Up to now, there is no generally accepted satisfactory explanation for the meaning of superhard materials. In 1981, the International Conference on Hard Material Science held that all materials with hardness greater than 1000HV could be called hard materials, which naturally included diamond and cubic boron carbide. Later, this definition was supplemented, and it was considered that materials capable of processing such substances as cemented carbide (hardness 1600-1800HV), corundum (-2000HV), silicon carbide (-2200HV) were called superhard materials.
At present, because diamond and cubic boron nitride materials have very high hardness, so they are collectively called superhard materials, which have high hardness, good wear resistance and heat conductivity, low thermal expansion coefficient and other excellent properties. Classification of superhard materials
It can be divided into single crystal superhard material and polycrystalline superhard material (also known as “composite superhard material”) and diamond film.
The main differences between single crystal superhard materials and polycrystalline superhard materials are as follows: single crystal diamond/cubic boron nitride materials are characterized by higher hardness, better heat resistance, but smaller size, which are mostly used for cutting tools such as saw blades; polycrystalline diamond/cubic boron nitride is mainly used for single crystal superhard materials such as diamond and cubic boron nitride powder. Polycrystalline composites are prepared by sintering under ultra-high pressure and high temperature with metal or non-metal binders as raw materials. Its hardness and heat resistance are slightly lower than those of single crystal materials, but because polycrystalline superhard materials are compact diamond bodies with compact internal structure, they can increase cutting area of tools, and overcome the disadvantage that single crystal superhard materials easily fall off from the surface of saw blades due to small bonding area, so they have higher wear resistance.
Diamond films are superhard films prepared on non-diamond substrates by chemical vapor deposition (CVD) or other methods. It can be used not only for making various diamond cutting tools, but also for making sound sensors, loudspeaker vibration film, infrared windows, X-ray detection windows, etc. as functional materials. It has a wide range of applications. Since the early 1970s, the trial production of diamond films has been carried out in the world, and the upsurge of research and development of diamond films has been set off rapidly. Since the mid-1980s, this research has been included in the “863 Plan” of China. Now, 80 mm and 2 mm thick diamond films have been prepared, and many achievements have been made in the application research. However, the research is still in the stage of development on the whole and has not reached the stage of industrial application. It is predicted that diamond film will be the main material of diamond industry in the 21st century. Scientists from all over the world are making unremitting efforts to industrialize diamond film.