1 Introduction
Diamond has excellent physical and chemical properties. It has the highest hardness among all substances in nature and has the highest thermal conductivity at room temperature. At the same time, it has extremely low thermal expansion coefficient, low friction coefficient, good chemical stability, and large band gap. Width (5.5eV), highest sound propagation speed, high semiconductor doping, and optical transmission from the far-infrared region to the ultraviolet region, so many excellent properties make it in machining, microelectronics, optics Many fields have broad application prospects.
However, there are few natural diamonds in nature. Synthetic diamonds synthesized by high temperature and high pressure are limited in size and expensive, making it difficult to use diamonds with excellent properties for practical applications. In 1982, Matsumto et al. used chemical vapor deposition (CVD) to prepare diamond films, which opened a new way for the application of diamonds, and thus set off an upsurge of CVD diamond film research worldwide. At present, China has also increased investment in diamond film research. Many research institutes have invested a lot of human and material resources in the development and application of diamond films. According to the status quo of domestic technology development and economic development characteristics, the application of diamond thin film coating tools, diamond heat sink substrates, field emission display devices, SAW devices, and nano-diamond films will be expected to enter the market one after another. Among them, the use of diamond's high hardness, high thermal conductivity, low coefficient of friction and other excellent properties, the diamond film coating for the production of simple indexable inserts and complex shapes of tools, can solve non-ferrous metals and their alloys and high wear resistance Complex materials and other processing problems. Therefore, CVD diamond thin film coating tools have broad application prospects in the field of cutting processing.
2. Diamond film CVD preparation method and quality evaluation
2.1 diamond film CVD preparation method
At present, there are many methods for synthesizing diamond films by CVD, which mainly include: hot wire CVD method, electron acceleration CVD method, direct current discharge plasma CVD method, direct current plasma jet CVD method, microwave plasma CVD method, electron cyclotron resonance CVD. Method, high frequency plasma CVD method, combustion flame method, laser induced CVD method, hollow cathode plasma CVD method, and the like. Among the various CVD methods, the synthesis index is better than the microwave CVD method and hot wire CVD method widely used by the research unit.
2.2 Diamond Film Quality Evaluation Method
The commonly used diamond film quality detection methods are as follows: 1 Raman spectroscopy is used to measure the structure, purity, and stress state of the film. As a result, if the characteristic peak of 1332/cm of natural diamond is shifted toward the low wave number direction, it indicates that the diamond internal stress is tensile stress; otherwise, the internal stress of the film is compressive stress. 2 The X-ray diffraction analysis of the diamond surface structure of the thin film layer. 3 Scanning electron microscope was used to observe the surface morphology, nucleation rate and growth rate of the film. 4 The infrared transmittance of the film was analyzed by infrared spectroscopy. 5 The indentation method is used to measure the interfacial bonding force at the membrane substrate (bonding force at the membrane-based interface is an important evaluation index for diamond film tool performance). Recent studies have shown that the elastic modulus, Poisson ratio, and the elastic modulus of the diamond film are measured by the bubble method. Residual stress, etc., is a very promising method for measuring the mechanical properties of diamond films.
3. Overview of CVD Diamond Film Coating Tools
3.1 CVD diamond film coating tool substrate pretreatment technology
The ideal tool material should have excellent wear resistance to extend tool life; it has high fracture toughness to withstand high cutting forces. However, most tool materials with good fracture toughness (such as high-speed steel) do not usually have good wear resistance, while materials with good wear resistance (such as ceramic materials) tend to have poor fracture toughness. Due to the good wear resistance and high fracture toughness of cemented carbide (WC-Co) materials, it is the substrate material for CVD diamond thin film coating tools commonly used at home and abroad. However, due to the large difference in thermal expansion coefficient between the diamond film and the cemented carbide, the bond strength of the film base after deposition is poor, and the cohesive phase Co in cemented carbide plays a role of promoting graphitization in the deposition process and has nucleation of diamond. Inhibition. In order to improve the deposition quality of the diamond film on the surface of carbide tools, the surface of the substrate must be properly pretreated (the mechanical and thermal properties of the commonly used hard coating materials and substrates are shown in the table below).
Mechanical and thermal properties of commonly used hard coating materials and substrates
Material - Melting Point or Decomposition Temperature (°C) - HV Hardness (MPa) - Young's Modulus (KN/mm2) - Thermal Expansion Coefficient (10-6/K) - Thermal Conductivity (W/mK)
Diamond-3800-80000-1050-1.3-1100
Cu-1084-/-98-16.6-386
Si-1420-/-/-2.5-84
WC-2776-23000-720-4.0-35
Al2O3-2047-21000-400-6.5-25
SiC-2760-26000-480-5.3-84
Si3N4-1900-17000-310-2.5-17
TiC-3067-28000-460-8.3-34
TiN-2950-21000-590-9.3-30
The substrate surface pretreatment methods commonly used at present are: 1 Surface decoking treatment: etching of Co in the surface layer of the substrate with HCl, HNO3, H2SO4, etc.; etching with hydrogen plasma or oxygen-containing hydrogen plasma Corrosion Co; the use of chemical reagents passivation and other methods to deactivate the Co in the surface layer of the substrate; use chemical reaction to replace Co, put the cemented carbide substrate cutter into the chemical reagent, replace the Co in the surface layer by displacement reaction Into other substances (such as Cu).
2 Pre-deposition of intermediate transition layers between the diamond film and the substrate. These transition layers should meet the requirements of moderate thermal expansion coefficient, stable chemical properties, good adhesion with cemented carbide and diamond, and reaction with Co to form stable compounds. . Currently used transition layer materials are: Ti, B, TiC, TiN, Cu, etc.; composite transition layer: WC / W, TiN / TiCN / TiN, TiCN / Ti and so on. Due to the existence of the intermediate transition layer, the internal stress caused by the lattice mismatch and the difference in thermal expansion coefficient between the diamond film and the cemented carbide substrate can be eliminated, and the carbon can be prevented from excessively infiltrating into the substrate during the deposition process or from the depth of the substrate. The surface diffuses to enhance nucleation density and adhesion. 3 surface seeding treatment. The surface of the cemented carbide substrate is subjected to ultrasonic treatment with a diamond-containing suspension (such as acetone) or the nano-sized diamond powder is evenly dispersed on the surface of the substrate by acetone, and then rapidly heated by the laser so that the diamond powder is embedded in the surface binder phase. , can increase the nucleation density. In addition, surface chemical cleaning, liquid ultrasonic cleaning, and hydrogen plasma bombardment are also basic approaches to substrate pretreatment. R. Bichle found that when the Co content is in the range of 3% to 10%, the nucleation rate of the diamond film decreases with the increase of the Co content; when the Co content exceeds 6%, the nucleation rate is the lowest. The results of the study indicate that the proper two-step etching process, ie, the first step of etching the WC phase with the Murakami agent, and then removing the Co phase by the acid etching, has a good effect of removing the Co phase.
3.2 Effect of diamond film structure on tool performance
A number of research institutes at home and abroad studied the use of cemented carbide substrates to make simple indexable inserts and carried out turning tests. Studies have shown that the adhesion strength of diamond thin film coated tools decreases as the thickness of the coating increases. For diamond thin-film coated cutters with WC-1.5 %Co cemented carbide as substrate, when the coating thickness is in the range of 5 ~ 10 μm, the adhesion strength decreases with the increase of the coating thickness. The trend is not very obvious; when the coating thickness After more than 10 μm, the adhesion strength decreased significantly with the increase of the coating thickness. Therefore, from the viewpoint of improving the adhesion strength, the coating thickness of the diamond thin film coated tool should not exceed 10 μm. It was also reported that the diamond thin film prepared on the surface of the cemented carbide substrate by CVD method is uneven, usually the surface roughness is Ry4. At ~10μm, the surface shape of the diamond-coated cutting tool during machining of the aluminum alloy affects the roughness of the surface to be machined, and it is difficult to obtain the desired surface finish of the finishing. OSG Corporation of Japan developed ultra-fine crystalline diamond thin film carbide cutting tools, which have excellent anti-adhesive properties, high processing accuracy, durability, and film toughness after cutting tests, and have been widely used for cutting diamond coatings developed by OSG Corporation. Tools are favored by users. Sun Fanghong et al.: A new process of flattening the diamond film by hot filament CVD in the later stage of deposition by simultaneously raising the concentration of the carbon source and lowering the reaction pressure, growing on the WC-Co6% hard alloy (YG6) in the early and middle stages of deposition Layer 10 ~ 15μm thick smooth diamond film, turning machining test shows that the coating tool life and cutting performance have improved significantly.
The main wear and tear loss modes of SiC diamond film coated cutters for cutting high silicon aluminum alloys are abrasive wear, cracking and peeling of diamond films. Abrasive wear is mainly caused by the "micro-cutting" effect of hard point Si particles in the workpiece material. The early exfoliation of diamond film is mainly due to insufficient bonding strength between diamond film and matrix, excessive depth of decobalt layer, and low strength of matrix. Cutting force and cutting thermal shock are the main reasons for the exfoliation of diamond film in the middle and later stages. The adhesion strength of different base material diamond film coating cutters is different. Intermittent cutting experiments were carried out on tools prepared on the W, WC-1.5%Co, WC-3%Co, and WC-6%Co substrates by the combustion flame method: WC- The 1.5% Co matrix tool had higher adhesion strength, while the WC-3% Co and WC-6% Co matrix tool had lower adhesion strength. The diamond film was deposited on the surfaces of cemented carbide and Si3N4 ceramic cutting tools by hot wire CVD. The results show that the bonding strength of the deposited diamond film on Si3N4 ceramic is far greater than the bonding strength of diamond film on cemented carbide. This is due to the easy surface of the carbide. The formation of loose layers of graphite, WC, etc. reduces the bonding properties of the film, and the diamond film can easily fail in the form of exfoliation. However, the Si3N4 ceramic substrate may form a SiC transition layer at the film-based interface, which can significantly enhance the bonding strength of the film. However, under the compressive stress, the diamond film on the Si3N4 ceramic substrate will fail due to cracking and crack propagation.
Preparation of 3.3 CVD diamond film coating drill
Cemented carbide is tougher and easier to machine into a tool with a complex shape compared to ceramics and is therefore used as the base material for the main diamond-coated diamond drill bit. Shanghai Jiaotong University Chen Ming et al. deposited diamond film on a cemented carbide YG6 drill bit. The drill diameters were φ2mm, φ3mm, φ4mm, φ6mm. The workpiece material was SiC particle reinforced aluminum matrix composite (35Vol%SiC, 14μm). The bit rotation speed was 1400～9000r/min, diamond deposition equipment is EACVD, reaction gases are acetone and hydrogen. The matrix pretreatment adopts oxidation treatment, that is to say, the drill bit is placed in a microwave plasma device of a CO2 atmosphere, so that WC and Co elements on the surface of the tool substrate oxidize. Due to the different oxidation rates, the binder phase Co between the WC particles is rapidly oxidized. With the removal of oxides (the addition of alkaline solution to remove the oxides of the surface of the drill bit and Co), the WC particles on the surface of the tool substrate are exposed, so as to increase the surface roughness, which is favorable for the nucleation and initial stages of the diamond. Grow. The cutting machining test shows that in the roughening process of the tool substrate surface, the oxidation treatment method is suitable for complex shape tools, which can ensure the cutting edge is intact and is convenient for mass production. It is a tool substrate pretreatment method with promising development; During the process of CVD deposition of diamond, adding a suitable amount of adhesion promoter can significantly increase the adhesion of the diamond film and thus increase the tool life. The CVD coating process is suitable for the preparation of diamond thin film coated tools with a complex diameter of φ4mm and above.
3.4 The effect of tool geometry on the performance of diamond thin film coating tools
The peeling of diamond film is not only related to its adhesion strength on the tool body, but also related to the geometric parameters of the tool. The study shows that the radius of the tool tip arc is an important geometric parameter that influences the change of cutting force and the heat dissipation condition in the cutting zone. Under the conditions of the tool matrix material, surface pretreatment, deposition process and coating thickness, the radius of the tool tip radius cut The peeling of the diamond film during the process has an important effect. The impact resistance of the diamond film coating tool increases with the increase of the arc radius of the tool tip, but when the tool tip arc radius is greater than 1.5 mm, the impact resistance of the tool decreases. Under the condition that the cutting system has sufficient rigidity, appropriately increasing the arc radius of the tip can effectively improve the impact resistance of the diamond thin film coated cutter. Hanyu et al. studied diamond-coated drills for cutting high-silicon aluminum alloys. The results show that by changing the shape of the cutting edge and the thickness of the coating, the drill bit structure can be optimized to improve the cutting effect. During the rotation of the drill bit, the mechanical load acting on the blade edge decreases with the increase of the blade inclination angle, and at the same time, the cutting load tends to concentrate on the blade tip edge as the blade inclination angle increases, resulting in the coating Local stress concentration. The test shows that the diamond coated drill shows the best cutting effect when the blade inclination angle is 20°.
4. Conclusion
In summary, the factors influencing the quality of the CVD diamond film tools include substrate materials, substrate pretreatment methods, and the shape of the tool substrate. Various factors have a close relationship with the cutting performance and service life of the diamond film tools.