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The formation of gases due to the sliding friction of teflon on steel in ultrahigh vacuum
Wear -
Elsevier Sequoia S.A., Lausanne - Printed
THE FORMATION OF GASES DUE TO THE SLIDING ON STEEL IN ULTRAHIGH VACUUM
W. WILKENS
AND
Deutsche ForschungsBraunschweig,
215
in The Netherlands
FRICTION
OF TEFLON
0. KRANZ und Versuchsanstalt fiir Luft- ulad
Raumfahrt, Forschungszentrum
(Deutschland)
(Received December
12, 1969)
SUMMARY
The formation of gases by sliding friction of self-lubricating material, Teflon filled with 25% carbon black, was investigated and the quantity and the composition of the gases evolved determined. By mass spectrometry, it was shown that all gases produced by the sliding process can be attributed to fragmentation of the Teflon molecule. At least 60% of the gases evolved during the sliding process condense on a copper surface cooled by liquid nitrogen. INTRODUCTION
Materials producing gases in vacuum which condense or react chemicallyon sensitive surfaces such as the surfaces of electrical contacts, optical systems, temperature control coatings, etc. have, as far as possible, to be excluded from spacecraft applications. The formation of gaseous products due to sliding friction is a troublesome phenomenon in vacuum techniques, especially with rotary feed-throughs but no published information concerning quantity and composition is available. BUCKLEY AND JOHNSON, working on friction and wear in vacuum, have reported on decomposition mechanisms of synthetics caused by friction in ultrahigh vacuum 112but their measurements only refer to the composition of the residual gases during the sliding process. The aim of the present work was to measure the quantity of gas produced by sliding friction, to obtain information on the composition of the gas by mass spectrometry and to look for condensable or adsorbable components. The coefficient of friction at different levels of total pressure, especially in ultrahigh vacuum, was also determined. The best Teflon composition with respect to friction and wear behaviour contained, according to BUCKLEY AND JOHNSONI, 25% coke flour. Thus, a similar material, a 25% carbon-black-filled Teflon, was used in the present experiments. The investigation of gas production due to friction is not only important because of the danger of contamination in spacecraft, but also because of the degradation mechanism of the bearing material and the undesirable changes in the composition of residual gases in ultrahigh-vacuum systems. Wear, 15 (1970) 215-227
216
W. WILKENS,
EXPERIMENTAL
The test
0. KRANZ
EQUIPMENT
facility
enables
the simultaneous
measurement
of load,
frictional
forces, temperatures, and total and partial pressures of the residual gases. It is possible to change the test geometry and load and to unload the specimens during test. The ultrahigh-vacuum test facility The apparatus used is shown in Fig. I. The main part is a double-walled vacuum system, (Leybold-Heraeus Type UP 30) in which total pressures in the low 10-s torr range are achievable by oil diffusion pumps, well baffled with liquid nitrogen
Auxiliary vocuum:p~10-5
Ultmhighvocuum: p=2.KPg
torr-
tow
Fig. I. Ultrahigh vacuum test facility for friction experiments. I, Drive for rotary feedthrough; 2, drive control; 3, revolution counter, 4; axle-shaft with disk; 5, device for measuring the coefficient of friction; 6, electron gun for cleaning surfaces; 7, Bayard12, omeAlpert gauge ; 8-1 I, amplifiers and recorder- - for measuring forces and temperatures; gatron with its magnet; 13-17, power supplies, amplifiers, recorders.
and filled with DC 705 diffusion pump oil. The disc in the inner chamber, driven directly by a rotary feed-through, is covered with any test material and slides over a pin screwed into an electro-mechanical measuring device. The total pressure is measured with an uncovered Bayard-Alpert ionisation gauge and the partial pressures with an Edwards mass spectrometer of the omegatron type. As indicated in Fig. I, the head of the analysing omegatron tube is connected with the inner chamber of the ultrahighvacuum system by a pipe of z-cm width in the clear and 65-cm long. The rotary feed-through, directly led through the outer and inner wall of the system, is usable up to 300 rev./min without changing the environmental conditions. During repeated test runs, controlled for a matter of hours, the variation of total pressure in the range below 5. IO-9 torr did not exceed &-5.10-10 torr at 300 rev./min. The electromechanical device for measuring the coefficient of friction in ultrahigh vactium For measuring friction outside the vacuum chambers, experimenters often Wear, rg (1970) 215-227
SLIDING
FRICTION OF TEFLON ON STEEL
217
transfer the forces through the chamber walls by bellows and levers which make the measuring system relatively inert. The device used in the present work allows the pick up of friction and load forces inside the vacuum vessel, the resolution in time being better than 10-3 set, and the noise signals from electrical and mechanical origin not exceeding 10-3 kp. The principle of the device is shown in Fig. 2. The load is imposed on the rider by the spring F, connected to the pin by the levers Hi, HZ, H3, andtheload-measuringgauge QN between Hi and Hz. The frictional forces are supported by the measuring gauge QR, which is connected with the pin by levers Ha and Ha. All levers are equipped with ball bearings, treated, before installation, with MOSS
Fig. 2. Principle of the friction-measuring
arrangement.
TEST PROCEDURE
The friction tests and the mass-spectrometry measurements were made in the UP 30 vacuum system. In a second system equipped with ion getter pumps, comparative mass-spectrometry measurements were made with a quadrupole gas-analyzer, the exposed head of which was mounted inside the vacuum vessel. In this facility, a Varian type VT 106,the measurement of friction forces will also be made as soon as the necessary equipment is completed. Specimen material and preparation The pins were made from Teflon filled with 25% carbon black (Pampus Company, Germany, quality AL 5125). The discs were made from stainless steel. The pins were machined to give z.o-mm-radius spherical heads for the friction measurements and flat heads of 1.3 ) o.z-mm diam. for combined friction and gas-production experiments. The discs were ground and highly polished with moist levigated alumina to a surface roughness of less than 0.1 ,um (c.1.a. readings < 0.05 pm). Before mounting the specimens in the vacuum chamber, they were thoroughly cleaned with distilled water and ethyl alcohol. Wear, 15 (1970) 215-227
218
W. WILKENS,
Friction
0. KRANZ
measurements
The coefficient of friction in ultrahigh vacuum was determined with the electromechanical device described above (Fig. 2). If N and R are the load and the frictional force and the corresponding forces measured by gauges QN and QR are N, and Rm, the measuring geometry yields R ,I.&=N = The maximum
1.24sN m
deviation
or uncertainty
of the friction
measurements
is given by the
relationship $!
< (2
+ 2
+4.8+2.5,~)10-”
where ,u is the coefficient of friction and AN and AR are the recorder deflections measured in mm. The first and second terms in the brackets refer to uncertainties in reading the recorder graphs, the third concerns geometric deviations, and the fourth refers to the influence of the frictional force on the loads. Due to the finite angle between the normal direction on the disc and the axis of the beam H3 (about o.~“), the load is supported partially by the gauge QR which measures the friction force. This yields an apparent frictional force RO that can easily be eliminated by measuring it when the disc is at rest: R=R’-Ro
(3)
The value of Ro is about I y0 of the load, N. Measurement
of total and partial pyessuyes
The total pressures and the pressure changes due to sliding friction in the pressure range between lo-9 torr and 10-7 torr are measured with an uncovered BayardAlpert ionisation gauge mounted inside the vacuum chamber. The uncertainty of the pressure measurements does not exceed &-5 %. As the gauge calibration refers to nitrogen pressure equivalents, the true pressures can only be calculated taking into account the ionization probabilities of the gases to be measured4p5. For analysing the gases produced by the sliding process, the omegatron mass spectrometer was calibrated with nitrogen for the masses 14 and 28, with CO2 for the masses 12,16,28 and 44 and with argon for the mass 40. The uncertainty of the concentration determinations of the molecular masses did not exceed + 20%. The resolution was better than one mass unit up to the mass 50 and about I .5 units up to the mass 70. The comparative mass-spectrometry measurements were performed with an uncovered quadrupole analysing system having a measuring range up to the mass 250. The system is mounted inside the ultrahigh-vacuum-facility VT 106. Additional information on the gases produced by the sliding process was obtained by mass spectrometry in the warming up period of a copper sheet surrounding the pin-disc arrangement cooled with liquid nitrogen during the previous friction experiments. This was done to determine the condensable or adsorbable part of the friction-produced gases and to look for their desorption characteristics. All experiments for determining the gases produced by friction were performed with pins having a flat Wear, rg (1970)215-227
SLIDING FRICTION OF TEFLON ON STEEL
surface of 1.3fo.z-mm 0.5 ‘z:: kp/mma.
219
diameter. Applying 700 p, the specific load on the pin is
of the pumping speed The pumping speed of the ultrahigh-vacuum oil diffusion pump of the facility UP 30, nominal 500 l/set, was reduced by baffles and experimental devices mounted in the vacuum-chamber. Thus it was necessary to control the pumping speed for the determination of gas-production rates. By means of a special valve between the inner and outer chamber of the doublewalled ultrahigh-vacuum system, quickly opened and closed from outside, the total pressure in the chamber was increased from the IO-9 torr to the IO-* torr range. After closing the valve, the decrease of pressure, p, was recorded by an oscilloscope and the pumping speed, S, determined by the relationship The determination
S=-dp
v (4) dt P-9, where p, is the residual total pressure and V the volume of the vacuum chamber. The residual pressure achieved after 24 h pumping was 4.8. IO-9 torr with all cooling traps at room temperature. The volume, V, was 22.0 1. The pumping speed, SN, for nitrogen only was (65 f 2) l/set. Conducting the pumping speed measurements, the pressure in the outer chamber of the double walled system was raised from I. 10-5 torr to 7.10-5 torr by continuously admitting nitrogen through a needle valve*. During the pumping speed measurements, the cold traps were kept at room temperature to eliminate additional, but unknown, pumping speeds. RESULTS
The coefficient
of friction of Teflon filled with carbon black on stainless steel After about IOOrev. on the highly polished discs of stainless steel, the mean friction forces became steady. The average coefficients of friction over three runs in air at 760 torr, and over three runs in ultrahigh vacuum, 4. IO-~ torr, were pair=o.r6 fo.02 and ,&h”=o.r7fo.o2. For all comparative measurements at ro-ztorr, IO-5torr andro-Vorrthecoefficient p was less than 0.22. No correlation with the total pressure in thetest chamberwas found. In Fig. 3, p is plotted for a test in air and a test in ultrahigh vacuum for the first I00 reversals. Gas production
rates caused
by sliding friction
in ultrahigh
vacuum
The pressure increase due to the sliding friction of carbon-black-filled Teflon on steel in ultrahigh vacuum as a function of time and sliding velocity is plotted in Figs. 4 and 5. After loading the pins, the pressure increase attained more than 509/oof * Admitting the residual gases present in the outer unbaked chamber into the inner vacuum vessel, the pumping speed, Sa, for the residual gases is only (38 f3) l/set. This corresponds to the higher mean molecular weight of the residual gases according to the relationship S,2M,=Sb=M~ (5) S, and SC being the pumping speeds for gases with the molecular weights M, and Mb. With SN = 65 l/set, Mb is about 80. Wear, 15 (1970) 2r5--227
W. WILKENS,
220
I-1
I
-X-
Test in air, 760 tow, maximum
-
Test in uhv, 4,10e9 tow,
0. KRANZ
deviation f 15%
maximum
deviation t12%
Load: 0.7 kp Sliding velocity: 9gc
0.05
r=n”mber of
0 2 4 6 6 IO’
20
30
40
50
60
70
P 30 rpm
re”erSQlS
60
90
100 z
Fig. 3. Coefficient of friction, p, of Teflon, filled with 25% carbon black on stainless steel in normal atmosphere and in ultrahigh vacuum.
IO*.
Ap Eor.4 0
t 30
Sllding velocity:
90%
0 Sliding veloaty.
60sc
X Sliding velocity.
30$$
A
Sliding velocity:
15cm set
0
Sliding vebclty
7.5%
25 20 15 10 5 5
10
15
20
t [mid
25
Fig. 4, Increase of total pressure Ap after loading the carbon-black-filled sliding velocities.
1Oe.G h
Teflon pin at different
A lOa.Ap~or~ Coefficient
of friction: Load
Specific
,....l,c
50
vs cm.sec100
0.7 kp
mm2
o.sJ=
load:
1’s:
10
pzO.17
:
3.3rpm
3
Fig. 5. Increase of total pressure Ap and of the gas-production rate 4 due to increasing sliding velocity vg. Carbon-black-filled Teflon sliding on stainless steel in ultrahigh vacuum. w&Z+‘, 15
(1970)
215-227
SLIDING
FRICTION
OF TEFLON
221
ON STEEL
its average equilibrium value in a few seconds. These values were reached after about IO min sliding in the velocity range 7,5-90 cmjsec. The pressure increase averaged over the last IO min running time is plotted as a function of sliding velocity in Fig. 5. The pressure increase, Ap, rises rapidly up to 15 cm/set, reaching 6 .IO-* torr, then increases only slowly up to 60 cm/set. In the velocity range 60-90 cm/set, there is a sharp rise in Ap to 2.3. IO-~ torr. A pressure increase, A$, in a vacuum system having a pumping speed, S, means a rate of gas influx or internal gas production, 4, (1torr/sec), that is simply related to A$ and S by 4=ApS
(6a) The pumping speed of the ultra high-vacuum system UP 30 being 65 l/set, the pressure increase at 60 cm/set, for example, corresponds to a gas-production rate, 4, of 5.10-e 1torr/sec. As previously stated, all data derived from pressure measurements are related to nitrogen equivalents. The true data can be calculated knowing the composition of the gases produced and their ionizing probabilities. If the number of molecules is 3.34.1019/l at I torr and 300°K (state equation), and S is 65 I/set, the number ri of molecules set free in I set is ii = 2.17.1ozlAp (6b) For a sliding velocity of 60 cm/set, 1.5.1014 molecules/set are changing from the solid into the gaseous state. Regarding the sliding pin as a source of molecules which can move freely in all directions, one can estimate the flux, rir (cm-%ec-l), of molecules at the distance Y from the source. A,=r.7.1020~
Ap
The number of molecules in a monolayer is about 1015, and is 5 -10~~ for nitrogen to which all pressure measurements refer. Assuming all molecules to stick on the surface, the number of monolayers, ti (see-l), built up in a second is ti=3.5.105$
Ap
(7b)
Referring to the danger of contamination in the vicinity of a sliding bearing in a spacecraft, eqn (7b) means that a monolayer of gas molecules is built up in about I h at a distance of IO cm from the carbon-black-filled Teflon bearing run at 60 cm/set under the conditions described above. The composition
of gases produced
by sliding friction
in ultrahigh
vacuum
The determination of gas-production rates in ultrahigh vacuum was completed by mass spectrometry measurements restricted to a sliding velocity of 60 cm/set (200 rev./min). The results of this investigation are given in Table I. Comparative measurements with a quadrupole gas analyser indicate that the part of gases with molecular masses exceeding the mass 69 is less than 5 o/o of the total gas produced by friction. The mass spectra listed in Table I were recorded under the following conditions. (a) Run without liquid-nitrogen cooling. ao: rotary feed-through at rest; pr=5.4.10-8 torr. al: rotary feed-through at 200 rev./min; pin unloaded; pr=5.6.10-8 torr. Wear,I5
(1970)
7.19-227
222 TABLE
W. WILKENS, 0. KRANZ
I
RELATIVE
CONCENTRATIONS
f
Molecular weight
position
12
C
0.4
0.5
0.8
0.3
0.7
0
0.2 < 0.1 0.8 1.6 5.5
0.5 0.4 I.1 1.h 5.5
0.4 0.2 I.3 I.9 6.8
--0.X - 0.2 0.2 0.3 I.3
0.5 < 0.1 0.9 0.5 2.”
0.3 7.3 0.1
0.3 8.9 “.I
0.4 19.5 0.2 -
“.I 10.6 0.1 -
0.2 12.9 < 0.1
4.0
4.”
‘3 74 I5 16
‘7 Hz0
18
a0
a2
I9
27 28 29 3” 3’ 32 4” 43 44 45 5” 51
N.&CO CF
CF2
3.0
3.7
5.2 I.1
-
bz
h
-
co2
b,
a21
0.9 I.0
I.3 2.1 0.5 2.2
2.2
2.2 -
ba
0.6 -
0.9
I.3 2.0 2.8 0.5 2.0
0.3 0.7 0.7 0.”
I.” 1.8 2.0
-
0.2
‘3.4 <“.I
1.5 I.1
bzl I.5
0.2
0.0 0. I <
0.1
0.4 3.2 0.0 35.” I.9 -
0.5
35.5
0.8
6.6
0.7
17.8
6.5
45.5
-
62
CFa
69 7”
-
7.9
7.9 -
108 p
5.4
5.6
II.”
9.1
Run a, spectra without liquid-nitrogen cooling; run b, spectra with liquid-nitrogen cooling. Subscripts: “, rotary feed-through (RF) at rest; I, RF at 200 rev./min, pin unloaded; 2, RF at 2OOrev./min, pin loaded 0.7 kp; 3, RF at rest, gas analysing near desorption maximum. Aasi (Abzl) difference between thef-values of runs a2 and ai (bs and bi) ; p, total pressure (torr) ; dashes indicate f < 0.1.
az: rotary
feed-through
at 200 rev./min;
pin loaded with 0.7 kp; p, = 1.1.10-7
torr. (b) Run with liquid-nitrogen cooling. bo: rotary feed-through at rest; p,= 1.7. IO-9 torr. bi: rotary feed-through bz : rotary feed-through
at 200 rev./min; pin unloaded; &=3.0+10-9 torr. at 200 rev./min; pin loaded with 0.7 kp; p, = 6.8. IO-9
torr. bs : rotary feed-through at rest ; liquid-nitrogen cooling interrupted ; trap warm ing up; gas-analysis near the maximum pressure rise (0.9.10-7 torr) The spectra of run (a) and those of run(b) had to be measured with different adjustments of the spectrometer due to the different pressure ranges. Thus, the massfrequency data are only comparable within run (a) or run (b). The main conclusions drawn from Table I and partially shown in Fig. 6 are given below. (I) The molecular masses 31, 50, and 6g only appear during sliding friction, the mass 6g with high concentration. (2) The concentration of mass 28 increases significantly by the sliding process. (3) The rise of the total pressure is caused predominantly by gas components with the molecular masses 28, 31, 50, and 69, respectively by components which yield these masses in the analyser tube of the spectrometer. With liquid-nitrogen cooling, the part of these components is about go% and without liquid-nitrogen cooling about 80% of the total of gas produced by friction. WCW,15 (197”) 215-227
223
SLIDING FRICTION OF TEFLON ON STEEL
Similarly, for the gas which desorbs from ,the copper sheet around the pindisc device in the ultrahigh-vacuum vessel after 30 min friction runs and after stopping the liquid-nitrogen cooling of the sheet. 90% of the pressure increase due to the desorption of the gas is caused by components with the molecular masses 28,31, 50, and 69. The total pressure during desorption and concentration of the masses 31, 50 and 6g at different times are plotted in Fig. 7. From the desorption experiment one can draw the following conclusions. Coefficient
Af A
of friction
Specifac Sliding
: p =0.17
Load
:
0.7 kp
load
:
0.5 2
velocity
: CO= f 200
rpm
B
El
I
20
4
40
60
I
-m
80
moss Fig. 6. Increase of the molecular mass concentrations Aj caused by the friction of carbon-blackfilled Teflon on stainless steel in ultrahigh vacuum. (The molecular masses m 31, m 50 and m 6g are only present during friction.) m=moleculor
+IO*.
p&of-d
t=Time ofter stopping the LN-cooling
Fig. 7. Total pressure, p, and relative concentrations, from the trap.
f, of typical
molecular
masses desorbing
(I) Because the proportions between the concentration of the molecular masses 31, 50, and 6g only present during sliding friction, change with time, more than one gas component is formed by the friction process. It is most probable that these components are the molecular fragments CF (mass 31), CFB (mass 50) and CFs(mass
69).
(2) More than 90% of the total amount of gas desorbed from the copper sheet, respectively more than 4.8.10-s 1 torr equivalent to 1.6.1017 molecules, are
w&&V, 15 (1970)
215-227
W.
224
WILKENS,
0. KRANZ
derived from the frictional process. For an adsorption time of 30 min, during which the pin was sliding over the disc (60 cmjsec, A$ =7 .IO-~ torr), the rate of adsorption was 9.4.1013 molecules/set. On the other hand, the pressure increase during the experiment without liquid-nitrogen cooling yielded a gas production rate of I .5.1014 molecules/set. Thus, the adsorbable (and/or condensable) part of the friction-produced gas is at least 60:/, relative to the surface of the copper sheet at liquid-nitrogen temperature. This part must be even larger because not all molecules leaving the pin-disc interface will be caught by the copper sheet surrounding the pin-disc apparatus. Comparative measurements with the uncovered quadrupole system mounted inside the facility VT 106 confirm the results gained with the omegatron mass spectrometer, especially the occurrence of the masses 31, 50, and 69. The masses, their frequencies and the possible molecules or molecular fragments are listed in Table II.
DISCUSSIOiY
Teflon,
or polytetrafluoroethylene,
has the chemical
formula
,-I-...-_L_,; I
F
I I
F
F
with an average molecular weight of about 1000. In the vacuum facility UP 30, as well as in the system VT 106, all molecular masses found only during friction can be attributed to fragments of the Teflon molecule. These fragments originate, at least partially, from the friction process itself. This conclusion is not only drawn from the desorption measurements described above, but also by further, as yet incomplete, experiments on the “lifetime” of the fragments in the vacuum system, for example experiments on the determination of the time in which the frequency of a mass reduces to a given value. The masses so(CFz), 62(CzFz), 6g(CFa), and 81(CzFa) have different lives, the fragment CFa has a long life exceeding those of the other components by more than one order of magnitude. Comparing the measurements made with the omegatron mass spectrometer in the facility UP 30 and with the quadrupole in the system VT 106, the high concentration of the molecular mass 6g is obvious. In the facility UP 30, however, the concentration of mass 69 is greater than that of mass 31. In the system VT 106 the opposite is true. This difference is possibly caused by the long life of the molecular fragment Wear, r5 (1970) 215-227
SLIDING
FRICTION
OFTEFLON
ON STEEL
225
CF3. The omegatron-analysing tube is relatively far from the sliding pin and optically shielded from it. Thus, the probability of the gas with mass 69 not reaching the omegatron analysing head is greater than for the other components. BUCKLEYAND JOHNSON, in their report on friction, wear and decomposition mechanisms for various polymer compositions, also found the molecular masses 31 and 50 during the sliding of pure and glass-filled Teflon, on steel1 but they did not find the mass 69, typical of friction of Teflon on steel found in both facilities UP 30 and VT 106. Another interesting experiment in BUCKLEYAND JOHNSON’Spaper is that on the influence of sliding velocity on the frequency of the molecular masses 50 and 31, which decreases unexpectedly with increasing sliding velocity. This finding seems to be unlikely as, in the present experiments, the amount of friction-produced gas rises with sliding velocity. BUCKLEY AND JOHNSONexplain the decrease of the concentration of the masses 50 and 31 with an increase of wear from 2.7.10-s cma/cm at 2.0 mjsec to 3.5 .IO-~ cma/cm at 5.0 mjsec by assuming that the additional wear particles carry away more than the additional frictional heat caused by the rise of sliding velocity. Taking into account the coefficient of friction, ,u=o.25, and the load, N = I kp, in the authors’ experiments, the wear particles had to carry away more than 2.106 watt sec/cma. This exceeds the heat-carrying capacity of Teflon by two orders of magnitude, even taking into account the energy required for breaking all C-bonds of the Teflon molecule. The effect observed could be explained by the influence of specific load (kp/mm2) on the process, because, starting with a hemispherical slider, the specific load decreases due to wear of the slider head and the specific energy input (watt sec/cm2) also decreases. Regarding the gas-production rate, not mentioned by BUCKLEYAND JOHNSON,the present measurements show that the increase of the rates with increase of sliding velocity consists of two, possibly three, stages with linear slopes (Fig. 5). This indicates, that the rate of gas production is more likely to be connected with wear and not with a temperature-dependent process. The different slopes are believed to correspond to different stages of wear. As explained above, the gas-production rates are related to nitrogen, that means to its ionizing probability or specific ionization, and to the pumping speed for this gas. The specific ionization of the molecular fragments CF, CF2, and CFa are not present. The specific ionization of CO and CO2 which significantly increase during friction, is about 25:/o respectively 35% greater than that for Nz. Thus, it is likely that the total pressure measurements are overvalued by about 50:/o. Considering the increase of the frequencies Afi of the molecular masses mi the pumping speed Sg for the continuously produced gas is S,=&
CAj’i (mN/mi) 4 XAfi -
(8)
Using eqn. (8) and the data from Table I, S, is 56.5 l/set but, regarding the specific ionization of the gas components, the frequencies Afi are also overvalued compared to nitrogen. Thus, S, can be regarded as the lower limit of the true pumping speed. Taking into account a 50% reduction of the overvalued total pressure data, the gasproduction rate cjg=.%A&, determined by eqn. (6a), is also overvalued by up to 60;/,. The same is true for the adsorption rate ~sd=SpApsd where Ap=r/TjT Apdt, (T WtWf’,1.5 (1970) 215-7.27
226
W. WILKENS, 0. KRANZ
being the adsorption time). The ratio of gad and da gives the adsorption coefficient, k,
The errors caused by relating all pressure measurements to nitrogen largely compensate each other. The statement that at least 60% of the friction-produced gases adsorb on liquid-nitrogen-cooled copper surfaces in ultrahigh vacuum remains valid. This finding, however, has to take into consideration that, during the 30 min friction run before the desorption measurements, about 1017 molecules are adsorbed. This corresponds to nearly a monolayer on a copper sheet surface of 250 cm2. The adsorption and/or condensation of further layers is under investigation. However, contamination film thicknesses even less than one monolayer can cause large changes in physical properties of solid surfaces”. Assuming, for example, that the contamination by friction-produced gases does not exceed that of one monolayer at a distance of IO cm from the sliding bearing after a one years running time in ultrahigh vacuum, the adsorption coefficient, k, may be estimated from eqn. (7b). k < 8.10-12
-&
This means that k < I . IO-J for a sliding velocity of 60 cmjsec. Thus, in the vicinity of sensitive and cooled surfaces, Teflon, despite being a very good self-lubricating material, must be excluded. The influence of surface tenlperature and surface material on the adsorption coefficient of the gases produced by sliding friction of Teflon materials has still to be investigated. CONCLUSIONS
The coefficient of friction of Teflon, filled with 2594 carbon black is not influenced by vacuum and its value is about 0.17. Up to sliding velocities of I m/set, the friction-induced gas-production rate rises to I -10-5 1 torr/sec under a specific load of 0.5 kp/mmz (total load 0.7 kp). More than 60% of the friction-produced gas adsorbs on copper surfaces at liquid nitrogen temperature, at least up to film thicknesses of one monolayer. It is, therefore, dangerous to use Teflon as a bearing material in the vicinity of sensitive surfaces of spacecraft. The molecular masses only detected during friction of the Teflon material are all attributed to fragmentation of the Teflon molecule. The fragments originate from the friction process itself and can only be partially formed in the analyser tube. ACKNOWLEDGEMENTS
This work was supported under contract number RFF 3ozg by the Bundesminister fur Bildung und Wissenschaft (Federal Minister of Education and Research), to whom we are very grateful. We are indebted to Prof. W. THIELEMANN, head of the structures and materials department, for his encouragement. We thank H. SWOBODA for his sk~fuldesign work and D. PAPE for assisting throughout allexpe~ments. Wear,15 (1970)215-227
SLIDING FRICTION OF TEFLON ON STEEL
227
REFERENCES D. H. BUCKLEYAND R.L.JOHNSON, NASA Tech.NoteTND-zo73,1963. D. H. BUCKLEY, NASA Tech. Note TN D-.?261,1o66. W. WILKENS, DFVLR-Innternal Working Piper-F166-o8, DFVLR Braunechweig, 1966. S. DUSHMAN, Scientific Foundalion of Vacuum Technique, Wiley, New York, 1962. M. ARDENNE, Tabellen zur Angewandten Physik, VEB Deutscher Verlag der Wissenschaften, Berlin, 1962. H. MAYER, Physik Diinner Schichten, Teil I und II, Wissenschaftliche Verlagsgesellschaft, Stuttgart, 1955. WeUP', 1.5 (1970) 215-227
Elsevier Sequoia S.A., Lausanne - Printed
THE FORMATION OF GASES DUE TO THE SLIDING ON STEEL IN ULTRAHIGH VACUUM
W. WILKENS
AND
Deutsche ForschungsBraunschweig,
215
in The Netherlands
FRICTION
OF TEFLON
0. KRANZ und Versuchsanstalt fiir Luft- ulad
Raumfahrt, Forschungszentrum
(Deutschland)
(Received December
12, 1969)
SUMMARY
The formation of gases by sliding friction of self-lubricating material, Teflon filled with 25% carbon black, was investigated and the quantity and the composition of the gases evolved determined. By mass spectrometry, it was shown that all gases produced by the sliding process can be attributed to fragmentation of the Teflon molecule. At least 60% of the gases evolved during the sliding process condense on a copper surface cooled by liquid nitrogen. INTRODUCTION
Materials producing gases in vacuum which condense or react chemicallyon sensitive surfaces such as the surfaces of electrical contacts, optical systems, temperature control coatings, etc. have, as far as possible, to be excluded from spacecraft applications. The formation of gaseous products due to sliding friction is a troublesome phenomenon in vacuum techniques, especially with rotary feed-throughs but no published information concerning quantity and composition is available. BUCKLEY AND JOHNSON, working on friction and wear in vacuum, have reported on decomposition mechanisms of synthetics caused by friction in ultrahigh vacuum 112but their measurements only refer to the composition of the residual gases during the sliding process. The aim of the present work was to measure the quantity of gas produced by sliding friction, to obtain information on the composition of the gas by mass spectrometry and to look for condensable or adsorbable components. The coefficient of friction at different levels of total pressure, especially in ultrahigh vacuum, was also determined. The best Teflon composition with respect to friction and wear behaviour contained, according to BUCKLEY AND JOHNSONI, 25% coke flour. Thus, a similar material, a 25% carbon-black-filled Teflon, was used in the present experiments. The investigation of gas production due to friction is not only important because of the danger of contamination in spacecraft, but also because of the degradation mechanism of the bearing material and the undesirable changes in the composition of residual gases in ultrahigh-vacuum systems. Wear, 15 (1970) 215-227
216
W. WILKENS,
EXPERIMENTAL
The test
0. KRANZ
EQUIPMENT
facility
enables
the simultaneous
measurement
of load,
frictional
forces, temperatures, and total and partial pressures of the residual gases. It is possible to change the test geometry and load and to unload the specimens during test. The ultrahigh-vacuum test facility The apparatus used is shown in Fig. I. The main part is a double-walled vacuum system, (Leybold-Heraeus Type UP 30) in which total pressures in the low 10-s torr range are achievable by oil diffusion pumps, well baffled with liquid nitrogen
Auxiliary vocuum:p~10-5
Ultmhighvocuum: p=2.KPg
torr-
tow
Fig. I. Ultrahigh vacuum test facility for friction experiments. I, Drive for rotary feedthrough; 2, drive control; 3, revolution counter, 4; axle-shaft with disk; 5, device for measuring the coefficient of friction; 6, electron gun for cleaning surfaces; 7, Bayard12, omeAlpert gauge ; 8-1 I, amplifiers and recorder- - for measuring forces and temperatures; gatron with its magnet; 13-17, power supplies, amplifiers, recorders.
and filled with DC 705 diffusion pump oil. The disc in the inner chamber, driven directly by a rotary feed-through, is covered with any test material and slides over a pin screwed into an electro-mechanical measuring device. The total pressure is measured with an uncovered Bayard-Alpert ionisation gauge and the partial pressures with an Edwards mass spectrometer of the omegatron type. As indicated in Fig. I, the head of the analysing omegatron tube is connected with the inner chamber of the ultrahighvacuum system by a pipe of z-cm width in the clear and 65-cm long. The rotary feed-through, directly led through the outer and inner wall of the system, is usable up to 300 rev./min without changing the environmental conditions. During repeated test runs, controlled for a matter of hours, the variation of total pressure in the range below 5. IO-9 torr did not exceed &-5.10-10 torr at 300 rev./min. The electromechanical device for measuring the coefficient of friction in ultrahigh vactium For measuring friction outside the vacuum chambers, experimenters often Wear, rg (1970) 215-227
SLIDING
FRICTION OF TEFLON ON STEEL
217
transfer the forces through the chamber walls by bellows and levers which make the measuring system relatively inert. The device used in the present work allows the pick up of friction and load forces inside the vacuum vessel, the resolution in time being better than 10-3 set, and the noise signals from electrical and mechanical origin not exceeding 10-3 kp. The principle of the device is shown in Fig. 2. The load is imposed on the rider by the spring F, connected to the pin by the levers Hi, HZ, H3, andtheload-measuringgauge QN between Hi and Hz. The frictional forces are supported by the measuring gauge QR, which is connected with the pin by levers Ha and Ha. All levers are equipped with ball bearings, treated, before installation, with MOSS
Fig. 2. Principle of the friction-measuring
arrangement.
TEST PROCEDURE
The friction tests and the mass-spectrometry measurements were made in the UP 30 vacuum system. In a second system equipped with ion getter pumps, comparative mass-spectrometry measurements were made with a quadrupole gas-analyzer, the exposed head of which was mounted inside the vacuum vessel. In this facility, a Varian type VT 106,the measurement of friction forces will also be made as soon as the necessary equipment is completed. Specimen material and preparation The pins were made from Teflon filled with 25% carbon black (Pampus Company, Germany, quality AL 5125). The discs were made from stainless steel. The pins were machined to give z.o-mm-radius spherical heads for the friction measurements and flat heads of 1.3 ) o.z-mm diam. for combined friction and gas-production experiments. The discs were ground and highly polished with moist levigated alumina to a surface roughness of less than 0.1 ,um (c.1.a. readings < 0.05 pm). Before mounting the specimens in the vacuum chamber, they were thoroughly cleaned with distilled water and ethyl alcohol. Wear, 15 (1970) 215-227
218
W. WILKENS,
Friction
0. KRANZ
measurements
The coefficient of friction in ultrahigh vacuum was determined with the electromechanical device described above (Fig. 2). If N and R are the load and the frictional force and the corresponding forces measured by gauges QN and QR are N, and Rm, the measuring geometry yields R ,I.&=N = The maximum
1.24sN m
deviation
or uncertainty
of the friction
measurements
is given by the
relationship $!
< (2
+ 2
+4.8+2.5,~)10-”
where ,u is the coefficient of friction and AN and AR are the recorder deflections measured in mm. The first and second terms in the brackets refer to uncertainties in reading the recorder graphs, the third concerns geometric deviations, and the fourth refers to the influence of the frictional force on the loads. Due to the finite angle between the normal direction on the disc and the axis of the beam H3 (about o.~“), the load is supported partially by the gauge QR which measures the friction force. This yields an apparent frictional force RO that can easily be eliminated by measuring it when the disc is at rest: R=R’-Ro
(3)
The value of Ro is about I y0 of the load, N. Measurement
of total and partial pyessuyes
The total pressures and the pressure changes due to sliding friction in the pressure range between lo-9 torr and 10-7 torr are measured with an uncovered BayardAlpert ionisation gauge mounted inside the vacuum chamber. The uncertainty of the pressure measurements does not exceed &-5 %. As the gauge calibration refers to nitrogen pressure equivalents, the true pressures can only be calculated taking into account the ionization probabilities of the gases to be measured4p5. For analysing the gases produced by the sliding process, the omegatron mass spectrometer was calibrated with nitrogen for the masses 14 and 28, with CO2 for the masses 12,16,28 and 44 and with argon for the mass 40. The uncertainty of the concentration determinations of the molecular masses did not exceed + 20%. The resolution was better than one mass unit up to the mass 50 and about I .5 units up to the mass 70. The comparative mass-spectrometry measurements were performed with an uncovered quadrupole analysing system having a measuring range up to the mass 250. The system is mounted inside the ultrahigh-vacuum-facility VT 106. Additional information on the gases produced by the sliding process was obtained by mass spectrometry in the warming up period of a copper sheet surrounding the pin-disc arrangement cooled with liquid nitrogen during the previous friction experiments. This was done to determine the condensable or adsorbable part of the friction-produced gases and to look for their desorption characteristics. All experiments for determining the gases produced by friction were performed with pins having a flat Wear, rg (1970)215-227
SLIDING FRICTION OF TEFLON ON STEEL
surface of 1.3fo.z-mm 0.5 ‘z:: kp/mma.
219
diameter. Applying 700 p, the specific load on the pin is
of the pumping speed The pumping speed of the ultrahigh-vacuum oil diffusion pump of the facility UP 30, nominal 500 l/set, was reduced by baffles and experimental devices mounted in the vacuum-chamber. Thus it was necessary to control the pumping speed for the determination of gas-production rates. By means of a special valve between the inner and outer chamber of the doublewalled ultrahigh-vacuum system, quickly opened and closed from outside, the total pressure in the chamber was increased from the IO-9 torr to the IO-* torr range. After closing the valve, the decrease of pressure, p, was recorded by an oscilloscope and the pumping speed, S, determined by the relationship The determination
S=-dp
v (4) dt P-9, where p, is the residual total pressure and V the volume of the vacuum chamber. The residual pressure achieved after 24 h pumping was 4.8. IO-9 torr with all cooling traps at room temperature. The volume, V, was 22.0 1. The pumping speed, SN, for nitrogen only was (65 f 2) l/set. Conducting the pumping speed measurements, the pressure in the outer chamber of the double walled system was raised from I. 10-5 torr to 7.10-5 torr by continuously admitting nitrogen through a needle valve*. During the pumping speed measurements, the cold traps were kept at room temperature to eliminate additional, but unknown, pumping speeds. RESULTS
The coefficient
of friction of Teflon filled with carbon black on stainless steel After about IOOrev. on the highly polished discs of stainless steel, the mean friction forces became steady. The average coefficients of friction over three runs in air at 760 torr, and over three runs in ultrahigh vacuum, 4. IO-~ torr, were pair=o.r6 fo.02 and ,&h”=o.r7fo.o2. For all comparative measurements at ro-ztorr, IO-5torr andro-Vorrthecoefficient p was less than 0.22. No correlation with the total pressure in thetest chamberwas found. In Fig. 3, p is plotted for a test in air and a test in ultrahigh vacuum for the first I00 reversals. Gas production
rates caused
by sliding friction
in ultrahigh
vacuum
The pressure increase due to the sliding friction of carbon-black-filled Teflon on steel in ultrahigh vacuum as a function of time and sliding velocity is plotted in Figs. 4 and 5. After loading the pins, the pressure increase attained more than 509/oof * Admitting the residual gases present in the outer unbaked chamber into the inner vacuum vessel, the pumping speed, Sa, for the residual gases is only (38 f3) l/set. This corresponds to the higher mean molecular weight of the residual gases according to the relationship S,2M,=Sb=M~ (5) S, and SC being the pumping speeds for gases with the molecular weights M, and Mb. With SN = 65 l/set, Mb is about 80. Wear, 15 (1970) 2r5--227
W. WILKENS,
220
I-1
I
-X-
Test in air, 760 tow, maximum
-
Test in uhv, 4,10e9 tow,
0. KRANZ
deviation f 15%
maximum
deviation t12%
Load: 0.7 kp Sliding velocity: 9gc
0.05
r=n”mber of
0 2 4 6 6 IO’
20
30
40
50
60
70
P 30 rpm
re”erSQlS
60
90
100 z
Fig. 3. Coefficient of friction, p, of Teflon, filled with 25% carbon black on stainless steel in normal atmosphere and in ultrahigh vacuum.
IO*.
Ap Eor.4 0
t 30
Sllding velocity:
90%
0 Sliding veloaty.
60sc
X Sliding velocity.
30$$
A
Sliding velocity:
15cm set
0
Sliding vebclty
7.5%
25 20 15 10 5 5
10
15
20
t [mid
25
Fig. 4, Increase of total pressure Ap after loading the carbon-black-filled sliding velocities.
1Oe.G h
Teflon pin at different
A lOa.Ap~or~ Coefficient
of friction: Load
Specific
,....l,c
50
vs cm.sec100
0.7 kp
mm2
o.sJ=
load:
1’s:
10
pzO.17
:
3.3rpm
3
Fig. 5. Increase of total pressure Ap and of the gas-production rate 4 due to increasing sliding velocity vg. Carbon-black-filled Teflon sliding on stainless steel in ultrahigh vacuum. w&Z+‘, 15
(1970)
215-227
SLIDING
FRICTION
OF TEFLON
221
ON STEEL
its average equilibrium value in a few seconds. These values were reached after about IO min sliding in the velocity range 7,5-90 cmjsec. The pressure increase averaged over the last IO min running time is plotted as a function of sliding velocity in Fig. 5. The pressure increase, Ap, rises rapidly up to 15 cm/set, reaching 6 .IO-* torr, then increases only slowly up to 60 cm/set. In the velocity range 60-90 cm/set, there is a sharp rise in Ap to 2.3. IO-~ torr. A pressure increase, A$, in a vacuum system having a pumping speed, S, means a rate of gas influx or internal gas production, 4, (1torr/sec), that is simply related to A$ and S by 4=ApS
(6a) The pumping speed of the ultra high-vacuum system UP 30 being 65 l/set, the pressure increase at 60 cm/set, for example, corresponds to a gas-production rate, 4, of 5.10-e 1torr/sec. As previously stated, all data derived from pressure measurements are related to nitrogen equivalents. The true data can be calculated knowing the composition of the gases produced and their ionizing probabilities. If the number of molecules is 3.34.1019/l at I torr and 300°K (state equation), and S is 65 I/set, the number ri of molecules set free in I set is ii = 2.17.1ozlAp (6b) For a sliding velocity of 60 cm/set, 1.5.1014 molecules/set are changing from the solid into the gaseous state. Regarding the sliding pin as a source of molecules which can move freely in all directions, one can estimate the flux, rir (cm-%ec-l), of molecules at the distance Y from the source. A,=r.7.1020~
Ap
The number of molecules in a monolayer is about 1015, and is 5 -10~~ for nitrogen to which all pressure measurements refer. Assuming all molecules to stick on the surface, the number of monolayers, ti (see-l), built up in a second is ti=3.5.105$
Ap
(7b)
Referring to the danger of contamination in the vicinity of a sliding bearing in a spacecraft, eqn (7b) means that a monolayer of gas molecules is built up in about I h at a distance of IO cm from the carbon-black-filled Teflon bearing run at 60 cm/set under the conditions described above. The composition
of gases produced
by sliding friction
in ultrahigh
vacuum
The determination of gas-production rates in ultrahigh vacuum was completed by mass spectrometry measurements restricted to a sliding velocity of 60 cm/set (200 rev./min). The results of this investigation are given in Table I. Comparative measurements with a quadrupole gas analyser indicate that the part of gases with molecular masses exceeding the mass 69 is less than 5 o/o of the total gas produced by friction. The mass spectra listed in Table I were recorded under the following conditions. (a) Run without liquid-nitrogen cooling. ao: rotary feed-through at rest; pr=5.4.10-8 torr. al: rotary feed-through at 200 rev./min; pin unloaded; pr=5.6.10-8 torr. Wear,I5
(1970)
7.19-227
222 TABLE
W. WILKENS, 0. KRANZ
I
RELATIVE
CONCENTRATIONS
f
Molecular weight
position
12
C
0.4
0.5
0.8
0.3
0.7
0
0.2 < 0.1 0.8 1.6 5.5
0.5 0.4 I.1 1.h 5.5
0.4 0.2 I.3 I.9 6.8
--0.X - 0.2 0.2 0.3 I.3
0.5 < 0.1 0.9 0.5 2.”
0.3 7.3 0.1
0.3 8.9 “.I
0.4 19.5 0.2 -
“.I 10.6 0.1 -
0.2 12.9 < 0.1
4.0
4.”
‘3 74 I5 16
‘7 Hz0
18
a0
a2
I9
27 28 29 3” 3’ 32 4” 43 44 45 5” 51
N.&CO CF
CF2
3.0
3.7
5.2 I.1
-
bz
h
-
co2
b,
a21
0.9 I.0
I.3 2.1 0.5 2.2
2.2
2.2 -
ba
0.6 -
0.9
I.3 2.0 2.8 0.5 2.0
0.3 0.7 0.7 0.”
I.” 1.8 2.0
-
0.2
‘3.4 <“.I
1.5 I.1
bzl I.5
0.2
0.0 0. I <
0.1
0.4 3.2 0.0 35.” I.9 -
0.5
35.5
0.8
6.6
0.7
17.8
6.5
45.5
-
62
CFa
69 7”
-
7.9
7.9 -
108 p
5.4
5.6
II.”
9.1
Run a, spectra without liquid-nitrogen cooling; run b, spectra with liquid-nitrogen cooling. Subscripts: “, rotary feed-through (RF) at rest; I, RF at 200 rev./min, pin unloaded; 2, RF at 2OOrev./min, pin loaded 0.7 kp; 3, RF at rest, gas analysing near desorption maximum. Aasi (Abzl) difference between thef-values of runs a2 and ai (bs and bi) ; p, total pressure (torr) ; dashes indicate f < 0.1.
az: rotary
feed-through
at 200 rev./min;
pin loaded with 0.7 kp; p, = 1.1.10-7
torr. (b) Run with liquid-nitrogen cooling. bo: rotary feed-through at rest; p,= 1.7. IO-9 torr. bi: rotary feed-through bz : rotary feed-through
at 200 rev./min; pin unloaded; &=3.0+10-9 torr. at 200 rev./min; pin loaded with 0.7 kp; p, = 6.8. IO-9
torr. bs : rotary feed-through at rest ; liquid-nitrogen cooling interrupted ; trap warm ing up; gas-analysis near the maximum pressure rise (0.9.10-7 torr) The spectra of run (a) and those of run(b) had to be measured with different adjustments of the spectrometer due to the different pressure ranges. Thus, the massfrequency data are only comparable within run (a) or run (b). The main conclusions drawn from Table I and partially shown in Fig. 6 are given below. (I) The molecular masses 31, 50, and 6g only appear during sliding friction, the mass 6g with high concentration. (2) The concentration of mass 28 increases significantly by the sliding process. (3) The rise of the total pressure is caused predominantly by gas components with the molecular masses 28, 31, 50, and 69, respectively by components which yield these masses in the analyser tube of the spectrometer. With liquid-nitrogen cooling, the part of these components is about go% and without liquid-nitrogen cooling about 80% of the total of gas produced by friction. WCW,15 (197”) 215-227
223
SLIDING FRICTION OF TEFLON ON STEEL
Similarly, for the gas which desorbs from ,the copper sheet around the pindisc device in the ultrahigh-vacuum vessel after 30 min friction runs and after stopping the liquid-nitrogen cooling of the sheet. 90% of the pressure increase due to the desorption of the gas is caused by components with the molecular masses 28,31, 50, and 69. The total pressure during desorption and concentration of the masses 31, 50 and 6g at different times are plotted in Fig. 7. From the desorption experiment one can draw the following conclusions. Coefficient
Af A
of friction
Specifac Sliding
: p =0.17
Load
:
0.7 kp
load
:
0.5 2
velocity
: CO= f 200
rpm
B
El
I
20
4
40
60
I
-m
80
moss Fig. 6. Increase of the molecular mass concentrations Aj caused by the friction of carbon-blackfilled Teflon on stainless steel in ultrahigh vacuum. (The molecular masses m 31, m 50 and m 6g are only present during friction.) m=moleculor
+IO*.
p&of-d
t=Time ofter stopping the LN-cooling
Fig. 7. Total pressure, p, and relative concentrations, from the trap.
f, of typical
molecular
masses desorbing
(I) Because the proportions between the concentration of the molecular masses 31, 50, and 6g only present during sliding friction, change with time, more than one gas component is formed by the friction process. It is most probable that these components are the molecular fragments CF (mass 31), CFB (mass 50) and CFs(mass
69).
(2) More than 90% of the total amount of gas desorbed from the copper sheet, respectively more than 4.8.10-s 1 torr equivalent to 1.6.1017 molecules, are
w&&V, 15 (1970)
215-227
W.
224
WILKENS,
0. KRANZ
derived from the frictional process. For an adsorption time of 30 min, during which the pin was sliding over the disc (60 cmjsec, A$ =7 .IO-~ torr), the rate of adsorption was 9.4.1013 molecules/set. On the other hand, the pressure increase during the experiment without liquid-nitrogen cooling yielded a gas production rate of I .5.1014 molecules/set. Thus, the adsorbable (and/or condensable) part of the friction-produced gas is at least 60:/, relative to the surface of the copper sheet at liquid-nitrogen temperature. This part must be even larger because not all molecules leaving the pin-disc interface will be caught by the copper sheet surrounding the pin-disc apparatus. Comparative measurements with the uncovered quadrupole system mounted inside the facility VT 106 confirm the results gained with the omegatron mass spectrometer, especially the occurrence of the masses 31, 50, and 69. The masses, their frequencies and the possible molecules or molecular fragments are listed in Table II.
DISCUSSIOiY
Teflon,
or polytetrafluoroethylene,
has the chemical
formula
,-I-...-_L_,; I
F
I I
F
F
with an average molecular weight of about 1000. In the vacuum facility UP 30, as well as in the system VT 106, all molecular masses found only during friction can be attributed to fragments of the Teflon molecule. These fragments originate, at least partially, from the friction process itself. This conclusion is not only drawn from the desorption measurements described above, but also by further, as yet incomplete, experiments on the “lifetime” of the fragments in the vacuum system, for example experiments on the determination of the time in which the frequency of a mass reduces to a given value. The masses so(CFz), 62(CzFz), 6g(CFa), and 81(CzFa) have different lives, the fragment CFa has a long life exceeding those of the other components by more than one order of magnitude. Comparing the measurements made with the omegatron mass spectrometer in the facility UP 30 and with the quadrupole in the system VT 106, the high concentration of the molecular mass 6g is obvious. In the facility UP 30, however, the concentration of mass 69 is greater than that of mass 31. In the system VT 106 the opposite is true. This difference is possibly caused by the long life of the molecular fragment Wear, r5 (1970) 215-227
SLIDING
FRICTION
OFTEFLON
ON STEEL
225
CF3. The omegatron-analysing tube is relatively far from the sliding pin and optically shielded from it. Thus, the probability of the gas with mass 69 not reaching the omegatron analysing head is greater than for the other components. BUCKLEYAND JOHNSON, in their report on friction, wear and decomposition mechanisms for various polymer compositions, also found the molecular masses 31 and 50 during the sliding of pure and glass-filled Teflon, on steel1 but they did not find the mass 69, typical of friction of Teflon on steel found in both facilities UP 30 and VT 106. Another interesting experiment in BUCKLEYAND JOHNSON’Spaper is that on the influence of sliding velocity on the frequency of the molecular masses 50 and 31, which decreases unexpectedly with increasing sliding velocity. This finding seems to be unlikely as, in the present experiments, the amount of friction-produced gas rises with sliding velocity. BUCKLEY AND JOHNSONexplain the decrease of the concentration of the masses 50 and 31 with an increase of wear from 2.7.10-s cma/cm at 2.0 mjsec to 3.5 .IO-~ cma/cm at 5.0 mjsec by assuming that the additional wear particles carry away more than the additional frictional heat caused by the rise of sliding velocity. Taking into account the coefficient of friction, ,u=o.25, and the load, N = I kp, in the authors’ experiments, the wear particles had to carry away more than 2.106 watt sec/cma. This exceeds the heat-carrying capacity of Teflon by two orders of magnitude, even taking into account the energy required for breaking all C-bonds of the Teflon molecule. The effect observed could be explained by the influence of specific load (kp/mm2) on the process, because, starting with a hemispherical slider, the specific load decreases due to wear of the slider head and the specific energy input (watt sec/cm2) also decreases. Regarding the gas-production rate, not mentioned by BUCKLEYAND JOHNSON,the present measurements show that the increase of the rates with increase of sliding velocity consists of two, possibly three, stages with linear slopes (Fig. 5). This indicates, that the rate of gas production is more likely to be connected with wear and not with a temperature-dependent process. The different slopes are believed to correspond to different stages of wear. As explained above, the gas-production rates are related to nitrogen, that means to its ionizing probability or specific ionization, and to the pumping speed for this gas. The specific ionization of the molecular fragments CF, CF2, and CFa are not present. The specific ionization of CO and CO2 which significantly increase during friction, is about 25:/o respectively 35% greater than that for Nz. Thus, it is likely that the total pressure measurements are overvalued by about 50:/o. Considering the increase of the frequencies Afi of the molecular masses mi the pumping speed Sg for the continuously produced gas is S,=&
CAj’i (mN/mi) 4 XAfi -
(8)
Using eqn. (8) and the data from Table I, S, is 56.5 l/set but, regarding the specific ionization of the gas components, the frequencies Afi are also overvalued compared to nitrogen. Thus, S, can be regarded as the lower limit of the true pumping speed. Taking into account a 50% reduction of the overvalued total pressure data, the gasproduction rate cjg=.%A&, determined by eqn. (6a), is also overvalued by up to 60;/,. The same is true for the adsorption rate ~sd=SpApsd where Ap=r/TjT Apdt, (T WtWf’,1.5 (1970) 215-7.27
226
W. WILKENS, 0. KRANZ
being the adsorption time). The ratio of gad and da gives the adsorption coefficient, k,
The errors caused by relating all pressure measurements to nitrogen largely compensate each other. The statement that at least 60% of the friction-produced gases adsorb on liquid-nitrogen-cooled copper surfaces in ultrahigh vacuum remains valid. This finding, however, has to take into consideration that, during the 30 min friction run before the desorption measurements, about 1017 molecules are adsorbed. This corresponds to nearly a monolayer on a copper sheet surface of 250 cm2. The adsorption and/or condensation of further layers is under investigation. However, contamination film thicknesses even less than one monolayer can cause large changes in physical properties of solid surfaces”. Assuming, for example, that the contamination by friction-produced gases does not exceed that of one monolayer at a distance of IO cm from the sliding bearing after a one years running time in ultrahigh vacuum, the adsorption coefficient, k, may be estimated from eqn. (7b). k < 8.10-12
-&
This means that k < I . IO-J for a sliding velocity of 60 cmjsec. Thus, in the vicinity of sensitive and cooled surfaces, Teflon, despite being a very good self-lubricating material, must be excluded. The influence of surface tenlperature and surface material on the adsorption coefficient of the gases produced by sliding friction of Teflon materials has still to be investigated. CONCLUSIONS
The coefficient of friction of Teflon, filled with 2594 carbon black is not influenced by vacuum and its value is about 0.17. Up to sliding velocities of I m/set, the friction-induced gas-production rate rises to I -10-5 1 torr/sec under a specific load of 0.5 kp/mmz (total load 0.7 kp). More than 60% of the friction-produced gas adsorbs on copper surfaces at liquid nitrogen temperature, at least up to film thicknesses of one monolayer. It is, therefore, dangerous to use Teflon as a bearing material in the vicinity of sensitive surfaces of spacecraft. The molecular masses only detected during friction of the Teflon material are all attributed to fragmentation of the Teflon molecule. The fragments originate from the friction process itself and can only be partially formed in the analyser tube. ACKNOWLEDGEMENTS
This work was supported under contract number RFF 3ozg by the Bundesminister fur Bildung und Wissenschaft (Federal Minister of Education and Research), to whom we are very grateful. We are indebted to Prof. W. THIELEMANN, head of the structures and materials department, for his encouragement. We thank H. SWOBODA for his sk~fuldesign work and D. PAPE for assisting throughout allexpe~ments. Wear,15 (1970)215-227
SLIDING FRICTION OF TEFLON ON STEEL
227
REFERENCES D. H. BUCKLEYAND R.L.JOHNSON, NASA Tech.NoteTND-zo73,1963. D. H. BUCKLEY, NASA Tech. Note TN D-.?261,1o66. W. WILKENS, DFVLR-Innternal Working Piper-F166-o8, DFVLR Braunechweig, 1966. S. DUSHMAN, Scientific Foundalion of Vacuum Technique, Wiley, New York, 1962. M. ARDENNE, Tabellen zur Angewandten Physik, VEB Deutscher Verlag der Wissenschaften, Berlin, 1962. H. MAYER, Physik Diinner Schichten, Teil I und II, Wissenschaftliche Verlagsgesellschaft, Stuttgart, 1955. WeUP', 1.5 (1970) 215-227