Metallized ceramic needles

Metal ceramics have the advantages of both metal and ceramics, it is small density, high hardness, wear resistance, good thermal conductivity, will not be brittle because of sudden cold or heat. In addition, in the metal surface coating a layer of airtight, high melting point, heat transfer performance is very poor ceramic coating, but also can prevent the metal or alloy oxidation or corrosion at high temperatures. Metal ceramics have both the toughness of metal, high thermal conductivity and good thermal stability, but also has the ceramic properties of high temperature, corrosion and wear resistance. Metal ceramics are widely used in rockets, missiles, supersonic aircraft shells, combustion chamber flame nozzle and other places.

It is a mechanical mixture of two-phase metal, each phase of the metal phase to retain the original physical properties. Two phases of metal in a phase for the refractory phase, its high hardness, high melting point, at high temperatures and impact does not deform under the action of the arc does not melt, so this phase of the metal in the material in the role of the skeleton. This type of metal has tungsten, molybdenum, metal oxides, etc.. Another phase of metal for the current-carrying phase, it mainly plays a role in electrical and thermal conductivity. This type of metal silver, copper and so on. Current-carrying phase metal melting point are relatively low, under the action of the arc high temperature melted into liquid, retained in the refractory phase of the metal skeleton composed of the void, preventing a large number of molten metal splash, so that the contact electric wear is greatly reduced. The following describes several commonly used metal ceramic materials:

Silver - cadmium oxide This material has good resistance to electrical wear, anti-melt welding and low and stable contact resistance. It is widely used in medium power appliances.

The reasons for these excellent properties of this material are:

1 under the action of the arc cadmium oxide decomposition, sublimation from the solid state into a gaseous state (decomposition temperature of about 900 ° C), resulting in violent evaporation, play a role in blowing the arc, and clean the contact surface;

2 cadmium oxide decomposition absorbs a large amount of heat, is conducive to the cooling of the arc and extinguished;

3 diffuse cadmium oxide particles can increase the viscosity of the molten material, reduce the splash loss of metal;

4 cadmium vapor part of the re-oxygen combination with the formation of solid cadmium oxide, deposited on the surface of the contact, the organization of the contact welding.

Cadmium oxide content of 12% to 15% can get the best performance. If the silver - cadmium oxide in the addition of some trace elements, such as silicon, aluminum, calcium, etc. can further refine the grain, improve resistance to electrical wear performance.

Silver - tungsten This material has the respective advantages of silver and tungsten. With the increase of tungsten content, resistance to arc wear and anti-melt welding performance, but the conductivity decreases. Low-voltage switches often use materials containing 30% to 40% tungsten, and high-voltage switches use materials containing 60% to 80% tungsten.

Silver - the disadvantage of tungsten is that the contact resistance increases with the increase in the number of contacts opened and closed, the serious case can reach more than ten times the initial value, because in the breaking process, the contact surface will produce tungsten trioxide (WO3) or silver tungstate (Ag2WO4) film, this film does not conduct electricity, so that the contact resistance increases dramatically.

Copper - tungsten The performance of this material is similar to that of silver - tungsten, but it is easier to oxidize than silver - tungsten, forming copper tungstate (CuWO4) film, which increases the contact resistance dramatically. It is not suitable for air switch contacts, but can be used for oil switch contacts.

Silver - graphite It has good electrical conductivity, small contact resistance, good resistance to welding performance, the disadvantage is that the electrical wear and tear. General graphite content does not exceed 5%.

Silver - iron has good electrical conductivity, thermal conductivity, resistance to electrical wear and other properties, used in medium and small current contactors than the pure silver contacts in the electric life exponentially increased. The main disadvantage is easy to rust spots in the atmosphere.