Bhushan B. Handbook of Micro/Nano Tribology, Second Edition

Подождите немного. Документ загружается.

various doses of C

, B

, N

, and Ar

ion species at 200 keV energy. The coefﬁcient of friction and wear

factor of C

-implanted silicon samples as a function of ion dose is presented in Figure 16.20 (Gupta et al.,

1993). The friction and wear tests were conducted using a ball-on-ﬂat tribometer (Gupta et al., 1993).

Each data bar represents the average value of four to six measurements. The coefﬁcient of friction and

wear factor decrease drastically with ion dose. Silicon samples bombarded above 10

–2

exhibit

extremely low values of coefﬁcients of friction (typically 0.03 to 0.06 in air) and the wear factor (reduced

by as much as four orders of magnitude). Gupta et al. (1993) reported that a decrease in coefﬁcient of

friction and wear factor of silicon as a result of C

ion bombardment occurred because of formation of

silicon carbide rather than amorphization of silicon. Gupta et al. (1994) also reported an improvement

in friction and wear with B

ion implantation.

For magnetic disk drive applications, macroscale friction and wear experiments have been performed

using a magnetic disk drive with unoxidized, oxidized, and implanted pins sliding against amorphous

carbon–coated magnetic disks lubricated with perﬂuoropolyether lubricant (Bhushan and Venkatesan,

1993; Venkatesan and Bhushan, 1993, 1994). The data of silicon samples were compared with the

commonly used slider materials in disk drives: Al

–TiC and Mn–Zn ferrite. Representative proﬁles of

the variation of the coefﬁcient of friction with number of sliding cycles for Al

–TiC slider and uncoated

and coated silicon pins are shown in Figure 16.21. Friction data obtained from the various tests conducted

in ambient air in terms of the initial coefﬁcient of friction, the contact life, i.e., the number of revolutions

before the coefﬁcient of friction increased by a factor of two, the maximum value of the coefﬁcient of

friction for cases when the increase was more than by a factor of two are presented in Table 16.5. For

the case of oxidized samples, a signiﬁcant increase (by a factor of two or more) was not observed and

so the range of variation of the coefﬁcient of friction for the duration of 50,000 cycles is indicated. It

was found that crystalline orientation of silicon has no effect of friction and wear. For bare silicon, after

initial increase in the coefﬁcient of friction, it drops to a steady state value of 0.1 following the increase,

as seen in Figure 16.21a. The rise in the coefﬁcient of friction and damage on the pin surface for Si(111)

is associated with the transfer of amorphous carbon from the disk to the pin, oxidation-enhanced fracture

of pin material followed by tribochemical oxidation of the transfer ﬁlm. The drop is associated with the

formation of a transfer coating on the pin, Figure 16.22a. The mechanism of transfer and tribochemical

oxidation was seen to be also operative for the friction increase for Al

–TiC and Mn–Zn ferrite

pin/slider materials tested in ambient air. As seen in Table 16.5 and Figure 16.21b, dry-oxidized Si(111)

exhibits excellent characteristics and this behavior has been attributed to the chemical passivity of the

oxide and lack of transfer of DLC from the disk to the pin. The behavior of PECVD was comparable to

that of dry oxide, but for the wet oxide there was some variation in the coefﬁcient of friction (0.26 to

0.4). The difference between dry and wet oxide has been attributed to increased porosity of the wet oxide

(Bhushan and Venkatesan, 1993; Pilskin, 1977).

Since tribochemical oxidation was determined to be a signiﬁcant factor, experiments were conducted

in dry nitrogen (Venkatesan and Bhushan, 1993, 1994). The variation of the coefﬁcient of friction for a

silicon pin sliding against a thin-ﬁlm disk is shown in Figure 16.21a. For comparison the behavior in

ambient is also shown. It is seen that in a dry nitrogen environment, the coefﬁcient of friction of Si(111)

sliding against a disk decreased from an initial value of about 0.2 to 0.05 with continued sliding. Similar

behavior was also observed while testing with Al

–TiC and Mn–Zn ferrite pins and sliders. Based on

SEM and chemical analysis this behavior has been attributed to the formation of a smooth amorphous-

carbon/lubricant transfer patch and suppression of oxidation in a dry nitrogen environment,

Figure 16.22b.

The experiments in dry nitrogen indicated that low friction conditions can be achieved in dry nitrogen

although transfer of carbon from the disk to the pin occurs. An experiment was performed with a

hydrogenated amorphous carbon–coated (~18 nm thick) silicon pin sliding against a lubricated thin-

ﬁlm disk in dry nitrogen. The friction variation with sliding for this experiment is shown in Figure 16.23.

No damage for the pin and disk surfaces could be detected after this experiment.

Based on macrotests using disk drives, we ﬁnd that the friction and wear performance of bare silicon

is not adequate. With single and polyscrystalline silicon sliding against an amorphous carbon–coated

magnetic disk, transfer of amorphous carbon from the disk to the pin/slider and oxidation-enhanced

fracture of pin/slider material followed by oxidation of the transfer coating (tribochemical oxidation) is

responsible for degradation of the sliding interface and consequent friction increase in ambient air. With

dry-oxidized or plasma-enhanced CVD SiO

-coated silicon, no signiﬁcant friction increase or interfacial

degradation was observed in ambient air. In the absence of an oxidizing environment (in dry nitrogen),

the coefﬁcient of friction decreased from 0.2 to 0.05 following amorphous carbon transfer for the

materials tested.

16.3.4 Boundary Lubrication Studies

The classical approach to lubrication uses freely supported multimolecular layers of liquid lubricants

(Bhushan, 1995). The liquid lubricants are chemically bonded to improve their wear resistance. To study

depletion of boundary layers, the microscale friction measurements were made as a function of number

of cycles on virgin Si(100) surface and silicon surface lubricated with about 2-nm-thick Z-15 and Z-DOL

perﬂuoropolyether (PFPE) lubricants, the data of which are shown in Figure 16.24 (Koinkar and Bhushan,

1996a). Z-DOL is a PFPE lubricant with hydroxyl end groups. Its lubricant ﬁlm was thermally bonded

at 150°C for 30 min and washed off with a solvent to provide a chemically bonded layer for the lubricant

ﬁlm. In Figure 16.24, the unlubricated silicon sample showed a slight increase in friction force followed

by a drop to a lower steady-state value after 20 cycles. Depletion of native oxide and possible roughening

of the silicon sample are believed to be responsible for the decrease in friction force after 20 cycles. The

initial friction force for Z-15 lubricated sample is lower than that of unlubricated silicon and increases

gradually to a friction force value comparable to that of unlubricated silicon after 20 cycles. This suggests

depletion of the Z-15 lubricant in the wear track. In the case of the Z-DOL-coated sample, the friction

force starts out to be low and remains low during the cycle of 100 tests. It suggests that Z-DOL does not

get displaced or depleted as readily as Z-15. Additional studies of freely supported liquid lubricants

showed that either increasing the ﬁlm thickness or chemically bonding the molecules to the substrate

with a mobile fraction improves the lubrication performance (Koinkar and Bhushan, 1996a,b).

For lubrication of microdevices, a more effective approach involves the deposition of organized, dense

molecular layers of long-chain molecules on the surface contact (Bhushan et al., 1995b). Such monolayers

and thin ﬁlms are commonly produced by Langmuir–Blodgett (LB) deposition and by chemical grafting

of molecules into self-assembled monolayers (SAMs). Based on measurements, SAMs of octodecyl (C

)

compounds based on aminosilanes on oxidized silicon exhibited a lower coefﬁcient of friction (0.018)

and greater durability (Figure 16.25) than LB ﬁlms of zinc arachidate adsorbed on a gold surface coated

with octadecylthiol (ODT) (coefﬁcient of friction 0.03) (Bhushan et al., 1995b). LB ﬁlms are bonded to

the substrate by weak van der Waals attraction, whereas SAMs are chemically bound via covalent bonds.

Because of the choice of the chain length and terminal linking groups that SAMs offer, they hold great

promise for boundary lubrication of microdevices.

16.3.5 Component Level Studies

Friction and wear studies have been conducted on actual or simulated MEMS components. One of the

MEMS devices in which friction and wear issues are critical, is a micromotor. To measure the in situ

static friction of a rotor-bearing interface in a micromotor, Tai and Muller (1990) measured the starting

torque (voltage) and pausing position for different starting positions under a constant-bias voltage. A

friction-torque model was used to obtain the coefﬁcient of static friction. To in situ measure the kinetic

friction of the turbine and gear structures, Gabriel et al. (1990) used a laser-based measurement system

FIGURE 16.15 Gray-scale maps of surface height corresponding friction force maps of Si(100), undoped polysilicon

ﬁlm (polished), n

-type polysilicon ﬁlm (polished), and SiC ﬁlm (polished) samples. (From Bhushan, B. et al., 1998,

in Tribology Issues and Opportunities in MEMS, B. Bhushan, ed., Kluwer Academic, Dordrecht. With permission.)

to monitor the steady-state spins and decelerations. Lim et al. (1990) designed and fabricated a polysilicon

microstructure to in situ measure the static friction of various ﬁlms. The microstructure consisted of

shuttle suspended above the underlying electrode by a folded beam suspension. A known normal force

was applied and lateral force was measured to obtain the coefﬁcient of static friction. Mehregany et al.

(1992) developed a quantitative method for in situ wear measurements in micromotors. They used a

wobble micromotor under electric excitation for quantitative wear measurement. Since the gear ratio of

the wobble micromotor depends on the bearing clearance, changes in the gear ratio can be a direct

measure of wear in the motor bearing.

FIGURE 16.16 (a) Scratch depths for 10 cycles as a function of normal load and (b) wear depths as a function of

normal load and as a function of number of cycles for various samples. (From Bhushan, B. et al., 1998, in Tribology

Issues and Opportunities in MEMS, B. Bhushan, ed., Kluwer Academic, Dordrecht. With permission.)

16.4 Closure

Silicon-based MEMS devices are made from single-crystal silicon, LPCVD polysilicon ﬁlms and other

ceramic ﬁlms. For high-temperature applications, SiC ﬁlms are being developed to replace polysilicon

ﬁlms. Tribology in MEMS devices requiring relative motion is of importance. AFM/FFM and nanoin-

dentation techniques have been used for tribological studies on the micro- to nanoscale on materials of

FIGURE 16.17 Scratch proﬁles for various samples.

(From Bhushan, B. et al., 1998, in Tribology Issues and

Opportunities in MEMS, B. Bhushan, ed., Kluwer Aca-

demic, Dordrecht. With permission.)

interest. These techniques have been used to study surface roughness, friction, scratching/wear, inden-

tation, and boundary lubrication of bulk and treated silicon, polysilicon ﬁlms, and SiC ﬁlms. Macroscale

friction and wear tests have also been conducted using the ball-on-ﬂat tribometer.

Measurements of microscale and macroscale friction forces show that friction values on both scales

of all the silicon samples are about the same among different silicon materials and higher than that of

SiC. The microscale values are lower than the macroscale values as there is less plowing contribution in

the microscale measurements. Surface roughness has an effect on friction. In microscale and macroscale

FIGURE 16.18 3D wear proﬁles of undoped polysilicon and SiC ﬁlms (polished) after 1, 5, 10, and 20 wear cycles

at 20 µN normal load. (From Bhushan, B. et al., 1998, in Tribology Issues and Opportunities in MEMS, B. Bhushan,

ed., Kluwer Academic, Dordrecht. With permission.)

tests, C

-implanted, oxidized, and PECVD oxide-coated single-crystal silicon samples exhibit much larger

scratching and wear resistance as compared with untreated samples. Polysilicon ﬁlms and undoped single-

crystal silicon show similar friction and wear characteristics. Doping of polysilicon ﬁlm does not affect

its tribological properties. Microscratching, microwear and nanoindentation, and macroscale friction

and wear studies indicate that SiC ﬁlms are superior when compared to the other materials currently

used in MEMS devices. Higher hardness and fracture toughness of the SiC ﬁlm is believed to be respon-

sible for its superior mechanical integrity and lower friction. Chemically grafted self-assembled mono-

layers and chemically bonded liquid lubricants show promising performance for boundary lubrication

in MEMS devices.

FIGURE 16.19 Representative load–displacement curves for various samples.

TABLE 16.4 Summary of Micro/Nanotribological Properties of the Sample Materials

Sample

rms

Roughness

(nm)

P-V

Distance

(nm)

Coefﬁcient

of Friction

Scratch

Depth

(nm)

Wear

Depth

(nm)

Nanohardness

(GPa)

Young’s

Modulus

(GPa)

Fracture

Toughness,

(MPA )Micro

Macro

Undoped Si(100) 0.09 0.9 0.06 0.33 89 84 12 168 0.75

Undoped polysilicon

ﬁlm (as deposited)

46 340 0.05 — — — — — —

Undoped polysilicon

ﬁlm (polished)

0.86 6 0.04 0.46 99 140 12 175 1.11

-type polysilicon ﬁlm

(as deposited)

12 91 0.07 — — — — — —

-type polysilicon ﬁlm

(polished)

1.0 7 0.02 0.23 61 51 9 95 0.89

SiC ﬁlm (as deposited) 25 150 0.03 — — — — — —

SiC ﬁlm (polished) 0.89 6 0.02 0.20 6 16 25 395 0.78

Measured using AFM over a scan size of 10 × 10 µm.

Measured using AFM/FFM over a scan size of 10 × 10 µm.

Obtained using a 3-mm-diameter sapphire ball in a reciprocating mode at a normal load of 10 mN and average sliding speed of 1 mm/s after 4 m

sliding distance.

Measured using AFM at a normal load of 40 µN for 10 cycles, scan length of 5 µm.

Measured using AFM at normal load of 40 µN for 1 cycle, wear area of 2 × 2 µm.

Measured using Nanoindenter at a peak indentation depth of 20 nm.

Measured using microindenter with Vickers indenter at a normal load of 0.5 N.

TABLE 16.5 Macrofriction Data for Various Hemispherical Pins with Radius of 100 mm Sliding against

Lubricated Thin-Film Rigid Disks in Ambient Air

Pin Material

Initial Coefﬁcient

of Friction

Cycles to Friction

Increase by a Factor of 2

Max. or Ending Value of

Coefﬁcient of Friction

–TiC

Mn–Zn ferrite

Single-crystal silicon

Polysilicon

PECVD-oxide-coated Si(111)

Dry-oxidized Si(111)

Wet-oxidized Si(111)

-implanted Si(111)

0.20

0.22

0.20

0.28

0.22

0.26

0.16

~2,200

~5,500

~1,200

~3,000

>50,000

~1,000

0.78

0.45

0.40

0.23–0.28

0.20

0.26–0.40

0.60

Normal load = 0.5 N; sliding speed = 0.9–1.2 m/s; ambient air (RH 45 ± 5%).

FIGURE 16.20 Inﬂuence of ion doses on the (a) coefﬁcient of friction and (b) wear factor on C

ion bombarded

single-crystal and polycrystalline silicon slid against alumina ball. S and P denote tests that correspond to single-

and polycrystalline silicon, respectively, and V corresponds to virgin single-crystal silicon. (From Gupta, B.K. et al.,

1993, ASME J. Tribol. 115, 392–399. With permission.)