Coated tools and their rational use

The coated tool effectively solves the contradiction between the higher the hardness and wear resistance of the tool material and the lower the strength and toughness. This paper reviews the coating methods, coating materials, pre-coating tool handling and rational use of coating tools.
introduction

The coated tool is coated on the surface of a hard alloy or high speed steel (HSS) substrate with good strength and toughness by vapor deposition to coat a thin layer of refractory metal or non-metallic compound with good wear resistance (also coated) Obtained on blades of superhard materials such as ceramics, diamond and cubic boron nitride. As a chemical barrier and thermal barrier, the coating reduces the diffusion and chemical reaction between the tool and the workpiece, thus reducing crater wear. The coated tool has the characteristics of high surface hardness, good wear resistance, stable chemical properties, heat and oxidation resistance, low friction factor and low thermal conductivity. It can improve tool life by more than 3~5 times compared with uncoated tools during cutting. The cutting speed is 20%~70%, the machining accuracy is improved by 0.5~1, and the tool consumption cost is reduced by 20%~50%. As a result, coated tools have become the hallmark of modern cutting tools, and their use in tools has exceeded 50%. At present, various tools used in cutting, including turning tools, boring tools, drills, reamers, broaches, taps, thread combs, rolling heads, milling cutters, forming tools, gear hobs and shaper cutters, etc. Coating processes can be used to improve their performance.

There are four types of coated knives: coated high speed steel knives, coated carbide knives, and coated knives on ceramic and superhard materials (diamond or cubic boron nitride) inserts. But the previous two coated tools used the most. The coating on the ceramic and super-hard material blades is a material with a lower hardness than the substrate, in order to improve the fracture toughness of the blade surface (more than 10%), reduce chipping and breakage of the blade, and expand the application range. .

a) coated blade

b) coating tool
Figure 1 coated blade and its tool

Coating method

There are currently two coating methods commonly used in production: physical vapor deposition (PVD) and chemical vapor deposition (CVD). The former has a deposition temperature of 500 ° C and a coating thickness of 2 to 5 μm; the latter has a deposition temperature of 900 ° C to 1100 ° C, a coating thickness of 5 to 10 μm, and a simple device and a uniform coating. Since the PVD method does not exceed the tempering temperature of the high-speed steel itself, the high-speed steel tool generally adopts the PVD method, and the hard alloy is mostly the CVD method. When the cemented carbide is coated by CVD, due to its high deposition temperature, a brittle decarburization layer (η phase) is easily formed between the coating and the substrate, resulting in brittle fracture of the blade. In the past decade or so, with the advancement of coating technology, the cemented carbide can also adopt the PVD method. In foreign countries, a composite coating process, called PCVD (plasma chemical vapor deposition), has been developed using a combination of PVD/CVD techniques. That is, using plasma to promote chemical reaction, the coating temperature can be lowered to below 600 ° C (the current coating temperature can be reduced to 180 ° C ~ 200 ° C), so that there is no diffusion between the cemented carbide substrate and the coating material. , phase change or exchange reaction, can maintain the original toughness of the blade. This method is reported to be particularly effective for coating diamond and cubic boron nitride (CBN) superhard coatings.

When coating by CVD method, the cutting edge needs to be passivated in advance (the blunt radius is generally 0.02~0.08mm, and the cutting edge strength increases with the radius of the blunt circle), so the blade has no sharp edges. Therefore, the PVD method should be used for tools that produce thin chips for finishing and require sharp cutting edges. In addition to being applied to conventional cutting inserts, the coating can be applied to integral tools and has been developed to be applied to welded carbide tools. According to reports, Japan's Mitsubishi Corporation adopted the PCVD method on the welded carbide drills. As a result, the life of the drill when machining steel is 10 times longer than that of the high-speed steel drill, and the efficiency is increased by 5 times.

Table 1 Various new coatings introduced by Swiss PLATIT
coating colour hardness
GPa 		Thickness μm Coefficient of friction Maximum operating temperature °C Description
TiAlN single layer Purple black 35 1-4 0.5 800 Universal high performance coating
TiAlN multilayer Purple black 28 1-4 0.6 700 Suitable for interrupted cutting
TiCN-MP Red-copper 32 1-4 0.2 400 High toughness universal coating
MOVIC Green-grey - 0.5-1.5 0.15 400 MoS2 base coating
CrN Silver bright 18 1-4 0.3 700 Suitable for processing copper and titanium
TiAlCN Red-purple 28 1-4 0.25 500 High performance universal coating
CBC (DLC) gray 20 0.5-4 0.15 400 Lubricating coating
GRADVIC gray 28 1.5-6 0.15 400 TiAlCN+CBC
AlTiN black 38 1-4 0.7 800 High performance coating
μAlTiN black 38 1-2 0.3 800 Good coating surface quality
AlTiN/SiN Purple orchid 45 1-4 0.45 1100 Nano-structure
Coating material

The coating material must have high hardness, good wear resistance, stable chemical properties, no chemical reaction with the workpiece material, heat and oxidation resistance, low friction factor, and strong adhesion to the substrate. Obviously, a single coating material is difficult to meet the above requirements. Therefore, the hard coating material has been initially coated with only a single TiC, TiN, Al 2 O 3 into a new stage of developing thick film, composite and multi-component coatings. The newly developed TiCN, TiAlN, TiAlCN multi-component, ultra-thin, ultra-multi-layer coating and TiC, TiN, Al 2 O 3 and other coatings, plus a new plastic deformation resistant matrix, in improving the toughness of the coating, coating Significant progress has been made in the bonding strength of the layer to the substrate and in improving the wear resistance of the coating. At present, it has broken through the technology of coating diamond film on the cemented carbide substrate, which has improved the performance of the tool.

The most mature and widely used hard coating material is TiN, but the bonding strength of TiN to the substrate is inferior to that of TiC coating. The coating is easy to peel off and the hardness is not as high as TiC. When the cutting temperature is high, the film is easy to oxidize. Ablated. TiC coatings have high hardness and wear resistance, and good oxidation resistance, but they are brittle and not resistant to impact. TiCN combines the advantages of both TiC and TiN materials. It can control the TiCN properties by continuously changing the composition of C and N during the coating process, and form a multi-layer structure with different compositions, which can reduce the internal stress of the coating and improve the toughness. Increase the thickness of the coating, prevent crack propagation and reduce chipping. Therefore, some of the blades currently produced, such as the GC4000 series blades recommended by Sweden Sandvik for processing steel, the CN series blades produced by Zhuzhou Cemented Carbide Factory of China, and the T715X and T725X coated blades of Toshiba of Japan have TiCN coating. Layer composition. TiCN-based coatings are suitable for processing ordinary steel, alloy steel, stainless steel and wear-resistant cast iron. The material removal rate can be increased by 2 to 3 times when machining workpieces.

TiAlN, CrC, CrN, and TiAlCN are new hard coating materials developed in recent years. TiAlN coated blades are commercially available. It has good chemical stability and oxidation and wear resistance. Its tool life can be 3-4 times higher than that of TiN coating when it is used to process high alloy steel, stainless steel, titanium alloy and nickel alloy. In addition, if there is a suitable aluminum concentration in the TiAlN coating, a hard inert protective film will be formed on the interface between the rake face and the chip during cutting. The film has better heat insulation and can be more effective. Ground for high speed cutting. For example, the H7 blade from Kennametal, USA, is a TiAlN coating designed for high-speed milling of high-performance materials such as alloy steel, high alloy steel and stainless steel. CrC and CrN are titanium-free coatings suitable for cutting titanium and titanium alloys, copper, aluminum and other soft materials with good chemical stability and no sticking. TiAlCN is a gradient structure coating that not only has high toughness and hardness, but also has a small friction factor. It is suitable for milling cutters, hobs, taps and other tools, and its cutting performance is significantly better than TiN.

Germany's CemeCon has developed the Supernitride coating series, in which the super-aluminum-titanium coating has a high aluminum content and forms a stable oxide layer (oxidation temperature up to 1000 ° C), which is harder and more dense than the general TiAlN coating. Dense, more high temperature resistant, suitable for high speed cutting, dry cutting and hard cutting tools, can process hardened steel with hardness up to 58HRC.

In addition, the nano-ultra-film coating process has become increasingly mature. According to reports, Sumitomo Electric Co., Ltd. has introduced a high-speed and powerful drill bit, which is coated with 1,000 layers of TiN and AlN ultra-thin film coatings on a tough K-type (WC+Co) cemented carbide substrate. The thickness is about 2.5 μm. The use shows that the bending strength and fracture toughness of the drill can be greatly improved, and the hardness is equivalent to CBN, and the tool life can be increased by about 2 times. The company has also developed ZX coated end mills with 2,000 layers of ultra-thin coatings, each with a thickness of about 1 nm. The 60HRC high-hardness material is machined with the end mill, and the tool life is much higher than that of TiCN and TiAlN coated tools. At the 8th China International Machine Tool Show (CIMT2003), the nanostructured coating (AITiN/SiN) end mill introduced by PLATIT of Switzerland has a coating hardness of 45GPa and an oxidation temperature of 1100°C. The cutting comparison test shows its life. It is 3 times higher than TiN coated end mills and 2 times higher than TiAlCN coated end mills. Table 1 is a variety of new coatings introduced by PLATIT. In addition to the above new coatings of AIINN/SiN and TiAlCN, there are also special coatings, such as CBC and DLC lubricated coatings, which have a small friction factor (0.15). For the application of tools such as taps and drills, it can improve chip removal performance or as a surface coating for composite coatings, reducing chip adhesion.
Nano-structure

Japan's Hitachi Tool Co., Ltd. introduced GM20, GM25 multi-layer thick film coated blades, which are carried out at a slightly lower temperature than ordinary CVD coatings (about 800 ° C ~ 900 ° C) to form a column with high wear resistance Crystallization, in order to improve the adhesion resistance of the blade, a layer of Al 2 O 3 film is coated on the surface of the tool. It is said that the coating has a large thickness, high toughness, tight bonding with the substrate, and good chipping resistance. It is especially suitable for interrupted cutting work, and the tool life can be 1.5~2 times higher than that of the general coated blade.

The Kennametal Hertel Company of the United States has a thick coating of 16μm thick on the KC9315 insert. This insert is especially suitable for processing high-strength cast iron (such as ductile iron and compacted graphite iron) with a cutting speed of up to 400m/min and can be dried. Used under cutting and interrupted cutting conditions. The blade coating has a total of three layers: alumina (Al 2 O 3 ), titanium carbonitride (TiCN), and titanium nitride (TiN).

At present, the application of diamond film coated tools has entered a practical stage. It is formed by depositing a layer of membranous diamond composed of polycrystals on a cemented carbide substrate (usually a K-type alloy), often referred to as a CVD diamond tool (CD tool for short). Because the base is easy to make complex shapes, it is suitable for tools with complex geometries. Both the United States and Japan have introduced diamond-coated taps, drill bits, end mills and indexable inserts with chipbreakers (such as Sandvik's CD1810 and Kennametal's KCD25) for non-ferrous and non-metallic applications. High-speed precision machining of materials, tool life is nearly ten times or even dozens of times higher than uncoated cemented carbide tools. Another CBN coating suitable for processing steel materials has also been developed and is moving into the industrial trial phase. A few years ago, Wuhan University developed a C3N4 film. The hardness of the film is close to that of super-hard materials. It can be coated on a high-speed steel drill bit to greatly improve the life of the drill bit. In addition, a coating company in the United States uses hot cathode evaporation technology to deposit carbon onto the surface of a high speed steel tool to obtain a well-bonded diamond-like carbon coating (DLC). Diamond-like diamonds are amorphous, but they have many diamond-like properties, such as high compressive strength and hardness, low friction factor and good corrosion resistance. The advent of diamond-like tools has shown a application for coated tools. New prospects.

In addition to the various hard coating materials described above, MoS 2 based soft coating materials and WC/C "medium hard" slip coating materials have been developed. The former can greatly improve the cutting performance of the cutter and prevent the formation of built-up edge; the latter has a slightly higher friction factor than the MoS2 coating, but its wear resistance is better. The soft coating can be used alone, or it can be hard coated on the surface of the tool, and then coated with MoS 2 soft coating. Whether it is cutting steel or processing high silicon aluminum alloy, it has good effect (see Figure 2). ), and it is equally effective for castings. For example, a carbide drill with a (Ti, Al)N+ MoS 2 soft coating is used to dry drill a deep hole in a gray cast iron engine block with a tool life of up to 1600 min and a TiN or TiCN coated drill. Their lifetimes are 19.6 min and 44 min, respectively.

At present, some industrialized countries have begun to design the coating composition, coating thickness and matching matrix materials according to the nature of the materials to be processed in order to obtain the best composition and obtain the best coating effect. At the same time, smart coatings are being developed that can cause electrical resistance changes in the event of wear, deformation or exposure to high temperatures, and emit electrons that can be recognized by the machine tool.

Workpiece material: 42CrMo4V (1000MPa)
Tool solid carbide; D=8.5mm
Drilling speed: 70m/min, feed rate: 0.15mm/r
Drilling depth 42.5mm, dry cutting

Workpiece material: AlSi9
Tool solid carbide; D=8.5mm
Drilling speed: 90m/min, feed rate: 0.315mm/r
Drilling depth 25.5mm, micro cooling lubrication
Fig. 2 Comparison of cutting stroke lengths when drilling drills of different coating materials on steel and aluminum parts

Rational use of coated tools

The use of coated tools is related to the coating method and equipment, coating process and coating materials, as well as the following.

  • Surface quality of the tool before painting
    The surface of the coated tool shall be a bright polished surface. There shall be no defects such as rust, grinding, oxidation and chipping on the working surface of the cutter, and no burr is required on the cutting edge. The surface roughness of the front and back flank should reach Ra<0.8~1.25μm. The smaller the surface roughness value, the better the degree of bonding of the coating. In addition, the quality of the cleaning of the tool surface is also very important.

  • Tool base material
    The base material of the coated tool and the coating material should be reasonably matched and must be selected according to different processing requirements. The base of the coated HSS tool can be either W6Mo5Cr4V2 (M2) general-purpose high-speed steel or cobalt-containing super-hard high-speed steel and powder metallurgy high-speed steel (PM HSS). Because the matrix of powder metallurgy is uniform, the effect is good. When processing titanium alloys, it is recommended to use cobalt super hard high speed steel such as W2Mo9Cr4VCo8 (M42) as the base material of the tool. For coated hobs, when machining gears at normal cutting speed (less than 45m/min), chipping is the main cause of hob wear. Therefore, W6Mo5Cr4V2 high-speed steel with better toughness should be selected as the base material of the tool; When hobbing (cutting speed is greater than 100m/min), crater wear is the main cause of hob wear. Therefore, cobalt-containing super-hard high-speed steel or CW9Mo3Cr4VN high-speed steel with high heat resistance and wear resistance should be selected as the base of the cutter. material.

    For the base of coated cemented carbide tools, when machining steel, it is advisable to choose hard alloys for processing steel, such as WC-TiC-Co or WC-TiC-TaC-Co alloys (P30 is used more); For non-ferrous metals, WC-Co alloys should be selected (K20 is used more).

    The hardness and machinability of the material to be processed also have an effect on the use of the coated tool. Tests have confirmed that coated tools are best suited for cutting difficult-to-machine materials such as high hardness and wear-resistant alloys.

  • Geometric angle of the tool
    Due to the good lubricity of the coating, the coated tool often slips on the surface of the workpiece, so the back angle on the coated tool should be slightly larger than the back angle of the uncoated tool. Practice has shown that for a type of finishing tool such as a reamer, after increasing the back angle, the cutting edge can be sharp, the chip formation is easy, the slip phenomenon is significantly reduced, and the use performance of the tool is improved.

  • Cutting amount and cutting fluid
    In order to fully utilize the performance of the coated tool, the cutting amount and cutting fluid must be selected correctly. Due to its good heat resistance and strong resistance to crater wear, coated tools can work with larger feed rates and cutting speeds, but first a larger feed rate should be selected. Generally, coated high speed steel tools use 10% to 100% more feed than uncoated tools, and a 20% to 30% increase in cutting speed is appropriate. In order to improve work efficiency, coated carbide tools can also be cut at a cutting speed of 25% to 70% higher than uncoated tools. At present, the cutting speed of the medium carbon steel with the coated carbide general-purpose tool can reach 100~150m/min for the end mill and 80~100m/min for the drill bit; the cast iron for tap processing is 20~40m/min.

    Practice has proved that when using No. 20 mechanical oil plus 10% kerosene cooling, the life of the coated high speed steel boring tool can be increased by 1~2 times. TiN coated high speed steel hob machining 20CrMnTi (197HBS) steel helical gear (modulus m=5), using 20# mechanical oil and kerosene mixed lubrication, the tool life can be increased by about 5 times, even after regrind It can be increased by 2~3 times, and the life of dry cutting is only doubled.

    When the coated tool is used, the precision of the machine tool is required, the rigidity is high and the vibration is small, and the clamping of the tool or the blade should also be firm.

  • Re-grinding and recoating of coated tools
    The coated tool must be reground after it has worn out. When the coated tool is reground, the worn parts of the tool must be completely worn away. For tools that only need to regrind the rake face (such as broaches, gear hobs, and pinion knives) or tools that only need to regrind the flank (such as drills and reapers), if they are adjacent to the cutting edge The coating on the other face (such as the spiral flute of the drill bit) is not damaged and the wear resistance of the tool can be improved. The re-sharpened coated tool has a tool life of about 50% or more of the original new coated tool life and still has a higher life than the uncoated tool.

    The grinding wheel used for the sharpened coated carbide tool can be a diamond grinding wheel. However, when sharp-coated high-speed steel tools are used, grinding with cubic boron nitride (CBN) grinding wheels has a good effect. The wear of the tool should be completely removed, the coating should not peel off, and the tool should not be annealed.

    An important issue with the use of coated tools is the problem of recovery of the cutting performance of the tool after re-grinding, that is, the problem of repeated coating (recoating) after each sharpening (opening) of the tool. For reground grinding tools, only the recoating can ensure that the total life of the tool is increased by more than 3 to 5 times. Where the recoating tool must first grind the geometric parameters according to the process requirements, the polishing part is not allowed to have various quality defects such as grinding, burrs and the like. For recoating, the localized shielding technique can be used to coat only the sharpened surface. For recoating without shielding technology, after 4~6 times of recoating, the coating thickness of the non-sharpening surface of the tool will be too large, which will affect the accuracy of the tool and local spalling. Recoat after stripping treatment. After recoating, the cutting performance of the tool is generally not lower than that of the first new coated tool, and the tool can be recoated several times until it is scrapped.

    As can be seen from the above, recoating has great potential for improving the wear resistance and productivity of the tool. However, whether to recoat after regrind depends on whether the tool can be repainted technically and economically.

Application of coated tools in China

China's research and application of coated tools began in the 1980s. At present, Hunan Zhuzhou Cemented Carbide Factory has been able to provide various coated tools for YB series, CN series and CA series. The YB series is made of imported equipment to produce the equivalent of the Swedish Sandvik GC series coated blades. The coating materials are TiC, TiC+Al 2 O 3 and TiC+Al 2 O 3 +TiC; CN series is mainly used for steel finishing. Processing, the coating material is TiC + Ti (C, N) + TiN; CA series coating material is TiC + Al 2 O 3 , suitable for processing of cast iron and non-ferrous materials. Sichuan Zigong Cemented Carbide Plant produces ZC series coated blades with US coating equipment. However, compared with the industrialized countries with advanced manufacturing technology, the proportion of applied coating tools in China and the level of coating technology are lower. To this end, Shanghai Tool Works Co., Ltd. and Hunan Zhuzhou Cemented Carbide Group have recently introduced coating equipment from PLATIT and CemeCon from Switzerland and Germany to make the coating technology and coating tool performance of Chinese tools rapid. Catch up and approach the international advanced level. At the same time, Sino-Swiss joint venture Shanghai Nawei Coating Co., Ltd., a large-scale coating service center for coating, re-grinding and recoating of knives, was established. The company introduced 4 sets of coating equipment. It can be coated with various coatings such as S-AlTiN (super aluminum aluminum oxynitride), S-TiN (super titanium nitride), TiCN, DLC, CrN. It can be expected that with the promotion and application of coating tools, it will promote the development of advanced cutting technology such as high-speed cutting, dry cutting and hard cutting in China, and the development of advanced cutting technology will further promote the application of coating tools.

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