Lubricative Property of Graphite Fluoride

228 Chapter 7 Lubricative Property of Graphite Fluoride 7.1 Introduction Graphite is composed of carbon monolayers stacked by van der Waals forces in the direction of the c-axis. As the interlayer force between monolayers is small, graphite is used as a solid lubricant. When carbon or graphite is directly fluorinated at a high temperature, a strong C-F covalent bond is formed, resulting in loss of aromaticity. The bond energy between fluorine and carbon is so strong as not to be ruptured even by high temperature and pressure. It was stated in Chapter 3 that graphite fluoride has a lower surface energy than poly(tetrafluoroethylene) because of the strong covalency and small polarizability of the C-F bond. In graphite fluoride, the chemical species facing other monolayers is fluorine bonded to tertiary carbons. It is therefore expected that graphite fluoride exhibit lubricity in the same way as graphite itself and molybdenum disulfide (M0S2). The first patent for lubrication by graphite fluoride appeared in 1961 (1). However, no data was included in this patent, and quantitative information was not available prior to 1969 (2,3). After these, several papers were published in Japan and in the USA on the lubricity of graphite fluoride. This chapter summarizes the results of reported lubrication tests. 7.2 Graphite Fluoride as a Solid Lubricant Fusaro and Sliney reported lubrication tests for graphite fluoride-rubbed films (2) and polyimide-bonded (CF)n films (1). Figure 7.1 shows the result for (CF)n rubbed on sand-blasted 440-C stainless steel in dry air in comparison with M0S2. The temperature limits are 480°C for (CF)n and 400°C for M0S2. As the decomposition temperature of the graphite fluoride used was 420°C, the temperature limit of 480°C may be due to an increase in the friction coefficient and a decrease in the wear life of the film. The friction coefficients of (CF)n films are slightly higher (0.03-0.15) than those of M0S2 films (0.02) in the temperature range 25-250°C. However, the wear lives of (CF)n films are more than six times those of M0S2. The same experiments were performed using 301 stainless steel disks and 440-C stainless steel riders. 3G1 disks are not as hard as 440-C disks. Figure 7.2 shows the results. Under the same conditions as in the above case, neither M0S2 nor graphite give continued lubrication. Only graphite fluoride shows excellent lubricity, though its wear life decreases compared with the case for 440-C disks. The friction coefficient is 0.015-0.03 in the temperature range 25-260° C. The variation in the friction coefficient with time at 25°C and 400°C, as shown in Figure 7.3, indicates that graphite fluoride is a better lubricant than M0S2 or graphite.

229

Unlubricated metal

/Decomposition ' of ( C F x) n

100

20 0

30 0

40 0

50 0

60 0

Temperature/°C

Figure 7.1 Effect of temperature on wear life and friction coefficient of graphite fluoride ((CFi.i2)n) and molybdenum disulfide powders burnished on sand-blasted 440-C stainless steel disks Riders, 440-C stainless steel; linear sliding speed, 1.6 meters per second; load, 500 grams; atmosphere, dry air (moisture content, 20 ppm) Powder O: (CFi.i2)n Δ: MoS2

U n l u b r i c adt e m e tla D e c o m p o s in tio of ( C F x) n 0

010

20 0

30 0

40 0

50 0

Temperature/°C

Figure 7.2 Effect of temperature on wear life and friction coefficient of graphite fluoride ((CFi.i2)n), graphite, and molybdenum disulfide powders burnished on glass-peened 301 stainless-steel disks Riders, 440-C stainless steel; linear sliding speed, 1.6 meters per second; load, 500 grams; atmosphere, dry air (moisture content, 20 ppm) Powder O: (CFi.i2)n V: Graphite Δ: MoS2

230

ff

Powder

ι 4,5

ϋ

3

0

1

440-C

2

301 440-C

( C F 1 1 )2n ( C F i . i 2) n 3 MoS 2 4 M0S2

Γ

=

Stainlessstee 1 disk

I

1 /

301 301

5 Graphite

1

ί 2

I

I

I

I

I

I

J_

_J_

I

I

(a) Temperature, 25°C.

120

160

J__ 200

J_

240

J__ 280

_L

320

JL

360

400

440

J_

480

520

Time/min (b) Temperature, 400°C

Figure 7.3 Variation of friction coefficient with time for burnished films of graphite fluoride ((CFi.i2)n), molybdenum disulfide, and graphite at 25° and 400°C Riders, 440-C stainless steel; linear sliding speed, 1.6 meters per second; load, 500 grams; atmosphere, dry air (moisture content, 20 ppm) Powder Stainless-steel disk 1 (CFi.i2)n 440-C 2 3

(CFi.i2)n M0S2

301 440-C

4 5

M0S2 Graphite

301 301

The lubrication tests performed in dry argon show that the friction coefficient of (CF)n is about the same (0.02-0.04) as that of films tested on 301 disks in dry air, however, the wear life decreases to 50 min compared with 250 min in dry air at 25°C. M0S2 and graphite don't lubricate in dry argon. Though it is known that graphite shows a better lubricating ability in moist air than in dry air, (CF)n has a lower friction coefficient (0.05) than graphite (0.09) in moist air, and its wear life is also longer (>700 min) than that of graphite (350 min)(Table7.1).

231 Table 7.1 Comparison of friction coefficient and wear life of burnished films of graphite fluoride, graphite, and molybdenum disulfide in three different atmospheres at 25°C Powder

Disk substrate (stainless steel)

Minimum friction coefficient Atmosphere Moist air Dry air

Dry argon Moist air Dry air

301 301

0.05

Graphite

.09

Immediate Immediate failure3 failure

M0S2

301

—

440-C 440-C

.06

Immediate Immediate failure failure .15 — .02 —

(CFi.i2)n

(CF 1 1 2 ) n M0S2

Wear life, min.

.15

0.02

0.025

Dr>r argon

700+ 350

250 0

50 0

—

0

0

1200 30

450

— —

70

Criterion for failure was a frictional force equal to that of unlubricated metal combination.

Lubrication tests were made using CFx(x = 0.7-1.12). With decreasing x, the amount of (C2F)n increased. However, no appreciable difference in the friction coefficients and wear lives was observed probably because the interlayer structures of (CF)n and (C2F)n are the same. The lubricity of graphite fluoride was examined by using a polyimide resin (PI) as the binder (1). Polyimides are characterized by a high thermal stability (400°C in air, 500°C in inert atmospheres), due to multiple bonds between the aromatic and heterocyclic rings. They are resistant to most common chemicals and solvents, except alkalis. Figure 7.4 shows the friction coefficients of Pi-bonded (CF)n and PIbonded M0S2 as a function of the temperature. The friction coefficient of Pi-bonded (CF)n remains constant at 0.08 in the temperature range 25-500°C. That of PIbonded M0S2 is 0.04 at 25-300°C, increasing to 0.05 at 350°C and to 0.08 at 400°C. The wear lives of four solid lubricant films are compared in Figure 7.5. The wear life of rubbed-on films of (CF)n is at least four times that of rubbed-on M0S2 over the whole temperature range. Pi-bonded (CF)n is up to ten times better than (CF)nrubbed films. At 25°C, the wear life of the Pi-bonded (CF)n is about twice that of the Pi-bonded M0S2 film. With increasing temperature, the difference in the wear lives increases, being about a factor of 60 at 400°C.

232

60 r

Pl-bonded

(CFi.i)n

Pl-bonded

M0S2

Unlubricated steel

•5

440C

stainless

20

300

200

400

500

Temperature/°C

Figure 7.4 Friction coefficients as a function of temperature for Pl-bonded graphite fluoride ((CFi.i)n) and Pl-bonded molybdenum disulfide (M0S2) films applied to 440-C stainless steel disks Riders, 440-C stainless steel disks; linear sliding speed, 2.6 meters per second; load, 1 kg; atmosphere, dry air (moisture content, 20 ppm) · : Pl-bonded (CFi.i)n

■ : Pl-bonded M0S2

10*FZ

V : Unlubricated 440C stainless steel

Pl-bonded

(CFi.!)n

Pl-bonded

M0S2

Rubbed-on

(CFi.!)n

Rubbed-on

M0S2

? 10*b

300

400

Temperature/°C

Figure 7.5 Wear life as a function of temperature for 440-C stainless steel disks lubricated with rubbedon films and polyimide (Pl)-bonded films of graphite fluoride ((CFi.i)n) and molybdenum disulfide (MoS2) Riders, 440-C stainless steel; linear sliding speed, 2.6 meters per second; load, 1 kg; atmosphere, dry air (moisture content 20 ppm); failure criterion, a friction coefficient of 0.30 · : Pl-bonded (CFi.i)n

■ : Pl-bonded M0S2

Δ : Rubbed-on (CFi.i)n

O: Rubbed-on M0S2

233

Gisser et al. examined the lubricity of graphite fluoride using silicate and epoxyphenolic binders (4). Falex machine tests showed that the wear life of graphite fluoride was superior to that of graphite with both types of binders. With silicate films, graphite fluoride gave 50% greater wear life than graphite, and with epoxy phenolic films, graphite fluoride had a 40% greater wear life than graphite. The wear life tests were made on a Four-Ball machine at 27 °C and 200° C. The wear scar diameters on the stationary balls were measured. The results indicate that at room temperature, there is little difference between graphite and graphite fluoride in a silicate binder, but there is an appreciable difference in wear in an epoxy-phenolic binder, namely, graphite fluoride gives lower wear. At 200°C, there is a sizable decrease in wear with graphite fluoride in both silicate and epoxy-phenolic binders. The extent of wear at 200°C is considerably lower than at 27°C. Table 7.2 shows the friction coefficients of graphite fluoride in dry, rubbed films and as an additive in grease, in comparison with those of graphite. Data were obtained from room temperature to 344°C. The friction coefficient of rubbed graphite fluoride is relatively constant (0.10-0.13) from 27°C to 344°C, but for rubbed graphite films, a rapid increase in the friction coefficient occurs at temperatures lower than 250°C. Friction failure of the grease alone occurs at 215°C, due to the thermal and oxidative decomposition of the grease. However, the addition of 2% graphite fluoride to grease prevents friction failure in spite of the decomposition of the grease at high temperatures. Table 7.2 Coefficient of friction when sliding a 52100 steel rider on a 1020 steel disk covered with rubbed and grease films T/°C

27 93 215 260 320 344

Graphite Rubbed

Graphite Fluoride Rubbed

0.19 0.19 0.11 0.48

0.12

0.53

0.13 0.11 0.10 0.10

—

0.11

Greaseb 0.14 0.12 SSC

Grease + 2% Grease + 2% Graphite Fluoride Graphite 0.15 0.17

0.13 0.13

SS

SS SS

0.13 0.12

— —

— —

0.15 0.08

Coefficient of friction of unlubricated slider on disk at 27°C = 0.74 Lithium grease (8%) in bis(2-ethylhexyl) sebacate Stick-slip motion, coefficient of friction could not be measured.

The lubricating ability of graphite fluoride was also examined by several Japanese companies. Figure 7.6 shows the friction coefficients as a function of the load (5). Graphite fluoride-added lithium grease lubricates much better than MoS2-added grease. The friction coefficient, about 0.12, is unchanged up to 20 kg/cm2 with the addition of 10% graphite fluoride. The temperature increase from sliding friction

234

is shown in Figure 7.7 (5). In the case of lithium grease alone, the temperature increases to around 160°C. However, the addition of 10% (CF)n suppresses the temperature increase to about 60°C. Figure 7.8 shows a comparison of the lubricities of graphite fluoride and M0S2 added to a carbon material (5). (CF)n lubricates well under high PV values while M0S2 causes burning thirst through frictional heat. Figure 7.9 illustrates the effect of addition of (CF)n to a PTFE-coated carbon fiber packing (5). For a packing without (CF)n, burning thirst occurs due to the increase in frictional heat over 5500 kg/cm2-m/min. However, a (CF)n-incorporated packing shows better lubricity as the amount of (CF)n added is increased.

0.4 Lithium grease

M0S2 10%

0.3 c

V

Φ

O

o c o

0.2

0.1

I

0

I

I

5

10

15

20

Load / k g / c m 2 Figure 7.6 Friction coefficient as a function of load, obtained by a four ball machine

235

Figure 7.7 Temperature increase with friction

M0S2 / M0S2 PV=1200/ PV=1000

CF

0

60

PV=1000

120 Time/min

Figure 7.8 Temperature increase with friction under high PV values

400

300

-

4*% 5%

200

A<

« ^

> ^ —

w

_____

Δ25%

°

> ·

100

^y

V

x

4500

5500

PV v a l u e / k g cm 2-m

6500 min"1

Figure 7.9 Effect of graphite fluoride on temperature increase with friction

236 When graphite fluoride is added to the carbon slider of a rotary pump, the wear loss, temperature increase and noise are all decreased (Fig. 7.10)(6).

x>

5 ^

4 3 CO

0)

s

2

J ^ ^ ^

^ s ^

1

^s^

0

^

o ^

Carbon

Carbon+ ( C F ) n 1 %

_

β

1000

2000

— %

Time/hr

Carbon OQ Ό

ω O

z

_rr^^^'—*""

80 \ 70 0Ίr

\f

φ

Carbon+(CF)n 1% •

1

1000

. .1

2000

·— 1

3000

Time/hr

o

o

Q) ft-

60

3

Φ

a E o

Carbon+(CF) n 1%

50 0"t0

1000

2000

3000

Time/hr Figure 7.10 Effect of addition of graphite fluoride to carbon slider

237

7.3 Lubricity of (CF)„-Co-Deposited Metal Film It is difficult to disperse graphite fluoride powder in plating baths because of the low surface energy of graphite fluoride. The dispersion of graphite fluoride is achieved by adding a cation surface-active agent to the electrolytic bath. The surface-active agent used is (C8Fi7S02NH(C2H5)2R)+I~ which gives hydrophilicity to graphite fluoride. Plating occurs in a Watts nickel bath in which (CF)n is suspended. Figure 7.11 shows the amount of co-deposited (CF)n as a function of the (CF)n content in the electrolyte (7). Co-deposited (CF)n increases with increasing (CF)n content in the electrolyte, reaching a constant value at around 30 g/1. The amount of (CF)n in the nickel matrix is shown in Figure 7.12 as a function of the current density. The (CF)n content in a nickel deposit decreases with increasing current density. However, with an increase in the (CF)n suspended in the electrolyte, the co-deposited (CF)n becomes constant independent of the current density. Co-deposition of (CF)n in a copper matrix gives similar results (8).

x

*·»hm CO E

"Ξ C IL·

O

(CF)n

content

in electrolyte / g/l

Figure 7.11 Variation in (CF)n content in a nickel deposit with increase in (CF)n content in the electrolyte at current density of 4 A/dm 2

238

Current density / A/dm 2

Figure 7.12 Variation of (CF)n content in a nickel deposit as a function of current density and contents of (CF)n in the electrolyte a:2, b:4, c:15, d:30, e:50 (g/1)

A (CF)n-co-deposited nickel film has been applied to the horizontal continuous casting of steels (9). The friction coefficients of a (CF)n-Ni composite film are shown in Figure 7.13. At room temperature, the difference in the friction coefficients is not so large. However, with increasing temperature, the difference rapidly increases. At 300°C, the friction coefficients of Cr and Ni platings are 0.95 and 0.65, respectively, while a (CF)n-Ni plating still has a low value, 0.1-0.2. The casting of steel using a mould plated with (CF)n-Ni film has a withdrawal force less than 0.5 t, but with a Cr-plated mould, it ranges from 0.5 to 2.0 t (Fig. 7.14).

1.0

—(J^PIating

/

SUS 304

/

/ C r plating

c

/

0.5

/

/

ft /

/

o

/ /

/

/

/

/

// 'Ni

/

P plating

N i + (CF) n plating

^^o10%(CF)n ^ ^ - - -. €l5%(CF) n

I

R.T

300 °C Temperature/°C

Figure 7.13 Coefficient of friction of the respective plating layers

239

- - 0 - - (Ni + graphite fluoride) plating — · - Cr plating 2.0 [

191 0 SUS 304

Casting length/m

Figure 7.14 Withdrawl force during casting of stainless steel

REFERENCES 1. Fusaro RL, Sliney HE (1972) NASA Technical Note D-6714 2. Fusaro RL, Sliney HE (1969) NASA Technical Note D-5097 3. Ishikawa T, Shimada T (1969) 5th International Symposium on Fluorine Chemistry, Moscow 4. Gisser H, Petronio M, Shapiro A (1971) International Conference of Solid Lubrication, Aug. 24 5. Ishikawa T, Takeda Y (1973) In: Watanabe N (ed) Fluorine Chemistry and Industry. Kagaku Kogyosha, p 82 6. Watanabe N, Matsuo K, Fujii R, Hoshikawa T, Arai M (1975) Abstract of Annual Meeting of Carbon Society of Japan, p 92 7. Yamaguchi F, Kurosaki S, Watanabe N (1975) Denki Kagaku 43:57 8. Yamaguchi F, Okamoto Y, Kurosaki S, Watanabe N (1975) Denki Kagaku 43:106 9. Umeda Y, Sugitani Y, Miura M, Nakai (1981) Tetsu to Hagane 67:1377