Wear, 171 (1994) 25-32
The sliding friction and wear behavior of single-crystal, polycrystalline and oxidized silicon Sreekanth
Computer Microtribology and Contamination Laboratov, OH 43210 (USA) (Received
8, 1992; accepted
Department of Mechanical Engineering, Ohio State University, Columbus,
The dynamic friction and wear behavior of single-crystal, polycrystalline and oxidized silicon has been studied using the pin-on-disk geometry. For single-crystal and polycrystalline silicon, even under low loads (0.05 N), damage in the form of grooving associated with debris generation is initiated early on the disks. This damage is associated with an increase in the coefficient of friction. With further sliding, the experiments suggest that oxidation modifies the sliding interface and the coefficient of friction decreases. Tests in a vacuum (0.13 Pa) show that the rise in the coefficient of friction is minimal; however, damage of the interface occurs. With oxidized silicon samples, the damage in the form of grooving is not very severe but the coefficient of friction is high in ambient air compared to that in dry nitrogen, probably due to strong interaction between oxide surfaces in the presence of air or humidity. These preliminary measurements also show similarities between the values of friction coefficients measured with macroscopic samples (as used in the present study) and those reported by other researchers using micromechanical devices.
Silicon has been the dominant material in use in the electronics industry for the past three decades. In recent years the field of micromechanical systems based on silicon has been receiving increasing attention. Micromechanical device fabrication is based on integratedcircuit (IC) technology. The advantage of this technology is that it affords the possibility of manufacturing micronsized structures with a high degree of precision and reproducibility and at a low cost. The second advantage derives from the fact that IC technology involves batch processing and hence a number of components can be produced simultaneously. Another advantage is that the micromechanical components can be integrated with electronic circuits on the same chip. A description of micromechanical devices based on silicon can be found in the articles by Petersen [l]and Angel1 ef al. . Friction and wear are important issues to be addressed in the design of mechanical systems involving moving parts. The effects of friction and wear on the performance of micromotors has been discussed by Mehregany and Tai . Friction presents an unwanted load on the micromotor and, in view of the small torques (10 pN m) generated by the micromotors, frictional forces have to be kept to a minimum in these devices. Wear of various micromotor parts such as the bushing
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and bearing can either result in complete breakdown of the device or affect its performance severely by generating wear debris which could be trapped at the interface between moving parts. Gabriel et al.  have observed significant wear of the hub of a micromotor in a scanning electron microscope. The materials of interest to micromechanical systems are presently single-crystal silicon, polycrystalline silicon films and silicon compounds such as silicon dioxide and silicon nitride. Friction and wear data on various combinations of these materials while sliding against each other are scarce. Such data, together with an understanding of the dynamic friction and wear behavior of these materials, are essential for the design and development of micromechanical systems based on silicon. Deng and Ko  have investigated the static coefficient of friction between millimeter-size samples made of silicon and silicon compounds (silicon dioxide and silicon nitride). Their results indicate that the coefficient of static friction depends on the material pair as well as the environment. For silicon dioxide/silicon dioxide and silicon dioxide/silicon contacts the coefficient of static friction increased with increasing humidity. The coefficient of static friction was reported to be higher in an oxygen atmosphere and lower in a nitrogen atmosphere compared to the values obtained in an ultrahigh-vacuum environment. Suzuki ef al.  have studied
the dynamic friction and wear of polysilicon, diamondlike carbon, sputtered S&N, and fluorocarbon-type liquid lubricant thin-film coatings with macroscopic samples. Their work indicates that a diamond-like carbon/&N, sliding pair is very effective in reducing both friction and wear. The coefficient of friction and wear were high for a Si,N,/polysilicon combination, whereas the coefficient of friction is high but the wear low for a Si,N,/Si,N, combination. Gabriel et ul.  have estimated dynamic coefficients of friction for polysilicon on silicon in the range of 0.25-0.35. These measurements were made on microcomponents (microturbines and gears), and dry friction was reported to be the dominant retarding force for operational speeds of less than 12 000 rev min I, whereas viscous drag (which arises from shear stresses due to velocity gradients between the bottom of the rotating structures and the substrate) were dominant at speeds greater than 12 000 rev min..‘. The objective of the present investigation is to study the dynamic friction and wear behavior of single-crystalline silicon, polycrystalline silicon and oxidized silicon. Sliding pairs of various combinations of these materials have been tested. The effect of the normal load and sliding speed has been investigated by conducting selected tests at various loads and speeds. The effect of the environment has been investigated by carrying out experiments in ambient air, reduced pressures (0.13 Pa) and in dry nitrogen. Experiments have been conducted with macroscopic samples using a pin-on-disk geometry. Besides monitoring friction, wear of the tested samples has been studied in a scanning electron microscope and with a surface profiler. Apart from shedding light on the mechanisms of dynamic friction and wear of the materials tested, comparison of the results obtained from this work with measurements made on actual micro-devices could validate testing with macroscopic samples and simple geometries.
2.1. Description of apparatus The experimental apparatus consists of a modified magnetic-disk drive capable of rotating the silicon disks at variable speeds from 100 to 3600 rev min-‘. The silicon disks were glued to an aluminum plate which in turn was attached to the drive. Following testing, the silicon disk could be detached from the aluminum plate for further analysis by heating on a hotplate. The pin to be tested was glued onto a IBM 3380-type suspension and mounted on a crossed I-beam fixture. The crossed I-beam fixture was instrumented with semiconductor strain gauges (gauge factor = 115) to measure
the normal and frictional forces exerted on the pin or slider. Frictional forces as small as 1 mN could bc measured with this arrangement. The output from the strain gauges was fed to a personal computer via an analog-to-digital converter. The entire setup was housed inside a bell-jar assembly which could be evacuated to 0.13 Pa (10 -’ Torr) and backfilled or purged with a gas to allow experiments to be conducted in desired environments. The humidity of the environment was monitored with a Vaisala humidity probe (accuracy, + 1%). The normal load used in most of the experiments was 0.1 N, and the rotational speed was 200 rev min’. The sliding speeds at track radii ranging from 20 to 35 mm varied from 0.4 to 0.7 m s- ‘. The 0.1 N load used in the experiments corresponded to a maximum hertzian contact pressure in the range 20-30 MPa for the materials tested. For comparison, Suzuki et al.  have estimated the average contact pressure at the shaft/bearing-rotor interface in a micromotor to be about 7.5 MPa. A high-speed test corresponding to a sliding speeds of approximately 10 m s-- ’ was also conducted.
2.2. Disk and pin materials The disks used in the experiments were single-crystal silicon wafers of orientation (1 lo), polycrystalline silicon wafers, and single-crystal silicon wafers of orientation (110) with an oxide coating approximately 1 pm thick. The disks were 75 mm in diameter and approximately 450 Frn thick. The pins used in the experiments consisted of single-crystal silicon of orientations (loo), (110) and (1 ll), polycrystalline silicon, and single-crystal silicon of orientation (110) with an oxide coating approximately 1 pm thick. The pins were 3.5 mmX3.5 mmX 850 pm thick. The surface of the pin in contact was provided with a spherical radius of curvature of 100 mm to avoid edge effects during testing. The grain size of the polysilicon samples tested was of the order of a few millimeters. The surface roughness of the samples was about 2 nm r.m.s. as measured with a non-contact optical profiler. Thermal oxidation of the silicon samples to produce a silicon dioxide coating was carried out in a quartz furnace at temperatures of 900-1000 “C in a dry oxygen atmosphere for about three days in order to grow a film approximately 1 pm thick.
3. Results and discussion 3.1. The fiction
and wear of single-crystal silicon
Figure 1 shows the variation of the coefficient of friction with the number of sliding cycles for a silicon
I Sliding friction
and wear behaviour
Fig. 1. Variation of the coefficient of friction with number of sliding cycles in ambient load and 0.5 m s-‘, and 0.2 N load and 9 m s- ’ for a single-crystal pin of orientation (110).
pin of orientation (110) sliding against a disk of orientation (110) at a load of 0.1 N and a speed of 0.5 m s-l. The initial coefficient of friction was around 0.25 and increased to 0.70 in about 3000 cycles of sliding and then decreased to a steady-state value of around 0.30 in about 6000 cycles. The rise in friction was associated with the appearance of a wear track on the disk (visible to the naked eye). The variation of the coefficient of friction with sliding cycles for (lll)and (lOO)-oriented pins sliding against (1 lO)-oriented disks was similar. Examination of the disk in the SEM prior to the maximum friction increase (i.e. after sliding for 2000 cycles) indicated damage in the form of grooving (Fig. 2(a)). A higher-magnification micrograph of one of the grooves is shown in Fig. 2(b). The profiler trace of the wear track also indicates grooves and ridges, with the grooves about 150-250 nm deep (Fig. 3). Wear debris of the order of a few micrometers in size is seen in Fig. 2(b) and could conceivably be aiding the grooving process [7,8]. The damage on the pin following 2000 cycles of sliding is shown in Fig. 4(a). The damage does not appear to be as extensive (no significant grooving) as the disk. A possible transfer fragment from the disk at the trailing edge of the wear scar is shown in Fig. 4(b). The wear tracks in our experiments indicate grooving. In wear experiments with silicon, other researchers [9-111 have reported that lateral cracking
air at 0.10 N load and 0.5 m s-l, 0.05 N (110) sliding against a disk of orientation
beneath the grooves was responsible for debris generation. However, we have seen no evidence of cracking. In our experiments, at loads of 0.05 N the variation of the coefficient of friction with sliding cycles and the damage morphology of the disk and pin were similar to those in the 0.10 N load test. This low-load test was performed to see whether a low-load threshold exists below which the rise in friction and the associated wear track formation and grooving do not occur. Loads below 0.05 N could not be used, as proper contact between the pin and disk could not be ensured at lower loads. SEM images of a pin and disk following sliding for 10 000 cycles (when the coefficient of friction had dropped down to about 0.3) are shown in Figs. 5(a) and 5(b), respectively. Both the disk and pin surfaces appear smoother, with the disk surface being covered by a white layer. Very fine charged debris is seen around the wear scar. At this point it is appropriate to study the variation in the coefficient of friction with sliding from experiments conducted in dry nitrogen and at reduced pressures (0.13 Pa) at a load of 0.10 N and a speed of 0.5 m s-’ (Fig. 6). The behavior in dry nitrogen is similar to the behavior in ambient air, but the rise in the coefficient of friction is from 0.25 to 0.60, followed by a decrease to a value of 0.20, whereas at 0.13 Pa the coefficient of friction increased from an initial value of 0.25 to 0.35 and remained at that level for the duration of the test. In both cases the damage
1 Distance across
Fig. 3. Profiler
the wear track (mm)
in Fig. 2(a).
(Fig. 1). However, at 3600 rev min-’ the friction was high (0.8) right at the start and then decreased to 0.30 within a couple of revolutions, and this could be due to rapid oxidation at the higher temperatures generated at the higher speeds.
(bi Fig. 2. SEM images: (a) worn region on a single-crystal silicon disk of orientation (110) after sliding against a single-crystal pin of orientation (110) at 0.10 N load and O.Sm sm.’ sliding speed in ambient air after 2000 cycles; (b) higher magnification of one of the grooves seen in Fig. 2(a).
on the pins and disks as observed in the SEM again indicated grooving, but a smooth coating as seen in Fig. 5(b) was not observed. The larger friction increase and development of a coating in ambient air could be due to oxidation of debris generated at the interface (by mechanical means) leading to rougher sliding, followed by development of the coating and filling of the grooves by the fine oxidized debris and a reduction in friction. A complete absence of oxygen in the dry nitrogen or 0.13 Pa environments is not expected, and so some of the effects of oxygen mentioned above can still be operative in these environments, although to a lesser extent. Tests under ultra-high-vacuum conditions could help clarify the effects of oxygen better. Tests conducted at speeds of 600 rev min’ (1.5 m SC’) and 3600 rev min’ (9.0 m SC’) indicated the damage mechanism to be similar to the 0.5 m s-’ test
3.2. Polycrystalline silicon and oxidized silicon The experiments with single-crystal silicon indicated that the damage to the rotating component (disk) is more severe than the damage to the pin. The variation of the coefficients of friction with sliding for singlecrystal pins of orientation (110) against polycrystalline disks at loads of 0.05 and 0.10 N in ambient air is shown in Fig. 7. In contrast to single-crystal silicon, the initial increase in the coefficient of friction is not very significant. The coefficient of friction fluctuates between 0.3 and 0.4 for the 0.05 N load test and between the same values after an initial increase to 0.45 at a load of 0.10 N. This range of values is similar to the range of 0.25-0.35 reported by Gabriel et al.  for polysilicon sliding on silicon in an actual micro-device. Surface damage on the disk was again in the form of grooving. The difference in the initial friction increase between polycrystalline silicon and single-crystal silicon suggests that the initial scale of damage (size of initial debris) is smaller in the polycrystalline case. The variation in the coefficient of friction with sliding for an oxidized disk sliding against an oxidized pin in ambient air and dry nitrogen is shown in Fig. 8. The steady-state coefficient of friction is approximately 0.70 in air and 0.1-0.2 in nitrogen. These results suggest that the interaction between oxide surfaces is strong in air (either oxygen or water vapor present in ambient air could promote strong interaction between silicon dioxide surfaces), resulting in higher friction in air
S. Venkatesan, B. Bhushan i Sliding friction and wear behaviour of silicon
(b) Fig. 4. SEM images: (a) worn region on a single-crystal silicon pin of orientation (110) after sliding against a single-crystal disk of orientation (110) at 0.10 N load and OSm SC’ sliding speed in ambient air after 2000 cycles; (b) transfer fragment from the disk.
compared to nitrogen. Such large differences in the friction values in ambient air and dry nitrogen environments were also reported by Deng and Ko  in their static friction experiments. They reported static friction coefficients of 0.45 in air and 0.15 in dry nitrogen. An optical micrograph, Fig. 9, of the worn oxide surface after sliding in ambient air indicates extensive cracking. The track from the nitrogen experiment is also very similar, suggesting the wear mechanism to be similar in air and dry nitrogen, although the friction is higher in air. The unworn surface also was much rougher and had cracks compared to the polished and smooth unoxidized samples. It is likely that these cracks were generated during cooling of the oxidized samples by residual stresses arising from differences in the coefficients of thermal expansion between the oxide layer and the silicon substrate. The coefficient of thermal
Fig. 5. SEM images: (a) worn region on a single-crystal silicon pin of orientation (110) after sliding against a single-crystal disk of orientation (110) at 0.10 N load and OSm s-’ sliding speed in ambient air after 10 000 cycles; (b) corresponding disk track.
expansion of cy-SQ has been measured as between 11.4 and 50.0X 10e6 Y--l, depending on the temperature and crystallographic structure (quartz, cristobalite, tridymite), while that of Si ranges between 2.9 and 4.3 X 1O-6 “C-l [12-141. Deep grooving as seen for the unoxidized samples is not seen for the oxidized samples. Our experiments with oxidized samples suggest that thin oxide films which possess a smooth untracked surface might work well in dry nitrogen.
4. Summary The dynamic friction and wear behavior of singlecrystal and polycrystalline silicon is associated with grooving and oxidation in the load and speed ranges tested. The values of the coefficients of friction measured using macroscopic samples in the present study are
I Sliding friction arid wear hehaviour of sd~cor~
I 10 (Thousands)
REVOLUTIONS Fig. 6. Variation of the coefficient of friction with number of sliding cycles in ambient air, dry nitrogen and reduced pressures (0.13 Pa) at 0.10 N load and 0.5 m s-’ sliding speed for single-crystal silicon pins of orientation (110) sliding against a single-crystal silicon disk of orientation (110).
REVOLUTIONS Fig. 7. Variation of the coefficient of friction with number of sliding cycles in ambient air at loads of 0.05 N and 0.1 N and at a sliding speed of 0.5 m s-’ for single-crystal pins of orientation (110) sliding against a polycrystalline silicon disk.
reasonably similar to the values obtained by testing with actual micro-samples. Based on our experiments, for applications involving zero-wear conditions, such as in micromechanical systems, it appears that single-crystal
and polycrystalline silicon are not suitable candidate materials for sliding-contact situations. Between the two, however, the initial increase in the coefficient of friction is higher for single-crystal silicon. Even at loads
S. Venkatesan, B. Bhushan f Sliding friction and wear behaviow
REVOLUTIONS Fig. 8. Variation of the coefficient of friction with number of sliding cycles in ambient air and dry nitrogen at 0.10 N load and 0.05 m s-’ sliding speed for thermally oxidized single-crystal silicon pins of orientation (110) sliding against thermally oxidized singlecrystal silicon disks of orientation (110).
The authors would like to thank Professor George Valco and Mr. Jim Jones for assistance with silicon oxidation experiments. This research was funded by the membership of the ~mputer ~icrotribo~o~ and Contamination Laboratory. The authors would like to thank reviewers for their constructive comments.
Fig. 9. Optical micrograph showing the worn areas (top region in micrograph) and unworn areas (bottom region) on a oxidized disk after sliding against an oxidized pin in ambient air at a load of 0.1 N load and 0.5 m s-’ sliding speed.
of 0.05 N, damage in the form of grooving and debris generation has been observed. From the viewpoint of wear, oxide coatings offer promise, but in this case the friction in air is high. Further testing involving coatings such as diamond-like carbon and silicon nitride on silicon is necessary to deveIop materials/coatings for mi~omechanical applications.
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S. Venkatesan, B. Bhushan
I Sliding friction and wear behaviour of silicm
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