24th Leeds Lyon Symposium on Tribology

Citation

(1999), "24th Leeds Lyon Symposium on Tribology", Industrial Lubrication and Tribology, Vol. 51 No. 1. https://doi.org/10.1108/ilt.1999.01851aab.012

24th Leeds Lyon Symposium on Tribology

24th Leeds Lyon Symposium on Tribology

London

"Tribology for energy conservation"

In contradiction to what one might expect from the title of this series of Symposia, the 24th event was actually held in London for the very good reason that, since the World Tribology Congress was also being held in London the following week, it could reasonably be expected that the turnout for both events would be augmented.

The symposium consisted of the usual format with all delegates invited to attend the keynote address and the presentations by the invited speakers after which the series of presentations were divided into two groups.

The presentations described below are of necessity limited to those given by the invited speakers, and also to those presentations covering the general subject of surface coatings.

Fuel economy improvement by engine and gear oils

Wilfried J. Bartz, Technische Akademie Esslingen, In den Anlagen 5, 73760 Ostfildern, Germany

The fuel consumption of a car depends on a set of parameters partly related to lubricants. Their influence mostly is much more pronounced than the lubricant influence. Only the mechanical losses can be decreased by lubricant-related measures; the fuel economy improvement which possibly might be realised is therefore rather limited. Only about 1/3 of the total friction losses occur in the mixed-film or boundary regime, whereas 2/3 are fluid-film friction losses. This ratio has to be kept in mind if the relative influences of friction modifiers or lower viscosities are estimated and evaluated. Changing viscosities will vary this ratio. When evaluating the viscosity influence on fuel consumption, the so-called effective viscosity must be taken into account. This is most important for non-Newtonian oils. Reducing the engine-oil viscosity by one SAE viscosity grade will result in fuel consumption reductions of 0.6 to 5.5 per cent at high temperatures and 1.0 to 7.5 per cent at low temperatures. The corresponding data for gear oils are 0.2 to 1.5 per cent (high temperatures) and 0.4 to 2.5 per cent (low temperatures). Using friction modifiers, fuel-consumption reductions between 0.7 and 4.0 per cent in engine oils and 1.0 and 6.0 per cent in gear oils are realistic. On the basis of a 50 per cent friction reduction, maximum-fuel-consumption reductions between 2.7 and 5.8 per cent by other engine oils and between 1.0 and 5.1 per cent by other gear oils are possible, considering the different driving programs. The total reduction is between 3.7 and 10.9 per cent.

Engine oil fuel efficiency ­ practical issues

Stefan Korcek, Ford Motor Company, Powertrain and Vehicle Research Laboratory, Dearborn, MI 48121

Engine oil plays an important role in improving fuel efficiency of gasoline internal combustion engines. Engine manufacturers would like to maximise oil contributions to fuel economy to meet future, more stringent targets of governmental regulations, improve energy resource utilisation, minimise CO₂ emissions, and last but not least, to satisfy their customers. To achieve these improvements engine manufacturers are developing or sponsoring development of engine tests to evaluate fuel efficiency of engine oils. In the USA, it was the Sequence VI test that was first used for this purpose; now, it is the Sequence VIA test, introduced in 1997. Work is already in progress on development of the Sequence VIB test which is planned for the year 2001. These tests, during the period of their development and introduction, are the major driving force leading to improvement of engine oil-derived fuel efficiency. Fundamental requirements for introduction of these tests include a correlation with "field" performance and use of an engine which is representative of the current population of engines. The "field" performance can be expressed by the correlation with the EPA M/H (Metro/Highway) test, used to determine the corporate average fuel economy (CAFE). The engine used in Sequence VIA is a 4.6L 2V Ford Modular engine, which will also be used in the Sequence VIB. The most important improvement in Sequence VIB will be in oil ageing conditions and the extent to which these conditions will match oil ageing reached at the end of 4000-6000 accumulated miles mandated prior to running the EPA M/H fuel economy test for CAFE. Incorporating more extensive ageing in Sequence VIB will require substantially improved retention of engine oil fuel efficiency. This improvement of retention can be mainly achieved through minimising of viscosity increase and achieving and maintaining of low boundary friction. Factors that affect the accomplishment of these improvements are discussed in this paper. Special attention is being given to interactions of additives among themselves and with base oil during the ageing process. Our studies of chemical interactions and their effects on friction reducing capabilities of a molybdenum dithiocarbamate/zinc dialkyldithiophosphate additive system, used in formulating advanced low friction/low viscosity engine oils, will be reviewed to illustrate the importance of the above interactions under oxidative conditions. Results of these investigations clearly show that further improvements and optimisation of fuel efficiency are possible and require understanding of changes occurring during oil ageing. However, it also can be concluded from these studies that tribologists, in studying various lubricated systems, besides investigating these systems with initial components, should develop an understanding of processes and changes occurring in these systems during their operation. This is the only way in which energy conservation can be accomplished.

Some challenges to tribology posed by energy efficient technology

Hugh Spikes, Tribology Section, Imperial College, London

A major preoccupation of modern industry and, indeed, society is to develop and continually improve energy efficient technology. For society as a whole this helps preserve natural resources and minimises the negative environmental impact of unbridled fuel consumption. So far as industry is concerned it satisfies the consumer and the regulator and thus maintains competitiveness. The science of tribology underpins this quest for energy efficient technology at many different levels. In manufacture, tribology plays a key role in the forming, cutting and shaping of metals, ceramics and polymers. Effective application of tribological principles enables not only reduced energy consumption and reduced material losses in manufacture but also improved and more tightly-controlled surface finish. The latter plays an important role in the development of low friction components. Another aspect of energy efficient technology is the requirement for engineering components such as bearings and piston assemblies to have long life, thus reducing both the energy cost implicit in replacement and the period of inefficient operation which precedes component failure. Since life is often limited by wear, rolling contact fatigue or other forms of surface damage, tribology obviously has a direct contribution to make in this sphere.

A third and perhaps the most obvious contribution of tribology to energy efficient technology is simply to design rubbing systems with low friction, so that these provide energy-efficient service over their lifetime. Most effort so far has gone into designing fuel efficient crankcase engines but there is growing concern to achieve low friction losses in transmissions, compressors and components such as vehicle tyres.

Perhaps the most fundamental and challenging level at which tribology is required to contribute to energy efficiency is to meet the needs of new energy-saving technologies, for example, in the development of very high temperature engines. These promise large energy saving benefits but only if concomitantly large technical problems are solved; such as how to effectively lubricate loaded, rubbing components at temperatures in excess of 500°C.

Five specific areas were examined where tribology has met or is meeting challenges posed by energy efficient technology. The first addressed the operation of systems with thin lubricant films to permit the use of energy efficient, low viscosity lubricants. The second two concerned the feasibility of new, energy efficient technology, traction drives and high temperature engines. The fourth discussed the energy saving implications of extending the life of rolling element bearings and finally, the benefits of developing simulations of complex tribological systems such as engines and transmissions were discussed.

Amongst the variety of other papers presented, three on the subject of surface coatings were of particular interest, and are briefly described as follows.

Energy efficiency through surface engineering

P.A. Dearnley, University of Leeds, UK, H. Weiss, University of Siegen, Germany

Recent innovations in the application of surface engineering technologies has led to significant energy savings in a variety of application sectors. Three types of material have benefited from recent developments:

• •

  carbon fibre reinforced polymers (CFRP);

• •

  aluminium alloys; and

• •

  titanium alloys.

Efforts have been made to replace steel rolls used in the German paper-making industry by CFRP. This is an attractive approach because a substantial reduction in roll mass is achieved and, consequently, less power is needed to drive the rolls during production. Hence, a tangible reduction in energy saving can be achieved. Unfortunately, CFRP rolls are subject to high wear rates through abrasion. A method has been developed that allows the surface to be coated, first with electroless nickel (an autocatalytic deposition method) and then finally with electroplated chromium. The key to the success of this approach is in the surface preparation of the CFRP prior to coating. This involves surface roughening, using SiC grit blasting, chemical cleaning, chemical activation and conditioning. Early results are encouraging and a number of trial rolls have been produced. The same method is presently under consideration for low mass components used in turbomolecular vacuum pumps.

Related innovations include the development of autocatalytic nickel-silicon carbide and nickel-chromium carbide deposition methods. These are now being applied for the surface protection of aluminium alloy cylinder surfaces, as a replacement for cast-iron sleeves. This represents a significant vehicle weight saving. Also of note is the development of lightweight disc-brake rotors made from aluminium alloys that are surface protected with a copper-SiC composite coating applied by arc spraying. The heating rate of both the disc rotor and the friction material are reduced. Without the coating there is significant risk of seizure, a factor that has inhibited the development of aluminium rotors to date. Microarc (MAO) oxidation is a new approach to the surface engineering of aluminium alloys. It enables layers of alumina to be grown on the surfaces of aluminium alloys that are thicker, harder (>800Hv) and denser than conventionally anodised aluminium. Tests in both abrasive and lubricated sliding contacts demonstrate an improvement in wear resistance, e.g. compared to untreated and laser alloyed aluminium alloys. Accordingly, MAO could significantlv increase the wider use of aluminium alloys in tribological applications.

Another possibility to achieve energy efficiency gains is to apply lubricious coatings of WC-C, diamond-like-carbon (DLC) or MoS₂ to metallic surfaces using, for example, the magnetron sputter deposition technique. Such coatings significantly reduce the coefficient of friction in both sliding and rolling contact situations. Some variants of DLC also appear tougher (damage resistant) in abrasive wear media. Accordingly they may have greater potential for the protection of bearing surfaces in both automotive engine and bio-medical implant applications, than more established coating materials like TiN.

Behaviour of diamond coatings on cutting tools

D. Paulmer, J. Rousseau*, T. Mathia*, T. Le Huu, H. Zadi ; F. Govin*, J.F. Larose**

LPMM Laboratory ­ CNRS URA 1215 ENSEM-NPL 2 avenue de la Forêt de Haye 54516 Vandoeuvre Les Nancy ­ France.

*Laboratoire de Tribologie et Dynamique des Systèmes ECL, ENISE, CNRS UMR 5513 ENISE Département de Physique des Interfaces 58 rue Jean Parot 42023 St Etienne ­ France

** ENISE Pôle de Productique

The best conditions for high and very high speed machining require cutting tools covered by ultra hard ceramic coatings. Here, we operated with pure diamond coatings deposited on turning plates and investigated two families of diamond coatings: the "commercial" coatings (usually formed by the CVD processes) and the "laboratory" coatings formed by the flame method. The latter method enables diamond coating deposits to be formed at high growth speed and with reductions of energy consumption and cost compared to the usual commercial coatings.

The laboratory system for diamond deposition by the flame method is based on a blowtorch with a circular commercial-type nozzle which is used for generating and conducting the flames. The pressures are monitored by highly accurate mass flow-indicators. Diamond coatings are deposited on cutting plates made in tungsten carbide which is also the substrate of the commercial plates. This process allows diamond-type coating deposits with a high growth speed and an excellent quality at low energy consumption.

Pure diamond cubic crystals [111] and [100] faces are identified by SEM and by the diamond specific Raman peak at 1,334cm^-1. The initial topography of the different coatings on the cutting plates was controlled by AFM and conventional tactile devices. Machining tests of the cutting plates were carried out by turning AU4 G Al samples in controlled and constant conditions, with a cutting-speed of about 1,000m/mn.

To demonstrate the differences in the behaviour of the two different families of coatings, we controlled the substrate-coating adhesion after machining and then examined both the cutting quality and the wear of the coatings. To achieve this, the 3D topographic state of the turned A1 samples surfaces was checked for the first and for the last one (50th) of the series. The values of the pertinent Root mean square (R_q), Skewness (S_k) and Kurtosis (E_k) were obtained. 3D surface statistical microroughness parameters are reported in Table I.

Table I

Parameter P P50 P1 P50 P1 P50

Commercial plates H10 type 1810 type

Rq	1.88	1.08	0.96	0.98
Rsk	0.87	0.15	0.43	0.98
Rek	1.88	1.80	1.88	1.66

Laboratory coatings plates

	Type 1			Type 2-1			Type 2-2
Rq	0.82	0.74	2.58		1.32	0.71		0.59
Rsk	0.20	0.30	0.14		0.50	0.63		0.86
Rek	1.66	1.79	1.43		1.91	2.22		2.59

For the three laboratory deposits, results show the roughness parameters developing in the same direction but with different amplitudes. This indicates a "softening of the machining" through the wear of the cutting materials. The slight variations of the roughness statistical parameters led us to conclude that this wear is simply due to a blunting of the sharpest asperities of the diamond crystals in cutting positions.

For the commercialised cutting tools, the concomitant decreases in R_sk and the R_ek revealed a trend to a depth decrease in the micro-roughness relief. This should result in a poorer cutting quality on the external side of the turning scratches. To understand this anomalous behaviour we controlled by AFM the 3D morphology of the diamond coating, close to the cutting line. The image showed a local scaling of the deposit with limited tearing areas due to inhomogeneity of the coating-substrate adhesion.

Since this did not occur with the laboratory depositions on the tungsten carbide plates, we conclude that diamond coatings deposited on cutting tools by the flame method are at least comparable with, if not better, than diamond cutting tools available on the market.

Physical characterisation of the antiwear performance of phosphate coatings. Application to engine parts

G. Monteui*, G. Meunier*, C. Roques-Carmes**

*PSA Peugeot Citroën Centre Technique de Belchamp 25420 Voujeaucourt** ENSMM 26 Chemin de l'Epitaphe 25030 Besançon Cedex France

It is well-known that manganese phosphate coatings are very effective in protecting the surface of mechanical parts against wear processes (Khaleghi et al., 1979; Wan et al., 1988). Consequently these coatings are widely used in the manufacturing of automotive parts, mainly on high loaded components like gearboxes pinions or engine tappets. This latter example of the application of phosphatation will be used as a demonstrative tool in order to show which physical properties of the manganese phosphate coatings are responsible for their antiwear ability.

Among many different methods of determining physical characterisations, X-ray spectroscopy in particular has been extensively used in this work to describe the crystallographic structure of various phosphate coatings on engine tappets.

Simultaneously a set of tappets treated by these different phosphate coating techniques, leading to different structural results, have been submitted to an endurance test in a cylinder head rig in order to evaluate their relative antiwear efficiency.

The main criterion determined by X-ray diffraction technique was the texture of the crystallites of manganese phosphate built up on the tappet frictional surface (Hureaulite). It was chosen because earlier work indicated that a relevant correlation existed between the texture of the manganese phosphate coatings and their antiwear ability (Monteil, 1987). From the different figures recorded by X-ray spectroscopy it is possible to plot diagrams showing the density of presence of the phosphate crystals for each direction of the half space delimited by the tappet surface. Consequently these diagrams allow the texture of the coating to be determined. Comparison of the different rate of texturing between the different coatings is therefore possible.

The wear properties were evaluated by means of a specific endurance test rig. This rig was built up around a V6 engine cylinder head electrically driven with a set of 12 tappets in each test. After completion of the tests the ranking of wear was carried out using a visual examination of the frictional surface of the worn tappets ranging from 0 (whole tappet surface scuffed) to 5 (no wear traces).

A comparison of the two set of results (X-ray and wear tests) shows that when a great texture in a particular direction is noticed in the manganese phosphate coatings a low wear is to be expected.

These conclusions allow the different manganese phosphate coatings to be qualified since their other physical and chemical properties should be similar. It gives also a new tool to the researchers in phosphating products to qualify their new techniques and to production engineers to follow the variations in their phosphating processes.