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Tribology International 40 (2007) 1680–1695

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Compatibility between tribological surfaces and lubricantadditives—How friction and wear reduction can be controlled

by surface/lube synergies

A. Nevillea,�, A. Morinaa, T. Haquea, M. Voonga,b

aCorrosion and Surface Engineering Research Group, School of Mechanical Engineering, University of Leeds, Leeds LS 2 9JT, UKbCummins Turbocharger Technologies Ltd, Huddersfield, UK

Received 1 October 2006; received in revised form 17 January 2007; accepted 18 January 2007

Available online 26 March 2007

Abstract

This paper reviews the recent trends in materials technology and lubricant additive technology in engines. The paper will review key

developments in surface engineering, application of nanocomposite materials and other advanced materials (including light alloys). It

will also assess the trends towards ‘‘greener’’ lubricant additives, driven by environmental legislation and will discuss the implications for

lubrication in the next decade. The key part of the paper will be to review the extent to which materials and lubricants are being used in

partnership in engineering systems to capitalize on the synergies, which can exist between surfaces and lubricants in boundary

lubrication. In a similar manner there are some important antagonisms that need to be identified—an appreciation of such compatibility

issues can assist engineers in selecting a lubrication system.

The paper will review existing literature from outside the work conducted by the authors and will substantiate some of the important

aspects of boundary lubrication surface/lubricant compatibility through reference to some recent work conducted by the authors.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Tribochemistry; Surface coatings; Lubrication

1. Introduction and background

Environmental protection, resource utilization andcustomer satisfaction are the three main drivers fortechnology development in the automotive sector. Ad-vances in technology related to engines, fuels, materialsand engine oils, are all required if optimum ‘‘green’’, fuelefficient and durable systems are to be developed [1].Introducing new materials in IC engines in isolation willnot itself yield the improvements required—a fact demon-strated by the introduction of ceramics in engine parts inthe 1980s which was not accompanied by optimized enginedesigns or appropriate lubrication strategies. Focusing onlubricants and their ability to efficiently lubricate a contact,it is now becoming increasingly apparent that lubricantformulations currently used are designed and tailored to

ee front matter r 2007 Elsevier Ltd. All rights reserved.

iboint.2007.01.019

ing author. Tel.: +44 113 343 6812; fax: +44 113 242 4611.

ess: A.Neville@leeds.ac.uk (A. Neville).

work mainly on Fe-based materials, used traditionally inengines. Obtaining optimum durability (wear) and fueleconomy (friction) of tribological systems, especially wherenew materials and surfaces or surface treatments are usedrelies on compatibility between the surface and thelubricant. In this paper, a review of the current situationin terms of use of new materials and surfaces (treatmentsand coatings) in the IC engine is reviewed. Results from anexperimental programme are reported which focus on twomain aspects:

�

How different tribocouples respond to single and binarylubricant additives? � How the interactions between additives affects the

tribofilm and the tribological response—highlightingsynergies and antagonisms?

The experimental programme in this paper covers fourdifferent tribocouples and four different lubricants and as

ARTICLE IN PRESSA. Neville et al. / Tribology International 40 (2007) 1680–1695 1681

such cannot offer a universal appraisal of surface/lubricantcompatibility. It does however, offer an illustration of theimportance of such reactions and an assessment of themechanisms by which surfaces and additives can interactsynergistically or antagonistically. A detailed appraisal ofthe openly available literature dealing with (a) lubricationof non-Fe surfaces and surface/additive interactions and(b) of additive/additive interactions on Fe-based surfacessets the scene for the results presented later in the paper.

The literature dealing with lubricant action at non-Febased surfaces 5-years ago was sparse but is growingrapidly as more researchers appreciate the importance ofoptimizing surface/additive performance. In the latestreview by Erdemir [2] the importance of considering thesurface and lubricant as a single system was also high-lighted. Nanostructured coatings and laser texturing ofsurfaces were both reported to improve the lubrication.Also, improvements in both friction and wear were seen tooccur when materials with high lubricity (solid lubricants)are used in systems operating in the boundary lubricationregime.

It is in the boundary lubrication regime (and mixedregime) where friction and wear reducing additivesnormally exhibit their functionality. The high load bearingcapacity and lubricity of solid lubricants in this regimeprovide a back-up function to lubricants [2]. However, it isnot common for the surface and lubricant to purposely beused in partnership to optimize performance. It is knownthat wear and friction performance in the boundarylubrication regime is controlled mainly from the lubricantadditives which form tribofilms in the contacting surfacesbut surface treatments and coatings have an important roleto play in providing an improved performance or they canin fact eliminate the benefits of the additives. Knowing thedetails of how surfaces and additives react is paramount inunderstanding how to achieve optimal lubrication in theboundary (and mixed) regimes.

Table 1

Commonly used tribo-materials for piston/cylinder and valve train assembly

Piston ring Piston skirt Cylind

Conventional

materials

Grey cast iron (CI) Grey cast iron Mono

Carbidic CI

Malleable/nodular

CI

Recently used

materials

Nitrided steel Copper and nickel-

based aluminium

alloy (e.g. Al 336)

Si-con

alloy (

Tool steel Cast A

matrix

Steel l

Comp

graphi

1.1. Materials and surfaces in IC engines

A wide range of tribo-materials, tribo-coatings andsurface treatment techniques for engine tribo-componentshave been used in the last few decades, many of whichare being replaced by newly developed materials andcoatings to meet the increasing demands of increased fueleconomy. The most commonly used tribo-materials, wearresistant coatings and running-in coatings are summarizedin Tables 1–3, respectively.

1.1.1. Piston ring

The components of the piston assembly are consideredto be the most complicated tribological components toanalyse because they experience large variations in load,speed, temperature and lubrication regime (which extendsfrom boundary to hydrodynamic lubrication regime [3]).Traditionally, low cost and readily available castirons namely grey CI, carbidic cast iron and malleable/nodular CI were used as piston ring materials [4–7].However, because of high strength and good fatigueproperties, recently nitrided stainless-steel [7] and tool steel[4] are widely being used as piston ring materials. Thecompression ring experiences extremely high tempera-tures resulting in severe wear. Arc-ion plating of Cr–N orCr on steel or cast iron compression rings has beenreported to give low friction loss [5,8] However, Cr–Nplating gives much lower friction compared to Cr platingresulting in 90% reduction in ring wear and 15% reductionin bore wear [4]. This is because the electrodeposited Crcoatings are very densely packed and the low porositycompared to Cr–N and give poor oil entrapment andtherefore, it is very difficult to maintain oil film at thesliding interface [6]. Piston rings made of cast iron or steelcan also be treated by gas or ion nitriding, Si3–N2 dispersedNi–P plating, etc [4]. More recently, plasma sprayedmolybdenum, cermet, ceramics and diamond like carbon

er bore/liner Camshaft Shim or follower

lithic grey CI Grey CI Nodular iron, high Cr-

containing ferro-based

powdered sintered metal

Nodular CI

Chilled hardened

CI

High chromium cast iron

Silicon nitride ceramic

taining Al

AA 390)

Forced steel Low chromium steel

l-based metal

composite

Titanium Carburized steel

iner Ceramic

acted

te iron

Composite Material Powder sintered alloy

(Fe–Cr–Mo–C)

ARTICLE IN PRESS

Table 2

Commonly used wear resistant tribo-coatings/surface treatments for piston/cylinder and valve train assemblies

Piston ring Piston skirt Cylinder bore Camshaft Shim or Follower

Chromium plating Cr plating on CI Hard chromium coating on CI Induction/flame of

medium carbon steel

Chromium plated steel

Plasma or flame

sprayed molybdenum

on CI

Ni-ceramic (SiC–Ni–P)

coating

Chromium plated steel liner Carburizing of low

carbon steel

Carbo-nitriding of nodular iron

DLC coating on steel

piston ring

Pb–Sn plating Coating on Si-containing Al

alloys:

Ion plating of Cr2N on steel

SiC dispersed nickel plating;

Plasma sprayed coating of hard

ferrous or non ferrous alloy

Gas/iron nitriding of

steel

Solid lubricants:

polytetrafluoroethylene

graphite-based material

MoS2

Coating on hypereutectic Al

alloy:

Ion plating of TiN/Ti on tool

alloy steel

Laser hardening of cast iron;

Surface heat treatment of

Al–MMC

Si3N2 dispersed Ni–P

plating

DLC coating on steel shim

Plasma sprayed

molybdenum

Ceramic/cermet coating

DLC coating

Table 3

Commonly used running-in coatings and surface treatment techniques for piston/cylinder and valve train assembly

Running-in coating/

surface treatment

techniques

Tribo-components

Piston ring Cylinder bore Camshaft Follower/shim

Phosphating Phosphates of iron, zinc or

manganese is formed by immersion

Manganese phosphate

is formed by immersion

oxide coating treating

in steam

Same as camshaft

Oxidizing Iron oxide (with/without surfer

compound)

Sulfinuz Sulphur, nitrogen and carbon

containing layer formed by

diffusion

Same as piston ring

Tufftriding Harder than sulfinuz layer and

absence of sulphur into the treated

surface

Same as piston ring Non-brittle surface layer of carbon

bearing epsilon FeN

Electroplating Electroplated layer of cadmium, tin

or copper on cast iron piston ring

Induction/flame

hardening

Hard martensitic

surface layer with

tough core Diffusion of

carbon on the surface

of low carbon content

steel and case

hardening

Carburizing

A. Neville et al. / Tribology International 40 (2007) 1680–16951682

(DLC) coatings are also becoming popular as piston ringcoatings [5]. The DLC coating on piston ring significantlyimproves the friction properties, engine reliability andwork life, however, high internal stress of DLC whichlimits the thickness of DLC coatings further reduces thelonger working life [9].

1.1.2. Piston skirt

Grey cast iron has been widely used as a piston materialbut recently, because of high thermal conductivity, castiron piston materials are being replaced by different kindsof light weight and high strength aluminium alloys. Thealuminium alloys experience high thermal expansion,

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which requires maintaining high clearance between pistonskirt and cylinder bore. The copper and nickel-basedaluminium alloy (Al 336) provides 13% lower thermalexpansion coefficient than pure Al and thus helps tomaintain low clearance between piston skirt and cylinderliner [7]. One major limitation of piston skirts made ofaluminium alloys sliding against cast iron cylinder liners ishigh friction. Polytetrafluoroethylene (PTFE), graphite-based materials, MoS2, etc., are used as coating materialsto reduce friction losses [4]. The nickel–ceramic compositecoatings or molybdenum coatings on aluminium alloyshave also been recommended for piston skirts because oftheir improved thermal as well as friction behaviour [7].

1.1.3. Cylinder bore

Monolithic grey cast iron and cast aluminium–siliconalloys are used as cylinder bore materials [4,7,10]. How-ever, the hard Si particles in the Al matrix can act asabrasive particles and increase wear. Therefore, cast ironcylinder liners are often used on aluminium alloy cylinderbores—still retaining a 30% reduction of weight comparedto cast iron cylinder bore [4]. For further reduction ofweight of the engine blocks, more recently, cast aluminiumalloy is being used without any cast iron cylinder linerwhere hypereutectic Al–Si alloys, especially AA390 iscommonly used as cylinder bore material and the innersurface of the cylinder bore is modified by SiC dispersedNi–P plating, plasma sprayed coating by hard ferrous ornon-ferrous alloys, etc. Conversely, scuffing resistance ofhypereutectic alloys is very poor because the interfacesbetween Al and Si-rich phases acts as the potential site forfatigue crack initiation and propagation [9] giving rise tosevere wear in low lubrication conditions. Therefore, morerecently, hypereutectic Al–Si alloys are being coated withelectrolytic iron coating, Nikasil (nickel and silicon carbidematrix coating) plating or thermal sprayed coatings. Shortcarbon fiber or SiC particulates containing Cast Al-basedMetal Matrix Composite (MMC) is another kind ofmaterial that facilitates significant weight reduction.Special honing techniques are used to etch the surface sothat hard particulates are exposed to the piston ring thusreducing friction [10]. More recently, because of theincreasing demand of high cylinder pressure, especiallyfor diesel engines, the development of Compacted Graphi-tic Iron (CGI) has been evolved as an emerging materialthat gives 75% higher tensile strength and double thefatigue strength of unalloyed grey CI and aluminium[11,12].

1.1.4. Valve train

Friction plays an important role in the selection of valvetrain materials; efficient selection of cam/follower materialsand coatings can significantly reduce the friction loss. It isgenerally considered that the cam/follower is operated inthe boundary lubrication regime. However, according toTaylor [13], the elasto-hydrodynamic lubrication regimealso significantly affects the friction characteristics at the

cam/follower interface. The failure mechanisms at the cam/follower interface, which are strongly influenced by theregime of lubrication, are pitting, polishing and scuffing.Therefore, the selection of material combination, surfaceproperty and lubrication packages for cam/follower con-figuration in relation to boundary lubrication and elasto-hydrodynamic lubrication is crucial.Traditionally, different types of cast iron are used to

make camshafts followed by hardening the sliding surfaceof the cam lobes by various hardening techniques[4,5,7,14]. The most commonly used cast irons are greycast iron [7], nodular cast iron [4] and chilled hardenedalloy cast iron [4,14]. Recently there have been moves toreplace these with forced steel, composite materials, etc.[14]. However, cast iron is still used for the large volume ofproduction because it is cheap and readily availablewhereas steel is used for low volume production becauseit is required to be machined to make camshafts resulting inhuge material loss. To improve the running-in performanceas well as to prevent the early failure of the cam, severalsurface treatment techniques such as induction hardeningor flame hardening of cast iron, carburizing of low carbonsteel, induction hardening of medium carbon steel, etc., areperformed. In addition, phosphate coatings, oxide coat-ings, carbon bearing epsilon FeN layers, etc., can bedeposited on the cam surface to improve the wetting andspreading of the oil and thus gives improved running-inperformance [15].The cam/follower is mainly operated in the boundary

lubrication regime; due to asperity contact wear rates canbe high. As such, it is necessary to make the cam/followercomponents from high wear resistant materials. Because oftheir excellent wear resistance properties, ferro-basedpowder sintered metal with high chromium, high chro-mium cast iron or silicon nitride ceramics are convention-ally used as shim materials [16]. The use of ceramics as ashim material has not proven to be economically viable todate [7]. Recently, steel or light weight forged aluminiumhave been used as shim materials [11]. Careful selection ofthe surface topology of the shim is very important becauseit greatly influences the running-in property and thetransition between boundary lubrication to elasto-hydro-dynamic lubrication and consequently, further increases ofthe volume of wear. Ion plated Cr–N and Cr2N on shimsgives excellent scuffing resistance under boundary lubrica-tion but the smooth surface of Cr plating gives poorlubrication characteristics [15]. Ion-plating of TiN/Ti ofseveral microns thickness [17] or the use of DLC coatingon steel shim can remarkably reduce the friction loss incam/follower arrangement [18].Apart from the use of different kinds of wear-resistant

and friction reduction coatings, several expendable coat-ings or surface treatment techniques are also used toimprove the running-in performance of the contactingsurfaces of the engine components. Some of the wearresistant coatings show good running-in properties whileothers need to be deposited with running-in coatings.

ARTICLE IN PRESSA. Neville et al. / Tribology International 40 (2007) 1680–16951684

Running-in performance is usually improved by modifyingthe contacting surfaces by a chemical conversion methodsuch as phosphating, oxidizing, sulfinuz, tufftriding, etc.,where the immediate surface of metal is converted to metalcompound [4,13]. Such metal compounds provide soft andporous surfaces and thus promote wettability and oilentrainment. In addition, electroplating, induction/flamehardening, carburizing, etc., are also used to improve therunning-in performance of the sliding surfaces. The mostcommonly used techniques to modify the surface with aview to improving running-in performance of the slidingsurfaces of the piston-cylinder and valve train assemblyhave been given in Table 3. In recent years, because of theexcellent running-in properties of amorphous carboncoatings [19,20], several attempts have been made to useDLC coatings on the shims in automotive valve trainapplication [16,21,22].

1.2. Light alloy integration

The increasing demand for increased fuel-economyrequires reduction of energy consumption, which canconsequently reduce air pollution. The weight reductionof a vehicle is directly related to the fuel and oilconsumption and to reduction in emissions. As mentionedearlier, because of the excellent characteristics of alumi-nium alloys such as their light weight, high strength, highthermal conductivity and corrosion resistance, the engineblock, which is one of the heavier parts of the engine, isbeing manufactured using aluminium alloys. On the otherhand, because of excellent high strength to weight ratio,good formability, good corrosion resistance and recyclingpotential, existing steel or copper-based sheet metals canpotentially be replaced by sheet aluminium. In 2000,reported by Miller et al. [23], aluminium casting was usedto manufacture 100% of pistons, about 75% of cylinderheads and for many other power train and chassisapplications. However, poor wear resistance and lowseizure loads have restricted the direct use of aluminiumalloys for cylinder bores and piston skirts giving rise tobore distortion and poor piston ring sealing and so causes ahuge oil consumption [9]. To improve the wear resistanceand to reduce friction, cast iron is used as liner material butsuch use further increases the dimension and wear ofengines. Several approaches have been taken to overcomethe drawbacks of aluminium alloys. The use of Al metal-matrix composites (MMC) reinforced with solid lubricant(graphite, MoS2, WS2, etc.), hard ceramic particles (e.g.SiC, Si3N4, Al2O3, etc.) and short fibres not only reducesthe friction and wear but also reduces the weight andthereby reduces fuel consumption and vehicle emissions[10]. Nanocoatings are developed by deposition of ironoxide or a mixture of iron oxide and aluminium on thealuminium alloy followed by fusion using a laser beam andthis approach, which is widely known as Laser SurfaceEngineering (LSE), is being considered as a promisingsurface treatment technique for the aluminium cylinder

bore [8]. Traditionally, a cast iron liner was used on thebores of the aluminium alloy engine blocks and this waslater replaced by iron plating. The traditional iron platingprocesses require the use of cyanides, which is furthereliminated owing to environmental restrictions [24]. Be-cause of attractive tribological behaviour, several research-ers tried to deposit DLC coatings on aluminium alloysusing different deposition techniques [24–26]. The naturallyformed oxide layer on the aluminium alloys has beenidentified as one of the main limitations, which causes pooradhesion of the DLC coating to the substrate. Malaczynskiet al. [24] recommended that the aluminium alloy surfaceshould be cleaned by argon sputtering and then theadhesion promoter Si:C bond layer of 50 nm can beformed on the Al alloy by carbon implantation followed bydeposition of the DLC coating. Nie et al. [26] claimed thatthe deposition of DLC on the alumina intermediate layerby plasma-immersion ion implantation provides excellentwear resistance and low and stable friction coefficient.However, despite the excellent tribological behaviour ofDLC coatings, they cannot be used in automotiveindustries until the mass production of such coatings iseconomically feasible. Commercially available engine oilsare basically designed/optimized for the metallic surfaces,and engine oils optimized for DLC or Al-MMC surfacesare yet to be commercialized. Therefore, engine oils,basically designed for metallic (Fe-based) surfaces, haveto be used for the components made of newly developedcomposite materials or coated with DLC, and thereplacement of these oils is still posing a technologicalchallenge.

1.3. DLC as a low friction coating

DLC is a kind of amorphous carbon coating having anetwork composed of sp2 (graphite like) and sp3 (diamondlike) bonds, and its physical as well as chemical propertygreatly varies with the varying ratio of sp2 and sp3 bondedatoms. DLC coatings usually show high hardness, excellentwear resistance, high corrosion resistance, high thermaland chemical stability, low friction property [27,28] andexcellent running-in property [19,20,27]. Because of theexcellent tribological performance and to meet the increas-ing demand of fuel economy and clean environment,recently, DLC coatings are extensively being used forautomotive engine components as well as for othertechnical and medical applications [29,30]. Gahlin et al.[30] claimed that about 30 million coated parts are beingused in automotive industry with an annual increase ofaround 50%. Since the tribo-performance of DLC isgreatly influenced by the environmental parameters such astemperature, relative humidity, etc. [31,32], the use of DLCin automotive engine is very crucial because the automotivecomponents are operated at high temperature, high load,partial lubrication and in an environment favourable foroxidation. Therefore, modifications of DLC by dopingwith hydrogen, different kinds of metals, nitrides and

ARTICLE IN PRESSA. Neville et al. / Tribology International 40 (2007) 1680–1695 1685

carbides have been developed to improve the mechanicaland tribological properties and to enhance adhesionstrength of the coating [30,33].

With the increase of temperature the crystalline graphitictransfer layer is formed by a sp3-sp2 phase transforma-tion of the DLC coatings in dry sliding conditions [34]. Thesteady-state low friction of DLC in ambient air is due tothe temperature increase at contact asperities facilitatingthe formation of a graphitized tribolayer [35]. Thus DLCprovides self-lubrication in dry air or in inert atmosphericconditions. But degradation of DLC increases withtemperature and the friction and wear performancedeteriorates because of the damage of DLC coating atelevated temperature [36]. It has been reported that thetribological behaviour of DLC starts to change around100 1C [37] and it starts to delaminate and lose itseffectiveness above 300 1C [38]. High temperature alsofacilitates the release of hydrogen from the DLC matrix.The engine tribo-components, responsible for the majorportion of friction loss, especially piston/cylinder and valvetrain assembly, are operated at relatively high temperature,pressure and sliding velocity and the maximum operatingtemperatures at the sliding interfaces of those componentsare 300 and 150 1C, respectively [3]. Therefore, the use ofDLC coating in those tribo-components is quite challen-ging. On the other hand, the increase of humidity level inthe environment reduces the rate of graphitization[27,31,39] and thereby, increases friction and wear, andthe friction coefficient may go as high as 0.1 in highhumidity conditions. Thus the temperature and environ-mental parameters greatly influence the stability of frictionand wear.

1.4. Lubricated contacts—steel/lubricant and coating/

lubricant interactions

1.4.1. Steel/lubricant interactions—focus on ZDDP and

MoDTC

Interactions between the lubricant and the lubricatedcomponent surface can be of physical in nature and/orchemical in nature. In terms of physical interactions,nanostructured coatings and laser texturing of surfaces

100 to1000 nmapprox.

Iron/Steel Substra

GlassPolyp+ ZnO

Fig. 1. ZDDP film

were seen to improve lubrication [2]. However, wear andfriction performance in the boundary lubrication regime iscontrolled mainly from the chemical reactivity of lubricantadditives, which form tribofilms in the contacting surfaces.In the following sections, interactions of the zinc dialkyldithiophosphate (ZDDP) and molybdenum dialkyl dithio-carbamate (MoDTC) additives with the lubricated steelsurface material in improving wear and friction arereviewed.

1.4.1.1. ZDDP. Most common anti-wear additives usedin practice are organochlorine, organosulphur, organopho-sphorus (tricresyl phosphate—TCP and dibutyl phos-phite—DBP), organometallic (ZDDP, MoDTP, MoDTC)and organic borate compounds [40,41]. To deliver theiranti-wear functionality, anti-wear additives, through tribo-chemical reactions, facilitate the formation of a very thinfilm (now commonly referred to as the tribofilm) on thelubricated surface.The zinc dialkyl dithiophosphate (generally referred to

as ZDDP or ZDTP) additive is one of the most widely usedin engine lubricants. This is because it is shown to havemultifunctional properties, namely anti-wear and anti-oxidant action. ZDDP has been subject of several reviewpapers [42–44], but in this review the focus will be on theZDDP/Fe-based surface interactions. The composition ofZDDP tribofilms in Fe-based materials is analysed with awide range of surface analytical techniques and shown tobe comprised of different layers. In general, it is agreed thatZDDP form a glassy phosphate film, with different chainlength as a function of depth, on top of an oxy/sulphidelayer. A schematic representation of a ZDDP tribofilm isgiven in Fig. 1.Considering that rubbing is necessary to produce the

speciation of phosphorus and sulphur, Martin [46,47],proposed a model for film formation with metal dithiopho-sphates in which both chemical and mechanical aspects arelinked together. According to Martin [46,47], reduction ofpolyphosphate chain length is a result of the chemicalprocess between the long-chain polyphosphate formedfrom ZDDP decomposition with the iron oxide wearparticles. On the basis of the hard and soft acids and bases

te

Iron Sulphideand/or Oxides

Inorganic Layer

yhosphate, ZnS

Hydrocarbon-rich Layer

OrganicRadicalsIncreasing

Fe, FeOFeSIncreasing

structure [45].

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(HSAB) principle a set of tribochemical reactions betweenthe polyphosphates and the oxides were proposed [48].According to this approach, a possible route for theelimination of 1mol Fe2O3, and formation of short-chainpolyphosphates, will be

5ZnðPO3Þ2ðZnO; P2O5Þ

þFe2O3! Fe2Zn3P10O31ð3ZnO; 5P2O5; Fe2O3Þ

þ2ZnO: (1)

By this, three-body abrasive wear is practically elimi-nated, suggesting that ZDDP effectiveness in wear reduc-tion is a result of its interaction with the substrate debris, inthis case iron oxide. This reaction develops because theFe3+ is a harder Lewis acid than Zn2+ and the cationexchange is energetically favourable from the point of viewof the HSAB principle. Phosphates are hard bases and willspecifically react with hard acids, resulting in reactionbetween iron and the phosphate.

According to some authors [49], the ZDDP tribofilmformation is not dependent on the nature of the surface.From XANES spectra made on thermal films and anti-wear films it was concluded that ZDDP-derived anti-wearand thermal films on steel are chemically similar [49–51]. Inthese studies, the presence of Fe2O3 is not seen as being arequirement for ZDDP anti-wear film formation. Theformation of short-chain polyphosphates, observed inZDDP tribofilms, is proposed to be as a result of hydrolysisof polyphosphates [49]. A linkage isomer of ZDDP (LI-ZDDP) was proposed as an important precursor for filmformation after analysis of thermal films and lubricantinsoluble ZDDP decomposition products. Thus a mechan-ism of film formation from ZDDP is suggested and isexplained with the reactions below:

ZnððROÞ2PS2Þ2 ðsolutionÞ ) ZnððROÞ2PS2Þ2

ðZDDP adsorbedÞ;

ZnððROÞ2PS2Þ2 ðsolutionÞ ) ZnðO2PðSRÞ2Þ2

ðLI�ZDDP in solutionÞ;

ZnðO2PðSRÞ2Þ2 ðsolutionÞ ) ZnðO2PðSRÞ2Þ2

ðLI�ZDDP adsorbedÞ;

ZnðROÞ4P2S4 þO2 ðor ROOHÞ ) ZnðPO3Þ2 ðpolyphosphateÞ

þ sulphur species;

7ZnðPO3Þ2 þ 6H2O) Zn7ðP5O16Þ2 þ 4H3PO4;

2ZnðPO3Þ2 þ 3H2O) Zn2P2O7 þ 2H3PO4;

short�chain polyphosphates: (2)

During steady state, where there is reduced formation ofsubstrate debris, wear protection is suggested to be a resultof wear debris from ZDDP tribofilm re-entering thecontact and by that reduce wear significantly [46,52]. TheZDDP debris is generated as a result of ZDDP tribofilm

delamination. Another mechanisms by which the ZDDPphosphate films reduce wear are that the film material canbehave as a viscous lubricant in boundary conditions [46].The viscosity of a glass varies continuously with thetemperature, and can decrease appreciably under shear.The mechanisms by which ZDDP tribofilms are formed areimportant if new surfaces are to be effectively protected byexisting lubricants. With there still being some doubtsabout whether the production of Fe-containing species is anecessary requirement for tribofilm formation, it is notcertain whether non-Fe-based surfaces will be effectivelyprotected from wear by ZDDP.Another interaction between the surface and ZDDP

additive is seen in the formation of iron sulphide. Running-in wear, when ZDDP was used, is believed to take placeprincipally by adhesive and oxidative processes [45].Localized adhesion and metal transfer between contactingasperities is prevented from growing into larger scaleadhesion, by reacted iron sulphide films formed by thesulphide products that occur early in the frictionaldecomposition process of ZDDP.Formation of iron sulphide and Zn/Fe phosphate shows

the interaction of ZDDP additive with the surface resultingin formation of the tribofilm.

1.4.1.2. MoDTC. Engine efficiency can be increased byreducing the mechanical losses, which are mostly causedby friction. One of the ways to reduce mechanical losses isby the use of friction reduction additives, which operate inboundary and mixed lubrication conditions. These addi-tives generally fall into two classes: the physically adsorbedmolecules such as fatty acids and amides, and thechemically reactive species such as molybdenum dithiocar-bamates [53–55].A very important class of friction-reducing additives,

which are used extensively in lubricant formulations aremolybdenum–sulphur containing compounds. Mo–S com-pounds are introduced in late 1950s [56] and interests onthem increased since the 1970s. These compounds aredocumented to reduce friction by forming a MoS2 film onmetal surfaces [57–66]. The layer-lattice structure of themolybdenum disulphide makes it possible for thesecompounds to facilitate low friction [57]. In molybdenumdisulphide there is powerful covalent bonding betweenatomic species, but between lattice layers there is only veryweak Van der Waals attraction. The weak Van derWaals forces between MoS2 layers maintain easy shearwithin the molecule and are responsible for the low frictionproperties.Analyses of MoDTC films formed on non-rubbing

surfaces [65,67], films that are formed mainly due tothermal decomposition of MoDTC, have shown that onlyMoO3 is formed. Surface analyses with X-ray Photoelec-tron Spectroscope (XPS) [58,67–69] have shown that onlyMoO3 forms outside the wear scar, while in the wear scarMoS2 and MoO3 are detected. The factors associated withrubbing and which could stimulate MoS2 formation in the

ARTICLE IN PRESSA. Neville et al. / Tribology International 40 (2007) 1680–1695 1687

wear scar are (a) contact temperature and (b) exposure ofthe metal surface due to the removal of oxide film. Thecontact temperature and mechanical removal of the oxidelayer, which will form a nascent iron surface, will initiateformation of MoS2 at the contact points. This mechanismof MoS2 formation explains why no MoS2 is formedoutside the wear scar, since there is no a rubbing and henceno high contact temperature or nascent iron surfaceproduced.

Grossiord et al. [65] in addition to finding MoS2 andMoO3, also found some residual MoDTC in the tribofilm.From the chemical point of view, they suggested theMoDTC decomposition occurs by a two-step process, asshown in Fig. 2:

�

R

R

R

R

c

Mo–S chemical bond breakage in MoDTC additive dueto the electron transfer, leading to three free radicals:one corresponding to the core of the molecule and theother two to the chain ends.

� Recombination of chain end radicals to form thiuram

disulphide, whereas the core radical decomposes intoMoS2 and MoO2. MoO2 can oxidize in the presence ofair and form MoO3.

This is in agreement with the XPS detection of MoS2 andMoO3 in the tribofilm but the role of the two chain free N-containing radicals is not clarified. A recent work, usingXPS, has shown the presence of N and S-containing speciesin the MoDTC tribofilms [70]. It is observed that thesespecies are formed before the formation of any Mo speciesinfluencing the evolution of the low friction film formation[70,71]. It is suggested that one possible mechanismfor nitrogen being at the surface is that the S in the

O

N CR

S

S

RMo

S

SMo

S

SC N

R

R

O

electron transfer

+

Mo

S

SMo

S

SMo

S

S

+ 21 O2

S S

N CS S

C NR

R +

21N C N

R

RN

thiuram disulfide

MoS2 sheet

MoS2 MoO2

MoO3molybdenum

oxide

+ Mo*

S

S

Mo*O ON CR

S

S*

R2

b

a

Fig. 2. MoDTC decomposition [65].

N-containing radical, formed from breakdown of MoDTC,reacts with the nascent surface according to the HSABprinciple, since S2� and metal atoms are known to be softbase and acid, respectively, forming FeSx and leaving the Npart to deposit on the surface [70]. The formation of ironsulphide from MoDTC has been observed in other studies,too [56,72], showing another interaction between theMoDTC additive and the surface material. Formation ofiron sulphide will act as a protective layer reducing wear[56,72,73] and by that allowing the formation of a friction-reducing layer of MoS2 from the other radical.Interaction between MoDTC and surface material is

seen on formation of MoS2 linked with rubbing andformation of iron sulphide species.

1.4.2. Coating/lubricant interactions

DLC is considered as a promising non-ferrous coatingbecause of its excellent tribological properties as mentionedearlier. Since the temperature and environmental para-meters greatly influence the stability of friction and wear ofDLC coatings, the use of lubricating oil will isolate thecoating from the surrounding hostile environment and actas a coolant to keep the temperature in the allowable limit.In addition, the lubricant additives may interact with theDLC coating and produce low friction and wear resistanttribofilms. Most of the lubricants developed so far arecustomized to form the tribofilm that will adhere to ferrousmaterials and until now no lubricants have been designedfor the non-ferrous coatings. To design new lubricants fornon-ferrous coatings, especially for the DLC coating, it isessentially important to understand how this coatinginteracts with the existing lubricant additives. Knowledgeof how DLC interacts with existing lubricant additives isscarce and contradictory. For example, Kano and Yasuda[74] showed that no stable tribofilm was found on the DLCsurface, when additive containing oils were used, whileother research groups observed tribofilm formation onDLC at boundary lubrication condition [75,76]. Thephysical properties of DLC coatings will significantly varywith deposition techniques, type of dopant used and thehydrogen content in the DLC matrix. Hence, any researchon the interactions between DLC and lubricant additivesshould take into account the intrinsic properties of DLC.

1.4.2.1. Role of hydrogen in DLC/lubricant interac-

tions. DLC coatings are generally described as hydro-genated amorphous carbon (a-C:H) and non-hydrogenatedamorphous carbon (a-C). Hydrogen stabilizes the randomcovalent network of DLC and prevents its collapse into agraphitic phase during deposition [77,78]. In the dryatmospheric condition, release of hydrogen occurs due tohigh contact temperature resulting in the conversion of sp3

to sp2 structure and provides high wear of a-C:H coating.However, in oil lubricated condition, DLC-coated surfacesare believed to be separated by thin oil film and thepresence of oil at the sliding contact may act as coolant andreduce contact temperature. Thus the formation of

ARTICLE IN PRESSA. Neville et al. / Tribology International 40 (2007) 1680–16951688

graphitic layer is believed to be suppressed and thetribochemical interactions of oil additives and DLCsurfaces become more dominant.

Yasuda et al. [79] observed that the a-C:H exhibited highfriction (0.11) and the a-C gave low friction coefficient(0.07) while they slid against steel pin (AISI 52100) usingengine oil 5W-30 (SJ grade) to lubricate the surface.Surface analyses performed with XPS on a-C did not showany tribofilm formed, although a tribofilm was detected inthe steel counterbody [74]. The low friction observed in a-Cis assumed to be related to the low surface energy of a-C:Has compared to a-C, effecting the adsorption of oiladditives. Now the question arises why the surface of a-C:H is chemically inert and why the surface energy is low.To explain such characteristics of a-C:H, it is necessary toinvestigate the atomic structure of DLC first. The structureof DLC is inherently meta-stable and the heat generatedfrom the tribo-friction can easily transform the sp3

structure into sp2 structure (graphite-like). In sp2 config-uration, three of the four valance electrons of a carbonatom are arranged trigonally and forms strong covalentbond with the nearest neighbouring atoms (Fig. 3b). Thefourth electron which is known as p electron perpendicu-larly lies on the sp2 hybrids as shown in Fig. 3b. Thus itforms the perfect graphite lattice where the atoms inhexagonal network are staked in the sequence of ABAB(Fig. 3a). The dangling covalent bond of the sp2 bondedcluster remains at high-energy state and can easily bepassivated by the adsorbed species from the surroundingenvironment. The possible reason of low friction exhibitedby a-C, as observed in [74,79], is because the oil additiveswere easily adsorbed and passivated the surface resulting inlow friction. In contrast, in case of a-C:H, the danglingbonds are passivated by the hydrogen atoms within thecoating and thus the hydrogen–carbon combination gives anon-polar inert surface having low surface energy whichfurther causes poor wettability and probably little or noadsorption of oil additives on the sliding surface. Mabachi

Fig. 3. (a) Atomic arrangement of lamellar graphite, (b) atomic structure of sp2

et al. [80] performed experiments in cylinder head cam/follower rig using oil containing MoDTC and found thata-C coatings provided low friction torque as compared toa-C:H.A completely opposite picture was portrayed by Barros-

Bouchet et al. [75] where they explained the positive role ofhydrogen in the formation of tribofilm on the DLCcoating. In their observation, the a-C:H showed lowfriction as compared to a-C. They found that the MoDTCand ZDDP are more active on the a-C:H facilitating theformation of MoS2. It is interesting to note that no P wasfound in the tribofilm and consequently no ZDDP-derivedanti-wear film was noticed. However, it has been claimedthat ZDDP enhanced the formation of MoS2 on theamorphous carbon surface by supplying S atoms. FromXPS analysis, they observed that the ratio of MoS2/MoO3

for hydrogenated DLC is five times higher than thatobserved on non-hydrogenated DLC. They argued that thehydrogen-terminated surface was damaged by the slidingaction and gave nascent dangling bonds, which reactedwith lubricant additives. They explained such phenomenonby making assumption based on chemical hardnessapproach (HSAB). According to their argument, the softbase hydrogenated carbon reacts with soft acid Mo+4 andforms low friction MoS2 layer while hydrogen-free carbonmaterial which is considered as intermediate base reactswith hard acid Mo+6 and forms high friction MoO3 layer.Thus the a-C:H promotes the formation of MoS2 andreduces friction efficiently while the a-C gives detrimentaleffect to the friction performance by forming large amountof MoO3.

1.4.2.2. Role of ferrous and non-ferrous dopants in DLC/

lubricant interactions. Metal is usually doped in DLC toimprove durability, reliability [81] and to maintain lowfriction and wear under severe operating conditions[82–84]. Metal doped in DLC (Me-C:H) forms metalcarbide and improves the strength of DLC films, and it is

hybridized graphite layer transformed from sp3 hybridized DLC structure.

ARTICLE IN PRESSA. Neville et al. / Tribology International 40 (2007) 1680–1695 1689

reported that boundary lubrication performance can beimproved if a high strength metal carbide is formed in thedefective part of the cross-linked carbon network of DLC[76]. In comparison to the tribological performance of non-metal-doped DLC, metal (Ti, Mo and Fe)-doped DLCoutperformed at boundary lubrication condition using oilcontaining both ZDDP and MoDTC, and Ti-DLC showedsuperior performance over Mo-DLC and Fe-DLC becauseit provided favourable surface to form low shear strengthfriction layer from MoDTC and ZDDP additives.

Ban et al. [81] performed the XPS analysis of thetribofilms formed by the oil containing ZDDP on the Si-doped DLC and found the presence of ZnO, ZnS, FePO4,FeS and FeS2 compounds, and it was believed that thosecompounds contributed to achieve low friction and highwear resistance. Although the presence of Si in DLCreduces the friction property, they observed that thefriction coefficient linearly increased with Si percentage inDLC matrix. On the other hand, Gangopadhyay et al. [21]investigated the performance of Si-containing amorphoushydrogenated DLC coating as the coating on shim slidingagainst cast iron cam lobes. Manganese phosphate coatingwas deposited on the top of DLC coating to improve thebraking-in performance. Interestingly, the DLC-coatedshim showed higher friction torque than that of productionsteel shim. It was argued that the use of oil prevented theformation of friction layer as found on DLC at dry slidingcondition but no elemental analysis was performed tosupport their argument.

In case of W containing DLC, low friction tribo-layer ofWS2 nanocrystals on the steel counter surface is formedwhen S containing EP additive was used in oil underboundary lubrication condition [85,86]. It has been arguedthat S of EP additives react with the nanocrystalline W andWC, transferred from the W-DLC coating, and forms WS2tribofilm, and consequently provides low friction beha-viour. Podgornik and Vizintin [87] showed that the increaseof the concentration of S containing EP additive has noeffect on the tribological behaviour of a-C:H while it givessignificant improvement in friction performance of W-DLC coating by forming low friction lamellar WS2tribofilm. The P containing anti-wear additive gave highfriction for both a-C:H and W-DLC but no tribochemicalanalysis was performed to find if any anti-wear film wearformed on the DLC coatings.

Kalin [88] observed that doped and undoped DLCshowed different tribological behaviour in presence of EPand AW additives under boundary lubrication condition.The steel/undoped DLC (a-C:H) combination showed thatthe use of base oil provided low wear of a-C:H while EP/AW containing oil gave high wear. In case of doped DLC(Ti–C:H, Si–C:H), the additive containing oils showed lowwear as compared to base oil.

Miyanaga et al. [89] used fully formulated oil and foundthat Ti-DLC showed the highest wear resistance and thelowest friction coefficient among all other coatings (a-C:H,MoS2, TiN, Mn-phosphate). Using the Secondary Ion

Mass Spectroscopic (SIMS) analysis, they claimed thatmore adsorbed extreme pressure agent was observed on theTi-DLC than that of undoped-DLC.

1.4.2.3. Selection of the tribo-materials/coatings. Theselection of tribo-metal and/or tribo-coating combinationis a challenging issue because the reactivity of the additivecontaining oil mainly varies with the property of frictionsurfaces as summarized in Table 4. The most possiblecombinations of coatings and substrate materials operatedunder boundary lubrication condition using wide range ofoil formulations have been compiled from the publishedpapers, and based on the friction and wear performance,suitable combinations have been sorted out. In two cases,friction coefficient of steel–steel combination was foundhigher than a-C/steel combination [27,90] while it waslower than a-C/steel combination in one case [75]. On theother hand, in two cases, a-C/steel showed lower frictioncoefficient than a-C:H/steel [27,90] whereas in one case a-C:H showed better friction performance than a-C/steelcombination [75]. Therefore, higher friction performanceof a-C/steel was supported by more researchers than thoseof steel/steel and a-C:H /steel combinations.No measurable wear of DLC coating was observed in the

study of Ronkainen et al. [27], and the possible reason forsuch results might be because of the application of lowforce and sliding velocity. Stallard and Teer [90], whoapplied comparatively high force and sliding velocity,noticed significant wear of DLC coatings. A similar trendof wear of DLC coatings sliding against cast iron counter-body has been reported by Haque et al. [91]. In terms ofwear performance, the wear of steel counterbody slidingagainst a-C:H was found to be lower than in steel/steel anda-C/steel combinations [27,75,90]. However, when the wearof DLC coatings are compared, a-C sliding against steelshowed lower wear than those of a-C:H and steel [90]. Thiscould be because of the high hardness of a-C compared toa-C:H which results in high plastic deformation in thesofter counterbody.Results from a study by Barros-Bouchet et al. [75], as

shown in Table 4, clearly indicate that a-C:H/a-C:H andTi–C:H/ Ti–C:H combination exhibit lower wear perfor-mance than a-C:H/steel, Ti–C:H/steel and steel/steelcombination but the improvement of frictional perfor-mance using DLC/DLC combination is found to be lowcompared with the DLC/steel combination. XPS analyseson a-C:H and Ti–C:H sample showed that a MoDTC-tribofilm is formed [75], suggesting that formation of lowfriction tribofilm from MoDTC is not affected by thepresence of iron in the contact. The XPS results alsosuggest MoDTC and ZDDP are more active on the a-C:H/a-C:H contact than on Ti–C:H.Conversely, Podgornik et al. [19,20] reported that, under

boundary lubrication conditions using additive-containingoil, the running-in time of WC-C:H/WC-C:H combinationwas much longer resulting in a higher wear rate than thatof WC-C:H /steel combination. This is to be suggested

ARTIC

LEIN

PRES

S

Table 4

Selection of tribo-metal and tribo-coating combination: role of hydrogenated and metal-doped DLC

Source Experimental condition Lubricant and

lubrication

Counterbody Coatings on the plate/disc Friction Total wear

Plate/disc Counterbody

Ronkainen et al. [27] Test: pin-on-disc Mineral base oil+EP

additive

AISI 52100 (100Cr6) a-C 0.08 No wear 300� 10�9mm3/Nm

Force: 10N a-C:H 0.13 No wear 80� 10�9mm3/Nm

Hz. Pr.:0398Gpa Boundary lubrication

Cond.

a-C:H (Ti) 0.10 No wear 50� 10�9mm3/Nm

V: 0.004m/s AISI 52100 0.13 No wear 90� 10�9mm3/Nm

Stallard et al. [90] Test: pin-on-disc Semisynthetic oil

(10W40)

AISI 52100 a-C �0.07 0.03� 10�9mm3/Nm 4.3� 10�9mm3/Nm

Force: 40N a-C:H �0.1 0.12� 10�9mm3/Nm 0.6� 10�9mm3/Nm

Hz. Pr.: �1.6Gpa AISI 52100 �0.09 0.35� 10�9mm3/Nm 7.4� 10�9mm3/Nm

V: 0.2m/s

Barros’ Bouchet et al.

[75]

Test: cylinder-on-flat PAO+MoDTC+ZDDP AISI 52100 a-C �0.08 Not mentioned 50� 10�9mm3/Nm

Force: 50–350N a-C:H (50 at.% H) �0.05 Not mentioned 2� 10�9mm3/Nm

Hz. Pr.: 0.6Gpa (Max) Ti-C:H (35 at.% H) �0.05 Not mentioned 2� 10�9mm3/Nm

V: 0.2m/s AISI 52100 �0.06 Not mentioned 6� 10�9mm3/Nm

a-C:H (50 at.% H) a-C:H (50 at.% H) �0.04 Not mentioned 0.07� 10�9mm3/Nm

Ti–C:H (35 at.%H) Ti–C:H (35 at.%H) 0.06 Not mentioned 0.4� 10�9mm3/Nm

Podgornik et al. [19] Test: two crossed

cylinder

GL-4 (fully formulated

oil)

WC-C:H WC-C:H 0.07 0.98� 10�3mm3/min

Force: 140–1700N Steel WC-C:H 0.07 0.6� 10�3mm3/min

Hz. Pr.: 2.4–5.6GPa

Tung and Gao. [92] Test: Modified high

frequency reciprocating

friction machine

PAO/engine oil Steel piston ring Cast iron cylinder bore �0.11 7.40.6� 10�9m3 3.7� 10�9m3

Force: 80N DLC-coated piston ring Cast iron cylinder bore �0.1 7.4� 10�9m3 0.8� 10�9m3

Oscillation frequency:

10Hz

PAO+MoDTC Steel piston ring Cast iron cylinder bore �0.04 0.8� 10�9m3 2.8� 10�9m3

V: 0.138m/s Sliding

distance: 6.9mm

DLC-coated piston ring Cast iron cylinder bore �0.08 0.8� 10�9m3 0.4� 10�9m3

Haque et al. [91] Test: pin-on-plate test Base oil+ZDDP+Moly

Dimer (MoDTC)

Cast iron a-C:H �0.07 9.18� 10�19m3/Nm 9.45� 10�19m3/Nm

Force: 326N

Hz. Pr.: 0.56–0.64GPa

A.

Neville

eta

l./

Trib

olo

gy

Intern

atio

na

l4

0(

20

07

)1

68

0–

16

95

1690

ARTICLE IN PRESSA. Neville et al. / Tribology International 40 (2007) 1680–1695 1691

because with the WC-C:H/WC-C:H combination, morecontact cycles were necessary to smoothen the surfaces andprovide conformity whereas, for the DLC/steel combina-tion, smoothening of the steel with the harder DLC countersurface facilitated good running-in performance. It wassuggested that the WC-C:H/WC-C:H combination showedpoor frictional performance because no tribologicallyactivated chemical reaction occurred on the WC-C:H. Onthe other hand, formation of the synergistic tribofilm onsteel at WC-C:H /steel was claimed and it was believed thatthe tribofilm facilitated low friction and wear.

As mentioned earlier, the use of steel as a piston ring orshim material and cast iron as cylinder liner or cammaterial is still popular in the automotive industries.However, some research has been performed to assess thefeasibility of using DLC coatings on steel shims or pistonrings sliding against cast iron. Tung and Gao [92]performed bench tests to investigate the possibility ofusing DLC coating on steel instead of using bare steel ringsliding against cast iron cylinder bore. They found that theuse of PAO gives higher friction coefficient than that ofusing PAO with MoDTC for both DLC/cast iron and steel/cast iron combination (Table 4). However, no significantimprovement in friction behaviour was noticed by usingDLC coated steel ring over the uncoated steel ring. It isinteresting to note that the results of material wear clearlyindicate that the use of DLC can significantly reduce thewear of cast iron counter surface. In addition, MoDTCfurther can reduce wear in both DLC/cast iron and steel/cast iron combinations. Therefore, it is quite reasonable touse DLC as a tribo-coating for steel piston ring in pistoncylinder assembly. Although the contact pressure at pistonring and cylinder liner (�10MPa) is much lower than thatof cam/follower contact (�0.6GPa), similar approach ofthe selection of material couple can be checked for cam/follower application.

This review has shown that the range of surface coatingsand treatments used in automotive components is diverseand opens up opportunities to improve friction and wearperformance. There is so far little published literature todemonstrate how lubricant additive/surface combinationscan be designed to optimize performance. In this next partof the paper, some experimental results using a limited sub-set of oils/additives and tribocouples are presented toillustrate how synergistic or antagonistic effects can existand that there is real potential if the compatibility ismatched.

2. Experimental details

Two ferrous-based materials (BS EN1452 cast iron andBS EN31 Cr-bearing steel) and an Al–Si piston alloy werestudied. Tribological properties of these materials inlubricating oils were examined using a pin-on-reciprocatingplate system. The pin was loaded using a static load appliedthrough a lever arm and the plate was reciprocatedunderneath through a crank mechanism.

The pins used were 20mm in length; 6mm in diameterand the ends of the pins were machined to a 20mm radiusof curvature. The rectangular plate measured 17� 6�3mm3. The test-stroke and test frequency are 10mm and1Hz, respectively. The load applied on the pin was 185N,which corresponds to a Hertzian pressure of between 0.5and 0.9GPa (depending on the material couple). A bi-directional load cell measured the friction force on thestatic pin during the course of the experiment. The frictionforce was recorded every 30min for 3 cycles during the testduration of 8 h. Tests were conducted at 100 1C.Using the friction force data, the friction coefficient (m)

was calculated by first calculating the average absolutefriction force and then dividing this by the applied load.This is then plotted as a function of time and the averagefriction coefficient is calculated for the 8-h period. Threeseconds of data (friction force versus time) were recordedat 30-min intervals. A square wave trace was recorded asexpected for a reciprocating motion.Typical friction coefficient values of 0.07–0.18 were

recorded suggesting that the contact regime is in boundarylubrication and this is verified from the square wave traces.The nature of the contact regime is further verified by thecalculation of the lambda ratio, L, calculated from theexpression by Dowson and Hamrock [93]. The lambdaratio (based on the starting values of Rq) for all thematerials is below unity (0.055oLo0.176).At the start of each experiment, the pin and plate were

cleaned using acetone to degrease and remove anyimpurities from the surface. They were then weighed usinga Mettler AJ150 micro-balance with an accuracy of70.1mg. Lubricating oils supplied by Infineum UK Ltd.were used in each test. The oil charge for each test was2.5ml.After each test the pins and plates were dipped in

heptane for 1–2 s to remove excess oil from the surface andthen weighed to calculate the volume of material loss fromthe plate, pin and the pin/plate system. The plates and pinswere then analysed using optical microscopy, Environ-mental Scanning Electron Microscopy (ESEM) with EDXattachment for chemical analysis and XPS.

2.1. Material and lubricants characteristics

This study focuses on the combined effects of materialcouple and the nature of the lubricant additives.

2.1.1. Materials

In this paper, current representative engine materialswere tested. The materials tested included plates of fourmaterials; BS EN31 Cr-bearing steel, Al–Si alloy (suppliedby Federal Mogul Ltd.), hydrogenated DLC coating(Diamonic—produced by Teer Coatings LTD) and pinsof EN 1452 cast iron and steel pins with DLC coating. TheEN31 has a measured hardness and density of 185HV and7.8 g/cm3, respectively while the EN1452 has a measuredhardness and density of 150HV and 7 g/cm3, respectively.

ARTICLE IN PRESS

Table 5

Synergistic–antagonistic effects of lubricant additives on friction on four

tribocouples

Tribocouple Adding ZDDP–MoDTC Adding MoDTC–ZDDP

CI/EN 31 Reduction—synergy Reduction—synergy

CI/Al-Si Increase—antagonism Increase—antagonism

CI/DLC Small increase—antagonism Reduction—synergy

DLC/DLC Increase—antagonism Reduction—synergy

A. Neville et al. / Tribology International 40 (2007) 1680–16951692

The Al–Si piston alloy has a hardness measuring 120HVand density of 2.6 g/cm3. The Al–Si alloy contains 23wt%Si and trace amounts of Cu (1.1wt%) and Mg (1.3wt%) inaddition to C, O and Al. The DLC coating hardness isspecified by Teer Coatings to be 1400HV and has athickness of approximately 5 mm. The material couplestested (pin/plate) were cast iron/EN 31, cast iron/Al–Si,cast iron/DLC and DLC/DLC.

2.1.2. Lubricants

Test were run using four lubricants referred to as: baseoil, base oil+MoDTC, base oil+MoDTC+ZDDP, baseoil+ZDDP. The names are self-explanatory and the levelof additive was 500ppm in the case of the MoDTC additiveand 1.2wt% of ZDDP (at 0.05%P). The base oil used wasa Group III base oil. The group designation relates to thesaturate level, the sulphur content and the viscosity indexof the crude oil. The base oil has a dynamic viscosity of3� 10�3N-s/m2 at 100 1C.

3. Results and discussion

The tests were run for 8 h and the friction was monitoredas a function of time during that period. In this paper, thefriction coefficient at the end of the test (assuming the traceis stable at this point) is compared. In this way the transienteffects often seen in friction-modified oils as the additive isactivated are not considered.

Fig. 4 summarizes all of the final friction coefficient datafor the four material couples lubricated by the four oils. Itis clearly seen that the friction coefficient depends both onthe material couple and the lubricant, as expected. DLCprovides inherently lower friction coefficients than theother tribocouples using three of the four oils but withOil+MoDTC+ZDDP the lowest friction is observed bythe ferrous system.

What is of interest in the context of this current paper isthe interactions between lubricant additives and as im-portantly how these interactions depend on material

0

0.05

0.1

0.15

0.2

0.25

base

oil

base

oil+

MoD

TC

base

oil+

ZDDP

base

oil+

MoD

TC+Z

DDP

Fin

al fr

iction c

oeffic

ient

CI/Al-Si

CI/EN-31

CI/DLC

DLC/DLC

Fig. 4. Friction coefficients (final values) measured for four tribocouples

with four oil types.

couple. To assess this, the information in Table 5 sets outwhether by adding a second additive (here ZDDP orMoDTC) to a single additive oil (again MoDTC or ZDDP)increases or decreases friction. As an example, where it saysthe effect of ‘‘adding ZDDP to MoDTC’’ then leads to acomparison between the friction in the Oil+MoDTC andOil+MoDTC+ZDDP system. The main message con-veyed from these results is that there is not a consistenteffect across all four tribocouples. The interactions betweenthe additives are also dependent on the nature of therubbing surfaces. The results for the ferrous system areconsistent with the literature—confirming the synergisticeffects between ZDDP and MoDTC in reducing friction[94–96].Table 5 and Fig. 4 summarize how the additives and

their interactions affect friction. In this study, the oilscontaining ZDDP and tribocouples were assessed inrelation to durability and tribofilm formation from theZDDP additive. The characterization of the polyphosphateglass, formed at the tribological interface during rubbing inpresence of ZDDP, has been by assessing the nature of theoxygen detected by XPS. The oxygen peak at 132 eV wasanalysed and the ratio of Bridging (P–O–P) to non-bridging

(–P ¼ 0 and P–O–Zn) oxygen, which is equal to BO/NBO ¼ (n�1)/2(n+1) [47]. From this the glass polymer-ization number (n) is calculated. In the cases where n ¼ 1,the glass is an orthophosphate, n ¼ 2 is a pyrophosphateand in the case that n is higher than 2, it is a metaphosphate[97].Fig. 5 shows the change in n as a function of material

tribocouple, oil and as a function of depth. The depthprofiling is given as an etch time in minutes and no attempthas been to quantify the exact depth given the uncertaintiesabout the nature of the tribofilm and how the removal rateby etching changes as a function of depth into thetribofilm. Hence the comparison at this stage is qualitativeonly. However, some very clear trends are shown (andthese are also shown in Table 6):

�

In all cases, the polyphosphate glass has a higherpolymerization number at the outer edge, consistentwith the theory that long-chain polyphosphates aretransformed during the rubbing process to shorter chainpolyphosphates � On addition of MoDTC to ZDDP a mixed effect on the

value of n is observed. For CI/EN 31, Ci/DLC there is a

ARTICLE IN PRESS

0

1

2

3

4

5

6

7

0 10 15

Etching time (mins)

n

Base oil+ZDDP

Base oil+MoDTC+ZDDP

CI/EN31 CI/Al-Si CI/DLC DLC/DLC

5

Fig. 5. Polymerization number, n, as a function of the tribocouple

material, oil and etching time.

Table 6

Effect of MoDTC on the nature of the phosphate film formed at the

tribocouple (plate) surface from the ZDDP additive

Tribocouple Effect on ‘‘n’’/wear

CI/EN 31 Reduction/decrease

CI/Al-Si Increase/increase

CI/DLC Reduction/decrease

DLC/DLC Small reduction/no measurable change

A. Neville et al. / Tribology International 40 (2007) 1680–1695 1693

significant reduction in n, for CI/Al–Si there is asignificant increase in n. For these couples the higher n

is associated with higher wear and vice versa.

� For the DLC/DLC couple the differences were small—a

small reduction in n with no measurable change in whatwas a very small wear rate.

4. Conclusions

The first part of this paper reviewed the current range ofmaterials that are employed in various components in theinternal combustion engine and from this review it is clearthat there is a general trend towards the use of surfaceengineering to improve durability. This is in parallel withincreasingly stringent legislation, which is ensuring that oilformulators must reduce (and eventually eliminate) thelevel of P (and S) in engine oils. To provide thefunctionality required without conventional additive sys-tems is going to require that surfaces are going to have towork in cooperation with oil additives and this work hasdemonstrated that to predict these interactions, and alsointeractions between additives, is not simple. The effective-ness of additives in reducing friction and wear and theinteractions between the two additives ZDDP and MoDTChave been shown to depend on the nature of thetribocouple. The major challenges, and also opportunities,

in term of providing optimum performance in terms of fueleconomy (reduced friction) and durability (reduced wear)can be summarized as:

�

To optimize synergies between additives for a specific

tribocouple—in the small subset of materials andlubricants investigated here it has been demonstratedthat synergies between MoDTC and ZDDP are max-imized for the ferrous system.

� To exploit the attributes of both the surface and lubricant

additives to optimize system performance—there areseveral developments in lubricious coatings and thesecan be used to provide a proportion of friction-reducingcapability of a system as oil formulations are changed.

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