

Programma(i)

IC-COST - European cooperation in the field of scientific and technical research (COST), 1971-

Argomento(i)

2 - Metallurgy and Materials Sciences

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