1. general physical properties. The contact material should have suitable hardness, smaller hardness can increase contact area under certain contact pressure, reduce contact resistance, reduce the contact heating and welding tendencies in static contact, and reduce the dynamic contact in the closing process. Higher hardness can reduce weld area and improve mechanical wear resistance.
The contact material should have the appropriate elastic modulus. The higher elastic modulus is easy to reach the limit of plastic deformation, so the surface film is easy to destroy and can reduce the resistance of the surface film. The lower elastic deformation can increase the contact area of the elastic deformation.
2. electrical performance. Contact materials should have higher electrical conductivity to reduce contact resistance, low two emission and light emission to reduce arc current and arcing time.
3. thermal physical properties. High thermal conductivity, so that the heat generated by the arc or Joule heat source can be transmitted to the contact base as quickly as possible. High specific heat capacity, high melting, gasification and decomposition latent heat. High ignition point and boiling point to reduce the trend of arc ignition. Low vapor pressure limits the density of metal vapor in the arc.
Chemical properties
Contacts should have high chemical stability, that is, the ability of corrosion resistant gases to material loss, and even if they produce surface films, their volatiles should be high.
Electrical contact performance
The electrical contact properties of contacts are the comprehensive embodiment of physical and chemical properties, and all kinds of characteristics are interacted with each other. Generally speaking, the electrical contact properties of contacts include:
1. surface condition and contact resistance. The contact resistance is greatly influenced by the surface condition, and the surface condition is closely related to the arc erosion process of the contact. Therefore, the erosion of the contact is basically uniform, in order to ensure the smooth surface of the contact, and the contact resistance is low and stable.
2. resistance to arc resistance and resistance to material transfer. The contact material has high melting point, boiling point, specific heat capacity and melting, vaporization heat and high thermal conductivity, although it is beneficial to improve the arc erosion resistance of the contact, but the above physical parameters can only improve the quenching condition of the arc between the contacts, or consume the heat flow of the arc input contact in large amount, however, once the contact surface is fused. The corrosion resistance of the contact is guaranteed only by the metallurgy characteristic of the contact material at high temperature, which involves the wettability of liquid silver to the contact surface of the contact. The viscosity of molten pool and the thermal stability of second and third components.
The transfer of contact materials is also closely related to the conventional thermal physical parameters of materials. However, these parameters can only reduce the material transfer caused by the fracture of the liquid metal bridge, and the material transfer caused by the arc action is related to the asymmetry of various physical parameters of the two pair contact and the asymmetry of the arc characteristics.
3. resistance to welding. The welding resistance of contact materials includes two aspects. One is to reduce the tendency of fusion welding as far as possible. From the angle of contact materials, it is mainly to improve its thermal physical properties. The two is to reduce the fusion welding force after the welding of molten metal together. The welding force mainly depends on the welding cross section and the tensile degree of the contact material. Obviously, the contact area and the conductive area can be increased to reduce the tendency of the static fusion welding to increase, but the welding force will increase once the fusion welding occurs. Therefore, in order to reduce the fusion welding force, or to improve the welding resistance of contact materials, the components with silver affinity are often added to contact materials.
Characteristic of arc
1. the ideal contact material should have good arc movement characteristics to reduce the arc's heat flow to the contacts which are too concentrated.
2. should also have a higher minimum arc voltage and minimum arc current: the minimum arc voltage depends largely on the work function of the electrical contact material and the ionization voltage of its vapor. The minimum arcing current is related to the binding energy required by the electrode material when the scattering atoms are released from the contact surface.
The arc of 3. contacts can have two forms of metal vapor state and gas state. Different forms of arc have different mechanism of action on the electrode. Contact material should make the arc from metal vapor transition from metal vapor to gas state as soon as possible.
In addition to the above requirements, contact materials should be processed as easily as possible and have a high performance price ratio.
In view of this, the requirements for electrical contact materials are extensive and demanding, and many requirements still have contradictions. The metal with high conductivity has lower hardness and melting point and boiling point. Therefore, it is impossible to obtain the contact material with high electrical conductivity and high hardness. Similarly, the atomic polymerization force in the metal crystal lattice determines the hardness, modulus of elasticity, melting point and boiling point of the material, which are always basically unified. In order to improve the fusion welds, it requires high thermal and physical parameters such as melting point and boiling point. But at the same time, to reduce contact resistance, the hardness is low. So there is no electrical contact material to meet the demand of Qian He. The development, production and selection of contact materials can only meet the most critical requirements according to the specific conditions of use.