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Tribology International 39 (2006) 1564–1575

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The feasibility of using electrostatic monitoring to identify diesellubricant additives and soot contamination interactions

by factorial analysis

J.E. Bootha,�, K.D. Nelsonb, T.J. Harveya, R.J.K. Wooda, L. Wanga,H.E.G. Powriec, J.G. Martinezb

aSchool of Engineering Sciences, University of Southampton, Highfield, Southampton. SO17 1BJ, UKbChevron Oronite Company, LLC, 100 Chevron Way, P.O. Box 1627, Richmond, CA 94802-0627, USA

cSmiths Aerospace, Information Systems—Southampton, School Lane, Chandler’s Ford, Hampshire, SO53 4YG, UK

Received 12 October 2005; received in revised form 31 March 2006; accepted 2 April 2006

Available online 8 June 2006

Abstract

Electrostatic monitoring is a condition monitoring technique, which has been used for monitoring lubricated sliding contacts. The

electrostatic charge is dependent on material wear and charge species in the lubricant (additives and contaminants). This paper presents

work carried out on a pin-on-disc (PoD) tribometer to investigate additive–additive and additive–carbon black interactions. Online

electrostatic charge and coefficient of friction (CoF) measurements were recorded. Post-test electro-kinetic sonic amplitude (ESA)

measurements were taken and pin and disc material loss was measured using 3D profilometry. Statistical examination of results was

conducted using Analysis of Variance (ANOVA) to reveal interactions. The primary conclusions include: primary zinc dialkyldithiopho-

sphate (ZDDP) was found to increase pin wear due to immature antiwear film formation. Interactions between carbon black and

detergent, and carbon black and dispersant, were observed in electrostatic charge data and ESA measurements. A complex between

ZDDP and dispersant was highlighted by measured electrostatic charge, ESA, and pin material loss.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Soot; Diesel oil; Electrostatic charge; Oil chemistry; Wear

1. Introduction

Present day lubricant chemists are trying to achieveconflicting demands of tighter emission regulations, in-creased fuel economy and extended drain time. Thedemands on the lubricant formulator are ever increasingand thus the rate at which lubricant formulations aresuperseded is increasing. The rate of product developmentand increased number of tests required for approval meansthat development costs are escalating. It is thereforeimportant that the lubricant formulator extract themaximum amount of information from each test. Lubri-cant chemists are interested in the interactions between oiladditives and try to suppress any antagonistic effects that

ee front matter r 2006 Elsevier Ltd. All rights reserved.

iboint.2006.04.004

ing author. Tel.: +44 23 8059 7667; fax: +44 23 8059 3230.

ess: jeb301@soton.ac.uk (J.E. Booth).

may be encountered to improve oil formulation. Additiveinteractions can be related to the charging mechanisms ofthe additive components within a lubricant. Electrostaticmonitoring as an on-line monitoring technique, has beenable to detect charges which are correlated with frictionand wear [1]. It also has the potential to inform thelubricant chemist about the mobility of charged particles inthe lubricant film such as additives, contaminants (soot,etc.) interactions between additives. Preliminary workinvestigating the use of electrostatic monitoring forevaluating lubricant performance has been carried out byWood et al. [2].

1.1. Aim

This paper presents research carried out to investigateadditive–additive and additive–contaminant interactions

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SteelIron oxides

(FexOy)

Iron Sulphide

(FexSy)Polyphosphate

Glass

Organic ZDDP decomposition products

and undegraded ZDDP replenishes thepolyphosphate layer once it has been

stripped

Film removal

Fig. 1. Three layers of a fully developed ZDDP antiwear film.

J.E. Booth et al. / Tribology International 39 (2006) 1564–1575 1565

by measuring on-line electrostatic charge. Of particularinterest is the ability of dispersants to prevent carbon blackfrom agglomerating (causing adverse wear) and in knowingwhether sufficient anti-wear films are formed at lowtemperature. Although the primary function of detergentis not being directly assessed, it is necessary to know ifthere are any interactions between these three (detergent,dispersant(s) and zinc dialkyldithiophosphate (ZDDP))common types of additive components.

1.2. Diesel engine lubricant additives and soot contamination

To achieve improvements in fuel efficiency lowerviscosity oils are used, which with the increase in contactloads result in thinner oil films separating components,giving rise to durability concerns. Tighter emission regula-tions; specifically for diesel engines, has lead to the use ofexhaust gas re-circulation systems to reduce NOx emission.Also the deliberate ignition retardation to reduce peakcylinder combustion temperature and therefore reduceNOx production increases the amount of incompletecombustion products, such as soot. Both of these systemsincrease the amount of soot finding its way into thelubricating oil. The consumer demand for longer serviceintervals and the increased cost for oil disposal have lead tolengthening of engine oil drain times. Soot levels as high as5wt%, in certain types of duty cycles, have been noted [3].Therefore lubricant additives need to minimise the effect ofcontaminants and thinner oil film for longer periods oftime.

This study focuses on interactions between three additivecomponents, namely detergent, dispersant and ZDDP, aswell as their interactions with carbon black (a sootsurrogate) contamination.

Zinc dialkyldithiophosphate is an anti-wear agent, oxida-tion and corrosion inhibitor. Under mixed lubrication, thetwo surfaces can intermittently penetrate the thin lubricantfilm causing decomposition of ZDDP through asperitycontact. ZDDP decomposition products react with metalsurfaces and form a tribo-film, which can be load bearingand reduce metal-to-metal contact. The thickness andcomposition of this anti-wear film are directly related to thetemperature and the extent of surface rubbing [4].

A fully developed ZDDP tribo-film formed on a ferroussurface includes three layers (see Fig. 1). Initially ZDDP isreversibly absorbed onto the metal surface at lowtemperatures. As the temperature increases the ZDDPstarts to decompose and sulphur will react with the exposedmetal surface to form a thin iron sulphide layer close to themetal surface [5]. Subsequently phosphate forms anamorphous layer of short-chain ortho- and meta-phos-phates with minor sulphur incorporation. The phosphatechains become longer towards the surface. This region issometimes described as ‘phosphated glass’ in which zincand iron cations act to stabilise the glass structure. Theoutermost layer of the anti-wear film, the phosphate chainscontain increasingly more organic ligands, eventually

giving way to a region comprised of organic ZDDPdecomposition products and undegraded ZDDP itself.These surface-active compounds form a thin layer thatpreferentially shears under boundary lubrication condi-tions. As this layer needs to be constantly replenished, theconcentration of ZDDP in the lubricant is critical.

Detergents control corrosion and resin build-up inengines. They are metal salts of organic acids that containan excess base, usually in the form of carbonate.Detergents neutralise inorganic acids from combustionproducts and organic acid products from lubricant oxida-tion and thermal decomposition resulting in a decrease inthe corrosive tendency of the acids. The neutralisation ofacidic blow-by gases is critical to diesel lubricant perfor-mances particularly for fuels with high sulphur content (10and 500 ppm for Europe and US, respectively). The organicproportion of the detergent enables solubilisation of thesalts formed by neutralisation and keeps them suspended inthe bulk lubricant. Thus oil-insoluble combustion pro-ducts, such as sludge or soot and oxidation products aredispersed. Detergents are also effective corrosion inhibi-tors, especially basic detergents [6,7] because they not onlyneutralise corrosive acidic products but also form surfacefilms that isolate metal surfaces from corrosive agents [8].The main action area for detergents in an engine is aroundthe piston, reducing lacquer, carbon and varnish depositson the pistons as well as preventing piston–ring stickingunder high-temperature operating conditions.Before 1955 only detergents were used to keep engines

clean, this was effective provided that the engine wasoperated at relatively high temperatures. Low operatingtemperatures as a result of short-distance/stop-and-go typedriving does not allow engine oil temperature to risesufficiently to vapourise water from the oil, thus resultingin highly viscous oil due to severe oil oxidation and a highlevel of insolubles in the oil. In diesel engines the majorcontribution to insolubles comes from carbon (soot) andoxidation products of the fuel and oil. More recently

ARTICLE IN PRESSJ.E. Booth et al. / Tribology International 39 (2006) 1564–15751566

European OEMs have identified low-temperature wear asan area of concern [9]. This is a major issue for small dieselengines used in urban environments where start/stopdriving is typical. Europe particularly has a large propor-tion of small urban diesel vehicles.

Dispersants control sludge and reduce formation ofdeposits. They typically consist of a polar group usuallyoxygen or nitrogen based, and a high molecular weightnon-polar group. The polar group associates with sludge ordeposits (soot) and the non-polar group keeps the materialsuspended in the bulk lubricant. The mechanism wherebythis occurs can be represented by a basic dispersantmolecule attaching itself to an acidic site on a surfaceand the non-polar groups (hydrocarbon chains) prevent thesoot particles from coming together. The stability of thedispersion is influenced by the molecular weight of the non-polar group and the level of reactive sites within the polarregion. By associating with colloidal particles dispersantsprevent them from agglomerating and falling out ofsuspension. Coalescence is prevented through either steric-or electrostatic factors [8,10] thus the charge helps toprevent agglomeration. Dispersants also lower the surface/interfacial energy of the polar species and reduce theiradherence to metal surfaces.

Soot is formed due to incomplete combustion of the fueland contaminates the lubricating oil by travelling past thepiston rings [11]. Soot production is mainly a concern indiesel engines, but can occur in direct injection gasolineengines. Soot is made up of hydrocarbon fragments, whichform aromatic molecular networks [12]. Unless there areelectrical and steric barriers to keep them apart, sootparticles will agglomerate as they approach one another.The agglomeration is caused by both van der Waals forcesand electrostatic attraction of the charges present on thesoot surface [10]. The particles, typically less than 200 nm,which have a tendency to agglomerate into a largermacrostructure, will lead to a significant rise in oilviscosity, with fuel economy consequences. When aggre-gates occur on surfaces, such as those of the combustionchamber, soot deposits result. These deposits are soft andflaky in texture. Wear of engine components could be dueto chemical reactions taking place on the surface (such ascorrosion) or abrasive action on the material or anti-wearfilm by soot particles. Gautam et al. [13] examined theeffects of soot-contaminated engine oil on wear of enginecomponents as well as other oil properties includingphosphorus level, dispersant level and sulphonate substratelevel.

1.3. Charge particle detection

1.3.1. Electrostatic charge monitoring

A condition monitoring technique has been developedbased on detecting electrostatic charge generation fromdeteriorating contacts. Electrostatic sensing was originallydeveloped for detection of debris in the gas path of jetengines and gas turbines [14–16]. The fundamental

principle of operation is detection of electrostatic chargeassociated with the debris present in the gas path. Changesin the charge level can then be related to the presence ofincreased amounts of debris and hence the onset of gaspath component deterioration. The advantage of thistechnique is that it measures a direct product of the fault,rather than secondary effects, such as increased vibrationor temperature exceedance. Thus, it is a useful tool forearly detection and progressive monitoring of componentdeterioration.Collaborative work by the University of Southampton

and Smiths Aerospace has investigated the application ofelectrostatic monitoring to oil systems. It has beendemonstrated that this system can detect charge relatedlubricated contact breakdowns including metal-on-metalcontacts and ceramic on steel contacts [1,17–21]. For thefirst time, this technique has been used to investigate chargechanges due to oil chemistry as well as wear in this study.

1.3.2. Electro-kinetic sonic amplitude (ESA)

The separation of charge, which exists at the particleliquid interface gives rise to several dynamic phenomenaassociated with colloidal systems. The driving force forelectro-kinetic phenomena is the net charge at the interfacebetween the liquid, which is bound to the particle, surfaceand the bulk fluid. The potential of the interface, known asthe plane of shear, is the zeta potential. When analternating electric field is applied to a colloidal dispersion,the particles will move in the electric field because of theirnet zeta potential. If there is a density difference betweenthe particles and the liquid, this oscillatory motion ofthe particles will result in the transfer of momentum to theliquid and the development of an acoustic wave. ESA is thepressure amplitude generated by the colloid per unitelectric field strength and is analogous to electrophoreticmobility.

1.4. The use of statistical methods for multiple variables and

measured parameters

Statistical methods can be used to determine thesignificance of variable interactions and variable effectson observed measurements. When experimental costs arehigh, these methods allow extraction of unbiased informa-tion, regarding the factors affecting a variable, from as fewobservations as possible. Fractionated factorial matricesare used to reduce the number of tests required to evaluatevarying order interactions, which is defined as the effect ofone component as a result of the level (e.g. presence orabsence) of another component.The aim of Optimal Design is to derive unbiased (or least

biased) main effects and interactions with a minimumnumber of observations. The D- and A-optimal designprocedures are methods used to select from a list ofcandidate points (combinations of factors), those pointsthat will extract the maximum amount of information fromthe experimental region (the n-dimensional space where the

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Table 1

Elemental composition of steels

Material BS534A99 (En 31)

Composition 0.95–1.10% C, 0.10–0.35% Si, 0.40–0.70% Mn,

1.20–1.60 Cr, Remainder Fe

Table 2

J.E. Booth et al. / Tribology International 39 (2006) 1564–1575 1567

model is applicable). D-optimal design maximises thedeterminant D of a matrix, which indicates factor effectsare maximally independent of each other. A-optimaldesigns also seeks independence, but by maximising thediagonal elements of the matrix, while minimising the off-diagonal elements.

Searching for the best design is not an exact method, butrather an algorithmic procedure that employs D- and A-optimal criteria along with the model (e.g. fractionatedfactorial) required to fit to the data and the number of testsdesired to find the best design.

Analysis of Variance (ANOVA) is used to uncover theinteractive nature of reality, as manifested in high-orderinteractions of factors for design procedures describedabove. ANOVA is a statistical method used to determinewhether two or more means are different. This proceduretries to identify sources of variability from one or morefactors and uses these variances to decide whether themeans are significantly different. Further details of thestatistical methods used can be found in [22].

2. Experimental procedure

Fig. 2 shows a schematic of the pin-on-disc (PoD) weartest rig with the associated instrumentation used. Theelectrostatic probe, to monitor charge, was positioned 901away from the pin/disc contact and 0.5mm above the discwear track, a strain gauge was installed to monitor thecontact friction force, and a tachometer to monitor the discrotating speed. A data acquisition system was used tocollect the electrostatic charge and coefficient of friction(CoF) signals in real-time. Suitable signal conditioning wasapplied to the sensors.

2.1. Test conditions and procedure

PoD tribometer described above was used to simulatethe wear of diesel engine valve train components. Experi-ments were carried out under ambient conditions(temperature ¼ 15–23 1C, relative humidity ¼ 20–60%) ata sliding speed of 5m/s and load of 30N (2.05GPa initial

Button Type Wear SiteElectrostatic Sensors

(10 mm φ)B.S. 534A99(En31) Disc

Load (N) B.S. 534A99(En31) Pin

LVDT

Lubrication

ω

ForceTransducer

Pin

Sensor

190°

(b)(a)

Fig. 2. (a) Schematic of the PoD test set-up and (b) position of

electrostatic sensor.

contact pressure). Hertzian contact stress within the valvetrain in the range 1.7–2.07GPa has been reported for lowemission diesel engines [23]. Thus the PoD conditions aremildly accelerated compared with typical valve-trainentrainment velocities and contact pressure. Tests werecarried out with a bearing steel ball loaded against abearing steel disc, lubricated by different blends of oilscontaining various additive combinations in a basestock.The half factorial test matrix is shown in Table 3. Theproperties of the test material BS534A99 (pin and disc) aregiven in Tables 1 and 2, these are similar properties to thevalve train components of interest. Bearing steels ratherthan cast iron have been used by other researchers lookingat the fundamentals of diesel lubricant interactions with aniron-based surface [24]. Homogeneous bearing steels areused to enhance the reproducibility and repeatability ofexperiments; both of which are integral to validatestatistical analysis.The test started with rotating the disc without contacting

the pin for two minutes in order to record backgroundsignals before the oil was sprayed onto the disc using apneumatic spray at a rate of 120ml/h. Five minutes later,once the disc surface was fully lubricated, the pin wasbrought into contact with nominal initial load. This loadwas then ramped up to the maximum load of 30N, using ahydraulic loading system, over a period of about 12.5min.This was considered sufficient running-in and once themaximum load was reached the tests were run for 1 h.The test pins were ultrasonically cleaned in a mixture of

acetone and ethyl acetate for 15min before testing. The

Details of test conditions

Ball Disc

Material BS534A99 (En 31)

Hardness (Hv30) 640 220

Elastic modulus, E (GPa) 210

Poisson’s ratio, n 0.3

Density, r (kg/m3) 7.80� 103

Dimensions (mm) Ø6 Ø100� 10

Roughness, Rq (mm) 0.09 0.02

Absolute viscosity @ 30 1C (cP) �133

Pressure–viscosity index (1/Pa) �9� 10�8

Oil flow rate (ml/h) �120

Oil temperature (1C) 16–21

RPM Variable

Pure sliding speed (m/s) 5

Load (N) 30

Contact pressure (GPa) 2.05

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disc surfaces were also cleaned with the same solvent usingcotton wool balls before testing.

2.2. Oil chemistry and test matrix

This paper investigates the interactions between 4additives and carbon black. Oil blends were prepared frombaseline formulations that contained 7.3wt% viscosityindex improver concentrate (ethylene/propylene copoly-mer) with an API group I base oil. The four additivesincluded one detergent, two dispersants and one type ofZDDP.

�

Ta

Fa

Oil

De

Dis

Dis

ZD

Ca

Raven 1040 carbon black was used as a surrogate fordiesel engine soot and blended to 2wt%. Raven 1040has a primary aggregate particle of size 28 nm, andaverage agglomerated particle size just under 300 nm.

� The detergent chosen was an overbased sulphurised

calcium phenate, which was blended to 50 milli-molarcalcium (about 2wt%).

� The two dispersants studied were both post treated

conventional succinimides made from polyisobutene.Dispersant 1 was a bis-succinimide, which has one polarhead per molecule and dispersant 2 was a polymericsuccinimide, which has about 10 polar heads permolecule. Both dispersants were blended to 8wt%.However, when the two dispersants were used together,they were blended at an equal percent of 8wt% in total.

� A primary ZDDP was blended to 0.1wt% phosphorus.

(Further details regarding the structures of the chemicalscan be found in [2,25]).

Oils containing carbon black were homogenised for1min using a rotor stator type-high shear mixer, model T25Basic Dispenser w/S25N-18G, manufactured by IKALaboratory Analytical and Processing Equipment, andthen the oil was degassed in a vacuum oven for 15min atabout 55 1C. The non-carbon black containing blends werestirred for 10min followed by 60min in an ultrasonic bathat 30 1C. All blends were ultrasonically shaken for 30minprior to testing on the PoD tribometer.

Standard analysis would require all 5 variables to beseparated by running 32 tests. For the work presented inthis paper a fractionated (half) factorial matrix 2(5–1) (16test oils) allowed the main effects and 2-factor interactionsto be evaluated by ANOVA. The half factorial matrixdescribed above has the disadvantage of insensitivity to

ble 3

ctorial matrix oil blends

run 1 2 3 4 5 6 7

tergent | | |persant 1 | | |persant 2 | | |DP | | | |rbon black

three or more factor interactions, however, it was decidedthat this number of interactions was not a key concern atthis early research stage. The original 16 blends weredesigned so that the factor effects are maximally indepen-dent of each other (A- and D-optimality), which are shownin Table 3 as the test matrix.

2.3. On-line signal processing

The real time data, including COF and rms charge wereaveraged for the 60min duration to assess correlation overthe whole test period. The data were also averaged forevery 5min interval (described as interval electrostaticcharge hereafter) to assess correlations of dynamic real-time features. All average data were statistically assessed atthe completion of the test matrix.

2.4. Off-line analysis

2.4.1. Surface profilometry

The volume loss for the pin and disc were measured byTaiCaan 3D laser Profilometry. The volume loss for eachtribo-couple was then inserted into Eq. (1) to calculate thespecific wear rate (SWR):

SWR ¼VLðmm3Þ

F ðNÞ � SDðmÞ; (1)

where VL is the volume loss of the disc or pin, F is the loadand SD is the sliding distance.

2.4.2. Electro-kinetic sonic amplitude (ESA)

A Matek Instruments MBS 8000 system was used tomeasure the ESA of the 16 oils. The instrument wasconfigured for non-aqueous measurement, and the phasewas referenced to use diesel engine oil that containedapproximately 6wt% of soot by thermogravimetric analy-sis. Ten measurements were performed for each sample andthe results averaged. All phase measurements were within151 of the reference.

2.4.3. X-ray photoelectron spectroscopy (XPS)

Post-test XPS analysis was carried out on three worn pinsamples by Evans Analytical Group (California, USA)using a PHI Quantum 2000 instrument. The X-ray beamhas a spot size of 5 mm across and 95% of the analysedsignal originates from a depth of �50–100 A.

8 9 10 11 12 13 14 15 16

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oeffi

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n

Lubricant Blends

Fig. 4. Bar chart of the 60min COF average.

J.E. Booth et al. / Tribology International 39 (2006) 1564–1575 1569

3. Results

The results for the measured on-line and off-lineparameters are shown graphically in Figs. 3–5. ANOVAwas carried out on both the on-line and off-line para-meters. Only a few blend component interactions weredetected for the 60min electrostatic charge average.Electrostatic monitoring is sensitive to dynamic eventsand thus when ANOVA was carried out for each 5mininterval of the 60min period, interactions were found,and in some cases persisted for the duration of the test.The statistical analysis yielded many interactions andtherefore narrowing down these statistical results wasnecessary. This was achieved by focusing on the electro-static results and calculating the significance of suchinteractions; p-values.

3.1. p-values–reduced model

It is important to identify the most significant effect(s) bya single factor or by two factor interactions. The residualerror is required for estimating the probability ofsignificance. The source of variability is separated todetermine whether the factor(s) or interaction affects aresponse. A model with only the most probable significanteffects was introduced to overcome insufficient degrees offreedom for error estimating required by a full model (i.e.randomised repeat tests). Extra degrees of freedom createdby this model were used to estimate the error. Theprobability of errors, i.e. p-value, is used to indicate thesignificance of the interactions on the variable and itsassociated response. For p-values less than 0.05, theinteractions between the factors are considered significantbecause only 5% of the interaction could be explained byother factors. The most interesting interactions withthe lowest p-values discovered by ANOVA are listed inTables 4 and 5.

Fig. 3. Bar chart of the 60min average electrostatic c

4. Discussions

A marginal correlation between ESA and 60min averagesurface charge detected by an electrostatic sensor wassuggested by a relatively high p-value of 0.15. Althoughboth are charge related techniques, ESA is a measurementof the pre-existing charge in the oil, whereas the electro-static sensor detects the total charge as a result oftribological activity, which is likely to be a combinationof charging mechanisms. Thus the correlation is marginalrather than significant.

4.1. Carbon black interactions with other additives and the

effects on charge responses

Carbon black increased the electrostatic charge detectedand ESA measured (see Tables 4 and 5). Carbon blackparticles are known to be chargeable. In oil containing

harge values and the off-line ESA measurements.

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Table 4

Statistical results from ANOVA for On-line parameters

Factor Variable Response p-value

Carbon black Average charge Increase 0.13

Dispersant 2 without detergent Interval charge Decrease o0.05

ZDDP with dispersant 1 Interval charge Increase o0.05

Carbon black without detergent Interval charge Increase o0.05

Carbon black without dispersant 2 Interval charge Increase o0.05

Table 5

Statistical results from ANOVA for post-test parameters

Factor Variable Response p-value

ZDDP Ball SWR Increase 0.06

Carbon black with ZDDP Disc SWR Decrease 0.01

Dispersant 2 without dispersant1 Disc SWR Decrease 0.04

Carbon black ESA Increase 0.01

ZDDP without dispersant 2 ESA Increase 0.04

Detergent with dispersant 1 ESA Decrease 0.09

ZDDP with dispersant 1 ESA Increase 0.12

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R [m

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m]

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1.0x10-13

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Dis

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WR

[m

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Fig. 5. Bar chart of disc and ball SWR.

J.E. Booth et al. / Tribology International 39 (2006) 1564–15751570

2wt% of carbon black, considering an average diameter of300 nm for spherical carbon black agglomerates, there are7� 1012 carbon black particles in one ml of oil sample.Agglomerated particles, if large, could retain a high enoughlocalised charge to be detected by the electrostatic sensor.

Fig. 6 illustrates the statistical finding that carbon blackwith detergent will reduce interval electrostatic charge andcarbon black with dispersant 2 will also reduce intervalelectrostatic charge. The statistical analysis indicates thatboth dispersant 2 and detergent reduce the effect thatcarbon black contributes towards increasing electrostaticcharge; dispersant 1 does not have this effect. For dispersedcarbon black to acquire a charge, dispersant and/ordetergent must de-adsorb from the surface taking a charge

with it and leaving an equal but opposite charge on theaforementioned surface.Typically, dispersant molecules perform this function,

and as they are usually amine-based they acquire a positivecharge from the formation of an acid–base pair at the sootsurface. Thus the dispersants in this study will effectnegative surface charge formation on dispersed carbonblack.Relative to dispersant 2, dispersant 1 is more surface

active, and as there are fewer polar regions per non-polarhydrocarbon tails—there are 2 hydrocarbon tails per polarregion in dispersant 1—it is more likely to de-adsorb fromdispersed carbon black. Dispersant 2 may bind very tightlyto a surface; there are about 10 polar regions per molecule,and the ratio of polar to non-polar regions is about 1:1.For these reasons, dispersant 2 does not support chargeformation as well as dispersant 1.Detergents generally employ organic acid-based surfac-

tants. These ‘‘soaps’’ are negatively charged. Consequentlycolloidal particles associated with detergents may obtain apositive surface charge. As there may be both positive andnegative charges present when dispersant and detergent areused, these positively charged particles serve to offset, orneutralise the measured static charge.

4.2. Specimen wear

Fig. 7(a–d) shows 3D topographical ball scar mapsgrouped in relation to the presence or absence of ZDDPand carbon black, which have historically had the greatestimpact on wear performance. The four maps are repre-sentative of the four different ball scar shapes seen for all16 tests, but with varying severity. For oils without eitherZDDP or carbon black (e.g. run 5, see Fig. 7(a)) the ballSWR was small and conformal. Oils containing carbonblack, but no ZDDP exhibit slightly greater (abrasive) ballwear, the SWR for run 9 (see Fig. 7(b)) is double that of

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(b) Time / Mins

QR

MS /

pC

(a)

Fig. 6. Real-time electrostatic charge for (a) Run 9 (carbon black) (b) Run 14 (detergent, dispersant 2, carbon black).

Fig. 7. (a) Run 5 (dispersant 2), (b) Run 9 (carbon black), (c) Run 16 (detergent, dispersant 1, dispersant 2, carbon black, ZDDP) and (d) Run 1 (ZDDP).

J.E. Booth et al. / Tribology International 39 (2006) 1564–1575 1571

run 5. For oils which contain ZDDP and carbon black theball shoulders are severely worn leaving a ‘Mohawk’ typefeature in the middle of the ball (see Fig. 7(c)). A

comparison of run 16 to run 9 shows 14 times greater ballSWR for run 16. All oils containing both ZDDP andcarbon black exhibit this Mohawk feature; but with varying

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Steel

Small Phosphate

layer

Film removal

Iron Sulphide(FexSy) Iron oxides

(FexOy)

Fig. 8. Schematic diagram of proposed under-developed film which leads

to a pro-wear characteristic—ZDDP reacts with the surface to form initial

stage of tribofilm but constant removal of this iron rich film will promote

wear (Tribochemical wear).

J.E. Booth et al. / Tribology International 39 (2006) 1564–15751572

widths. The Mohawk width for run 16 (see Fig. 7(c)) is220 mm, this is larger than the initial maximum Hertziancontact pressure diameter of 167 mm. For all other oilswhich contain ZDDP and no carbon black, the ball scar isapproximately flat, with small ridges and has the greatestball SWR, a fact shown in the statistical analysis (see Table5). Run 1 (see Fig. 7(d)) has the highest ball SWR out ofthe 16 test oils, it has 4 times more wear than run 9 andover 100 times more wear than run 5. This effect of ZDDPto exacerbate wear is contrary to what is generally thoughtof as an antiwear additive; in following sections thisnegative effect is termed ‘pro-wear’.

The wear scar shapes; in particular those oils containingcarbon black and ZDDP, can be explained by elastohy-drodynamic theory. The surface of a point contact, evenunder small loads, will deform, initially elastically.Although these deformations may be very small they canplay a vital role in promoting the generation of a fluid filmto separate two surfaces in relative motion. In slidingelastohydrodynamic point contacts, deformation of the(originally) spherical surface will cause a constriction in theoil film with an associated pressure spike, towards thedownstream end and around the side of the contact,forming a ‘horse-shoe’ shaped constraint [26]. This pressurespike can be considerably higher than the maximumHertzian contact pressure. Optical interference patterns[27] have confirmed that the contours of constant filmthickness follow the contours of constant pressure [28],with the areas of minimum film thickness occurringdownstream and around the side (the horse-shoe) of thecontact [29].

Wear for run 1 started around the area of maximumpressure/minimum film thickness and progresses to thecentre of the ball, flattening the spherical surface. For oilscontaining ZDDP and other factors (additives and/orcontaminants) the pro-wear effect of ZDDP is slowed—preventing the progression of wear from the ball shouldersto the centre. The following sections discuss reasonsbehind the pro-wear effect and how this is minimised dueto interactions with other additives, in particular disper-sants.

4.2.1. ZDDP’s pro-wear effect

XPS was conducted on three worn ball surfaces and anunusual tribo-film was found for those surfaces run withoils containing ZDDP. Table 6 shows the elementalcompositions of these films.

Table 6

XPS analysis

Ball specimen C N O Na Si P

Run 5 40.2 2.0 40.9 0.8 1.8 —

Run 7 39.8 1.7 40.9 0.7 1.3 0.

Run16 (centre) 35.5 0.4 44.1 0.9 — 0.

Run 16 (side) 34.8 0.7 43.4 0.8 0.9 0.

The results show low concentrations of zinc, sulphur andphosphorous compared to other ZDDP antiwear films [30].The low phosphorous level especially relative to sulphurindicates that a minimal amorphous-polyphosphate layerhad formed. This layer is critical for an effective ZDDPantiwear film because it shears preferentially to the metalcomponent under boundary lubrication conditions. Thisamorphous layer is constantly replenished by the thin toplayer of organic decomposition products and undergradedZDDP.The pro-wear effect of ZDDP is due to the formation of

an under-developed anti-wear film. Primary ZDDP hashigh thermal stability and the tests were run at roomtemperature, which limited the decomposition of primaryZDDP, thus prevented the formation of a fully developedantiwear film. However, the reaction of sulphur withinZDDP, with iron to form iron-sulphide is less temperaturedependent. Active sulphur has a significant influence onpromoting wear [3].The thin phosphate layer is easily stripped away leaving

an iron sulphide layer, which is also readily sheared leavinga nascent metal surface (see Fig. 8). This nascent metalsurface is able to form iron oxide, which reacts withsulphur to form iron sulphide. This cyclic repeats topromote ball wear.

4.2.2. Interactions between ZDDP and dispersants

The SWR for run 1 and run 7 clearly shows thatdispersants reduces the pro-wear effect of ZDDP. TheZDDP–dispersant interaction is thought to be a combina-tion of three mechanisms: increasing the thermal stability

S Cl K Ca Fe Zn Pb

— 0.6 0.5 1.1 10.3 1.0 0.9

9 0.4 0.4 — 0.8 10.0 2.5 0.7

2 0.5 0.4 — 0.6 13.2 3.6 0.7

4 0.6 0.8 — 0.5 13.2 3.1 0.8

ARTICLE IN PRESS

D1

D1

D1

D1

D1

D1

D2D2

D2

D2 D2

D2

Energy Required for De-adsorption

Steel Steel

d1 d

ZDDPZDDP

ZDDP

ZDDPZDDP ZDDP

ZDDP

ZDDPZDDP

ZDDP

ZDDP

ZDDP

ZDDP

ZDDP

d1>>d2

2

D1

D1

D1

D1

D1

D1

+++

+

+

+

+

+

+

+

+ +

+

++

+

+ +

+

(a) (b)

ZDDP ZDDPZDDP

ZDDPZDDP

ZDDP

ZDDP ZDDP

ZDDP ZDDP

ZDDP

ZDDP

ZDDP

ZDDP

D2 D2

D2

D2+

+

+ +

D1+

D1+

D1+

D1+

D1

D1+

Fig. 9. Schematic of the (a) dispersant 1 and (b) dispersant 2 interaction with ZDDP which leads to a reduction in the pro-wear effect of ZDDP.

J.E. Booth et al. / Tribology International 39 (2006) 1564–1575 1573

of ZDDP, suspension of ZDDP and ZDDP decompositionproducts, and competition for surface sites.

Harrison et al. [31] reported formation of a complexbetween ZDDP and succinimide polyamine. The basicnitrogen from the dispersant was shown to form a stablecomplex with ZDDP. It has been demonstrated that theZDDP–amine complex makes the ZDDP more resistant tothermal degradation by retarding the rate of peroxidedecomposition [32]. Usually a reduction in ZDDP decom-position products would reduce the ability of ZDDP toform an effective film to minimise wear. However, underthe test conditions in this study a reduction in the amountof ZDDP decomposition products, especially sulphurcontaining, is beneficial as it minimises the pro-wear effect.The presence or concentration of sulphur is thought to bethe cause of the pro-wear effect.

Apart from reacting with ZDDP, the dispersants arelikely to suspend undegraded ZDDP and ZDDP decom-position products. The significance of the latter is thatsuspension prevents sulphur containing decompositionproducts from reacting with the steel surface, furtherreducing the pro-wear effect of ZDDP. The prevention ofdecomposed ZDDP from reaching the steel surface is alsoenhanced by surface competition between the dispersantmolecules and ZDDP decomposition products. The moreadsorbed dispersant molecules on the steel surface, thefewer ZDDP decomposition products can reach thesurface. This effect is discussed in Section 4.2.3. Fig. 9shows a simplification of three mechanisms associated withZDDP–dispersant interaction.

The interaction between ZDDP and both dispersants isalso shown by electrostatic charge and ESA and is thoughtto be related to the same interaction found for ball SWR.

Fig. 10 illustrates the statistical finding that ZDDP withdispersant 1 increases interval electrostatic charge. Theinteraction between ZDDP and dispersant 2 is shown bythe statistical analysis to be only significant for ESA.However, online electrostatic data shown in Fig. 10(b)indicates that there maybe a marginal reduction inelectrostatic charge due to the presence of dispersant 2.This follows the marginal correlation between ESA andelectrostatic charge discussed earlier.The suspension of ZDDP and ZDDP decomposition

products by dispersant 2 and the corresponding reductionin charge, maybe due to the reasons discussed for thecarbon black—Dispersant 2 interaction. The tenaciousbond formed between the 10 polar regions of dispersant 2and ZDDP requires more energy to de-adsorb dispersant 2from ZDDP than dispersant 1 from ZDDP. Thus thesuspension of ZDDP by dispersant 2 is more stable hencethe ESA measurement is lower. Dispersant 1 in thepresence of ZDDP however, increased the charge detectedby both electrostatic sensor and ESA. Dispersant 1molecules can form multiple layers, but are more easilyde-adsorbed; especially at the extremities. Thereforedispersant 1 has the ability to form a higher localisedcharge even if the retention of this charge is comparativelyunstable.Fig. 10(b) exhibits oscillating charge features and

interval charge was used to identify the transient natureof a wearing contact. However, the statistical analysis doesnot take into account the oscillating characteristic thatoccurs in a few experiments, neither does it assess if theoscillations have different periods. Future research work isrequired to investigate the effect of single additives on suchtransient electrostatic signals.

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0 5 10 15 20 25 30 35 40 45 50 55 60

0.012

0.014

0.016

0.018

0.020

0.022

0.024

0.026

QR

MS /

pC

Time / mins

0 5 10 15 20 25 30 35 40 45 50 55 60

0.012

0.014

0.016

0.018

0.020

0.022

0.024

0.026

Time / Mins

QR

MS /

pC

(b)(a)

Fig. 10. Real-time electrostatic charge for (a) Run 4 (Detergent, Dispersant 1, ZDDP) (b) Run 6 (Detergent, Dispersant 2, ZDDP).

J.E. Booth et al. / Tribology International 39 (2006) 1564–15751574

4.2.3. Effect of dispersant on wear

For the oils which did not contain ZDDP, it appears thatthe additives in particular dispersants provided a filmwhich minimised the wear on the ball (see Fig. 7(a)). Bothdispersants employed in this study are expected to formsurface films capable of small wear reductions. The surfacepacking efficiency of the two dispersants is very different. Amolecule of dispersant 1 will have two hydrocarbon tailsstanding out from the surface. Each of these tails willoccupy space at the surface based on the area occupied bythe polar region, and the volume filled by the motion of thehydrocarbon tail in the oil medium. The packing efficiencyof dispersant 1 is limited by the volume required for eachmolecule. Dispersant 2, on the other hand, containshydrocarbon tails linked together through each polarregion; the distance between the anchor points for thehydrocarbon tails is of the order of several carbon–carbonsingle bonds. The molecule is expected to form comb-likeadsorbed layers [33]. The close proximity of the hydro-carbon tails serves to thicken the surfactant layer in thecontact (relative to dispersant 1) and reduce wear. Whenboth dispersants were employed, the anti-wear effect ofdispersant 2 was not observed (see Table 5); clearly,competition for the surface favours the lower molecularweight and more mobile dispersant 1.

5. Conclusions

A five factor test matrix 2(5–1) was carried out toinvestigate the interactions between different factors onmeasured parameters including: charge, wear and friction.The five factors include four additives (detergent, disper-sant 1, dispersant 2 and ZDDP) and one oil contaminant(carbon black a soot surrogate). Analysis of Variance(ANOVA) revealed single factor effects and two factorinteractions on the measured parameters of electrostaticcharge and ESA and specimen SWR.

The presence of carbon black was found to increaseaverage electrostatic charge and ESA. Where steric or

electrical barriers are insufficient to keep carbon blackparticles dispersed, agglomeration will occur; these largeparticles could retain a high enough charge to be detectedby the electrostatic sensor. Dispersant and detergents canalso charge carbon black through de-adsorption.Both dispersant 2 and detergent in the presence of

carbon black were found to reduce the interval electrostaticcharge. Both dispersant 2 and detergent are known to bindtightly to the carbon black surface; minimising chargeformation through de-adsorption.ANOVA revealed that ball wear was increased in the

presence of ZDDP. Investigations using XPS analysisidentified that under the test conditions used in this study,the presence of primary ZDDP did not generate a fullydeveloped antiwear film. Instead, formation and strippingof iron sulphide promoted ball wear.This paper has demonstrated the use of electrostatic

sensors and ESA to detect changes in charge associatedwith the presence of additives and a contaminant, as well asdetecting binary interactions on a PoD tribometer.Electrostatic sensors that correlate or show anti-correlationwith off-line electrokinetic measurements such as ESA,could be extremely useful for lubricant performanceevaluation. There is potential for electrostatic sensors andESA to be used in a test programme to monitor theinteraction between additives and contaminants, as con-taminant loading varies during fired engine tests. Bycomparing the pre-existing charge in the lubricant mea-sured by ESA with the electrostatic charge detected fromthe trib-contact, further understanding of additive andcontaminant interactions could be gained.

Acknowledgements

The authors would like to thank Dr. S. Stults, Dr. E.S.Yamaguchi and Dr. J.J. Harrison from Chevron OroniteCompany for financial and technical assistance; Dr. V.Palekar from Chevron Global Lubricants; Tai Caantechnologies for use of a 3D laser profilometer, and

ARTICLE IN PRESSJ.E. Booth et al. / Tribology International 39 (2006) 1564–1575 1575

RAEng for support through their International TravelGrant scheme.

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