saving solutions in designing coatings for fuel injection systems

BRAMAT20117 th International Conference on Materials Science and Engineering – BRAMAT 2011Braşov, 24 – 26 February 2011A GENERAL OVERVIEW ON PERFORMANT AND COST – SAVINGSOLUTIONS IN DESIGNING COATINGS FOR FUEL INJECTIONSYSTEMSCorina COJOCARIU, Adnana MARIN, Camelia GABOR, Daniel MUNTEANUTransilvania University of Brasov, RomaniaAbstract. According to C. Treutler [1], in Europe, more than 40% of all new cars are equipped with Diesel engines,including also the high class cars. High-pressure injection systems with pressure values up to 2000 bar (200 MPa) givethe best results for combustion. At the same time, high - hard carbon coatings hold the key to improved performance forthese kinds of systems. Regarding this, Diamond-like carbon (DLC)-coatings present crucial advantages in terms oftribological properties (low friction coefficient and low wear for both coated part and counter body). Using a cheapervariant of alloyed steel for manufacturing fuel injection parts and combining it with a DLC deposition, the results couldbe very performing in terms of quality – cost aspects. Specially designed plasma processes for DLC coatings depositionallow keeping the component temperature below 200°C, in order to not modify by annealing the structure of steel usedand to decrease its hardness. This paper represents a theoretical study of a debutant Ph.D. students group and it aims topresent a general overview regarding some advantages of using DLC films; at the same time, there are presented fewpractical applications of these kinds of films in manufacturing certain parts of Diesel fuel injection systems.Keywords: fuel injection system, DLC coatings, tribology, efficient solutions1. IntroductionIf we look at the applications today, it is fair tosay that all majorities of diesel cars that havecommon rail or high-pressure injection use this typeof coating technology on their diesel injectioncomponents. According to http://vehicleengineer.comsite, this technology was first usedsome ten years ago on the Volkswagen TDI engine,and this Diamond-like Carbon (DLC) coatingtechnology has been constantly fine tuned duringthe last decade through both motorsport andproduction vehicle applications.For modern environmentally friendly cars, weneed engines with low emissions and with reducedfuel consumption.On the other hand, we would like to have a fundriving experience and we want to buy the car at thelowest price possible. Gasoline and diesel fuelinjection systems provide the basis for the carmanufacturer to fulfil these targets.Diesel engines have a higher degree ofefficiency in their use of the fuel energy. In Europe,more than 40% of all new cars are equipped with adiesel engine, including high-class cars [1].According to the research in the field anddifferent practical applications, High-pressureinjection systems with pressures up to 2000 bar(200 MPa) give the best results for combustion [1].The challenges for high-pressure pumps forpassenger cars are the high pressure of up to 2000bar (200 MPa), low power consumption,lightweight, low noise and long lifetime.For low cost reasons, it should not need aseparate lubrication system. Unfortunately, thequality of diesel fuel differs concerning the abilityto lubricate a mechanical contact of differentcomponents. Therefore, we need solid lubricantcoatings for several parts to protect them fromfriction and wear [1, 2].Thin film coatings, deposited by the help ofplasma technology, are very hard and wearresistant.Intense plasma cleaning and ionbombardment during deposition result in excellentadhesion and high density of the films.Thin films are highly suitable for parts withlow tolerances.Specially designed plasma processes allowkeeping the component temperature below 200 o C inorder not to anneal the steel used and decrease itshardness.On the other hand, the need for a moderatetemperature of the parts to be coated will limit thehigh deposition rate [1, 3].DLC coatings have brought valuable costefficientbenefits to engines in both the racing andautomotive markets, and as such we anticipate thatwe will see DLC coatings used more and more inboth of these industries.23

RECENT, Vol. 12, no. 1(31), March, 2011According to C. Treutler [1], for the protectionof component parts against friction and wear,diamond-like-carbon (DLC)-coatings show crucialadvantages in a low wear, in the low coefficient offriction (even for a non lubricated contact with steelas counterpart) and in the low wear of the counterbody.Hence, for example, only the pump pistonneeds to be coated and not the inner surface of thepump cylinder, which would be very difficult.Besides this, the costs for the coating of thecounterpart can be saved [1].Despite the current economic crisis, it isessential for the industry and market leaders to startcontemplating the recovery [13].These leaders need to determine how theircompany and products will be profiled during therecovery to make sure they are back on track whenbusiness picks up again.The European Commission has recentlydeveloped a programme to reduce vehicle CO 2emissions, which motor manufacturers must adheretoo. Failure to meet these emission limits will in thefuture result in financial penalties of at least €95 pergram above the limit set by the EC [13].Many studies showed that using DLC coatingsreduces the friction losses, which translates intoreduced consumption and, subsequently, loweremissions. Essentially, this means that DLCcoatings will be one way to help meet future CO 2limits.PSA Peugeot Citroën (previously PeugeotSociété Anonyme - PSA Group) conducted a test tomeasure the actual value of DLC coatings in thisprocess. It revealed that DLC coatings applied ontappets reduce overall fuel consumption by 1-2%,which equates to a saving of somewhere between 2-3g/km of CO 2 [13].The biggest advantage of DLC coatings is thatthey reduce CO 2 emissions in existing engines, sothere is no need for a drastic redesign of the engine.Like an example, tappets can be coated, which canlead to a 1-2% reduction of emissions without aredesign of the valve-train.According to Mark Boghe [13] (Bekaert DLCcoatings), if a manufacturer wants to take fulladvantage of this technology, however, thesecoatings need to be integrated from the start of thedesign. At the same time, by applying DLC coatingsto components not only increases the lifetime of thecomponents, but also extends the lifetime of theentire engine.Considering a camshaft, by applying a DLCcoating, the profile of the cams is protected againstwear (longer lifetime) but it also guarantees that, aslong as the coating is applied, the performance ofthe camshaft is consistent (no gradual deteriorationin performance).On the other hand, the emissions outlets sufferbecause the natural by-product of combustion arecarbon deposits. As small, super hard particulatesflow through the filters, they come in contact withthe moving parts and gouge them in a significantmanner [13].If these parts are harder than the particulates, theyare crushed and absorbed; DLC coatings are usuallystronger than whatever goes into the engine [15].2. Theoretical background on DLC coatingsIn terms of Diamond-like carbon properties, itis often believed that the presence of sp 3 sites is theonly criterion for diamond-like properties. This isthe source of much confusion. Many softtransparent polymers contain high sp 3concentrations, but are obviously not considered asdiamond-like [4]. In that case, only the optical gapis similar to that of diamond. In pure diamond, allcarbon atoms reside in sp 3 sites, but the importantpoint is that these sites are interlinked by strongcovalent bonds (about 7.02 eV), with no weak chainelement (except at a few crystalline point defects).This is one of the explanations for diamond'shardness.In polymers, most of the chains of covalentbonds are terminated by H atoms (C–H covalentbonds have energies of 4.1 eV for H–C sp 3 , 4.5 eVfor H–C sp 2 , and 5.4 eV for H–C sp 1 ). Between thechain elements, discontinuities of much lowerbinding energy are produced (hydrogen bonds of adifferent kind than H–C covalent bonds, and vander Waals bonds).This explains their “plastic” properties. Ingraphite, the carbon atoms reside in sp 2 sites, andare interlinked by a higher binding energy of 7.03eV within the dense plane only, and the fourthelectron of the outer carbon atom electron orbitalcontributes to a weak binding energy of 0.86 eVbetween planes, explaining why it is much softer,and somewhat brittle.An amorphous mixture of exclusively carbonatoms will correspond mostly to sp 3 and sp 2 sites(with a few sp 1 sites), and the local binding energieswill be distributed around the energies of 7.02, 7.03and 0.86 eV (often reported with an average of 3.6eV) [4].According to the literature, many different24

RECENT, Vol. 12, no. 1(31), March, 2011categories of DLC films have been reported. S.Neuville presents a very professional synthesis ofthese in [4]. According to this, the main groups area-C (non-hydrogenated amorphous carbon), a-C:H(hydrogenated amorphous carbon) and polymeric a-C:H (highly hydrogenated amorphous carbon).Within this classification, the concentration of sp 3sites should also be considered. However, as wehave seen above, the sp 3 concentration alone is not asufficient criterion by which to determine propertydifferences among different amorphous carbontypes, which may have a similar concentration ofsp 3 sites. In practice, the mechanical and physicalproperties are the factors, which define anddifferentiate DLCs.- Polymeric amorphous carbon. a-C:H type DLCscontaining more than 40 at.% hydrogen, are usuallycalled polymeric amorphous carbon materials [4, 5],which are very similar to “plastic” polymers.- Soft graphitic a-C:Hs and a-Cs. Amorphouscarbon films with a majority of sp 2 sites (>90 at.%)have properties close to graphite and a hardnesswhich can be lower than 10 GPa [4, 5, 6].- Hard a-C:H and ta-C. Hydrogenated amorphouscarbon (a-C:H) can reach high hardness (about 50GPa) when the H content is not higher than 20 at.%and they contain a high sp 3 /sp 2 site ratio [5, 7].These films have been considered for a long time asvery promising because of their low frictioncoefficient (e.g., against stainless steel) in someinert atmospheres [4, 8, 9].- Tetrahedral amorphous carbon and polycrystallinediamond. The differentiation between thepreviously described hard a-C and ta-C can be verysubjective when the carbon sp 3 sites are rarelybonded to other types of atoms and carbon sites,with a low bonding energy. In this case, it is nolonger necessary to consider the difference betweenstrongly bonded sp 3 and weakly bonded sp 3 , sincestrong bonds (explaining their hardness as alreadydiscussed) interlink almost all of them. This ispractically the only case where the diamond-likeproperties can be described with the sole criterion ofthe sp 3 concentration. Tetrahedral amorphouscarbon (ta-C) films contain a majority ofinterconnected sp 3 sites (50 to 100% with hardnessof 70 GPa up to 100 GPa, which is close todiamond) [4, 10]. Polycrystalline diamond iscomposed of crystallites containing nearly 100%sp 3 . They have hardness close to bulk diamond.According to S. Neuville at al., ta-C films areable to support higher levels of stress, in an elasticstate, without fatigue effects leading to materialfailure. This is because the yield stress is higherthan conventional DLCs; this benefit is only ofvalue as long as the coating can accommodate thesubstrate deformation without debonding and aslong as no cohesive cracking or delamination of thecoating or substrate occurs, (e.g., when the surfaceis subjected to the Hertzian contact pressures andshearing forces usually encountered in practicalcontacts) [4].When the substrate material deforms, then thestress distribution is modified at the coating–substrate interface and in the film material.For this reason, different research programstried to find solutions for decreasing gradient ofproperties between surface (DLC layer) andsubstrate and design properly the whole system:substrate – interface – film (figure 1). Table 1presents 2 solutions founded by Bosch, in the caseof DLC coatings deposited on certain fuel injectionparts [1].DLC coatingIntermediate (gradient) layerAdhesion layersubstrateFigure 1. Schematic arrangement of a DLC superficialsystem with gradient of propertiesTable 1. Substrate improving variants of load capacity ofDLC coatings [1]System A. B.FunctionallayerIntermediatelayerAdhesionlayerDLC (free ofmetal)C layer/WC/CrChromiumDLC (free ofmetal)CrCChromiumSubstrate Steel SteelThe tribological properties of DLC coatings donot depend only on the type of coating, but also onworking conditions or contact parameters. Becauseof that coefficient of friction depends on thecoating’s thermal stability, which is subject to their,as well as on atmosphere in which coating isoperating.One very important factor in DLC coatingsperformance is also humidity and the type of gas in25

RECENT, Vol. 12, no. 1(31), March, 2011which coating is operating [11].According to M. Sedlakek at al., doping ofDLC coating with different metals (Ti, Nb, Ta, Cr,Mo, W, Ru, Fe, Co, Ni, Al, Cu and Ag) leads tochanges in their mechanical properties (lowerinternal stress and hardness, improved adhesion) aswell as in their tribological properties, which mainlyreflect in decrease of atmosphere dependence [11].Several methods have been developed forproducing diamond-like carbon films.Plasma Enhanced (assisted) CVD techniques(PECVD) employing RF and DC glow dischargesin hydrocarbon gas mixtures produce smoothamorphous carbon and hydrocarbon films (ontosubstrates negatively biased), which have mixed sp 2and sp 3 bonds. The CVD processes will generallyrequire deposition temperatures of at least 600°C togive the required combination of properties;however, low temperature deposition is possible.The CVD technique gives good deposition rates andvery uniform coatings, and is suited to very largescaleproduction. High-power pulsed-DC sourcesmay use as an alternative of r.f. PECVD method.Pulsed-DC PECVD technologies provide higherdeposition rates, DLC films with high adherence,and do not require matching networks, resulting in areduction of production cost.Another technique for DLC deposition is basedon Ion Beam deposition. This has the advantage ofbeing able to deposit high quality coatings at verylow temperatures (near room temperature).The Closed Field Unbalanced MagnetronSputter Ion Plating Process, is a technique that canreadily apply a-C:H films (> 4 µm) to substrates ofany shape. The process is based on closed fieldunbalanced magnetron sputter ion plating,combined with plasma assisted chemical vapourdeposition.The new technique combines the benefits ofboth plasma CVD and ion beam deposition. Thedeposition is carried out between 150 and 200°C ina closed field unbalanced magnetron sputter ionplating system.Different companies (for instance Bosch) haveelaborated several proprietary processes to makediamond-like-carbon (DLC) - coatings. [1, 12].These are combinations of physical vapourdeposition (PVD) and plasma-enhanced chemicalvapour deposition (PECVD).The highly ionized plasma, driven by energyfrom high frequency electromagnetic excitation anda hydro-carbon gas inlet provide the carbon ions,which form the coating. An intense ionbombardment ensures the film quality and anunbalanced magnetron cathode supplies metal ionsfor the adhesion layer and the metal-containingfilms.3. DLC coatings used for fuel injectionsystem partsAnalysing the technical results in this field, wecan see that many internal combustion engines,whether compression ignition or spark ignitionengines, use fuel injection systems to provideprecise and reliable fuel delivery into the enginecombustion chambers.Such precision and reliability are needed toimprove fuel efficiency, maximize power output,and reduce undesirable emissions. Generally, fuelinjection systems will include a fuel pump and oneor more fuel injectors [16, 17, 18].The fuel pump supplies fuel to the injectors,which subsequently provide precise control of thefuel supply and timing to engine cylinders [16].According to the literature, Diesel engines havehistorically used various forms of fuel injection.Two common types include the unit injectionsystem and the distributor/inline pump system.While these older systems provided accurate fuelquantity and injection timing control, they werelimited by several factors [17].The common rail injector system is used ingasoline direct injection for modern two and fourstrokegasoline-run engines, but is more popularlyused in diesel engines; in gasoline engines, thegasoline is pressurized and injected into thecombustion chamber in each cylinder by thecommon rail fuel line. In diesel engines, the highpressurefuel rail line feeds individual solenoidvalves (figure 2) [17, 18].Newer common rail diesel systems now usepiezoelectric injectors (injectors that can generatean electric field when mechanical stress is appliedto them) which results in increased precision andhigher pressure.Figure 3 shows two examples of parts whichare coated with DLC [1]. There is the Common-Rail-High-Pressure-Pump, in which a polygonshapedroller on an eccentric tappet drives the threepistons of the pump. The supporting areas of theroller have to withstand a high load, which isperpendicular to the surface [1]. They need aprotection for the running-in process and for thesmooth operation during lifetime.The injector needle opens and closes the outletof the valve. It has to move very precisely within26

RECENT, Vol. 12, no. 1(31), March, 2011the valve and needs a coating with a low frictioncoefficient to guarantee a reproducible amount offuel per each individual cycle of the engine. Thesetwo examples represent the two reasons for usingDLC-coatings for high wear resistance and lowfriction.a.b.Figure 2. Schematic arrangement of a common rail fuelinjection system (a) and a photo of it (b)b.Figure 3. Parts coated with DLC films inside of acommon rail fuel injection system [1](a) injector needle; (b) polygon-shaped roller4. Economic aspects and technicaladvantages of DLC coatingsThe cost of DLC coating depends on, amongother things, the size of the part to be coated.According to Mark Boghe [13] (Bekaert DLCcoatings), generally, it could be estimate the cost ofa.the coating to be approximately 30% of the cost ofthe component. The main advantage of the coatingis that it immediately extends the lifetime of thecomponent. You can use it longer and it will wearless. In general, the lifetime is increased by aminimum of 50 per cent. This means that aninvestment of 30 per cent is offset by an immediate50 per cent gain – not a bad deal, especially in thecurrent climate.Another use for DLC coatings is to refurbishcomponents [13]. According to Michael De Maegt,the General Manager of Bekaert DLC, byrefurbishment it is possible to taking back coatedand used components and recoating them [13]. Thisway, there is no need to buy a new component. Inday-to-day use, most engines do not suffer anyexcessive wear.At the same time [13], DLC coatings still havean important role to play as they allow a reductionin friction losses and therefore consumption.According to [13], a good example is coatingtappets in small gasoline cars. By coating thetappets, the friction losses are reduced by no lessthan 40 per cent, which translates into a reduction infuel consumption of around 1 to 2 per cent.An application where this technology is focussedat present is in the area of friction reduction, mainlyin the valve train assembly and components.By considering the energy that is wastedthrough friction, the friction reducing DLC coatingswill contribute to a more efficient engine, therebydecreasing emissions.Preventing wear-related damage to biodieseldriven engine parts is currently a high profile issuein the automotive industry, along with efforts toreduce fuel consumption [14].Thereby protecting the part by reducing thefriction, it is also possible to reduce the wear impacton the parts and their counterparts as well.In addition, DLC coatings are very inert whichmeans that they do not react with the counterpart asthe coating forms an amorphous carbon layer on thesurface of the component. The DLC coating alsohas no affinity with the counterpart therebyeliminating the tendency of seizing or weldingwhen one rubs the surfaces of two similar materialstogether under pressure. One of the biggestadvantages in the application of DLC coatings tovalve train components is that this technology canbe introduced without changing anything in theirlogistics or vehicle assembly process [15].Manufacturers can use the same engine designand install a coated component instead of an27

RECENT, Vol. 12, no. 1(31), March, 2011uncoated component.According to Mark Boghe [13] investing inDLC coatings essentially buys you two majorbenefits. First, the hard and low friction layer willimprove the wear resistance of the component. Thisresults in a considerably longer life for your engineand its parts, and a stable performance for thelifetime of the coated component.5. Future research objectives of the groupHaving in mind this theoretical study, theauthors will try in the proximity future to find newsolutions for developing carbon-based coatings(including here also DLC types), with performingmechanical and tribological properties, which canbe prepared at low temperatures, in good efficiencyconditions.6. ConclusionsDLC films may possess exceptional mechanical(high hardness), optical (high optical band gap),electrical (high electrical resistivity), chemical(inert) and tribological (low friction and wearcoefficient) properties and can be deposited at lowtemperature (< 200°C).The DLC-coatings could be considered anenabling solution for high-pressure fuel injectionsystems, for components with higher performanceand higher lifetime and for cost efficient solutions.7. Tamor, M.A., Vassell, W.C., Carduner, K.R. (1991) Atomicconstraint in hydrogenated “diamond-like” carbon.Applied Physics Letters, vol. 58, no. 6, p. 592-594, ISSN0003-69518. Erdemir, A., Switala, M., Wei, R., Wilbur, P. (1991) ATribological Investigation of the Graphite-to-DiamondlikeBehavior of Amorphous Carbon Films Ion-Bean-Depositedon Ceramic Substrates. Surface and Coatings Technology,Vol. 50, p. 17-23, ISSN: 0257-89729. Enke, K. (1988) Amorphous Hydrogenated Carbon Films.EMRS Meeting Strasbourg XVII, Les Editions de laPhysique Z.I. Courtaboeuf F-les Ulis France, p. 11710. Neuville, S. (2002) The enhancement of interconnected sp 3sites by chemical effects during ta-C film growth. Diamondand Related Materials, Vol. 11, Issue 10, p. 1721-1730,ISSN 0925-963511. Sedlacek, M., Podgornik, B., Vizintin, V. (2008)Tribological properties of DLC coatings and comparisonwith test results: Development of a database. MaterialsCharacterization, Vol. 59, Issue 2, p. 151-161, ISSN: 1044-580312. Reuter, W., Voigt, J., Neuffer, A., Lunk, A. (1997) Plasmaconditions for the deposition of wear-resistant carboncoatings.Surface and Coatings Technology, Vol. 93, no. 1,p. 93-98, ISSN 0257-897213. http://www.ae-plus.com/keytopics/cs-motorsport -Bekaert.htm14. http://www.engineeringtalk.com/news/15. http://vehicle-engineer.com16. http://en.wikipedia.org/wiki/Fuel_injection17. http://en.wikipedia.org/wiki/Common_rail18. http://en.wikipedia.org/wiki/Diesel_engineReferences1. Treutler, C. (2005) Industrial use of plasma-depositedcoatings for components of automotive fuel injectionsystems. Surface and Coatings Technology, Vol. 200, Issues5-6, p. 1969-1975, ISSN 0257-89722. Donnet, C., Erdemir, A. (2004) Solid lubricant coatings:recent developments and future trends. Tribology Letters,Vol. 17, no. 3, p. 389-397, ISSN 1023-88833. Grischke, M., Bewilogua, K., Dimigen, H. (1993)Preparation, properties and structure of metal containingamorphous hydrogenated carbon films. Materials andmanufacturing processes, vol. 8, p.407-417, ISSN 1042-69144. Neuville, S., Matthews, A. (2007) A perspective on theoptimisation of hard carbon and related coatings forengineering applications, Thin Solid Films, vol. 515, Issue17, p. 6619-6653, ISSN 0040-60905. Weissmantel, C., Bewiloga, K., Breuer, K., Dietrich, K.,Ebersbach, D., Erler, U., Rau, H.J., Reisse, G. (1982)Preparation and properties of hard i-C and i-BN coating.Thin Solid Films, vol. 96, Issue 1, p. 31-44, ISSN 0040-60906. Holloway, C., Kraft, O., Kelly, M.A., Nix, W.D., Shuh,D.K., Hagstrom, S., Pianetta, P. (1999) Interpretation of X-ray Photoelectron Spectra of Elastic Amorphous CarbonNitride Thin Films. Applied Physics Letters, vol. 74, p.3290 - 3292, ISSN 0003-695128