#22- Comparison of Coefficient of Friction and Contact Resistance During Sliding Wear on Clad Gold-Nickel Surfaces
Abstract: This Materion Technical Materials paper presents an analysis of experimental sliding wear data for four hard-gold flash over gold-diffused-nickel material systems (GF-DGNi). The surface hardness of the DGNi material was altered by varying both the gold surface concentration and the amount of cold reduction after the diffusion anneal. These material systems were evaluated for unlubricated and lubricated sliding wear durability. The lubricant used was a 6-ring polyphenyl ether, OS-138, that was applied by dipping the specimens in a 2.5% solution of OS-138 in a propylene glycol monomethyl ether carrier. A gold flash palladium connector pin and a strip of metal that had been clad with the gold-nickel material system were mated to form a pseudo-crossed-rod configuration for each of these sliding wear experiments.
Sliding Wear Experiments
Sliding wear experiments were conducted at 50 grams and 150 grams normal load. The amplitude of the motion was 1.14 mm with a velocity of 1 mm/second. Each experiment was conducted for a minimum of 2000 wear cycles and five replications were made for each experimental test condition. A new sliding wear test machine and software program were developed for this test series to appropriately handle the clad metal samples and to allow simultaneous measurement of coefficient of friction and electrical contact resistance during each wear cycle.
When clean noble metal contacts are subjected to sliding wear, the coefficient of friction typically increases from an initial value of 0.2 to a value of 0.5, or greater, followed later by contact failure when the resistance increases to unacceptable values. Previous investigators have suggested this increase in coefficient of friction is a direct indication of failure.
We observed this same type of increase in coefficient of friction, but our simultaneous measurements of electrical contact resistance clearly showed that this increase is not a direct indication of contact failure. Rather, our experiments confirmed the initial increase in the coefficient from 0.2 to 0.5 was due to work hardening.
Contact Resistance Load
At light normal contact loads and for the materials we considered, contact failure occurs much later by slow oxidation of nickel as surface gold is worn away by sliding wear. At high normal contact loads a different failure mechanism driven by metallic bonding occurs which, in our case, transferred copper into the wear track leading to predominant failure by copper oxide formation. The application of a contact lubricant was able to retard the metallic bonding mechanism and subsequent copper oxide formation in some cases and thereby extend contact lifetime. In other cases, the application of contact lubricant was ineffective.