Supporting InformationWear particle dynamics drive the difference between repeated and non-

repeated reciprocated sliding

Feng-Chun Hsia1,2, Fiona M. Elam1, Daniel Bonn2, Bart Weber1,2 and Steve E. Franklin1,3,4*

1 Advanced Research Center for Nanolithography (ARCNL), Science Park 106, 1098XG

Amsterdam, The Netherlands

2 Van der Waals-Zeeman Instituut, IoP, University of Amsterdam, Science Park 904, 1098XH

Amsterdam, The Netherlands

3 Department of Materials Science and Engineering, The University of Sheffield, Sheffield S1 3JD,

UK

4 ASML Research, De Run 6501, 5504DR Veldhoven, The Netherlands

* s.franklin@arcnl.nl

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Fig. S1 The friction force measured during the first 12 cycles in repeated and non-repeated

experiments, emphasizing the run-in behavior in the repeated experiments. (a) Sapphire-on-

Si wafer and (b) SiC-on-Si wafer experiments.

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Fig. S2 Third body formation during non-repeated sliding. A thin third body that could be

washed off by sonicating the sapphire sphere in acetone was observed on the wear scar on the

sapphire ball after a non-repeated experiment. The friction behavior for this particular

experiment is consistent with that observed in all repeated experiments. Scale bars, 50 µm.

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Fig. S3 Friction measurements for non-repeated sliding of sapphire-on-Si wafer and

corresponding variation in ball topography. (a) Friction force for 6 m sliding distance (same

data as Fig. 1a) (b) The RMS surface roughness measured by optical profilometry of Ball 2, Ball 3

and Ball 4 are 78 nm, 44 nm and 18 nm, respectively. Scale bar, 20 µm. There is a substantial

ball-to-ball variation in the measured friction forces as well as the ball topography: the highest

friction forces were measured using the smoothest ball, as expected based on the

interpretation of the experiments that is presented in the main text.

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Fig. S4 Optical images of sapphire balls. (a),(d) Measured before the friction experiment. (b),(e)

Measured after the friction experiment. (c),(f) Measured after ultrasonic cleaning of the ball

after the friction experiment. Scale bar, 100 m.

Fig. S5 Ex-situ height profiles of SiC ball. (a) Measured before and (b) after the friction

experiment. (c) Measured after ultrasonic cleaning of the ball after the friction experiment.

Scale bar, 50 µm.

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Fig. S6 Optical images and SEM-EDX analysis of the wear track on the Si substrate of the

repeated sapphire-on-Si wafer experiment. Optical images of (a) Left and (c) Right ends of the

wear track. (b),(d) Optical and SEM images of the wear track, respectively. The presence of the

oxygen (O) signal in (e) and the absence of the Si signal in (f) indicate the formation of the SiO x

debris.

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Fig. S7 Optical image of the sapphire ball after repeated sliding. Scale bar, 100 m.

Fig. S8 Comparison of the surface topography of the sapphire and SiC balls before and after

non-repeated sliding experiments. The blue and red shaded areas indicate where the AFM

height mapping was performed on the unworn and worn regions on the balls. The scale bars for

optical and AFM images are 100 µm and 1 µm, respectively.

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Fig. S9. Contact gap profiles (a) before and (b) after sliding. The surface height profiles for the

BEM calculation are recorded by the profilometry. The normal force in the BEM calculation was

scaled with the size of the height profiles of the ball compared to the area of the Hertzian

contact before sliding and the apparent area of contact after sliding, ~21 mN in both cases. The

color scale shows the gap between contact surfaces where dark red regions indicate the

contact part; 8.3 m2 before sliding and 60.5 m2 after sliding. Scale bars, 5 µm.

Fig. S10 The surface topography of wear scar on the sapphire ball after milling. Wear debris,

highlighted by dashed lines, can be observed outside the wear scar but not in the contact zone;

the worn region on the ball is higher than the debris. The local elastic modulus measured during

PeakForce tapping mode AFM imaging of the worn ball signals contrast between the wear

debris and the (worn) sapphire. Scale bars, 25 µm.

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Table S1 Wear mechanisms predominantly involved at the contact interface with sliding

distance.

Sapphire-on-Si waferSliding mode Contact surface Sliding distance [m] Wear mechanisms

Repeated

Ball 0—1 Run-in behavior; Third body is formed and adheres to ball

4—6 Third body on ball minimized wear

Flat 0—1 Abrasive wear; Ploughing from pristine surface asperities on ball

4—6 Third body on ball minimized wear

Non-repeatedBall

0—1 Run-in behavior; flattening of sharpest asperities

4—6 Mild wear (adhesive wear in some cases)

Flat0—1 Abrasive wear (only first ~cm)4—6 Mild wear

SiC-on-Si waferSliding mode Contact surface Sliding distance [m] Wear mechanisms

Repeated

Ball

0—1 Run-in behavior; Removal of sharpest asperities, mild wear

4—6Mild wear, Abrasive wear resulting

from compressed debris ridge formed on flat

Flat0—1

Abrasive wear and formation of compressed debris ridge in wear

track

4—6 Abrasive wear at either side of compressed debris ridge

Non-repeatedBall

0—1 Abrasive wear4—6 Predominantly mild wear

Flat0—1 Abrasive wear4—6 Predominantly mild wear

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