The cermet is a composite material composed of ceramic hard bonded with metal or alloy. Cermets not only maintain some excellent properties of ceramics, such as high hardness, wear resistance, high-temperature resistance, oxidation resistance and chemical stability, but also have good toughness and plasticity of metals.
WC-Co, the earliest cermets, which have been applied in many fields due to its high hardness. However, the research and development of tungsten-free cermets have gone through three generations due to the shortage of W and Co resources. The first generation was World War II, when Germany produced cermets using Ni-bonded TiC; the second generation was in the 1960s, when Ford Motor Company added Mo to Ni bonding phase to improve the wettability of TiC and other carbides, thus improving the toughness of materials; the third generation of cermets introduced nitride into the hard phase of the alloy and changed the single phase into the composite phase, and the bonding phase is also improved by adding Co phase and other elements. Now, another new direction in the development of cermets is the boride-base cermet. Because the boride ceramics have high hardness and melting point, excellent conductivity and corrosion resistance, the boride-base cermet are the most promising cermets.
The cermet with metal matrix is made by adding fine oxide powder to metal matrix, also known as dispersion reinforcing material, which mainly includes sintered aluminum (alumina), sintered beryllium (beryllium oxide), TD nickel (thorium ni-oxide), etc. In a broad sense, cermets also include refractory compound alloys, hard alloys, and metal-bonded diamond tool materials. Ceramic phase in cermets is an oxide or refractory compound with high melting point and high hardness, and the metallic phase is mainly the transition element (iron, cobalt, nickel, chromium, tungsten, molybdenum, etc.) and its alloy.
The cermet has the advantages of both metal and ceramic, it has low density, high hardness, good wear-resisting and heat conductivity, and it will not crack easily because of sudden cold or heat. Besides that, coating the metal surface with the ceramic coating with good air tightness, high melting point, and poor heat transfer performance can also prevent the metal or alloy from oxidizing or corroding at high temperature. Cermets possess not only the toughness, high thermal conductivity and good thermal stability of metals but also the high temperature, corrosion and wear resistance of ceramics, which are widely used in rockets, missiles, supersonic aircraft shells, combustion chamber flame vents and other aspects.
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