Experimental investigation of physical adhesion, measurement of contact area during adhesion process, and effect of adsorbed or impurity species on metal-metal bondin

Keller, D. V., Jr.

English

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ff 653 July 65 
BS 
D. V. Keller, Jr. 
Associate Professor of Metallurgy 
Department of Chemihal Engineering am4 Metallurgy 
Prepared for 
National Aeronautics and Space Administration 
Office of Grants and Research Contracts; Code SC 
Washington, D. C. 
Grant No. RSG-483-1 
e"* * 
SYRACUSE UNIVERSITY RESEARCH INSTITUTE 
https://ntrs.nasa.gov/search.jsp?R=19650026795 2020-03-16T22:05:45+00:00Z
9 
Semi-Annual Report-2 
Approved by : 
ADHESIOI4 BE!CWEEH ATOMICALLY PURE METALLIC SURFACES 
PART IV 
Prepared for  
National Aeronautics and Space Administration 
Office of Grants and Research Contracts; Code CS 
Washington, D.C. 
Submitted by: 
Douglas V. Keller, Jr., Director 
This report w a s  produced under a sponsored 
contract. The conclusions and recommendations 
expressed are those of the  Author and are not 
necessarily endorsed by the  Sponsor. 
duction of t h i s  report, o r  any portion thereof, 
must bear reference t o  the  or iginal  source and 
Sponsor. \ 
Repro- 
SYRACUSE UNIVERSITY RESEARCH INSTITOTE 
Department of Chemical Engineering and Metallurgy 
Sponsored by : 
Douglas V. Keller, Jr. N.A.S.A. - NsG-483 
ProJect Director Supplement 1 
Date: August 1965 
S.U.R.I. Report 100. Met. 1100-6588A 
EXPERIMENTAL INVESTIGATIONS OF SOLID ADHESION 
Experimental investigation of solid-solid adhesion a t  Syracuse University 
is  directed toward the three main problem areas c i ted  in  the  theoretical  review 
submitted on 1 Mey 1965 t o  NASA as a semi-annual report (1). Briefly these areas 
include: a) physical adhesion, i.e. no experimental evidence of a t t rac t ive  forces 
between clean metal surfaces pr ior  t o  physical contact have been cited; b) mea- 
surement of true contact area during the  adhesion process, i.e. no known experi- 
mental technique has been developed which w i l l  provide true contact area of ad- 
hesion; c)  effect  of adsorbed, or  impurity, species on metal-metal adhesion, i.e. 
no adhesion techniques have been developed t o  date f o r  surfaces with a measured 
degree of puri ty  concentration. 
The following is a brief progress report of these ac t iv i t ies :  
Physical Adhesion - As indicated i n  the  previous report (1) , physical adhesion 
is  defined as dispersion forces of a t t ract ion occurring at separation distances 
l e s s  than one micron which exis t  between para l le l  plates pr ior  t o  physical con- 
t ac t .  
s i m i l a r  metal plates separated by small distances, c 1 u ,  which are e lec t r ica l ly  
short-circuited giving rise t o  an electrostat ic  f i e l d  a t t ract ion dependent on 
the contact potential  of the  two metals. 
been studied t o  a limited extend fo r  quartz and certain metal specimens ( i n  air)  , 
and these data are  in  agreement with the theoret ical  equations (1) developed t o  
date. 
One may also include i n  t h i s  category forces of a t t ract ion between dis- 
The dispersion and f i e l d  forces have 
Davis and Engelke (2)  proposed t h a t  the observed s t a t i c  adhesion force data 
from the  quartz-quartz system, which they investigated, coincided w i t h  an ex- 
trapolated l i n e  of the force of a t t ract ion vs distance of separation plot used 
normally t o  justif3. the dispersion force of a t t ract ion theory. 
theory proposed by Lifshitz (3) i s  only dependent upon the  opt ical  properties of 
Since the  force 
the material under investigation and since the opt ical  properties of most ma- 
terials are obtainable over most of the c r i t i c a l  wave lengths, the ultimate proof 
of t he  proposed theories as w e l l  as a confirmation of the  experimental observa- 
t ions could w e l l  lead t o  a direct  mechanism of estimating the  adhesion proper- 
t i es  of two unknown materials using only the  opt ical  properties of those mate- 
rials. FUlYthermore, the general nature of the  theoret ical  equation allows the  
inclusion of 
specif ical ly  metals which is certainly t o  the advantage of t h e  study. 
materials, i.e. inorganic, organic, sol id  o r  l iquid,  rather than 
The system designed and constructed t o  make physical adhesion measurements 
between atomically clean metal plates i s  shown i n  Figure 1. The apparatus con- 
sists of a vacuum micro-balance t o  which is attached one metal sample plate ,  o r  
receptor fo r  a metal vapor film. 
balance capable of sens i t i v i t i e s  t o  0.1 pgrams and produces a DC output pro- 
portional t o  the  balance load which is  recorded on a 10 mil l ivol t  electronic re- 
corder. 
t rac t ion  between it and the  movable sample p la te  (HI thus may be recorded. The 
movable sample plate  (H) consists of an aluminum substrate supported on a short  
section of 0.007" x 0.187" bi-metallic s t r i p  which acts  as a res i s tor  i n  a high 
current c i rcu i t .  By varying t h e  current through the  s t r i p ,  e.g. varying i ts  t e m -  
peratures, the movable sample plate can be made t o  move at a l inear  rate ( ca l i -  
brated beforehand) in to  contact with the sample p la te  on the balance. 
progression of the  movable p la te  toward t he  balance plate  variation i n  the  
forces on tha t  p la te  w i l l  be recorded as an apparent mass change. 
the rate of closure, plate-plate, and the  rate of mass change are simultaneously 
known, a correlation between separation distance and a t t rac t ive  force may be 
determined. 
The balance is  a Cahn RG automatic electro- 
Changes i n  apparent mass of t he  sample piate  (F) due t o  forces of at-  
During 
Since both 
The vacuum loop system i l lus t ra ted  i n  Figure 1 appeared t o  be the  only 
solution for obtaining recorded microgram measurements i n  a s t a t i c  ultra-high 
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vacuum (UHV) system, pressures below 10-l' Torr, since the balance i tself  con- 
s t i t u t e s  a primary gas source w e l l  above 10 
and s l igh t ly  modified Cahn balances have been operated t o  pressures i n  the  10 
-8 Torr. Overdesigned pumping systems 
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Torr range (as claimed by the  producers); however, since the proposed experiments 
were t o  include atomically clean and control contaminated surfaces it w a s  fel t  
t ha t  t he  elimination of all possible internal  contamination sources, ever present 
i n  dynemic vacuum systems, ought t o  be eliminated. The loop design consists of a 
balance c e l l  and a test c e l l  which m a y  be isolated from the other through valves, 
Ja and Jb, since the balance w i r e  passes from one c e l l  t o  the other through a low 
conductivity bypass. 
J and Jb, are open. a 
(C)  
is  used t o  establish t h e  lower pressure i n  the  test  c e l l ,  while t he  main pumping 
system maintains as low a pressure as possible in the balance chamber (loo8 Torr 
esti~nsted).  The pumping system consist8 of two 2" Granville-Phillips l iquid ni- 
trogen pumps, two 2" CVC o i l  diffusion pumps and a Welch duo-seal mechanical pump. 
Upon establishment of the pressure i n  the test  c e l l  a fresh film of aluminum 
During the  45OoC bakeout cycle of the test c e l l  both valves, 
The balance system pressure is recorded on the hot ion gauge 
Then after bakeout , valve Ja is closed and the  titanium sublimation pump (I) 
is deposited on the sample plates and the  movable plate  is  set into motion. 
a t t rac t ive  force is  then recorded. 
The 
The materials t o  be tes ted i n  t h i s  apparatus include various combinations 
of similar  and dissimilar atomically clean metals and the  effect  of various 
contaminating gases. 
vacuum system, including the ce l l s ,  hsve undergone tests t o  pressures below 10-l' 
Torr without the presence of t he  balance mechanism. 
standardized and mounted; Bowever, due t o  d i f f i cu l t i e s  t he  balance had t o  be re= 
moved for  repairs at the Cahn Company. 
mounting and recalibration are expected t o  be accomplished wi th in  a month or  so. 
Contact Area Measurements - The discussions i n  the previous report indicated that 
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The apparatus is i n  the  f i n a l  stages of assembly. The 
The balance mechanism w a s  
The repairs having been completed, re- 
the very nature of s t a t i c  chemical adhesion, i.e. variations from a welded junc- 
t ion  t o  a non-welded junction, was grossly dependent on the  atomic species 
actually present on the  surface irrespective of the nature of the bulk material 
constituting the substrate. The direct  observations and l imitations of t h i s  
statement are  only quali tative i n  nature; and if  a substantial  mechanism of ad- 
hesion is t o  be proposed, quantitative data must be available i n  i ts  supporD. 
t h i s  time most solid-solid adhesion investigations have been directed toward 
A t  
go - no-go information between various materials under such widely varying vacuum 
and surface conditions that a r ea l i s t i c  analysis of the output data appears t o  be 
impossible. Direct quantitative data are s t i l l  required t o  establish the e f fec ts  
of mono1a;yers and contaminants on the force of adhesion. 
ta ining these quantitative data is the lack of a reliable technique t o  estimate 
The main problem i n  ob- 
t h e  amount of surface contaminant a t  the instant a force measurement is i n  
progress. When these data become available the i r  correlation w i l l  be invaluable 
i n  the  prediction of possible lubricating mechanisms t o  prevent bulk adhesion. 
A system has been studied i n  t h i s  laboratory which seems t o  show great promise 
i n  t h i s  direction. 
presence or  absence of an adsorbed layer between two metals in  contact has a large 
Numerous investigators (4,6) i n  the  past have shown t h a t  the 
effect  on the  contact resistance of  t h a t  system; furthermore, these data seem t o  
be related t o  the t rue  area of contact between the specimens. Since the previous 
investigators were not concerned direct ly  w i t h  the precision force measurements 
nor the  perfection,!of the  surface, experimental techniques are lacking; however, 
the system designed and tes ted i n  t h i s  laboratory allows fo r  s t a t i c  force measure- 
ments below a 5 gm load and can readily be coupled with contact resistance measure- 
ments a t  t h e  junction. 
used t o  examine these relationships i n  several refractory m e t a l  systems. 
The system i l l u s t r a t ed  schematically i n  Figure 2 w i l l  be 
The apparatus consists of a t o r s i o n  balance with a 23 cm quartz beam mounted 
on a tungsten torsion wire which is, in turn,  supported on two 3/16" stainless 
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steel support rods (HI welded t o  a conflat blank-off flange. 
samples AB and CD, one set (CD) is fixed through insulator supports t o  the con- 
f lat  base plate,  while the  other set is mounted on one arm of the balance beam. 
The samples are connected e lec t r ica l ly  t o  outside of the ce l l .  Thus the elec- 
t r i c a l  c i rcu i t  shown i n  Figure 2 represents the test  system. 
O f  the two m e t a l  
The torsion balance is not actually used as a balance device, i n  t ha t  during 
a test the beam is held i n  a fixed force f ield supplied by the  permanent magnet 
at the end of the  2" pyrex housing tube interacting wi th  t h e  iron core at the 
beam end. 
t h e  ample8 t o  the desired point of contact, yet  at a separation distance of 
about 0.1 cm. Sample contact and force measurements are accomplished by in- 
creasing the current i n  solenoid (L) which, i n  turn,  causes an a t t rac t ive  force 
between it and the  iron slug (MI) fixed t o  the  end of the s t r a in  gauge wire (N). 
This force is required t o  shear t h e  a t t rac t ive  force between the iron slug (M) 
fixed t o  the balance beam and the positioning magnet. 
wire constantan s t r a in  gauge (N) is attached t o ,  but e lec t r ica l ly  insulated from 
t h e  balance beam, and has a residual resistance of about 80 ohms. Therefore, 
t h e  force required t o  bring the samples in to  contact, i.e. t he  shearing force 
of t he  fixed magnet field, is recorded as a strain. 
t he  current-voltage properties in the  crossed metal samples indicates the  point 
of physical contact; thereafter, force versus contact resistance properties are 
recorded as the contact force is increased by means of the external solenoid. 
A f t e r  the  max imum contact force is achieved, the contact force is  reduced ( s t i l l  
recording contact resistances) t o  t he  point of contact separation. 
torsion system required a force t o  shear t he  fixed magnetic f ie ld  and t h i s  f ie ld  
is superimposed constantly on the contact system, the force of adhesion, or  the 
breaking force, can be measured as the change in zero point at separation. The 
balance system as described can produce a maximum load of about 5 gms w i t h  a 
This is called t h e  "positioning magnet" i n  tha t  it is used t o  align 
me 0.00095" x 5" straight 
Simultaneous observation of 
Since the 
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sensi t ivi ty  of about 10 milligrams. 
ages i n  the range of 6 mil l ivol ts  i s  used t o  detect the resistance values in  the 
system. 
s i s t o r  is used t o  detect the n u l l  point i n  the bridge c i rcu i t .  
A precision Kelvin bridge u t i l i z ing  volt- 
A Keithley nanovoltmeter (lo-' volts)  i n  conjunction with a 30 n re- 
Modifications in  the beam support mechanism are i n  the  process of com- 
pletion, and are expected t o  be complete within a month. 
molybdenum system w i l l  be investigated i n  order t o  compare these data with those 
of the  molybdenum-copper (immiscible) system. 
Surface Impurities - As indicated in  the previous section, the existence of un- 
known contaminants on a surface t o  be engaged in  an adhesion study may have a 
gross effect  on the results. It is  hoped tha t  the contact resistance technique 
w i l l  provide one means of detecting the,presence of these layers during a tes t ,  
thus enabling an investigator t o  c i t e  a point of reference in  describing the 
surface under investigation. The purpose of the th i rd  area of investigation is  
t o  examine the u t i l i za t ion  of contact potential  measurements as a second method 
of surface characterization prior t o  adhesion measurements. Since the  measure- 
ments of the  contact potential  of a m e t a l  or  alloy i s  a very simple procedure 
yet  extremely sensit ive t o  surface condition ("$1, t he  technique seemed most 
worthy t o  consider. 
The molybdenum- 
The contact potential  apparatus shown in Figure 3 consists of an ultra-high 
vacuum system (K-10) u t i l i zed  t o  evacuate a 5" O.D. x 8" test  c e l l  i n  which a 
polycrystalline tungsten plate ,  standard (F2), is  supported pa ra l l e l  t o  a mov- 
able test sample p la te  (F1). Cyclic variation of the position of the  tungsten 
p la te  normal t o  the test  sample p la te  produces an AC signal due t o  the contact 
potent ia l  f i e ld  between the  two plates which may be detected on an ocilloscope. 
If  a known DC voltage is  supplied t o  the plates which is  opposite, but equal t o  
the contact potential ,  the observed AC current w i l l  disappear; and, the impressed 
DC potential  i s  equal t o ,  and recorded as, the observed contact potential. The 
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use of t h i s  technique in  conjunction with adhesion studies w i l l  be examined 
after the standardization of the apparatus. 
This apparatus is i n  the early stages of construction and is expectedto 
be in operation within a month or so. 
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BIBLIOGRAPHY 
1) Keller, D.V., "Adhesion Between Atomically Pure Metallic Surfaces - IV", 
Semi-Annual Report (1100-653SA), March 1965, NASA - NSG-483-1. 
2) Davis, R.S., and W e l k e ,  J.L.,  "Solid Surfaces i n  Contact Under Extreme 
Environmental Conditions", Abstract 985C, SAE' Internationel Conference, 
Detroit, January, 1965. 
3) 
4) 
Lifshitz,  E.M., Soviet Physics , 2, 73 (1956) 
Went, J.J., Physica, VIII, 233 (1941) 
5) K i s l i u k ,  P . ,  B e l l  System Tech. J., - 37, 925 (1958). 
6 )  Holm, R., "Electrical Contacts Handbook", Springer, Berlin (1958) . 
7) Delchar, T., Eberhagen, A . ,  and Tampkine, F.C., J. Sci. Ins t . ,  40, 105 
(1963) 
8) 
9) 
Hopkins, B.J. ,  and Riviere, J.C., B r i t .  J. App. Phy., 15, 941 (1964). 
Anderson, P.A. ,  Phy. Rev., re, 1034 (1941). 
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Adhesion between atomically pure metallic surfaces, part iv semiannual report no. 2

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