This article is partly taken from the "Metal Cutting Tool Design Manual", which comprehensively describes the knowledge of tool coating. Let's first show the state of TIN-coated high-speed steel tools when cutting different metals through a short video, slow motion:

0 1 Tool coating

By vapor deposition or other methods, a thin layer (generally only a few microns) of refractory metal (or non-metal) compound with high wear resistance is coated on the hard alloy (or high-speed tool steel tool) substrate, which is to improve the tool material One of the effective ways to 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).

02Coating method and characteristics

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

CVD method:

The CVD method belongs to the category of atomic deposition. It uses the vapor, hydrogen and other chemical components of metal halides to decompose, heat, and other gaseous and solid reaction deposits at the atomic scale of atoms, ions, and molecules at a high temperature of 950 to 1050 ℃. A method of forming a solid deposition layer on the surface of the heated substrate. The process includes three stages: material vaporization, transportation to the vicinity of the substrate and the formation of a covering layer on the substrate.

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

CVD technology is mainly used for the surface coating of carbide turning tools. The coated tools are suitable for medium- and heavy-duty high-speed rough machining and semi-finish machining.

Compared with other coating methods, 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 carbon nitrides, etc.

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

• The deposition rate is high and easy to control.

• The coating has high purity, fine and dense grains.

• Strong adhesion and thicker coating can be obtained.

• Low process cost, suitable for mass production.

Medium temperature chemical vapor deposition (MTCVD) at 700~900℃ can obtain dense fibrous crystalline TCN coating, the coating thickness can reach 8~10μm, and A2O3, TiN and other resistant coatings can be deposited on the surface by CVD process technology. Material with 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 approximately doubled compared to ordinary coated inserts. The main disadvantage of the CVD method is that the deposition temperature is relatively high. When the high-speed tool steel cutting tool is coated, the cutting tool will be annealed and deformed. Therefore, the deposited tool needs to be quenched.

PVD method

The PVD method uses physical forms such as evaporation or sputtering to remove the material from the target source, and then deposits these energy-carrying vapor ions on the surface of the substrate or part through a vacuum or semi-vacuum space to form a film. Through a gas phase reaction process, The vaporized 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-pressure electron beam evaporation (LVEE), cathodic electron arc deposition (CAD), transistor high-voltage electron beam evaporation (THVEE), unbalanced magnetron sputtering (UMS), ion beam assisted deposition Method (IAD) and Dynamic Ion Beam Mixing (DIM). The main difference is that the vaporization method of the deposited material and the method of generating plasma are different, which makes the film formation speed and the film quality different.

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

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

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

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

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

• Uniform coating, no thickening or rounding at the cutting edges and arcs, so complex tools can also obtain uniform coatings.

• There is no decarburization phase, and there is no CVD method that is easily brittle due to chlorine erosion and hydrogen embrittlement deformation, and the coating 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 the coating composition has also developed from a single coating to TiC, TiCN, ZrN, CrN, MoS2, TIAIN, TiAICN, TiN-AIN, CN and other multiple composites. Coating, and due to the emergence of nano-level coatings, the quality of PVD coated tools has 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 accuracy of the cutting edge of the tool are not inferior to uncoated tools in high-precision machining.

Other coating methods: Plasma Chemical Vapor Deposition (PVCD), Ion Beam Assisted Deposition (IBAD), Laser Strengthening, etc., etc.

Coating characteristics

• The use of coating technology can greatly increase the surface hardness of the tool without reducing the strength of the tool. The current hardness that can be achieved is close to 100GPa;

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

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

• Coating technology, as the final process of tool manufacturing, has almost no impact on tool accuracy and can be repeated coating processes.

03 Coating technology and tool coating knowledge

Titanium Carbide (TiCN):

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

CVD diamond coating:

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

Although the normal temperature hardness of TiAlN coating may be lower than that of TiCN coating, it has been proved that it is much more effective than TiCN in high temperature processing. The reason why the TiAlN coating can 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 carbide tools is usually higher, which makes TiAlN the preferred coating for carbide tools. Carbide drill bits and end mills usually use this PVDTiAlN coating stone coating Knives have become a good choice for cutting non-ferrous and non-metallic materials.

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

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

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

Selection criteria for coating materials:

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

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

The reason why the TiAlN coating can 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 carbide tools is usually higher, which makes TiAlN the preferred coating for carbide tools. Carbide drills and end mills usually use this PVDTiAlN coating.

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

04 Progress of commonly used coating materials and super hard 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 tools in industrially developed countries has accounted for 50% to 70% of high-speed steel tools, and the utilization rate of some complex tools that cannot be reground has exceeded 90%.

Due to the high technical requirements of modern metal cutting tools, TiN coatings are increasingly unable to adapt. The oxidation resistance of TiN coating is poor. When the service temperature reaches 500°C, the film is obviously oxidized and ablated, and its hardness cannot meet the needs. TiC has higher microhardness, so the wear resistance of this material is better. 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 common coating material in coated tools.

The development of TiCN and TiAlN has brought the performance of coated tools to a higher 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 increase the life of the tool.

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

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

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

CBN is second only to diamond in terms of hardness and thermal conductivity. It has excellent thermal stability and does not oxidize when heated to 1000°C in the atmosphere. CBN has extremely stable chemical properties for iron group metals. Unlike diamond, which is not suitable for processing steel, it can be widely used for 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 a relatively high cutting speed, and can cut high-hardness chilled rolls, hardened materials mixed with carbon, and wear on tools. Very serious Si-Al alloys, etc. The methods of low-pressure gas phase synthesis of CBN films mainly include CVD and PVD methods. 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 the basic research and application technology of 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 may have a hardness that meets or exceeds the hardness of diamond. The success of the synthesis of carbon nitride is an 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 materials science in the world.