A. Definition of cermets
Cermet is a structural material composed of ceramic hard phase and metal or alloy bonding phase. Cermets, from the English word Cermets, is composed of Ceramic and Metal. Ceramics not only maintain the characteristics of high strength, high hardness, wear resistance, high temperature resistance, oxidation resistance and chemical stability, but also have good metal toughness and plasticity. Since there is no clear boundary between the two terms of "cermet" and "cemented carbide", it is difficult to divide the boundary between specific materials. From the perspective of the composition of materials, "cemented carbide" should be classified as "cermet", and Ie. Campbell classified "cemented carbide" as "cermet".
B. The history of cermets
WC-Co based ceramics, as the earliest studied cermets, have been used in many fields due to their high hardness (HRC80~92) and extremely high compressive strength of 6000MPa (600kg.N/mm2). However, due to the shortage of W and CO resources, the research and development of tungsten-free cermets have gone through three generations. The first generation is during World War II, Germany produced cermets with Ni bonded to TiC. In the second generation, in the 1960s, Ford Motor Company of the United States added Mo to Ni binder to improve the wettability of TiC and other carbides, thus improving the toughness of the material. The third generation of cermet introduced nitride into the hard phase of the alloy and changed the single phase to composite phase. The bonding phase was improved by adding CO phase and other elements. In recent years, another new direction in the development of cermets is boride based ceramics. Boride based ceramics have become the most promising cermets because of their high hardness, melting point, excellent electrical conductivity and corrosion resistance.
C.The composition of cermets
In order to make ceramics resistant to high temperatures and resistant to breakage, they are made by adding metal powder to the clay in which they are made. Metal-based ceramics are made by adding fine oxide powder in metal matrix, also known as dispersion reinforcing material. There are mainly sintered aluminum (aluminum-alumina), sintered beryllium (beryllium-beryllium oxide), TD nickel (nickel - thorium oxide), etc. A composite material consisting of one or more ceramic phases and metallic phases or alloys. The broad cermet also includes refractory compound alloy, hard alloy, metal-bonded diamond tool materials. Ceramic phase in cermets is oxide or refractory compound with high melting point and hardness, and the metal phase is mainly transition elements (iron, cobalt, nickel, chromium, tungsten, molybdenum, etc.) and their alloys.
D. Classification of cermets
According to the percentage of each component phase, cermets can be divided into two categories: ceramic matrix and metal matrix.
1. Ceramic foundation is mainly composed of:
① Oxide is a ceramic. With alumina, zirconia, magnesium oxide, beryllium oxide as the matrix, and metal tungsten, chromium or cobalt composite, has the characteristics of high temperature resistance, chemical corrosion resistance, good thermal conductivity, high mechanical strength, can be used as missile nozzle liner, metal melting crucible and metal cutting tools.
② Carbide is a ceramic. With titanium carbide, silicon carbide, tungsten carbide as the matrix, and metal cobalt, nickel, chromium, tungsten, molybdenum and other metals composite, with high hardness, high wear resistance, high temperature resistance and other characteristics, used in the manufacture of cutting tools, high temperature bearings, sealing rings, wire picking die sleeves and turbine blades.
③ nitride foundation belongs to ceramics. With titanium nitride, boron nitride, silicon nitride and tantalum nitride as the matrix, it has super rigidity, thermal vibration resistance and good creep properties at high temperature, and has few applications.
④ Boride is a kind of ceramic. With titanium boride, tantalum boride, vanadium boride, chromium boride, zirconium boride, tungsten boride, molybdenum boride, niobium boride, hafnium boride as the matrix, and some metal materials composite.
(5) Silicide is a ceramic. With manganese silicide, iron silicide, cobalt silicide, nickel silicide, titanium silicide, zirconium silicide, niobium silicide, vanadium silicide, niobium silicide, tantalum silicide, molybdenum silicide, tungsten silicide, barium silicide as the matrix, and part or trace metal materials composite. Molybdenum silicide cermets are widely used in industry.
2. Metal-based ceramics are made by adding fine oxide powder in the metal matrix, also known as dispersion reinforcing material. There are mainly sintered aluminum (aluminum-alumina), sintered beryllium (beryllium-beryllium oxide), TD nickel (nickel - thorium oxide), etc. The content of alumina in sintered aluminum is about 5% ~ 15%. Compared with alloy aluminum, it has high high temperature strength, small density, easy processing, corrosion resistance and good thermal conductivity. It is often used in the manufacture of structural parts of aircraft and missiles, engine pistons, chemical machinery parts, etc.
E.The properties of cermets
Ceramics have the advantages of both metal and ceramic, it is small density, high hardness, wear resistance, good thermal conductivity, not because of sudden cooling or sudden heat and brittle crack. In addition, the metal surface coated with a layer of good air tightness, high melting point, heat transfer performance is very poor ceramic coating, also can prevent metal or alloy oxidation or corrosion under high temperature. Cermets have not only the toughness of metal, high thermal conductivity and good thermal stability, but also the high temperature resistance, corrosion resistance and wear resistance of ceramics.
F. The use of cermets
Ceramics are widely used in rocket, missile, supersonic aircraft shell, combustion chamber flame nozzle and other places.
Application of electrical contact head
In order to meet all kinds of complicated and even contradictory requirements of contact materials, cermet materials have been developed.
It is a mechanical mixture of two phase metals, each phase of the metal retains the original physical properties. Two-phase metal in the refractory phase, its high hardness, high melting point, under the effect of high temperature and impact does not deform, does not melt under the effect of arc, so this phase metal in the material of its skeleton role. This kind of metal has tungsten, molybdenum, metal oxide and so on. The other phase metal is the current carrying phase, it mainly plays the role of electrical and thermal conductivity. These metals silver, copper, etc. The metal of current carrier phase has a low melting point, and melts into liquid under the effect of high temperature of arc, which remains in the gap formed by the refractory metal skeleton, which prevents a large number of splashing of molten metal and greatly reduces the electric wear of the contact. Let's introduce several commonly used cermet materials:
(1) Silver - Cadmium Oxide material has good resistance to electrical wear, welding resistance and low contact resistance and stable characteristics. It is widely used in medium power electrical appliances.
This material has these excellent properties for the following reasons:
1) Cd oxide decomposes under the action of arc and sublimates from solid state to gaseous state (decomposition temperature is about 900℃), resulting in violent evaporation, which plays the role of arc blowing and cleats the contact surface;
2) Cadmium oxide absorbs a lot of heat when decomposed, which is beneficial to the cooling and extinguishing of arc;
3) Dispersed Cd oxide particles can increase the viscosity of the molten material and reduce the metal splash loss;
4) Part of the cadmium vapor is recombined with oxygen to form solid cadmium oxide, which is deposited on the surface of the contact to organize the welding of the contact. The best performance can be obtained when the cadmium oxide content is 12%~15%. If some trace elements, such as silicon, aluminum and calcium, are added to the Ag-Cd oxide, the grain can be further refined and the resistance to electric wear can be improved.
(2) The silver - tungsten material has the advantages of silver and tungsten. With the increase of tungsten content, the resistance to arc wear and welding is improved, but the electrical conductivity is decreased. Low-voltage switches are commonly made of materials containing 30% ~ 40% tungsten, and high-voltage switches are made of materials containing 60% ~ 80% tungsten. The disadvantage of silver-tungsten is that the contact resistance increases with the increase of the opening and closing times of the contact, which can reach more than ten times of the initial value in severe cases. Because in the splitting process, the surface of the contact will produce tungsten trioxide (WO3) or silver tungstate (Ag2WO4) film, which is non-conductive, so that the contact resistance increases dramatically.
(3) Copper-tungsten is a material with similar properties to Ag-tungsten, but it is easier to oxidize than Ag-tungsten to form CuWO4 film, which makes the contact resistance increase dramatically. It is not suitable for air switch contact, but can be used as oil switch contact.
(4) silver - graphite has good electrical conductivity, small contact resistance, good welding resistance, and the disadvantage is that electrical wear is large. Generally the graphite content is not more than 5%.
(5) Silver - iron has good electrical conductivity, thermal conductivity, resistance to electrical wear and other properties, used in medium and small current contactors than pure silver contact electric life doubled. The main disadvantage is in the atmosphere easy to grow rust spots.
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