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Wear 271 (2011) 994– 998

Contents lists available at ScienceDirect

Wear

j o ur nal ho me p age: www.elsev ier .com/ locate /wear

hort communication

urface texture changes followed-up in real time during the initial wear transientf dry sliding of steel against several metals using laser light scattering

. Martinez-Fuentesa, I. Dominguez-Lopezb, A.L. Garcia-Garciab,∗

Universidad Politecnica de Queretaro, Carretera 420 s/n, El Marques, Qro, MexicoCICATA-IPN Cerro Blanco 141, Queretaro, Qro. Mexico

r t i c l e i n f o

rticle history:eceived 1 September 2010eceived in revised form 6 January 2011ccepted 6 January 2011

a b s t r a c t

Previous experimental work has shown the possibility of applying laser light scattering (LLS) methods tostudy the evolution of surface topography during the initial transient period in a wear experiment on apin-on disk apparatus. A specific set up of a laser source, pointing towards anywhere on the wear-trackof the pin on the disk, and a detector to sense the light scattered from the surface, located off the plane ofincidence, providing an LLS signal whose changes are related to the surface damage due to the pass of the

eywords:nitial wear transientLSunning-inin-on-disk

pin as it runs over the disk. When applied to monitor the surface changes during the running-in stage,the LLS signal shows a characteristic signature for each material and wear regimen. While typical surfaceanalysis using the pin-on-disk apparatus is done ex situ, after wear took place, our experimental setupis designed to detect the minute changes suffered by the surface of the disk due to mechanical contactwith the pin, showing the onset stages of wear during the initial transient, in real time. Ex situ analysisusing optical microscopy (OM) and surface roughness measurements provides an insight as to the eventsleading to the changes in surface texture observed in LLS.

. Introduction

Surface texture modifications may enhance or degrade the per-ormance of tribological pairs. Sometimes, at the beginning of aear test, a running-in period has been observed, where incipi-

nt wear is present at small, and constant wear-rate. Running-ins considered as a transition from the time a system shows a highriction coefficient and severe wear, to the condition of low-frictionnd minimal wear [1]. In some cases, the initial state does not reachtability, turning into catastrophic wear.

During a wear test the change in surface texture is caused by sev-ral wear-mechanisms acting either simultaneously or separately.t is well known that during sliding-wear adhesive, fatigue and cor-osion wear take effect. Characterization of this process can only beone through the analysis of changes occurred on the sample sur-ace. For the pin-on-disk apparatus, this analysis is performed exitu, after causing enough damage to the sample surface to be able to

se gravimetric methods in order to determine the amount of mate-ial removed. Also, it relies on the measurement of the transversalross-section of the wear track made by the pin on the disk surface,sing conventional surface roughness measurements to estimate

∗ Corresponding author. Tel.: +52 442 2290804x81024.E-mail addresses: victor.martinez@upq.edu.mx (V. Martinez-Fuentes),

dominguezl@ipn.mx (I. Dominguez-Lopez), agarciag@ipn.mx (A.L. Garcia-Garcia).

043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2011.01.053

© 2011 Elsevier B.V. All rights reserved.

the volume of the material removed for the purpose of calculatingthe wear-rate. Wear on the pin-tip is also determined, accordingto the ASTM G99-05 standard. Obviously, this method does notallow for the observation of the dynamics of incipient wear at thebeginning of the test, determinant of the ulterior behavior of thetribological system, which enhances the relevancy of studying theevolution of wear since the initial stages. This task requires the useof non-invasive techniques, able to probe the surface under testingin situ, in real-time. One of these, proven to be fit for the intendedpurpose, is known as laser light scattering (LLS), which forms partof a family of optical techniques widely used for surface texturecharacterization [2–4].

For the present application, LLS is used to correlate the minutechanges occurred over the sample surface, caused by the loaded pin,with the change in intensity of the scattered light at a fixed positionof a light detector, located somewhere off the plane of incidence,as reported by Domínguez et al. [5]. When light is incident on arough surface, it may be reflected specularly, diffusely or both. Theproportion of diffuse to specular reflection depends on the relationbetween the roughness of the surface and the wavelength of theincident radiation. Upon contact with the pin, at the beginning of

a pin-on-disk wear test, the height and slope of the asperities onthe pin and disk surfaces change at the point of contact. Duringthis period, the changes on surface texture, characterized by theheight of the asperities, are of the same order of magnitude as theinitial surface roughness (<0.3 �m). Due to the pin-on-disk contact,

V. Martinez-Fuentes et al. / Wear 271 (2011) 994– 998 995

Table 1Materials used for sample disks. The reported roughness (Rq) was measured aftergrinding.

Materials Diameter (mm)a Thickness (mm)a Rq (�m)

Bronze 65 10 0.12 ± 0.01Steel AISI 1018 100 10 0.13 ± 0.01

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Aluminium 100 10 0.26 ± 0.02

a Nominal.

owever, the slopes of the asperities show major changes whichre easily detected by optical means via the measurement of theight scattered away from the surface at the point of incidence, asetermined by Martinez-Fuentes [6] and Martinez-Fuentes et al.7]. Most LLS applications are focused on correlating the LLS signalith surface texture on a static surface (rms roughness Rq < 1 �m),

ut not with the rapid changes in surface texture caused by wearue to dry-sliding, during a pin-on-disk test.

The expediency of laser light is the non-invasive way in whicht allows indirect observations of the evolution of surface texturesing dynamical parameters for wear characterization [8–13]. Theurface roughness measurement is extracted from the spatial inten-ity distribution of laser light scattering from the surface to benspected as has been done for standards of roughness [8] and

achined surfaces [9]. A previous attempt to use laser light totudy changes on surface texture on a pin-on-disk apparatus waseported by Patton and Zabinski [10], who tried to monitor theexture changes in the wear track using the specularly reflectedeam instead of the scattered light field. However, the intensity ofhe light reaching the detector was too-high to show the desiredffect.

The combination of experimental techniques developed by theuthors provides a more sensitive way to detect the minute changesn surface texture at the onset of a wear test carried on in ain-on-disk apparatus, wear, without interfering with the devel-pment of the test. Furthermore, with this method is possible totop the test at known points on the LLS vs. time curve to furtherharacterize the changes on the worn surface in order to make a cor-elation between changes in surface texture and wear mechanismsnd regimes. These are the main advantages of the experimentalethod described in this work.

. Experimental procedure

.1. Sample preparation

Experiments were performed on three different materials, listedn Table 1. Sample disks were cut from as-received commercialars. The disks were turned and ground using subsequently higherrades of abrasive paper, from 120 to 600 grits. After grinding,urface rms roughness Rq was measured using a Mitutyo SJ-400tylus profiler with 0.8 mm sampling-length and 4 mm evaluation-ength. Roughness measurements were performed complying withhe ISO-4288 standard. At least five measurements were taken,t different radii on the sample disk. Average rms roughness and

ncertainty measurements for each material specimen are indi-ated in Table 1. In all cases, Rq was kept bellow the ASTM G99-05equirement (0.8 �m).

able 2ptical parameters for LLS measurements.

Laser source to point of incidence Angle of incidence D

L = 20.0 ± 0.2 cm � = 45 ± 2◦ D

Fig. 1. The LLS system consists of a laser source (L) and a light detector (D), bothlocated on the plane of incidence at angles � and with respect to normal N,respectively. The plane of incidence is tangential to the wear track on the disk (2) atthe incidence point, opposite to the pin (1). The disk rotates at constant ω0.

2.2. Optical arrangement

The optical arrangement for the LLS technique consists of a lasersource (Lasermate, � = 630 nm), a 3.5 mm silicon light-detector(UDT, model UV-035), and a mechanical stand to hold them bothin position. Such an arrangement was adapted to an in-house builtpin-on-disk apparatus, schematically shown in Fig. 1.

The mechanical stand provides all the necessary degrees of free-dom to place the laser beam anywhere on the sample disk, withthe incidence point opposite to the contact point of the pin onthe disk. The plane of incidence is tangential to the wear trackat the incidence point and perpendicular to the plane formedby the normal N and the pin. The laser (L) and light detec-tor (D) were positioned in the plane of incidence 20 cm awayfrom the incidence point, at angles � and from the normal N,respectively. An operational amplifier TL-071 was used for signalconditioning, in order to use phase sensitive amplification tech-niques to increase the signal-to-noise ratio. A lock-in amplifier(Stanford Research Systems, model SR830) was fed with the light-detector conditioned signal. Data acquisition was done over theRS232 interface of the lock-in amplifier. The optical parametersfor the LLS experiments are indicated in Table 2. These parame-ters were selected as optimum values using the model of LLS ofMartinez-Fuentes [6].

2.3. Wear apparatus

The in-house built tribometer, described elsewhere [5], is

of an optical encoder located at the end of the disk axis and anelectronic controller, in the range of 0–600 rpm. The disk is held inplace using a three-jaw chuck. The frictional force is measured by

etector to point of incidence Angle of detection of scattered light

= 20.0 ± 0.2 cm = 52 ± 2◦

996 V. Martinez-Fuentes et al. / Wear 271 (2011) 994– 998

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ig. 2. Typical LLS vs. time graph of dry sliding of steel/bronze at 0.88 m/s and 2 Noad.

eans of a linear velocity differential transducer (LVDT) that senseshe flexional displacement of the mechanical arm holding thein.

.4. Experimental measurements

All measurements were performed under existing laboratoryonditions of temperature (24 ± 1 ◦C) and humidity (58 ± 3% HR).ll three materials were tested the same load (2 N) and linear veloc-

ty (0.88 m/s). Computer data acquisition was performed at a ratef 2 points/s. For the pin tip, a 7.15 mm diameter steel ball bear-ng was used. The wear tests were performed on the same disk,djusting the angular velocity to keep constant the tangential veloc-ty. Following the wear test, wear tracks were characterized usingrofilemetry and OM.

. Results and discussion

.1. Steel/bronze

Fig. 2 shows a typical plot of the LLS normalized-signal vs. num-er of cycles, obtained for the steel/bronze counterparts. The initialounting starts at the time the pin is set on the disk. The arrows indi-

Fig. 4. Surface roughness measurements and microscopy

Fig. 3. First derivative of the LLS signal vs. cycles for different runs of the steel/bronzeexperiments.

cate positions of interest where the rate of change of the LLS signalindicated an obvious change in surface texture. For every experi-mental point a fresh wear track was started on the same disk. Thetrend of the LLS signal was repeatable in general terms, as shownin Fig. 3.

For a short period of time, about 700 cycles, the LLS signaldecreases from the initial point indicating the average smoothing ofthe surface on the wear-track, which according to Fig. 4a is barelya few micrometers deep and about 100 �m wide. After reachingpoint A, the LLS signal increases rapidly, as the surface deterioratesdue to wear, reaching point B in Fig. 2. At this point, however, thewear track is also a few micrometers deep and about 0.5 mm wide(Fig. 4b). At about 1300 cycles (Fig. 2) a peak is observed in the LLSsignal, indicating again a smoothing of the surface, due to strainhardening of the bronze surface. Finally, at point C in Fig. 2, at about1700 cycles, the LLS signal starts increasing again, consistent withthe damage suffered by the surface. Still, after 1700 cycles, the wear

track is still a few microns deep and about 0.7 mm across (Fig. 4c).Notice how after point C the LLS signal shows a structure, a seriesof humps, indicating reversal in surface texture change, giving riseto running-in stages.

of the wear track at the points A, B and C in Fig. 2.

V. Martinez-Fuentes et al. / Wear 271 (2011) 994– 998 997

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the LLS signal, almost at a constant rate. As shown in the micro-

ig. 5. Typical LLS vs. time graph of dry sliding of steel/steel at 0.88 m/s and 2 N load.

.2. Steel/AISI 1018 Steel

Fig. 5 shows the results obtained for the Steel/AISI 1018-Steelystem. In this case, a slight decrease in the LLS normalized sig-al, resulting from the smoothing of roughest surface peaks, is

ollowed by a quick raise of the LLS signal at point A, about 700ycles after the beginning of the test. The profile ran across theear track barely shows any damage on the surface, but is more

isible under the optical microscope (Fig. 6a). After point A, the LLSignal grows steadily up to point B, where the LLS signal decreasesnd reaches a plateau, at about 1200 cycles. The width of the trackas grown now to about 0.4 mm and is <3 �m deep (Fig. 6b). Thisattening of the LLS signal is probably due to work-hardening ofhe surface, which is associated with extensive plastic deformationf the microstructure evidenced as elongated grains or low-angleub-grain boundaries, as shown by Barceinas-Sanchez and Rain-orth for other material [14,15]. Cold working generally results on

higher yield strength. At about 3800 cycles, a new jump in the

LS signal is indicated at point C. The profile measurement shows aear track about 0.6 mm wide and <2 �m deep, with a modulation

n peak height across the width of the track (Fig. 6c).

Fig. 6. Profilemetry and microscopy of the we

Fig. 7. Typical LLS vs. time graph of dry sliding of steel/aluminium at 0.88 m/s and2 N load.

3.3. Steel/aluminium

Steel/aluminium is a typical example of adhesive wear. Fig. 7shows the LLS signature obtained for this system. Immediately afterthe pin is set on the rotating aluminium disk, a rapid raise in theLLS signal is observed, which changes at point B. Profilemetry andoptical microscopy were performed at point A, after about a 100cycles, just to get an idea of the appearance of the track beforereaching the B point (Fig. 8a). At point A the wear track showsa more disrupted surface than for bronze and steel surfaces. Themicrograph in Fig. 8b, taken just before the “knee bent” shows awear track more than 0.8 mm wide, with an inner surface severelydamaged completely destroyed, but rather “flat”, the slope distri-bution being more or less uniform. Towards the end of the run, atpoint C (Fig. 8c), surface roughness measurement shows an increasein the roughness of the inner surface, but a homogeneous dis-tribution of slopes, which accounts for the uniform decrease of

graph of Fig. 8c, where a big flake of aluminium has been removed,adhesive wear and fatigue are the main mechanisms operatingin the tribological system at this point, in contrast with the two

ar track at the points A, B and C in Fig. 5.

998 V. Martinez-Fuentes et al. / Wear 271 (2011) 994– 998

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revious systems, where the main wear mechanism is surfaceatigue.

. Conclusions

The application of LLS – in conjunction with profilemetry andM – provides useful information for the understanding of the wearrocesses that take place during the initial wear transient of dryliding in the tribological systems studied.

By following-up the dynamics of the LLS signal, it is possible todentify the moments at which major changes in surface textureccur, as shown in Fig. 3. The wear test can then be stopped toarry on ex situ characterization of surface topography using pro-lemetry and OM. This information is essential to know if one is toetermine the wear-mode causing the texture changes observedia LLS during the early stages of wear.

The three materials under study produced LLS signals quite dif-erent from one another. The correlation of the LLS signal withrofilemetry and OM measurements indicate a fatigue wear modeominating during the early stages of wear in the case of bronzend steel. For aluminium, however, the observations indicate that

combination of adhesive and fatigue wear takes place during theumber of cycles studied.

One of the main benefits of having LLS signals during a wear tests the short time required to obtain useful information. It shoulde desirable when studying thin films – solid or liquid – on metalubstrates. An extension of this technique to materials other thanetals should be possible with an appropriate change in laser

ource wavelength and light detector. Also, it should be possibleo compare the performance of different tribological pairs in lessime than the standard currently used for a pin-on-disk wear-test.n this case, the use of the derivative of the LLS signal represents aetter choice.

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ar track at the points A, B and C in Fig. 7.

References

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[2] D.J. Whitehouse, Handbook of Surface Metrology, IOP Publishing, USA, 1994.[3] D.J. Whitehouse, Handbook of Surface and Nanometrology, IOP Publishing,

London, 2003.[4] S.I. Chang, J.S. Ravathur, Computer vision based non-contact surface rough-

ness assessment using wavelet transform and response surface methodology,Quality Engineering 17 (2005) 435–451.

[5] I. Domínguez, J.A. Huerta, R. Montes, J.D.D. Ortiz, J. Pichardo, A.L. García, M.Aguilar, D. Jaramillo, C-7 Mediciones de cambio de intensidad en luz láser espar-cida, aplicada al monitoreo de desgaste. Simposio de Metrología 2006, CENAM,Querétaro, México.

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[9] R.S. Lu, G.Y. Tian, On-line measurement of surface roughness by laser lightscattering, Measurement Science and Technology 17 (2006) 1496–1502.

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13] J.C. Le Bosse, G. Hansali, J. Lopez, J.C. Dumas, Characterisation of surface rough-ness by laser light scattering: diffusely scattered intensity measurement, Wear224 (1999) 236–244.

mation at the Surface of a Worn Zirconia, EMAG97, Cambridge, 1997, pp.499–502.

15] J.D.O. Barceinas-Sanchez, W.M. Rainforth, Transmission electron microscopystudy of a 3Y-TZP Worn under dry and water-lubricated sliding conditions,Journal of the American Ceramic Society 82 (6) (1999) 1483–1491.