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Contact resistance, adhesion and wear of tarnished and untarnished silver subjected to an oscillating friction load
Wear, 69 (1981) 157 - 165 0 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
157
CONTACT RESISTANCE, ADHESION AND WEAR OF TARNISHED AND UNTARNISHED SILVER SUBJECTED TO AN OSCILLATING FRICTION LOAD H. BRESGEiV
Siemens A.G., Hofmnnnstrasse
51,D800
Miinchen (F.R.G.)
(Received July 29, 1980)
Summary As a follow-up to earlier investigations, silver chips with and without AgaS tarnish layers were subjected to an oscillating friction load from a hard gold stud. Contact resistance, adhesion and wear were investigated with contact forces of 10 cN at a friction frequency of 150 Hz using frictional movements smaller than the diameter of the stud-plate contact area (i.e. smaller than 100 pm). The increased contact resistance caused by the tarnish layer dropped appreciably as a function of movement after only a short frictional contact. For larger (e.g. 50 pm) ~plitudes the contact resistances were only slightly higher than on the un~~h~ chip. In the presence of tarnish layers, the breakaway forces which increase with increasing frictional movement (10 - 70 pm range) may be attributed partly to adhesion due to tarnishing and partly to cold welding. However, none of the tests with the tarnished chips produced any significant silver surface wear. Without a tarnish layer typical forms of transfer wear between chip and stud occurred.
1. ~trodu~tion The increased contact resistance caused by tarnish layers can generally be reduced by mechanical treatment. An earlier investigation [ 1 ] was aimed especially at reducing the contact resistance RD by means of an oscillating friction load. Frictional movements smaller than the radii of the mutual contact areas and contact forces below 100 cN were used to simulate the characteristics of electromechanical communications contacts. Evidence that tarnish layers on contacts do not necessarily impair their operation is provided firstly by the fact that the contact resistance of such layers is frequently much less critical than might be concluded from meas~rnen~ conducted without frictional movement and secondly by the fact that these layers can also give protection against mechanical wear 12J .
158
In the present paper we concentrate forded by tarnish layers against wear. The tance characteristics of silver surfaces with are compared utilizing the same apparatus ]Y [Il.
2. Specimens
primarily on the protection afadhesion, wear and contact resisand without Ag,S tarnish layers parameters as were used previous-
and test procedure
The friction tests were conducted on two plates of solid silver, one with an AgsS tarnish layer (approximately 0.8 pm thick [l] ) and the other without a tarnish layer. The surface was kept as clean as possible by using airtight packing directly after manufacture. The hardness of the silver chips was about 80 HVs.aes. The companion electrodes consisted of hard gold studs similar to those used earlier [l] , with a radius of 1.5 mm and a hardness of approximately 200 HV0.ac5. The contact force of the studs on the flat silver chips was 10 cN. The silver chips were moved backwards and forwards over the stationary studs in the same sinusoidal pattern as was used previously (frequency, 150 Hz; movements of 10,30, 50 and 70 pm) [1] . Each test was carried out with a new clean stud (mechanically polished and ion etched) with a defined contact force, frequency and movement, and up to about lo5 frictional movements were applied at a particular point of contact on the chip. When the oscillating friction was stopped the contact resistance was measured at a voltage of less than 20 mV. The studs were then lifted off the chips and the breakaway forces were registered. The tests were carried out in air at about 25 “C with approximately 45% relative humidity. At the end of a test series, the points of friction were investigated by scanning electron microscopy and the maximum amounts of erosion in the silver chips were measured.
3. Adhesion 3.1. Results 3.1 .I. Silver chip with a tarnish layer: increased adhesion for larger frictional movements About lo5 frictional movements were made at each of a number of points on the surface of the sulphide-coated silver chip. Ten tests with 10, 50 and 70 pm movements each (a total of 30 tests) were carried out at a variety of points on the chip. The breakaway forces FPb were measured at fixed numbers of frictional movements (the rate of increase in the force to breakaway was approximately 0.6 cN s-l). Figure 1 shows the adhesion coefficients (Yobtained from FJF,, where F,, is the contact force.
159
,
Mean value &=30pl
Fig. 1. Adhesion coefficient as a function of the number of frictional movements for chips with and without tarnish layers. 10' Q 2.5
10%
IO'
1
2
'8
15 I a
fk! I I 10.' 05
10-l
Frictionat movements-
Fig. 2. Contact resistance RD and adhesion coefficient o ss a function of the number of frictional movements. 30 X lo* frictional movements were made at a certain point on the sulphide-coated chip, first with a 10 pm movement and then with a 70 I.trnmovement.
Adhesion increases with frictional movement, which is also demonstrated by the following test (Fig. 2). The silver chip with a tarnish layer was exposed to 15.4 X lo* frictional movements, first with a movement of 10 Mm and then with 70 pm at the same point of friction (without using ti new stud): (Yrose shortly after changing from 10 to 70 pm (and the contact resistance dropped appreciably).
160
uuL..Lu.L
d-i...
-LLLauL
10'
10
1
’
10
Fss,*x--
Fig. 3. Structure of the resistance RU: R,, constriction tion resistance of At; RF, coating resistance of Af. Fig. 4. Maximum contact resistance
resistance
CN
10'
‘-
of A,;
RE*, constric-
due to cold welding as a function of the possible bonding force F,, RD (measured in the cold-welded condition), according to eqn, (1).
3.1.2. Silver chip without a tarnish layer; reduced adhesion for larger frictional movements
The mean values of the adhesion coefficient (Yfor 30 and 70 pm movements are given in Fig. 1 for the silver chip without a tarnish layer. The test conditions were identical to those described ,in Section 3.1 .l. The increased adhesion for the larger frictional movement (70 pm) only occurs up to about 0.7 X lo* frictional movements. After this, (Ydrops slightly for larger movements. 3.2. estimation of the m~imum pothole bonding forces due to cold welding In instances where breakaway forces are measured at a frictional contact which is thinly or partly covered by a tarnish layer it can be assumed that the corresponding bonding forces are caused not only by cold welding (metal to metal) but also by adhesion due to tarnishing. The predominant holding force can be determined by measuring the contact resistance RD before breakaway. The partly tarnished true contact area A, can be subdivided into a metallically clean portion and a section covered by a coating. These may ideally be described by two circular areas A, and Af so that A, =A, +A* where A,,, is the metallically clean contact area and Ai is the contact area with a coating. The contact resistance RU = RD -R, where R, represents the resistance of the leads, i.e. 1.4 mS2 in this case, then consists of the parallel circuit of Fig. 3. The relationship between RE and A,,, is governed by the equation RE = p/Za where p (or (pl + p&/Z) is the resistivity of the conductors and a the radius of the circular area A,. Then
161 2
2
4RE2
4RU2
&=a2*=P
The bonding
force F, due to cold welding is represented
by
F, = ZA, where 2 is the tensile strength. The welded area A, lies within the metallic contact area, i.e. A, < A,, which means that
(1) Given a certain contact resistance R,, , the breakaway come cold welding will thus not exceed F, max where F
force needed to over-
zp211 Smax = 4(R,-, - R,)2
If, however, the breakaway force F measured on the basis of RD is greater than F, mpx it must be assumed that the surface layer causes additional adhesion of at least approximately F - F, ,,,=. The relationship (1) is represented graphically in Fig. 4. In the measurements it was found that (pr + p2)/2 was 7.4 X 1O-6 Q cm for a hard gold stud on a silver chip. Values of 200 - 500 dN mmm2 were used for the tensile strength (curves 1 and 2). Curve 3 shows the deviation due to more widely differing p and 2 values [ 31. 3.3. The allocation of the breakaway forces to cold welding and adhesion due to tarnishing After frictional contact with a 10 pm movement the tarnished chip showed practically no bright spots within the friction area. Thus the adhesion coefficients (up to about unity) may be attributed to adhesion due to tarnishing. This statement may be retained even when consideration is given to the RD values measured before breakaway, which have a median value of about 80 mS1 after approximately 2 X lo4 frictional movements (Fig. 5). According to Section 3.2 and Fig. 4 the adhesive bonding forces due to cold welding do not exceed 0.05 cN for a contact resistance of this magnitude. However, the average reading was about 5 cN (Fig. 1) which means that the difference 5 - 0.05 = 5 cN can be explained by adhesion due to tarnishing. This also explains why higher breakaway forces occurred generally for larger frictional movements; these (in the range investigated of up to 70 pm) increase the removal of tarnish from the area subjected to friction and therefore reduce the contact resistance (Fig. 5). Thus there are more bright spots within the friction area. Cold welding increases and the adhesion coefficient rises in all cases. (Breakaway forces of up to about 10 cN were measured for Ag,S adhesion and of up to about 45 cN for cold welding for the untarnished chip (Fig. l).)
162
0
1
5
10~10"
FrIctIonal mo"ements-
Fig. 5. Contact resistance RD as a function of the number of frictional movements. Median RD values of -ten readings each for silver chips with a tarnish layer and of six readings each for silver chips without a tarnish layer are shown.
The untarnished chip showed higher adhesion for initial friction (up to about 0.7 X lo4 frictional movements), again using a larger frictional movement. This indicated the presence of surface layers on the chip and/or the stud, which were removed after a low number of frictional movements (thus cleaning the point of friction). After this, larger movements tended to reduce adhesion slightly.
4. Contact resistance The behaviour of the contact resistance RD has been reported earlier [l] for a tarnished chip subjected to frictional movements up to 100 pm. It was found that RD decreased with increasing number of frictional movements and, after a fairly low number (about 2 X 104) of movements of 30 100 pm, reached a stable value that was lower for the larger frictional movements. Figure 5 shows the median RD values for various movements (up to 70 ym) as a function of the number of frictional movements. This diagram also shows the central RD curve for a movement of 30 pm on the untarnished
163
(4
tb)
Fig. 6. Place of friction on (a) a silver chip (without an AgzS layer) and (b) a hard gold stud: contact force, 10 cN; frictional movement, 70 pm; about lo4 frictional movements. Abrasive wear of the chip with silver transferred to the stud is shown.
chip (the contact resistances for 30 and 70 I.trn movements were practically identical). It appears that, having reached a stable level with a movement of 70 pm, the contact resistance of the tarnished chip is only slightly higher (about 2.5 ma) than that of the untarnished chip.
5. Wear 5.1. Silver chip with a tarnish layer In no case did frictional movements on the tarnished silver chip cause erosion of the silver surface or of the hard gold stud (investigations up to 30 X lo4 frictional movements). These movement merely removed the tarnish layer, increasingly so with increasing extent of movement (expressed again in terms of R,, behaviour). The tarnish layer thus acted as a wear-inhibiting lubrication layer. This effect even persisted after most of the lubrication had been removed, e.g. with a movement of 70 pm and R, = 4.4 mS2. Another explanation of this wear-reducing phenomenon is that the initially completely AgaS-covered silver surface was passivated by the frictional action to such an extent as to make it resistant to wear. 5.2. Silver chip without a tarnish iayer The frictional movements on the un~~h~ chip ~h~acte~stic~ly transported silver from the softer chip (hardness about 80 HVo.ooa) to the harder gold stud (hardness about 200 HV,,,,) as shown in Fig. 6. Using a new stud each time a series of friction tests was carried out to show how the amount of erosion depended on the number of n frictional
164
Excursmn=30pm v
Fig. 7. Erosion h due to friction on a silver chip without a tarnish layer as a function of the number of frictional movements. The curves of the median value h* are given for 30 pm and for 70 pm movements.
movements. A light-section microscope was used to measure the depth of erosion at each point of friction on the chip. Figure 7 shows the erosion depth h with a frictional movement E = 30 pm plotted against the number of n frictional movements. The median erosion h* curves for E = 30 I.tm and E = 70 pm are also shown. Initially, h* increases considerably with increasing number of frictional movements but settles down at about 11 -16 pm after approximately 3 X lo4 movements. Thus the amount of erosion does not increase monotonically with the number of frictional movements. The maximum h* value is reached faster for E = 70 pm than for E = 30 ym, i.e. the amount of material transferred as a function of n is initially greater for a larger frictional movement (70 pm). That the amount of erosion is confined and does not rise monotonically with the number of frictional movements may be explained by the fact that silver transferred to the stud is re-transferred to the silver chip, especially with increasing erosion depth. When h* is plotted against the product en (total frictional travel) the curves for E = 30 I.cm and for e = 70 Humagree better than when h* is shown as a function of n. Agreement is very good particularly in the initial range
165
(up to about en = 250 mm) but diminishes as more silver is re-transferred to the chip. A monotonic rise with en has been described [4] for the wear of crossed wires as a result of a smaI1 amplitude of frictional movement. This applies equally well to the present tests, in the initial range where IZis not too large.
6. Discussion For a sulphide-coated silver chip subjected to an oscillating friction load no metal surface wear of any type was found, even though considerable breakaway forces for cold welding (up to about 20 cN) were measured. In contrast, typical silver surface abrasive wear occurs in the absence of a sulphide coating. Very thin tarnish layers and remains of tarnish layers (e.g. with contact resistances of about 4 ma ) sufficed to stop wear under these mechanical conditions. The breakaway forces and corresponding bonding effects were attributed partly to cold welding and partly to Ag,S adhesion. The breakaway forces found in the presence of surface layers were higher for larger frictional movements than for smaller movements (in the range investigated of up to 70 ,um). However, in instances where surface layers were generally absent the breakaway forces dropped slightly with larger movements after initial friction of a relatively brief duration. This may be attributed to the fact that the surface layers in the area of friction are removed faster with larger frictional movements so that cold welding increasingly occurs (and hence leads to higher breakaway forces). A criterion for cleanness (fiction in air) can be obtained by plotting the breakaway forces against frictional movement (10 70 pm): the contact areas are relatively clean if for increased movements the breakaway forces do not increase.
References 1 H. Bresgen, Reducing the contact resistance of tarnish layers on silver by means of an oscillating tangential motion, IEEE Trans. Compon., Hybrids Manuf. Technol., 1 (1) (1978) 65 - 68. 2 A. W. de Gee, Der Einfluss von Oxidschichten auf den Reibungs- und Verschleissvorgang bei Gleitreibung, Materialpriifung, 9 (5) (1967) 166 - 169. 3 E. Diirrwgchter (ed.), Doduco Datenbuch, Doduco KG, Pforzheim, 1974. 4 R. G. Bayer and J. L. Sirico, Wear of electrical contacts due to small-amplitude motion, ZBM J. Res. Dew., 15 (March 1971) 103 - 107.
157
CONTACT RESISTANCE, ADHESION AND WEAR OF TARNISHED AND UNTARNISHED SILVER SUBJECTED TO AN OSCILLATING FRICTION LOAD H. BRESGEiV
Siemens A.G., Hofmnnnstrasse
51,D800
Miinchen (F.R.G.)
(Received July 29, 1980)
Summary As a follow-up to earlier investigations, silver chips with and without AgaS tarnish layers were subjected to an oscillating friction load from a hard gold stud. Contact resistance, adhesion and wear were investigated with contact forces of 10 cN at a friction frequency of 150 Hz using frictional movements smaller than the diameter of the stud-plate contact area (i.e. smaller than 100 pm). The increased contact resistance caused by the tarnish layer dropped appreciably as a function of movement after only a short frictional contact. For larger (e.g. 50 pm) ~plitudes the contact resistances were only slightly higher than on the un~~h~ chip. In the presence of tarnish layers, the breakaway forces which increase with increasing frictional movement (10 - 70 pm range) may be attributed partly to adhesion due to tarnishing and partly to cold welding. However, none of the tests with the tarnished chips produced any significant silver surface wear. Without a tarnish layer typical forms of transfer wear between chip and stud occurred.
1. ~trodu~tion The increased contact resistance caused by tarnish layers can generally be reduced by mechanical treatment. An earlier investigation [ 1 ] was aimed especially at reducing the contact resistance RD by means of an oscillating friction load. Frictional movements smaller than the radii of the mutual contact areas and contact forces below 100 cN were used to simulate the characteristics of electromechanical communications contacts. Evidence that tarnish layers on contacts do not necessarily impair their operation is provided firstly by the fact that the contact resistance of such layers is frequently much less critical than might be concluded from meas~rnen~ conducted without frictional movement and secondly by the fact that these layers can also give protection against mechanical wear 12J .
158
In the present paper we concentrate forded by tarnish layers against wear. The tance characteristics of silver surfaces with are compared utilizing the same apparatus ]Y [Il.
2. Specimens
primarily on the protection afadhesion, wear and contact resisand without Ag,S tarnish layers parameters as were used previous-
and test procedure
The friction tests were conducted on two plates of solid silver, one with an AgsS tarnish layer (approximately 0.8 pm thick [l] ) and the other without a tarnish layer. The surface was kept as clean as possible by using airtight packing directly after manufacture. The hardness of the silver chips was about 80 HVs.aes. The companion electrodes consisted of hard gold studs similar to those used earlier [l] , with a radius of 1.5 mm and a hardness of approximately 200 HV0.ac5. The contact force of the studs on the flat silver chips was 10 cN. The silver chips were moved backwards and forwards over the stationary studs in the same sinusoidal pattern as was used previously (frequency, 150 Hz; movements of 10,30, 50 and 70 pm) [1] . Each test was carried out with a new clean stud (mechanically polished and ion etched) with a defined contact force, frequency and movement, and up to about lo5 frictional movements were applied at a particular point of contact on the chip. When the oscillating friction was stopped the contact resistance was measured at a voltage of less than 20 mV. The studs were then lifted off the chips and the breakaway forces were registered. The tests were carried out in air at about 25 “C with approximately 45% relative humidity. At the end of a test series, the points of friction were investigated by scanning electron microscopy and the maximum amounts of erosion in the silver chips were measured.
3. Adhesion 3.1. Results 3.1 .I. Silver chip with a tarnish layer: increased adhesion for larger frictional movements About lo5 frictional movements were made at each of a number of points on the surface of the sulphide-coated silver chip. Ten tests with 10, 50 and 70 pm movements each (a total of 30 tests) were carried out at a variety of points on the chip. The breakaway forces FPb were measured at fixed numbers of frictional movements (the rate of increase in the force to breakaway was approximately 0.6 cN s-l). Figure 1 shows the adhesion coefficients (Yobtained from FJF,, where F,, is the contact force.
159
,
Mean value &=30pl
Fig. 1. Adhesion coefficient as a function of the number of frictional movements for chips with and without tarnish layers. 10' Q 2.5
10%
IO'
1
2
'8
15 I a
fk! I I 10.' 05
10-l
Frictionat movements-
Fig. 2. Contact resistance RD and adhesion coefficient o ss a function of the number of frictional movements. 30 X lo* frictional movements were made at a certain point on the sulphide-coated chip, first with a 10 pm movement and then with a 70 I.trnmovement.
Adhesion increases with frictional movement, which is also demonstrated by the following test (Fig. 2). The silver chip with a tarnish layer was exposed to 15.4 X lo* frictional movements, first with a movement of 10 Mm and then with 70 pm at the same point of friction (without using ti new stud): (Yrose shortly after changing from 10 to 70 pm (and the contact resistance dropped appreciably).
160
uuL..Lu.L
d-i...
-LLLauL
10'
10
1
’
10
Fss,*x--
Fig. 3. Structure of the resistance RU: R,, constriction tion resistance of At; RF, coating resistance of Af. Fig. 4. Maximum contact resistance
resistance
CN
10'
‘-
of A,;
RE*, constric-
due to cold welding as a function of the possible bonding force F,, RD (measured in the cold-welded condition), according to eqn, (1).
3.1.2. Silver chip without a tarnish layer; reduced adhesion for larger frictional movements
The mean values of the adhesion coefficient (Yfor 30 and 70 pm movements are given in Fig. 1 for the silver chip without a tarnish layer. The test conditions were identical to those described ,in Section 3.1 .l. The increased adhesion for the larger frictional movement (70 pm) only occurs up to about 0.7 X lo* frictional movements. After this, (Ydrops slightly for larger movements. 3.2. estimation of the m~imum pothole bonding forces due to cold welding In instances where breakaway forces are measured at a frictional contact which is thinly or partly covered by a tarnish layer it can be assumed that the corresponding bonding forces are caused not only by cold welding (metal to metal) but also by adhesion due to tarnishing. The predominant holding force can be determined by measuring the contact resistance RD before breakaway. The partly tarnished true contact area A, can be subdivided into a metallically clean portion and a section covered by a coating. These may ideally be described by two circular areas A, and Af so that A, =A, +A* where A,,, is the metallically clean contact area and Ai is the contact area with a coating. The contact resistance RU = RD -R, where R, represents the resistance of the leads, i.e. 1.4 mS2 in this case, then consists of the parallel circuit of Fig. 3. The relationship between RE and A,,, is governed by the equation RE = p/Za where p (or (pl + p&/Z) is the resistivity of the conductors and a the radius of the circular area A,. Then
161 2
2
4RE2
4RU2
&=a2*=P
The bonding
force F, due to cold welding is represented
by
F, = ZA, where 2 is the tensile strength. The welded area A, lies within the metallic contact area, i.e. A, < A,, which means that
(1) Given a certain contact resistance R,, , the breakaway come cold welding will thus not exceed F, max where F
force needed to over-
zp211 Smax = 4(R,-, - R,)2
If, however, the breakaway force F measured on the basis of RD is greater than F, mpx it must be assumed that the surface layer causes additional adhesion of at least approximately F - F, ,,,=. The relationship (1) is represented graphically in Fig. 4. In the measurements it was found that (pr + p2)/2 was 7.4 X 1O-6 Q cm for a hard gold stud on a silver chip. Values of 200 - 500 dN mmm2 were used for the tensile strength (curves 1 and 2). Curve 3 shows the deviation due to more widely differing p and 2 values [ 31. 3.3. The allocation of the breakaway forces to cold welding and adhesion due to tarnishing After frictional contact with a 10 pm movement the tarnished chip showed practically no bright spots within the friction area. Thus the adhesion coefficients (up to about unity) may be attributed to adhesion due to tarnishing. This statement may be retained even when consideration is given to the RD values measured before breakaway, which have a median value of about 80 mS1 after approximately 2 X lo4 frictional movements (Fig. 5). According to Section 3.2 and Fig. 4 the adhesive bonding forces due to cold welding do not exceed 0.05 cN for a contact resistance of this magnitude. However, the average reading was about 5 cN (Fig. 1) which means that the difference 5 - 0.05 = 5 cN can be explained by adhesion due to tarnishing. This also explains why higher breakaway forces occurred generally for larger frictional movements; these (in the range investigated of up to 70 pm) increase the removal of tarnish from the area subjected to friction and therefore reduce the contact resistance (Fig. 5). Thus there are more bright spots within the friction area. Cold welding increases and the adhesion coefficient rises in all cases. (Breakaway forces of up to about 10 cN were measured for Ag,S adhesion and of up to about 45 cN for cold welding for the untarnished chip (Fig. l).)
162
0
1
5
10~10"
FrIctIonal mo"ements-
Fig. 5. Contact resistance RD as a function of the number of frictional movements. Median RD values of -ten readings each for silver chips with a tarnish layer and of six readings each for silver chips without a tarnish layer are shown.
The untarnished chip showed higher adhesion for initial friction (up to about 0.7 X lo4 frictional movements), again using a larger frictional movement. This indicated the presence of surface layers on the chip and/or the stud, which were removed after a low number of frictional movements (thus cleaning the point of friction). After this, larger movements tended to reduce adhesion slightly.
4. Contact resistance The behaviour of the contact resistance RD has been reported earlier [l] for a tarnished chip subjected to frictional movements up to 100 pm. It was found that RD decreased with increasing number of frictional movements and, after a fairly low number (about 2 X 104) of movements of 30 100 pm, reached a stable value that was lower for the larger frictional movements. Figure 5 shows the median RD values for various movements (up to 70 ym) as a function of the number of frictional movements. This diagram also shows the central RD curve for a movement of 30 pm on the untarnished
163
(4
tb)
Fig. 6. Place of friction on (a) a silver chip (without an AgzS layer) and (b) a hard gold stud: contact force, 10 cN; frictional movement, 70 pm; about lo4 frictional movements. Abrasive wear of the chip with silver transferred to the stud is shown.
chip (the contact resistances for 30 and 70 I.trn movements were practically identical). It appears that, having reached a stable level with a movement of 70 pm, the contact resistance of the tarnished chip is only slightly higher (about 2.5 ma) than that of the untarnished chip.
5. Wear 5.1. Silver chip with a tarnish layer In no case did frictional movements on the tarnished silver chip cause erosion of the silver surface or of the hard gold stud (investigations up to 30 X lo4 frictional movements). These movement merely removed the tarnish layer, increasingly so with increasing extent of movement (expressed again in terms of R,, behaviour). The tarnish layer thus acted as a wear-inhibiting lubrication layer. This effect even persisted after most of the lubrication had been removed, e.g. with a movement of 70 pm and R, = 4.4 mS2. Another explanation of this wear-reducing phenomenon is that the initially completely AgaS-covered silver surface was passivated by the frictional action to such an extent as to make it resistant to wear. 5.2. Silver chip without a tarnish iayer The frictional movements on the un~~h~ chip ~h~acte~stic~ly transported silver from the softer chip (hardness about 80 HVo.ooa) to the harder gold stud (hardness about 200 HV,,,,) as shown in Fig. 6. Using a new stud each time a series of friction tests was carried out to show how the amount of erosion depended on the number of n frictional
164
Excursmn=30pm v
Fig. 7. Erosion h due to friction on a silver chip without a tarnish layer as a function of the number of frictional movements. The curves of the median value h* are given for 30 pm and for 70 pm movements.
movements. A light-section microscope was used to measure the depth of erosion at each point of friction on the chip. Figure 7 shows the erosion depth h with a frictional movement E = 30 pm plotted against the number of n frictional movements. The median erosion h* curves for E = 30 I.tm and E = 70 pm are also shown. Initially, h* increases considerably with increasing number of frictional movements but settles down at about 11 -16 pm after approximately 3 X lo4 movements. Thus the amount of erosion does not increase monotonically with the number of frictional movements. The maximum h* value is reached faster for E = 70 pm than for E = 30 ym, i.e. the amount of material transferred as a function of n is initially greater for a larger frictional movement (70 pm). That the amount of erosion is confined and does not rise monotonically with the number of frictional movements may be explained by the fact that silver transferred to the stud is re-transferred to the silver chip, especially with increasing erosion depth. When h* is plotted against the product en (total frictional travel) the curves for E = 30 I.cm and for e = 70 Humagree better than when h* is shown as a function of n. Agreement is very good particularly in the initial range
165
(up to about en = 250 mm) but diminishes as more silver is re-transferred to the chip. A monotonic rise with en has been described [4] for the wear of crossed wires as a result of a smaI1 amplitude of frictional movement. This applies equally well to the present tests, in the initial range where IZis not too large.
6. Discussion For a sulphide-coated silver chip subjected to an oscillating friction load no metal surface wear of any type was found, even though considerable breakaway forces for cold welding (up to about 20 cN) were measured. In contrast, typical silver surface abrasive wear occurs in the absence of a sulphide coating. Very thin tarnish layers and remains of tarnish layers (e.g. with contact resistances of about 4 ma ) sufficed to stop wear under these mechanical conditions. The breakaway forces and corresponding bonding effects were attributed partly to cold welding and partly to Ag,S adhesion. The breakaway forces found in the presence of surface layers were higher for larger frictional movements than for smaller movements (in the range investigated of up to 70 ,um). However, in instances where surface layers were generally absent the breakaway forces dropped slightly with larger movements after initial friction of a relatively brief duration. This may be attributed to the fact that the surface layers in the area of friction are removed faster with larger frictional movements so that cold welding increasingly occurs (and hence leads to higher breakaway forces). A criterion for cleanness (fiction in air) can be obtained by plotting the breakaway forces against frictional movement (10 70 pm): the contact areas are relatively clean if for increased movements the breakaway forces do not increase.
References 1 H. Bresgen, Reducing the contact resistance of tarnish layers on silver by means of an oscillating tangential motion, IEEE Trans. Compon., Hybrids Manuf. Technol., 1 (1) (1978) 65 - 68. 2 A. W. de Gee, Der Einfluss von Oxidschichten auf den Reibungs- und Verschleissvorgang bei Gleitreibung, Materialpriifung, 9 (5) (1967) 166 - 169. 3 E. Diirrwgchter (ed.), Doduco Datenbuch, Doduco KG, Pforzheim, 1974. 4 R. G. Bayer and J. L. Sirico, Wear of electrical contacts due to small-amplitude motion, ZBM J. Res. Dew., 15 (March 1971) 103 - 107.