Coating, that is, cemented carbide or high-speed steel tool, can not only improve the wear resistance of tool material, but also not reduce its toughness by coating a layer of refractory metal compound with good wear resistance on its upper surface through chemical or physical methods.
（1） Type of tool coating
According to different coating methods, coated tools can be divided into chemical vapor deposition (CVD) coated tools and physical vapor deposition (PVD) coated tools.
The principle of CVD coating is shown in Fig. 2-19, and the principle of PVD coating is shown in Fig. 2-20.
Fig. 2-19 CVD coating principle
Fig. 2-20 PVD coating principle
According to the different base materials of coated tools, coated tools can be divided into cemented carbide coated tools, high-speed steel coated tools and coated tools on ceramics and superhard materials (diamond and cubic boron nitride).
Coated cemented carbide tools generally adopt chemical vapor deposition method, and the deposition temperature is about 1000 ℃.
Coated high-speed steel tools generally adopt physical vapor deposition method, and the deposition temperature is about 500 ℃.
Diamond coating was synthesized on cemented carbide substrate by CVD (chemical vapor deposition).
The synthesized coating has similar hardness and thermal conductivity to natural diamond, and plays an excellent role in the processing of non-ferrous materials.
Because of its good cutting performance, diamond coated tools have broad application prospects in the field of cutting and machining.
They are ideal tools for machining graphite, metal matrix composites, high silicon aluminum alloys and many other wear-resistant materials.
At present, their main application fields are automotive and aerospace industries.
See Fig. 2-21 for the structure of diamond coated tools.
Fig. 2-21 structure of diamond coated tools
DLC is an amorphous hard carbon film with the intermediate properties of graphite and diamond composed of the same carbon atoms.
DLC coating generally has low friction coefficient with non-ferrous metals and is used for processing non-ferrous materials such as aluminum alloys.
The characteristics of DLC coating are shown in table 2-21.
Table 2-21 characteristics of DLC coating
|Type||Hardness (HV)||Friction coefficient|
According to the properties of coating materials, coated tools can be divided into two categories, namely “hard” coated tools and “soft” coated tools.
The main goal of “hard” coated tools is high hardness and wear resistance.
Its main advantages are high hardness and good wear resistance.
The typical coatings are tic and tin.
Various coated tools are shown in Fig. 2-22.
“Soft” coated tools are made of solid lubricants such as MoS2 and WS2.
The goal of “soft” coating is to achieve low friction coefficient, also known as self-lubricating tools. Its friction coefficient with workpiece material is very low, only about 0.1, which can reduce adhesion, friction, cutting force and cutting temperature.
Fig. 2-22 coated tool (piece)
（2） Characteristics of tool coating
1. Good mechanical and cutting performance
The coated tool combines the excellent properties of the base material and the coating material, which not only maintains the good toughness and high strength of the base material, but also has the high hardness, high wear resistance and low friction coefficient of the coating.
Therefore, compared with uncoated tools, the cutting speed of coated tools can be increased by 2~5 times, and the use of coated tools can obtain obvious economic benefits.
2. Strong versatility
Coated tools have wide versatility and significantly expanded processing range.
One coated tool can replace several uncoated tools, which can greatly reduce the variety and inventory of tools, simplify tool management and reduce the cost of tools and equipment.
See table 2-22 for the emergence age and applicable processing fields of various tool coating technologies.
Table 2-22 age of various tool coating technologies and applicable processing fields
|Time||Coating composition||Coating method||Main application fields|
|1968||TiC, TiN||CVD||Cemented carbide tools|
|1973||TiCN, TiC+Al2O3||CVD||Cemented carbide tools|
|1979||TiN||PVD||High speed steel tool coating|
|1981||TiC+Al2O3+ TiN Al-O-N||CVD||Cemented carbide tool coating|
|1982||TiCN||MT-CVD||Cemented carbide tool coating|
|1984||TiCN||PVD||Coating of carbide, high speed steel milling cutter and drill bit|
|1986||Diamond, CBN||CVD, PVD||Cemented carbide tool coating|
|1989||TiAIN||PVD||Cemented carbide milling cutter coating (processed with dry steel and cast iron)|
|1990||TiC, TiN, TiCN||PCVD||Moulds, thread cutters, milling cutters, etc|
|1991||TiAIN+CrC||PVD||Turning and milling titanium alloy|
|1993||TiN+TiCN (CVD) +TiN (PVD)||CVD+PVD||Carbide milling tools|
|1993||CrN||PVD||Titanium alloy and copper alloy processing|
|1994||MoS2||PVD||High speed complex tool coating|
|1995||TiN+AiN||PVD||Coating of carbide milling insert|
|1996||Thick film fibrous TiCN||MT-CVD||Coating of carbide turning tools (for rough and fine machining)|
|1996||CNx||CVD, PVD||High speed steel tool coating|
|2000||TiAICN||PVD||Cemented carbide tool coating|