According to the Koehler epitaxial heterostructure theory, when depositing two multilayer films of different elastic modulus metals M(1) and M(2) (EM(1)<EM(2)>, the thickness of a single layer film When it reaches a small order of magnitude, it can effectively block the proliferation of the dislocation source in the layer; when subjected to external stress, the dislocation in M(1) will move to the M(1)/M(2) interface, M( The elastic strain formed in 2) produces a repulsive force that hinders the passage of dislocations through the interface, so the hardness of the multilayer film is much higher than the hardness calculated by the mixing rule. Based on the Koehler theory, the research of nano-multilayer films has gradually It has become a development hotspot in the coating industry. Its design idea is to control the one-dimensional periodic structure of the multilayer film, effectively adjust the dislocations, defects and movements in the film, thereby obtaining a film with high hardness, high modulus and high temperature performance.
According to its composition, nano-multilayer superhard membranes can be divided into the following categories:
Nitride/nitride: TiN/VN, 56 GPa; TiN/VNbN, 41 GPa; TiN/NbN, 51 GPa; TiN/SiN, 80-105 GPa
Nitride/Carbide: TiN/CNX, 45-55GPa; ZrN/CNX, 40-45GPa
Carbide/Carbide: TiC/VC, 52GPa; TiC/NbC, 45-55GPa; WC/TiC, 40GPa
Nitride or carbide/metal: TiN/Nb, 52GPa; TiAlN/Mo, 51GPa
Nitride/Oxide: TiAlN/Al2O3
Boride/nitride or carbide: TiB/TiN, TiB/TiC, 50-70 GPa
Metal/metal etc.
The super-hardness characteristics and high modulus of the nano-multilayer film indicate the broad application prospects in the field of tools, thus stimulating the development enthusiasm of relevant domestic researchers. Since 2000, Wuhan University, Jilin University, Shanghai Jiaotong University, and Xi'an Jiaotong University have carried out similar research work. The focus of the research is on the exploration of the hardening mechanism of various nano-multilayer films. The membranes studied involve C3N4/TiN, TiN/NbN, TaN/NbN, TiN/Al2O3, TiN/Si3N4, TiN/AlN, TiN/TiB2, TiN/SiO2, TIN/SiC, AlN/VN, Si3N4/CrN, etc., and the desired hardness and Young's modulus were obtained, and the preparation method was mainly based on PVD magnetron sputtering technology. Nano-multilayer AlN/(Ti,Al)N film made by the State Key Laboratory of Metal Matrix Composites of Shanghai Jiaotong University, when the one-dimensional modulation period is 10nm, the highest hardness is 29GPa; and when the one-dimensional modulation period is 1.3nm At the time, the highest Young's modulus of 377.8 GPa is obtained.
However, it should be noted that in order to achieve the super-hard effect, laboratory studies have confirmed that the one-dimensional period of the multilayer film should generally be around 10 nm, while the film preparation technology based on magnetron sputtering is based on the target. The purpose of adjusting this period is to control the position and length of the workpiece to be plated. For a complex profile cutting tool, it is obviously difficult to obtain a uniform nano-scale multilayer film at each profile, so the super-hardness and high modulus of the nano-multilayer film are expected to be greatly improved. The overall performance of the tool, but so far has not been able to enter commercial applications.
Veprek et al. of the Technical University of Munich, Germany, proposed the theory and design concept of nanocomposite superhard films according to Koehler's theory of epitaxial heterostructures, and prepared Ti-Si-N (nc-TiN/) by plasma chemical vapor deposition. The α-Si3N4) film is said to have a film hardness of 105 GPa. The domestic Xi'an Jiaotong University, Qingdao Institute of Chemical Technology, and Chengdu Tool Research Institute also cooperated with them to carry out related research work and obtained valuable experimental data. As far as the preparation method is concerned, PCVD is more suitable for the preparation of nano-scale films of complex profile workpieces. However, the current process repeatability is a key issue affecting its commercial application.
4 Problems in the research and application of domestic PVD technology
4.1 Changes in the tool coating market
The application of coated tools is closely related to the cutting technology. Unlike the 1990s, the current applications are more focused on carbide indexable inserts, high speed gear cutters, carbide rod cutters and some special-shaped cutters. For CNC machine tools, the cutting speed is usually higher than 100m/min, and a single TiN film is difficult to meet the requirements. At present, the domestic commercial application of PVD technology is still dominated by high-speed steel tool coating. Although the demand for coated tools is increasing, for example, the total amount of tool coating processing in Chongqing has increased from 1 million yuan in 2000 to 1000 this year. More than 10,000 yuan, but the application field is still dominated by motorcycle gear processing industries, cutting speed is mostly less than 50m / min; and in the field of cemented carbide tools, most of its high-end coated tool products are coated abroad. The monopoly of the company, especially the emergence of foreign tool coating processing service centers, has made domestic coating processing service enterprises face more severe competition.
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