Experimental Study on Cutting Performance of CVD Diamond Thick Film Tool

1 Introduction The CVD diamond thick film material is a fully crystalline pure polycrystalline diamond with excellent mechanical properties such as high hardness, large thermal conductivity, low friction coefficient, isotropy, etc. It not only makes up for the scarcity of natural diamond, is expensive and anisotropic. Such defects, but also overcome the artificial synthetic single-crystal diamond particles, polycrystalline diamond (PCD) chemical thermal stability is poor, CVD diamond film interface bonding strength and other defects, is an ideal material for manufacturing cutting tools. In foreign countries, CVD diamond thick film tools have entered the stage of commercial application. In China, the research, development, and industrialization of such tools are also accelerating. It can be foreseen that the application of CVD diamond thick film cutting tools will have a positive and far-reaching impact on the machinery manufacturing industry (especially the automotive industry). 2 CVD diamond thick film tool performance characteristics Hard CVD diamond thick film is pure polycrystalline diamond material, carbon atoms are covalently bonded by SP3 type, its hardness is close to natural diamond, higher than the use of cobalt binder polycrystalline diamond (PCD) . The influence of the grain size of the wear-resistant CVD diamond on the wear resistance is different from that of the PCD. The larger the grain size of the PCD tool is, the better the wear resistance is, and the more favorable it is to the rough machining. For the fine machining that requires a higher surface quality of the workpiece, fine grain PCD is required, but the tool life is reduced. The wear resistance of CVD diamond does not depend on the size of the grain size. The large grain and the small grain material have the same wear resistance (2 to 10 times higher than the PCD), which is very advantageous for finishing. Friction coefficient Because the dangling bonds on the diamond surface are chemically inert due to hydrogen saturation, the polishing surface of the CVD diamond thick film tool has a very low coefficient of friction (0.06 to 0.1), which can effectively reduce the cutting temperature. Thermal conductivity diamond has higher thermal conductivity than all other materials. The thermal conductivity of CVD diamond is 8-20 W/cmK, much higher than the thermal conductivity of silver (4.29 W/cmK). The high thermal conductivity of the CVD diamond thick film tool is conducive to the heat conduction of the cutting edge, which can avoid sticking knife phenomenon due to tool heating, thus increasing the tool life, reducing the thermal damage and thermal deformation of the workpiece. Thermal stability The thermal stability of CVD diamond is much higher than that of polycrystalline diamond (PCD) synthesized under high temperature and high pressure. Due to sintering requirements, a catalytic metal (such as cobalt) is added to the PCD, which generally begins to oxidize at 600°C and graphitizes at 700°C. However, CVD diamond needs to be oxidized at temperatures above 700°C in air, and its thermal stability in vacuum or inert gas can reach more than 1200°C. The extremely high thermal stability and chemical stability of CVD diamond thick film tools can greatly improve their cutting performance, especially for high wear resistant new composites at higher cutting temperatures. The different performance characteristics of single crystal diamond, polycrystalline diamond, and CVD diamond determine their application range. In addition to fracture toughness, other properties of CVD diamond are better than PCD, the application scope of the two tools is basically the same, but PCD tool is more suitable for rough machining, semi-finishing and machining applications with large cutting impact, while CVD diamond thick film tool It has advantages in finishing and continuous cutting.

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Processed material: GFRP
Cutting conditions: ap=0.33mm, f=0.1mm/r, v=80m/min, dry cuttingFig. 1 Comparison of wear of CVD diamond, PCBN and YG8 when turning GFRP

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Material to be processed: Aluminum-based SiCp particle reinforced composite (substrate ZL109, SiCp 20%, particle size about 35μm)
Cutting conditions: ap = 0.3mm, f = 0.1mm/r, v = 76m/min, dry cutting Figure 2 Wear condition of CVD diamond thick film tool when lathing aluminum-based SiCp particle reinforced composite (particle size 35μm)

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Material to be processed: Al-SiCp particle reinforced composite (matrix ZL109, SiCp 20%, particle size about 28μm)
Cutting conditions: ap=0.3mm, f=0.1mm/r, v=70m/min, dry cutting Fig. 3 Wear status of CVD diamond, PCD and PCBN when turning aluminum-based SiCp particle reinforced composites (particle size 28μm) Compared

3 CVD Diamond Thick Film Cutting Tools Test Objectives and Conditions The aim of the cutting test is to study the precision and ultra-precision machining performance of CVD diamond thick film cutting tools and the cutting performance when machining difficult composites. Cutting tests were conducted at a space factory and Beijing Institute of Technology. The test machine used was a Swiss-built Shaublin 125 precision lathe; four CVD diamond thick car blades for testing were welded to a hard alloy body using a CVD diamond thick film with a thickness of 0.6 mm. The total blade thickness was 1.6 mm. 600 times universal tool inspection microscope blade, cutting edge smooth, intact. Test Results and Analysis Hard aluminum alloy (LY12) was machined from a CVD diamond thick car blade. The cutting amount is: cutting depth ap=5-6 ​​μm, feed rate f=0.01 mm/r, cutting speed v=120 m/min. Cutting test results: The surface roughness of the machined surface is Ra = 0.05 μm (the machine performance and useful life limit the further improvement of the surface quality), indicating that the CVD diamond thick film turning tool can be used for precision machining and has the ability to replace natural single crystals. The possibility of diamond turning tools for ultra-precision cutting. Three other CVD diamond thick-film car blades were used to cut three types of composite materials: phenolic plastic based glass fiber reinforced composite (GFRP) and aluminum-based SiCp particle reinforced composites with particle sizes of 35 μm and 28 μm, respectively. The blade geometry angles are: g=0°, a=6°, kr=75°, kr′=15°, and the tip arc radius r=0.3 to 0.35 mm. The J19 universal tool microscope was used to detect the wear width VB of the back face of the tool, and compared with the cutting wear of other types of turning tools. The test results are shown in Figures 1 to 3, respectively. It can be seen from Fig. 1 that when cutting phenolic plastic based glass fiber reinforced composite (GFRP) materials with CVD diamond, PCBN, and cemented carbide (YG8) three kinds of tools, respectively, if the tool wear surface width VB=0.15mm For the blunt standard, the service life of the YG8 turning tool is only 30 minutes; the PCBN turning tool wears after 50 minutes; and the flank wear width VB of the CVD diamond thick turning tool after cutting for 50 minutes is only 0.06 mm. From Figure 2, it can be seen that when turning a typical hard-to-process material—aluminum-based SiCp (particle size about 35μm) reinforced composite material, the flank wear width VB after 30 minutes of CVD diamond thick film turning tool is only about 0.075mm. , Its wear resistance and service life are significantly higher than carbide (YG8) tools and PCBN tools. As can be seen from Fig. 3, when aluminum-based SiCp particle reinforced composites with a particle size of about 25 μm were cut with CVD diamond, PCD, and PCBN respectively, the PCBN tool was blunt after cutting for 5 minutes; the PCD tool was turned after 36 minutes. The surface wear width VB = 0.12 mm, which is nearly blunt; at this time, the flank wear width VB of the CVD diamond thick film turning tool is only 0.08 mm. The test results show that the wear resistance of the CVD diamond thick film tool is the best, followed by the PCD tool, and the PCBN tool is the worst. The above cutting test results are close to the cutting test results given in other documents. In summary, CVD diamond thick film tools have better wear resistance and longer service life than PCD and PCBN tools in semi-precision and precision machining. In addition, CVD diamond tools are also suitable for high-speed machining because CVD diamonds have a low coefficient of friction and high thermal conductivity, allowing higher cutting speeds without harmful heat build-up. 4 Conclusions When CVD diamond thick film tools are used to cut hard aluminum alloy (LY12), surface roughness Ra of 0.05μm can be achieved, so this tool is suitable for precision machining and may replace natural diamond tools for ultra-precision cutting. The CVD diamond thick film cutters are significantly superior to hard alloy cutters, PCBN cutters, and PCD cutters when cutting hard-to-machine materials such as phenolic plastic based glass fiber reinforced composites, aluminum-based SiCp reinforced composites, and the like. The CVD diamond thick film tool has excellent cutting performance and has a wide application prospect in the field of superhard cutting.

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