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on a great deal of interrelated and independent parameters. These parameters can be sub-divided into two elementary groups: Those which are internally occurring and subsequently, those which are externally occurring.

The category of internal factors represents the influence of mechanical effects, such as contact loading, sliding velocity and reciprocation and characteristics of motion, are commonly grouped under the term ‘fretting’. As well as electrical based factors, such as the type and strength of current and the operating voltage.

Mechanical Factors RC changes from: Electrical Factors Heat _Arc Wear Mechanical Reaction Film Formation Film Rupture Surface Damage Film Formation Erosion Material Transfer Film Rupture Film Formation Rc ↑ Rc ↓ Rc ↑ Rc ↑ Rc ↑ Rc ↓ Rc ↑↓

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External based factors are representative of environmental effects and are often uncontrollable; effects such as time-temperature variation, humidity, atmospheric pressure as well as contaminants from air board particles.

These factors affect the performance of the contact by changing the properties of the contact material and depositing surface films. This in turn leads to physical and chemical changes in the contact, resulting in the build-up of wear with a subsequent effect of degradation of the contact interface, increase in contact resistance and eventual failure.

Relating these and previously discussed effects from arcing allows the quantification of process and measurement noise in the experimental results which are essential later on in the prediction process.

4.6.1 Fretting

A material that is subjected to small oscillatory movements at the interface of contacting materials can suffer from accelerated surface damage, this process is known as ‘fretting’. A unified model to explain the fretting process remains unfounded, however, the effect is dependent on numerous factors and many theories have been proposed, each is plausible to the exclusion of any other. The main factors that contribute to fretting can be roughly categorised as a) contact conditions b) environmental conditions and c) the properties and behaviour of the contact material, respectively. This may be summarised in figure 4.24 below.

An absence in literature of the effects of failure due to fretting is not without reason. As fretting is a time related process, the effects only become noticeable after long periods of time due to the build-up in the contact zone of wear based debris. As well as this, the destruction of the contact zone due to arcing and melting can shield the observer from any recognisable effects, especially at early stages.

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Figure 4.24 showing the effects contributing to process noise in measurements.

A few authors have carried out work into trying to measure the effects of fretting. The effects of fretting on the contact resistance of different contact plating materials and aluminum–tin-plated-copper and aluminum–copper was explored by Antler and Sproles, 1982; and Braunovic´, 1992. The results which are show in figure 4.25 a&b below. Contact Conditions  Load  Frequency  Amplitude  Design  Duration Environmental Conditions  Temperature  Humidity  Chemical  Lubrication

Fretting Rates and Mechanisms Material Behaviour/Properties

 Hardness  Ductility  Strength  Corrosion  Fatigue  Adhesion  Oxidation

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Figure 4.25 a&b, showing the effects of fretting on the contact resistance of different contact plating materials; aluminum–tin-plated-copper and aluminum–copper. Antler, M. and Sproles, E. S., IEEE Trans. CHMT, 5(1),158–166, and 1982; Braunovic´, M., IEEE Trans. CHMT, 15, 204–214, 1992.

The loading on the contact also exhibits considerable significance on the contact resistance due to fretting conditions. A small contact force can result in larger asperities in the measured contact resistance and even result in eventual open circuit. This result may be intuitive, as a greater loading results in less movement and hence less fretting.

At loads less than 1 N, when contacts are made, the surface asperities of harder materials penetrate the oxide films naturally present on the material establishing localized metallic contacts and setting up conducting paths. Fretting causes the shearing of these metallic bridges which in turn causes the formation of wear products. Interestingly, a small fraction of these will oxidise, however, the majority will remain as metallic particles; thus a good metallic contact between the conducting surfaces is established. This effect is manifested by a decrease in contact

resistance. This process is cyclic, and the temporary rupture of the insulating layer and appearance of localized metallic contacts and conductive paths is soon

eradicated. Oxidation due to high current density will quickly eliminate the conducting paths resulting in a rapid rise in contact resistance. This effect is associated with low contact force only.

Hence contact force becomes important and subsequent decline within the components of the relay, such as the electromagnetic field in the coil and wear to associated mechanical components will mean force is also reduced further.

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Other effects that can increase fretting are the frequency. The rate of fretting is shown to be dependent on the oscillation frequency. Oxidation will occur at lower frequency as the process is time dependant, hence, a decrease in the number of conduction paths and again an increase in contact resistance.

Environmental effects can cause changes in chemical reaction rates. Moisture can cause changes which affect the physical characteristics such as collection of debris and the surface mechanical properties of the contact material may be attributed to relative humidity. Related to this is the effect of temperature on the fretting process. The rate at which a chemical reaction takes place, such as oxidation and corrosion and the subsequent damage resulting, is in part, temperature governed.

4.6.2 Effect of Current

The effect of current across the contact interface has already been discussed in the literature review. Changes attributed to surface film formation and contact face asperities can cause localised heating and structural changes, resulting in changes in the contact resistance. Subsequently, the effect of current on the contact

resistance behaviour of tin-plated copper contacts under fretting corrosion conditions was investigated in detail by (Lee and Mamrick, 1988).

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Figure 4.26 a&b. Lee, A., Mao, A., and Mamrick, M. S., Proceedings of 34th IEEE Holm Conference on Electrical Contacts, San Francisco, 87–91, 1988.

The results shown in Figure 4.26 a&b depict the contact resistance (a) and contact voltage (b) as a function of fretting cycles and electrical current. The conclusions drawn are that contact resistance behaviour can be explained by the presence of resistance plateaus that fluctuate delaying a further resistance rise. As the applied voltage and current are increased, it was found that the resistance plateaus become lower and longer.

To obtain a physical understanding of these resistant plateaus, the current through the contact constriction as discussed in (Chapter 2) causes the contact spot to thermally runaway until the melting of the material occurs (this constitutes the first plateau). Further damage to the contact in the form of corrosion gives higher resistance and more heating, the temperature can rise further to the melting, sublimation, and decomposition of the oxides, and even up to the vaporization of the material, collectively forming the second contact resistance or voltage plateau.