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Fig. 1 Progress in the research of superhard tool coatings (in chronological order) Table 1 summarizes the advantages of superhard wear-resistant coated tools. For example, superhard coated tools can reduce tooling costs by regrinding and recoating compared to PCBN or PCD tools. Table 1 Advantages of superhard coated tools Advantages of coated tools (compared to uncoated tools) Advantages of superhard tools (compared to other tools) Advantages of coated tools (compared to PCD tool tools) Tool life Long high hardness (good hot hardness) Applicable to complex tools Cutting time Short wear resistance Good cutting edge Arc radius Small cutting amount Large friction coefficient Small back angle Cutting force Small resistance to oxidation Strong no chamfering Friction coefficient Small chemical stability Lower cost Adaptability to specific processing operations No coolant needed, no cooling No Binder-Free Diffusion Diamond Application Diamond is a naturally pure crystalline carbon with its atomic structure covalently bonded to a face-centered cube. Diamonds have exceptionally high physical properties, such as the highest hardness in nature, extremely high thermal conductivity, and excellent insulating properties. Because diamond has excellent wear resistance and chemical stability for non-ferrous materials, it is very suitable as a cutting tool for cutting applications. In the past, diamond was mainly used as an abrasive or cutting edge in production. In the second half of the 20th century, with the increasing use of light alloy materials in the automotive industry, the importance of diamond tools in the field of cutting is increasingly evident. The ever-increasing demand for diamond has led to the need for synthetic diamond to replace (supplement) natural single crystal diamond (MCD). There are two kinds of thermodynamic methods for artificially synthesizing diamond: one is a high temperature and high pressure synthesis (HPHT) method with a pressure of 6 GPa and a temperature of 1700 °C; the other is a low pressure synthesis method with a pressure of 10 OhPa and a temperature of 1000 °C. In 1955, General Electric made a breakthrough in artificial diamond synthesis using high temperature and high pressure synthesis. In the mid-1970s, PCD was introduced as a composite tool material. PCD is formed by bonding diamond grains together with a cobalt binder and can be used as an integral tip material that is welded to the body. Replacing PCD with CVD Diamond While studying PCD materials, low pressure synthesis of diamonds was achieved using a chemical vapor deposition (CVD) coating process. In 1975, scientists of the former Soviet Union deposited for the first time a CVD diamond tantalum layer on a non-diamond substrate. This process significantly improves the performance of diamond as a tool material. CVD diamond coated knives can be classified into thick film tools and thin film knives. When a CVD diamond thick film tool is prepared, a thick diamond film having a thickness of several hundred micrometers is first deposited on the substrate, and is then cut into small pieces by a laser beam and welded to the body. In contrast, CVD diamond film tools deposit diamond films of a few microns thick directly onto a shaped tool in a complex manner. Because no metal bond is used, CVD diamond-coated tools outperform PCD tools in hardness, wear resistance and high temperature resistance. In addition, diamond coated tools also have a great advantage in the flexibility of tool geometry design. In 1993, the commercialized diamond coated indexable blades were first introduced. Today, there are a variety of specially designed diamond tool coatings. In addition to the standard microcrystalline diamond film mainly composed of cubic crystal grains, a nanocrystalline diamond film composed of a Baras structure and a multi-layered diamond film alternately coated with a micrometer and a nanocrystalline film layer were also prepared. In addition, the microstructured microcrystalline diamond coating that locates the diamond facets on the tool cutting edge surface is also being developed and analyzed. 
Tool: VCGT160412; Cutting amount: f=0.1mm, Vc=600m/min, ap=1mm; Coolant: 5%
Figure 2 The relationship between flank wear band width and cutting length when CVD diamond coated tool and uncoated carbide tool turning AlSi20 cast iron workpiece outer ring CVD diamond tool wear resistant coating shown in Figure 2, When processing highly corrosive hypereutectic Al-Si alloys, the tool life of nanocrystalline diamond-coated carbide-coated carbide tools is increased by more than 20 times compared to uncoated carbide tools, thereby reducing the cost of removing the unit volume of material. Indexable blade costs. In addition, because diamond-coated tools do not produce built-up edge, the surface roughness of the machined upper part is significantly improved compared to ordinary carbide tools. Development of CBN Tool Coatings On the one hand, various physical vapor deposition (PVD) and plasma accelerated chemical vapor deposition (PACVD) processes have been successfully applied to CBN coating preparation. Typically, the thickness of the deposited CBN coating is limited to less than 500 nm, mainly due to the presence of large residual compressive stress inside the over-thick CBN coating, poor adhesion, and lack of long-term stability under ambient conditions. On the other hand, the development of CBN coatings on application-related silicon-free substrates remains a technical challenge. Nonetheless, the Fraunhofer Surface Engineering and Film Research Institute (IST) in Braunschweig, Germany, has developed a method for obtaining a thicker CBN coating based on the combination of a B4C coating and a BCN gradient coating. Improve the adhesion and stability of the plant CBN coating system without substantial reduction. The Fraunhofer Institute for Ceramics and Sintering Technology (IKTS) in Dresden provided a CBN coating solution with a thickness greater than 2 MICRO;M on a silicon substrate while also depositing on a carbide cutting insert, at IST below 600°C. The temperature was successfully implemented and analyzed at the Institute of Machine Tools and Factory Management (IWF) of the Technical University of Berlin through friction and turning tests. Table 2 Performance of Various Coatings Coating Type WC-Co CBN
(Si matrix) CBN
(WC-Co Matrix) Diamond
(WC-Co matrix) TiN
(WC-Co matrix) TiAlN
(WC-Co matrix) Maximum thickness - 2.5 2 >3 >3 >3 Microhardness 8 to 25 60 to 65 55 to 60 80 to 100 25 to 30 22 to 28 Vickers hardness [HV0.01] 800 to 2500 to 5800 ～5500 - ～2800 ～2400 Elasticity [GPa] 550～650 500～550 500～550 1050 300 330～350 Abrasion Resistance
[m3m-1N-110-15] n/a ~ 0.4 ~ 0.6 - 5 ~ 7 5 ~ 7 Friction coefficient [-] n/a 0.4 0.4 << 0.7 0.6 Scratch test [N] - 18 to 20 ~ 35 - 50 >25 Carbide-based CBN coating system hardness is more than 2 times that of standard TIN coating, wear resistance increased by more than 10 times, and the coefficient of friction with mating material steel is only half of TIN coating ( See Table 2). These CBN-coated tools have been successfully tested in the turning of alloy steels, hardened steels, ductile irons and nickel-base superalloys. Turning test of diamond-coated tools CBN coating system is usually polished with K10 grade cemented carbide with the smallest grain size as a substrate, coated with titanium atomic layer to enhance adhesion, and then depositing B4C coating and BCN. Gradient coating and finally coating the surface with a CBN coating. However, the most advanced coating system used previously was to use TiN or TiAlN thick coatings instead of Ti atomic layers to provide better adhesion between the substrate and the CBN coating system. Fig. 3 compares the tool life of a TiN coating with TiN and TiAlN coatings with (B4C-RCN-) CBN added to the surface when turning hardened steel. As shown in Figure 3, by depositing an additional CBN surface layer on the TiN base coating, the tool life can be significantly improved. In particular, at the maximum cutting speed of 100 m/min, the tool life can be increased by almost 3 times. In contrast, the TiAlN-CBN coating performs even better. When the cutting speed is increased from 80m/min to 100m/min, the service life of TiAlN-CBN coated tools is greatly increased, although there may be reasons for the softening of the workpiece material due to the increase of cutting temperature. In a comparative test using an expensive PCBN tool, when the cutting speed was 100 m/min, the life of the PCBN tool was only 54 minutes, which was only increased by 33%. It is worth noting that, unlike used PCBN tools, a coating indexable insert designed with a CNMA geometry can be used (indexed) four times, so it is cost-effective. 
Tool: CNMA120408 Workpiece Material: 1.2344(52HRC); Cutting amount: f=0.1 mm, ap=0.5mm; Dry cutting 
Fig. 3B-G Wear process of rake face of TiAlN-CBN coated tool Fig. 3A Relationship between tool life of different coating and cutting speed in turning hardened steel Fig. 3B-G shows TiAlN-CBN coated tool (unpolished) The substrate was coated with 2 μm thick TiAlN and 1.2 μm thick CBN) at different stages of the wear process in turning (Vc=80 m/min, ap=0.25 mm). Initially, some of the workpiece material adhered to the contact area to protect the tool; then the built-up edge began to form (Figure 3D) and was subsequently removed (Figure 3F). This process is a disadvantage because it fills the rough surface of the unpolished substrate (usually a rough surface which is advantageous for the adhesion of the coating and was tested for turning due to the very good milling effect). However, it can be seen in FIG. 3F that the cutting edge is penetrated in some small areas, thereby resulting in greater breakage of the cutting edge shown in FIG. 3G. Turning tests again demonstrate that the developed coating system has good wear resistance because the uncoated substrate can only stand for a few seconds under such severe processing conditions. The diamond-coated tool milling test is followed by the performance evaluation of the CBN coating system under interrupted cutting conditions. In order to carry out the milling test, IKTS (Ferwa Hefei Institute of Ceramics and Sintering Technology) specially developed various cemented carbide substrates with different grain sizes and different cobalt contents. In Figure 4, the superior cutting performance of TiAlN-CBN coatings is demonstrated by comparison with standard TiN, TiAlN coatings and commercial CVD multilayer coatings. For example, when the feed length reaches 1800mm, the flank wear of the two TiAlN-CBN coated tools used in the test is still only 0.1mm. At this time, the flank wear of the pure TiAlN coated tool is approximately twice that of the former. . In contrast, CVD coated tools and TiN coated tools perform significantly worse than milled hardened steels. In addition, the wear progression of the TiAlN-CBN coated tool shown in Figure 4B-G shows once again that the newly developed CBN coating system has good enough adhesion and hardness when milling hardened steel because the coating achieves The tool rake face is fully protected until the tool life criteria are met. If a rotating device is installed in the deposition chamber of the IST coating equipment, the cutting performance of the coating is likely to be further improved, because at this time the CBN surface layer may be mainly deposited on the rake face of the tool. 
Face milling; cutter: SNUN150612, Dc=53mm, z=l; Workpiece material: 1.2080 (365HV62.5); dry cutting; cutting amount:, Vc=70m/min, f=0.1mm, ap=0.5mm, ae= 5mm 
Figure 4B-G Wear process of front and back face of TiAlN-CBN coated tool Fig. 4A Relationship between wear width and cutting speed of flank wear of different coated cutters when milling hardened steel Conclusion Due to special requirements for superhard materials With the application, the following conclusions can be drawn: Successful cutting (turning and milling) tests on various workpiece materials using the innovative TiN-CBN and TiAlN-CBN coatings. The results show that in the turning and milling of hardened steel, the tool performance is significantly improved due to the additional CBN surface layer on TiN and TiAlN standard coatings. Compared to uncoated carbide tools, CVD diamond coated tools have several times longer life. In addition, unlike cemented carbide and PCD, CVD diamond does not contain a cobalt binder material, so it has better wear resistance and adhesion, and can obtain a better processing table and quality. This article describes the development trend of combining the good wear resistance of superhard cutting tool materials with the advantages of tool coating. At present, the research of CBN coatings focuses on multi-layer composite coating systems with CBN as the surface; and CVD diamond coating. The research focus of the layer is mainly on the pure single crystal or polycrystalline diamond coating.
Super hard tool coating
The deposition of an ultra hard thin layer composed of CVD diamond and PVD cubic boron nitride (CBN) on the surface of the tool has great application potential, because of the highest hardness of the two functional wear-resistant coatings and tool geometry parameters in all materials. The combination of flexibility to meet the specific requirements of a given machining task enables efficient machining strategies (such as high-speed cutting) to be implemented more efficiently, with the result that competitive advantage is enhanced by reducing production time and manufacturing costs. Therefore, it is important to develop innovative tool coatings that combine the advantages of coatings with superhard cutting materials. At the same time, both superhard tool coatings perform well under continuous and interrupted cutting conditions. This article gives some test results on CBN and diamond coatings and comparison of different stages of development. According to the ISO 513 standard, superhard tool materials include polycrystalline cubic boron nitride (PCBN) and polycrystalline diamond (PCD). Industrial tooling manufactured from superhard materials is worth about $4 billion. As shown in Fig. 1, the main trend in promoting the development of super-hard tool materials is reflected in two aspects: the tip material used as a tool and the wear-resistant coating applied to the surface of the tool body in order to make the tool get stronger. The features and flexibility of higher geometry design. However, the study of non-natural material CBN does not progress as fast as diamond research.