Research on Sharpening Characteristics of Single Crystal Diamond Tools
1 Introduction
In ultra-precision machining, the main factors to ensure the quality of the machined surface are high-quality tools, in addition to high-precision machine tools and ultra-stable machining environments. Natural diamond has high hardness, good wear resistance, high strength, good thermal conductivity, low friction coefficient with non-ferrous metals, good anti-adhesion and excellent corrosion resistance and chemical stability. It can sharpen sharp edges and be sharpened. It is considered to be the most ideal tool material for ultra-precision cutting, and has an important position in the field of machining, especially in the field of ultra-precision machining.
2 Physical properties of single crystal diamond
Diamond is a crystal of a single carbon atom, and its crystal structure belongs to the equiaxed face-centered cubic system (a system with the highest atom density). Since the bond between carbon atoms in diamond is sp3 hybrid covalent bond, it has strong binding force, stability and directionality. It is the hardest substance known in nature at present, and its microhardness can reach 10000HV. Other physical properties are shown in the table.
Table 1 Physical properties of diamond Physical properties - numerical hardness -60,000 to 100,000 MPa, depending on crystal direction and temperature Bending strength -210 to 490 MPa
Compressive strength -1500～2500MPa
Elastic modulus - (9 ~ 10.5) × 10 12 power MPa
Thermal conductivity -8.4～16.7J/cm·s·°C
Mass heat capacity -0.156J/(g·°C) (normal temperature)
Start oxidation temperature -900~1000K
Start graphitization temperature -1800K (in inert gas)
Friction coefficient between aluminum alloy and brass -0.05 to 0.07 (at normal temperature)
In the late 1970s, in the research of laser nuclear fusion technology, it was necessary to process a large number of high-precision soft metal mirrors, which required the surface roughness and shape accuracy of soft metals to reach ultra-precision levels. If the traditional grinding and polishing methods are used, not only the processing time is long, the cost is high, the operation is difficult, and the required precision is not easily achieved. Therefore, there is an urgent need to develop new processing methods. Driven by real-world demand, single-crystal diamond ultra-precision cutting technology has been rapidly developed. Due to the physical properties of single crystal diamond, it is not easy to stick to the knife and produce built-up edge when cutting. The surface quality is good. When processing non-ferrous metals, the surface roughness can reach Rz0.1~0.05μm. Diamond can also effectively process non-ferrous metal materials and non-metallic materials, such as non-ferrous metals such as copper and aluminum and their alloys, ceramics, unsintered hard alloys, various fiber and particle reinforced composite materials, plastics, rubber, graphite, glass. And a variety of wear-resistant wood (especially solid wood and plywood, MDF and other composite materials).
3 Sharpening characteristics of natural single crystal diamond tools
In ultra-precision machining, the two basic accuracies of a single crystal diamond tool are the accuracy of the blade profile and the blunt radius of the cutting edge. It is required that the circular tool cutting edge for machining an aspherical lens has a roundness of 0.05 μm or less, and the blade straightness for processing a polyhedral mirror is 0.02 μm; the blunt radius of the cutting edge (Ï value) indicates the cutting edge of the tool. Sharpness, in order to adapt to various processing requirements, the blade edge radius ranges from 20nm to 1μm.
3.1 Crystal face selection of single crystal diamond tools
Diamond crystals belong to the cubic system. Due to the difference in atomic arrangement and atomic density on each crystal plane and the difference in the distance between crystal faces, the anisotropy of natural diamond crystals is caused. Therefore, the physical and mechanical properties of diamond not only on the crystal faces are exhibited. Different, its manufacturing difficulty and service life are different, and the microscopic damage strength of each crystal face is also significantly different. The microscopic strength of diamond crystals can be determined by the Hertz test method. Since diamond is a typical brittle material, its strength value generally deviates greatly, mainly depending on the shape and distribution range of the stress distribution, so it is suitable for analysis by probability theory. When the applied stress is the same, the (110) crystal plane has the highest damage probability, the (111) crystal plane is the second, and the (100) crystal plane has the smallest probability of damage. That is, under the action of external force, the (110) crystal plane is most likely to be broken, the (111) crystal plane is second, and (100) is the most difficult to break. Although the grinding rate of the (110) crystal plane is higher than that of the (100) crystal plane, the experimental results show that the (100) crystal plane has higher resistance to stress, corrosion and thermal degradation than other crystal planes. Combined with the micro-strength comprehensive consideration, using the (100) surface as the front and back knives of the tool makes it easy to sharpen the high-quality cutting edge and is less prone to micro-cracking.
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