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Correlation between friction and wear properties and electrical performance of silver coated electrical connectors
Wear 330-331 (2015) 400–405
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Correlation between friction and wear properties and electrical performance of silver coated electrical connectors Jian Song n, Vitali Schinow Precision Engineering Laboratory, Ostwestfalen-Lippe University of Applied Sciences, Liebigstrasse 87, 32657 Lemgo, Germany
art ic l e i nf o
a b s t r a c t
Article history: Received 20 August 2014 Received in revised form 3 February 2015 Accepted 10 February 2015
The wear resistance of coatings is a very important factor which not only influences the reliability but also the lifetime of electrical connectors. The coefficient of friction of the coating, along with the normal force of the contacts, determines the insertion and extraction force of connectors and in addition represents an important indicator for the state of the contact surfaces. The main finding of this study is the disproportionate increase in lifetime with increasing thickness of silver coating. The difference between wear and friction curves of thin and thick silver coatings and the correlation between the different phases of friction curves and different stages of wear curves are analyzed, with the adhesive friction being the dominating type of friction in all four phases of friction, which peaks to its maximum in the phase II. & 2015 Elsevier B.V. All rights reserved.
Keywords: Friction Wear Electrical resistance Contacts Silver Thickness
1. Introduction With an increasing number of electrical connectors in electronics, machines and vehicles, the requirements regarding reliability and lifetime of connectors is also rising. Electrical contacts of connectors are usually coated in order to prevent corrosion of the base copper alloy. One of the important coating materials of electrical connectors is silver which is not only highly resistant to oxidation and corrosion but also much less expensive than gold. Coatings of electrical connectors experience wear because of the motion between contacts due to vibration, different thermal expansion coefficients on both sides of connectors or mating and unmating of connectors. Once the protective coatings have been worn through, electrical contacts will fail rapidly due to corrosion or fretting corrosion. Therefore the wear resistance of the coatings is a vital factor which influences the reliability and lifetime of electrical connectors. The coefficient of friction of the coating, together with the normal force of the contacts, determines the insertion and extraction force of connectors, which is one of the important handling characteristics of connectors. The coefficient of friction of the coating varies immensely due to the wear of the coating and is therefore also an important indicator for the state of the contact surfaces [2,5,6,13–16]. Many aspects of the tribology of electrical contacts have been investigated in recent years, including the influence of lubrication
films, modifications of coatings and influence of vibrations. Most studies focus on the conventional thickness of coatings of up to 5 mm [13–18]. Even within this very small range of coating thickness, the nonlinearity of the correlation between the lifetime of electrical contacts and the coating thickness was observed. The authors of previous studies defined a threshold thickness and identified the linear correlation between the lifetime and the coating thickness [15,18]. The requirements for many new applications have however been increased, for instance in the case of extremely high wear resistance of the coatings used for charging devices for electrical vehicles. We have therefore investigated a larger range of coating thickness up to more than 10 mm. Due to this large range of the coating thickness the nonlinearity of the correlation between the lifetime and the coating thickness can be clearly identified. The online measurement of wear also shows a continuous decreasing wear rate with the increasing sliding distance which reveals the disproportionate increase of the wear resistance and accordingly the lifetime with the increasing coating thickness. Our study deals with the following issues:
The friction and wear characteristics of silver coatings of different thicknesses as function of sliding distance.
The wear rate in the wear-in stage and in the steady-state wear stage.
The correlation between the friction and wear characteristics n
Corresponding author. Tel.: þ 49 177 2131820; fax: þ 49 5261 70285028. E-mail address: [email protected] (J. Song).
http://dx.doi.org/10.1016/j.wear.2015.02.026 0043-1648/& 2015 Elsevier B.V. All rights reserved.
and the state of silver coatings on the contact surfaces, which are analyzed using microscopical methods in order to identify
J. Song, V. Schinow / Wear 330-331 (2015) 400–405
the mechanisms responsible for changes in friction and wear properties. The correlation between friction, wear characteristics and electrical performance of contacts, these being measured with self-developed test rigs.
The findings obtained in this study provide important reference points for the improvement of connector performance. In the case of the application in electrical contacts, the disproportionate increase in lifetime with the increasing thickness of the coating creates new possibilities for silver coated electrical contacts e.g. in charging connectors for electrical vehicles, in addition to providing new models for the calculation of lifetime.
401
lower due to the plastic deformation in the middle. The contacts are wired for a four-wire resistance measurement, with a computer- controlled data acquisition system. The wear measurement is conducted with a laser distance sensor, which measures the relative movement of the sample holder. The online measurement has the advantage that it differentiates the run-in stage of wear at the beginning of the test from the normal stage of wear after the run-in stage, which consequently enables a precise forecast of the development of the wearing process. An optical microscope is also used for the surface analysis.
3. Theory 2. Materials and methods
3.1. Contact resistance, surface protection, motion and wear
2.1. Contacts and materials
According to Holm the contact resistance RC is proportional to the specific electrical resistance ρ and inversely proportional to the contact area [1]:
The contacts used for the investigation were stamped and the base material is CuSn4. The size of the contacts is shown in Fig. 1. The contacts are coated with pure silver of varied thickness ranging from 3 to11 mm. 2.2. Test rig and analysis of contact areas A test rig is used for the tribological and fretting tests, which enables a defined displacement of fretting motion at the contact interface, Fig. 2. A piezo actuator is programmed to move forwards and backwards with an amplitude of 50–300 mm and cycle duration of 1 s. The contact normal force is provided with a dead load and various normal forces can be applied. The average pressure can be estimated with the Hertz's equation for a ball-on-plate setup, Table 1. The true value of the average pressure is somewhat
RC ¼
ρ
ð1Þ
2a
where a is the radius of the contact area. Copper or copper alloys are commonly used as base metals for electrical contacts due to their high conductivity. However copper and copper alloys have low standard electrode potentials and therefore corrode and oxidize easily. Corrosion and oxidation products lead to a marked increase of the specific electrical resistance. Different materials are used for the coatings of electrical contacts in order to protect copper or copper alloys against corrosion and oxidation with silver being one of the important coating materials used for electrical contacts. In many cases the motion of contacts is unavoidable as a result of vibration or thermal expansion and in these cases the wear properties of the coating determine the duration of the surface protection and effect the lifetime of contacts [2]. 3.2. Wear The volumetric wear is the quantity of worn material [mm3]. It is more convenient to use the linear wear for coatings of electrical contacts, which is the volumetric wear divided by the apparent contact area, and this is therefore used to quantify wear in this paper. The linear wear rate is the linear wear per unit sliding distance [m]. Table 1 Normal force and estimated average pressure. Normal force [N]
1
2
5
Average pressure [MPa]
240
300
400
Fig. 1. Size of contacts.
Fig. 2. Test rig for wear and fretting corrosion tests.
402
J. Song, V. Schinow / Wear 330-331 (2015) 400–405
Wear takes place according to a definite pattern as illustrated in Fig. 3. The initial wear rate (run-in stage) is relatively high as a result of the high stress due to the microscopic surface asperities. Normal wear (steady-state, stage II) begins when the true area of bearing contact (total asperity-contact area) has been substantially increased by plastic deformation and wear. From this point onwards, wear occurs at a low rate. The determination of the duration of the run-in stage and the parameters which influence its duration are of extreme interest, since these are very important for a precise calculation of wear. The severe wear (stage III) phase is normally not relevant because of the wear-through of coatings. 3.3. Friction The parameters for friction can be rather different, depending on the state between the contacts and in most cases dry friction prevails between contacts. In the case of dry friction, there are an adhesive and an abrasive component and third body friction can also be of relevance due to wear particles in the contact area, Fig. 4 [4–8]. The adhesive friction force depends on the adhesive energy of contact surfaces. In order to separate contacting surfaces, the minimal energy Wad required is [4] W ad ¼ γ a þ γ b γ ab
ð4Þ
with W ad
γa γb γ ab
mJ adhesion energym 2 mJ surface energy of surface am 2 mJ surface energy of surface bm 2 mJ boundary surface energym 2
The surface energy γ of metals is about 1000 mJ/m². The surface energy of metal oxides is several 100 mJ/m² and the surface energy of polymers is 20–40 mJ/m². The boundary surface energy γab of contacts made of the same material has the lowest value and has the highest value, if the atoms are repellent. Since most electrical contacts are made of the same surface material, a high adhesive friction is expected. The acting radius of adhesive force is small and therefore the effective zone of adhesion includes only the real contact area, which is proportional to the hardness of the material. As a result
Fig. 3. Generalized pattern of the wear process [2,3].
we have the following relationship for the coefficient of adhesive friction fad [4,9,12]: f ad ¼
dW ad M H dx
ð5Þ
with M H
factor proportional to the surface roughness hardness
This means the oxidation of the surface material leads to an increasing coefficient of adhesive friction. Since the oxidation of the surface material increases the contact resistance, a change in the coefficient of adhesive friction can be a very important indicator for the state of electrical contacts. The abrasive friction is the friction force due to deformation and fraction of the material. The abrasive friction force Fab can be expressed by the following relationship [9]: K Ic pffiffiffiffiffiffi F ab ¼ c pffiffiffiffi F N E H
ð6Þ
with c KIc E H FN
factor depending on the surface roughness ductility Young's modulus hardness normal Force
The surface film in the contact area generally reduces both adhesion and abrasion and at the same time results in an additional friction component F sh , which is basically determined by the shear strength of the film [6]. Wear particles, as third bodies, play an important role in the friction of contacts and they can reduce friction and wear by forming transfer films and slippery materials [8]. Fig. 5 shows a typical friction curve as a function of time or number of motion cycles. It can be divided into the initial coefficient of friction and four phases [10,11]: Initial coefficient of friction: It is determined by the shear strength of the surface film and has, according to Popov, an almost constant value of 0.19 [10,11]. Phase I: The real contact area increases with increasing motion cycles which then leads to an increasing adhesive friction. The increasing coefficient of friction documents the dominating proportion of the adhesive friction. Phase II: The adhesive friction reaches its maximum. Phase III: Two mechanisms can lead to a reduction of friction. Relevant amounts of wear particles develop in the contact area and the third body friction reduces the coefficient of friction. In addition, the buildup of the oxidation film increases the boundary surface energy in Eq. (4) and reduces the adhesive friction. Phase IV:The coefficient of friction reaches equilibrium.
Fig. 4. Different friction mechanisms of electrical contacts [4,7,8].
J. Song, V. Schinow / Wear 330-331 (2015) 400–405
4. Results and discussion Figs. 6 and 7 show the wear, coefficient of friction and contact resistance curves as functions of sliding distance of coatings with 3 mm and 11 mm of silver. Both the run-in stage and the steadystate can be identified in the wear curves and it can be observed that the duration of the run-in stage increases disproportionately with increasing coating thickness. The run-in stage of the 3 mm coating is completed after approximately 300 mm, whereas the run-in stage of the 11 mm coating lasts for more than 3000 mm. The key issue with regard to electrical contacts is the development of the electrical contact resistance, which is essential for the functionality aspect. Knowledge about the correlation between friction, wear and electrical resistance can be of assistance with regard to improving and optimizing the coatings for use in electrical contacts. Some correlations are evident when the development of the wear curves, the friction curves and the contact resistance curves are compared: 1. The coefficient of friction curve of the 3 mm coating clearly shows the initial coefficient of friction, and phase I to phase IV of friction (Fig. 6). 2. The first instability of contact resistance was at no time observed in the run-in stage of the wear curves (Figs. 6 and 7). Since the run-in stage is basically the process of smoothing surface asperities, the wear-through of coating is not expected at this stage and surface protection therefore exists. 3. In the case of 3 mm silver coating, a good correlation between the transition of the run-in stage to the steady-state stage of
the wear curve and the beginning of phase III of the friction curve after approximately 300 mm was observed (Fig. 6). 4. The first instability of contact resistance was at no time observed before phase III of the friction curves (Fig. 6). However, there is a large difference between 3 mm and 11 mm coatings. In the case of the 3 mm coating, phase III began after approximately 300 mm of travel and the electrical resistance displayed the first instability after approximately 800 mm. This implies that at this stage wear-through damage of the silver coating has occurred and the oxidation of the base metal has taken place. It can also be observed on the contact surface. Fig. 8 shows the degradation of the contact area and illustrates the surface after 300 mm. In the case of the 11 mm coatings, phase III was not observed after 60,000 mm, which would mean that with a less than 4-fold coating thickness, a more than 75-fold lifetime was achieved. The disproportionate increase in lifetime with the increasing thickness of the silver coating is a relevant result of this study, with these mechanisms contributing to this disproportionate increase:
The wear-through of the coating occurred shortly after the
Fig. 5. Friction curve as function of number of cycles [9].
403
beginning of phase III in the case of the 3 mm coating (Fig. 8), whereas the wear-through of the 11 mm coating did not occur after a disproportionate long distance of travel (Fig. 9). An X-ray fluorescence spectroscopy (XRF) measurement showed a remaining thickness of more than 1 mm silver in the contact area after 60,000 mm. Since the wear rate in the run-in stage was 4 mm/m, after a travel of 300 mm in the run-in stage, the linear wear was 1.3 mm. It should be mentioned at this point that the surface roughness (RZ) of the coating is more than 1 mm and a partial wear-through of the 3 mm coating can therefore be expected shortly after 300 mm. The marked decrease of the coefficient of friction was the result of the increased boundary surface energy (Eq. (4)) due to the wear-through of the silver coating, which in turn leads to oxidation of the base metal. The wear-through and the oxidation of the base metal are confirmed by microscopical image of the wear surface (Fig. 8). In the case of the 11 mm silver coating, the coating was still far from wear-through by the end of the run-in stage. The wear rate was less than 0.3 mm/m in the steady-state stage of wear. The reason for the low wear rate was the increasing third body friction which was enabled by the silver debris in the contact area.
Fig. 6. Wear, friction and contact resistance curves of coating with 3 mm of silver, normal force 5 N.
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J. Song, V. Schinow / Wear 330-331 (2015) 400–405
Fig. 7. Wear, friction and contact resistance curves of coating with 3 mm of silver, normal force 2 N.
Fig. 8. Contact area after a given sliding distance: a) 0, b) 30 mm, c) 300 mm, d) 600 mm, and e) 1800 mm, thickness of coating 3 mm, normal force 5 N.
Fig. 9. 11 mm Ag coating after 60,000 mm travel, 1.2 mm Ag still remains on the surface.
No relevant decrease of the coefficient of friction was observed in phase III (Fig. 7), which means that the adhesion friction is still the dominating friction type and there was no change in the boundary surface energy and the fretting interface remained silver on silver. This condition leads to a disproportionate increase in lifetime and as a result neither a large change of contact resistance (Fig. 7) nor oxidation was observed (Fig. 9).
Some new impulses regarding the design of coatings for electrical contacts can be derived from these results: The sum of the linear wear in the run-in stage and the surface roughness can be defined as the critical thickness. A coating within the critical thickness range will be worn through very fast due to the high wear rate in this stage. With regard to the design of coatings this result means that if fretting is expected in the application, the thickness of the silver coating should be much higher than the critical thickness. Furthermore, the run-in stage and the steady-state stage must be calculated separately and different wear rates must be used in order to forecast the wear progression. Three to four measurements were conducted for every combination of parameters and all of them showed a similar development. The scatter at the beginning of phase II is less than 50 mm and the scatter at the beginning of phase III is approximately 150 mm, in the case of the 3 mm coating (Fig. 10). The analysis of the friction curves of different thicknesses and varied normal force in the first cycles furthermore reveals the following findings: 1. The initial coefficient of friction of about 0.2 cannot be proved true in every case. In the case of 11 mm the initial coefficient of friction is approximately 1 (Fig. 7).
J. Song, V. Schinow / Wear 330-331 (2015) 400–405
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the duration of this phase can be disproportionately increased by increasing the thickness of the silver coating. It is presumed that third body friction already begins in phase II for a thick silver coating (11 mm) which subsequently leads to a disproportionate increase in lifetime. Adhesive friction is the dominating friction type in all four phases of the friction process, which peaks to its maximum in phase II.
Acknowledgment
Fig. 10. Repeatability of the measurements – COF – curves of 3 mm coating.
This work forms a part of two research projects financed by the German Federal Ministry for Education and Research (BMBF, 1758X07), the European Union and the German State NorthrhineWestfalia (w0804nm001) and Phoenix Contact in Blomberg. The University of Paderborn supported the projects with materials and measurements. References
Fig. 11. Friction curves in the first cycles with various normal forces.
The fact that the coefficient of friction with a normal force of 2. 5 N increases much faster than the coefficient of friction with a normal force of 1 N is the result of the faster increase of the real contact area at a higher normal force. This reveals the dominating role of the adhesive friction, also in phase I of the friction curve. 3. The proportion of the abrasive friction in the coefficient of friction can be analyzed by comparison of the curves in Fig. 11. According to Eq. (6) the abrasive friction force should increase with the increasing normal force. However no relevant difference in coefficients of friction can be observed in the silver coating and therefore underlined the dominating role of the adhesive friction.
5. Conclusion The friction, wear and contact resistance curves of silver coating of various thicknesses were investigated and analyzed in our study. The main finding is the disproportionate lifetime increase with the increasing thickness of the silver coating. The typical friction phases I to IV can always be identified in the case of a thin silver coating. For a thin silver coating (3 mm), the transition area of the wear-in stage to the steady-state stage correlates to the transition area of phase II to phase III of the friction curve with the first electrical instability being expected shortly after the beginning of phase III. Only phase I and an extremely long phase II are identified in the case of an 11 mm silver coating. The lifetime of the electrical contacts is determined by the duration of phase II of friction and
[1] R. Holm, Electric Contacts, Springer Verlag, Berlin, 2000. [2] J. Song, et al., Wear of electrically conductive surfaces, in: Proceedings of the Fracture Fatigue and Wear (FFW) Conference, Kitakyushu, Japan, 2014. [3] N.D. Spencer, Surfaces, Interfaces, and their Applications II – Introduction to Wear, ETH, Zurich, Switzerland, 2014. [4] J. Fontaine, C. Donnet, A. Erdemir, Fundamentals of the tribology of DLC coatings, in: C. Donnet, A. Erdemir (Eds.), Tribology of diamond - like carbon films, first ed.,Springer Science, Berlin, 2008, pp. 139–154. [5] J. Song, et al., Tribological Properties and lifetime of electrical contacts, in: Proceeding of the World Tribology Congress (WTC), Torino, Italy, 2013. [6] J. Song, et al., Reibverhalten von elektrischen Kontaktoberflächen, in: Proceedings of the Gesellschaft für Tribologie (GfT) Conference, Göttingen, Germany, 2014. [7] J.A. Seitz, Charakterisierung hoch belasteter amorpher wasserstoffhaltiger Kohlenstoffbeschichtungen zur tribologischen Anwendung auf Ventiltriebskomponenten (Ph.D. thesis), University Augsburg, Germany, 2012. [8] I.L. Singer, K.J. Wahl, Role of third bodies in friction and wear, US Naval Research Laboratory, Code 6176, Washington, D.C., 2013. [9] K.H. zum Gahr, Abrasiver Verschleiß metallischer Werkstoffe, VDI-Verlag, Düsseldorf, Germany, 1981. [10] K.H. Habig, Tribologie-Handbuch, Vieweg þ Teubner Verlag, Berlin, Germany (2010) 246. [11] V.L. Popov, Kontaktmechanik und Reibung, Springer-Verlag, Berlin, Germany (2010) 148. [12] I.V. Kragelski, et al., Grundlagen der Berechnung von Reibung und Verschleiß, VEB Verlag Technik, Berlin, 1982. [13] Wanbin Ren, Yingwei Du, Li Cui, Peng Wang, Jian Song, Research on fretting Regimes of gold-plated copper alloy electrical contact material under different vibration amplitude and frequency combinations, Wear 321 (2014) 70–78. [14] S. Noël, D. Alamarguy, L. Baraton, Influence of contact interface composition on the electrical and tribological properties of nickel electrodeposits during fretting tests, in: Proceedings of the International Conference on Electrical Contacts-International Conference on Reliability of Electrical Products and Electrical Contacts (ICEC-ICREPEC), 2012, (9.3-142). [15] S. Fouvry, P. Jedrzejczyk, P. Chalandon, O. Alquier, From fretting to connector vibration tests: a transfer function approach to predict electrical contact resistance endurance, in: Proceedings of the 27th International Conference on Electrical Contacts, 22 – 26 June, Dresden, 2014, pp. 255–260. [16] R.E. Abdi, N. Benjemaa, The effect of the temperature on the wear and resistance of automotive connectors subjected to vibration tests, J. Automob. Eng. 229 (2) (2015) 189–196. [17] P. Jedrzejczyk, S. Fouvry, O. Alquier, Quantification of the electrical contact endurance of thin platted silver coatings subjected to fretting wear: influence of coating thickness, in: Proceedings of the International Conference on Electrical Contacts-International Conference on Reliability of Electrical Products and Electrical Contacts (ICEC-ICREPEC), Beijing, 2012. [18] S. Fouvry, P. Jedrzejczyk, P Chalandon, Introduction of an exponential formulation to quantify the electrical endurance of micro-contactsenduring fretting wear, Wear 271 (2011) 1524–1534.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Correlation between friction and wear properties and electrical performance of silver coated electrical connectors Jian Song n, Vitali Schinow Precision Engineering Laboratory, Ostwestfalen-Lippe University of Applied Sciences, Liebigstrasse 87, 32657 Lemgo, Germany
art ic l e i nf o
a b s t r a c t
Article history: Received 20 August 2014 Received in revised form 3 February 2015 Accepted 10 February 2015
The wear resistance of coatings is a very important factor which not only influences the reliability but also the lifetime of electrical connectors. The coefficient of friction of the coating, along with the normal force of the contacts, determines the insertion and extraction force of connectors and in addition represents an important indicator for the state of the contact surfaces. The main finding of this study is the disproportionate increase in lifetime with increasing thickness of silver coating. The difference between wear and friction curves of thin and thick silver coatings and the correlation between the different phases of friction curves and different stages of wear curves are analyzed, with the adhesive friction being the dominating type of friction in all four phases of friction, which peaks to its maximum in the phase II. & 2015 Elsevier B.V. All rights reserved.
Keywords: Friction Wear Electrical resistance Contacts Silver Thickness
1. Introduction With an increasing number of electrical connectors in electronics, machines and vehicles, the requirements regarding reliability and lifetime of connectors is also rising. Electrical contacts of connectors are usually coated in order to prevent corrosion of the base copper alloy. One of the important coating materials of electrical connectors is silver which is not only highly resistant to oxidation and corrosion but also much less expensive than gold. Coatings of electrical connectors experience wear because of the motion between contacts due to vibration, different thermal expansion coefficients on both sides of connectors or mating and unmating of connectors. Once the protective coatings have been worn through, electrical contacts will fail rapidly due to corrosion or fretting corrosion. Therefore the wear resistance of the coatings is a vital factor which influences the reliability and lifetime of electrical connectors. The coefficient of friction of the coating, together with the normal force of the contacts, determines the insertion and extraction force of connectors, which is one of the important handling characteristics of connectors. The coefficient of friction of the coating varies immensely due to the wear of the coating and is therefore also an important indicator for the state of the contact surfaces [2,5,6,13–16]. Many aspects of the tribology of electrical contacts have been investigated in recent years, including the influence of lubrication
films, modifications of coatings and influence of vibrations. Most studies focus on the conventional thickness of coatings of up to 5 mm [13–18]. Even within this very small range of coating thickness, the nonlinearity of the correlation between the lifetime of electrical contacts and the coating thickness was observed. The authors of previous studies defined a threshold thickness and identified the linear correlation between the lifetime and the coating thickness [15,18]. The requirements for many new applications have however been increased, for instance in the case of extremely high wear resistance of the coatings used for charging devices for electrical vehicles. We have therefore investigated a larger range of coating thickness up to more than 10 mm. Due to this large range of the coating thickness the nonlinearity of the correlation between the lifetime and the coating thickness can be clearly identified. The online measurement of wear also shows a continuous decreasing wear rate with the increasing sliding distance which reveals the disproportionate increase of the wear resistance and accordingly the lifetime with the increasing coating thickness. Our study deals with the following issues:
The friction and wear characteristics of silver coatings of different thicknesses as function of sliding distance.
The wear rate in the wear-in stage and in the steady-state wear stage.
The correlation between the friction and wear characteristics n
Corresponding author. Tel.: þ 49 177 2131820; fax: þ 49 5261 70285028. E-mail address: [email protected] (J. Song).
http://dx.doi.org/10.1016/j.wear.2015.02.026 0043-1648/& 2015 Elsevier B.V. All rights reserved.
and the state of silver coatings on the contact surfaces, which are analyzed using microscopical methods in order to identify
J. Song, V. Schinow / Wear 330-331 (2015) 400–405
the mechanisms responsible for changes in friction and wear properties. The correlation between friction, wear characteristics and electrical performance of contacts, these being measured with self-developed test rigs.
The findings obtained in this study provide important reference points for the improvement of connector performance. In the case of the application in electrical contacts, the disproportionate increase in lifetime with the increasing thickness of the coating creates new possibilities for silver coated electrical contacts e.g. in charging connectors for electrical vehicles, in addition to providing new models for the calculation of lifetime.
401
lower due to the plastic deformation in the middle. The contacts are wired for a four-wire resistance measurement, with a computer- controlled data acquisition system. The wear measurement is conducted with a laser distance sensor, which measures the relative movement of the sample holder. The online measurement has the advantage that it differentiates the run-in stage of wear at the beginning of the test from the normal stage of wear after the run-in stage, which consequently enables a precise forecast of the development of the wearing process. An optical microscope is also used for the surface analysis.
3. Theory 2. Materials and methods
3.1. Contact resistance, surface protection, motion and wear
2.1. Contacts and materials
According to Holm the contact resistance RC is proportional to the specific electrical resistance ρ and inversely proportional to the contact area [1]:
The contacts used for the investigation were stamped and the base material is CuSn4. The size of the contacts is shown in Fig. 1. The contacts are coated with pure silver of varied thickness ranging from 3 to11 mm. 2.2. Test rig and analysis of contact areas A test rig is used for the tribological and fretting tests, which enables a defined displacement of fretting motion at the contact interface, Fig. 2. A piezo actuator is programmed to move forwards and backwards with an amplitude of 50–300 mm and cycle duration of 1 s. The contact normal force is provided with a dead load and various normal forces can be applied. The average pressure can be estimated with the Hertz's equation for a ball-on-plate setup, Table 1. The true value of the average pressure is somewhat
RC ¼
ρ
ð1Þ
2a
where a is the radius of the contact area. Copper or copper alloys are commonly used as base metals for electrical contacts due to their high conductivity. However copper and copper alloys have low standard electrode potentials and therefore corrode and oxidize easily. Corrosion and oxidation products lead to a marked increase of the specific electrical resistance. Different materials are used for the coatings of electrical contacts in order to protect copper or copper alloys against corrosion and oxidation with silver being one of the important coating materials used for electrical contacts. In many cases the motion of contacts is unavoidable as a result of vibration or thermal expansion and in these cases the wear properties of the coating determine the duration of the surface protection and effect the lifetime of contacts [2]. 3.2. Wear The volumetric wear is the quantity of worn material [mm3]. It is more convenient to use the linear wear for coatings of electrical contacts, which is the volumetric wear divided by the apparent contact area, and this is therefore used to quantify wear in this paper. The linear wear rate is the linear wear per unit sliding distance [m]. Table 1 Normal force and estimated average pressure. Normal force [N]
1
2
5
Average pressure [MPa]
240
300
400
Fig. 1. Size of contacts.
Fig. 2. Test rig for wear and fretting corrosion tests.
402
J. Song, V. Schinow / Wear 330-331 (2015) 400–405
Wear takes place according to a definite pattern as illustrated in Fig. 3. The initial wear rate (run-in stage) is relatively high as a result of the high stress due to the microscopic surface asperities. Normal wear (steady-state, stage II) begins when the true area of bearing contact (total asperity-contact area) has been substantially increased by plastic deformation and wear. From this point onwards, wear occurs at a low rate. The determination of the duration of the run-in stage and the parameters which influence its duration are of extreme interest, since these are very important for a precise calculation of wear. The severe wear (stage III) phase is normally not relevant because of the wear-through of coatings. 3.3. Friction The parameters for friction can be rather different, depending on the state between the contacts and in most cases dry friction prevails between contacts. In the case of dry friction, there are an adhesive and an abrasive component and third body friction can also be of relevance due to wear particles in the contact area, Fig. 4 [4–8]. The adhesive friction force depends on the adhesive energy of contact surfaces. In order to separate contacting surfaces, the minimal energy Wad required is [4] W ad ¼ γ a þ γ b γ ab
ð4Þ
with W ad
γa γb γ ab
mJ adhesion energym 2 mJ surface energy of surface am 2 mJ surface energy of surface bm 2 mJ boundary surface energym 2
The surface energy γ of metals is about 1000 mJ/m². The surface energy of metal oxides is several 100 mJ/m² and the surface energy of polymers is 20–40 mJ/m². The boundary surface energy γab of contacts made of the same material has the lowest value and has the highest value, if the atoms are repellent. Since most electrical contacts are made of the same surface material, a high adhesive friction is expected. The acting radius of adhesive force is small and therefore the effective zone of adhesion includes only the real contact area, which is proportional to the hardness of the material. As a result
Fig. 3. Generalized pattern of the wear process [2,3].
we have the following relationship for the coefficient of adhesive friction fad [4,9,12]: f ad ¼
dW ad M H dx
ð5Þ
with M H
factor proportional to the surface roughness hardness
This means the oxidation of the surface material leads to an increasing coefficient of adhesive friction. Since the oxidation of the surface material increases the contact resistance, a change in the coefficient of adhesive friction can be a very important indicator for the state of electrical contacts. The abrasive friction is the friction force due to deformation and fraction of the material. The abrasive friction force Fab can be expressed by the following relationship [9]: K Ic pffiffiffiffiffiffi F ab ¼ c pffiffiffiffi F N E H
ð6Þ
with c KIc E H FN
factor depending on the surface roughness ductility Young's modulus hardness normal Force
The surface film in the contact area generally reduces both adhesion and abrasion and at the same time results in an additional friction component F sh , which is basically determined by the shear strength of the film [6]. Wear particles, as third bodies, play an important role in the friction of contacts and they can reduce friction and wear by forming transfer films and slippery materials [8]. Fig. 5 shows a typical friction curve as a function of time or number of motion cycles. It can be divided into the initial coefficient of friction and four phases [10,11]: Initial coefficient of friction: It is determined by the shear strength of the surface film and has, according to Popov, an almost constant value of 0.19 [10,11]. Phase I: The real contact area increases with increasing motion cycles which then leads to an increasing adhesive friction. The increasing coefficient of friction documents the dominating proportion of the adhesive friction. Phase II: The adhesive friction reaches its maximum. Phase III: Two mechanisms can lead to a reduction of friction. Relevant amounts of wear particles develop in the contact area and the third body friction reduces the coefficient of friction. In addition, the buildup of the oxidation film increases the boundary surface energy in Eq. (4) and reduces the adhesive friction. Phase IV:The coefficient of friction reaches equilibrium.
Fig. 4. Different friction mechanisms of electrical contacts [4,7,8].
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4. Results and discussion Figs. 6 and 7 show the wear, coefficient of friction and contact resistance curves as functions of sliding distance of coatings with 3 mm and 11 mm of silver. Both the run-in stage and the steadystate can be identified in the wear curves and it can be observed that the duration of the run-in stage increases disproportionately with increasing coating thickness. The run-in stage of the 3 mm coating is completed after approximately 300 mm, whereas the run-in stage of the 11 mm coating lasts for more than 3000 mm. The key issue with regard to electrical contacts is the development of the electrical contact resistance, which is essential for the functionality aspect. Knowledge about the correlation between friction, wear and electrical resistance can be of assistance with regard to improving and optimizing the coatings for use in electrical contacts. Some correlations are evident when the development of the wear curves, the friction curves and the contact resistance curves are compared: 1. The coefficient of friction curve of the 3 mm coating clearly shows the initial coefficient of friction, and phase I to phase IV of friction (Fig. 6). 2. The first instability of contact resistance was at no time observed in the run-in stage of the wear curves (Figs. 6 and 7). Since the run-in stage is basically the process of smoothing surface asperities, the wear-through of coating is not expected at this stage and surface protection therefore exists. 3. In the case of 3 mm silver coating, a good correlation between the transition of the run-in stage to the steady-state stage of
the wear curve and the beginning of phase III of the friction curve after approximately 300 mm was observed (Fig. 6). 4. The first instability of contact resistance was at no time observed before phase III of the friction curves (Fig. 6). However, there is a large difference between 3 mm and 11 mm coatings. In the case of the 3 mm coating, phase III began after approximately 300 mm of travel and the electrical resistance displayed the first instability after approximately 800 mm. This implies that at this stage wear-through damage of the silver coating has occurred and the oxidation of the base metal has taken place. It can also be observed on the contact surface. Fig. 8 shows the degradation of the contact area and illustrates the surface after 300 mm. In the case of the 11 mm coatings, phase III was not observed after 60,000 mm, which would mean that with a less than 4-fold coating thickness, a more than 75-fold lifetime was achieved. The disproportionate increase in lifetime with the increasing thickness of the silver coating is a relevant result of this study, with these mechanisms contributing to this disproportionate increase:
The wear-through of the coating occurred shortly after the
Fig. 5. Friction curve as function of number of cycles [9].
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beginning of phase III in the case of the 3 mm coating (Fig. 8), whereas the wear-through of the 11 mm coating did not occur after a disproportionate long distance of travel (Fig. 9). An X-ray fluorescence spectroscopy (XRF) measurement showed a remaining thickness of more than 1 mm silver in the contact area after 60,000 mm. Since the wear rate in the run-in stage was 4 mm/m, after a travel of 300 mm in the run-in stage, the linear wear was 1.3 mm. It should be mentioned at this point that the surface roughness (RZ) of the coating is more than 1 mm and a partial wear-through of the 3 mm coating can therefore be expected shortly after 300 mm. The marked decrease of the coefficient of friction was the result of the increased boundary surface energy (Eq. (4)) due to the wear-through of the silver coating, which in turn leads to oxidation of the base metal. The wear-through and the oxidation of the base metal are confirmed by microscopical image of the wear surface (Fig. 8). In the case of the 11 mm silver coating, the coating was still far from wear-through by the end of the run-in stage. The wear rate was less than 0.3 mm/m in the steady-state stage of wear. The reason for the low wear rate was the increasing third body friction which was enabled by the silver debris in the contact area.
Fig. 6. Wear, friction and contact resistance curves of coating with 3 mm of silver, normal force 5 N.
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Fig. 7. Wear, friction and contact resistance curves of coating with 3 mm of silver, normal force 2 N.
Fig. 8. Contact area after a given sliding distance: a) 0, b) 30 mm, c) 300 mm, d) 600 mm, and e) 1800 mm, thickness of coating 3 mm, normal force 5 N.
Fig. 9. 11 mm Ag coating after 60,000 mm travel, 1.2 mm Ag still remains on the surface.
No relevant decrease of the coefficient of friction was observed in phase III (Fig. 7), which means that the adhesion friction is still the dominating friction type and there was no change in the boundary surface energy and the fretting interface remained silver on silver. This condition leads to a disproportionate increase in lifetime and as a result neither a large change of contact resistance (Fig. 7) nor oxidation was observed (Fig. 9).
Some new impulses regarding the design of coatings for electrical contacts can be derived from these results: The sum of the linear wear in the run-in stage and the surface roughness can be defined as the critical thickness. A coating within the critical thickness range will be worn through very fast due to the high wear rate in this stage. With regard to the design of coatings this result means that if fretting is expected in the application, the thickness of the silver coating should be much higher than the critical thickness. Furthermore, the run-in stage and the steady-state stage must be calculated separately and different wear rates must be used in order to forecast the wear progression. Three to four measurements were conducted for every combination of parameters and all of them showed a similar development. The scatter at the beginning of phase II is less than 50 mm and the scatter at the beginning of phase III is approximately 150 mm, in the case of the 3 mm coating (Fig. 10). The analysis of the friction curves of different thicknesses and varied normal force in the first cycles furthermore reveals the following findings: 1. The initial coefficient of friction of about 0.2 cannot be proved true in every case. In the case of 11 mm the initial coefficient of friction is approximately 1 (Fig. 7).
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the duration of this phase can be disproportionately increased by increasing the thickness of the silver coating. It is presumed that third body friction already begins in phase II for a thick silver coating (11 mm) which subsequently leads to a disproportionate increase in lifetime. Adhesive friction is the dominating friction type in all four phases of the friction process, which peaks to its maximum in phase II.
Acknowledgment
Fig. 10. Repeatability of the measurements – COF – curves of 3 mm coating.
This work forms a part of two research projects financed by the German Federal Ministry for Education and Research (BMBF, 1758X07), the European Union and the German State NorthrhineWestfalia (w0804nm001) and Phoenix Contact in Blomberg. The University of Paderborn supported the projects with materials and measurements. References
Fig. 11. Friction curves in the first cycles with various normal forces.
The fact that the coefficient of friction with a normal force of 2. 5 N increases much faster than the coefficient of friction with a normal force of 1 N is the result of the faster increase of the real contact area at a higher normal force. This reveals the dominating role of the adhesive friction, also in phase I of the friction curve. 3. The proportion of the abrasive friction in the coefficient of friction can be analyzed by comparison of the curves in Fig. 11. According to Eq. (6) the abrasive friction force should increase with the increasing normal force. However no relevant difference in coefficients of friction can be observed in the silver coating and therefore underlined the dominating role of the adhesive friction.
5. Conclusion The friction, wear and contact resistance curves of silver coating of various thicknesses were investigated and analyzed in our study. The main finding is the disproportionate lifetime increase with the increasing thickness of the silver coating. The typical friction phases I to IV can always be identified in the case of a thin silver coating. For a thin silver coating (3 mm), the transition area of the wear-in stage to the steady-state stage correlates to the transition area of phase II to phase III of the friction curve with the first electrical instability being expected shortly after the beginning of phase III. Only phase I and an extremely long phase II are identified in the case of an 11 mm silver coating. The lifetime of the electrical contacts is determined by the duration of phase II of friction and
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