Research on Surface Coating Technology of Cemented Carbide Cutting Tool

With the development of the modern machining industry towards high precision, high speed cutting, hard machining instead of grinding, dry machining (no coolant) to protect the environment and reduce costs, there are quite high requirements for tool performance. It is an inevitable development trend to develop various cutting materials with superior wear resistance and stable processing for a long time.

In the cutting process, the performance of the cutting tool has a decisive influence on the efficiency, precision and surface quality of the cutting process. There are always contradictions between the two key indicators of tool performance—hardness and strength. Materials with high hardness have low strength, and increasing strength is often at the expense of reducing hardness. In order to further improve the wear resistance of cemented carbide tools, a cemented carbide coating has been developed. The coating of cemented carbide acts as a chemical barrier and thermal barrier, reducing the wear of the crescent of the carbide tool. The coating has a high coefficient of friction, which can significantly increase its service life.

The so-called cemented carbide coating refers to coating a layer of material with high hardness and wear resistance on the surface of cemented carbide. Coating one or more layers of metal or non-metallic compound films (such as TiC, TiN, Al2O3, etc.) with high hardness and good wear resistance on the tool substrate can better solve the problem of the strength and The contradiction between hardness, this is a revolution in the development of cutting tools.

Here, the characteristics of coated cemented carbide, the mechanism, advantages and disadvantages of cemented carbide coating methods, and the types, characteristics and applications of cemented carbide coatings are reviewed.

1. Characteristics of cemented carbide coating

Coated carbide has the following characteristics:

(1) The coated tool combines the advantages of high strength, high toughness of the substrate and high hardness and high wear resistance of the coating, which improves the wear resistance of the tool without reducing the toughness of the substrate. Therefore, coated cemented carbide has high hardness and excellent wear resistance;

(2) The coating film has little effect on the toughness of the substrate;

(3) Reduce the friction coefficient between the tool and the workpiece;

(4) Long service life of coated cemented carbide, etc.

2. Carbide coating method

The coating methods of cemented carbide include high temperature chemical vapor deposition coating (CVD), physical vapor deposition coating (PVD), plasma chemical vapor deposition coating (IBVD), medium temperature chemical vapor deposition coating (MTCH) and ion-assisted physical Vapor deposition coating (IBVD), etc., the most commonly used methods are high temperature chemical vapor deposition coating and physical vapor deposition coating, and different coating methods have different coating mechanisms, advantages and disadvantages.

2.1 High temperature chemical vapor deposition coating

Since the appearance of high temperature chemical vapor deposition coating in the 1960s, it has been widely used in cemented carbide tools.

2.1.1 Mechanism of High Temperature Chemical Vapor Deposition Coatings

High-temperature chemical vapor deposition coating refers to the interaction of the mixed gas of the coating material on the surface of the hard alloy under certain temperature conditions to decompose some components in the mixed gas and form a coating of metal or compound on the surface of the hard alloy. layer. The key to this method:

(1) The interaction between the mixed gas of the coating material and the surface of the cemented carbide, that is, the reaction between the mixed gas of the coating material on the surface of the cemented carbide to produce deposition, or through a combination of the mixed gas of the coating material Particles react with the cemented carbide surface to produce deposits;

(2) The deposition reaction must be carried out under certain energy activation conditions.

2.1.2 Advantages of High Temperature Chemical Vapor Deposition Coatings

High temperature chemical vapor deposition coatings have the following advantages:

(1) The preparation of the coating source required for high temperature chemical vapor deposition is relatively easy;

(2) Single-layer and multi-layer composite coatings such as TiC, TiN, TiCN, TiB, Al2O3 can be realized;

(3) The bonding strength between the coating and the substrate is high;

(4) The coating has good wear resistance.

2.1.3 Defects of high temperature chemical vapor deposition coatings

Although the high-temperature chemical vapor deposition coating has the above advantages, it also has inherent defects. Its flaws:

(1) The coating temperature is 900 ° C ~ 1100 ° C, that is, the coating temperature is high, so that a brittle decarburization layer (η phase) is easily formed between the coating and the substrate, resulting in brittle fracture of the tool and a decrease in bending strength ;

(2) The interior of the coating is in a state of tensile stress, which can easily lead to microcracks during use;

(3) The exhaust gas and waste liquid discharged during the coating process will cause industrial pollution and have a greater impact on the environment, which is in conflict with the current green industry advocated. constraints.

2.2 Physical vapor deposition coating

Physical vapor deposition coatings appeared in the late 1970s. The successful application of its technology in the field of high-speed steel cutting tools has attracted great attention from all over the world. While people are competing to develop high-performance and high-reliability coating equipment, they are also paying attention to The expansion of its application field has carried out more in-depth research, especially in the application of cemented carbide and ceramic cutting tools.

2.2.1 Mechanism of Physical Vapor Deposition Coatings

Physical vapor deposition coating refers to the physical method of vaporizing the coating material into atoms, molecules or ionization into ions under vacuum conditions, and then depositing a coating on the surface of the cemented carbide through a gas phase process. The process of this method can be divided into three steps: the first step is to vaporize the coating material, that is, the coating material evaporates, sublimes and decomposes, etc., and becomes the coating source; the second step is the migration of atoms, molecules or ions of the coating material The process to the surface of cemented carbide, in which atoms, molecules or ions may collide, resulting in a series of complicated processes such as ionization, recombination, reaction, energy change and change of running direction; the third The first step is the adsorption, accumulation, nucleation and growth of the atoms, molecules or ions of the coating on the surface of the cemented carbide to finally form a coating.

2.2.2 Advantages of Physical Vapor Deposition Coatings

Physical vapor deposition coatings have the following advantages:

(1) Compared with the chemical vapor deposition layer, the temperature of the physical vapor deposition coating is low, generally below 600 ° C, and has little effect on the bending strength of the tool material;

(2) The inside of the coating is in a state of compressive stress, which is more suitable for the coating of hard alloy precision and complex tools;

(3) No pollution to the environment, in line with the current development direction of green industry;

(4) Due to the emergence of nano-scale coatings, the quality of physical vapor deposition coating tools has made a new breakthrough. This coating not only has high bonding strength, high hardness and good oxidation resistance, but also can effectively control precision tool edges. Mouth shape and precision.

2.2.3 Defects of physical vapor deposition coatings

Although physical vapor deposition coating has the above advantages, there are also some defects as follows:

(1) Coating equipment is complex, expensive, high process requirements, long coating time and increased cost of tools, etc.;

(2) The impact resistance, hardness and uniformity of the tools produced by this method are worse than those produced by high-temperature chemical vapor deposition coatings, and the service life is shorter than those produced by high-temperature chemical vapor deposition coatings;

(3) The geometric shape of the coated tool is single, which limits its superiority;

(4) When the coating and the substrate are cooled, internal stress and microcracks are generated due to different shrinkage rates.

2.3 Plasma chemical vapor deposition coating

Since both high-temperature chemical vapor deposition coatings and physical vapor deposition coatings have certain defects, in recent years, foreign countries have developed plasma chemical vapor deposition coatings by combining high-temperature chemical vapor deposition coatings and physical vapor deposition coatings. Deposit coating.

2.3.1 Mechanism of plasma chemical vapor deposition coating

Plasma chemical vapor deposition coating refers to the generation of high-energy electrons through electrode discharge to ionize the gas into plasma, or the introduction of high-frequency microwaves into carbide-containing gases to generate high-frequency high-energy plasma. The active carbon atoms or carbon-containing groups in the hard A method of depositing coatings on the surface of high-quality alloys.

2.3.2 Advantages of plasma chemical vapor deposition coatings

Advantages of plasma chemical vapor deposition coatings:

(1) It uses plasma to promote chemical reactions, which can lower the coating temperature below 600°C;

(2) Due to the low temperature of the coating, no diffusion, phase change or exchange reaction occurs between the cemented carbide substrate and the coating material, so the substrate can maintain its original strength and toughness.

2.3.3 Disadvantages of plasma chemical vapor deposition coatings

Disadvantages of plasma chemical vapor deposition coatings:

(1) The equipment investment is large, the cost is high, and the purity requirements of the gas are high;

(2) Severe noise, strong light radiation, harmful gas, metal vapor dust, etc. generated during the coating process are harmful to the human body;

(3) It is difficult to coat the inner surface of the small hole diameter.

2.4 Medium temperature chemical vapor deposition coating

2.4.1 Mechanism of medium temperature chemical vapor deposition coating

The mechanism of medium temperature chemical vapor deposition coating is the same as that of chemical vapor deposition coating, except that the coating temperature of the former is lower than that of the latter, and the coating temperature of medium temperature chemical vapor deposition coating is generally 700 ℃ ~ 900 ℃.

2.4.2 Advantages of medium temperature chemical vapor deposition coatings

Advantages of moderate temperature chemical vapor deposition coatings:

(1) Fast deposition rate;

(2) Coating thickness;

(3) Uniform coating for workpieces with complex shapes;

(4) High coating adhesion;

(5) The internal residual stress of the coating is small.

These make this method easy to industrialize. Therefore, it is a coating method superior to high temperature chemical vapor deposition coatings.

2.4.3 Disadvantages of MTCVD coatings

Disadvantages of MTCVD coatings:

(1) The inside of the coating layer is in a state of tensile stress, which is easy to cause microcracks during use;

(2) The exhaust gas and waste liquid discharged during the coating process will cause industrial pollution and have a greater impact on the environment, which is in conflict with the current green industry advocated. Therefore, the development of this method has been restricted to a certain extent.

2.5 Ion Assisted Physical Vapor Deposition Coatings

2.5.1 Mechanism of ion-assisted physical vapor deposition coating

Ion-assisted physical vapor deposition coating means that while the coating is deposited in the cold phase, an ion beam with a certain energy is used to bombard the continuously deposited material, so that the deposited atoms and the matrix atoms are continuously mixed, and the atoms at the interface penetrate and dissolve into one, thus Greatly improved the bonding strength of the coating and the substrate.

2.5.2 Advantages of ion-assisted physical vapor deposition coatings

Ion-assisted physical vapor deposition coating has the advantages of vapor deposition and ion implantation. The deposition temperature can be lowered to 200°C-500°C, so C, N, and B compounds can be prepared at lower temperatures, which can be used for hard alloys that are difficult to coat due to poor bonding.

2.5.3 Disadvantages of ion-assisted physical vapor deposition coatings

The disadvantages of ion-assisted physical vapor deposition coatings are the same as those of plasma chemical vapor deposition coatings.

3. Types of coated cemented carbide

3.1 Single infiltration layer coated cemented carbide

Commonly used single-layer coated cemented carbide materials include TiC, TiN, Al2O3, TiAlN, TiCN, etc. Due to their different characteristics, the suitable processing conditions are also different.

3.1.1 TiC coated cemented carbide

The color of TiC is gray, the surface hardness is HV3000, the initial oxidation temperature is 400°C, and the adhesion strength is average. Although its surface hardness is high, its adhesion and heat resistance are relatively low.

Carbide coated with TiC first appeared as a coated tool. Its coating has a thickness of 5-7μm and a hardness of HV2300-3250. The main features of TiC-coated cemented carbide: the coating has high hardness and is easy to diffuse into the matrix, so the coating has high bonding strength with the matrix, anti-sticking, high durability and good oxidation resistance. This kind of coated cemented carbide tool has a small friction coefficient against steel, which is suitable for processing steel, and the tool wear is small during cutting. However, there will be a decarburization layer (brittle phase) between the coating and the substrate, and the decarburization layer will increase with the thickness of the coating, resulting in a decrease in the bending strength of the blade, an increase in brittleness, and easy collapse during cutting knife.

3.1.2 TiN coated cemented carbide

The color of TiN is golden yellow, the surface hardness is HV2000, the initial oxidation temperature is 600°C, and the adhesion strength is very good. Its thermal expansion coefficient is similar to that of high-speed steel, and it has good adhesion to high-speed steel.

The thickness of the hard alloy coating coated with TiN is 8-12 μm, and its main characteristics are: the hardness of the coating is lower than that of TiC, and the bonding strength with the substrate is worse than that of TiC. However, the thermal conductivity of TiN coating is good, and the friction coefficient with iron base material is smaller than that of TiC coating, so the anti-crescent wear performance is better. In addition, brittle phases are not easily produced between the TiN coating and the substrate, so the allowable thickness of the coating is larger than that of the TiC coating.

3.1.3 TiCN coated cemented carbide

The color of TiCN is purple, the surface hardness is HV2700, the initial oxidation temperature is 450°C, and the adhesion strength is good. It has the advantages of good adhesion strength of TiN and good wear resistance of TiC, higher hardness than TiN, and small friction coefficient, which has a certain inhibitory effect on cohesion.

Coating cemented carbide with TiCN can reduce the internal stress of the coating, improve the toughness of the coating, increase the thickness of the coating, prevent the spread of cracks, and reduce tool chipping. Using TiCN as the main coating of the coated tool has both the good toughness of TiC and the good hardness of TiN, which can significantly improve the service life of the tool.

3.1.4 Al2O3 coated cemented carbide

Alumina (Al2O3) is characterized by good thermal stability and chemical properties at high temperatures, and the highest mechanical strength.

In terms of oxidation resistance and diffusion wear resistance, no material can compare with alumina (Al2O3), but because the physical and chemical properties of alumina (Al2O3) and the cemented carbide substrate are too different, a single alumina ( Al2O3) coatings cannot be made into ideal coated carbide tools. The emergence of multi-layer coatings and related technologies makes the coating not only improve the bonding strength with the cemented carbide substrate, but also has the comprehensive properties of various materials. The cemented carbide tools coated with it are suitable for cutting hot field