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Impact wear of electroplated coatings
353
Wear, 155 (1992) 353-362
Impact wear of electroplated
coatings
Zhuan-Ke Chen Xi’an Jiaotong University, Xi’an 710049 (China) (Received
August 5, 1991; revised and accepted
November
12, 1991)
Abstract The mechanism of impact wear of electroplated rhodium and gold has been investigated. Two different kinds of contact specimens were used. One was blades of nickel-iron alloy
covered with rhodium on a gold underplate, and the other was gold alloy cylinder-to-plate contacts which were electroplated with gold. Scanning electron microscopy, X-ray fluorescence and Auger electron spectroscopy were used to identify the wear craters, elemental compositions and the failure process. Different wear mechanisms were found in these two kinds of specimen. Surface fatigue wear was found to occur in impacts on rhodium coating; adhesive wear was suggested for impacts on gold coating. Three stages could be distinguished during the increase in the number of impacts on rhodium coating. Deep impacted craters were characteristic of the cylinder with plated gold. Contamination of both kinds of contact surfaces
under
impacting
action
was observed.
1. Introduction
Electroplated coatings have been widely used in the electrical industry, as in relays, connectors, etc., to obtain low and stable contact, corrosion and wear resistance. Thin composite layers of gold and rhodium are commonly used on nickel-iron alloy substrates in reed relay contacts to inhibit the soft sticking of pure gold coating and to maintain low contact resistance. The thin gold plate on gold alloy substrates in hermetically sealed relay contacts, however, mainly plays the role of preservation during the manufacturing process and storing period. It is also useful for maintaining good contact resistance during operation. Much work has been done on maintaining good electrical contact using electroplated coatings, and most of it has dealt with sliding wear [l-5] and fretting wear [6, 71. The tribological behavior of relay contacts with noble metal coatings, however, is considered only in refs. 8-11. Owing to the make-break action of the relay contacts, the operation of the relay causes contact wear; this can dominate the electrical behavior of the contact, especially for relays which have contacts with coatings and are used in low load applications. For the mechanism of impact wear, the earlywork of Rabinowicz 112,131 investigated the wear of ductile metals using a pendulum-type impact tester. He found that the dominant wear mechanism of ductile materials was adhesion. Other workers, using a variety of materials, have favored the idea that impact wear is surface fatigue wear [14, 151, surface fracture wear [16, 17] and a wear mechanism which changes during the impact process [18]. The objective of the present work is to investigate the mechanisms of impact wear of electroplated coatings of relay contacts. Scanning electron microscopy (SEM),
0043-1648/92/$5.00
0 1992 - Elsevier
Sequoia. AU rights reserved
354
X-ray fluorescence (EDX) and Auger electron spectroscopy (AES) were used to examine the surface topographies and compositions of the impact craters of the contact surfaces.
2. Experimental
details
2.1. Materials and specimens Two kinds of relays were obtained at random from a production line. One was the reed relay with blades made of nickel-iron alloy (SO Ni-5OFe) covered by 0.5 pm electroplated rhodium over 2.5 pm gold underplate. The two blades were symmetrical and moved towards each other by a magnetic energizing force. The other was a hermetically sealed relay with cylinder (stationary contact) to plate (movable contact) contacts made of gold alloy (50Au-20Ag-30Cu) with 2 pm electroplated gold. 2.2. Apparatus and experimental procedures The standard relay life testing system which included energizing circuit, load circuit and testing circuit was set up and used with the impacting apparatus. The relays were energized by square and sine wave electrical signals, and no electrical current passed through the contacts, i.e. the contacts make and break without switching load. The relevant testing conditions are listed in Table 1. Contact resistance was measured after a certain number of impacts by using the four-terminal technique. After having been impacted, the relay was opened and the contacts were examined with SEM, EDX and AES.
3. Results 3.1. Impacting of electroplated rhodium Figure 1 shows the typical SEMs of the reed blades covered with rhodium plate after 104-lo7 impacting cycles. In these micrographs impacted craters, such as the dark region (Fig. l(a)), cracks (Fig. l(b)), pits or holes (Fig. l(c)) where the rhodium coating was torn apart and the contact surface of the blades were observed. It is also noted that the gold underneath the rhodium was extruded through cracks under the action of dynamic and static contact pressure, as shown in Fig. l(d), resulting in the soft sticking of contacts in the testing process. AES survey was taken at the contact surfaces as shown in Fig. 2. Carbon, chlorine, sulfur and oxygen were detected at the dark region (Fig. 2(a)), and the gold was found in pit areas (Fig. 2(b)). The EDX results showed that the gold was dominant TABLE
1
Impact test conditions
Impacting frequency (Hz) Static contact force (N) Temperature (“C)
Reed relay
Sealed relay
100/200 0.06 2&25
5 0.03 20-25
355
(cl
(4
Fig. 1. Impact craters of reed relay contacts: (a) dark region after impacting 10 000 times (f= 100 Hz, square wave signal); (b) cracks after impacting 60 000 times (f= 100 Hz, sine wave signal); (c) pits formed after impacting 10’ times (f=ZOO Hz, square wave signal); (d) sticking spot after impacting 60 000 times (f= 100 Hz, sine wave signal). at pits compared with that in non-impacted zone (Fig. 3(a) (b)), so it was further confirmed that the rhodium coating was removed at pitted areas (Fig. l(c)). That the gold underneath the rhodium was extruded through cracks was verified with EDX, as shown in Fig. 3(c). 3.2. Impacting of electroplated gold The morphology of cylinder to plate contacts with gold coating after impact is shown in Fig. 4. It was observed that the impact wear occurred on the touching asperities of the cylinder (Fig. 4(a)), and material transfer from cylinder to plate was found on the plate surface (Fig. 4(b)). Figure 5 is the AES analysis results which demonstrate that the substrate materials, such as silver and copper, appear either on cylinder or on plate surfaces. Auger spectra show that severe contamination caused by chlorine, carbon and oxygen existed on the contact zones.
4. Discussion Adhesion, surface fatigue and surface fracture are the three basic mechanisms of impact wear. In the impact of relay contacts with electroplated coatings, the impacting
356
r00 (a)
200
300 Klnetlc
500
400 Ezergy
Fig. 2. Elemental compositions of impacted l(a); (b) compositions of Fig. l(c).
becomes more complicated. hypotheses can be made.
failure
+-ii52
UeVI
craters
by AES analysis: (a) compositions
Based on the observation
of Fig.
in this study, the following
4.1. Failure mechanism of impacting rhodium coating The impact failure of electroplated rhodium contacts can be divided into three stages. Figure 6 schematically shows the mechanism of failure in different stages. In stage 1 (up to about 10 000 impacting cycles), plastic deformation due to the impacting action smoothes the touching asperities, resulting in island-like plate patches on the reed surfaces. The contamination is characteristic of dark organic deposit on islandlike plate patches on the reed surfaces. The contamination is characteristic of dark organic deposit on island-like plate patches due to a mechanochemical effect [ll, 191. The contact resistance shown in Fig. 7 increases with numbers of impact cycles. In stage 2 (up to 40 000 impacting cycles), cracks may initiate on the rhodium coating or at the rhodium-gold interface. Since the gold underplate is softer than the rhodium top coating, the further impacting will cause high stress and strain at contact
357
(b)
(4
I i
1Y 8.9’tO FS= 2K .rEtM:**HH*
ch ZWVd=LS
ktu ‘Vi?=
19.2
: Cf!
*)ia&
Cc) Fig. 3. Surface compositions of impacted craters by EDX analysis: (a) compositions impacted surface; (b) compositions in Fig. l(c); (c) compositions in Fig. l(d).
in non-
(4 Fig. 4. Impacted craters of sealed relay contacts: (a) contact spots on cylinder after impacting 5000 times (f-5 Hz, square wave signal); (b) contact spots on plate after impacting 5000 times (f=S Hz, square wave signal).
@I Fig. 5. Elemental compositions of impacted 4(a); (b) compositions in Fig. 4(b).
craters
by AES analysis: (a) compositions
in Fig.
spots of the rhodium-gold interface, resulting in cracks at these areas. As the impacting continues, these cracks propagate and gold is extruded through the cracks from the interface to the surface. Because of the good chemical and electrical characteristics of gold, the contact resistance decreases with impact as the number of cycles increases. In stage 3, the gold is extruded and extended on to the contact spots, and the contact resistance will remain stable. However, because of the larger adhesive strength of gold contacts, the rhodium coating between the cracks can be delaminated and pits will be formed in contact zones. On the other hand, when the adhesive force of gold is at about the same level as the mechanical retractile force of blades, then slow release of the contacts takes place. Otherwise, if the adhesive force is larger than retractile force, soft sticking will occur, as shown in Fig. l(d). As mentioned above, it is suggested that the mechanism of impacting rhodium coating on gold underplate under the make-break action is primarily a manifestation of surface fatigue.
359
I
miqtxracks
\
Fig. 6. Schematic diagrams of the mechanisms of faiIure in different
stages.
(a) Stage I, (b)
state II, (c) state III.
4.2. Failure mechanism of impacting gold coating The impacting failure of electroplated gold contacts can be assumed to be due to adhesive wear. Loose wear particles cause the deep impacted craters on cylinder, and the gold coating in contact zones is removed, resulting in exposure of substrate materials to the atmosphere (before the relay is sealed). Therefore, corrosion (oxidation and chlorination) and polymerization take place. It is because of wear and contamination that the contact resistance changes during impact. The variation of contact resistance is very small (less than 10 mn) in the measurement, and this demonstrates two facts: (a) that the electrical resistivity of gold and gold alloy are similar; (b) that the corrosion film formed in the contact zones can be punctured by impact. To analyze this inter-facial failure process, the Hertz contact theory (201 is used. For the cylinder to plate contacts, i.e. line contact condition as seen in Fig. 8(a), the
Fig. 7. Contact samples).
resistance
of reed relay VS. impacting
static
/’
(a)
/
movable
numbers
contact
contact
‘X
Fig. 8. Schematic stress distribution
(average value of 50 different
(b) diagrams of contact and stress distribution: (a) contact diagram; (b) normal on contact surface and normal and shear stress along the 2 direction.
stress along the normal be given by
direction
of contact
surface
under
the action
of force
P can
361
where h=&5642
q=2
l_uz;
E
b=l.l28&j??i
and a,, a,, and 0; are the component of normal stress, T,,,~~is the shear stress, E is Young’s modulus and u Poisson’s ratio. The distribution of normal and shear stress is shown in Fig. 8(b). It is obvious that the normal stress is concentrated on the surface, while the maximum shear stress is located at the subsurface, i.e. 2 equals 0.786. That means plastic deformation occurs more easily at the surface, while delamination is easier at the subsurface. Wear and material transfer depend on the normal force and the characteristics of adhered junctions. Because of the good ductility of the electroplated gold coating, the impact wear mechanism is suggested to be adhesion.
5. Conclusion
(1) Based on the surface topographies, EDX and AES analysis, two different failure mechanisms are suggested i.e. surface fatigue for impact wear of electroplated rhodium coatings, and adhesive wear for the impact wear of electroplated gold contacts. (2) Three stages can be distinguished in impact behavior of electroplated rhodium coating. (a) In stage 1, the touching asperities were smoothed, the island-like plate patches were formed and dark organic deposits were produced on plate patches due to mechanochemical action. (b) In state 2, cracks may initiate on the rhodium coating or at the rhodium-gold interface. (c) In stage 3, the gold under-plate is extruded to the surface through the cracks and pits and sticking problems may occur. (3) The impact wear of cylinders with gold coatings is characteristic of severe plastic deformation, i.e. deep cratering. Material transfer from cylinder to plate was observed, and this is considered to be due to adhesion of the gold coating at touching asperities. (4) Electrical contact resistance is a good indicator of impact contacts with the rhodium coating on gold underplate, while this is not true in impact wear of gold
362 coating on gold alloy substrate. This difference arises from the difference in electrical resistivity between the coating and underplate or substrate. (5) Further work is needed to understand fully the difference of impacted craters caused by different impacting forces due to different energizing signals.
Acknowledgments The author is grateful to Qun Li Radio Appliance Factory for supplying the test specimens. Thanks are owed to senior engineer Tian-Pei Zheng, Mr. Jian-Qi Wang, Mr. Ping-B0 Yang and Mr. Shu-Yang Wang for their assistance in the experiments. The author would like to express his thanks to Dr. K. Sawa for his help in revision.
References 1 M. Antler, Wear of electroplated gold, ASLE Trans., 11 (1968) 248-260. 2 M. Antler, Tribology of metal coatings for electrical contacts, Thin Solid Films, 84 (1981) 245-256. 3 M. Antler, Tribological properties of gold for electrical contacts, IEEE Trans. Parts, Hybriu!s, PuctUJg., 9 (1973) 4-14. 4 K. Miyoshi, T. Spalvins and D. H. Buckley, Tribological Characteristics of solid films deposited on metals by ion plating and vapor deposition, Wear, 108 (1986) 169-184. 5 H. Tian, N. Saka and E. Rabinowicz, Friction and failure of electroplated sliding contacts, Wear, 142 (1991) 57-85. 6 M. Antler and M. H. Drozdowicz, Fretting corrosion of gold-plated connector contacts, Wear, 74 (1981/1982) 27-50. 7 H. Tian, N. Saka and E. Rabinowicz, Fretting failure of electroplated gold contacts, Wear, 142 (1991) 265-289. 8 H. Grossmann, M. Huck, G. Schaudt and F. J. Wagner, Make and break properties of electrodeposits, IEEE Trans. Comp., Hybrids, Munf Technol., 8 (1985) 70-79. 9 Z. K. Chen, Impact wear of electrical contacts. In Proc. hat. ConjI on Electrical Contacts, Chicago, IL, 1991, The Institute of Electrical and Electronics Engineering, Inc., USA, pp. 172-175. 10 Z. K. Chen, Analysis of the surface characteristics of electrical contact using SEM and AES. In Proc. ISVTS, WuXi, China, 1991, to be published. 11 M. Hasegawa and K. Sawa, Formation of palladium oxides by mechanical reaction on Pd and Ag-Pd alloy contacts, IEEE Trans. Comp., Hybrids, Munf: Technol., 13 (1990) 33-39. 12 E. Rabinowicz, Metal transfer during static loading and impacting, Pmt. Phys. Sot., Land. Sect. B, 65 (1952) 630-640. 13 E. Rabinowicz and K. Hozaki, Impact wear of ductile metals. In Proc. of Japan Society of Lubrication Engineers Znt. Tribol. Cor$, Tokyo, Japan, 1985, pp. 263-268. 14 R. S. Montgomery, The mechanism of percussive wear of tungsten carbide composites, Wear, 12 (1968) 309-329. 15 E. B. Iturbe, Surface layer hardening of polycrystalline copper by multiple impact, /. Muter. Sci., 15 (1980) 2331-2334. 16 C. J. Studman and J. E. Field, A repeated impact testing machine, Wear, 41 (1977) 373-381. 17 I. R. Sare, Repeated impact abrasion of ore-crushing hammers, Wear, 87 (1983) 202-25. 18 K. Wellinger and H. Breckel, Deformation and wear in the impact of metallic workpieces, Wear, 13 (1969) 257-281. 19 H. W. Hermance and T. F. Egan, Organic deposits on precious metal contacts, Bell Syst. Tech. J., 37 (1958) 739-841. 20 C. Q. Gao and J. J. Liu, Adhesive and fatigue wear of materials, Mech. Engineering, 1989 (in Chinese):
Wear, 155 (1992) 353-362
Impact wear of electroplated
coatings
Zhuan-Ke Chen Xi’an Jiaotong University, Xi’an 710049 (China) (Received
August 5, 1991; revised and accepted
November
12, 1991)
Abstract The mechanism of impact wear of electroplated rhodium and gold has been investigated. Two different kinds of contact specimens were used. One was blades of nickel-iron alloy
covered with rhodium on a gold underplate, and the other was gold alloy cylinder-to-plate contacts which were electroplated with gold. Scanning electron microscopy, X-ray fluorescence and Auger electron spectroscopy were used to identify the wear craters, elemental compositions and the failure process. Different wear mechanisms were found in these two kinds of specimen. Surface fatigue wear was found to occur in impacts on rhodium coating; adhesive wear was suggested for impacts on gold coating. Three stages could be distinguished during the increase in the number of impacts on rhodium coating. Deep impacted craters were characteristic of the cylinder with plated gold. Contamination of both kinds of contact surfaces
under
impacting
action
was observed.
1. Introduction
Electroplated coatings have been widely used in the electrical industry, as in relays, connectors, etc., to obtain low and stable contact, corrosion and wear resistance. Thin composite layers of gold and rhodium are commonly used on nickel-iron alloy substrates in reed relay contacts to inhibit the soft sticking of pure gold coating and to maintain low contact resistance. The thin gold plate on gold alloy substrates in hermetically sealed relay contacts, however, mainly plays the role of preservation during the manufacturing process and storing period. It is also useful for maintaining good contact resistance during operation. Much work has been done on maintaining good electrical contact using electroplated coatings, and most of it has dealt with sliding wear [l-5] and fretting wear [6, 71. The tribological behavior of relay contacts with noble metal coatings, however, is considered only in refs. 8-11. Owing to the make-break action of the relay contacts, the operation of the relay causes contact wear; this can dominate the electrical behavior of the contact, especially for relays which have contacts with coatings and are used in low load applications. For the mechanism of impact wear, the earlywork of Rabinowicz 112,131 investigated the wear of ductile metals using a pendulum-type impact tester. He found that the dominant wear mechanism of ductile materials was adhesion. Other workers, using a variety of materials, have favored the idea that impact wear is surface fatigue wear [14, 151, surface fracture wear [16, 17] and a wear mechanism which changes during the impact process [18]. The objective of the present work is to investigate the mechanisms of impact wear of electroplated coatings of relay contacts. Scanning electron microscopy (SEM),
0043-1648/92/$5.00
0 1992 - Elsevier
Sequoia. AU rights reserved
354
X-ray fluorescence (EDX) and Auger electron spectroscopy (AES) were used to examine the surface topographies and compositions of the impact craters of the contact surfaces.
2. Experimental
details
2.1. Materials and specimens Two kinds of relays were obtained at random from a production line. One was the reed relay with blades made of nickel-iron alloy (SO Ni-5OFe) covered by 0.5 pm electroplated rhodium over 2.5 pm gold underplate. The two blades were symmetrical and moved towards each other by a magnetic energizing force. The other was a hermetically sealed relay with cylinder (stationary contact) to plate (movable contact) contacts made of gold alloy (50Au-20Ag-30Cu) with 2 pm electroplated gold. 2.2. Apparatus and experimental procedures The standard relay life testing system which included energizing circuit, load circuit and testing circuit was set up and used with the impacting apparatus. The relays were energized by square and sine wave electrical signals, and no electrical current passed through the contacts, i.e. the contacts make and break without switching load. The relevant testing conditions are listed in Table 1. Contact resistance was measured after a certain number of impacts by using the four-terminal technique. After having been impacted, the relay was opened and the contacts were examined with SEM, EDX and AES.
3. Results 3.1. Impacting of electroplated rhodium Figure 1 shows the typical SEMs of the reed blades covered with rhodium plate after 104-lo7 impacting cycles. In these micrographs impacted craters, such as the dark region (Fig. l(a)), cracks (Fig. l(b)), pits or holes (Fig. l(c)) where the rhodium coating was torn apart and the contact surface of the blades were observed. It is also noted that the gold underneath the rhodium was extruded through cracks under the action of dynamic and static contact pressure, as shown in Fig. l(d), resulting in the soft sticking of contacts in the testing process. AES survey was taken at the contact surfaces as shown in Fig. 2. Carbon, chlorine, sulfur and oxygen were detected at the dark region (Fig. 2(a)), and the gold was found in pit areas (Fig. 2(b)). The EDX results showed that the gold was dominant TABLE
1
Impact test conditions
Impacting frequency (Hz) Static contact force (N) Temperature (“C)
Reed relay
Sealed relay
100/200 0.06 2&25
5 0.03 20-25
355
(cl
(4
Fig. 1. Impact craters of reed relay contacts: (a) dark region after impacting 10 000 times (f= 100 Hz, square wave signal); (b) cracks after impacting 60 000 times (f= 100 Hz, sine wave signal); (c) pits formed after impacting 10’ times (f=ZOO Hz, square wave signal); (d) sticking spot after impacting 60 000 times (f= 100 Hz, sine wave signal). at pits compared with that in non-impacted zone (Fig. 3(a) (b)), so it was further confirmed that the rhodium coating was removed at pitted areas (Fig. l(c)). That the gold underneath the rhodium was extruded through cracks was verified with EDX, as shown in Fig. 3(c). 3.2. Impacting of electroplated gold The morphology of cylinder to plate contacts with gold coating after impact is shown in Fig. 4. It was observed that the impact wear occurred on the touching asperities of the cylinder (Fig. 4(a)), and material transfer from cylinder to plate was found on the plate surface (Fig. 4(b)). Figure 5 is the AES analysis results which demonstrate that the substrate materials, such as silver and copper, appear either on cylinder or on plate surfaces. Auger spectra show that severe contamination caused by chlorine, carbon and oxygen existed on the contact zones.
4. Discussion Adhesion, surface fatigue and surface fracture are the three basic mechanisms of impact wear. In the impact of relay contacts with electroplated coatings, the impacting
356
r00 (a)
200
300 Klnetlc
500
400 Ezergy
Fig. 2. Elemental compositions of impacted l(a); (b) compositions of Fig. l(c).
becomes more complicated. hypotheses can be made.
failure
+-ii52
UeVI
craters
by AES analysis: (a) compositions
Based on the observation
of Fig.
in this study, the following
4.1. Failure mechanism of impacting rhodium coating The impact failure of electroplated rhodium contacts can be divided into three stages. Figure 6 schematically shows the mechanism of failure in different stages. In stage 1 (up to about 10 000 impacting cycles), plastic deformation due to the impacting action smoothes the touching asperities, resulting in island-like plate patches on the reed surfaces. The contamination is characteristic of dark organic deposit on islandlike plate patches on the reed surfaces. The contamination is characteristic of dark organic deposit on island-like plate patches due to a mechanochemical effect [ll, 191. The contact resistance shown in Fig. 7 increases with numbers of impact cycles. In stage 2 (up to 40 000 impacting cycles), cracks may initiate on the rhodium coating or at the rhodium-gold interface. Since the gold underplate is softer than the rhodium top coating, the further impacting will cause high stress and strain at contact
357
(b)
(4
I i
1Y 8.9’tO FS= 2K .rEtM:**HH*
ch ZWVd=LS
ktu ‘Vi?=
19.2
: Cf!
*)ia&
Cc) Fig. 3. Surface compositions of impacted craters by EDX analysis: (a) compositions impacted surface; (b) compositions in Fig. l(c); (c) compositions in Fig. l(d).
in non-
(4 Fig. 4. Impacted craters of sealed relay contacts: (a) contact spots on cylinder after impacting 5000 times (f-5 Hz, square wave signal); (b) contact spots on plate after impacting 5000 times (f=S Hz, square wave signal).
@I Fig. 5. Elemental compositions of impacted 4(a); (b) compositions in Fig. 4(b).
craters
by AES analysis: (a) compositions
in Fig.
spots of the rhodium-gold interface, resulting in cracks at these areas. As the impacting continues, these cracks propagate and gold is extruded through the cracks from the interface to the surface. Because of the good chemical and electrical characteristics of gold, the contact resistance decreases with impact as the number of cycles increases. In stage 3, the gold is extruded and extended on to the contact spots, and the contact resistance will remain stable. However, because of the larger adhesive strength of gold contacts, the rhodium coating between the cracks can be delaminated and pits will be formed in contact zones. On the other hand, when the adhesive force of gold is at about the same level as the mechanical retractile force of blades, then slow release of the contacts takes place. Otherwise, if the adhesive force is larger than retractile force, soft sticking will occur, as shown in Fig. l(d). As mentioned above, it is suggested that the mechanism of impacting rhodium coating on gold underplate under the make-break action is primarily a manifestation of surface fatigue.
359
I
miqtxracks
\
Fig. 6. Schematic diagrams of the mechanisms of faiIure in different
stages.
(a) Stage I, (b)
state II, (c) state III.
4.2. Failure mechanism of impacting gold coating The impacting failure of electroplated gold contacts can be assumed to be due to adhesive wear. Loose wear particles cause the deep impacted craters on cylinder, and the gold coating in contact zones is removed, resulting in exposure of substrate materials to the atmosphere (before the relay is sealed). Therefore, corrosion (oxidation and chlorination) and polymerization take place. It is because of wear and contamination that the contact resistance changes during impact. The variation of contact resistance is very small (less than 10 mn) in the measurement, and this demonstrates two facts: (a) that the electrical resistivity of gold and gold alloy are similar; (b) that the corrosion film formed in the contact zones can be punctured by impact. To analyze this inter-facial failure process, the Hertz contact theory (201 is used. For the cylinder to plate contacts, i.e. line contact condition as seen in Fig. 8(a), the
Fig. 7. Contact samples).
resistance
of reed relay VS. impacting
static
/’
(a)
/
movable
numbers
contact
contact
‘X
Fig. 8. Schematic stress distribution
(average value of 50 different
(b) diagrams of contact and stress distribution: (a) contact diagram; (b) normal on contact surface and normal and shear stress along the 2 direction.
stress along the normal be given by
direction
of contact
surface
under
the action
of force
P can
361
where h=&5642
q=2
l_uz;
E
b=l.l28&j??i
and a,, a,, and 0; are the component of normal stress, T,,,~~is the shear stress, E is Young’s modulus and u Poisson’s ratio. The distribution of normal and shear stress is shown in Fig. 8(b). It is obvious that the normal stress is concentrated on the surface, while the maximum shear stress is located at the subsurface, i.e. 2 equals 0.786. That means plastic deformation occurs more easily at the surface, while delamination is easier at the subsurface. Wear and material transfer depend on the normal force and the characteristics of adhered junctions. Because of the good ductility of the electroplated gold coating, the impact wear mechanism is suggested to be adhesion.
5. Conclusion
(1) Based on the surface topographies, EDX and AES analysis, two different failure mechanisms are suggested i.e. surface fatigue for impact wear of electroplated rhodium coatings, and adhesive wear for the impact wear of electroplated gold contacts. (2) Three stages can be distinguished in impact behavior of electroplated rhodium coating. (a) In stage 1, the touching asperities were smoothed, the island-like plate patches were formed and dark organic deposits were produced on plate patches due to mechanochemical action. (b) In state 2, cracks may initiate on the rhodium coating or at the rhodium-gold interface. (c) In stage 3, the gold under-plate is extruded to the surface through the cracks and pits and sticking problems may occur. (3) The impact wear of cylinders with gold coatings is characteristic of severe plastic deformation, i.e. deep cratering. Material transfer from cylinder to plate was observed, and this is considered to be due to adhesion of the gold coating at touching asperities. (4) Electrical contact resistance is a good indicator of impact contacts with the rhodium coating on gold underplate, while this is not true in impact wear of gold
362 coating on gold alloy substrate. This difference arises from the difference in electrical resistivity between the coating and underplate or substrate. (5) Further work is needed to understand fully the difference of impacted craters caused by different impacting forces due to different energizing signals.
Acknowledgments The author is grateful to Qun Li Radio Appliance Factory for supplying the test specimens. Thanks are owed to senior engineer Tian-Pei Zheng, Mr. Jian-Qi Wang, Mr. Ping-B0 Yang and Mr. Shu-Yang Wang for their assistance in the experiments. The author would like to express his thanks to Dr. K. Sawa for his help in revision.
References 1 M. Antler, Wear of electroplated gold, ASLE Trans., 11 (1968) 248-260. 2 M. Antler, Tribology of metal coatings for electrical contacts, Thin Solid Films, 84 (1981) 245-256. 3 M. Antler, Tribological properties of gold for electrical contacts, IEEE Trans. Parts, Hybriu!s, PuctUJg., 9 (1973) 4-14. 4 K. Miyoshi, T. Spalvins and D. H. Buckley, Tribological Characteristics of solid films deposited on metals by ion plating and vapor deposition, Wear, 108 (1986) 169-184. 5 H. Tian, N. Saka and E. Rabinowicz, Friction and failure of electroplated sliding contacts, Wear, 142 (1991) 57-85. 6 M. Antler and M. H. Drozdowicz, Fretting corrosion of gold-plated connector contacts, Wear, 74 (1981/1982) 27-50. 7 H. Tian, N. Saka and E. Rabinowicz, Fretting failure of electroplated gold contacts, Wear, 142 (1991) 265-289. 8 H. Grossmann, M. Huck, G. Schaudt and F. J. Wagner, Make and break properties of electrodeposits, IEEE Trans. Comp., Hybrids, Munf Technol., 8 (1985) 70-79. 9 Z. K. Chen, Impact wear of electrical contacts. In Proc. hat. ConjI on Electrical Contacts, Chicago, IL, 1991, The Institute of Electrical and Electronics Engineering, Inc., USA, pp. 172-175. 10 Z. K. Chen, Analysis of the surface characteristics of electrical contact using SEM and AES. In Proc. ISVTS, WuXi, China, 1991, to be published. 11 M. Hasegawa and K. Sawa, Formation of palladium oxides by mechanical reaction on Pd and Ag-Pd alloy contacts, IEEE Trans. Comp., Hybrids, Munf: Technol., 13 (1990) 33-39. 12 E. Rabinowicz, Metal transfer during static loading and impacting, Pmt. Phys. Sot., Land. Sect. B, 65 (1952) 630-640. 13 E. Rabinowicz and K. Hozaki, Impact wear of ductile metals. In Proc. of Japan Society of Lubrication Engineers Znt. Tribol. Cor$, Tokyo, Japan, 1985, pp. 263-268. 14 R. S. Montgomery, The mechanism of percussive wear of tungsten carbide composites, Wear, 12 (1968) 309-329. 15 E. B. Iturbe, Surface layer hardening of polycrystalline copper by multiple impact, /. Muter. Sci., 15 (1980) 2331-2334. 16 C. J. Studman and J. E. Field, A repeated impact testing machine, Wear, 41 (1977) 373-381. 17 I. R. Sare, Repeated impact abrasion of ore-crushing hammers, Wear, 87 (1983) 202-25. 18 K. Wellinger and H. Breckel, Deformation and wear in the impact of metallic workpieces, Wear, 13 (1969) 257-281. 19 H. W. Hermance and T. F. Egan, Organic deposits on precious metal contacts, Bell Syst. Tech. J., 37 (1958) 739-841. 20 C. Q. Gao and J. J. Liu, Adhesive and fatigue wear of materials, Mech. Engineering, 1989 (in Chinese):