Friction force microscopy (FFM) was used to study microscale friction between a sharp tip and various samples. Effect of normal load and tip material on the ...
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Thin Solid Films 278 (1996) 49-56
Effect of normal load on microscale friction measurements Bharat Bhushan *, Ashok V. Kulkarni Cwnputer Microtribology
and Contamination
Laboratory, 206 W. 18th Avenue, The Ohio State University, Columbus, OH 43210-1107,
USA
Received 15 May 1995; accepted 2 October 1995
Abstract Friction force microscopy (FFM) was used to study microscale friction between a sharp tip and various samples. Effect of normal load and tip material on the coefficient of friction has been studied. Friction force as a function of normal load of virgin silicon with a thin film of native oxide and dry-oxidized SiOZ coating showed two distinct slopes. The coefficient of friction in the low load region of less than about 1.5mN is lower than that in the high load region. The critical load at which the coefficient of friction starts to increase corresponds to the specimen hardness. Ploughing at high loads is believed to he responsible for high values of the coefficient of friction. The coefficient of friction of polished natural diamond remains virtually independent of normal load because no ploughing occurs. The coefficient of friction on a macroscale is higher than that on a microscale for comparable contact stresses. When measured for the small apparent area of contact and very small loads used in microscale measurements, the indentation hardness and modulus of elasticity on a microscale are higher than that at the macroscale. This reduces the degree of wear at the microscale. In addition, small apparent areas of contact in microscale measurements reduces the number of particles trapped at the interface and thus minimizes the ploughing contribution to the friction force. Based on this study, it is concluded that measured values of the coefficient of friction on a microscale are a strong function of normal load and the apparent area of contact. Ultralow values of the coefficient of friction and near-zero wear can be achieved with microscale components at very light loads in the absence of significant ploughing. Keywords:
Atomic force microscopy;
Nanostructures;
Tribology
1. Introduction The atomic force microscope (AFM) /friction force microscope (FFM) and its modifications have demonstrated effectiveness as a tool for micro/nanotribological studies [ 1,2]. These techniques have gained increasing acceptance as a means to study surface topography, adhesion, friction, scratching and wear, and boundary lubrication and for measurements of nanomechanical properties. There is a growing interest in microtribological studies of silicon material as it is used in fabrication of micromechanical components such as micromotors, microactuators, microsensors, and magnetic heads [ 1,3-lo]. An important issue in microtribological studies is the factors affecting friction behavior. Friction and wear are significantly affected by surface films. For example, in the case of silicon, it is known that a few monolayers ( - 1.5-2 nm) of native oxide is formed on even freshly cleaved silicon which gradually grows to a thickness of about 4 nm [ 111. The layer * Corresponding author: Robinson Laboratory, 206 W 18th Avenue, The Ohio State University, Columbus, OH 43210-I 107, USA; Tel.: (614) 2920651, Fax: (614) 292-0325. 0040.6090/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO40-6090(95)08138-O
of native oxide alters the true friction behavior of silicon at low loads. In addition to the removal of surface layers at high loads, plastic deformation at high loads is expected to affect the friction properties. Previous studies [ 1,2] have reported that the coefficient of friction on a macroscale is higher than that on a microscale. This difference may arise from the difference in the normal forces (contact stresses) and contact sizes used in the macro and micro measurements. Objectives of this research are to study the effect of normal load on the microscale friction and to compare the coefficient of friction values measured on a micro- and macroscales.
2. Experimental Microscale friction measurements were made using a FFM developed at NTT [ 12,131 and a commercial atomic force microscope/friction force microscope (AFM/FFM) produced by Digital Instruments [ 1] . The latter instrument can be used for measurements of topography and friction simultaneously. With the Nn/FFM instrument, friction force is measured as a function of normal force while scanning the sample in the x direction for a scan length of 8 pm with a
50
B. Bhushan, A. V. Kulkarni / Thin Solid Films 278 (1996) 49-56
Square pyramidal silicon nitride tip
Three-sided
pyramidal
(natural) diamond tip
Fig. 1. SEM micrographs of a PECVD S&N4cantilever beam with a S&N4tip and a stainless cantilever beam with a diamond tip used in commerci al AFM/ FFM and NTT/FFk instruments.
scanning speed of 800 nm s- ’ [ 131. The method used to measure friction with the commercial instrument is described elsewhere [ 11. The sample is scanned with a scan rate of 2 Hz and a scanning speed of 4 p.m s- ’ on a 1 pmxl pm scan area. For this study, microfabricated S&N4 and polished diamond tips were used. S&N, cantilever beams with integrated square pyramidal tips with a radius of about 30-50 nm are produced by plasma-enhanced chemical vapor deposition (PECVD) (Fig. 1). The normal spring constant of these beams ranges from 0.06 to 0.58 N m-’ and a beam with spring constant of 0.58 N m- ’ was used for the present study. These tips are used in the commercial instruments for surface topography and friction measurements at low loads (typically up to 150 nN) . Single-crystal natural diamond tips are typically used for measurements at high loads ( l-100 p,N) . Diamond tips are also used in the NTT/FFM instruments for friction measurements. The tips are ground to shape of a three-sided pyramid with an apex angle of 80” whose point is sharpened to a radius of about 100-150 nm (Fig. 1). The
radii of the tips used for N’IT/FFM and AFM/FFM instruments were 100 nm and 150 nm, respectively. The length of the tip used for the AFM/FFM instrument was 280 pm. The tips are bonded with conducting epoxy to a gold-plated 304 stainless steel spring sheet (length = 20 mm, width = 0.2 mm, thickness = 20 pm) which acts as a cantilever. The calibration procedure for the normal and friction forces for the AFMIFFM instrument has been described by Bhushan [ 11, Cantilever stiffness value and friction force conversion factors for S&N4 tip are presented by Bhushan [ 11. The diamond tip is mounted on a cantilever beam with a rectangular cross-section. In the case of the NlT/FFM instrument, the length of the cantilever beam is constant with a corresponding spring constant of about 16 N m-l. For the commercial instrument, the free length of the beam can be varied to change the spring constant. The spring constant of an endloaded cantilever beam of rectangular cross-section for the AFM/FFM instrument was calculated using the following equation.
51
B. Bhushan. A.V. Kulkarni / Thin Solid Films 278 (1996) 49-56
where E is the elastic modulus, t is the thickness, w is the width and I is the length of the cantilever beam. The spring constant of the beam with a diamond tip at its end can be varied by choosing the appropriate length of the cantilever. For high load experiments (l-40 ~.LN), the length of the cantilever was 1.3 mm while for the low load experiments it was 2.25 mm. The spring constants calculated using Eq. (1) for the cantilever length of 1.3 mm and 2.25 mm are 36 N m ~ ’ and 6.94 N m- ‘, respectively. The calibration procedure for normal and friction forces for the AFM/FFM instrument using the diamond tip is the same as described by Bhushan [ 1] for S&N4 tip. The conversion factors for a friction force with a lower load range (O-18 PN) for Si( 111)) SiO, and diamond specimens are 36 p,N V-‘, 47.7 )IN V-’ and 13.8 yN V-‘. respectively. The friction force as a function of normal force is then plotted and slope of the line gives the value of the coefficient of friction.
Virgin Si(l1 I)
10
0
I -
30
20 Normal
50
40
Force @.N)
@)
Natural diamond
3. Results and discussion 3.1. Microscale friction measurements 3.1.1. NTUFFM machine with diamond tip Friction measurements were made on as-received singlecrystal Si( ill), HF-cleaned Si( 111) and SiOJSi (0.7 p,m thick dry-oxidized SiO,). For the etched Si( 111) sample, the surface films were removed in dilute hydrofluoric acid ( 1 DI: 1 HF) using a standard etching procedure. Thermal oxidation of the silicon wafer was carried out in a quartz furnace at a temperature of 900-1000 “C in dry oxygen ambient. Friction force as a function of normal force for the three samples is shown in Fig. 2(a). For the SiO, specimen, the friction force increases linearly in the range of O-15 yN with a coefficient of friction (p) of about 0.05. ‘Ihe coefficient of friction increases to about 0.06 above a critical load of about 15 FN. In the case of Si( 111) specimen, the friction force curve also shows two distinct slopes. In the range of applied normal force of O-15 pN, a slope of 0.05 was observed while between 15 and 50 p,N, a slope of 0.2 was obtained. The slope of 0.05 observed in Si( 111) in the range of O-15 p.N corresponds to the coefficient of friction value of the SiOZ specimen. The friction profile of Si( 111) at small values of applied load corresponds to that of the native oxide film present. The presence of a native oxide film of thickness about 3.2 nm was confirmed by ellipsometric measurement of the Si( 11111as-received specimen. To understand the reasons for the two slopes in the friction force, we calculated the contact stresses at different loads. If we assume that the tip can be modelled as a sphere of radius R against a flat specimen surface, we can estimate the radius of the contact area a and maximum contact pressure pmax for an elastic contact by the Hertz equation
00 0
10
20 Normal
Force
30 (pN)
40
50
,
Fig. 2. Friction force as a function of normal force for (a) virgin Si( 111) I-IF-cleaned Si( 111) and 0.7 Frn thick dry oxidized SiOz grown on Si( 111)) and (b) polished natural diamond. p is the coefficient of friction. Measurements were made on a scan length of 8 pm. Inset in Fig. 2(a) shows the data on magnified scale and the arrows in the inset indicate the break in the slope.
(2) (3) where 1 -= E’
-(l-u:)+(l-v;) E,
1 1 -_=-+R’ R,
1 R,
(da) E2
(4b)
W is the normal load, E,, E2 and vi, u2 are the Young’s modulus of elasticity and the Poisson ratios of two contacting bodies, respectively, and R, and R2 are the radii of curvature of the two bodies. For a summary of contact stresses at various loads and with various tips see Table 1. Consider the case of a diamond tip of radius 100 nm and a Si( 111) surface. For a normal load of 1 p,N, the average contact stress is 5.1 GPa while at 15 p,N normal load, the contact stress is 12.7 GPa. The nanoindentation hardness of
B. Bhushan, A. V. Kulkarni / Thin Solid Films 278 (1996) 49-56
52
Table 1 Calculated average contact stresses of Si3N4, diamond tips and a sapphire ball in contact with Si( 111) and natural diamond a Sample
Tip/ball
b
Contact Stresses ( GPa) 1OOnN
Silicon
(111)
Natural diamond
S&N, Natural diamond Sapphire ball S&N4 Natural diamond Sapphire ball
1OOmN
3.3 1.9
10pN
30 pN
15.2 9.0
22.0 13.0
24.4 20.2
35.1 29.0
0.23 5.3 4.3 0.41
188 GPa, Y~~(,,,) =0.28, .EtiamDnd=1140 GPa, ~,,,,,=0.07, a&ic,H,= EsilNl = 3 10 GPa, vsllNl = 0.22, Esapphire= 440 GPa, usapPhire = 0.23’ RSilNl = 50 nm, Rtiamon,,= 150 nm, Rsapphire = 3 mm.
Si( 111) is about 11.5 GPa [ 141. In the present study we observe that the critical load of about 15 P.N, with a contact stress of about 12.7 GPa which is comparable with the hardness of Si( 111) . Therefore, sliding at loads higher than the critical load, the tip not only ploughs through the native oxide but the contact stresses are high enough to cause significant plastic deformation at the interface. Significant ploughing of the silicon surface is responsible for a higher value of the coefficient of friction at higher loads. To substantiate the occurrence of ploughing at high loads, microfriction measurements were performed on Si( 111) at a normal force of l50 pN, inside a marked region using the NTI’/FFM, and surface profiles of the marked region were obtained using the AFM/FFM. The marked area was carefully scanned using S&N4 tip at a normal force of 50 nN. The surface profile of the marked region is shown in Fig. 3. The surface plot clearly shows the scratches generated during the microfriction measurements using NTT/FFM, particularly at higher loads. This observation further confirms that friction is higher at higher loads because of ploughing contribution in the measurements. To study the contribution of the native oxide layer, friction force as a function of normal force for Si( 111) specimen after cleaning with HF solution was measured (Fig. 2(a)). A standard cleaning procedure was employed for removing the native oxide overlayer and the surface film thickness was again measured by ellipsometric technique. Even a freshly cleaned specimen showed the presence of native oxide film of - 1.5 nm. Fig. 2(a) shows the critical load to be around 9 pN with corresponding contact stress of 10.7 GPa. Since the oxide layer thickness has decreased in the cleaning procedure, the ploughing of the diamond tip has occurred at a lower normal force close to Si( 111) hardness. Friction data of silicon specimen after 24 h of cleaning is also shown in Fig. 2(a). The critical load observed in this case is about 13 pN. The native oxide is formed when silicon is exposed to air. The native oxide film thickness must have increased within 24 h, resulting in the increase of the critical load. The inset in Fig. 2(a) shows an enlarged portion of the breakdown region.
Fig. 3. AFM surface profile of the Si( 111) after microfriction measurement at two adjacent locations using using NTT/FFM at a normal force of 0 pN (bottom) to 50 t.cN (near top).
To further understand the mechanisms of load dependence, natural diamond which is extremely hard and does not form an oxide film, was studied. Fig. 2(b) shows the plot of friction force as a function of normal force for this specimen. The normal force was applied in the range O-50 PN. The friction force increased linearly with increasing applied normal force and there was no load dependence on the coefficient of friction. 3.2. Commercial AFWFFM machine with Si,N, and
diamond tips The coefficient of friction measurements of Si( 111)) SiOz coating, and a natural diamond specimen were next made by AIM&FM using a S&N4 tip with a cantilever spring constant of 0.58 N m-‘. Fig. 4 shows the plot of friction force as a function of normal force in the range of 0 to about 100 nN (external force) or about 50 to 150 nN (total force) for these specimens. All plots show a linear variation of friction force with normal force. The negative intercept of the force line with the normal force axis gives the intrinsic adhesive forces 10 0 l
Si(lll) SiO2
l
~=0.06 \
Diamond
0
20 Normal
40 60 Force (nN)
SO
loo
,
0
Fig. 4. Friction force as a function of normal force for virgin Si( 111) , SiOz coating and polished natural diamond. Measurements were made on 1 pm X 1 pm scan area using a SisN., tip.
B. Bhushan, A.V. Kulkarni /Thin Solid Films 278 (1996) 49-56
53
Table 2 Typical rms roughness,
coefficients
of
microscaleand macroscalefriction values of various samples
Coefficientof microscalefriction Rms roughnessa AFM/FFM S&N4 tip ’ NlT/FFM diamond tip b (nm)
Sample
Silicon( 111) Dry-oxidized SiOZ Polished natural diamond a Measured b Obtained ’ Obtained ’ Obtained ’ Obtained
0.14 0.14 2.3
1-15 p,N
20-50 PN
50-150
0.05 0.05 0.11
0.2 0.06 0.11
0.03 0.05 0.06
/LN
Coefficientof macroscalefriction e AFM/FFM
diamond tip d
0.2-15 )LN
20-35 p,N
0.1 N
1N
0.05 0.04 0.023
0.13 0.08 0.023
0.18 0.19 0.07
0.60 0.21 0.07
on 1 mm X 1 mm scan area using AFM. using a diamond tip with radius of 100 nm and at a scanning speed of 800 nm s-l on a 8 mm scan length. using a S&N, tip with radius of about 50 nm and at a scanning speed of 4 mm s-l on a 1 mm X 1 mm scan area. using a diamond tip with radius of about 150 nm at a scanning speed of 4 mm SK’ on a 1 mm X 1 mm scan area. using a sapphire ball with 3 mm radius at an average sliding speed of 0.8 mm s-‘.
resulting from the meniscus effect. The coefficient of friction values obtained are listed in Table 2. The calculated average contact stresses for Si( 111) at a load of 150 nN is 3.7 GPa which is far below the hardness value of SiOZ coating ( 17 GPa) [ 151. We therefore observe a linear plot of friction force as a function of normal force. No measurable ploughing is observed at such small loads.
cl
In order to study friction at higher loads (0.240 p,N) , the diamond tip mounted on a stiff cantilever beam was used in the AFM/FFM. In order to examine the effect of tip material, first the experiments were conducted at the low loads with a cantilever spring constant of 6.94 N m-‘. Friction plots are shown in Fig. 5 (a) and the coefficient of friction values are summarized in Table 2. We note that the coefficient of friction 0.2%
Si(l1 I)
Natural diamond CO-
2 ‘Z
0.15-
(a)
q
Si(ll1)
.
SiO2
l
Natural diamond
2 % t
O.l-
.ii E 8
o!
0
,
,
,
,
200
400
600
800
Normal
1
4
1
0
Force
,
0.05-
0
loo0
n
n
l
..e--I
0
(nN)
ii
IO
z
” -
q 0
” 9
. 1
20 Normal
Force
A
a
+
I
I
30
40
(JLN)
Si(lll)
l
SiO2
l
Naturaldiamond
(b) 0
i0
i0
NormalForce
jo
40
i0
(pN)
Fig. 5. Friction force as a function of normal force for virgin Si( 111). SiOZ coating and polished natural diamond. Measurements were made on a fresh 1 p,rn X 1 km scan areas using a diamond tip with a maximum force to (a) about 1300 nN and (b) 40 pN.
0
1
10
I
1
1
20
30
40
Normal
Force
QLN )
Fig. 6. (a) Coefficient of friction as a function of normal force and (b) corresponding wear depth as a function of normal force for Si( 11 l), SiOl coating and natural diamond. All measurements were made on a fresh 1 em X 1 p.m scan area using a diamond tip.
B. Bhushan, A. V. Kukwni / Thin Solid Films 278 (1996) 49-56
54
SiOz
Si(ll1)
~------T~-----_~~ (a>
“U
Y”
r*
Fig. 7. Surface profiles of the (a) Si( 111)and SiOz coating and (b) natural diamond samples after friction measurements. Normal forces used for friction measurements and corresponding average wear depths are indicated with each profile. The profiles of worn surfaces were obtained at a normal force of 150 nN.
values of Si( 111) and SiOz coating using S&N4 and diamond tips are comparable. Next friction measurements were made at higher loads using the cantilever spring constant of 36 N m- ‘. The normal force applied is in the range of l-40 pN. Fig. 5(b) shows the friction force as a function of normal force for Si( 111) , SiO* coating and polished natural diamond specimen. In the case of the Si( 111) specimen, as the normal force is increased up to 17 p,N, the friction force shows a linear increase with coefficient of friction being 0.05. Beyond 17 p,N normal force, the friction force increases substantially
exhibiting a coefficient of friction value of 0.13. So the friction force shows two distinct slopes. The SiO* coating, on the other hand, shows a linear variation of friction force with a slight increase at about 22 pN. The coefficient of friction of SiOz is 0.04 at loads below 22 PN and increases to 0.08 at higher loads. Load dependence on coefficient of friction for Si( 111) and Si02 is consistent with the data (obtained by the NTT/FFM machine) presented in Fig. 2. To assess the surface damage, the wear profiles of measured areas was obtained and the average wear depths as a
B. Bhushan, A.V. Kulkarni/Thin
55
Solid Films 278 (1996) 49-56
28.8 pN
7.2 pN
750 “”
750
n*
Si(ll1)
750
750 “I4
n*
“”
SiO,
Natural diamond
“”
Fig. 8. Frhion force profiles of Si( 111) at 7.2 pN (mean = 0.42 pN, CY = 0.04 bN) and at 28.8 FN (mean = 2.0 p,N, (T= 0.13 kN) , SQ at 7.2 I.LN (mean = 0.43 0, m=o.o2 CLN) andat28.8 CLN (mean= 1.34ILN,u=O.O5 ~~N),andnaturaldiamondat7,2~N (mean=O.l4FN, g=O.Ol pN) audat28.8 pN (memEO.7 pN, CT= 0.07 pN)
function of normal force for different samples are plotted in Fig. 6(b). The wear profiles are presented in Fig. 7. In profiles at high loads, pile-up of the material is observed at both ends of the scan area. Non-uniformity in wear depth is generally observed. The coefficient of friction data as a function of load using a diamond tip obtained from the data in Fig. 5 are plotted in Fig. 6(a). The knee in the coefficient of friction as a function of normal force for Si( 111) and SiO* coatings corresponds to the knee in the wear depth as a function of normal force curves. Thus, an increase in the coefficient of friction is correlated to the surface damage. Representative friction force profiles of various samples at 7.2 and 28.8 p,N normal forces are shown in Fig. 8. We note that the local variation in the friction force at a higher force of 28.8 pN is
high as compared with that of 7.2 p,N. This increase is again associated with the ploughing damage at high loads. 3.3. Comparison between microscale and mucroscaie friction data Microscale friction data are compared with macroscale friction data in Table 2. We note that macroscale values are higher than those of microscale values at lower contact stresses. When measured for the small apparent areas of contact and very low loads used in the microscale measurements, the indentation hardness and modulus of elasticity are higher than that at macroscale. This reduces the degree of wear in microscale measurements. In addition, a small apparent area
56
B. Bhushan. A. V. Kulkarni/ Thin Solid Films 278 (1996) 49-56
of contact reduces the number of trapped particles at the interface, and thus minimizes the ploughing contribution to the friction force. These differences explain the lower friction and wear in microscale measurements.
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4. Conclusions
References
The coefficient of friction on a microscale is found to be a strong function of normal force. The critical load at which an increase in the coefficient of friction occurs corresponds to the surface hardness. The presence of native oxide films protects the interface thus increasing the critical load. Ploughing at high loads is responsible for high values of the coefficient of friction. The macroscale coefficient of friction at lower contact stress is higher than that on amicroscale. These trends can be explained by the differences in the mechanical properties at different scales and in the apparent areas of contact. Thus Amontons’ law of friction which states that the coefficient of friction is independent of the apparent area of contact and normal force does not hold for microscale measurements. These findings suggest that microcomponents sliding under lightly loaded conditions should experience ultralow friction and near-zero wear. Acknowledgements The research reported in this paper was sponsored by the Department of Navy, Office of the Chief of Naval Research.
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