Research and application of the hottest superhard

Research and application of superhard thin film material coating four

4 cubic boron nitride (c-BN) thin film cubic boron nitride (CBN) is second only to diamond in hardness, but has higher thermal and chemical stability than diamond. Therefore, as a cutting tool material, it is superior to diamond. Diamond cannot be used to process steel materials, while CBN tools are competent to process almost any material, including steel materials. In addition, cubic boron nitride has a large band gap, high thermal conductivity, good insulation, and is easy to be doped into n-type and p-type semiconductors. It is the same as diamond. Therefore, the static friction between the force measuring cylinder and the force measuring piston has changed into dynamic friction, which has a wide range of good optical transmission performance from ultraviolet to infrared. Therefore, it also has a good application prospect in optical applications and semiconductor applications

there are many methods to prepare cubic boron nitride films. Physical and chemical vapor deposition methods such as sputtering deposition, ion beam assisted deposition, ion plating, plasma assisted CV to avoid rail deformation D can be used to prepare cubic boron nitride films. A common feature of these preparation methods is that they cannot be separated from the bombardment of ions. Most of the cubic boron nitride prepared has amorphous structure, with very large stress and poor adhesion to the substrate. The preparation of c-BN is similar to that of DLC in many aspects. BN has two crystal forms, one is cubic e-BN (similar to diamond structure and properties), the other is hexagonal h-BN (similar to graphite structure and properties). The current research results tend to believe that ion bombardment is conducive to the deposition of c-BN, which is similar to the need for ion bombardment in the preparation of DLC. The research results of the preparation of crystal c-BN with preferred orientation by electron assisted hot wire CVD have been reported in the literature, but other researchers have not been able to repeat this result well. Foreign countries have also reported the preparation of c-BN by DC arc plasma jet method, but the obtained film is a composite film of c-BN and h-BN, not a pure phase c-BN film

c-bn thin films are still in the research stage. At present, the focus of research is to find technical ways to control and reduce the internal stress of c-BN Thin Films and methods to prepare high-quality crystalline c-BN Thin Films

5 carbon nitrogen film

since Cohen et al. Predicted in the early 1990s that there may be hardness that may exceed diamond in C-N system β- After the C 3N4 phase, there was a wave of synthesis in the world immediately β- C3N4 research frenzy. Researchers at home and abroad are scrambling to be the first to synthesize pure phase β- C3N4 crystal or crystalline film. However, after more than ten years of efforts, no one has achieved the above goals. In most cases, the obtained CNx film is amorphous, and the N/C ratio in the film is related to the preparation method and specific process of the film. Although the hardness of diamond predicted by Cohen et al β- C3N4 crystal, but the existing research shows that the hardness of CNx film can reach 15gpa-50gpa, which can be compared with DLC. At the same time, CNx films have very unique friction and wear characteristics. In air, the friction coefficient of CNx film is o.2-o.4, but in N2, CO2 and vacuum, the friction coefficient is o.01-o.1. The friction coefficient in N2 atmosphere is the smallest, which is 0.01. Even blowing nitrogen into the experimental area in atmospheric environment can reduce the friction coefficient to 0.017. Therefore, CNx films are expected to be applied in the field of friction and wear. besides. CNx thin films may also have a good application prospect in optics, thermology and electroacupuncture, which have strict production requirements for power train electronics

CNx films can be prepared by reactive magnetron sputtering, ion beam deposition, double ion beam sputtering, laser beam deposition (PLD), plasma assisted CVD and ion beam implantation if the number of scaffolds is sufficient. In most cases, the prepared films are amorphous, and the maximum n/C ratio is 45%, that is, CNx is always carbon rich. Similar to the situation of c-BN, the preparation of CNx films requires ion bombardment, and there is a great internal stress in the films. It is necessary to further reduce the internal stress of the films and improve the adhesion of the films in order to obtain practical applications. As for whether we can really obtain b-c3n4 with hardness higher than that of diamond, no conclusion can be made at present

6 nanocomposite films and nanocomposite multilayer films

the hardness of nanocomposite films obtained by alternating deposition of nano thickness films is related to the thickness (modulation period) of each layer of film, which may be higher than the hardness of each constituent film. For example, the hardness of tin is 2L GPA, and the hardness of NbN is only 14gpa, but the hardness of tin/NbN nanocomposite multilayer is 5lgpa. The hardness of tiyn/VN nanocomposite multilayers is as high as 78gpa, which is close to that of diamond. Recently, the hardness of nano grain composite tin/SiNx film material has reached a record 105gpa, which can be said to completely reach the hardness of diamond. This amazing result has been repeatedly verified by different researchers and different research groups in the same research group and proved to be correct. This may be the first time to obtain superhard film materials with hardness comparable to diamond. Its significance is obvious

there is no completely convincing conclusion about the explanation of why diamond hardness can be obtained. Some people believe that in the case of nano multilayer composite films, the interface of nano multilayer films effectively prevents the slip of dislocations, making it difficult for cracks to expand, resulting in an abnormal increase in hardness. In the case of nanocrystalline composite films, the strengthening effect may be caused by the strain field around the nanocrystalline grain boundary of TiN films and highly dispersed nano coherent SiNx particles, resulting in a sharp increase in hardness

whether the above theoretical explanation is completely reasonable or not, the application prospect of this nanocomposite multilayer film and nanocrystalline composite film is very clear. Nanocomposite multilayers have high hardness and low friction coefficient, so they are ideal tool (mold) coating materials. Their emergence poses a severe challenge to the status of diamond as the hardest material. At the same time, it also has very obvious advantages in economy, so it has a very good market prospect. However, because there are still some technical problems that have not been solved, it has not been widely used in industry for the time being

it is conceivable that with the further maturity of technology, this kind of material may be rapidly industrialized. Although nano multilayers and nano grain composite films have posed a severe challenge to the highest hardness of diamond, as far as I can see, I don't think they can completely replace diamond. Diamond film is a multifunctional material with a wide range of applications, and its applications are not limited to superhard materials. Moreover, diamond films can be made into self-supporting films with a great thickness (more than 2mm), which is impossible for nanocomposite multilayers and nanocomposites

(Lu Fanxiu, Beijing University of science and Technology)

China Vacuum