Survey of CVD diamond film coating tools

1. Introduction

Diamond has excellent physical and chemical properties, has the highest hardness among all materials in nature, has the highest thermal conductivity at room temperature, and has a very low coefficient of thermal expansion, low friction coefficient, good chemical stability, and large forbidden band. Width (5.5eV), highest acoustic 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, the amount of natural diamond in nature is extremely small, and synthetic diamonds synthesized by high temperature and high pressure are limited in size and expensive, so that diamonds having excellent properties are difficult to be widely used in actual production. In 1982, Matsumto et al. used chemical vapor deposition (CVD) to prepare diamond films, which opened up new avenues for the application of diamonds, which led to a worldwide CVD diamond film research boom. At present, China has also increased investment in research on diamond films. A number of research units have invested a large amount of manpower and material resources in the development and application of diamond films. According to the status quo of domestic technology development and economic development, the application of diamond film coating tools, diamond heat sink substrates, field emission display devices, surface acoustic wave devices and nano-diamond films are expected to enter the market. Among them, diamond film coating is used to make simple indexable inserts and complex shape cutters by using diamonds with high hardness, high thermal conductivity, low friction coefficient, etc., which can solve non-ferrous metals and their alloys and high wear resistance. Processing problems such as composite materials. Therefore, CVD diamond film coating tools have broad application prospects in the field of cutting.

2. Preparation method and quality evaluation of diamond film CVD

2.1 Diamond film CVD preparation method

At present, there are many methods for synthesizing diamond films by CVD, including hot filament CVD, electron accelerated CVD, DC discharge plasma CVD, DC plasma jet CVD, microwave plasma CVD, and electron cyclotron resonance CVD. Method, high-frequency plasma CVD method, flame method, laser induced CVD method, hollow cathode plasma CVD method, and the like. Among the various CVD methods, the comprehensive index is preferably the microwave CVD method and the hot wire CVD method widely used by the research unit. 

2.2 Diamond film quality evaluation method

At present, the commonly used diamond film quality testing methods are as follows: 1. The structure, purity and intra-membrane stress state of the film are measured by Raman spectroscopy. The result is that the internal stress of the diamond is tensile stress relative to the characteristic peak of the natural diamond of 1332/cm, indicating that the internal stress of the diamond is tensile stress; otherwise, the internal stress of the film is compressive stress. 2 The crystal plane structure of the thin film layer diamond was analyzed by X-ray diffraction. 3 The surface morphology, nucleation rate and growth rate of the film were observed by scanning electron microscopy. 4 Infrared spectroscopy was used to analyze the infrared transmittance of the film. 5 Determination of film-based interfacial adhesion by indentation method (film-based interfacial adhesion is an important evaluation index for diamond film tool performance). Recent studies have shown that the measurement of the elastic modulus, Poisson's ratio and residual stress of diamond films by bubbling method is a promising method for measuring the mechanical properties of diamond films. 

3. Survey 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 the life of the tool; it has high fracture toughness to withstand high cutting forces. However, most tool materials with good fracture toughness (such as high-speed steel) usually do not have good wear resistance, while materials with good wear resistance (such as ceramic materials) tend to have poor fracture toughness. Because of its good wear resistance and high fracture toughness, cemented carbide (WC-Co) materials are the substrate materials for CVD diamond film coating tools commonly used at home and abroad. However, due to the large difference in thermal expansion coefficients between diamond film and cemented carbide, the bond strength of the film base after deposition is poor, and the binder phase Co in the cemented carbide plays a role in promoting graphitization during the deposition process. Inhibition. In order to improve the deposition quality of the diamond film on the surface of the cemented carbide tool, the surface of the substrate must be properly pretreated (see Table 1 for the mechanical and thermal properties of the commonly used hard coating materials and substrates). 

Table 1. 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
At present, the substrate surface pretreatment methods commonly used are: 1 surface de-Co treatment: etching of Co in the surface layer of the substrate by using HCl, HNO3, H2SO4, etc.; using hydrogen plasma or oxygen-containing hydrogen plasma engraving Corrosion Co; using chemical reagent passivation and other methods to make Co in the surface layer of the substrate inactive; replace Co with chemical reaction, put the cemented carbide substrate cutter into the chemical reagent, and replace the Co in the surface layer by displacement reaction Become other substances (such as Cu). 2 Pre-depositing an intermediate transition layer between the diamond film and the substrate. These transition layers should satisfy the requirements of moderate thermal expansion coefficient, stable chemical properties, good adhesion to cemented carbide and diamond, and reaction with Co to form stable compounds. . Currently, the transition layer materials commonly used are: Ti, B, TiC, TiN, Cu, etc.; composite transition layers: WC/W, TiN/TiCN/TiN, TiCN/Ti, and the like. Due to the existence of the intermediate transition layer, the internal stress caused by the lattice mismatch between the diamond film and the cemented carbide substrate and the difference in thermal expansion coefficient can be eliminated, and the carbon can be prevented from excessively penetrating into the substrate during the deposition process or Co can be deep from the substrate. The surface diffuses, thereby enhancing nucleation density and adhesion. 3 surface phytolith treatment. The surface of the cemented carbide substrate is sonicated with a suspension containing diamond micropowder (such as acetone) or the nano-sized diamond micropowder is evenly dispersed on the surface of the substrate by acetone, and then rapidly heated by a laser to embed the diamond micropowder into the surface layer of the binder phase. Can increase the nucleation density. In addition, surface chemical cleaning, liquid ultrasonic cleaning, and hydrogen plasma bombardment are also basic means of substrate pretreatment. R.Bichle et al 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 Co content; when the Co content exceeds 6%, the nucleation rate is the lowest. The results show that the proper two-step etching process, that is, the method of etching the WC phase with the Murakami agent and then removing the Co phase by acid etching has a good effect of removing Co.

3.2 Influence of diamond film structure on tool performance

A number of research institutes at home and abroad have conducted research on the manufacture of simple indexable inserts using cemented carbide substrates and conducted turning tests. Studies have shown that the adhesion strength of diamond film coated tools decreases with increasing coating thickness. When the thickness of the coating is in the range of 5-10 μm, the adhesion strength decreases with the increase of the coating thickness when the coating thickness is in the range of 5-10 μm; when the coating thickness is After more than 10 μm, the adhesion strength decreases significantly as the thickness of the coating increases. Therefore, from the viewpoint of improving the adhesion strength, the coating thickness of the diamond thin film coating tool should not exceed 10 μm. It has also been reported that the diamond film prepared by the CVD method on the surface of the cemented carbide substrate is uneven, usually having a surface roughness of Ry 4 to 10 μm, and the surface shape of the cutting tool coated with the diamond film affects the processed aluminum alloy. The roughness of the surface makes it difficult to obtain the surface finish desired for finishing. Japan OSG Company has developed ultra-fine crystal diamond film cemented carbide tool, which has good anti-adhesion, high processing precision, durability and film toughness after cutting test. It has been widely used in diamond coating cutting developed by OSG. Tools are also favored by users. Sun Fanghong et al. used a hot-wire CVD method to grow a diamond film at the same time in the late deposition stage by increasing the carbon source concentration and lowering the reaction pressure. On the WC-Co6% cemented carbide (YG6), it grows in the early and middle stages of deposition. The surface of the layer is 10 to 15 μm thick with a smooth diamond film. The turning test shows that the service life and cutting performance of the coated tool are significantly improved.
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The main wear and breakage effects of CVD diamond film coated tools for cutting high silicon aluminum alloys include abrasive wear, diamond film cracking and spalling. Abrasive wear is mainly caused by the "micro-cutting" effect of hard-point Si particles in the workpiece material. The early diamond film spalling is mainly due to insufficient bonding strength between the diamond film and the substrate, the depth of the decobalted layer is too large, and the strength of the matrix is low. The cutting force and the thermal shock of cutting are the main reasons for the spalling of the diamond film in the middle and late stages. Different substrate materials have different adhesion strengths of diamond film coating tools. Intermittent cutting tests of tools prepared by flame method on W, WC-1.5%Co, WC-3%Co, WC-6%Co substrates show that: WC- The adhesion strength of the 1.5% Co matrix tool is higher, while the adhesion strength of the WC-3%Co and WC-6%Co matrix cutters is lower. The diamond film was deposited on the surface of cemented carbide and Si3N4 ceramic tool by hot wire CVD. The results show that the bonding strength of the deposited diamond film on Si3N4 ceramic is much higher than that of the diamond film on the cemented carbide, which is due to the easy surface of the cemented carbide. The loose layer of graphite, WC and the like are formed to reduce the bonding property of the film. The diamond film is easy to directly fail in the form of spalling; and the film-based interface of the Si3N4 ceramic substrate may form a SiC transition layer, which can significantly enhance the bonding strength of the film. However, under the action of compressive stress, the diamond film on the Si3N4 ceramic substrate will fail in the form of cracks and crack propagation. 

3.3 Preparation of CVD Diamond Film Coated Drills

Compared with ceramics, cemented carbides have better toughness and are easier to process into complex shaped tools, and are therefore used as the base material for the main deposited diamond film coated drill bits. Chen Ming, from Shanghai Jiaotong University, deposited diamond film on the cemented carbide YG6 drill bit. The drill diameters were φ2mm, φ3mm, φ4mm, φ6mm, and the workpiece material was SiC particle reinforced aluminum matrix composite (35Vol% SiC, 14μm). From 1400 to 9000 r/min, the diamond deposition equipment is EACVD, and the reaction gases are acetone and hydrogen. The substrate pretreatment is treated by oxidation, that is, the drill bit is placed in a microwave plasma device of a CO2 atmosphere to cause oxidation reaction of WC and Co elements on the surface of the tool substrate. The bond phase Co between the WC particles is rapidly oxidized due to different oxidation rates. With the removal of oxides (addition of alkali solution to remove the surface of the bit and the oxide of Co), the WC particles on the surface of the tool substrate are exposed to the surface, thereby increasing the surface roughness, which is beneficial to the nucleation and initial stage of the diamond. Growing. The cutting test shows that in the roughening treatment of the tool substrate surface, the oxidation treatment method is suitable for complex shape cutters, which can ensure the cutting edge is intact and is convenient for mass production. It is a promising tool substrate pretreatment method; In the process of CVD deposition of diamond, the addition of proper amount of adhesion promoter can significantly improve the adhesion of diamond film and improve the tool life. The CVD coating process is suitable for the preparation of complex shape diamond film coating tools with diameter of φ4mm and above. 

3.4 Effect of tool geometry on the performance of diamond film coating tools
The spalling of the diamond film is not only related to its adhesion strength on the tool substrate, but also to the geometry of the tool. The research shows that the radius of the tool tip is an important geometric parameter that affects the change of cutting force and the heat dissipation condition of the cutting zone. Under the condition of the tool base material, surface pretreatment, deposition process and coating thickness, the tool nose radius is cut. The peeling of the diamond film during the process has an important influence. The impact resistance of the diamond film coated tool increases with the increase of the radius of the tool nose arc. However, when the tool nose 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 radius of the cutting edge can effectively improve the impact resistance of the diamond film coated tool. Hanyu et al. studied diamond-coated drill bits 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 bit structure can be optimized to improve the cutting effect. During the rotation of the drill bit, the mechanical load acting on the blade decreases as the blade inclination increases, and at the same time the cutting load tends to concentrate on the tip of the blade as the blade inclination increases, which results in a coating Local stress concentration. Tests have shown that diamond coated drill bits show the best cutting results when the blade angle is 20°.

4. Conclusion

In summary, the factors affecting the quality of the CVD diamond film tool are substrate material, substrate pretreatment method, tool base shape, etc., and all aspects are closely related to the cutting performance and life of the diamond film tool.