Surface Contamination and Electrical Contact Resistance
Correlation of Surface Contamination Introduced by Feeding Equipment on Electrical Contact Resistance
Contact degradation caused by austenitic stainless steel feeder bowls has raised concern among manufacturers of electrical contacts. The surface contamination of three contact materials fine silver, silver cadmium oxide (90%Ag 10%CdO) and a silver alloy (75%Ag 24.5%Cu 0.5%Ni) with 15 microinches (Ra) surface finish were investigated. Contact resistance probing was employed and surfaces were analyzed to explain the degradation process. The investigation elucidated the effects of various vibratory feeder bowl coatings which reduced surface contamination. The attainment of low contact resistance was explained as interactions among the nobility of contact material, type of coating and time spent in the vibratory feeder bowl.
Austenitic stainless steel feeder bowls are the most used feeding equipment by manufacturers of electrical contacts in their assembly operations. Despite cleaning the vibratory feeder bowls after each production, some of the assemblies remained slightly contaminated. The presence of foreign matter on the surface of electrical contacts is usually the cause of high resistance failures. Electrical contact is a junction between two or more current-carrying members, which provides electrical continuity at their interface (1). Early research by various investigators has shown that the most common types of contaminations are oxides and corrosion products (2,3,4), particulate , wear and fretting [6), and contamination originating in the manufacturing processes.
In particular, feeding equipment (especially stainless steel feeder bowls), has been shown to enhance surface contamination and adversely affect contact resistance . Contact assembly often introduces contaminants via force and abrasive friction into feeding equipment. The latter process can quickly introduce contaminant film, prohibiting metal to metal contact between the current-carrying members and degrading contact performance. Minute amounts of base metal are generated from the shank during feeding and become embedded in the contact faces. It is easy to remove loose particles or unoxidized oil films, but the removal of metallic inclusions or highly compacted particles is difficult indeed. Field data shown in Figure 1.1.1, from several electrical contact users, suggest the need to minimize contamination during assembly.
Various types of contamination have received attention throughout the years. However, very little experimental work has been done with coatings and fiber materials developed for vibratory feeder bowl applications. The coatings were chosen after recommendations from several feeder bowls and contact assembly manufacturers. This paper aims at discussing the correlation of surface contamination introduced by feeding equipment on electrical contact resistance.
2. Experimental Procedure
2.1 Experimental Conditions
Three commercially available coatings for vibratory feeder bowl applications were employed for the experiment. The coatings were brushlon, Teflon and urethane.
Brushlon is a unique combination of synthetic brush fibers with 20 degrees of tilt uniformly distributed in a flexible backing. To assure that contacts were in continual motion in the vibratory feeder bowl, twenty-degree sectors were cut and attached to the substrate with a sprayed adhesive.
The contacts under consideration were rivets manufactured with radius head configuration, from the most widely used materials for electric contacts in the United States, as illustrated in Table I.
Contacts were manufactured under normal tool conditions and their surfaces contained spherical asperities from the tool surface. The surface finish of the test specimens was measured with a Mitutoyo surftest-201 surface measuring instrument. The arithmetic mean for the deviation of the departure of small scale surface irregularities from the mean line is called Ra.
Roughness measuring instruments which directly display RA are common and averaging the data minimizes the effect of scratches so that consistent results can be obtained. The approximate surface finish of contacts with radius head configuration was 15 microinches (Ra). A sample lot of fifty (50) contacts was submitted to coated vibratory feeder bowls for different intervals. Accelerated tests were performed to determine the interactions between coatings and contact materials. Once the contacts were in the vibratory feeder bowls for a prescribed time, the resistance of (7) contacts was measured employing an instrument called a contact resistance probe. The vibratory feeder bowls were cleaned with ethyl alcohol upon completion of the entire time cycle (360 minutes) before submitting new material to the test. A schematic diagram of the experimental flow chart is given in Figure 2.1.1.
2.2 Technique and Apparatus
Contact resistance was measured employing dry circuit techniques. The primary requirement for dry circuit testing of contacts has been previously described [9).
Figure 2.2.1 illustrates the contact resistance apparatus. The apparatus consists of fixtures for holding specimens of various sizes and shapes, and a Keithly 580 micro-ohm meter. A similar set up was used by Antler (10) and Nobel (11). A mechanism applies a measurable load which can be increased, decreased, or held constant. Only one contact spot was randomly assessed during testing. However, specific contact spots could be assessed employing the stage micrometers to move the probe holder. The reference surface (the probe) was manufactured from fine silver with a conical shape (gold diffused and gold treated).
The probe holder has been designed so force may be applied to the contact. An electrical load force gauge ACCU FORCE II is mounted on the top of the probe holder. A calibrated spindle on the side of the stage is advanced manually, while the electronic gauge displays the load applied to the sample. One of the micro-ohm meter voltage leads was attached to the probe holder and the other lead to the sample holder. A minimum of seven contacts from each interval was submitted to contact resistance probing. After each measurement, the probe was gently wiped with a tissue, assuring a debris-free surface similar to the technique used by Russell (12).
3. Experimental Results
3.1 Vibratory Feeder Bowls Coated With Brushlon Teflon and Urethane
As reported in a previous paper , contact resistance steadily increased as the time spent in the austenitic stainless steel vibratory feeder bowls increased, as a function of the material nobility. Contacts with various nobility levels and 15 microinches (Ra) surface finish were submitted to vibratory feeder bowls coated with (1) brushlon, (2) Teflon or (3) urethane 90 durometer and then subjected to contact resistance probing. The data represents rivets manufactured from fine silver, silver cadmium oxide (90Ag 10CdO) and a silver alloy (75AG 24.5Cu 0.5Ni) respectively.
Figure 3.1.1A depicts the resistance values of contacts submitted to a vibratory feeder bowl coated with brushlon. The interval values ranged from 15 minutes to 360 minutes. The three contact materials tested yielded average resistance readings, ranging from 1.1 milliohms after 15 minutes to 1.4 milliohms after 360 minutes at 100 grams load. Fine silver contacts manifested a gradual increase in contact resistance readings as the time spent in the vibratory feeder bowl increased
The Teflon coated vibratory feeder bowl also allowed attainment of low contact resistance. Fine silver and silver cadmium oxide (90Ag 10C1o) yielded the lowest contact resistance readings. The resistance readings ranged from 1.2 milliohms after 15 minutes to 1.0 milliohms after 360 minutes at 100 grams load. Silver cadmium oxide (90Ag 10CdO) produced very uniform resistance readings of 0.98 milliohms, as illustrated in Figure 3.1.1B. The resistance values of the silver alloy contacts gradually increased as a function of time spent in the vibratory feeder bowl.
Similar results were obtained for those contacts submitted to urethane-coated vibratory feeder bowls. Fine silver and silver cadmium oxide (90Ag 10CdO) produced the lowest resistance values of the three contact materials studied, as shown in Figure 3.1.1C.
Typical data distribution of the contact resistance readings is displayed in Figures 3.1.2A, 3.1.2B, and 3.1.2C. Scattering of the data was observed in some instances, due to test conditions and random formation of surface contamination on some contacts.
Figure 3.1.3 shows the average contact resistance and standard deviation of fine silver after spending one month in the coated vibratory feeder bowls. The graph in Figure 3.1.3 indicates that over time, the brushlon coating will rapidly degrade the surface and produce open circuit resistance. Teflon and urethane coatings yielded low contact.
Resistance values 2.0 milliohms and 1.5 milliohms respectively. The standard deviation was very narrow and (in some instances) there was no scattering of the contact resistance readings, as shown in Figure 3.1.3. On the other hand, fine silver contacts yielded open circuit resistance values after 45 minutes in the austenitic stainless steel vibratory feeder bowl .
surface, as shown in Figure 3.2.1D. Similar results were obtained for fine silver contacts after spending only 45 minutes in the austenitic stainless steel vibratory feeder bowl .
3.2 Scanning Electron Microscopy
Film contamination present on the surface of the contacts is shown in Figures 3.2.1 to 3.2.3. The contamination was evident after 6 hours in the coated vibratory feeder bowls for all contact materials studied. After spending one month in the coated vibratory feeder bowls, fine silver contacts were analyzed for contamination.
Specific attention was paid to the contaminant composition. The energy dispersion spectroscopy analysis was performed at an accelerating voltage of 10Kev. This provided minimal penetration of the electron beam and identification of typical contaminant material. Light elements such as carbon, oxygen, and nitrogen could be identified with the use of an ultra-thin window.
The analyses of the spot checks performed on the contacts surface reflected similar results. Isolated contamination was identified, and was comprised of hydrocarbon, as illustrated in Figure 3.2.1A, 3.2.1 B and 3.2.1C.
At one month in the brushlon coated vibratory feeder bowl, the fine silver contacts displayed a hydrocarbon film. The hydrocarbon film was thick and covered the entire contact surface, as shown in Figure 3.2.1D. Similar results were obtained for fine silver contacts after spending only 45 minutes in the austenitic stainless steel vibratory feeder bowl .
Minimal film contamination was detected on the contacts submitted to teflon coated vibratory feeder bowls. Only isolated fine dimple contamination was detected throughout the contact surfaces, as shown in Figures 3.2.2A, B and C. At one month in the teflon coated vibratory feeder bowl, an organic film (hydrocarbon) was detected on the contact surfaces. However, the film was thin and only the bottom of the contact apprentices was covered by contaminations as shown in Figure 3.2.2D.
The urethane-coated vibratory feeder bowl revealed similar film contamination, as was observed with the Teflon coating for the noble material tested. The population of the finely dispersed dimple contamination was limited and confined to the center of the contact as shown in Figure 3.2.3A and 3.2.3B. The silver alloy contact exhibited minimal surface contamination, as illustrated in Figure 3.2.3C. At one month in the urethane coated vibratory feeder bowl, the fine silver contacts exhibited surface conditions similar to the silver alloy contacts, as illustrated in Figure 3.2.3D.
The severity of surface contamination produced by austenitic stainless steel vibratory feeder bowls as a function of material nobility has already been demonstrated.
The results suggest a general picture of the behavior of the contacts when subjected to coated vibratory feeder bowls. The contact materials fine silver, silver cadmium oxide (90Ag 10CdO) and a silver alloy (75AG 24.5Cu 0.5Ni) with 15 microinches (Ra) surface finish exhibited little surface degradation up to 360 minutes intervals. Low contact resistance was obtained for all contact materials tested, at 100 grams load and for every interval used in the investigation. This was an improvement over the results obtained during the investigation with austenitic stainless steel vibratory feeder bowls under the same test conditions.
Surface contamination is best understood when comparing the different coatings. Brushlon is a unique combination of synthetic brush fibers uniformly distributed in a flexible backing. In an industrial environment, lubricant mists adhere to the brush fibers. Therefore, contact surfaces become susceptible to film contamination independent of the silver content (nobility) of the materials. Low contact resistance was attained due to the isolated and random formation of the contaminant film on the contact surface.
Coatings such as Teflon and urethane cured to 85 durometers and 90 durometers produced very little surface degradation. The noble materials did not show susceptibility to film contamination. Substantially clean sites were available throughout the contact surface for assessment during contact resistance probing. The silver alloy (75AG 24.5Cu 0.5Ni) rivets achieved the cleanest surface in both coatings.
The Teflon and urethane coatings were effective in reducing film contamination. However, the addition of copper to silver increases resistance due to surface film oxidation. Therefore, slightly higher resistance values were obtained from the silver alloy than noble materials.
As shown in figure 3.1.3 surface degradation was evident as contacts spent one month in the brushlon coated vibratory feeder bowls. The film contamination covered the entire contact surface. Loads exceeding 500 grams were not sufficient to break the contaminant film to allow metal to metal contact. Contamination was present on the surface of contacts submitted to teflon coated vibratory feeder bowls; however, the contamination was not sufficient to cover contact asperities. Therefore, metal to metal contact was achieved very readily. Urethane was most effective in preventing film contamination. At one month in the vibratory feeder bowl, the contacts showed minimal surface contamination. We must note that (1) after one month, the teflon coating did wear; (2) contact wear was observed with the urethane coating; (3) surface finish was not altered after one month of exposure to teflon and urethane coatings; and (4) the energy dispersion spectroscopy analysis did not detect wear debris from the coatings embedded on the contact surface.
Clearly, it was shown that coated vibratory feeder bowls did not produce significant surface contamination, in comparison to austenitic stainless steel vibratory feeder bowls. When contacts were submitted to the same accelerated test in the austenitic stainless steel vibratory feeder bowls, an organic film consisting of hydrocarbon formed on the contact surface after 45 minutes. This was difficult to break even at 500 grams load and prohibited metal to metal contact. The silver alloy (75AG 24.5Cu 0.5Ni) rivets displayed similar contact resistance values after spending 120 minutes in the austenitic stainless steel feeder bowl and 360 minutes in the coated vibratory feeder bowls.
In summary, the results show that brushlon, teflon and urethane coatings prevented surface degradation. However, long term usage makes coatings susceptible to wear and the introduction of surface contamination is manifested again as was observed with austenitic stainless steel vibratory feeder bowls. Further study is needed to correlate the effects of long-term usage of coated vibratory feeder bowls on contact performance.
The effect of surface degradation by a brush fiber material and coated vibratory feeder bowls, on contact resistance of the three contact materials studied can be explained as follows:
- The coated vibratory feeder bowls, significantly reduced surface contamination compared to austenitic stainless steel vibratory feeder bowls under the same test conditions.
- The brush fiber material and coatings effectively reduced surface degradation. Low contact resistance was achieved, up to 6 hours regardless of the material.
- After one month, contacts submitted to brushlon coated vibratory feeder bowls exhibited identical film contamination as contacts submitted to austenitic stainless steel vibratory feeder bowls for 45 minutes.
- SEM analyses revealed little surface contamination and no wear debris on contacts submitted to teflon and urethane coated vibratory feeder bowls.