Contact lubricant working principle and actual use effect

 

The electrical contact of an electrical switch is one of the important parts of the electrical contact system. Through the analysis of the contact failure factors and failure mechanism, the article discusses in detail the improvement effect and reliability of the electrical contact point by the new technology of contact protection and lubrication. The mechanism is improved, and the principle of selection and use of contact lubricants is proposed at the same time.

 

Factors affecting the current conductance between the contact surfaces: Before understanding the mechanism of contact lubricants on improving the electrical performance of the contacts, it is necessary to understand the following main factors that affect the current conduction between the contact surfaces.

 

1. Contamination and corrosion of the contact itself

Normally, the contacts are working in a sealed or unsealed environment. When the contacts work in a non-sealed environment, the exposed metal surface will inevitably be polluted to varying degrees because the air contains pollutants of different nature. Even if the contacts work in a sealed environment, due to the characteristics of the working medium, they will also decompose and release organic gases with different contents and properties, which will cause different degrees of pollution on the contact surface.

 

The direct harm caused by the contamination of the contact surface is that the surface film formed by the contaminants significantly increases the contact resistance of the contact surface. In order to make the electrical contact system that undertakes current-carrying work work properly, this protective film must be effectively removed.

 

2. Arc corrosion between contacts

 

In the electrical contact system, when the contact is closed or the load is opened, the arc discharge phenomenon usually occurs to the switching device with a current intensity of about 1A or more. The arc discharge will cause instantaneous local high temperature on the contact surface, which will cause the melting, vaporization and deformation of the metal surface. At the same time, it will also cause impurities in the air or the gas inside the electrical contact system to contaminate the metal surface and form an organic layer. Or inorganic membrane. The arc discharge phenomenon will cause the contact resistance of the contact surface to increase.

Arc discharge will also cause an electrochemical reaction in the air between the contact surfaces and produce nitric acid. First, a corrosive film is formed on the contact surface, and then a layer of non-conductive metal compound is formed.

 

The ionization of the air between the contact surfaces under the action of arc discharge and the high temperature after ionization are accompanied by the phenomenon of "migration" of metal ions between the contact surfaces. This kind of electrochemical phenomenon is particularly prominent in the process of completing the current carrying process of the DC circuit. The main surface of this "migration" phenomenon caused by arc discharge is the "point" and "crater" arc discharge corrosion damage caused by the contact.

 

It should be particularly pointed out here that in addition to the above-mentioned contact current carrying density which is the cause of arc discharge, the surface finish of the contact is also the cause of arc discharge. Regardless of whether it is a fixed or non-fixed contact, the arc discharge caused by the roughness between the two contact surfaces will also cause the contact surface to suffer premature arc discharge corrosion.

 

In arc discharge corrosion, the damage of resistive load to the contacts is the least. Because the resistive load is relatively fixed, the current is not affected by the pulsation when the contacts are carrying current. Inductive loads (such as motors, etc.) generate starting current nearly ten times the normal working current when starting, which makes the contacts more harmful to arc discharge corrosion when performing current-carrying work with sudden changes in current density. . The rapid increase in current density will also increase the heat generation of the contacts, and also make arc discharge corrosion and fusion on the surface of the contacts more serious. For capacitive loads (such as tungsten filament lamps, etc.), it has a starting current similar to that of inductive loads, and how to protect the contacts must be taken seriously.

 

As far as the above discussion is concerned, any electrical contact system that carries current work, if there is the possibility of arc discharge corrosion in its working environment, special attention must be paid when designing the electrical contact system, and necessary measures must be taken to make arc discharge Corrosion is kept to a minimum.

 

3. Mechanical wear and corrosion between contact surfaces

 

As we all know, no matter whether it is chemical coating or mechanical processing, it is impossible to make a contact with an absolutely smooth surface. Even if the surface is very smooth when observed with the naked eye objectively, there are still many tiny irregularities on the surface of the contact in the microscopic view. . That is to say, the actual contact area of ​​the contact is always smaller than the theoretical contact area, and it is precisely because the actual contact area is reduced that the operating temperature of the contact surface rises and the current-carrying efficiency decreases, which is the main reason for the contact burnout. one. This phenomenon is particularly obvious in power switching systems.

 

The electrical contact system of the mechanical structure will inevitably be accompanied by contact wear in the process of completing the conversion of the current-carrying signal. Under normal circumstances, wear is a physical phenomenon that is closely related to the smoothness of the contacted surface. The wear must be produced under a certain frictional resistance. For mobile or sliding contacts, due to their own operating characteristics, the lubrication and wear of the contacts are fully considered in the design of electronic devices. For the contacts used in relays, contactors, etc., the lubrication and wear of the contacts are often neglected in the design, manufacturing and use process, while the material and current carrying capacity of the contacts are poured in the design and manufacturing process. Main energy. Therefore, the cleaning and lubrication of the contacts in the whole process of design, manufacture and use of electronic devices must arouse special attention.

 

The wear degree of contacts in electronic devices will be mainly affected by the following five factors:

 

(1). The way the contact wears during work mainly depends on the shape of the contact itself, and different contact shapes affect the amount of contact wear to varying degrees.

 

(2). The smoothness of the contact surface is a direct factor that affects wear, because the uneven surface of the contact not only causes wear, but also the excess particles generated during wear and corrosion will gather in the depressions on the contact surface, giving the contact surface damage Cleanliness, accelerated wear of contacts and current carrying capacity bring certain hidden dangers

 

(3). Different contact materials have different wear characteristics. Because different metal materials have a certain degree of difference in wear resistance, rust resistance and oxidation resistance, the material of the contact not only affects the performance and cost of electronic devices, but also directly affects the service life of electronic devices. When two different metal materials contact each other as contacts, the problems of lubrication and wear on the surface become more important. This is because the asymmetry of the hardness of the two mutually contacting surfaces causes excessive wear on the high-hardness contact surface and the low-hardness contact surface, which causes premature failure of electrical components and affects the reliability of the control system. For the contact surface processed by electroplating, since the surface coating is prone to peeling (the electroplating layer is more likely to peel off than the electroless plating layer), the peeling of the surface coating will cause the surface contact resistance and excess to change suddenly, thereby intensifying the surface wear and tear. The temperature rises until the electrical components fail.

 

(4). There is a certain contact resistance on any contact surface, and a certain contact resistance must consume a certain amount of power, which will cause a certain temperature rise on the contact surface. When the contact is working in a changing temperature field, due to the physical properties of thermal expansion and contraction of the metal material itself, thermally induced fretting wear occurs on the surface of the contact. This thermal wear phenomenon will increase with the increase in the thermal expansion and contraction characteristics of the mating contact materials, and will also cause the relative movement between the two contacts to increase. Based on the same thermal wear mechanism, changes in temperature can cause changes in the degree of contact tightness, which depends on the difference in thermal expansion coefficients of the two materials. When the contact becomes loose due to temperature changes, the contact resistance increases and heat is increased. As the contact gap increases, the contact surface is more likely to be contaminated. When the contact becomes tight, any relative displacement that occurs between the surfaces of each contact will increase surface wear.

 

(5). Contact pressure (contact electric contact and static contact pressure): Normally, mechanical wear is proportional to contact pressure. In an electrical contact system, in order to achieve the purpose of reducing contact resistance and improving contact reliability, one of the important means is to increase Contact pressure, therefore, the existence of wear between the contacts will be inevitable.

 

4. Bounce of the contact itself

 

For any kind of electrical contact system with mechanical structure, due to its own mechanical characteristics, the contact will inevitably be accompanied by the occurrence of contact bounce in the work of completing the load conversion. In practical applications, the electrical contact system works in frequent startup and shutdown states, which not only increases the frequency of contact bounce, but also accelerates the wear of the contact. A more serious hazard is that each contact bounce may cause arc discharge. As the surface of the contact continues to approach and close, the intensity of the discharge becomes larger and longer, and the duration becomes longer and longer.

 

When the electrical contact system works in an inductive or capacitive load environment, the damage caused by contact bounce will be particularly serious. Contact bounce will not only cause repeated heating of the contact surface and be damaged by high-energy arcs, but also increase the possibility of corrosion and fusion.

 

Other hazards caused by contact bounce in addition to its own electrical contact system are often ignored. However, with the continuous development of science and technology, the requirements for the reliability of automatic control systems are becoming more and more stringent, so contact bounce The damage to the control circuit caused by the pulsating current impact has attracted more attention from engineers. On the other hand, in precision electronic measuring instruments, because the contact bounce makes the control circuit unable to cleanly open and close the detected object, this kind of bounce often leads to false readings or wrong measurement results. .

 

5. Micro-vibration in the actual use environment of the contact

 

The so-called fretting wear refers to a contact movement with a small amplitude, and the amplitude of fretting is generally in the range of tens of microns to 100 microns. The main causes of micro-vibration are as follows: temperature changes; electromagnetic induction vibration; sound wave vibration. In this regard, European and American experts have conducted more in-depth research.

 

The fretting phenomenon will inevitably produce metal "migration" and wear. When the contact surface is further corroded due to increased wear or ground debris, this process is called friction corrosion. For contact surfaces of different materials, the result of fretting wear and the process of fretting wear are different. For example, the common contact material-gold, due to its relatively stable chemical properties, the metal of the gold-plated contact surface generally only suffers fretting wear and aggravated chip wear. However, the friction and corrosion of the base material caused by the pores of the gold plating layer should be paid attention to. Under normal circumstances, in addition to gold, other contact materials such as copper, tin, silver or such alloys, friction corrosion will occur.

 

When the mating contacts use metal materials with different hardnesses, the friction and corrosion phenomenon becomes more serious due to the unidirectional nature of the metal "migration" phenomenon. For platinum group elements and their alloys, the latest research shows that due to the catalytic effect of platinum and platinum alloys, the contact surface will be covered with a relatively thick, difficult-to-remove organic polymer, which makes the contact surface contact The resistance is greatly increased.