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Biolubrifiants
(2011)
Un point de départ pour la mise en route des lubrifiants « neutre à l'environnement » ont été les huiles perdues et les fuites accidentelles, les lubrifiants perdus rejoignant les sols et les eaux (voir lac de Constance).
Même en prenant en compte les voies du recyclage, la combustion dans les moteurs et les fuites connues, environ 30 % (entre 20 % et 40 %, selon les études) du volume de lubrifiants entre dans l'environnement par des voies non attribuables ou est probablement mis en décharge de manière illégale.
Avec de tels chiffres publics, les arguments des lobbyistes de l'industrie pétrochimique, affirmant que, dans la plupart des applications, les fluides sont enfermés dans des réservoirs clos et étanches, ne convainquent pas les politiques. C'est aussi peut-être parce que, en Allemagne, environ 60 % des particuliers changent leur huile eux-mêmes.
OEMs perceive diamond-like carbon (DLC) coatings as part of a strategy for low frictional tribosystems. Especially the automotive industry anticipates a benefit in applying such coatings in association with the lightweight construction of mechanical parts, for instance in gears and engines. In previous investigations it was shown, that DLC, amorphous carbon (a-C) and tetragonal amorphous carbon (ta-C) coatings can be slip-rolling resistant at least up to ten million load cycles at Hertzian contact pressures P0max of 2.9 GPa lubricated by engine oil at 120°C, Further improvements in load carrying capacity up to P0mean of 2.62 GPa (FN= 5,000 N) were achieved with alternative, available Steel metallurgies as Substrates for thin film coatings. Additional factors, such as influences of the lubrication on the surface chemistry (additives and base oils), wear and friction behavior were investigated.
Leichtbaustrategien im Antriebsstrang erhöhen durch Downsizing die Hertz’schen Kontaktpressungen und die mechanischen Lasten. Dadurch müssen neben wälzbeständigen Stählen auch dazu passende reibungsmindernde Konzepte gefunden werden, in der vorliegenden Arbeit wurden alternative, hoch zähe Stähle mit einer Wälzbeständigkeit bis Pomax = 4,2 GPa untersucht, welche mit geeigneten Grundöl- und Additivkombinationen die Reibungszahlen herunter auf 0,025 reduzieren konnten. Diese alternativen Stahllegierungen erlauben nicht nur an der Zahnflanke sehr hohe Kontaktpressungen, sie bieten außerdem entsprechende Zähigkeits- und Festigkeitseigenschaften für den Zahnfuß. In vorangegangenen Untersuchungen wurde eine Vielzahl verschiedener Dünnschichten auf ihre Überrollbeständigkeit unter Extrembeanspruchungen validiert. Allerdings zeigte sich bei den meisten Dünnschichten ein Qualitätsunterschied von Charge zu Charge. Unter Verwendung verschiedener organischer, metallorganischer und anorganischer Additive konnten, sich daran anschließende Versuche, vielversprechende Ergebnisse erzielt werden. Sämtliche Versuche unter extremen Beanspruchungen wurden bei +120°C Öltemperatur auf Zwei-Scheiben-Tribometern vom Typ AMSLER und 2Disk von Optimol Instruments unter Misch-/Grenzreibungsbedingungen durchgeführt und die tribologischen Eigenschaften der neuartigen, unbeschichteten Stähle und in Relation zu leistungsstarken DLC-/ZrCN Dünnschichten gesetzt.
The dilution of biogenic fuels into lubricating engine oils often leads to a shortening of the recommended oil drains (between 30% and 60%) and an increase in wear. The large number of overlapping and influencing factors, of which dilution and polymerization of fuel components in the engine oil are emphasised, makes it difficult to find a uniform solution to prevent failures in the various applications.
Insofar single solutions for the different types of biofuels are needed. The contribution of base oil chemistry and additives as well as triboactive materials is featured to deal with the adverse effects of biofuels. In the frame of the European Commission (EC)-funded project 'cleanengine', tentative engine oils based on esters with a content of renewables and polyglycols are formulated to increase the lubricant's tolerance in engines fuelled with biofuel-based blends, with the aim of ensuring required lubricating and wear protection performance while keeping oil drain intervals unchanged.
The present paper focuses on four-stroke diesel applications, fuelled by biodiesel (fatty acid methyl ester — FAME) as well as by rapeseed oil and Jatropha oil (pure vegetable oils, triglycerides), together with relevant blends of those biofuels and conventional diesel fuel.
This paper screens the functional profile (in particular rheological, toxicological, bio-compatibility, tribological and biofuels affinity) of lube families with respect to biofuel contamination. Moreover, this is followed by the contributions of piston ring and liner materials as well as thin film coatings.
In order to reduce experimental effort, tribological tests are performed for coating selection, simulating the friction, wear and scuffing behaviour between the piston ring and cylinder liner, as well as lubricant evaluation. The creation of test standards in piston ring/cylinder liner simulation is discussed. PVD coatings, HVOF coatings and nitriding treatments have been applied to piston rings as alternative for chromium coatings. CrN/TiN coating seems very effective for increasing the wear and scuffing resistance. Biodegradable engine oils based on esters and polyglycols reduce friction and exceed the wear resistance of reference engine oils based on hydrocarbons for the new generation engines with high cylinder pressures.
Magnéli-type vanadium oxides form the homologous series VnO2n-1 and exhibit a temperature-induced, reversible metal–insulator first order phase transition (MIT). We studied the change of the adhesion force across the transition temperature between the cleavage planes of various vanadium oxide Magnéli phases (n = 3 ... 7) and spherical titanium atomic force microscope (AFM) tips by systematic force–distance measurements with a variable-temperature AFM under ultrahigh vacuum conditions (UHV). The results show, for all investigated samples, that crossing the transition temperatures leads to a distinct change of the adhesion force. Low adhesion corresponds consistently to the metallic state. Accordingly, the ability to modify the electronic structure of the vanadium Magnéli phases while maintaining composition, stoichiometry and crystallographic integrity, allows for relating frictional and electronic material properties at the nano scale. This behavior makes the vanadium Magnéli phases interesting candidates for technology, e.g., as intelligent devices or coatings where switching of adhesion or friction is desired.
Wear tests are often used in engine development in order to characterize the functional profile of surfaces interacting with lubricants. This paper proposes a new test procedure to determine Stribeck-type curves for the tribosystem piston ring/cylinder liner involving test pieces prepared from engine components. The Variation of the friction coefficient with sliding speed depends on the structure and Constitution of the base oil, the viscosity index, the surface topography of the liner and ring, and the friction modifiers used. Novel organic friction modifiers of low molecular mass not only reduce the friction coefficients under mixed / boundary lubrication, but also in the fluid film regime. In addition, the influence of surface roughness on the wettability of surfaces can be significant for both static and dynamic wetting. For this reason, the interaction of the lubricant with the surfaces was also investigated to enable a correlation between the surface properties of the liquid and solid phases and the tribological behavior.
Original equipment manufacturers (OEMs) and end-users perceive 'Zero Wear' differently. The 'Zero Wear' approach will be put into a general relation for different applications and illuminated by individual paths, either based on monolithic materials, thin film coatings or alternative base oils, featuring triboactive materials and lubricious oxides for tribological engine components and dry running foil bearings or specific DLC, ta-C and novel Zirconium-based thin film coatings for concentrated contacts above FZG 14 and alternative engine oils (NoSAP & bio-no-tox). The associated tribometric test equipments for these examples will also be detailed.
Diamond-like carbon (DLC) coatings are perceived as part of a strategy for low frictional tribosystems. Especially the automotive industry anticipates a benefit in applying such coatings in association with the lightweight construction of mechanical parts, for instance in gear and engine components. Therefore, in previous investigations amorphous carbon (a-C) and tetragonal amorphous carbon (ta-C) coatings were inspected. For up to ten million cycles at Hertzian contact pressures P0max of 2.9 GPa it was shown that these coatings are slip-rolling resistant not only at room temperature in the presence of unaddivated paraffin oil, but also up to 120 °C oil temperature. The steel substrates in this investigation were made of the hardened and tempered steels 100Cr6H and Cronidur 30. The aim of the current work is manifold. The Hertzian contact pressures should be increased up to P0max of 4.2 GPa by using new steel metallurgies as substrates for thin film coatings. These steel metallurgies are also assessed, if they can compete as uncoated couples. Two high toughness spring steels and an ultra-high toughness aerospace steel were tested in a twin disc tribometer of the Amsler type as well as in an Optimol 2Disk test rig under mixed/boundary conditions in a factory fill SAE 0W-30 engine oil. Different factors such as influences of the lubrication, surface chemistry and wear behaviour were investigated.
Some of the properties intrinsically present in the backbone of polyalkylene glycols (PAGs), which have nowadays attracted the automotive original equipment manufacturers (OEMs), are presented, such as bio-no-tox properties, heat capacity, extended drains, low friction, low AlSi-liner wear, fuel economy, and very high viscosity indices, and are achieved with metal-, ash-, and polymer-free formulations, which guarantee no adverse contamination of the lambda sensor and the active centers of the catalyzer as well as not to increase the exhaust back pressure of the particulate filter through ash deposits. This paper illuminates the latest results from OEM's engine bench and road testing supporting the following functional benefits: (a) bio-no-tox, (b) improvements of fuel economy, even when compared with latest hydrocarbon-based prototype oils, (c) reduced wear, (d) reduced engine friction, (e) extended drains, (f) suited for bio-fuels, (g) no adverse effects on after-treatment devices through ash-, metal-, and polymer-free formulations, etc.
A correlation of adhesion force and electrical conductivity is established for a vanadium oxide and highly oriented pyrolytic graphite (HOPG). Adhesion forces were determined by analysis of pull-off forces obtained from force–distance curves with atomic force microscopy in ultrahigh vacuum, on clean surfaces and with defined contact conditions. The investigated samples include (i) different stoichiometries of Magnéli-type vanadium oxides (V3O5, V4O7, V6O11, and V7O13), which undergo metal-insulator-transitions as a function of temperature, (ii) the (0001) basal plane and the (10–10) prism plane of highly oriented pyrolytic graphite (HOPG), which differ in their respective perpendicular-plane electrical conductivities by several orders of magnitude, and (iii) the (100) surfaces of pure metal single crystals, i.e., silver (Ag), copper (Cu), iron (Fe) and gold (Au). It is shown, that the vanadium oxides as well as the graphite exhibit significantly lower adhesion forces in their electrical conductive state than in their non-electrical conductive state. The values obtained for the electrical conductive states are quantitatively confirmed by the measurements on the single crystal metals.