Key words: thin film material; superhard multilayer film; superlattice film; nanocrystalline composite film
The term "superhard" material refers to a material having a microhardness Hv ≥ 40 GPa. With the advancement of modern manufacturing industry, more and more difficult materials are being processed, the development of metal cutting technology, especially the emergence of high-speed cutting, dry cutting and micro-lubricating cutting processes, has put more and more severe technical requirements on metal cutting tools. . The emergence of coated tools is considered a revolution in the history of metal cutting tool technology. The superhard film material is plated on the surface of the metal cutting tool, which is suitable for the high technical requirements of the modern manufacturing industry for metal cutting tools. The metal cutting tool base maintains its high strength, and the coating coated on the surface can exert its " The advantages of super-hard, tough, wear-resistant and self-lubricating greatly improve the durability and adaptability of metal cutting tools in modern processing. In addition, many components used in friction environments, such as steel collars on textile machines, piston rings in internal combustion engines, various molds, etc., can also greatly increase the service life of hard film materials. Therefore, hard film materials can be widely used in machinery manufacturing, automotive industry, textile industry, geological drilling, mold industry, aerospace and other fields.
In practical industrial applications, hardness is only one of many technical requirements, in addition to high temperature hardness and toughness, oxidation resistance, chemical stability, friction coefficient and wear rate of hard materials to workpieces, adhesion strength of coatings, thermal conductivity There are certain requirements. For different applications, the technical requirements of the film are different.
The film forming methods for hard films are mainly divided into physical vapor deposition (PVD) and chemical vapor deposition (CVD). In recent years, they have made great progress. Among the PVD technologies, arc ion plating and magnetron sputtering ion plating are the mainstream coating technologies for industrial production. Arc ion plating has a series of advantages such as high ionization rate, fast film growth rate and good adhesion of coating, which accounts for a large share of the coating market. In the mid-1990s, seven large-scale coating machines introduced from abroad in China were all arc ion plating, which greatly promoted the progress of China's coating industry. Zui near magnetron sputtering sputter ion plating, due to the emergence of new technologies such as non-equilibrium magnetic field, multi-target magnetic field coupling, twin magnetron target, pulse sputtering, medium frequency alternating current sputtering power source, etc., make magnetron sputtering technology in the preparation of multi-element composite Membrane, superlattice film and nanocrystalline superhard film exceed the arc ion plating method. The advanced magnetron sputtering technology provides technical guarantee for the deposition of super-hard films. The perfect coating equipment function is the basis for ensuring the quality of super-hard film materials. Super-hard film materials are very active in materials science and engineering.
1. Hard film materials are diversified. TiN is an industrialized and widely used hard film material. Subsequently, TiC and TiCN have been developed right, and the microhardness has also increased from 20 GPa to about 28 GPa. TiC has high resistance to mechanical friction and abrasive wear. Its coefficient of expansion is similar to that of cemented carbide, so it is firmly bonded to the substrate and is suitable as a base film for multi-coating of cemented carbide inserts. TiCN has strong toughness and resistance to breakage. Some famous foreign tool manufacturers have combined the above three materials to design a TiC-TiCN-TiN multilayer composite film. In addition, there are TiC-TiCN-Al 2 O 3 -TiN, TiAlN-MoS 2 and TiAlN-WC. /C and so on. These composite membranes, which exploit the advantages of several materials, greatly improve the durability of coated cemented carbide inserts and become a perfect design in multilayer film systems.
TiAlN is another widely used Ti-based hard composite film, which also has a microhardness of 28 GPa. Due to the addition of Al element, the tool coated with the film is subjected to high temperature during use, and a thin layer of very stable chemical properties of Al 2 O 3 is formed on the surface of the film to protect the coating from being continuously oxidized. The TiAlN film can operate at temperatures up to 800 °C. Generally, when the working temperature of TiN reaches 500 ° C, it is gradually oxidized, thereby affecting the service life of the coating. So TiAlN coated tools can be used for high speed cutting, dry cutting, and some difficult materials.
Chromium nitride (CrN) is a common hard film material. Although its microhardness is not very high (about 18GPa), it has strong abrasion resistance, and its anti-oxidation and adhesion ability is also good. It is often used as a superhard composite film for application. Processing difficult materials such as titanium alloys. The expansion coefficient of tantalum nitride (HfN) is very close to that of cemented carbide. As a coating of cemented carbide insert, it has high bonding strength; HfN has higher thermal stability and chemical stability than other hard film materials. High temperature hardness (30GPa), suitable for high-speed cutting tools, wear resistance is 2 times higher than TiN, and even exceeds Al 2 O 3 . Molybdenum nitride (Mo 2 N) coated tool processing all materials have greatly reduced friction, reduce cutting temperature, reduce wear, suitable for processing Ti alloy, Ni alloy, heat resistant alloy. The amorphous α-B 4 C microhardness prepared by the high frequency PCVD method can exceed 50 GPa. The microhardness of TiB 2 deposited by magnetron sputtering is as high as 70 GPa. The ternary compounds BC 4 N, B 12 C 2.88 Si 0.35 and Si 3 N 2.2 C 2.16 were prepared by the PVD method and their microhardness was 63-65 GPa. The transition metal nitrides, carbides and borides are good superhard film materials, and many thin film materials are still to be further developed in the field.
Diamond and cubic boron nitride (c-BN) are two hard film materials with excellent properties. It is well known that diamond is a hard solid substance in the world, and it is an allotrope of carbon with graphite, fullerene (C 60 ) and carbon nanotubes. Low-pressure chemical vapor deposition diamond films have been widely recognized by countries all over the world for more than two decades, and have become a research hotspot in the field of materials science. Jilin University, Beijing University of Science and Technology and Shanghai Jiaotong University have made gratifying achievements in diamond film research, and have taken a new step in practical application and industrialization of results. Professor Wang Jitao of Fudan University successfully applied the modern thermodynamic theory of non-equilibrium zero dissipation to the super-large theoretical significance. A large number of theoretical and experimental studies have basically clarified the mechanism of low-pressure vapor deposition of diamond films, the chemical environment of vapor deposition and surface processes. A deeper understanding of the nucleation and growth kinetics of diamond on heterogeneous substrates has been gained. From the depositional technology, the successful development of large-area, high-growth deposition equipment and processes has resulted in an increase in the deposition rate of diamond films by three orders of magnitude compared to the mid-1980s, and the cost of preparation has been greatly reduced. The preparation of high-quality diamond film has made remarkable progress. The "optical grade diamond film" is of sufficient quality to be compared with the high quality natural type IIa gem-quality diamond single crystal. Comparable. However, only mechanical properties, especially mechanical strength, are quite different from natural diamonds. The low pressure vapor deposited diamond film is often called DLC (Diamond-Like Carbon), and its microstructure and natural diamond are still quite different. In recent years, research on diamond films has focused on obtaining high hardness, low coefficient of friction, ultra low wear and self-lubricating DLC ​​films. In the 1990s, low-pressure vapor deposition of DLC in the presence of activated hydrogen was often used, and the membrane contained a large amount of hydrogen. A large number of experimental studies have shown that the low coefficient of friction of the DLC coating is caused by the low shear stress of the interface layer, and is also affected by the test environment. The friction coefficient of the DLC film has a large span and is composed of the structure and composition of the film. Caused by changes. The friction mechanism of hydrogen-containing DLC ​​and hydrogen-free DLC has a large difference. The DLC film has a self-lubricating effect, and the self-lubricating effect is enhanced by the introduction of hydrogen, and the addition of water or oxygen reduces the lubricating effect, and the hydrogen-containing DLC ​​in the ultra-high vacuum can obtain an ultra-low friction coefficient. However, too much hydrogen will reduce the bonding force and hardness and increase the internal stress. The hydrogen in the DLC is slowly released at a higher temperature, causing the coating to work unstable. The DLC without hydrogen has a friction coefficient of 0.6 in vacuum and the wear is severe. After adding water, the friction coefficient decreased from 0.6 to 0.07. The hydrogen-free DLC has higher hardness than hydrogen-containing DLC, has uniform structure, can be deposited in a large area, has low cost, and has a smooth surface. It has become a research hotspot of DLC coating in recent years. Since 1997, research on hydrogen-containing DLC ​​coatings has been declining. Although hydrogen-containing DLC ​​coatings are now widely used, due to their inherent defects, they will certainly be coated with hydrogen-free DLC in many future applications. Replaced. In the next step, research on the mechanism of film formation without hydrogen DLC coating can be carried out, and the existing magnetron sputtering equipment can be used to prepare DLC without hydrogen and high-quality Ta-C film.
In 1997, AAVoevodin of the United States proposed a Ti-TiC-DLC gradient transition film for the deposition of superhard DLC coatings, which gradually increased the hardness from a softer steel matrix to a superhard (60-70 GPa) DLC film. Such films maintain high hardness, low coefficient of friction, reduced brittleness, improved load bearing capacity, film-based bonding force and wear resistance. This structure is further developed by coating a Ti-TiC-DLC gradient layer with a nano-thickness composite structure layer composed of a plurality of layers of Ti and amorphous DLC to obtain Ti-TiC-DLC-n×(Ti-DLC). Multilayer film. Although the composite structure has reduced hardness, it acts as a buffering stress to prevent the initiation of cross-section microcracks, thereby improving the film-based bonding force and the overall toughness of the film.
DLC coatings are expected to be used in aerospace gyro bearings, solar cell windsurfing devices, spacecraft gears and bearings, cutting tools in machining, automotive engines, gas turbines and turbine blades. In addition, magnetic media protection (hard disk, magnetic head), optical infrared window, radome, solar cell anti-reflection film, infrared lens protection film, flat panel display, medical surgical instruments, human implant components (such as joints, valves, etc.) can be widely used. application.
2. Nano-superlattice superhard composite film alternately deposits two different materials M(1) and M(2), which have different elastic moduli EM(2)>EM(1), but their thermal expansion coefficient and chemical bond The intensity is similar. If the thickness of the two materials is so small that no dislocation source acts in the film. If the stress state is low, the dislocation will move from the softer M(1) layer toward the M(1)/M(2) interface. Deformation occurs in the second layer having a higher modulus of elasticity, which will cause repulsive forces, thereby preventing dislocations from passing along the interface. The strength or hardness of such a structured multilayer film should then be expected to be much greater than the state of mixing of such materials. The two materials alternately overlap in nanometer thickness to form a nano-superlattice composite film, and the composite film exhibits super-modulus super-hardness phenomenon. The micro-hardness of the superlattice composite film can reach 2-4 times that of the single-component composition material. Table 1 shows the microhardness of some nano-superlattice composite films.
M(1)/M(2) | Modulation period (nm) | Microhardness HV (GPa) | Preparation | literature |
TiN/VN | 4.8 | 55 | PVD | [1}{4} |
TiN/NbN | 4 | 51 | PVD | [4] |
TiN/CrN | 2--3 | 35 | PVD | [5][6] |
TiN/C 3 N 4 | 2--4 | 50--70 | PVD | [8] |
TiN/AlN | 3 | 40 | PVD | [7] |
TiAlN/CrN | 3.0--3.2 | 55-60 | PVD | [9] |
TiAlYN/VN | 3--4 | 42--78 | PVD | [9] |
NbN/CrN | 3--7 | 42--56 | PVD | [9] |
TiC/VC | 3--10 | 52 | PVD | [10] |
TiC/NbC | 3--10 | 45--55 | PVD | [10] |
The relationship between the yield stress (or hardness) and the grain size of the material based on the dislocation plugging theory of single crystal and polycrystalline materials is called the Hall-Petch relationship.
3, nanocrystalline-amorphous superhard composite material such a hard film is characterized by a hard phase, usually a transition metal nitride is embedded in another amorphous nano-scale grain. Ultra high hardness of over 40 GPa can be obtained. There are two types of superhard films, the main difference being that the second phase can be a hard phase, such as: nc-MeN/a-Si 3 N 4 , a-TiB 2 ; the other second phase is a soft phase. Such as: nc-MeN/Me', Me'=Cu, Ni, Y, etc. Table 2 contains the microhardness of some of these superhard composites.
/ | HvGPa | MeN grain size nm | Second rate% | Deposition method | literature |
nc-TiN/a-Si 3 N 4 | ≥ 100 | 6.8 | 5.44 | P-CVD | [1][11] |
nc-TiN/C-BN | 60--80 | 4--10 | 20 | P-CVD | [1][13] |
nc-TiN x /a-TiB 2 | 71 | <10 | 20 | P-CVD | [12] |
nc-W 2 N/a-Si 3 N 4 | 51 | 1--10 | 20 | P-CVD | [13] |
Zr-Ni-N | 57 | 20--50 | 6.2 | PVD | [14] |
Zr-Cu-N | 55 | 19--38 | 1--2 | PVD | [14] |
Zr-YN | 47 | 5--10 | 7.7 | PVD | [14] |
Al-Cu-N | 48 | <10 | 8.1 | PVD | [14] |
Cr-Cu-N | 35 | 50--90 | 7.7 | PVD | [15] |
W-Ni-N | 55 | ≈10 | ≈8 | PVD | [10] |
Ti-Al-N | 47 | 30 | Ti45, Al55 | PVD | [10] |
Ti-BC | 71 | 1--5 | TiB 2, 30 | PVD | [10] |
Cr-Ni-N | 46 | <10 | 5 | PVD | [14] |
The grain size of the hard phase is the main factor affecting the hardness of the composite film. Different composite films need to be optimized by the deposition process, usually with a grain size of d ≤ 10 nm. The second phase is amorphous, it should have sufficient plasticity, and the nano-grain forms a strong interface, and avoiding grain boundary slip is a key technology to improve the hardness of the composite film. This type of superhard composite film also conforms to the law described by Hall-Patch relationship. Its yield stress or hardness is proportional to its Young's modulus E and is restricted by the growth of microcracks in the deposition process. The ratio of the second phase in the composite film also has a large effect on the microhardness of the film.
Nc-TiN/a-Si 3 N 4 is a currently reported film material having a high microhardness of Zui. It uses titanium tetrachloride (TiCl 4 ) and silane (SiH 4 ) as raw materials, and is deposited by P-CVD in the presence of excess hydrogen and nitrogen. The measured microhardness is Plastic Hardnees, Hv=110 GPa when loaded at 30 mN, and HV=80 GPa when loaded at 70 mN. The film thickness is 6.1 μm, the film structure is nc-TiN/a-Si 3 N 4 /a-TiSi 2 , the grain size is 6.8 nm, and the silicon content is 5.44%.
4. Looking into the super-hard film material is a booming field, and only by opening its practical application can it enhance its vitality. The advanced foreign tool coating thickness has been increased from the traditional 3-5μm to 10-15μm, and the high-quality DLC coating is more than 30μm. For the high-speed steel tool PVD method to deposit a coating of 10-15um thickness, there is a high technical difficulty, and it is necessary to solve the stress and adhesion of the film growth. In addition to the hardness of the film, the oxidation resistance and chemical stability of the film, the friction coefficient and the wear rate of the film all set high technical requirements. Obviously, a single film material cannot meet the above technical requirements, and the functional film is developing toward a diversified multilayer film.
Advanced tool coatings often use oxide coatings such as Al 2 O 3 , which is a good measure to improve the oxidation resistance of the coating. Although oxides such as Al 2 O 3 , ZrO 2 , TiO 2 and SiO 2 have long been studied and matured, it is not an easy task to plate them on the surface of the tool and maintain high quality, high efficiency and stable production. The above-mentioned several hard oxides of Zui have a microhardness of only 10-20 GPa, and the microhardness of the oxide film is not a congenital defect. Through the technology of oxide superlattice film, its microhardness and toughness have been greatly improved, which has been tried abroad.
With these technical problems solved, hard film technology and its applications can take a new step.
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