In the 1950s, after the synthesis of diamond powder by high temperature and high pressure technology, the diamond-based cutting tool, polycrystalline diamond (PCD), was fabricated in the 1970s. The PCD grains were disorderly arranged, and they were not directional, so the hardness was uniform. It has a very high hardness (8000 ~ 12000HV) and thermal conductivity, low coefficient of thermal expansion, high modulus of elasticity and low coefficient of friction, the blade is very sharp. It can process a variety of non-ferrous metals and extremely wear-resistant high-performance non-metallic materials, such as aluminum, copper, magnesium and their alloys, hard alloys, fiber plastic materials, metal matrix composites, wood composites and so on. The average size of diamond grains contained in PCD cutters is different, and the impact on performance is also different. The larger the grain size, the higher the wear resistance. Under similar cutting edge processing, the smaller the grain size, the better the cutting edge quality. For example, a PCD cutter with a grain size of 10 to 25 μm can be used to process aluminum alloys with a Si content of less than 12% at a high-speed grain size of 8 to 9 μm from 500 to 1500 m/min: PCD processed plastics and wood with a grain size of 4 to 5 μm. Wait. For ultra-precision machining, PCD tools with small grain sizes should be used. Usually the PCD tool is sintered into a diamond-hard alloy composite blade for welding on the body. Using an ultra-high pressure device, at a high temperature of 50,000 to 60,000 atmospheres and a high temperature of 1400 to 1600 °C, single crystal diamonds with neat shapes and very few impurities can be artificially synthesized, the quality is uniform and stable, the crystal surface is very clear, and the identification is easy. It has the highest thermal conductivity of all materials and the same strength as natural diamond. The current maximum size is up to 8mm. The uniformity of the size, shape and performance of such single crystal diamonds is not possible in natural diamond products. It has better wear resistance than PCD. The wear resistance of PCD will be weakened when it exceeds 700 °C. Because its structure contains metal Co, it promotes the "reverse reaction", that is, the conversion of diamond to graphite. However, it has good fracture toughness and can be used for interrupted cutting. For example, an aluminum alloy having a Si content of 10% can be milled at a high speed of 2500 m/min. At present, the application of synthetic single crystal diamond tool materials has been rapidly developed, and its new application field is the wood processing industry. There is a growing demand for highly wear-resistant layered wood flooring with an alumina coating on the surface. During processing, the wear-resistant layer of the wood board will cause passivation of the cutting edge, which causes the wear-resistant layer of alumina to be broken. The blade must be sharpened or replaced frequently, and the performance of the artificial single crystal diamond is significantly better than that of the PCD tool.
Chemical vapor deposition CVD diamonds are currently being researched and developed, depositing PCDs with excellent inter-growth, columnar structure and very dense. CVD diamond also exhibits different grain sizes and structures depending on the growth conditions. It does not require a metal catalyst, so its thermal stability is close to that of natural diamond. Depending on the application requirements, different CVD deposition processes can be selected to synthesize PCDs with widely different grain sizes and surface topography. CVD diamond as a tool requires a variety of different grain sizes due to its application. CVD diamond is made in two forms: one is to deposit a thin film (CVD film) having a thickness of less than 30 μm on the substrate: the other is to deposit a substrate-free thick diamond film (CVD thick film) having a thickness of 1 mm. At present, there are not many CVD thin film diamond applications.
The CVD thick film can be brazed to the substrate by special but simple and feasible techniques, but the strength of the solder joint is guaranteed. Compared with PCD, it has good thermal stability but high brittleness and is non-conductive. Cannot be used in electrical discharge machining (EDM) technology. CVD thick film diamonds are popularized in woodworking tools and dressing tools. Due to the high purity and high wear resistance and thermal stability of CVD thick film diamond, it has great potential in the field of high speed machining of high wear resistant materials. CVD thick film diamond tool materials currently available for EDM cutting have also been successfully manufactured and are still to be tested and evaluated. The current cost of CVD thick film diamond is relatively high. With the development of technology, the cost is gradually reduced, and it will be a strong competitor of PCD.
The performance characteristics of the three main diamond tool materials - PCD, CVD thick film and synthetic single crystal diamond are: PCD weldability, mechanical grinding and fracture toughness, wear resistance and cutting edge quality, corrosion resistance Worst. CVD thick film has the best corrosion resistance, mechanical grinding, edge quality, fracture toughness and wear resistance are middle, and weldability is poor. Synthetic single crystal diamond edge quality, wear resistance and corrosion resistance are the best, weldability, mechanical grinding and fracture toughness is the worst.
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