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Wear 262 (2007) 1113–1122

Effect of diesel soot contaminated oil on engine wear

Sam George, Santhosh Balla, Mridul Gautam ∗Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown 26506, USA

Received 28 February 2006; received in revised form 22 September 2006; accepted 13 November 2006Available online 6 December 2006

bstract

Contamination of lubricating oil by diesel soot is one of the major causes of increased engine wear, especially with most engine manufacturerspting for Exhaust Gas Recirculation (EGR) technology to curb oxides of nitrogen (NOx) emissions. The diesel soot interacts with engine oilnd ultimately leads to wear of engine parts. Factors which can change or modify the characteristics of the soot surface are expected to play anmportant role in controlling the interactions with soot. Hence, it is important to study the interactions between soot and oil additives in order toevelop high performance diesel engine oils for engines equipped with EGR.

A statistically designed experiment was developed to study the effects of soot contaminated engine oil on wear. The variables that were consideredere the base stock (groups I and II), dispersant level, and zinc dithiophosphate (ZDP) level. The above three variables were formulated at two

evels: low (−1) and high (1), which resulted in 23 matrix (8 oil blends). In order to study the non-linear effect of soot, it was considered as aariable and was tested at three levels: low (−1), medium (0), and high (1). This resulted in testing of 24 oil samples.

A three-body wear machine was used to simulate and estimate the extent of wear quantitatively. The extent of wear was measured as the actualoss of material, in grams. A second set of experiments were performed on a milling machine (ball-on-flat disk setup). The wear scars formedn the steel balls were examined using a scanning electron microscope (SEM) and were analyzed qualitatively to determine the effect of sootontaminated oils on wear.

The results obtained were analyzed using the general linear model (GLM) procedure of the statistical analysis system (SAS) package to determine

he significance of variables on wear. The analysis indicated that wear increased nonlinearly as the amount of soot increased. Cumulative wearas more for samples with soot contamination than without soot contamination. This showed a detrimental effect of soot on the oil blends wearerformance. The SAS analysis showed that the base stock and soot content were the most significant variables affecting wear. Dispersant andDP levels were also found to be significant. The highest wear resulted from a sample that had 4% soot.2006 Elsevier B.V. All rights reserved.

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eywords: Diesel soot; Engine wear; Lubricating oil; Base stock; Dispersant

. Introduction

Diesel engines are extensively used in automotive systemsue to their better fuel economy. Despite these advantages, dieselngines suffer from environmental drawbacks, such as highevels of exhaust NOx, and particulate matter. The major contrib-tors of atmospheric NOx inventories are diesel engines. Somef the key technologies for controlling NOx emissions are con-rolling fuel injection system parameters, controlling in-cylinder

harge conditions, EGR, and controlling fuel formulation. EGRs one of the more attractive engine based technologies for reduc-ng NOx emissions. This reduction in NOx is accompanied by

∗ Corresponding author. Tel.: +1 304 293 3111x2312; fax: +1 304 293 6689.E-mail address: Mridul.Gautam@mail.wvu.edu (M. Gautam).

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043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2006.11.002

n increase in particulate matter and poor combustion perfor-ance. The contamination of lubricating oil by diesel soot is a

ey factor relating to the increased engine wear.The soot induced wear mechanism is still not fully understood

nd a more fundamental knowledge is needed in this area. Fiveoot induced wear mechanisms have been proposed since the970s: preferential adsorption of ZDP decomposition productsy soot, lessening the anti-wear film formation on metal surfacesesulting in metal to metal contact and soot abrasion, reductionn the surface coverage rate by ZDP because of the competi-ion for metal surface sites between ZDP and soot, weakeningnd removal of ZDDP (zinc dialkyldithiophosphate) reaction

lms by abrasion as a result of soot, and pumpability problemsue to soot agglomeration [1–13]. The first four wear mecha-isms involve surface interactions between soot and additives,oot and metal, or among soot particles. The factors, which can

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hange or modify the characteristics of the soot surface, arexpected to play an important role in controlling the interactionsith soot. Changes in the fuel composition may significantly

lter the physical structure and surface/bulk chemistry of soot.ence, it is important to study the interactions between engine

oot and oil additives in order to develop high performanceiesel engine oils for EGR equipped engines. Recent studies onngine wear involving moderately high soot levels have reportedhat abrasive action by soot is the major wear mechanism iniesel engines. Ryason et al. [2] have found evidence of fur-ows on wear scar surfaces whose widths are approximately00 nm, which is about three times the size of the individualarbon black particles under study. Kim et al. [3] showed thatear mechanism was a possible combination of the anti-wearlm removal and adsorption of ZDP by soot. Bardasz et al. [4]eported that wear in a GM 6.5 L test roller follower was mainlyaused by abrasive action of agglomerated soot particles. Ratoind Spikes [5], Chinas-Castillo and Spikes [6], and Van Dam etl. [7], from their studies concluded that dispersed carbon blackapidly removed ZDDP reaction films by abrasion. Gautam et al.11–13] reported that average wear was higher with soot con-amination than without soot contamination. They concludedhat diesel soot reduces the oil’s anti-wear properties, presum-bly by abrasive wear mechanism. Thus, soot induced abrasiveear is a key area for investigation.In the current study, a statistically designed experiment was

eveloped to study the effects of soot contaminated engine oiln wear. The variables that have been considered are the disper-ant level, ZDP level and base stock. These three variables wereormulated at two levels: low (−1), and high (1), which resultedn 23 matrix (8 oil blends). Group I base stock was consideredow (−1), and group II base stock was considered as high (1) forhe statistical analysis. Soot was also considered as a variablend was tested at three levels so as to study the non-linear effectf soot. The three levels tested were: low (−1), medium (0),nd high (1). This resulted in testing of 24 oil samples. Earliertudies [11–13] of similar nature considered only two levels ofoot and hence, the non-linear variation of wear with increasingoot levels went unnoticed.

Three-body wear mechanism occurs between the piston,ylinder, and the intermediate soot particles. In this mechanism,ear occurs at the particle–surface interface. A three-body wearachine was designed and developed to simulate and estimate

he extent of wear, as it is very difficult to test each oil samplen an engine. The tests were performed having a cast iron rounds the first surface and a tempered steel specimen as the secondurface with the intermediate soot particles in between them.he extent of wear was measured as the actual loss of material,

n grams.A second set of experiments were performed on a milling

achine (ball-on-flat disk setup) using a specially designedhuck and an aluminum cup. Wear tests were performed withhe oil blends forming an interface between a rotating steel ball

AISI 52100) and cast iron round. The wear scars formed onhe steel ball were examined using a scanning electron micro-cope (SEM). These wear scars were analyzed qualitatively toetermine the effect of soot-contaminated oils on wear.

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(2007) 1113–1122

The results obtained were analyzed using the general linearodel (GLM) procedure of the statistical analysis system (SAS)

ackage to determine the significance of variables on wear. Thetatistical analysis system also highlighted the significance ofarious interactions among the variables on wear.

. Background

Soot contaminated lubricating oil is one of the causes foriesel engine wear. Diesel soot interferes with the lubricating oilhereby leading to increased wear [3]. Also, soot contaminationf lubricating oil increases its viscosity and causes pumpabilityroblems. Walls of the combustion chamber fail to get evenlyoated with the lubricant and leads to increased wear.

Lubricant additives perform a number of diverse functions.hey can be classified into chemically inert and chemicallyctive types. Chemically inert additives improve the lubricanthysical properties and include emulsifiers, demulsifiers, pouroint depressants, foam inhibitors, and viscosity modifiers.hemically active additives interact with metals to form protec-

ive films, reducing wear. Chemically active additives includeispersants, detergents, anti-wear and extreme pressure agents,xidation inhibitors, and rust and corrosion inhibitors. Almostll commercial lubricants contain additives to enhance their per-ormances in amounts ranging from less than 1–25% or more.he function of these additives is to protect metal surfaces (rings,earings, gears, etc.), resist oxidation, minimize deposit forma-ion, prevent corrosion and wear, extend the range of lubricantpplicability, improve flow characteristics, improve lubricanttability and to extend the lubricant life.

The five major wear mechanisms in a diesel engine arebrasion, adhesion, fatigue, corrosion and lubricant breakdown.orrosion and lubricant breakdown involves a series of chemical

eactions that lead to wear while abrasion, fatigue, and adhesionnvolve mechanical damage of surfaces. For all the above fiveorms of wear, lubricant contamination is a predominant driveror wear.

There are various types of wear testing devices that haveeen reported in published literature. For wear measurements,t is very important to decide the variables that need to be con-rolled, the variables that may be ignored, and those that need toe measured. The significant variables in the friction and wearrocess are: load, velocity, temperature, contact area, geome-ry, and surface finish [14]. Load plays a very significant rolen wear testing as it directly influences the surface tempera-ure and can affect the real contact area. However, most wearxperiments are run at relatively low loads and this limitation ismposed by a need for more rugged equipment and higher powerequirement. In the current study, a load of 35 lb (15.9 kg) waspplied for the three-body wear tests. At higher speeds, tem-erature varies significantly, and velocity also influences bothhe surface temperature and the fluid film thickness in lubricantpplications [14]. In the current study, tests were performed

t a low speed of 200 rpm. Sliding distance is linearly pro-ortional to the wear. Hence, tests could be performed for ahorter period of time and could be extrapolated to longer times.urface film of the wearing surface determines the regime of

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ubrication and, thus, plays a significant role. Moderately roughurfaces operate in the boundary lubrication regime and allowore contact. In this study, the cast iron rounds were turned on

he lathe at a low speed of 250 rpm to obtain boundary lubricationondition.

Previous wear tests listed in this section were conducted onifferent types of wear configurations, depending on the pur-ose of study. The primary reasons for conducting the wear testsere to simulate the wear process in the diesel engine, evaluate

he lubricant properties, and to characterize the materials. Theajor wear testing machines and configurations that simulate

he wear process are: four-ball wear testing machine, flat-on-flatonfiguration, two-ball wear machine, crossed cylinders, andall-on-flat disk configuration.

Rounds [15] performed tests on a four-ball wear testingachine with soot contaminated oil samples. The oil samplesere obtained from a number of sources by normal oil drain pro-

ess. Tests were performed on a four-ball wear testing machinesing the collected samples to evaluate the oil properties inresence of soot. According to Rounds, soot did not act as anbrasive, but soot preferentially adsorbed the anti-wear additive.his was the plausible reason he provided for the wear takinglace in a diesel engine. He concluded that ZDP was the mostffective anti-wear additive in the presence of diesel soot. Helso performed hardness tests on soot and alumina, which is anown abrasive. Rounds disagreed with the concept that sootemoved the surface coating by abrasive phenomenon, since theardness of soot is lower than the hardness of alumina. He alsouggested that engine load and EGR have a large effect on theoot pro-wear characteristics.

Many authors have disputed the adsorption theory proposedy Rounds. Ryason et al. [2] performed wear tests on a ball-on-at-disk tribometer using carbon black and steel balls made ofISI 52100 steel. Wear tests were performed on carbon black,

lumina and silica. Investigations were carried out on the wearcars from the tests using scanning electron microscope (SEM)nd electron probe micro-analysis. The SEM pictures showedhat the scars on the surfaces of the balls worn in the presencef oils containing carbon black, alumina and silica were similar,nd differ from that of the ball worn in the presence of oil alone.yason concluded that the wear that occurred was abrasive inature. He also suggested that although the wear was abrasiven nature, the cutting of the material did not take place. The sootarticles ploughed through the surface, forming a groove with amooth curved cross-section, depressed at the center and raisedt the edges.

Nagai et al. [8] performed tests on valve train and studiedhe wear in the presence of soot. They concluded that the wearf cam noses and rocker arm tips was found to increase signif-cantly with the increase in EGR rate. The drain oil analysis athe end of each EGR test run indicated evidence of elementsuch as zinc and phosphorous. This contradicted the adsorptionheory proposed by Rounds initially. Tests were also performed

n four-ball wear testing machine and it was concluded that,oot strips off the anti-wear film formed on the lubricated metalurface and the subsequent metallic contact accelerated the wearrocess. They also concluded that soot might change to a very

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(2007) 1113–1122 1115

ard particle under the high-pressure conditions and might bebrasive to the metal.

Berbeizer et al. [16] investigated the role of carbon black onild lubricated wear. The test setup they used involved a plane-

n-plane tribometer to simulate lubricated mild wear betweening, cylinder and particles in suspension. They conducted a sys-ematic study of carbon black parameters on mild wear by eval-ating special test blends in which different types of commercialarbon black were used as model compounds. They concludedhat bore polishing is influenced more by the size, nature, andoncentration of carbon black rather than by the products ofil degradation. They suggested that decreasing the amount ofarbon black reaching the piston or suspension in the lubricantould reduce bore polishing. Bore polishing can also be reducedy reducing the elementary carbon black particles, or by com-letely changing the microstructure of graphitized carbon. Theseodifications were only feasible if the combustion parameters

uch as temperature, gas oil additives or the lubricant additivesere changed. They also suggested that abrasive wear is not the

ole factor contributing to increased wear, but two other impor-ant phenomena also play a role in increasing wear: a decreasef the surface coverage rate by ZDP molecules due to physi-al adsorption of carbon black on the surface, and a subsequentodification of the physical and mechanical properties of the

eaction film by the introduction of carbon in their composition.Kim et al. [3] conducted experiments using a statistically

esigned oil test matrix to investigate both oil viscosity andiesel engine oil additive components. They investigated theffects of oil formulations on diesel engine valve train wear.hey concluded that laboratory wear tests could properly differ-ntiate the anti-wear performance provided by different engineils. They concluded that an anti-wear additive film must formn the metal surface to reduce wear. The anti-wear properties ofhe diesel engine oil could be improved by increasing the ZDPoncentration. It was also suggested that improved specificationsere needed, as the existing diesel engine oil specifications wereot adequate to protect every engine.

Ratoi and Spikes [5] investigated the main factors that deter-ine the impact of soot on friction and ZDDP film formation

n formulated oils. Carbon black was used as a soot analogueor their studies. They concluded that dispersed carbon blackapidly removed ZDDP reaction films by abrasion, which coulde prevented by using appropriate dispersant additives.

Chinas-Castillo and Spikes [6] studied the behavior of dilutedoot containing oils in lubricated contacts. Carbon black col-oid, lampblack colloid, and used engine oil were used for theirtudy. Ultra-thin film interferometry and image analysis tech-iques showed that soot colloid particles are entrained in theubricated contact inlets where they can influence the frictionnd wear characteristics of the base stock. The study showed thathe entrainment of soot particles occur at slow speeds, affectinghe film characteristics of clean engine oils and the process is

ore pronounced at high temperatures.

Needelman and Madhavan [9] studied the effect of lubri-

ating oil components, and nature of contamination on engineear. They proposed the chain-reaction of wear and conductedsurvey of engine oil contamination and the necessary improve-

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ents that had to be accomplished to reduce this contamination.hey concluded that contamination of the lube oil causes wearf engine components and also suggested that, a special rela-ionship is present between the size of the contaminant particlesnd the thickness of dynamic oil films. The contaminant parti-les larger than the oil film cause wear of engine components byaking simultaneous contact with both the surfaces.Cadman and Johnson [10] studied the effect of EGR on engine

ear using analytical ferrography technique. The collected oilamples from the engine were analyzed for metal wear debrissing analytical ferrography technique. A 15% EGR showed aignificant increase in the concentration of the wear particles.quilibrium particulate concentrations with 15% EGR were 10

imes higher than normal baseline levels. They also believed thatoot acts as an abrasive to remove the anti-wear surface coatingrovided by the additives in the lubricant.

Corso and Adamo [17] suggested that soot contaminantsnteract with the adsorption/chemisorption mechanism of ZDPn metal surfaces inducing a transition from anti-wear Fe3O4o pro-wear FeO. This transition apparently occurs due to theresence of soot in the lubricant limiting the access of oxygeno the metal surfaces.

Akiyama et al. [18] studied the phenomena of abnormalylinder wear in EGR equipped diesel engines. They concludedhat the cylinder wear of a diesel engine, which is equippedith EGR, increases at low temperatures and suggested that the

bnormal wear may be due to corrosion of cast iron. Corrosionf cast iron is due to formation of sulfuric acid formed whenondensed water reacts with the combustion SOx (oxides of sul-ur). However, this may not be the primary reason for engineear as the sulfur content in diesel engines has been reduced to.05 wt.%.

Gautam et al. [11,12] investigated the effects of soot con-aminated engine oil on three-body wear. Phosphorous level,ispersant level and sulfonate substrate level were the three oildditives they tested and concluded that there is an interactionetween oil additives and soot in reducing the oil’s anti-wearroperties. They also concluded that wear increases with higheroot concentration and decreases with higher phosphorous con-entration. They also performed tests on the ball-on-flat-disketup with soot and alumina and compared their wear ratios.t was concluded that abrasion could be the major mechanismnvolved in the diesel engine wear.

Gautam et al. [13] also investigated the effects of base stock,ispersant level, and ZDP level on three-body wear. The studyonsidered soot at two levels and hence could not determinehe non-linear effect of soot on three-body wear. Results indi-ated that the oil’s anti-wear properties were reduced as a resultf soot. The statistical analysis led to the conclusion that basetock and dispersant levels were significant on oil’s wear per-ormance, while the effect of ZDP was negligible within theange of concentrations tested. Ball-on-flat-disk tests showedhat the wear scar diameter as a result of soot was similar to that

ue to alumina indicating that the wear due to soot is abrasiven nature. EDAX (energy dispersive X-ray analysis) tests per-ormed on soot samples showed that there was no adsorption ofnti-wear agents by soot.

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ig. 1. Schematic representation of the setup for the three-body wear machine.

. Experimental equipment and procedure

.1. Three-body wear machine

A three-body wear testing machine was designed and devel-ped to study the effect of soot on engine wear. The schematic ofhe three-body wear machine setup is shown in Fig. 1. The three-ody wear testing machine had a stationary surface (specimen)s the first body, a rotating surface (cast iron round) as the secondody and the entrapped intermediate particles (soot) as the thirdody. The wear tests were conducted at a constant speed of rota-ion, and a constant linear sliding distance was maintained. Fourrials were performed for each specimen with each oil sample.he first trial was done by wearing the specimen on the outer-ost track of the cast iron round. The radius of the track was

nput into the computer program and the test stopped when theequired sliding distance was reached. Subsequent trials wereone on inner tracks and the test time varied according to therack radius. The effect of temperature was not taken into con-ideration as the continuous flow of oil provided the necessaryooling. A 4140 tempered steel specimen of 0.25 in. (6.35 mm)iameter was used as the stationary body. Due to crossed cylin-ers or other configurations that could vary the contact areauring the test, thus, giving inconsistent results, a flat-on-flatonfiguration was taken into consideration. The specimen wasemoved from the chuck of the holder at the end of each testnd was thoroughly cleaned, first in a hexane bath, and then inn acetone bath to remove the soot particles and oil adheringo the specimen. The specimen was then dried to remove anyexane or acetone adhering to the surface and weighed. Thextent of wear was measured as the actual loss of material, inrams.

.2. Design of experiment

If the designed experiment consists of more than one fac-or, the factors can influence the response individually or as aombination. In order to take care of such responses, an appro-

S. George et al. / Wear 262

Table 1Lubricant composition matrix

Blend number Base stock Dispersant level ZDP level

WVU397 Group I (−1) Low (−1) Low (−1)WVU398 Group I (−1) High (1) High (1)WVU399 Group I (−1) Low (−1) High (1)WVU400 Group I (−1) High (1) Low (−1)WVU401 Group II (1) Low (−1) Low (−1)WVU402 Group II (1) High (1) High (1)WW

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VU403 Group II (1) Low (−1) High (1)VU404 Group II (1) High (1) Low (−1)

riate statistical model needs to be designed and developed toetermine the effects of the various factors and the interactionsetween them. The factors that were taken into considerationor this study were base stock, dispersant level, and ZDP level.he three factors were tested at two levels, high (1) and low−1). This resulted in a 23 matrix. The lubricant compositionatrix is shown in Table 1. The eight samples in Table 1 were

ested at three levels of soot. The levels of soot used wereow (−1), medium (0), and high (1). The amount of soot forevel low was 0 wt.% or no soot contamination, for medium,wt.%, and for high, 4 wt.%. This resulted in 24 samples for theear tests. Group I base stock was assigned a value of −1 androup II base stock, a value of 1. The factor–level combinationsor the designed experiments are shown in Table 2. To most

ccurately simulate actual lubricants, other components suchs anti-oxidant, viscosity index improver, calcium, magnesium,etergent, rust inhibitor, anti-foamant, and pour point depressantere kept constant. These functional intermediates are generally

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able 2actor–level combinations for the statistical analysis

ampleumber

Basestock

Dispersantlevel

ZDPlevel

Sootlevel

Base stock ×dispersant

BaseZDP

1 −1 −1 −1 −1 1 12 −1 1 1 −1 −1 −13 −1 −1 1 −1 1 −14 −1 1 −1 −1 −1 15 1 −1 −1 −1 −1 −16 1 1 1 −1 1 17 1 −1 1 −1 −1 18 1 1 −1 −1 1 −19 −1 −1 −1 0 1 10 −1 1 1 0 −1 −11 −1 −1 1 0 1 −12 −1 1 −1 0 −1 13 1 −1 −1 0 −1 −14 1 1 1 0 1 15 1 −1 1 0 −1 16 1 1 −1 0 1 −17 −1 −1 −1 1 1 18 −1 1 1 1 −1 −19 −1 −1 1 1 1 −10 −1 1 −1 1 −1 11 1 −1 −1 1 −1 −12 1 1 1 1 1 13 1 −1 1 1 −1 14 1 1 −1 1 1 −1

(2007) 1113–1122 1117

resent in all commercial lubricants. The 24 samples producedrom the factor–level combination for the designed experimentsere tested on the three-body wear machine and the resultsbtained were analyzed statistically using the general linearodel (GLM) procedure of the statistical analysis system (SAS)

ackage. The SAS analysis gave the individual effects and inter-ctions of the three variables, base stock, dispersant and ZDPith soot.

.3. Randomization

In any experimental design, all the factors that affect theesponse must be taken into consideration. But this is not thease always and there is a possibility that some factor might beeglected. In order to average out all these uncertain factors inhe experiment, the test runs were completely randomized. In

randomized design, all the factor–level combinations in thexperiment including the repeat tests were randomized. In thistudy, all the 24 oil samples were randomized first and the wearests were conducted according to the random sequence.

.4. Ball-on-flat-disk setup

A set of experiments were performed using a ball-on-flat-isk setup on a milling machine to qualitatively analyze theear process. AISI 52100 stainless steel ball, 0.5 in. (12.7 mm)

n diameter, was worn against a gray cast iron surface in the pres-nce of soot–oil formulation at 30 lb (13.6 kg) load for 30 min.he oil samples which resulted in highest and lowest wearn the three-body wear tests were considered for the ball-on-

stock × Base stock ×soot

Dispersant ×ZDP

Dispersant ×soot

ZDP ×soot

1 1 1 11 1 −1 −11 −1 1 −11 −1 −1 1

−1 1 1 1−1 1 −1 −1−1 −1 1 −1−1 −1 −1 1

0 1 0 00 1 0 00 −1 0 00 −1 0 00 1 0 00 1 0 00 −1 0 00 −1 0 0

−1 1 −1 −1−1 1 1 1−1 −1 −1 1−1 −1 1 −1

1 1 −1 −11 1 1 11 −1 −1 11 −1 1 −1

1118 S. George et al. / Wear 262 (2007) 1113–1122

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oiestttstsfttwdtcoefficients of variance are also presented in the table. Graphswere also plotted to study the effect of the considered variableson soot contaminated engine oils. Fig. 6 shows the variation ofwear with increase in soot level. The wear values used are aver-

Fig. 2. Schematic representation of the ball-on-flat-disk setup.

at-disk experiments. The schematic of the setup is shown inig. 2.

.5. Microscopic studies

A scanning electron microscope (SEM) was used for theicroscopic studies, in order to determine the wear scar diam-

ters. The wear scars were caused due to the abrasive nature ofoot particles on the stainless steel ball specimen. The scanninglectron microscope is a powerful tool for analyzing the surfacef materials. It is analogous to reflected light microscope, excepthat it uses electrons for image formation.

.6. Soot collection and oil sample preparation

Soot was not isolated from the lubricant as lubricant additivesre known to absorb on the surface of soot, which would eithernfluence or suppress its nascent surface chemistry [1,19–21].nstead, soot was collected from the walls of a surge tank thatas placed in the exhaust line of a Caterpillar 3304 engine. The

urge tank was part of a multi-tube mini-dilution tunnel system,nd being close to the exhaust manifold, the soot collected wasepresentative of soot that would enter the oil sump. Soot wasarefully scraped of the walls of the tank after it was allowed touild up over several weeks of engine operation in the engineest cell facility at West Virginia University.

The preparation of a stable soot suspension is a challengingask because the density of the soot particles (approximately.8 g/cm3) is higher than the density of oil (approximately.88 g/cm3). Soot particles generally tend to agglomerate if theispersant does not perform its function and this makes it veryifficult to prepare a stable soot suspension artificially. The sed-mentation of the soot particles is possible if the sample is storedor a long period of time before performing the tests. In ordero avoid this and to prepare stable soot suspensions, a procedureroposed by Ryason et al. [22] was followed. The procedure waslso used in prior studies at West Virginia University [11–13].he step-by-step procedure that was followed for the prepa-

ation of all the samples is given in Appendix A. An ounce

29.57 cm3) of the oil sample was measured using an electroniceighing machine and was poured into a glass vial. Soot was

lso weighed using the electronic weighing machine and wasdded into the glass vial to obtain the required soot–oil sample. F

ig. 3. Variation of actual wear for WVU404 (sample nos. 8, 16, and 24).

. Results and discussion

.1. Three-body wear data analysis

The three-body wear data consists of the wear values (g)btained for each of the four trials on the three-body wear test-ng machine. Figs. 3 and 4 show the variation of actual wear forach trial, and cumulative wear with increasing soot level for oilample WVU404. Similar plots were made for all oil samplesested. The cumulative wear curves showed a higher wear forhe oil samples with soot contamination than without soot con-amination as can be seen in Fig. 4. This was the case for all oilamples tested, which showed the detrimental effect of soot onhe oil blend’s wear performance. The graphs for the actual wearhowed that the wear values stabilized after the first two trialsor most of the samples tested. The average wear values overhe four trials were used for the statistical analysis. Fig. 5 showshe variation of average wear for all the oil samples with andithout soot contamination. The error bars represent two stan-ard deviations (±2σ). Table 3 gives the average wear values forhe different oil formulations. The standard deviations and the

ig. 4. Variation of cumulative wear for WVU404 (sample nos. 8, 16, and 24).

S. George et al. / Wear 262 (2007) 1113–1122 1119

Fig. 5. Average wear for all oil samples.

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Fig. 7. Effect of base stock on wear.

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Fig. 6. Effect of soot level on wear.

ge values for all the samples having the same soot level. Similarraphs were also plotted for individual oil samples tested. Theesults showed that the percentage increase in wear from 0% to% soot was less than the percentage increase in wear from 2%o 4% soot for all the oil samples tested. This can also be seen bynalyzing the graphs for the variation of cumulative wear. Thislearly indicated the nonlinear variation of wear for the three

evels of soot used in this study. A careful analysis of the datalso showed that the nonlinearity was more pronounced in casef samples with group II base stock and high dispersant level.ig. 7 shows the effect of base stock on wear. The wear values

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able 3verage wear values for the different oil formulations

ample ID 0% soot 2% soot

Averagewear (g)

Standarddeviation

COV Averagewear (g)

VU397 0.0024 0.0005 0.20 0.0031VU398 0.0023 0.0005 0.21 0.0035VU399 0.0022 0.0003 0.16 0.0032VU400 0.0026 0.0006 0.23 0.0039VU401 0.0035 0.0016 0.46 0.0045VU402 0.0032 0.0008 0.24 0.0043VU403 0.0029 0.0007 0.25 0.0039VU404 0.0031 0.0007 0.24 0.0049

Fig. 8. Effect of dispersant level on wear.

sed are average values for the four samples having the samease stock and corresponding to each soot level. The wear valuesere more for blends with group II base stock as compared to

hose with group I base stock, and the increase in wear is moret higher soot levels. Fig. 8 shows the effect of dispersant level

n wear. The wear values used are average values for the fouramples having the same dispersant level and corresponding toach soot level. Wear values were almost the same for samplesithout soot contamination, but were greater for samples hav-

4% soot

Standarddeviation

COV Averagewear (g)

Standarddeviation

COV

0.0007 0.24 0.0048 0.0008 0.170.0011 0.31 0.0057 0.0016 0.280.0004 0.12 0.0048 0.0009 0.180.0008 0.21 0.0068 0.0010 0.150.0012 0.26 0.0071 0.0009 0.130.0004 0.09 0.0065 0.0008 0.120.0008 0.20 0.0062 0.0008 0.120.0010 0.21 0.0079 0.0011 0.14

1120 S. George et al. / Wear 262 (2007) 1113–1122

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4

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Table 4p-Values from statistical analysis

Variables and interactions p-Value

Base 0.0001Dispersant 0.0018ZDP 0.0022Soot 0.0001Sootq 0.0010Base × dispersant 0.1072Base × ZDP 0.1671Base × soot 0.0647Dispersant × ZDP 0.3110Dispersant × soot 0.0088ZDP × soot 0.0647Base × sootq 0.6250Dispersant × sootq 0.8441Z

csp

iaffivhttsfTsciptD0wawwwtbal

hwwear. Similarly, the effect of dispersant on wear was more pro-

Fig. 9. Effect of ZDP level on wear.

ng a high dispersant level for higher soot levels. The reasonehind this could be the fact that dispersants generally increasehe viscosity of the oil samples. Increased viscosity would resultn pumpability problems and ultimately cause wear of enginearts. Alkylsuccinimides, and alkylsuccinic esters are typicalompounds that make up the dispersants and have high molecu-ar weights and viscosity. Dispersants make up a major portionf the additive package and hence its effect and the effect ofoot–dispersant interaction on viscosity are of particular impor-ance. A higher level of dispersant is desirable at high soot levelso as to keep the soot in suspension, which is a primary functionf the dispersant. A high dispersant level and high soot leveldversely affects the viscosity of the lubricant. Hence, the rightalance of dispersant in the lubricant composition is an impor-ant factor. The effect of soot and additive levels on viscosityas also investigated as part of this study and the results are pre-

ented in a separate paper [23]. Fig. 9 shows the effect of ZDP onear. The wear values used are average values for the four sam-les having the same ZDP level and corresponding to each sootevel. Wear values were low for samples having a high level ofDP as compared to a low level for higher soot levels. The wearalues were approximately the same for samples without sootontamination at the two ZDP levels. This result is obvious con-idering the anti-wear property of ZDP that reduces friction andear.

.2. Statistical analysis of the three-body wear data

The data obtained from the three-body wear machine testsas statistically analyzed. Base stock, dispersant and ZDP were

ested at two levels: high (1) and low (−1). Soot was tested athree levels: high (1), medium (0), and low (−1). Apart fromoot, another factor, sootq, was added to study the nonlinearehavior of wear. Sootq was tested at two levels: high (1) andow (−2). The two levels are unequally spaced about zero unlikeigh (1) and low (−1) for the other variables, since sootq is

hosen to represent the nonlinearity associated with viscosityue to composition of soot in oil. The average values of weariven in Table 3 were used for the statistical analysis.

nat

DP × sootq 0.4966

The general linear model (GLM) procedure of the statisti-al analysis system (SAS) package was used to determine theignificance of variables on wear obtained from experimentserformed using the 24 oil samples.

The output file from the SAS analysis gave the effects andnteractions of the various oil additives with soot. The output filelso gave the significance probability (p-value) for the variousactors and their interactions. Significance probability quanti-ed the degree of discrepancy between the estimated parameteralue and its hypothesized value. In the current study, the nullypothesis (H0) for the statistical analysis stated that none ofhe factors influence wear. The alternate hypothesis (Ha) statedhat the factors are significant. If the p-value is smaller than theignificance level (α), then the null hypothesis is rejected and theactors corresponding to that p-value are considered significant.he analysis gave a clear indication of the variables that wereignificant and those variables, which were not. The p-valuesorresponding to each variable and their interactions are shownn Table 4. From the output, it was clear that the base stock with a-value of 0.0001, and soot content with a p-value of 0.0001 werehe most significant factors at a 95% confidence level (α = 0.05).ispersant with a p-value of 0.0018 and ZDP with a p-value of.0022 were also significant at a 95% confidence level. Sootqith a p-value of 0.0010 clearly indicated that it is significant at95% confidence level and it confirms the nonlinear behavior ofear with varying soot levels. The only significant interactionas that between dispersant and soot whose p-value was 0.0088hich is significant at 95% confidence level. This agrees with

he discussion already made regarding Fig. 8. The interactionetween base stock and soot, and the interaction between ZDPnd soot could be considered significant only at 90% confidenceevel (α = 0.10).

A higher level of dispersant and group II base stock resulted inigher wear. It was also seen that the effect of group II base stockas more dominant than that of the higher dispersant level on

ounced than that of ZDP level. The presence of soot is alwaysssociated with increased wear. Two-factor interaction showedhat the interaction of all the variables considered with soot were

S. George et al. / Wear 262 (2007) 1113–1122 1121

Table 5Wear scar diameters and wear ratios from the ball-on-flat-disk tests

Blend number Wear scar diameter, WSD (mm) Wear ratio

WVU399 without soot 0.0170 0.0500WVU399 with 2% soot 0.0350 0.1029WW

sr

4

mlwvtwapdcrrpf

FWfwpswccsot

F

Fig. 11. SEM image of wear scar formed with sample WVU404 with 2% soot.

F

ittc

5

t

VU404 without soot 0.0200 0.0588VU404 with 2% soot 0.0400 0.1176

ignificant. The above analyses confirm with the analysis andesults obtained from the graphs.

.3. Ball-on-flat-disk results

The ball-on-flat-disk wear tests were performed on a millingachine using a specially designed chuck to qualitatively ana-

yze the wear scar on the steel ball specimen. The experimentsere performed on samples having the lowest and highest wearalues obtained from the three-body wear tests. The sample withhe lowest wear was WVU399 and that with the highest wearas WVU404. SEM photographs were taken for the wear scars

nd the wear scar diameter (WSD) was determined from thesehotographs. The WSDs were then normalized by the Hertziameter, which is calculated by the Hertz equation [24] to cal-ulate the wear ratio for each test. Wear ratio is defined as theatio of WSD to Hertz diameter. Kim et al. [3] had used wearatios and WSDs as indicators for determining the anti-wearroperties of test oils. Table 5 shows the wear ratios obtainedor the different oil samples tested.

The SEM pictures for sample WVU404 are shown inigs. 10–12. SEM pictures were also obtained for sampleVU399. The results showed that the wear ratios were higher

or oil samples with soot than oil samples without soot. Theear ratios, with and without soot contamination for oil sam-le WVU404, is higher than corresponding wear ratios for oilample WVU399 which is in agreement with the three-bodyear result. The results showed that the WSD due to soot

ontamination is approximately twice the WSD without soot

ontamination. This is also in agreement with previous studies ofimilar nature [11–13]. The wear scar tests were also performedn oil samples WVU399 and WVU404 at 4% soot. The SEM pic-ures showed that the stainless steel specimen had traces of cast

ig. 10. SEM image of wear scar formed with sample WVU404 without soot.

afiswtZlfrhttao

otp

ig. 12. SEM image of wear scar formed with sample WVU404 with 4% soot.

ron material. This could be a result of adhesive wear betweenhe two surfaces in contact. The reason behind this could be thathe contamination of the lubricating oil affected the anti-wearharacteristics, leading to the breakdown of the lubricant.

. Conclusion

The SAS analysis performed on the data obtained from thehree-body wear tests indicated that wear increased nonlinearlys the amount of soot increased. Cumulative wear was moreor samples with soot contamination than without soot contam-nation. This showed the detrimental effect of soot on the oilample’s wear performance. The analysis also indicated thatear decreased when group I base stock was used compared

o group II base stock. Oil samples with low dispersant and highDP performed better than samples having high dispersant and

ow ZDP at high soot levels. The reason behind this could be theact that high dispersant levels together with soot contaminationesults in lubricant thickening resulting in increased wear. ZDPas an anti-wear property that reduces friction and wear; hence,he lower wear at higher ZDP levels. The SAS analysis indicatedhat base stock, dispersant, ZDP, soot, and dispersant–soot inter-ction were significant and they affected the wear performancef the oil samples.

The wear scar diameters and the wear ratios from the ball-n-flat-disk tests confirmed with results obtained from thehree-body wear machine tests. At higher levels of soot, theerformance of the oil samples deteriorated.

1 ar 262

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[diesel engine soots; application to the analysis of soot in used engine oils,SAE 942030, 1994.

122 S. George et al. / We

ppendix A

Once an ounce (29.57 cm3) of the oil sample and the requiredmount of soot (wt.%) was measured and poured into a glass vial,he following steps were ardently followed to obtain stable sootuspensions for all the tests. It is the same procedure proposedy Ryason et al. [22]:

(1) Loosen the cap of the oil sample container.(2) Immerse the sample in a 200 ◦F (93 ◦C) water bath to just

beneath the threaded part at the top of the container for15–20 s.

(3) Remove the container and tighten the cap.(4) Shake vigorously for at least 30 s (time the shaking period).(5) Invert the container and repeat step (4).(6) Return the container to upright position.(7) Carefully remove the cap.(8) With a small, clean, dry stainless steel spatula, stir the

sample. Insert the spatula all the way to the bottom of thecontainer, drag the tip of the spatula along the bottom of thecontainer, and then withdraw it to check for the presence ofa thickened, viscous layer at the bottom of the container. Ifsuch a layer appears to be present, as judged by the appear-ance of the material on the tip of the spatula, re-insert thespatula and stir vigorously until the bottom sediment iscompletely mixed with the remainder of the sample. Thisshould be repeated until the oil clinging to the spatula hasa uniform appearance and drains uniformly.

(9) Recap the container loosely.10) Again, immerse the container in the 200 ◦F (93 ◦C) water

bath up to the threaded part for 15–20 s.11) Remove the container and tighten the cap.12) Shake vigorously for 15 s.13) Rotate the container 180◦ and again shake vigorously for

15 s.14) Invert the container.15) Repeat steps (12) and (13).

The above procedure should provide a stable soot suspensionor about 2 h. If the sample was not utilized within that time, thebove procedure was carried out again. The soot–oil sample washen used for the wear tests. The oil formulations were tested athree levels of soot: 0% or without soot contamination, 2% soot,nd 4% soot by weight.

eferences

[1] F.G. Rounds, Carbon: cause of diesel engine wear?, SAE 770829,1977.

[

[

(2007) 1113–1122

[2] P.R. Ryason, I. Chan, J. Gilmore, Polishing wear by soot, Wear 137 (1990)15–24.

[3] C. Kim, C. Passut, D. Zang, Relationships among oil composition,combustion-generated soot, and diesel engine valve train wear, SAE922199, 1992.

[4] E.A. Bardasz, V.A. Carrick, V.L. Ebeling, H.F. George, M.M. Graf,R.E. Kornbrekke, S.B. Pocinki, Understanding soot mediated oil thick-ening through designed experimentation-part 2: GM 6.5 L, SAE 952527,1995.

[5] M. Ratoi, H.A. Spikes, The influence of soot and dispersant on ZDDP filmthickness and friction, Lubric. Sci. 17 (1) (2004) 25–43.

[6] F. Chinas-Castillo, H.A. Spikes, The behavior of diluted sooted oils inlubricated contacts, Tribol. Lett. 16 (4) (2004) 317–322.

[7] W. Van Dam, W.W. Willis, M.W. Cooper, The impact of lubricant compo-sition and rheology on wear in heavy duty diesel engines, in: W.J. Bartz(Ed.), Proceedings of the 12th International Colloquium Tribology 2000-Plus, vol. 1, Technische Akademie Esslingen, January, 2000, pp. 515–525.

[8] I. Nagai, H. Endo, H. Nakamura, H. Yano, Soot and valve train wear inpassenger car diesel engines, SAE 831757, 1983.

[9] W. Needelman, P. Madhavan, Review of lubricant contamination and dieselengine wear, SAE 881827, 1988.

10] W. Cadman, J. Johnson, The study of the effect of exhaust gas recirculationon engine wear in a heavy-duty diesel engine using analytical ferrography,SAE 860378, 1986.

11] M. Gautam, K. Chitoor, M. Durbha, J.C. Summers, Effect of diesel sootcontaminated oil on engine wear – investigation of novel oil formulations,Tribol. Int. 32 (1999) 687–699.

12] M. Gautam, M. Durbha, K. Chitoor, M. Jaraiedi, N. Mariwalla, D. Ripple,Contribution of soot contaminated oils to wear, SAE 981406, 1998.

13] M. Gautam, K. Chitoor, S. Balla, M. Keane, Contibution of soot contami-nated oils to wear-part II, SAE 1999-01-1519, 1999.

14] R. Benzing, I. Goldblatt, V. Hopkins, W. Jamison, K. Mecklenburg, M.Peterson, Friction and wear devices, Am. Soc. Lubric. Eng. (1976).

15] F.G. Rounds, Soots from used diesel engine oils-their effects on wear asmeasured in four-ball wear tests, SAE 810499, 1981.

16] I. Berbeizer, J. Martin, P. Kapsa, The Role of Carbon in Lubricated MildWear, CNRS, France, 1986.

17] S. Corso, R. Adamo, The effect of diesel soot on reactivity of oil additivesand valve train materials, SAE 841369, 1984.

18] K. Akiyama, K. Masunaga, K. Kado, T. Yoshioka, Cylinder wear mecha-nism in an egr-equipped diesel engine and wear protection by the engineoil, SAE 872158, 1987.

19] F.G. Rounds, The generation of synthetic diesel engine oil soots for wearstudies, Lubric. Eng. 40 (1984) 394.

20] M.J. Covitch, B.K. Humphrey, D.E. Ripple, Oil thickening in the Mack T-7engine test—fuel effects and the influence of lubricant additives on sootaggregation, SAE 852126, 1985.

21] M.J. Covitch, Oil thickening in the Mack T-7 engine test II—effects of fuelcomposition on soot chemistry, SAE 880259, 1988.

22] P.R. Ryason, M.J. Hillyer, T.P. Hansen, Infrared absorptivities of several

23] S. George, S. Balla, V. Gautam, M. Gautam, Effect of diesel soot onlubricant oil viscosity, Tribol. Int. 40 (2007) 809–818.

24] M. Neale, Tribology Handbook, Butterworths, London, 1973.