ear is defined as the gradual loss ofmaterial from a solid surface due to

its interaction with a second contacting sur-face. For example, abrasive wear occurs par-ticularly when tall sharp asperities from ahard surface produce gouges on a softersurface.

STLE member Dr. Matthew Siniawski,assistant professor of mechanical engineer-ing at Loyola Marymount University in LosAngeles, says, “Surface wear is a majorproblem in reducing the lifetime of machin-ery. Efforts to better predict wear can pro-

vide a number of benefits,including greater energyconservation and improvedproductivity.”

Development of theoret-ical models to characterizewear has been a majorobjective because it canreduce the amount of test-

ing needed to verify the efficacy of anti-wearcoatings and potentially allow for smallermargins of safety in the design of machin-ery. A number of models have been pro-posed to explain surface wear.

Siniawski comments, “Most of thesemodels are good building blocks and pro-vide a greater understanding of wear. How-ever, they are only effective for specificmaterials under specific conditions andcannot be applied to other systems.”

The challenge in finding a universal wearlaw that works is due to the complexity ofthe rate of removal of material from a sur-face over time. Siniawski believes that theimpact of changes in the composition ofboth interacting surfaces over time at themicroscale has a significant impact on the

surface wear. Equation (1) developed in 1998 by

Stephen Harris and coworkers provided agood prediction for the wear rate of steelsliding against diamond-like-carbon (DLC)coatings.1 Siniawski and colleagues testedthis relationship by sliding 52100 steel ballbearings against coupons coated withboron carbide (B4C) and DLC in pin-on-diskexperiments. He says, “We tested the ballbearing over 105 cycles and obtained experi-mental validation for equation (1). Thisequation is shown below.

A(n) = V(n)/d= A1nB (1)

A(n) represents the wear rate averagedover the first n cycles. V(n) is the volume ofsteel removed from the ball during the firstn cycles of the test and d is the distance ofthe steel ball traveled over the disk. A1 rep-resents the wear rate of the first cycle.

The “B” term is a measure of the cycle ortime dependency of the wear rate. Siniawskiindicates that “B” can range from -1 to 0. Theformer would occur if all of the system’sability to generate wear is lost after the firstcycle. If “B” is 0, then the wear rate would beconstant during the process. For the initialexperiments of the 52100 steel ball on theDLC and B4C coatings, “B” was found to be -0.8. Siniawski explains, “There is an indirectrelationship between the wear rate and “B.”The wear rate cannot fall faster than inverse-ly with the number of cycles completed.”

Experimental validationSiniawski conducted work to determine ifequation (1) can be used to predict the sur-face wear of a system, given A1 and “B.” Anexample of this work, carried out by Sini-

12 O C T O B E R 2 0 0 7 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

By Dr. Neil Canter

Contributing Editor

Tech Beat

Empirical testing of a universal law for surface wear

W

The challenge in finding a universal

wear law that works is due to the

complexity of the rate of removal of

material from a surface over time.

12-17 techbeat 10-07 9/20/07 4:32 PM Page 12

awski using 52100 steel, is shown in Figure1. Siniawski says, “We looked at the wearrate of 52100 steel as a function of the cyclesusing boron carbide coatings of varying sur-face roughness (Ra). Equation (1) does avery good job of predicting actual perform-ance over a wide range of boron carbide sur-face roughness.”

The graph on the bottom left in Figure 1shows the decline in the surface roughnessof the steel, as the number of cycles increas-es to 1,000. Equation (1) shows good pre-dictability of abrasive wear, as seen in thegraph on the bottom right of Figure 1.

Scanning electron microscopy (SEM)images of the boron carbide coating arealso shown for the unworn (on the left) andthe worn (on the right) surfaces. The wornboron carbide coating surface image wastaken after 500 cycles.

Boron carbide is a very abrasive coatingwhile steel is a semisoft surface. Siniawskisays, “We found that 52100 steel chemo-mechanically polishes the boron carbidesurface at the same time that the muchmore abrasive boron carbide is mechanical-ly polishing the steel.”

Siniawski points out that the tall sharpasperities in any surface are much easier todull during the process of abrasive wear. Headds, “The really tall sharp asperities weardown quickly, while the remaining hiddenasperities on a surface wear down later.These asperities are shorter and tend to beprotected by the taller asperities. In addi-tion, those asperities that are duller take alot longer to wear down.”

All of the previous work described wascarried out without any lubricant. Siniawskievaluated the 52100 steel boron carbidesystem with a neat paraffinic oil exhibiting aviscosity of 20 cSt at 40 C. He says, “The useof a lubricant lowered the wear rate, and thewear of the system can still be accuratelypredicted with equation (1).”

Siniawski found that this decrease in thewear rate was only partly attributed to theuse of a lubricant. He adds, “The prettysevere boundary lubricant conditions makeit difficult for a neat base oil to be more of afactor in reducing wear.” Siniawski plans infuture work to evaluate other lubricants thatare formulated with anti-wear and extremepressure additives to see if equation (1) canbe used to predict surface wear of more sys-

tems with more complex lubricants. Additional testing also was carried out

with vegetable oil-based lubricants that arechemically different than paraffinic baseoils. Equation (1) was also found to be accu-rate in predicting the surface wear of steelwith vegetable oil-based lubricants.

Siniawski next examined literature dataand evaluated empirical work described in17 publications that cover systems with awide range of materials and contact condi-tions. In each case, equation (1) provides agood wear predictability for the results. Hesays, “We found the root-mean-square(RMS) deviation between the data andequation (1) to vary at the maximum by10%. In most cases, the deviation was lessthan 5%.”

Equation (1) shows promise to be a uni-versal law for surface wear and could possi-bly be one of the Holy Grails of tribology.Future work will involve learning moreabout how proper material selection, sur-face roughness and other parameters influ-ence both A1 and “B.”

Siniawski, says, “If that can be achieved,equation (1) could potentially be used topredict the surface wear of any system.Another approach would be to develop aconcise empirical database of values of A1

T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y O C T O B E R 2 0 0 7 1 3

Scanning electron micro-

scopy (SEM) images of

boron coatings are shown

on the top. The figure on

the top left is the unworn

coating and the figure on

the top right is the worn

coating after sliding

against 52100 steel for 500

cycles. The graph on the

bottom left shows the de-

cline in the surface rough-

ness of steel, as the number

of cycles increases to 1,000.

Good predictability of

equation (1) is seen on the

bottom right graph.

Figure 1.

CONTINUED ON PAGE 14

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12-17 techbeat 10-07 9/20/07 4:32 PM Page 13

he increasing requirements placed onlubricants to provide a high level of

performance over increasing long anddemanding operating time frames has ledthe industry to utilize more and more prod-

ucts derived from syn-thetic basestocks. Lu-bricants are marketedbased on such bases-tocks as polyalphao-lefins (PAOs), esters(di and polyol), poly-alkylene glycols (PAGs)and phosphate esters.

Many of these base-stocks have been made in the past by rawmaterials derived from petroleum oil (petro-chemicals). With the increasing trendtoward using environmentally friendly rawmaterials, the chemical industry has beenlooking to use renewable sources such ascarbohydrates, proteins and lipids.

One chemical intermediate that has beenused in the manufacture of polyester fibersis 1,3-propanediol, also known as PDO. Thisintermediate has been produced in the pastfrom propylene or via a hydroformylationprocess from ethylene oxide.

Recently, DuPont and Tate & Lyle havecommercialized an alternative process formanufacturing PDO from corn-derived glu-cose through a fermentation process. Glu-cose is converted into PDO through theassistance of a metabolically engineered ver-

sion of E. coli. Commercial production of Bio-PDO™ from glucose started up late in 2006.

Polyether diolsDuPont is now in the process of commer-cializing Cerenol™, a new family of renew-ably resourced, high-performance polyols(polyether diols). Certain grades of Cerenolwill be a type of synthetic basestock forfunctional fluids. The polyether diols areprepared by polymerization of biobasedPDO. Molecular weights for these polymersrange from 500 to 3,000.

Ray Miller, new venture manager forbiobased materials for DuPont, says, “Wehave taken a different approach in thedevelopment of a biobased lubricant bases-tock. Instead of conducting chemical trans-formations on a high molecular weight nat-ural raw material, we take a renewablysourced small molecule and build it up in apolycondensation reaction to form a poly-ether diol.”

One of the first objectives was to ensurethat these renewably sourced polyetherdiols exhibited a very low toxicity profile.Testing carried out indicates that the poly-ether diols are not skin and eye irritants,and they also are not skin sensitizers. A verylow acute oral mammalian toxicity (LD50 >2000 mg/kg) also was verified.

The polyether diols represent a family ofnew products that contain both homopoly-mers and copolymers. Miller says, “We can

CONTINUED FROM PAGE 13

New synthetic basestocks from a renewable sourceT

14 O C T O B E R 2 0 0 7 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

and “B” that can be used by tribologists topredict wear for a specific system. Use ofequation (1) could significantly reduce theamount of experimental testing needed todetermine wear rates for specific machinerysystems prior to use.”

Details about equation (1) including ref-erences covering its origin are included in arecently published paper.2 Additional infor-mation also can be obtained by contactingSiniawski at msiniawski@lmu.edu.

References1. Harris, S., Weiner, A., Olk, C., Grischke, M.(1998), “Effects of Nanoscale Morphologyon the Abrasion of Steel by Diamond-likeCarbon,” Wear, 219, pp. 98-104.

2. Siniawski, M., Harris, S. and Wang, Q.(2007), “A Universal Wear Law for Abrasion,”Wear, 262, pp. 883–888.

‘Instead of conducting chemical transforma-

tions on a high molecular weight natural

raw material, we take a renewably sourced

small molecule and build it up in a polycon-

densation reaction to form a polyether diol.’

12-17 techbeat 10-07 9/20/07 4:32 PM Page 14

tailor the polyether diol to meet specificapplications and customer requirements.”For example, the terminal hydroxyl groupspresent on these polymers can be convertedinto esters through treatment with organicacids.

Biodegradability testing is in the processof being carried out on the polyether diols.Hari Sunkara, research associate forDuPont, says, “Biodegradability depends onthe type of product and is determined byparameters such as molecular weight andwater solubility. Those polyether diols thatexhibit low molecular weights and are solu-ble in water are mainly biodegradable.”

The polyether diols useful in lubricantapplications are not all homopolymersmanufactured from PDO. Sunkara says, “Wefound that addition of ethylene glycol as aco-monomer in the polymerization leads toimproved low-temperature properties forpolymers with molecular weights rangingfrom 500 to 1,500.”

The pour point of polytrimethylene etherglycol (homopolymer prepared from PDO) is-24 C. Inclusion of ethylene glycol willenable the pour point to drop to between -40 C and -60 C.

Polytrimethylene ether glycol exhibitssuperior thermal and oxidative stabilityproperties. A homopolymer with a molecularweight of 500 displays a flash point of 235 C.

The polymer was evaluated for oxidativestability by the ASTM D2272 procedure,which is known as the RPVOT (RotatingPressure Vessel Oxidation Test). A knownantioxidant (phenothiazine) was added at a1% treat rate and the polyether diol evaluat-ed at 150 C. Failure was not seen until 415minutes. Sunkara says, “A conventionalmineral oil will fail at between 80 and 90minutes under the same test conditions.”

He adds, “Polyether diols do not containtertiary carbons in the backbone, whichenables them to exhibit superior oxidativestability as compared to other polyethers.This property is unexpected because it wasanticipated that the linear nature of poly-ether diols combined with the greater num-ber of ether linkages would lead to inferioroxidative-stability properties.”

Polyether diol homopolymers andcopolymers are being evaluated as synthet-ic lubricant basestocks in a number of appli-cations, including dielectric fluids, gear oils,

heat transfer fluids and hydraulic fluids.Miller says, “Our objective is to determine inwhich applications polyether diols displaysuperior characteristics.”

Initial work as a basestock in heat trans-fer fluids has shown promise. Sunkara says,“Polyether diols exhibit good thermal con-ductivity values that enable them to be suit-able for use in heat transfer fluids.”

Polyether diol homopolymers also havebeen evaluated as dielectric fluids. Blendingwith an ester or a vegetable oilis preferred to provide dielec-tric breakdown voltages greaterthan 30 kv.

Miller says, “Evaluation ofpolyether diols in lubricantapplications will be completedin 2008. At that time we will beable to provide additionalinformation about their performance char-acteristics and how best they can be used assynthetic basestocks. We see polyetherdiols as very attractive for the lubricant mar-ket because they are prepared from renew-ably sourced materials and can meet therequirements of high-performance function-al fluids.”

DuPont is in the process of commercial-izing this new line of renewably sourced,high-performance polyether diols. Addition-al information can be requested atcerenol@usa.dupont.com.

A new family of renewably

resourced, high-perform-

ance polyols (polyether

diols) has been developed

that can be used as a syn-

thetic basestock for func-

tional fluids.

Figure 2.

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‘Polyether diols exhibit good

thermal conductivity values that

enable them to be suitable for

use in heat transfer fluids.’

CONTINUED ON PAGE 16

T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y O C T O B E R 2 0 0 7 1 5

12-17 techbeat 10-07 9/20/07 4:33 PM Page 15

16 O C T O B E R 2 0 0 7 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

s a chemist who has worked on syn-thesizing specific molecules, I have

often been frustrated when a specific reac-tion approach did not work. Lessonslearned from experimentation haveproven to be useful in finally developingthe right strategy to prompt the reactantmolecules to act in what I would considerthe “right way.”

Often enough, I have wanted to stick myhand in the reaction flask and literally “beat”on the molecules to force them to react in amanner that I wanted them to do. This frus-tration from my past is brought up becausemost techniques available to convert one

molecule to another haveentailed the use of eitherheat or light to enable thereaction to succeed.

In lubrication, some addi-tives will not function unlessthey reach a specific temper-ature. An example of this,discussed in the September

TLT, which featured a special report onextreme pressure (EP) additives, is the needfor three of the specific EP additive classesto become activated when the temperatureof the lubricant application reaches a spe-cific point. EP additives will then form metalsalt barriers that reduce friction and wear.1

If a molecule could be activatedthrough a mechanical process, then theremay be an opportunity to develop newtechnologies that could be incorporatedinto metal surfaces for use when the forceor pressure reaches a specific level. Oncethe molecule is activated mechanically, itwould then be able to exhibit extremepressure characteristics.

The use of force on molecular systemshas led to the fracturing of molecules in arandom manner in the past. A systematicapproach to using mechanical force, as ameans to synthesizing molecules in a pre-dictable manner, has not occurred untilnow.

Use of mechanophoresJeffrey Moore, William H. and Janet Lycan,professor of chemistry at the University ofIllinois at Urbana-Champaign, led aresearch team that has developed a processfor driving chemical reactions through theuse of mechanical force. He says, “We con-sider this type of reaction to bemechanochemistry. The key to mechanicallyinducing reactions is to activate the mole-cule in a predictable manner. This approachcan be accomplished through the use of aspecific molecular unit activated bymechanical force, which is known as amechanophore.”

The mechanophore chosen by theresearch team is benzocyclobutene (BCB).Moore adds, “BCB contains a four-memberring that is sterically strained and suscepti-ble to a systematic bond breakage understress.”

BCB was incorporated into the center ofa polyethylene glycol (PEG) polymer withmolecular weights ranging from 4 to 60 kDa.The reaction was monitored by having theactivated BCB react with a maleimide toform a compound that could be detected bygel permeation chromatography (GPC)using ultraviolet light.

The mechanical force was provided byultrasound at a frequency of 20 kHz. Theresearchers detected a reaction at a temper-ature between 6 C and 9 C, too low for theprocess to occur thermally. Moore says,“The real reason we chose this system wasthat the chemistry for the thermal reactionis well known. We deliberately chose condi-tions under which heat could not induce thereaction.”

The reaction is known as an electrocyclicprocess in which one bond in the strained,four-member ring is broken. This enablesthe ring to be opened in a specific manner.Organic chemists know that the Woodward-Hoffmann rules have been developed todetermine how the ring should open in thepresence of heat and light.

A

Mechanochemistry: A newapproach to synthesize molecules

A systematic approach to using

mechanical force, as a means to syn-

thesizing molecules in a predictable

manner, has not occurred until now.

CONTINUED FROM PAGE 15

12-17 techbeat 10-07 9/20/07 4:33 PM Page 16

Two isomers (cis and trans) of the BCB-based polymer were evaluated. The researchteam found that mechanical stress forcedthe ring openings of both isomers to gener-ate the same intermediate isomer. This find-ing was consistent with a computationalanalysis carried out using the COGEF (COn-strained Geometries simulate ExternalForce) method.

In fact, the cis isomer produces a differ-ent intermediate isomer than is found withthe use of heat. In the case of the trans iso-mer, the same intermediate isomer isdetected with both the use of mechanicalenergy and with heat.

Mechanical force mechanismMoore indicates the mechanism for the useof mechanical force is not known. He says,“We believe that the acoustic field generat-ed by the ultrasound creates cavities in theorder of microns (up to 100 microns). Thesecavities form slowly but collapse rapidly.During the collapse, solvent molecules vio-lently rush past the polymer chains towardthe collapsing cavities to fill the voids. Theresulting stress on the polymer chain leadsto the bond breakage in the mecha-nophore.”

Moore indicates that the polymer chainis in a velocity gradient. Most of the stress isfelt in the middle of the chain, which is thelocation for the mechanophore. One end ofthe chain is sucked into the collapsing cavi-ty, while the other end is a drag on theentire chain.

Figure 3 is an overlay of three imagesshowing how mechanical force causes thereaction to occur. The colored image repre-sents the BCB group, while the two strainsrepresent the polymer chain. The blue col-ored image is how BCB looks at the begin-ning of the reaction, while the yellow col-ored structure shows the end of the reac-tion.

Moore sees a number of applications formechanochemistry. He says, “We envisionthat a color-generating mechanophore couldbe produced in a mechanically induced reac-tion by, for example, scratching a polymerfilm. The mechanophore could act as aprobe to assist with mapping out stressfields associated with the polymer film.”

A second application envisioned by

Moore is to use mechanophores as a cata-lyst that could, for example, release a pro-ton under stress to drive a specific process.This feature combined with the color-gener-ating mechanophore could lead to the useof a color-generating sensor that is triggeredby force to turn on and catalyze a reaction.

If the right type of mechanophore can befound, then it is not too difficult to envisionlubricant additives placed on a surface thatcan be mechanically activated by this typeof technology. Further information can befound in a recently published paper.2 <<

References1. Canter, N. (2007), “Trends inExtreme Pressure Additives,”Tribology and Lubrication Technolo-gy, 63 (9), pp. 10–18.

2. Hickenboth, C., Moore, J.,White, S., Sottos, N., Baudry, J.and Wilson, S. (2007), “BiasingReaction Pathways with Mecha-nical Force,” Nature, 446 (7134),pp. 423–427.

Neil Canter heads his own consulting company,Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him atneilcanter@comcast.net.

The BCB mechanophore

used in the process is

placed in the middle of a

polymer chain (representedby the attached strands).

The blue image of the

mechanophore represents

the start of the reaction.

The yellow image repre-

sents the end of the reac-

tion. Ultrasound is used to

generate cavities that col-

lapse rapidly as solvent

molecules (shown in thegreen gradient) rush past

the mechanophore.

Figure 3.

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T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y O C T O B E R 2 0 0 7 1 7

‘The key to mechanically inducing

reactions is to activate the mole-

cule in a predictable manner. This

approach can be accomplished

through the use of a specific

molecular unit activated by

mechanical force which is known

as a mechanophore.’

12-17 techbeat 10-07 9/20/07 4:33 PM Page 17