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Slip-rolling resistant thin film ta-C coatings for up to 3.000 MPa of Hertzian contact pressure
(2009)
Le secteur automobile doit faire face actuellement à de nouveaux challenges au niveau de la construction allégée, des économies de carburant et des coûts. Ces exigences motivent la mise au point de tribosystèmes pouvant résister à des pressions de contact de plus en plus élevées avec de faibles coefficients de frottement. L'optimisation de systèmes existants par l'application de revêtements de surface performants représente une alternative intéressante. Ce texte présente les performances de couches minces dans des conditions d'essais de slip-rolling (roulement à composante de glissement) en présence de lubrifiants liquides. Après une première sélection à température ambiante, les revêtements les plus performants ont été testés à 120 °C. Il s'agit de revêtements DLC en carbone hydrogéné (a-C:H) et en carbone tétraédrique (ta-C) de dernières générations ainsi qu'un nouveau système « revêtement/substrat ». Certains des revêtements DLC développés récemment sont résistants en slip-rolling au moins jusq'à 10 millions de cycles à 120 °C dans l'huile moteur sous des pressions hertziennes de contact de P0max = 2600 / 2940 MPa. De plus, le nouveau système revêtement Zr(C,N)x/substrat peut résister au moins à 1 million de cycles sous des pressions hertziennes initiales de contact allant jusqu'à P0max = 3500 MPa et à des températures de lubrifiant d'au moins 120 °C. -----------------------------------------------------------------------------------------------------------------------------------------------
The light-weight approach and fuel economy targets in today's automotive engineering require tribosystems, which can withstand higher contact pressures associated with low coefficients of friction. The application of surface coatings represents one approach among others. This paper presents thin film coatings performances in a bench mark test procedure exerting slip-rolling conditions in the presence of liquid lubricants. After a first selection at room temperature, the most resistant coatings were evaluated at 120 °C. These are newly developed DLC coatings (a-C:H & ta-C) as well as a novel coating-substrate system. Some of the newly developed DLC-coatings are slip-rolling resistant for at least up to 10 million cycles at 120 °C oil temperature) under Hertzian contact pressures of P0max = 2600 / 2940 MPa. Furthermore, this novel Zr-based thin film coating can withstand at least 1 million cycles under initial Hertzian contact pressures of up to P0max = 3500 MPa and oil temperatures of at least 120 °C associated with low coefficients of friction under mixed/boundary conditions.
Nowadays, diamond-like carbon (DLC) coatings are mainly used in many low loaded applications such as in machine tools, computer devices and many more.
Especially the automotive industry anticipates a benefit in applying such coatings in association with the lightweight construction of mechanical parts, for instance in gear components.
The ulterior motive is a global performance increase regarding environmental impact and power efficiency.
In recent years, the slip-rolling resistance of DLC, a-C and ta-C thin film coatings was improved considerably.
In view of the mechanical application of thin film coatings, there is still room for improvements.
It was experimentally shown, that a-C and ta-C coatings can be slip-rolling resistant at room temperature in unaddivated paraffin oil up to ten million cycles under Hertzian contact pressures up to P0max = 2.9 GPa.
Typically, the prime coated steel substrates were made of the hardened and tempered steels 100Cr6H and Cronidur 30.
The aim of the work this to overtake these results, i.e. at higher maximum Hertzian pressures up to P0max = 4.2 GPa.
Under such extreme conditions, it is necessary to choose special steels to avoid the failure of the substrate and to permit a higher load carrying capacity of the coating-substrate-system.
In consideration of the increased substrate properties, two high toughness spring steels and an ultra-high toughness aerospace steel were tested on a twin disc tribometer of the Amsler type as well as on an Optimol TwinDisc test rig under mixed/boundary conditions.
Different factors such as residual stresses of the substrate, influences of the lubrication, and wear behaviour were investigated.
The respective results are presented here.
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.
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.
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.
The use of high niobium alloyed cast iron alloys is a relatively new approach in which the niobium addition intends to improve the properties of the material by the precipitation of hard niobium carbides during solidification. Steels can be replaced by ductile cast iron in some rolling applications, such as gears and cams, in order to reduce material costs. The aim of this work is to evaluate ductile iron alloyed with 1 weight percent (wt.%) niobium for the as cast specimens and with 1.8 wt.% and 2.4 wt.% niobium for the austempered specimens under lubricated slip-rolling tests using mixed/boundary conditions in an Amsler-type machine. Austempered ductile iron (ADI) alloyed with 1 wt.% chromium, or Carbidic ADI, was tested for comparison. For the as cast conditions, the niobium addition resulted in an increase of wear resistance owing to the low contact pressure of these tests. However, for the austempered specimens, the best performance was found for unalloyed ADI. The main factor acting in the initiation and propagation of cracks in ductile iron is the presence of the graphite nodules. The coarse carbides also contributed to the initiation of cracks and spalling of the material.