In cutting processing, tool performance has a decisive impact on the efficiency, accuracy and surface quality of cutting processing. There seems to always be a contradiction between the two key indicators of tool performance - hardness and strength (toughness). Materials with high hardness tend to have low strength and toughness, and improving toughness often comes at the expense of a decrease in hardness. Coated tools composed of one or more layers of metal or non-metallic compound films (such as TiC, TiN, Al2O3, etc.) with high hardness and good wear resistance are coated on a soft tool substrate to better solve the problem of tool problems. The contradiction between strength and toughness is a revolution in the development of cutting tools.

1. Characteristics of tool coatings

(1) Good mechanical and cutting properties. Coated tools combine the excellent properties of the base material and the coating material. They not only maintain the good toughness and high strength of the base material, but also have the high hardness, high wear resistance and low friction coefficient of the coating. Therefore, the cutting speed of coated tools can be increased by 2 to 5 times compared with uncoated tools. Obvious economic benefits can be obtained by using coated tools.

(2) Strong versatility. Coated tools have wide versatility and significantly expand the processing range. One coated tool can replace several non-coated tools, thus greatly reducing the variety and inventory of tools, simplifying tool management, and reducing tool and equipment costs.

2. Classification of coatings

According to different coating methods, coated tools can be divided into chemical vapor deposition (CVD) coated tools, physical vapor deposition (PVD) coated tools, hybrid processes and combination technologies. The principle of CVD coating is shown in Figure 1a, and the principle of PVD coating is shown in Figure 1b. The hybrid process is an effective combination of plasma-assisted CVD technology and traditional PVD technology. For example, a traditional CrN hard coating is deposited first, and then a layer of DLC coating is deposited on top to reduce friction. Combination technology is to nitride the surface layer of tools or parts before coating, which can improve the effectiveness of the coating.

CVD can coat TiCN with excellent wear resistance and Al2O3 thick film with excellent heat resistance. Therefore, CVD coating can show long life in high-speed and high-efficiency cutting processes that generate high temperatures.

PVD is generally used under the same or higher cutting speed conditions as uncoated carbide and high-speed steel, with the goal of extending tool life. It has less restriction on the matrix and less damage, so it is particularly suitable for tools that require wear resistance and chipping resistance. It is also suitable for low-feed machining and finishing or threading tools that require sharp edges.

According to the different substrate materials of coated tools, coated tools can be divided into carbide coated tools, high-speed steel coated tools, and coated tools on ceramics and superhard materials (diamond and cubic boron nitride). Coated carbide cutting tools generally use chemical vapor deposition method, and the deposition temperature is around 1000°C. Coated high-speed steel cutting tools generally use physical vapor deposition method, and the deposition temperature is around 500°C.

The diamond coating is synthesized on the cemented carbide substrate using CVD (chemical vapor deposition). The synthetic coating has a hardness and thermal conductivity comparable to that of natural diamond, and exhibits excellent performance in the processing of non-ferrous materials. Due to its good cutting performance, diamond-coated tools have broad application prospects in the field of cutting processing. They are ideal tools for processing graphite, metal matrix composites, high-silicon aluminum alloys and many other wear-resistant materials. Currently, they are mainly used in is the automotive and aerospace industry.

According to the nature of the coating material, coated tools can be divided into two categories, namely "hard" coated tools and "soft" coated tools. The main goals pursued by "hard" coated tools are high hardness and wear resistance. Its main advantages are high hardness and good wear resistance. Typical ones are TiC and TiN coatings. Various coated tools are shown in Figure 4 . "Soft" coated tools are tools prepared with solid lubricants such as MoS2, WS2, etc. The goal of "soft" coating is low friction coefficient, also known as self-lubricating tool. Its friction coefficient with the workpiece material is very low. It is only about 0.1, which can reduce stickiness, friction, cutting force and cutting temperature.

Coating cutting tools is one of the important ways to improve the performance of cutting tools. The emergence of coated cutting tools has greatly improved the cutting performance of cutting tools, and the application fields are constantly expanding. Coated cutting tools have great potential in the field of CNC machining and will It is the most important tool variety in the field of CNC machining in the future. At present, the coating proportion of foreign carbide indexable inserts is more than 70%, and the coating proportion of European gear cutters is as high as 90%. Coating technology has been applied to end mills, reamers, composite hole machining tools, gear hobs, shaving cutters, forming broaches and various machine-clamped indexable inserts to meet the needs of high-speed cutting of various steels and cast irons, and resistant materials. Thermal alloys and non-ferrous metals and other materials are needed.

How many types of coating, do you know?

TIN Coating

TiN is a general-purpose PVD coating that has a high cost performance and is a fairly stable compound that can increase tool/tool hardness and have a high oxidation temperature. This coating can achieve very good processing results when used on high-speed steel cutting tools or forming tools.

The titanium nitride coating and its sintered body have a satisfactory golden color and can be used as gold-substitute decorative materials. They have good imitation gold effect and decorative value, and are anti-corrosion and extend the life of handicrafts.