micro/nanotribology and its applications to magnetic storage devices and mems

Tribology International Vol. 28, No. 2, pp. 85-%, 1995 Copyright 0 1995 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0301-679X/95/%10.00 +O.OO

Microlnanotribokgy and its appkk tions to magnetic

ge devices and

Bharat Bhushan

Scanning tunnelling microscopy and atomic force microscopy/ friction force microscopy are increasingly used for tribological studies ranging from atomic and molecular scales to microscales. These studies are needed to develop a fundamental understanding of interfacial phenomena on a small scale and to study interfacial phenomena in micro- and nanostructures used in magnetic storage devices, MEMS and other industrial applications. Although micro/ nanotribological studies are critical to study micro/nano structures, these studies are also valuable in understanding interfacial phenomena in macrostructures to provide a bridge between science and engineering. Friction and wear on micro- and nano- scales have been found to be generally smaller compared to that at macro-scales. Therefore micro/nanotribological studies may identify regimes for ultra-low-friction and zero-wear. In this paper we present examples of micro-scale tribological measurements made on magnetic storage and MEMS components.

Keywords: microhanotribological tools, magnetic storage devices, MEMS, A FM/FFM

Introduction

The advent of new techniques to measure surface topography, adhesion, friction, wear, lubricant film thickness, and mechanical properties all on a micro- to nanometre scale, to image lubricant molecules and the availability of supercomputers to conduct atomic- scale simulations has led to the development of a new field referred to as microtribology, nanotribology, molecular tribology or atomic-scale tribology. This field is concerned with experimental and theoretical investigations of processes, ranging from atomic and molecular scales to micro-scales, occurring during adhesion, friction, wear, and thin-film lubrication at sliding surfaces. The differences between the conventional or macrotribology and micro/nanotribology are

Computer Microtribology and Contamination Laboratory, Depart- ment of Mechanical Engineering, The Ohio State University, Col- umbus, OH 43210-1107, USA Received 3 November 1993; accepted I4 June 1994

contrasted in Fig 1. In macrotribology, tests are conducted on components with relatively large mass under heavily loaded conditions. In these tests, wear is inevitable and the bulk properties of mating components dominate the tribological performance. In micro/ nanotribology, measurements are made on components (at least one of the mating components) with relatively

Macrotribologv

Large Mass Heavy Load

Wear (Inevitable)

Bulk Material

Micro/nanotriboloau

Small mass ( g ) Light load ( g to mg)

No Wear (Few atomic layers)

Surface (few atomic layers)

Fig 1 Comparisons between macrotribology and micro/ nanotribology

Tribology International Volume 28 Number 2 March 1995 85

small mass under lightly loaded conditions. In this situation negligible wear occurs and the surface properties dominate the tribological performance.

Micro/nanotribological studies are needed to develop a fundamental understanding of inter-facial phenomena on a small scale and to study interfacial phenomena in micro- and nanostructures used in magnetic storage systems, microelectromechanical systems (MEMS) and other industrial applications. The components used in micro- and nanostructures are very light (of the order of a few micrograms) and operate under very light loads (of the order of a few micrograms to a few milligrams). As a result, friction and wear (on a nano- scale) of lightly loaded micro/nano components are highly dependent on the surface interactions (few atomic layers). These structures are generally lubricated with molecularly thin films. Micro- and nanotribological techniques are ideal for studying the friction and wear processes of micro- and nanostructures. Although micro/nanotribological studies are critical to study these structures, they are also valuable in gaining a fundamental understanding of interfacial phenomena in macrostructures to provide a bridge between science and engineering. Friction and wear on micro- and nano-scales have been found to be generally small compared to that at macro-scales. Therefore, micro/ nanotribological studies may identify regimes for ultra- low-friction and zero-wear.

The field of tribology is truly interdisciplinary. Until recently, it has been dominated by mechanical, materials and chemical engineers who have conducted macro tests to predict friction and wear lives in machine components and who have devised new lubricants to minimize friction and wear. Development of the field of micro/nanotribology has attracted many more physicists and chemists who have significantly contributed to a fundamental understanding of friction and wear processes on an atomic scale. Thus, tribology is now studied by both engineers and scientists. The micro/nanotribology field is growing rapidly and it has become fashionable to call oneself a ‘tribologist’. Recently, two conferences held in Morioka, Japan, in October 1992 and Laliki, Poland, in September 1993 have been dedicated to this emerging field of micro/ nanotribology. A handbook of micro/nanotribology is in press’. To give a historical perspective of the field, a scanning tunnelling microscope (STM) developed by Binnig et ~1.~ in 1981 is the first instrument capable of directly obtaining three-dimensional (3D) images of a solid surface with atomic resolution. This technique is limited to imaging of electrically conducting surfaces. G. Binnig and H. Rohrer received a Nobel Prize for Physics in 1986 for their discovery. Based on their design of STM, Binnig et aL3 developed an atomic force microscope (AFM) (also called a scanning probe microscope (SPM)) to measure ultra-small forces (less than 1 p,N) present between the AFM tip surface and the sample surface. An AIM can be used for the measurement of all engineering surfaces. It has become a popular surface profiler for topographic measurements on micro- to nano-scales4-6. Mate et al.’ were the first to modify an AFM in order to measure both normal and friction forces and this instrument is

Micro/nanotribology and applications to magnetic storage devices and MEMS: B. Bhushan

generally called a friction force microscope (FFM) or lateral force microscope (LFM). By using a sharp diamond tip mounted on a stiff cantilever beam, an AFM can be used for scratching, wear and indentation hardness measurements and nanofabrication purposes - 8 14. AFMs have also been used for studies of adhesionr5,r6, material manipulation”, and thin- film boundary lubrication18-23, as well as to measure lubricant film thickness4,24*25, surface temperatures26 and magnetic force, including application for magnetic recording 27. STMs have been used to image lubricant molecules28.

For completeness, we also mention a surface force apparatus (SFA) which has been used to study rheology of molecularly thin liquid films between atomically smooth and optically transparent surfaces29-31. How- ever, only AFMs can be used to study engineering surfaces in dry and wet conditions. The scope of this paper is limited to AFM/FFM.

Interest in the micro/nanotribology field grew from magnetic storage devices and its applicability to microelectromechanical systems (MEMS) is clear. In this paper we start with examples of magnetic storage devices and MEMS where micro/nanotribological tools and techniques are essential for interfacial studies. We then present examples of micro/nanotribological studies to conduct roughness, friction, scratching, wear, indentation, material manipulation, and lubrication studies and use of AFMs for nanofabrication purposes.

Magnetic storage devices and MEMS

Magnetic storage devices

A conventional magnetic recording process is accomplished by the relative motion between a magnetic medium against a read-write magnetic head. Under steady-state operating conditions, a load-carry- ing air film is formed, and only isolated asperity contacts may occur between magnetic components. However, during start-stop operations of the disk drive, physical contact takes place32,33. The ever- increasing need for high-density recording requires the development of drive systems that operate at ultra- low air gaps at the head-disk interface (HDI). Current drives operate at a head-disk spacing of about 0.1 pm, and several contact recording drives are under development that are expected to operate at a head-disk spacing of 0.05 km or less. High static friction (stiction) at the HDI after storage and increase in kinetic friction after use as a result of disk wear at HDI are the major impediments to commercialization of contact recording.

Figure 2 shows a schematic of a rigid disk drive with a 95 mm form factor. A non-removable stack of multiple disks mounted on a ball bearing spindle is rotated by an electric motor at a constant angular speed ranging from about 3000 to 10000 r-pm, dependent upon the disk size. The head slider-suspension assembly (allowing one slider for each disk surface) is actuated by a stepper motor or a voice coil motor using a linear or rotary actuator. Generally, a rotary actuator is used to save space, as shown in Fig 2.

86 Tribology International Volume 28 Number 2 March 1995

Fig 2 Schematic of the magnetic rigid disk drive con- sisting of the disk stack and the rotary actuator for the head-suspension assembly

Figure 3(a) shows the schematic of a thin-film head slider with a three-rail, taper-flat design. A nanoslider commonly used today has dimensions of 2 x 1.6 x 0.425 mm with a mass of about 7.7 mg. A normal load of about 3.5 g is applied during use. Figure 3(b) shows the sectional view of a thin-film rigid disk. Future trends are to use smaller drives with ultra-small sliders (half dimensions of a nanoslider) and an ultra-smooth (1-2 nm rms) and flat disk substrate (made of glass or glass ceramic) with about 5-10 nm thick diamondlike carbon (DLC) overcoat and 0.5-2 nm thick bonded perfhroropolyether lubricant. Horizontal thin-film heads with a single-crystal silicon substrate are under develoment which can be mass produced inexpensively and miniaturized using silicon integrated-circuit tecdnology34,35.

a

Ltqwd Lubricant 1-4 nm Lopt~~nal; Protective Overcoat 20.40 nm

- Electroless Ni-P

l-----l 10-20 .lm for meta, film

Anodized (alumde) Z-20 pm for oxide film

AI-MS (96-4) substrate 0.78-13 m m

b

Fig 3 (a) Schematic of a nanoslider with thin-film head construction (arrow shows the direction of disk rotation); (b) sectional view of a thin-film rigid disk


MEMS

In recent years, the emerging field of MEMS has received increasing attention. MEMS are fabricated using the technology developed for integrated circuits. The total size of the component can be as small as 100 pm or even less. Microsensors, fine-polishing microactuators, microgrippers, micromotors, micro- pumps, gear trains, cranks, manipulators, nozzles and valves, a fraction of a millimetre in size, are being fabricated in laboratories around the world36-39. Figure 4(a) shows an example of a surface micromachined structure - a micromotor driven electrostatically (by electrostatic attraction between positive and negatively charged surfaces) which has been fabricated at MIT and UC Berkeley. The UC Berkeley micromotor consists of 12 stators and a four-pole rotor of 120 pm diameter with an air gap between the rotor and stator of 2 Km. The structural material for the hub, stator and rotor is polysilicon film deposited by LPCVD. The vertical walls of the stator and rotor contain 340 nm thick LPCVD silicon nitride film for low friction and wear at high operating speeds (Fig 4(b)). The motor built at MIT delivered a torque of 12 pNm and synchronous motor speeds of up to 2500 rpm. The wear resistance of the rotor/stator interface and reduction of friction at the hub/rotor interface is critical for high-speed interfaces.

b

Fig 4 (a) An electrostatic (variable capacitance) micromotorfabricated ofpolysiliconfilm; (6) schematic cross-section of the micromotorj7



Magnetic storage devices and MEMS are the two examples where micro/nanotribological tools and techniques are essential for studies of micro/nano-scale phenomena.

Experimental

resolution -2.5 pm), non-contact optical profiler (lateral resolution - 1 Frn) and AFM (lateral resolution -10 nm) are shown in Fig 5. The figure shows that roughness is found at scales ranging from millimetre to nanometre. The measured roughness profile is dependent on the lateral and normal resolutions of the measuring instrument. Instruments with different lateral resolutions measure features with different scale lengths. It can be concluded that a surface is composed of a large number of length scales of roughness that are superimposed on each other.

Surface roughness is generally characterized by the standard deviation of surface heights which is the square root of the arithmetic average of squares of the vertical deviation of a surface profile from its mean plane. However, due to the multiscale nature

The AFM/FFM used in the studies conducted in our laboratory has been described in several publications 1@13,40.41. Briefly, the sample is mounted on a PZT tube scanner to scan the sample in the x-y plane and to move it in the vertical (z) direction. A sharp tip at the end of a flexible cantilever is brought into contact with the sample. Normal and frictional forces applied at the tip-sample interface are measured using a laser beam deflection technique. Simultaneous measurements of friction force and surface roughness can be made with our instrument. For surface roughness and friction measurements, a microfabricated square pyramidal Si3N4 tip (with a radius of about 30-50 nm) on a beam with a stiffness of about 0.4 N/m is used at a normal load of lo-150 nN. For scratching, wear and indentation studies and nanofabrication, a three- sided pyramidal diamond tip mounted on a stainless 0

steel beam. with a normal stiffness of about 25 N/m is used at relatively high loads (lo-150 P,N). -100

For measurement of surface topography and friction force on a micro-scale the sample is scanned in a direction orthogonal to the long axis of the cantilever beam typically over about 1 pm x 1 pm area with a scan rate of 5 Hz (scanning speed of 2.5 pm/s). For scratching and wear, the sample is scanned in a direction orthogonal to the long axis of the cantilever beam at a scanning speed of typically 1 pm/s. For wear, an area of 2 pm x 2 pm is generally scanned. Sample surfaces are scanned before and after the scratch or wear to obtain the initial and final surface topography at a load of about 0.5 ~.LN over an area larger than the scratched or worn region to observe the scratch or wear scars. The operation procedures 0

for nanoindentation are similar to those used for nanowear except that the scan size is set to zero in the case of nanoindentation in order for the tip to continuously press the sample surface for about 2 seconds. The surface is imaged before and after the indentation at a normal load of about 0.5 PN. e Nanohardness is then calculated by dividing the indentation load by the projected residual area.

-200 HIS =2.5 "m

a0

500 looa 1500 zoo0 WJ

Nanofabrication is conducted by scratching the sample surface with a diamond tip at specified locations and scratching angles. The normal load used for scratching (writing) is about 40 PN with a writing speed of about 500 rim/s.

Results and discussion

Surface roughness

All surfaces are rough on a micro-scale. Even the smoothest surfaces, such as those obtained by cleavage of some crystals, contain roughness on an atomic scale. Roughness profiles of an as-polished thin-film magnetic rigid disk measured using a stylus profiler (lateral

Fig 5 Sugace roughness profiles of an as-polished magnetic thin-film rigid disk measured using (a) a stylus profiler (lateral resolution - 2.5 p) (b) a non- contact optical profiler (lateral resolution - 1 pm) and (c) an atomic force microscope (lateral resolution - 10 nm)


Microlnanotribology and applications to magnetic storage devices and MEMS: B. Bhushan

of surfaces, it is found that the variances of surface height and its derivatives and other roughness parameters strongly depend on the lateral resolution of the roughness-measuring instrument32 (Fig 6). The scale dependence in Fig 6 suggests that instruments with different resolutions and scan lengths yield different values of these statistical parameters for the same surface. In order to use the roughness parameters for prediction of real area of contact, friction and wear it is first necessary to quantify the multiscale nature of surface roughness.

This can be characterized by fractal modelling. Fractal characterization of surface roughness is scale independent and provides information on the roughness structure at all length scales that exhibit the fractal behaviour, called here the M-B mode142+43 and G-8 mode16.

Friction

We have measured friction from micro- to nano-scales using FFM10-13~40~41. Ruan and Bhushan41 reported that atomic-scale friction of a freshly cleaved HOP graphite exhibited the same periodicity as that of corresponding topography. However, the peaks in

-600t--15t-

Fig 6 Scale dependence of standard deviation of surface height (a), u’ and CT” for an as-polished thin-film rigid disk

friction and those in corresponding topography profiles were displaced relative to each other. Bhushan et al.1o-‘3 reported that the coefficient of friction on a micro-scale is lower than the macrofriction. For typical values of coefficients of friction of an MP tape and an as-polished thin-film rigid disk see Table 1. Larger values for the macro coefficient of friction may be the result of the ploughing effect associated with macro- scale measurement.

In order to show elegantly any correlation between local values of friction force and surface roughness we measured the surface roughness and friction force of a gold-coated ruling with rectangular grids. Figure 7 shows the surface profile, the slopes of surface profile taken along the sliding direction, and the friction profile for the ruling. We note that friction force changes significantly at the edges of the grid. Friction force is high locally at the edge of the grid with a positive slope and low at the edge of the grid with a negative one. Thus, there is a strong correlation between the slope of the roughness profiles and the corresponding friction force profiles. (For a clearer correlation, see the grey-scale plots of slope and friction force profiles for an FFM tip sliding in either direction in Fig 8). This correlation can be explained by a ‘ratchet’ mechanism. Based on this mechanism, the local friction force is a function of the Iocal slope of the sample surface. The local variation in friction force should be of the opposite sign as the scanning direction is reversed, shown in Fig 8. In the right- hand plot of the bottom row of Fig 8 the sign of the friction plot is reversed. We note a slight difference between the bottom two profiles, suggesting that there is a directionality effect in the friction.

The directionality effect in friction on a macro-scale is observed in some magnetic tapes. In a macrotest, a 12.7 mm wide metal-particle (MP) tape was wrapped over an aluminium drum and slid in a reciprocating motion with a normal load of OS N and a speed of about 60 mm/s12. The coefficient of friction as a function of sliding distance in either direction is shown in Fig 9. We note that the friction for this tape

Table 1 S&ace roughness (rms) and micro- and macro-scale friction data of magnetic tapes and disk samples

Sample Micro-scale coefficient of Macro-scale coefficient of

rms (nm) friction friction

NOP AFM 1 pm 10 km Ni-Zn Si3N4 A1203-TiC X X ferrite

250 pm 1 pm 10 1 Fma 10k.m” wrn X X X

250pma 1 pma lOt.~rn=

Metal-particle tape 6.0 6.1 11.7 0.08 0.07 0.29 0.22 - Unlubricated, as-polished 2.2 3.3 4.5 0.05 0.06 - - 0.26 disk

Seen area NOP: Non-contact optical profiler AFM: Atomic force microscope


Micro/nanotribology and applications to magnetic storage devices and MEMS: 6. Bhushan

4.00 urn

shown in Fig 11. Corresponding grey-scale plots of slope of roughness profiles are also shown in this figure. The left-hand side of Fig 11 corresponds to the sample sliding from the left towards the right and vice versa. Again we note a general correspondence between the slope and friction force profiles and that, generally, the points which have high friction in the left-to-right scan have low friction as the sliding direction is reversed. This relationship is not true at some locations. At the bottom, the sign of friction force profile is reversed in the right-hand profile, but we still observe some differences in the two friction profiles. This may result from the asymmetrical asperit- ies and/or asymmetrical transfer of wipe material during manufacturing of the tape. Thus, there is a directionality effect in friction also on a micro-scale.

Scratching and wear

2.5

4.00

0 0

Scratches made on the MP tape and an unlubricated as-polished thin-film rigid disk at various loads are shown in Fig 12. All scratches were made with ten cycles. We note that scratch depth increases with an increase in the normal load. Figure 13 shows the wear profiles at 20 ~.LN load and at various cycles on an unlubricated thin-film disk. We note that the wear is not uniform and is largely initiated at the texture grooves present on the disk grooves. This suggests that surface defects act as initiation sites for wear.

Figure 14 shows the wear depth as a function of number of cycles for the unlubricated thin-film disk at 10 and 20 ~.LN loads. Wear initially takes place slowly with a sudden increase between 40 and 50 cycles at 10 pN and after about ten cycles at 20 p,N. The rapid increase is associated with the breakdown of the carbon coating.

30 RS

4.00 15

0

0

We note that scratches and wear profiles can be produced with very shallow depths, Thus the AFM technique can be used to measure scratch and wear resistance of ultra-thin films.

Indentation

-15

Fig 7 (a) Surface roughness profile, (b) slope of the roughness projile taken in the sample sliding direction (the horizontal axis) and (c) friction profle for a gold- coated ruling at a normal load of 155 nN

is different in different directions. Next, we made simultaneous measurements of surface roughness and friction force on a micro-scale. Surface profiles, slope of surface profiles along the sliding direction and the friction force profiles are shown in Fig 10. A strong correlation between the local variation in the surface slope and friction force is clearly observed. Grey-scale plots of local coefficients of friction of this MP tape as the FFM tip is scanned in either direction are

The mechanical properties of materials can be measured using AFM i1-i4. Bhushan and Ruan12 measured the indentability of magnetic tapes at increasing loads on a nano-scale (Fig 15). In Fig 15 the vertical axis represents the cantilever deflection and the horizontal axis the vertical (Z) position of the sample. The ‘extending’ and ‘retracting’ curves correspond to the sample being moved towards or away from the cantilever tip, respectively. The left part of the curve shows the tip deflection as a function of the sample travelling distance during sample-tip contact, which would be equal to each other for a rigid sample. However, if the tip indents into the sample, the tip deflection would be less than the sample travelling distance; in other words, the slope of the line would be less than 1. In Fig 15 we note that the line in the left part is curved with a slope of less than 1 shortly after the sample touches the tip, which suggests that the tip has indented the sample. Later, the slope is equal to 1, suggesting that the tip no longer indents


Fig 8G 6% :n in the Sam)

Micro/nanotribology and applications to magnetic storage devices and MEMS: 8. Bhushan

-scale plots of (a) the slope of the surface roughness, (b) the friction force and (c) the ? right-hand profile reversed) for a gold-coated ruling. The left-hand side of the figure c sliding from left to right and vice versa. Higher points are shown by lighter areas

f ric :tion force ‘OWfZ spar rzds to


3

‘s .- 2 .- i? c3 01

0 25 50 75 100 125 0

Number of drum passes

Fig 9 Macro coefficients of friction as a function of sliding cycles for an MP tape sliding over an aluminium drum in a reciprocating mode in both directions. Normal load = 0.5 N over 12.7 mm wide tape, sliding speed = 60 mmls

the sample. Since the curves in extending and retracting modes are identical, the indentation is elastic up to a maximum load of about 22 nN used in the measurements.

Bhushan et a1.14 have reported that indentation hardness with a penetration depth as low as 1 nm can be measured using AFM. Figure 16 shows the line plots of indentation marks generated on an unlubricated as- polished thin-film disk at normal loads of 100 PN and 140 pN. The hardness value at 100 PN is much higher than at 140 pN. This is expected since the indentation depth is only about 15 nm at 100 pN, which is smaller than the thickness of the carbon coating (-30 nm). The hardness value at lower loads is primarily the value of the carbon coating and at higher loads the value of the magnetic film, which is softer than the carbon coating 32. This result is consistent with the scratch and wear data discussed previously.

Material manipulation

Ruan and Bhushan17 conducted indentation on fullerene films using AFM and observed transfer of fullerene molecules to the AFM tip during indentation. The fullerene molecules transferred to the AFM tip were subsequently transported to a diamond surface when the diamond sample was scanned with the contaminated tip. This demonstrates the capability of material manipulation on a molecular scale using AFM.

Lubrication

AFMs have been used to measure film thickness of ultra-thin lubricant films (2 nm in thickness) with a lateral resolution on the order of AFM tip radius, about 100 nm or less, which is not possible by other techniques4J4T25.

Bonded lubricants are commonly used to reduce friction and wear of sliding surfaces such as in thin- film magnetic rigid disks3*. Blackman et aP8. and Mate*O studied the deformation of bonded lubricant using AFM. They reported that the bonded lubricants


Fig 10 (a) Surface roughness profile (IJ = 7.9 nm), (b) slope of the roughness profile taken along the sample sliding direction (mean = -0.006, u = 0.300) and (c) friction force profiie (mean = 5.5 nN, cr = 2.2 nN) of a MP tape at a normal load of 70 nN

behave as a soft polymeric solid while contacted with an asperity.

Meyer et al.*’ and Overney et al. ** showed that FFM can be used to image and identify compositional domains with a resolution of - 0.5 nm. Although the topography of the individual domains can be imaged with a standard AFM, it is the additional information provided by the friction measurement that allows them to be chemically differentiated.


Fig the and


.ey-sc land jersa

ale plot of (a) the slope of the surface roughness, (b) friction force and (c) friction yrofile reversed). The left-hand side of the figure corresponds to the sample sliding frc Higher points (in roughness slope or friction force) are shown by lighter areas

fo Iwe (sign in >rn left L to right



s.bi

f

9 5.00

P

Fig 12 Surface profiles for a scratched unlubricated as- polished thin-film rigid disk and MP tape

Nanofabrication

AFM can be used for nanofabrication by extending the nanoscratching operation. The letters OHIO were written on a (100) single crystal silicon using a diamond tip at 40 PN normal load (Fig 17). The writing was done at a slow speed (0.2 km/s for scratching each line).

Conclusions AFM/FFM can be used for measurements of roughness, friction, scratching/wear, indentation, material manipulation, and lubrication on a micro- to nano- scale and for nanofabrication purposes. Commonly measured roughness parameters are scale dependent, requiring the need of scale-independent parameters to characterize roughness. Local variation in microfriction can be explained by a ratchet mechanism. Microwear is initiated more rapidly near texture grooves. Break- down of thin films can be detected using AFM and indentation hardness with a penetration depth as low as 1 nm can be measured. Microscratching and nanoindentation are the powerful ways of screening thin films. Material manipulation on a nano-scale is possible with AFM. Measurement of lubricant film thickness with a lateral resolution on a nano-scale and

0

2om 5 cvc. ,

f :: 2

.oo

H

0

1.00 2Qw IO cvc.

+---- ! fkoo

1.00 I 2.00 20

3.00 ~.LN

20 CYC.

Fig 13 Surface profiles of an unlubricated as-polished thin-film rigid disk showing the worn region (centre 2 pm x 2 pm). The normal load and the number of test cycles are indicated in the figure


Microlnanotribology and applications to magnetic storage devices and MEMS: B. Bhushan

-9- Unlubricated as-polished disk 4 Lubricated as-polished disk

5ooi 400

z ; 300 FL $ k 200

s

100

1OpN

1 - 0 c--+

0 10 20 30 40 50

Number of cycles

Fig 14 Wear depth as a function of number of cycles for unlubricated as-polished, thin-film rigid disks at 10 PN and 20 PN loads

* Retracting - Extending

Z Position - 15 nmldiv

Fig 1.5 indentation curve for a MP tape. The spring constant of the cantilever used was 0.4 Nlm

boundary lubrication studies can be conducted using AFM. Finally, nanofabrication can also be performed.

Acknowledgements

We would like to thank Drs V. N. Koinkar and J. Ruan for AFM/FFM and S. T. Patton for macrofriction measurements. The research reported in this paper was supported in part by the Office of the Chief of Naval Research, Department of Navy (Contract No. NOO014-93-l-0067), National Storage Industry Consor- tium/Advanced Research Projects Agency (Grant MDA 972-93-l-0009) and the industrial membership of the Computer Microtribology and Contamination Laboratory (CMCL). The content of the information does not necessarily reflect the position or the policy of the government, and no official endorsement should be inferred. This keynote address was presented at the First International Colloquium - Microtribology ‘93, Laliki, Poland, 7-8 September, 1993.

Fig 16 Images with nanoindentation marks generated on an unlubricated as-polished thin-film disk at normal loads of 100 and 140 pN. The normal loads used in the indentation, the indentation depths and the hardness values are indicated in the figure

Fig 17 Example of nanofabrication. The letters OHIO were generated by scratching a (100) silicon surface using a diamond tip at a normal load of 40 FN


MicroInanotribology and applications to magnetic storage devices and MEMS: B. Bhushan

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