Scanning probe microscopy of automotive anti-wear films

WI3AR wear212 (1997) 254-264

Scanning probe inicroscopy of automotive anti-wear films A.J. Pidduck b, GC Smith a** aShell Intematimd

Oil Products Ltd, Shell Research and Technology Centre Thornton, P.U. Box 1. Chester CHI JSH, UK b Dqfence Evaluation and Research Agency, St, Andrew Road, M&em WR14 3PS. UK Received 18 September 19%; accepted IS April 1997

Atomic force microscopy (AF’M) and lateral force microscopy (LFM) have been used to examine the Iocal topography and frictional propertiesof anti-wearfilms formed during tribological testing using a reciprocatingA~sIer test - a laboratorymechanical test to simulate engine valve trainconditions - and a real engine component. Measurementswere made in ambient conditions, under a model hydrocarbon fluid (do&cane), and undera lubricatingoil. AFM gave high resolution images togetherwith an estimate of the distributionof film thicknesses and coverages in good agreement with previous electron microscopy results. Force-distance curves were sensitive to soft liquid-like surface overlayers found to be present in some cases. Using LFM, relative friction coefficients weE determined on microscopic smooth areas of Am&r specimens prepa using different lubricantadditives, and the values compared with macroscopic values measuredat the end of the An&r test. Absolute values for these micro-regions were also measured, with a lower degree of confidence. The possibitity of using the AFM lip to simulate a single asperitytriboiogical contact was investigated. 8 1997 Shell Research Ltd. Published by Elsevier Science S.A. Kepvords: ZDDP: AFM;LFM: Anti-wear film; Amslertest; Cam follower; Friction:Tribology;Asperitycontact

advantage of producing specimens in a form amenable to microscopy and surface analysis [ 21. The films generated in

1. Illtiuctlon An understanding of surface processes such as friction and

wear is fundamental to optimising design where engineering c0mponenr.sftlz& intakes Uibo~OgiC~ contact. This isespecially important iti wxiem engine design, which places greaterstresseson the lubricantwhile at the same timecalling for the low wear rates and friction reduction necessary for high performance coupled with low emissions. In the regime of boundary lubrication, protection against wear is given by the formation of surface films through the action of lubricant additives. In automotive engines, zinc dialkyl dithiophosphate (ZDDP) additives have been used for many years to give protection under extreme pressure (EP) conditions. Much effort has been devoted to elucidating the chemical composition and structure of the films, and successful models have been generated which give iesights into the growth mechanisms and the effect of lubricant formulation on the nature of the Tim [ I 1. Recent work has shown that the recip-

rocating Amsler rig accurately simulates the conditions in internal combustion engine valve-train systems and has the * Correspondingauthor.Presentaddress:Shell International Chemicals B.V., Sk11 Researchwd Technology Centre Amsterdam, P.0. Box 38OfKi. 1030 BN Am&dam. The Netherhds. Tel: + 3120 630 299; fax: f 3 120 630 4037; e-mail: smithI @sic.skll.nl 0043-1648/97/$17.00

this rig, and in vehicle valve-train systems, arc patchy on the scale of a few microns, vary in thickness over the l&l00 nm range, and are chemically complex in a manner which also depends on the surface topography [ 3 3, Their surfacemicrochemistry is a function of the operating conditions and the

lubricant formulation. Whilst studies of the dependence of macroscopic friction and wear performance on lubricant formulation have been carried out extensively, the relationship between the microscopic chemical and tribological properties of the heteroge-

neous films and their macroscopic lubrication performance is pa~rly understood, The aim of the present work is to explore the use of scanning probe microscopy techniques to provide new information on anti-wear films formed on realistic tribological specimens, to measuhe their microscopic frictional properties and to compare with the macroscopic behaviour, In the following we first present high resolution atomic force microscopy ( AFM) [4,5] topographic images from Amsler test specimens in air, and we report evidence, from images and from forctiistance curves, for the presence of soft surface overlayers in some cases. Secondly, we apply lateral force microscopy (LFM), sometimes called friction force microscopy (FFM) I&12], to study the relative frictional

Q 1997 Shell ResearchLtd.Publishedhy ElsevierScience S.A. All rightsreserved

PIISOO43-1648(97)00081-I

A. J. Pidduck, G, C. Smith / Wear 212 ( i 3Y7) 2542rW

Table

t

Summary of the Am&r

block samples. AFM measurements made, and some results from imaging in air. In all cases: base. refined mineral baseail: ZDDP.

zinc dialkyl dithiophosphate anti-wear addirivc: det. detergent; disp, dispersam: and FMA Amsler specimen number 41 wa!s produced using a full cammercial engine oil Amslcr block numb83

AFM in air

41

c,

43 50 Y? 53 54

,, H H * H

70 74

H

AFM under liquid

r/

LFhJl in air

lf H ti

fl

93

LFM

c,

r/

Summary oil formulation

Fully fonntrlaccr! bw + det + dihp basef ZDDP hasc+ ZDDP basef ZDDP + dct + disp iraaiz: ZE!P + dctf disp base + ZDDP base f ZDDPi-

e fl

f and FMA2

under liquid

I, c,

c/

95

255

FMAI basei ZDDP f dct + disp base + ZDDP f FMAZ

properties of surface micro-regions of Amsler specimens prepared using different lubricanf formulations. We then compare atomic force microscopy ! AFM I and LFM results in air with measurements made in the presence of a model lubricant (dodecane) . Finally, we perform AFM and LFM in a mineral oil containing ZDDP to assess the potential for direct simulation of the tribological behaviour of a single asperity contact.

2. Experimental

arc organo-molybdenum friction modifying additives.

Range of pit depths measured by AFM (nm)

lo&230 240-520 120-280 100-200 100-140 40-90 MJ-ix

Very high initial AFM force

surface covered by micro-/nanoislands

v

Y Y

dheSiW

&@

_d

v

v

fl

50-100

c,

c/

3o-60

e

friction and hence give a fuel economy benefit. In this work, two experimental organo-molybdenum compounds were used as FMAs, knowc as FMAI and FMA2. The precise details of the oil formulations are not relevant to the work reported here. The Amsler specimens were produced using the range of experimental oil formulations shown in Table I. After production, they were stored under fresh refinedmineral oil to preserve the anti-wear film structures and washed with HPLC grade n-heptane solvent immediateIy prior to AF’M analysis. A cam follower taken from an engine after completion of a standard Sequence VE test [ 131 was also investigated.

2. I. Specimen preparutiun 2.2. Atomic force microscq~y the reciprocating Amsler tribo1ogical testing machine [ 2 1, the specimen slides over a rotating disc in a reciprocating motion which accurately reflects a critical region in a cam/ foliower system in an internal combustion engine valve train. The specimensand rotating discs were machinedfrom EN3 1 steel and heat-treated to control their hardness. It was intended thatthe discs should be significantly harder than the test specimen blocks, so that most of the wear and topography modification would occur on the test specimen blocks. Hardnesses of H”==820f 10 kg mm-* and H,=5OOf 10 kg mm-* were used for the discs and specimen test blocks respectively. Prior to use, the test specimen surfaces were ground to a smooth finish (centre line average roughness R, =O. 14 kO.02 pm j and oriented so that the direction of sliding in the test was perpendicular to the original grinding direction [ 21. Test oils were formulated using blends of refined mineral base oil with various combinations of ZDDP anti-wear additive, detergent, dispersant and friction modifying additives. In commercial oils, detergents and dispersants are added to maintain engine cleanliness and control the formation of deposits, but their action is known to affect the formationof protective anti-wear films. Friction modifying additives (FMAs) are included in the formulation to reduce engine In

Contact AFM was carried out, in air and liquids, using Digital Instruments Nanoscope III and Dimension 3OtKl Large Sample microscopes. Microfabricated 100 pm long SiJ, V-shaped cantiIevers (theoretical bending force constant 0.4 N m - ’ ) with integrated pyramidal tips were used. AFM images were obtained under constant force conditions, where the cantilever z-deflection signal, sensed by opticaf beam deflection, is maintained nearIy constant by a feedback loop coupled with the z-axis piezoelectric scanner. The calibrated z-piezovoltage as a function of X-J’ position then directly represents the surface topography. Force curves (cantilever spring force as a function of zpiezo extension} were also determined at fixed positions on the specimens. A typical force curve obtained in air is shown in Fig. 1(a). Away from the specimen the lever maintains its free deflection, +:;A first makes surface contact at S ( “snapin”). In air, this initial contact is frequently with a surface layer of adsorbed water vapour or other contaminants, and is accompanied by formation of a meniscus around the tip surface contact. Retraction of the lever results in an increasing canti1ever spring force acting against the meniscus-related and other adhesive forces, until the “pull-off’ force (P) is reached, when the cantileverjumps back to its freedeflection

2%

A.J. Pidduck G.C. Smith / Wear 212 119971254-264

-7s

0

76

slm* Fig. I. AFM Furce-distancc

cures

obtained

2-b in

air from Art&r

-*so (nm)

120

60

0

Sampla Zposltlon

40

420

(nm)

specimens (a) 54 and (b 1 70. The applied force has been calculated from he lever deflection

signal assuming a force constant of 0.6 N m- ‘_

Additional effects are seen in some cases, for instance Fig. I (b). Hysteresis during contact indicates that some plastic deformation has occurred whilst load was

position.

lm::to the oresence of a relatively soft applied to the surfiic, dU & surface film. The value of tbe force constant was not independently verified in this work, and may vary between levers. Therefore whilst comparative force measurements between samples using the same lever are valid, force values measured with different levers can be only be compared qualitatively.

2.3. Lateral force microscopy LFM was carried out by scanning along an axis perpendicular to the long axis of the cantilever. The lateral force acting between the tip and the surface applies a torque to the cantilever, which, in optical beam deflection AFM using a fourquadrant photodetector, is detected simultaneously with collection of topographic data [ 6121. The lateral force depends on both tip-surface friction and surface slope. The lateral cantilever deflection signal due solely to friction was

obtained by selecting microscopically smooth areas and measuring the difference (offset) between forward (F) and reverse (R) LFM linescan signal levels. It was usual to allow a short period for the LFM signals to stabilise before recording the linetraces, which were measured as a function of applied cantilever spring force. Pull-off force and free fever deflection signal were determined, from force curves, immediately before and after the LFM tneasu~ments. Torsional force constants for the V-shaped cantilevers used are unknown, and thus absolute lateral forces were not determined. Nevertheless, slopes derived from plots of the lateral force signal (LFM F-R offset) vs. total normal force (applied force + pull-off force) are proportional to a local coefficient of friction. Measurements of relative friction coefficients at different positions on different specimens with the samt tip are valid so long as the nature of the tip and lever can be shown not to have changed during the measurements, and this is the approach which has been adopted in the present Work.

LFM images presented were obtained by subtraction of LFM data in F and R scan directions. The mean signal level was removed from each scan line in the subtracted LFM

image, which then purely shows the variation in lateral force signal around an arbitrary reference plane.

,7; Resultsand

discussion

3. I. AFM imaging in ambient atmosphere Specimens were initially examined by optical bright-field and Nomarski differential interference contrast microscopy to assess their overall uniformity. Under bright-field conditions, the specimen surfaces often appeared covered by microscopic patches of brown or blue coloration, many showing a micmn-scale texture, and interspersed by occasional brighter streaks extending along the sliding axis. AFM images were subsequently obtained for square areas of l-50 Frn side at scan rates of l-2 Hz. Generally the large area AFM images showed long smooth features aligned along the sliding direction interspersed by various densities of micron-sized pits. The bright streaks observed by optical microscopy correspond with the largest elongated smooth regions. Fig. 2 shows typical images obtained near the centreof specimen 53, comparing relatively smooth and pitted areas. The images are shown as grey-scafecoded height maps with the scale from darkest (low height) to brightest (highest points). Rms roughnesses of pitted and smoother regions, evaluated over IO0 pm* areas, were about 30nm and 8 nm respectively. Smooth patches several microns across can be seen, with sharp drops to a consistent lower height level between them. We interpret the smooth patches as the top of the anti-wear film, with the dark areas between them being areas where the anti-wear film has either failed to form, or been damaged to expose the metal substrate. Measurements of the pit depths then gives an estimate of the f&n thickness. This was consistently found to be in the range 10% 140 nm for specimen 53, in agreement with the results of previous electron microscopy estimates [ I 1. Fig. 3 shows a pitted region on specimen 53 in more detail. The anti-wear film surface is flaking in several places, confirming that local damage is occurring, and some debris is present. Small pits of various sizes are evident, possibly at different stages of formation. The close-up of the large pit in Fig. 3(b) shows the pit base to be very rough. In contrast,

A. J. Pidduck, G.C. Smith / Wtw 2 I2 I I P~7)2$$-264

Im

0

10

20

30

0

30

90

20

30

Irm

(rm

Fig. 2. Representative AFM images and cross-sections from near the center oi Am&r +L --kc:: 9 _ , with the tip positioned on (a) a typical region. and (b) a region appearing PSDbrightstreakunder the optical microscope. Arrows denote the cross-section positions. Tbc Amskrslidingclinch ibwiiC;;;i.

6

WInm

(4

0

3

2

4 Clm

a

i

i (rm

Fig. 3. AFMimages from a microscopically pitted area of Amsfer specimen 53. Theslidingdirectionis vtnical.

the anti-wear film surface, as shown

by Fig. 4(a) (with the tip positioned on a bright streak), can be very smooth. The surface texture comprises shallow (often sub-nm depth) parakl grooves, with spacings down to 3CL50nm. Some surface debris is also observed. Although the results described for specimen 53 were typical, some differences were seen on other Amsler specimens. Table 1 compares pit depth measurements from the different samples. Interpreting these as local film thicknesses, the results indicate substantial variations from position-to-position on the same sample, as well as from sample-to-sample

prepared with the same nominal oil formulation. There is however some evidence for a systematic dependence on formulation in that the largest values were obtained from a base oil without ZDDP, and the smallest values when components in addition to ZDDP were incorporated. Table I also summa&s effects encountered during AFM imaging. In some specimens the tipsurface adhesion on first

contact exceeded the pull-off force which could be applied by the AFM ( I pN approx.) preventing reliable imaging. Extended heptane washing (up to 1 h) was generally successful in removing the liquid-like film believed responsible for this effect, AFM pull-off forces measured, afterextended

solvent washing as necessary, were then in the range 15-200 nN. Several specimens appeared to be covered by a high density of micro-or nano- islands (mostly of height 30 nm or less) which were swept aside by the AFM tip under certain conditions. Smearing was observedat higher applied forces, as used for LFM measurements, indicating islands to be re!atively soft residues, rather than simply poorly adhered particulates. It is probable that these are the origin of forcecurves of the type in Fig. I (b), The interaction between the tip and

the surface deposits (not removed by extended beptane rinsing) is shown in Fig. 5(a), which was acquired from Amsler specimen 74 immediately after AFM and LFM imaging of smaller 3 pm scan areas with higher applied cantilever spring

258

Fig. 4. (a) Smatl area AFM image from a bright streak on specimen 53. The sliding dircclion is vcnical. (b) LFh+Icontrast over

thesameareaas in (a) +

Fig. 5. (a) 10 pm AFM image frttmAmsler specimen 74 after AFM and LFM imagingover 3 pm areas. The AFM scanning directionis harizonhl. and the Amsler wear direction Iflo from horizontal. (b) 3 pm LFM image obtained immediakly before (a). with ;Ltotal normal force of 580 rN The lower half of (bl had previously been scanned at forces

upIu 760 nN during LFh+llinescanmeasurements.whilstthe upper half had not been previouslyscanned.

force. The areas of the previous scans (such as shown in Fig. 5(b) ) have been swept clean of the soft deposits. Small island features were not observed previously in scanning electron microscopy (SEM) images of these specimens. It was thus concluded that the AFM measurements in the ambient environment were more sensitive to surface residues than the vacuum techniques such as SEM or X-ray photoelectron spectroscopy (XPS). To test this, one sample showing the sticky surface layer effect was left under vacuum

overnight prior to AFM imaging, but the contamination remained unaffected. This layer, which could be slowly removed by extended heptane washing, ruling out solvent contamination, presumably either originates directly from the Amsler test conditions or indirectly from subsequent ageing during storage in liquid or air. It was observed on specimens

in which the base oil was the only common component, and thus is probably hydrocarbon-derived. Lubricant residues may form by surface segregation of higher molecular weight compounds formed in the oil or by direct surface reaction and degradation under local extremes of temperature and pressure [ 141. Clearly, there is an optimum balance to be achieved between cleaning the specimen sufficiently to obtain good contact

AF’M images,

and over-cleaning

using aggressive

solvents which may damage the structure of the original antiwear Mm. AFM measurements were also made from the surface of a cam follower taken from an engine which had completed a standard industry test ( Sequence VE [ 131) Optical microsl

copy showed a clearly ridged topography

rather different from any of the Amsler test specimens, with a much greater height difference between peaks and valleys on the surface. Fig. 6 is a typical AFM image from the cam follower, show-

ing the surface ridges, the absence of microscopic pits, and the presence of small particles or islands on the surface, which can be smeared by the tip during scanning. Using “tapping mode” conditions, where the lever vibrates vertically during scanning, making only intermittent contact and avoiding shear forces Wween tip and sample, the surface islands were imaged without alteration. However, although tapping mode gave good surface images in air, it is not compatible with lateral force microscopy and friction force detemination, and

was not used further in this study. 3.2. LFM measttrements irt ambient atmosphere Initial measurements Typical

LFM linetraces

were made on Amsler specimen 53. along a single X4 pm line on a

A.J. Pi&it&

G.C. Smiih /Wear

212 (1947) 2%-2&f

259

A series of comparative LFM measurements were then made using Amsier blocks 54, 70, 74 and 95 with a single

Si,N, AFM tip, with the aim of comparing quantitatively the micro-frictional properties of the different specimens. Block

54 was used as a cuntrol sample, as it was produced using the same reciprocating Amsler conditions and lubricant as 53. which had given successful AFM and LFM results. The measurement sequence was 34 3 74 4 54 + 70 + 54 + 95 * 54. in each case, AFM and LFM images were made at a single

position chosen using the AFM optical microscope, force of forward-reverse LFM offset signals made as load was applied and then removed, the force curve repeated,and a topographicAFM image recorded to check for any surface damage. Fewer LFM data pints were measuredthan in the case of specimen53 in or&r to avoid risking the tip (which would invalidate the comparison) during scanning at higher applied loads. As for

curve measured, a series of measurements

53, LFM F and R direction linebaces were largely parallel in most cases, indicating predominantly uniform surface friction. Significant frictional variations were however freFig. 6. Reprc~ntarive AFMimageand cross-section from the surface of the cam follower. The wex directionand IIWAHM scanningdirectionare verlical in this image. Arrows denote cross-se&u

position.

quently associated with surface islands. In the case of specimen 74, LFM linetraces, as in Fig. 9, showed two levels, that with the larger F-R offsets being associati with the nanoislands. In conjunction with the AE’M image in Fig. 5 ( a), the LFM image in Fig. S(b) shows how these were smexed and swept aside at the highest applied toads. Slopes and intercepts determined from the LFM offset data versus total (applied + puil-off) force by linear regression anaiysis, are summarised in Table 2. Table 2 shows several important results. The measurements on specimen 54 are repeatabie, so long as the same type of area is chosen for the analysis. In this case, points close to one another on a region appearing brown under the optical microscope gave consistent resuIts. The precise nature of the anti-wear film corresponding with thiscoloration is not

known. The reproducibility of this result shows that the

Fig. 7. LFMforward( F) and reverse( R1linetracedala froma smootharea of Amsler specimen 53 at toral normal forces of (a) 22 nN and (b) 230 nN.

smooth region of Amsler specimen In general the LFM linemces

53 are shown in Fig. 7. were largeiy parallel, indicating

generally uniform surface friction. This is emphasised by the image of LFM signal variation in Fig. 4(b) which shows

little contrast except around highly sloped features such as surface islands. Friction signal (LFM F-R offset) is plotted versus total normal force (applied force+pull-off force) in Fig. 8. The behaviour appears to follow a straight line, with a slope of approximately 3.8 mV nN- ’ . The intercept is close to zero net force.

50

f00 f

150

200

250

ORCE [nN1

Fig. 8. LFM F-R offset signal (V) vs. total nocrnal force (nN) for a smooth part of the anti-wex film on Amsler block 53 in air. Different symbols denotedifferent foad-whadcyclcs, Fpdenotesthemeasuredpulhff (adhe-

sive) force (95 nN).

Table 2 Summary of LFM data from the analysis ~quence of .2mslcr blocks S-l. 74.70

and 94 in air

Opt ical appearance of position selected

Pull-off

LFM

LFM

force aftrr LFM

slope IX I04V

intercept (nN1

for LFM

(nN)

nN_‘)

51 51

blur area. posn. 0 brown urca. posn. I

45

4.1

+

19

0.949

0.120

35.0

5.5

+-W

0.973

0.120

-ls.ti

74

briphl stripe

30 105 lower LFM offs&?ts

2.4

0.86 I 0.952 0.9fH 0.939 0.977

53.3

9.1

121 tl-4 +60 +%I 4-53

0.045

higher LFM of&Is 18 36 18

0. ! 20 0.093

5 I .7 43.0 50.0

0.867

0.047 0.120

Sample (inordaof measurement

I

54

brown. posn. 2 shiny brown stripe brown posn. 3

70 54 9s 54

brown area brown pan. 4

6.2 4.0 6.0

2.1 5.5

170 30

-

-100 -ss

LFM regression coeff.

0,972

End-of-lest Amslrr friction co&.

LFM alopdp

($1

nN ‘1

0.12I)

( x IO’ v

Sl.l 45.H

coefficients indicated a reasonably good linear proportionality between the friction signal and the norma force. although

(a)

3

intercept values both signiticantly above and below zero total force were obtained. Plotting the combined data for Amsler specimen 54 in air (all four data sets) on log-log axes and performing a linear regression gave a proportionality of lo& LFM offset) ar ( I .032 & 0.040) log( load). with a regression coefficient of 0.958. A linear relationship between the friction signal (the LFM

offset) and the total load could arise from a plastic contact or a multi-asperity elastic contact. whereas a 213 power law dependence is predicted for a Hertzian sphere-flat single asperity elastic contact [ 15-I 7]. Significant plastic deformation of the sample in AFM is usually obvious by a change

Fig. 9.

i a) Height

( AFM 1 and ! b) LFM iinctrxe

drttu from a smooth ~rca

of Amslcr specimen 74 in air un&x ;t total normal f&rce of 400 nN. Table 3 Comparison of micro-friction coefticicnts determined by LFM for rhc zcas cnumined, and end-of-test macro-friction coefticicnts from the rcciprocaring Amrler lest Specimen

LFM

c-o-t

micro-friction

Amsler friction

LFM/macro

corfticienr.

coefticicnt

cwfkicnt

5-t ( brown aria) 74 (low) 70 95 52 ( low loi 1

0.16 0.06 0.10 0.06 0.17

0.120 O.MS 0.093 0.047 0.120

1.33 1.33 I.08

( high load 1

0.46

ratio

1.28 I.-I?

behaviour of the microscope, lever and tip combination has not changed significantly during the measurements under

applied load. It also implies that areas of similar appearance show similar friction properties. In all cases the regression

in the imaged surface topography as a result of scanning, whilst blunting of the tip. caused by wear of the tip or surface. leads to a deterioration in image resoIution and an increased pull-off force. Although there have heen reportsof the fatigue wear of Si,N, tips without apparent blunting [ 1S], the sim-

plest explanation for the reproducible behaviour observed on block 54, with no sign of the above effects during the extended series of measurements, is that mostly elastic deformations are involved. The AFM tip-specimen surface contact may, due to atomic-scale roughness, be strictly multi-asperity. The presence of load-bearing molecular layers in the tipsurface interspace, some evidence for which has been shown. would also affect the apparent power-law dependence of the lateral force [ 17,191. Table 2 also gives the macroscopic end of test ( e.o.t ) friction coefficient measured during the Amsler test. This is an average over the whoie sample surface and is generally found to be reproducible from rest-to-test within approximately IO%. Except for the area of blue appearance on specimen 54

and the higher data on specimen 74. the ratio between the LFM slope and the e.o.t friction coefficient is quite constant at an average value of 49 mV nN - ‘. This consistent propor-

tionality suggests a link between the macroscopic friction vahtes and the microscopic frictional behaviour of smooth regions of the anti-wear film. Table 3 compares approximate

261

Aif (ai

Fig. IO.

Dodscane 5Onm

( a1Comparison of AFM images from the sumc smooth arca of chc surface of Amsler spccimcn 52 obtained tirstly ( h ) subsequent LFM image in do&cane under an xpplied nrH-nlal force al- I80 IN

in air and

then under d4Idecane

using the smw tip.

plot slopes in this work. with macroscopically-determined values. The general level

values for CL,derived from the LFM

of contidence expected for the LFM measurement of p is f 50% and the mean ratio of the micro to the macro coefficients is I30%. Thus, within the certainty ofthe measurement. there is agreement between micro- and macro-scale values. The macroscopic measurement is. however. for a steel-onsteel contact (in the presence of the anti-wear film) whereas in the AFM it is for an SiJ$ tip on an anti-wear film/steel specimen, and so different coefficients of friction may be expected. Agreement between microscopic and macroscopic values suggests that the interfacial shearplane which predominantly determines friction. is the same in both cases. Our results suggest that the contacted surface in AFM is either the mechanically robust surface of smooth areas of the anti-wear film or an overlying molecular layer which may be displaced at the highest loads.

Table 2 also shows that the lower friction (LFM slope) values obtained with test specimens 74 and 95 are associated with relatively high pull-off forces. Low friction implies an easily sheared surface layer, whereas a high pull-off force implies either liquid meniscus formation or direct bonding between the tip and surface. in the former case. the liquidlike overlayer may be providing the readily-sheared molecular layer between the tip and surface. In the latter case, the

friction reduction may be due to sliding of crystalline planes such as occurs in graphite, or MO&. The formation of MO?& layers is one mechanism by which the Mo-containing friction reducing additives (present in the oil formulations used to generate specimens 74 and 95) are postulated to function. 3.3. A FM md L FM measurements under dudecane Amsler specimens 52 and 9.3 were imaged underdo&cane using the liquid cell of the Digital lnstnrments D3000 AFM. In the case of specimen 52, the same area was imaged first in air and then under dodecane. The images, shown in Fig. lO( a), are virtually indistinguishable, aithough the latera1 resolution is slightly better under dodecane. This is a result of the lower tipsurface force during imaging in the liquid ( 30 nN applied force in dodecane. as compared with I30 nN adhesive force in air) leading to less deformation and thcrcfore a smaller tip contact area. LFM images from hmsier specimen 52 in dodecane showed, as in Fig. lO( b), little frictional variation except around surface islands. The plot of LFM offset signal vs. applied load is shown in Fig. 1 i _The behaviour is linear and reproducible up to high loads, when a disproportionate increase in friction occurs. This may be due to removal of the last stage of an adherent lubricant boundary layer at the high-

A.4 Pidduck, G.C. Smith / Wear 212 (1#7)

2G!

254-264

est contact pressures, or may be a non-linearity in cantilever or detector response. Neverthetess, the result demonstrates,

at least at lower applied forces, the feasibility of relative friction force measurements on realistic l &bological surfaces under liquid. Force curves for Amslec specimen 52 under dodecane showed pull-off forces always below 3 nN. Occasionally, as shown in Fig. t2( a). discr&e force steps were observed.

These suggest displacement of distinct monolayers of liquid as the tip approaches the hard surface of the specimen. The result is similar to that observed by O’Shea et al. [ 201 in work on the idealised system of n-dodecanol liquid over a Fig. 11.lAcd force signal vs applied load for Amler specimen 52 measured under cLockcane.Arrows denote the Id-untd sequence.

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mica surface, where force oscillations on a 3,7 nm period were interpreted as displacement of dodccanol bilayers. In Fig. 12(a) the steps are less well-defined (as may be expected for a non-atomically smooth surface), but are consistent with displacement of layers of 1.2-l .5 nm thickness. In the case of Amsler block 93, the pull-off force in air was

too high to allow AFM imaging. However, the adhesion forces after immersion in do&cane were low (always 5 10 nN). In addition to the typical liquid force curves showing very small pull-off forces, curves having the form shown in Fig- 12(b) were sometimes obtained. On the tip approach stage, the first lever deflection (at 5’) occurred up to 100 nm away from the hard contact point ( ‘C’) of the specimen

surface. This behaviour is consistent with disruption of a very soft film contaminating either the surface or tip. Discrete steps

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in the pull-off force were then seen on tip retraction, the adhesion sometimes extending up to 200 nm from the hard contact position. This suggests elasto-plastic adhesion between the tip and surface, due possibly due to long chain (polymeric) molecules or a gel-like layer. This layer is likely to have been the origin of the large AFM adhesive forces on

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Srrnokz-P-b WI Fig. 12. Forcc-distmce curves obtained in dmkcane for Amsler block spcimens ( a) 52 and (b) 93 showing force-step bthaviour.

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force signal vs. applied load for the polished steel specimen mcpsuml u&r oil. Arrows denote the load-unload sequence.

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specimen 93 in air. Thus in liquid, AFM imaging is unaffected by this layer, whilst force curve measurements are able to probe its properties. 3.4. AFM/LFM experiments under he

oil with ZLN?P

The objectives of the experiments under oil were to establish the feasibility of imaging and friction force measurement, and to investigate the possibility of using the AFM tip to simulate a single asperity contact in a tribological situation, thereby providing a novel route to study wear and film formation processes. For these reasons, an alternative sample of a fresh diamond polished Amsler test piece was used. Prior to AFM imaging the specimen was ultrasonically cleaned in acetone and then in heptane, to mimic the cleaning procedure

used for test specimens prior to use in the Reciprocating Amsler machine. The specimen was first imaged in air and a topography of intersecting scratch marks, typical of mechanical polishing, seen. A few contamination spots were also seen, some of which appeared to smear easily under the action of the tip. The corresponding lateral force images showed uniform contrast, except at the contamination spots.

263

Height

Fig. 14. Height and tip deflection ( error signal 1 AFM imaps showingthe areasof AFM wear testing (two npprox.1 pm squares. offw ) on the polished steel specimenunder oil. Deflection image contrat is proportionalto surface slope. and tends to cmphasisc fincscale surface texture. Dashed white lines on the deflectionimage indicate tk approximateboundaries of the AFM wear-testedareas.

No difficulties were experienced in obtaining AFM images from the surface of the polished test piece under oil, even though the oil used was darker in colour and considembly more viscous than the dodecane previously used. Again, higher imaging resolution was obtained duz TVthe reduced tip-surface contact force. LFM images showed uniform contrast, without the small islands seen in the air images. The LFM signal F-R offset is shown plotted vs. applied load in Fig. 13. The LPM linetraces appeared slightly noisy due to a nanoscale polishing roughness (visible at the top of Fig. 14 for instance), and the LFM linetraces showed a more marked time dependence than usuai. On increasing the applied load the LFM offset first showed a rapid increase, followed by a slower decrease. On decreasing the applied load the LFM offset decreased slowly, to end at a value below the corresponding value obtained during the loading sequence. The reason for this time dependence is not known, but may relate to the presence of the anti-wear additive. However, at low loads the signal returned to levels simiiar to those initially observed. Following AFM and LFM measurement, the tip was used in simulated wear tests in oil on the polished Amsler block surface. Initially, a nominal scan size of 1 pm was scanned at 120 Hz for 15 min with an applied load of approximately 360 nN. In a second subsequent test a nominal scan size of 1 pm was scanned at 40 Hz for 50 minutes with an applied load of approximately 500 nN. A subsequent APM image with the two, slightly offset, heavily-scanned areas at the centre is shown in Fig. 14. Both height and deflection (error signal) images are shown. In the deflection signal image, which emphasises the finescale surface texture, slight modification of the two areas scanned is apparent due to local removal of the nanoscale background surface texture ( present over the remainder of the image). Careful comparisons of images of this region before and after the AFM wear test showed no evidence of any overall relative height change in the scanned region, within aconfidence limit of f 2 nm. Thus,

despite the polishing action, there is no evidence of further wear. The fine scale roughness outside the worn area is readily resolved by the tip, showing that the Si& tip has not been blunted during the experiment. In order to compare conditions in the AFM wear test with those in the Amsler test, we may crudely estimate the peak AFM local contact pressure, assuming an elastic Hertzian contact with a tip radius of I&50 nm and an applied load of 500 nN, to be of the order of 1GPa. in the Amsler rig running at 400 N load, the calcutated Hertzian contact width is 0.18 mm over an 8 mm length. With a semi-elliptical pressure distribution this gives a maximum pressure of 360 MPa, and a mean of 240 MPa [ 21, though peak stresses at asperity contacts may be several times higher. Despite the comparable contact pressures, there does not appear to be evidence for the presence of a reacted ZDDP-based layer in the APM images. This is perhaps not surprising given that the AFM test is carried out at room temperature with a maximum scan speedof0.2 mms-‘, whereas in a cam and tappet system in an internal combustion engine the parts may move with peak relative velocities of the order of several metres per second over several hours with oil temperatures in the range 80120 “C.

4. Conclusions In this work we have agqlied scanning probe microscopy ( SPM ) techniques to the important technological problem of the formation and action of automotive anti-wear fiIms, with the following conclusions. 1. AFM images from reciprocating AmsIer test pieces consistently reveal a smooth surface topography interspersed by micron-sized pits. Pit depths are in agreement with previous electron microscopy estimates of local anti-wear film thickness, and show evidence of a dependence on lubricant formulation. The images contain information on

264

A-J. Pi&u&

G.C. Smith/ Weur 212 (19971254-264

anti-wear film development. A real engine component was also imaged. 2. AFM allows surf&ce investigation with less perturbation compared with conventional vacuum-based surface analytical techniques. AFM uncler immersion in hydrocarbon gives higher resolution images than in air and removes the need for prolonged solvent rinsing. 3. AFM force-distance curves indicate theexistence of a soft viscous surface layer overlying the mechanicaI anti-wear film surface in many cases. Immersion in hydrocarbon allows the micromechanical properties of this layer to be directly studied, which may be important if it is related to the formation or action of anti-wear films [ 143. The most adherent residues, remaining after prolonged solvent rinsing, could k displaced by AFM scanning at the highest applied loads, and had acleareffect in the frictional hehaviour observed by LPM. 4. LPM can be used to determine relative frictioncoefficients on smooth microscopic regions of specimens prepared using differcut oil formulations if, by returning to a reference specimen, the frictional behaviour of the tip is fcund to be unaltered. The relative friction coefficients measured fkom our specimens by this means were found to be proportional to the macroscopic values measured at the end of the Amsler test. Absolute values were derived from the LPM data with a reduced degree of confidence. 5. APM scanning in base oil containing ZDDP, at high applied load and scan rate, ied to slight polishing of a steel surface by the SiJ’$, tip, but no evidence for the buiId-up of an anti-wear film, other than some hysteresis during LPM measurements. This could be because, whilst peak contact stresses during AFM may be comparable with those in a real engine environment, the l~al contact velocities, temperatures, deformations, strain rates and test times arc much lower. Overall, we believe that we have demonstrated that SPM holds great promise for gaining new insights into the microtribological and structural aspects of surface films, important in friction and wear, formed on realistic engineering specimens.

Adnowledgements Valuable discussioiss with J.C. Be11andG.W. Roper (Shell Research and Technology Centre Thornton) and R.E. van den Berg (Shell Research and Technology Centre Amsterdam) are gratefully acknowledged.

Rclereaces 1 i] P.A. Wilknnet. D.P. Dailey, R-0. Carter BI, W. Zhu, J.C. Bell. D. Park. T&ulogy Int. 28 ( 1995) 163-175. 121 G.W. Roper. J.C. Bell. paper SAB 952473, Society of Automotive Engineers Fuels and Lubricants Meeting and Exposition, Toronto, Ontario+ 16-19 October 1995. tqrintcd from Recent Snapshots and Insights into Lubricant Tribology, SAE special publication SF-1 I 16 (1995). [3] I.R. Batkshire. M. Rutton, CC. Smith. Appl. Surf. Sci. 84 ( 1995) 33 1-338. 141 E. Meyer. Rogr. Surf. Sci. 41 ( 1992) 3. (51 D.R. wr, B.A. Parkinson, Anal. Churn. 66 ( 1994) 84R-105R: Anal. Chcm. 67 ( 1994) 297A. [6] 0. Marti. Physica ScriptaT ( 1993) 599. 171 G. Haugstad, W.L.Gladfelter,E.B. Weberg,langmuir9 (1993) 3717. [ 81 S. Grafstrom, M. Neitzett, T. Hagen, J. Ackermann, R. Neumann, 0. Probsr. M. Wortgc. Nanotcchnology 4 ( 1993) 143. 191 S.Grafstrom.J.Ackerman n. T. Hagtn. R. Ncumarut,O. Frobst, J. Vat. Sei. Technol. B12 ( 1994) 1559. 1IO] M. Bingelli. C.M. Mate. Appl. Phys. Len. 65 ( 1994) 415. 1I I J B. Bhushan, J.N. lsraclachvili. U. Landman. Nature 374 ( 1995) 607. [ 121 B. Bhushan (ed.). Handbook of Micro/Nano Tribology, CRC Series Mechanics and Materials, CRC Rcss. Boca Raton, FL, 1995. 1131ASTM Sqmcc VE crankcase oil test, further details of which may bc fwnd in Refercncc Handbk for Crankcase Oils, Shell Additives, London. 1990. [ 141 SM. Hsu. Langmuir 12 ( 19%) 4482. [ 151J.D. Summers-Smith. An Introductory Guide to Industrial Tribology. MEP. London.1994. [ 161F.P. Bowden. D. Tabor. Friction and Lubrication. Methuen, London. 1956. 117 1 A-W. Adamson. Physica Chemistry of Surfaces. Chapt. XII, 15th cdn., Wiley, 1990. [ 181 A. Khurshu&v. K. Kate. Wltramicroscopy 60 ( 1995) ! 1. [ 191 J. Hu, X. Xiao, D.F. Ogletree, M. Sahneron, Surf. Sci. 327 (1995) 358-370. [ZO] SI O’Shea, M.E. Welland, T. Rayment, Appl. Phys. Lctt. 18 ( 1992) 224Q.