Key words: single crystal diamond tool selection orientation
Material Choice and Orientation of Single-crystal Diamond Cutters
Zhang Jingmin et al
Abstract: The principle and method of material choice and orientation of diamond in manufacturing single-crystal diamond cutters are introduced.
Keywords:single-crystal diamond cutter material choice orientation
In the manufacturing process of diamond tools, the first step is to select materials. According to different processing conditions, selecting the appropriate diamond raw material can avoid the increase of tool manufacturing cost due to the use of high grade rough stone under the premise of ensuring the tool performance. At the same time, for high-quality diamond tools for ultra-precision machining, the tool performance requirements can be fully guaranteed by selecting materials.
The anisotropy of single crystal diamonds varies greatly in different crystal planes and in different directions. Proper orientation not only simplifies the machining process, reduces manufacturing costs, but also increases tool life by a factor of (or even tens of times).
Therefore, when manufacturing single crystal diamond tools, scientific and rational selection and orientation are of great significance for giving full play to the excellent performance of diamond tools and improving processing quality and economic benefits.
First, the selection of diamond tools
1. Classification of diamonds According to the differences in impurities contained in diamond crystals, diamonds can be mainly divided into four categories:
(1) Most types of natural diamonds of type Ia belong to this type, and their nitrogen content is about 3000 ppm. Nitrogen exists in aggregate form, which causes a point defect in the diamond crystal.
(2) Type Ib is very rare in Ib type natural diamond, but almost all synthetic diamonds are Ib type, and its nitrogen content is 40-500ppm. Nitrogen exists in the form of a replacement solid solution, which is uniformly distributed in the diamond lattice to make diamond. It is yellowish green.
(3) Type IIa This type is only present in a small amount in natural diamond, and its nitrogen content is only 20 ppm, which is a high-purity diamond.
(4) Type IIb, that is, semiconductor diamond, which has a nitrogen content of 20 ppm, but contains sufficient boron to form a P-type semiconductor. This type is rarely found in natural diamonds, but can be obtained by special denitrification and boron addition methods when synthesizing diamond.
2. Material selection principle Type Ia is the most common in natural diamonds, and most single crystal diamond tools use this material. Since natural diamonds produce different natural conditions, their quality is highly discrete. Natural diamonds are generally classified into different categories and grades based on the particle size (weight), shape, integrity, transparency, number of cracks and inclusions, color and uniformity of the crystals. According to the use of natural diamond, China divides it into nine categories, such as gemstones, drawing dies, knives, grinding tools and abrasives. Some of them can be subdivided into several grades according to their quality or usage requirements (JC220-79).
The quality requirements of the diamond for the tool are: the crystal is complete, the shape is a dodecahedron, a curved octahedron or a transitional crystal. The minimum diameter of the crystal is not less than 4mm, and the color is colorless, light green, yellow, brown, etc. No crack is allowed. The surface of the crystal is allowed to have inclusions and etch pits of not more than 0.5 mm, and the weight is 0.7 to 3 carats.
With the increasing use of single crystal diamond tools and the continuous improvement of manufacturing technology, the actual range of raw materials to be selected is not limited to the above standards. If advanced single crystal diamond brazing technology is used, a diamond tool with a weight of only 0.05 carat can be made, so that the crystal size requirement is greatly reduced. For jewellery tools, piston knives, contact lens knives and most civilian products with lower precision requirements, only the blade part is required to be free from cracks, impurities and wraps, but its shape, color and quality of the tip of the cutter head are not strict requirements. Applicable diamonds can also be selected from the three-stage grinding wheel material. It also found that dark brown diamonds have a higher tool life.
For ultra-precision knives and ophthalmic scalpels with extremely high precision requirements, it is necessary to select materials from the drawing die class I or even gem-quality rough stones, and it is also necessary to use a polarizing microscope or a more precise instrument to select diamonds with small internal stress. raw material.
In recent years, breakthroughs have been made in the production of large-grain single-crystal diamond by synthetic methods. Both De Beers and Sumitomo have produced large-grain synthetic diamond products with a crystal length of 13 mm. The nitrogen atoms in the synthetic Ib type diamond are uniformly distributed in the diamond lattice in the form of carbon atoms in a single replacement diamond crystal, which reduces the possibility of nitrogen atoms accumulating on the cutting edge of the tool to form a micro-crack. The crystal lattice is uniformly distorted and the hardness of the diamond is increased. For synthetic diamond products specially designed for the manufacture of tools, the internal stress is also optimized to make the product quality more stable, reliable and less discrete. For general purpose synthetic diamond products, there is basically no need to select materials. Since the orientation of the crystal axis of the rough stone is precisely specified at the time of shipment, crystal orientation is not required.
Edge Technologies of the United States has used Sumicrystal from Sumitomo Corporation of Japan to manufacture cutting knives for optical parts such as high-precision mirrors. The life of various tools made with De Beers's monocrystalline (Monodite) is 20% to 200% higher than that of natural diamond.
The disadvantage of the Ib type artificial single crystal is that it is brittle and processing is more difficult, and a fine sharpening method is required to obtain a qualified blade quality, and the price is higher than that of natural diamond.
Second, the orientation of the diamond tool
The orientation of the diamond tool contains two concepts: (1) tool orientation: determine which surface of the diamond is placed on the front and back flank of the tool to achieve higher performance and longer tool life; Crystal orientation: For a diamond single crystal, how to find the direction of its crystal axis, so that the tool face of the tool is accurately placed on the designed crystal face.
1. Tool orientation The determination of the tool orientation scheme is related to the wear mechanism in the machining process.
Early theory believed that the wear form of diamond tools was fragmentation and mechanical wear, and the tool orientation was mostly (110-110), that is, the rake face and the flank face of the tool were placed on two mutually perpendicular (110) faces. At this time, the flow direction of the flank of the rake face, the rubbing direction of the flank face and the machined surface are all in the most difficult direction of the (110) plane. Since the soft direction of the (110) plane is the softest direction in the entire crystal, the process is the best.
Further research indicates that the wear form of diamond tools should also include chemical wear forms such as thermal degradation and corrosion, and in some cases, chemical wear forms dominate. A large number of experimental results also show that the (100) surface has higher resistance to stress, corrosion and thermal degradation than other crystal faces. Based on these facts, the (110-100) orientation scheme is widely adopted. In this scheme, the flank face of the tool is (110) face and the flank face is (100) face. Although the rubbing direction of the rake face and the chip is in the softest direction of the (110) face, the most severely worn back knives On the surface, the rubbing direction is in the most difficult grinding direction of (100), and the ability of the flank to resist other wear is improved while maintaining the resistance to mechanical wear. Another advantage of this orientation scheme is that diamond material can be saved because the diamond rough typically has a maximum length in the <100> direction of the (110) plane.
The wear of diamond tools is a very complicated physical and chemical process. When machining different workpieces under different conditions, the proportion of various wear forms will change. Therefore, the orientation of the diamond tools should be based on the main wear and tear in the machining. Form to choose a reasonable orientation plan.
When processing hard and brittle materials such as ceramics and glass, or when the vibration is large due to machine precision, etc., the wear of the tool is mainly caused by small cracks. Therefore, the tool orientation scheme should have the highest strength of the blade, which can be selected (100-100). ) Orientation.
When processing silicon-aluminum alloy workpieces such as pistons, since the material contains many hard points of silicon compounds, the mechanical wear of the tool has a large specific gravity, so the (110-110) orientation can achieve better processing results.
Chemical wear can be the most important form of wear when processing non-metallic materials with complex compositions, so most (110-100) or even (100-100) orientation schemes are used.
The determination of the orientation scheme also requires a comprehensive consideration of the manufacturability of the tool manufacturing. The easy-grinding direction of the (110) face is softer than the easy-grinding direction of the (100) face, so the (110) face has better workability. The (111) surface is not easy to grind in any direction, and should generally be avoided.
2. The method of crystal orientation crystal orientation can be divided into instrument orientation and manual orientation. The instrument orientation error can be controlled within 1°, and the orientation accuracy is high, but expensive equipment is required and the operation is complicated. In addition, the superiority of instrument-oriented tools compared to manual tool-oriented tools, how much life can be improved, and how much orientation error exceeds the tool performance, the research literature on this aspect is rare. At present, the general diamond tool manufacturers often use manual mesh measurement.
The manual direction is determined by determining the position and direction of the crystal axis of the crystal according to the number and relative position of the original stone faces. For an octahedral crystal, the three mutually perpendicular lines connected by three pairs of symmetric vertices are the X, Y, and Z axes of the crystal. Therefore, the crystal plane of the octahedron is the (111) plane; the eight squares obtained by grinding the vertex perpendicular to the axis are (100) planes; and the edges of the two sides intersecting the edges are grounded at equal angles to obtain (110) face.
The dodecahedral crystal has six vertices formed by four edges, and the line connecting the symmetry vertices is its crystal axis. Its twelve surfaces are (110) faces; the vertices where the six four edges intersect form the (100) face; the vertices where the eight three edges intersect form the (111) face.
Manual visual inspection of crystal orientation is a fast, easy, labor-saving and time-saving orientation method. When encountering defective and irregular crystals or processed tools, it can also be easy to grind and hard according to the crystal faces and crystal faces of diamond. The mutual positional relationship between the grinding directions finds the desired crystal faces through some local, incoherent crystal orientation clues.
Material selection and orientation of single crystal diamond tools
Abstract: The principle and method of diamond material selection and orientation in the manufacturing process of single crystal diamond tools are introduced.