As lubricants, polyphenyl ethers maintain low and stable contact resistance under fretting conditions.
Neil R. Aukland
Harry C. Hardee
Manuel E. Joaquim
The benefits of using connector lubricants have been known for decades, but the debate continues about their proper use and application. Adding to the confusion are numerous published and unpublished reports that make false claims about the data. Published research1-7 that discusses the desirable characteristics of contact lubricants clarifies some of these misconceptions.
A properly formulated contact lubricant decreases the dynamic coefficient of friction, reduces adhesive wear, provides atmospheric corrosion protection, prevents fretting corrosion and does not migrate away from the point of contact. The dynamic coefficient of friction for clean noble metals can be as low as 0.30 and can reach values in excess of 1.00.2 Adding a polyphenyl ether to the metallic interface decreases the coefficient of friction to somewhere between 0.15 and 0.20, without sig- nificant effect on contact resistance.2,3 Polyphenyl ethers have been used as connector lubricants for 35 years. They have proven to be safe to use, and have not demonstrated any known material compatibility problems.
This point about no significant effect on the contact resistance is true given that the conditions for hydroplaning were not present during the sliding wear experiments or that the resistance measurement is taken when the metallic interface is not moving. Hydroplaning can cause the metallic interface to separate and is a function of the viscosity of the lubricant and velocity.2 A reduction in the dynamic coefficient of friction minimizes insertion and withdrawal forces.2,3
Adhesive wear is a problem with connectors that are mated and unmated many times during their service life. Prow formation, metal transfer, debris formation and surface roughening characterize this type of wear.4 Under ideal conditions, the addition of a lubricant will result in burnishing.2 The surface shown in Figure 1 is an example of a lubricated surface that experienced burnishing. Typically, without lubrication, this metallic interface exceeds a dynamic coefficient of friction of 0.40 in less than 100 cycles. With the addition of a lubricant, the dynamic coefficient of friction of the burnished surface did not exceed 0.20 in 10,000 sliding wear cycles.
In some cases, a thin lubricant film at the point of contact provides a certain amount of atmospheric corrosion protection. Polyphenyl ethers are effective in reducing various types of corrosion, including galvanic, fretting, radioactive and chemical. In addition, the application of a thin film of polyphenyl ether lubricant prevents or minimizes tarnish film creep.5
Another desirable characteristic of connector lubricants is their ability to remain at the point of contact and not migrate into neighboring areas. This migration property, known as wetting, can cause the contamination of relays, switches and other electronic devices. Perfluoroalkypolyethers have this undesirable property,2 and for this reason have not been recommended as contact lubricants. Polyphenyl ethers are known to remain at the point of contact, which is another reason they have been recommended as effective contact lubricants for a number of years.2,3
Impact of Fretting
Fretting is a major cause of connector failure. Fretting causes the metallic interface inside a connector to erode and causes the buildup of oxide debris at the point of contact (asperities). This accumulation of oxide debris will eventually cause the contact resistance to become unstable. The most important characteristic of a metallic interface inside a connector is its ability to maintain a low and stable contact resistance, and a contact lubricant can help by assisting in the removal of debris.
Polyphenyl ethers have a history of being used for the purpose of maintaining a low and stable contact resistance under fretting conditions. They have been used to protect tin/lead, precious metals mated to tin or tin/lead, various types of gold material systems and other types of metal surfaces.1-7 Multicrystalline wax is not a good contact lubricant under fretting condition, because this solid lubricant has a tendency to be pushed away from the point of contact under fretting conditions.2,6,7 In addition, multicrystalline wax is severely temperature limited.2,3
The interpretation of fretting data collected on some lubricated material systems may be misleading and lead to incorrect conclusions. For example, one connector lubricant tested at the Advanced Interconnection Laboratory was able to maintain a low and stable contact resistance under fretting conditions, but was not able to prevent the premature wearing away of the protective outer surface. Fretting experimentation is an accelerated test and the exposure of the sublayer would probably result in the premature failure of the connector for another reason, such as chemical corrosion.
Experimental Procedures
Two series of experiments were conducted on tin/lead samples in a crossed-rod configuration. A baseline series of experiments was conducted on unlubricated tin/lead samples at 50 and 200 g normal force. A second series of experiments was conducted on lubricated samples at 50 and 200 g normal force to establish the effect of a 5-ring polyphenyl ether (OS 124) on clean tin/lead surfaces.
Unlubricated Fretting Experiments
Prior research has revealed an unusual contact resistance pattern at 50 g7 and at 100 g8 normal force for this tin/lead material system. Figure 2 shows the contact resistance data for this unusual pattern at 50 g normal force. Figure 3 shows the contact resistance pattern at 100 g normal force. In both cases, the contact resistance data is low and stable for the first few hundred cycles, then increases until a peak is reached. After the peak is reached, the contact resistance values return to a low and stable condition. This condition continues for a short period, which is longer than the initial stable period. In less than 5,000 cycles, the second period of stability ends and a second peak in contact resistance is observed. After the second peak, the contact resistance never returns to a low and stable condition.
Tin oxidation and the exposure of nonconductive intermetallic compounds that are formed at the tin/lead copper boundary are the cause of the first peak. As the tin/lead layer is worn away dur-ing fretting, the copper layer is exposed and a good copper-to-copper interface is formed, which explains the return to a low and stable resistance.6 Continued fretting motion causes the copper-to-copper interface to degrade, which explains the second peak.
Earlier research8 concluded that an increase in the normal force resulted in a decrease in the magnitude of the first peak. The contact resistance data pattern for 200 g normal force, shown in Figure 4, reinforces this conclusion. In this figure, the first peak at approximately 300 cycles is almost imperceptible.
Lubricated Fretting Experiments
Figures 5 and 6 show typical contact resistance patterns for tin/lead samples that were lubricated with the 5-ring polyphenyl ether at the beginning of the second series of experiments. Figure 5 shows the contact resistance data collected at 50 g normal force. Figure 6 shows the contact resistance data collected at 200 g normal force. The contact resistance data collected at 200 g normal force is lower than the contact resistance data collected at 50 g. Therefore, a higher normal force results in a lower and more stable contact resistance.
Conclusion
The unlubricated fretting experiments demonstrated that an increase in contact resistance results in an increase in the period that the contact resistance is low and stable (see Figures 4, 5 and 6). Fretting experiments on tin/lead surfaces lubricated with the 5-ring polyphenyl ether reveal that an increase in normal force results in a decrease in the maximum value of the recorded contact resistance. This indicates that an increase in normal force causes a decrease in contact resistance. Based on review of published research papers3 on contact lubricants and on the data presented here, the 5-ring polyphenyl ether is an effective contact lubricant.
References
1. M. Antler, The Lubrication of Gold Wear, Vol. 6, 1963, p. 44-65.
2. M. Antler, "Sliding Studies of New Connector Contact Lubricants," IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. CHMT-10 (1), 1987, p. 1-12.
3. M. Antler, "Electronic Connector Contact Lubricants: The Polyphenyl Ether Fluids," Proceedings of the IEEE Holm Conference on Electrical Contacts, 1986, p. 129-138.
4. M. Antler, "Sliding Wear of Metallic Contacts," IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. CHMT-4 (1), 1981, p. 15-29.
5. M.E. Joaquim, "Connector Contact Lubrication with Polyphenyl Ether: A Review," Proceedings of the 30th Annual Connector and Technology Symposium, 1997, p. 217-225.
6. M. Antler, "Survey of Contact Fretting in Electrical Contacts," IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. CHMT-4 (1), 1985, p. 87-104.
7. M. Antler, Neil Aukland, Harry Hardee, et al, "Recovery of Severely Degraded Tin-Lead Plated Connector Contacts Due To Fretting Corrosion," IEEE Transactions on Components and Packaging Technologies, Vol. CHMT-x (1), 1999, p. 87-104.
8. Neil Aukland, Harry C. Hardee, "Improving Fretting Performance of Tin-Lead Contacts," Connector Specifier, March 1999, p. 10-12.
NEIL R. AUKLAND, Ph.D., is a college associate professor and HARRY C. HARDEE, Ph.D., is a professor and head of the Advanced Interconnection Laboratory, New Mexico State University, P.O. Box 30001, Las Cruces, NM 88003-8001; (505) 646-6534; Fax: (505) 646-6111; E-mail: naukland@nmsu.edu. Mr. Aukland is also president of Connector Lubricant Technology Llc. MANUEL E. JOAQUIM is president and CEO, Santovac Fluids Inc. and Findett Corp., 8 Governor Drive, St. Charles, MO 63301-7311; (636) 723-0240; E-mail: mjoaquiml@cs.com.
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Figure 1. An example of a lubricated surface that experienced burnishing. Note the smoother surface area in the center in relation to the rougher outer area.
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Figure 2. A typical contact resistance pattern for 50 g normal force and 50 µm fret amplitude.
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Figure 3. A typical contact resistance pattern for 100 g normal force and 50 µm fret amplitude.
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Figure 4. A typical contact resistance pattern for 200 g normal force and 50 µm fret amplitude.
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Figure 5. Contact resistance pattern for a tin/lead interface lubricated with the 5-ring polyphenyl ether at 50 g normal force.
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Figure 6. Contact resistance pattern for a tin/lead interface lubricated with the 5-ring polyphenyl ether at 200 g normal force.
SPEC SHEET
End Applications: Connectors Related Products: Pins, contact lubricants
Main Point: A properly formulated contact lubricant decreases the dynamic coefficient of friction, reduces adhesive wear, provides atmospheric corrosion protection, prevents fretting corrosion and does not migrate away from the point of contact. Although some lubricants lower the dynamic coefficient of friction, polyphenyl ethers are the most effective.1 Results of experiments testing the contact resistance for fretted tin/lead interconnections with and without a 5-ring polyphenyl ether at 50 and 200 g normal force are presented.
