Properties and Characteristics of Coated Cutting Tool Materials and Application of Cutting Tools

Coating the tool is one of the important ways to improve the performance of the tool. The emergence of coated cutting tools has made a major breakthrough in the cutting performance of cutting tools. The coated tool is coated with one or more layers of refractory compound with good wear resistance on the tougher tool body, which combines the tool substrate with the hard coating, so that the performance of the tool is greatly improved. Coated cutting tools can improve processing efficiency, improve processing accuracy, prolong tool life and reduce processing costs.

About 80% of the cutting tools used in new CNC machine tools use coated tools. Coated cutting tools will be the most important tool variety in the field of CNC machining in the future. ⑴ Types of coated tools

According to different coating methods, coated tools can be divided into chemical vapor deposition (CVD) coated tools and physical vapor deposition (PVD) coated tools. Coated carbide tools generally use chemical vapor deposition, and the deposition temperature is around 1000 °C. Coated high-speed steel tools generally use physical vapor deposition, and the deposition temperature is about 500 °C;

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).

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, typically TiC and TiN coatings. The goal pursued by "soft" coating tools is a low coefficient of friction, also known as self-lubricating tools, and its friction with the workpiece material The coefficient is very low, only about 0.1, which can reduce bonding, reduce friction, reduce cutting force and cutting temperature.

Recently developed a nano-coating (Nanoeoating) tool. This coated tool can use different combinations of various coating materials (such as metal/metal, metal/ceramic, ceramic/ceramic, etc.) to meet different functional and performance requirements. A properly designed nano-coating can make the tool material have excellent anti-friction and anti-wear functions and self-lubricating properties, which is suitable for high-speed dry cutting.

⑵ Characteristics of coated tools

The performance characteristics of coated tools are as follows.

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, but also has the high hardness, high wear resistance and low wear resistance of the coating. coefficient of friction. Therefore, the cutting speed of the coated tool can be increased by more than 2 times than that of the uncoated tool, and a higher feed rate is allowed. Coated tool life is also increased.

② Strong versatility: Coated tools have wide versatility, and the processing range has been significantly expanded. One coated tool can replace several non-coated tools.

③ Coating thickness: With the increase of coating thickness, the tool life will also increase, but when the coating thickness reaches saturation, the tool life will no longer increase significantly. When the coating is too thick, it is easy to cause peeling; when the coating is too thin, the wear resistance is poor.

④ Regrindability: Coated blades have poor regrindability, complex coating equipment, high process requirements, and long coating time.

⑤ Coating material: Tools with different coating materials have different cutting performance. For example: when cutting at low speed, TiC coating has an advantage; when cutting at high speed, TiN is more suitable.

⑶ Application of coated tools

Coated cutting tools have great potential in the field of CNC machining, and will be the most important tool variety in the field of CNC machining in the future. Coating technology has been applied to end mills, reamers, drills, compound hole processing tools, gear hobs, gear shaper cutters, gear shaving cutters, forming broaches and various machine clamping indexable inserts to meet the requirements of high-speed cutting Steel and cast iron, heat-resistant alloys and non-ferrous metals and other materials.

Tool Coating

Coating a thin layer (usually only a few microns) of a refractory metal (or non-metal) compound with high wear resistance on the cemented carbide (or high-speed tool steel tool) substrate by vapor deposition or other methods is an important way to improve the tool material. One of the effective ways to increase wear resistance without reducing its toughness. It is also a good way to solve a pair of contradictions in the development of tool materials (the higher the hardness and wear resistance of the material, the lower the strength and toughness).

Coating method and characteristics

At present, the commonly used tool coating methods are chemical vapor deposition (CVD) and physical vapor deposition (PVD). In recent years, some new coating processes have emerged, which have good application prospects.

CVD method

The CVD method belongs to the atomic deposition category. It uses vapor, hydrogen and other chemical components of metal halides to decompose, heat-bond, etc. gas and solid reaction deposits at atomic scales such as atoms, ions, and molecules at a high temperature of 950~1050°C. A method of forming a solid-state deposition layer on the surface of a heated substrate, the process includes three stages: material vaporization, transportation to the vicinity of the substrate, and formation of a covering layer on the substrate.

Among various CVD methods, vacuum ion bombardment and magnetron ion reactive spraying are the most widely used.

CVD technology is mainly used for surface coating of carbide turning tools, and its coated tools are suitable for high-speed roughing and semi-finishing of medium-sized and heavy-duty cutting.

Compared with other coating methods, the CVD method not only has simple equipment and mature technology, but also has the following advantages:

There are many types of deposits, which can be coated with metals, alloys, carbides, nitrides, borides, oxides, carbonitrides, oxynitrides, hydrogen carbonitrides, etc.

It has a high degree of permeability and uniformity, and can obtain multi-layer coatings of different tissues, and the thickness of the coating is uniform.

The deposition rate is high and easy to control.

The coating has high purity and fine and dense grains.

Stronger adhesion allows thicker coatings to be obtained.

The process cost is low and suitable for mass production.

Medium-temperature chemical vapor deposition (MTCVD) at 700-900°C can obtain TCN coatings in the form of dense fibrous crystals, the coating thickness can reach 8-10 μm, and can be deposited on the surface layer by CVD technology. Good high-temperature oxidation performance, low affinity with the processed material, and good self-lubricating performance.

MT-CVD coated inserts are suitable for use under high speed, high temperature, heavy load, and dry cutting conditions, and their service life can be doubled compared with ordinary coated inserts. The main disadvantage of the CVD method is that the deposition temperature is high, and when the high-speed tool steel tool is coated, the tool will be annealed and deformed. Therefore, the tool after deposition must be quenched.

PVD method

The PVD method uses physical forms such as evaporation or sputtering to remove materials from the target source, and then deposits these energy-carrying vapor ions on the surface of the substrate or parts to form a film through a vacuum or semi-vacuum space. Through the gas phase reaction process, The evaporated or sputtered metal atoms undergo a gas phase reaction, thereby depositing the required compound on the surface of the tool. PVD coating can be coated with titanium nitride, titanium carbonitride, aluminum titanium nitride, and carbides and nitrides of various refractory metals.

At present, the commonly used PvD methods include low-voltage electron beam evaporation (LVEE), cathode electron arc deposition (CAD), transistor high-voltage electron beam evaporation (THVEE), unbalanced magnetron sputtering (UMS), and ion beam assisted deposition. method (IAD) and dynamic ion beam mixing (DIM). The main difference is that the vaporization method of the deposition material and the method of generating plasma are different, resulting in differences in film formation speed and film quality.

PVD technology is mainly used in the surface treatment of solid carbide tools and high-speed tool steel tools, and has been widely used in the coating treatment of carbide drills, milling cutters, reamers, taps, special-shaped tools, welding tools, etc.

Compared with the CVD method, the PVD method has the following advantages:

The coating temperature (300~500°C) is lower than the tempering temperature of high-speed tool steel, so it will not damage the hardness and dimensional accuracy of high-speed tool steel tools, and heat treatment is no longer required after coating.

The effective thickness of the coating is only a few microns, so the original precision of the tool can be guaranteed, and it is suitable for coating high-precision tools.

The coating has high purity and good compactness, and the combination of the coating and the substrate is firm, and the coating performance is not affected by the substrate material.

The coating is uniform, and there is no thickening or rounding at the cutting edge and arc, so complex tools can also obtain uniform coating.

There is no decarburization phase, and there is no brittle coating caused by CVD method due to chlorine corrosion and hydrogen embrittlement deformation, and the coated blade has high strength.

The working process is clean, pollution-free and pollution-free.

At present, PVD technology not only improves the bonding strength between the film and the tool matrix material, but also the coating composition has been developed from a single coating to a variety of multi-component composites such as TiC, TiCN, ZrN, CrN, MoS2, TIAIN, TiAICN, TiN-AIN, CN, etc. Coating, and due to the emergence of nano-scale coatings, the quality of PVD coating tools has made a new breakthrough. This thin film coating not only has high bonding strength, hardness close to CBN, good oxidation resistance, and can effectively control precision The shape and precision of the cutting edge of the tool, when performing high-precision machining, its machining accuracy is not inferior to that of uncoated tools.

Other coating methods: Plasma Chemical Vapor Deposition (PVCD), Ion Beam Assisted Deposition (IBAD), Laser Enhancement, etc., to name a few.

Coating Features

The use of coating technology can greatly increase the surface hardness of the tool without reducing the strength of the tool, and the hardness that can be achieved at present is close to 100GPa;

With the rapid development of coating technology, the chemical stability and high temperature oxidation resistance of the film are more prominent, thus making high-speed cutting possible;

The lubricating film has good solid-phase lubrication performance, which can effectively improve the processing quality and is also suitable for dry cutting;

Coating technology, as the final process of tool manufacturing, has little effect on tool accuracy and can be repeatedly coated.

Coating technology and tool coating knowledge

Titanium carbide nitride (TiCN)

The coating has a higher hardness than a titanium nitride (TiN) coating. Due to the increase of carbon content, the hardness of TiCN coating is increased by 33%, and its hardness range is about Hv3000-4000 (depending on the manufacturer).

CVD diamond coating

The application of CVD diamond coating with a surface hardness as high as Hv9000 on cutting tools has been relatively mature. Compared with PVD coating cutting tools, the life of CVD diamond coating cutting tools has increased by 10-20 times. The high hardness of the diamond-coated tool makes the cutting speed 2-3 times higher than that of the uncoated tool, so that the CVD diamond oxidation temperature refers to the temperature value when the coating begins to decompose. The higher the oxidation temperature value, the more favorable it is for cutting under high temperature conditions.

Although the room temperature hardness of TiAlN coating may be lower than that of TiCN coating, it has been proved to be much more effective than TiCN in high temperature processing. The reason why the TiAlN coating can still maintain its hardness at high temperatures is that a layer of aluminum oxide can be formed between the tool and the chips, and the aluminum oxide layer can transfer heat from the tool to the workpiece or chips.

Compared with high-speed steel tools, the cutting speed of cemented carbide tools is generally higher, which makes TiAlN the coating of choice for carbide tools. Carbide drills and end mills often use this PVDTiAlN coating stone coating The tool becomes a good choice for cutting non-ferrous metals and non-metallic materials.

The hard film on the surface of the tool has the following requirements for the material

①High hardness, good wear resistance; ②Stable chemical properties, no chemical reaction with workpiece materials; ③Heat and oxidation resistance, low friction coefficient, firm adhesion with the substrate, etc. It is difficult for a single coating material to fully meet the above technical requirements.

The development of coating materials has gone through the development stage of TiC-A12O3-TiN composite coating and multi-component composite coatings such as TiCN and TiAlN from the initial single TiN coating and TiC coating. Now the latest development is TiN/NbN, TiN/CN, and other multi-component composite film materials have greatly improved the performance of tool coatings.

Coating Material Selection Criteria

In the manufacturing process of coated tools, it is generally selected according to the hardness, wear resistance, high temperature oxidation resistance, lubricity and anti-adhesion of the coating, among which the oxidation of the coating is most directly related to the cutting temperature. technical conditions.

Oxidation temperature refers to the temperature value when the coating begins to decompose. The higher the oxidation temperature value, the more favorable it is for cutting under high temperature conditions. Although the room temperature hardness of TiAlN coating may be lower than that of TiCN coating, it has been proved to be much more effective than TiCN in high temperature processing.

The reason why the TiAlN coating can still maintain its hardness at high temperatures is that a layer of aluminum oxide can be formed between the tool and the chip, and the aluminum oxide layer can transfer heat from the tool to the workpiece or chip. Compared with high-speed steel tools, the cutting speed of cemented carbide tools is generally higher, which makes TiAlN the coating of choice for carbide tools. Carbide drills and end mills often use this PVDTiAlN coating.

From the perspective of application technology: In addition to cutting temperature, cutting depth, cutting speed and coolant may have an impact on the application effect of tool coating.

Advances in common coating materials and superhard coating technology

Among the hard coating materials, TiN is the most mature and widely used. At present, the utilization rate of TiN-coated high-speed steel cutting tools in industrially developed countries has accounted for 50% to 70% of high-speed steel cutting tools, and the utilization rate of some complex cutting tools that cannot be reground has exceeded 90%.

Due to the high technical requirements of modern metal cutting tools, TiN coating is increasingly unable to adapt. The oxidation resistance of the TiN coating is poor, and when the service temperature reaches 500°C, the film layer is obviously oxidized and ablated, and its hardness cannot meet the requirements. TiC has a higher microhardness, so the material has better wear resistance. At the same time, it adheres firmly to the substrate. When preparing multi-layer wear-resistant coatings, TiC is often used as the underlying film in contact with the substrate. It is a very commonly used coating material in coated tools.

The development of TiCN and TiAlN has brought the performance of coated tools to a new level. 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. Setting TiCN as the main wear-resistant layer of the coated tool can significantly improve the life of the tool.

TiAlN has good chemical stability and anti-oxidation wear. When processing high-alloy steel, stainless steel, titanium alloy, and nickel alloy, the service life of TiN-coated tools is 3-4 times longer than that of TiN-coated tools. If there is a higher Al concentration in the TiAlN coating, a thin layer of non-state Al2O3 will be formed on the surface of the coating during cutting, forming a hard inert protective film, and the coated tool can be used more effectively. High speed machining. Oxygen-doped titanium nitride carbide TiCNO has high microhardness and chemical stability, and can produce the effect equivalent to TiC + A12O3 composite coating. Metal processing WeChat, the content is good, it is worth paying attention to.

Among the above-mentioned hard film materials, there are three types whose microhardness HV can exceed 50GPa: diamond film, cubic boron nitride CBN, and carbon nitride.

The temperature requirement of many deposited diamond films is 600°C to 900°C, so this technology is often used to deposit diamond films on the surface of cemented carbide tools. The commercialization of diamond carbide cutting tools is a major achievement of coating technology in recent years.

CBN is second only to diamond in terms of hardness and thermal conductivity. It has excellent thermal stability and can be heated to 1000°C in the atmosphere.Oxidation also does not occur. CBN has extremely stable chemical properties for iron group metals. Unlike diamond, which is not suitable for processing steel, it can be widely used in finishing and grinding of steel products.

In addition to excellent wear resistance, CBN coating can also process heat-resistant steel, titanium alloy, and hardened steel at relatively high cutting speeds, and can cut high-hardness chilled rolls, carbon-doped hardened materials, and reduce tool wear. Very serious Si-Al alloy, etc. There are mainly CVD and PVD methods for low-pressure gas phase synthesis of CBN thin films. CVD includes chemical transport PCVD, hot wire assisted heating PCVD, ECR-CVD, etc.; PVD includes reactive ion beam plating, active reactive evaporation, laser evaporation ion beam assisted deposition, etc. There is still a lot of work to be done in terms of basic research and application technology for CBN synthesis technology, including reaction mechanism and film formation process, plasma diagnosis and mass spectrometry analysis, determination of optimal process conditions, and development of high-efficiency equipment.

Carbon nitride has the potential to match or exceed the hardness of diamond. The success of synthesizing carbon nitride is a very outstanding example of molecular engineering. As a superhard material, carbon nitride is expected to have many other valuable physical and chemical properties. The study of carbon chloride has become a hot topic in the field of material science in the world.