Development and Application of TiAlN Coated Tools

Abstract: As a new coating material, TiAlN coating has excellent properties such as high hardness, high oxidation temperature, good thermal hardness, strong adhesion, low friction coefficient and low thermal conductivity. It is expected to replace TiN partially or completely, especially for High speed cutting. In this paper, the development and application of TiAlN coated tools at home and abroad are reviewed, and the TiAlN coating process and its cutting performance are analyzed.

1 Introduction

Coating the tool is one of the important ways to improve the performance of the tool. With the appearance of the coated tool, the cutting performance of the tool has made a major breakthrough. Coated tools can increase machining efficiency and machining accuracy, extend tool life and reduce machining costs.

Since the 1970s, tool coating technology has developed rapidly and the coating process has become more mature. The proportion of coated inserts used in Western industrialized countries to indexable inserts has increased from 26% in 1978 to 50-60% in 1985. About 80% of the cutting tools used in the new CNC machine tools use coated tools. The proportion of coated blades from Sweden's Sandvik and Kennametal has reached 80 to 85%. The proportion of coated cemented carbide inserts on CNC machine tools in the United States is 80%, and coated blades for turning in Sweden and Germany account for more than 70%. Between 1981 and 1985, tool production in the former Soviet Union increased by 16%, carbide tools increased by 29%, and coated tools increased by a factor of five. Coated tools have become an important symbol of modern tools and will be one of the most important tool types in the field of CNC machining in the future.

As a new coating material, TiAlN coating has excellent properties such as high hardness, high oxidation temperature, good thermosetting, strong adhesion, low friction coefficient and low thermal conductivity. It is especially suitable for high-speed cutting of high alloy steel, stainless steel and Qin. Alloys, nickel alloys and other materials. In the case of high wear resistance, TiAlN is expected to partially or completely replace TiN in view of the shortcomings of TiN coating in high temperature performance. Therefore, TiAlN coated tools have extremely broad application prospects.

Since Knotek et al. first published research results on TiAlN coatings in 1985, people have expressed great concern about their excellent resistance to high temperature oxidation and good performance. TiAlN films have been successfully prepared by various PVD methods. . Since the preparation methods of TiAlN coatings are not the same, the properties of the reported TiAlN coatings are also different. Table 1 compares the main properties of several commonly used coatings (data from Balzers Coatings Ltd.). It can be seen from the table that the TiAlN coating has high hardness and oxidation temperature, and the performance of the TiAlN coating is gradually increasing with the multilayering and nanocrystallization of the coating. TiAlN has good chemical stability, strong resistance to oxidation and wear, its hardness is 3400~3600HV, and its wear resistance is only lower than that of diamond-like film. It is the most superhard coating currently in the international tool industry. When the Al content exceeds 50%, in order to distinguish it from TiAlN, it is called AlTiN. The hardness of the AlTiN coating increases with the increase of aluminum content, and the aluminum content can exceed 65%. When the aluminum content is increased, a softer AlN phase is formed to lower the hardness, but the aluminum content is not the only factor affecting the hardness. . AlTiN has a hardness of up to 4500 HV. At present, TiAlN/Al2O3 multi-layer PVD coating has been successfully researched. The hardness of this tool is up to 4000HV, the number of coating is 400 layers (thickness 5nm), and the cutting performance is better than that of TiC/l2O3/TiN coating tool.

Table 1 Comparison of main properties of several coatings

Coating material Micro hardness
(HV)
Elastic Modulus
(GPa)
Maximum applicable temperature (°C) Coating structure
TiN 2300 260 600 Single layer
TiCN 3000 352 400 Single layer
TiAlN 3300 380 900 Nano-structure

2 TiAlN coating process

In addition to the coating material, the cutting performance of the coated tool depends to a large extent on the technical level of the coating process, the bonding strength of the coating to the substrate, the coating and its interface structure, the preferred orientation, and the individual Coating thickness and total thickness are important factors in determining the performance of the coated tool. The following is a description of the selection of the substrate and coating materials and the coating process parameters.

1) Selection of substrate and coating materials

For coated tool materials, the degree of physical matching between the coating and the substrate and the different coatings affects the interfacial stress load transfer and material properties, especially the degree of thermal expansion coefficient matching between the layers and the difference in elastic modulus. It has the greatest impact on material properties. Due to the difference in the degree of thermal expansion mismatch and the modulus of elasticity, residual stress fields of different sizes and distributions will be formed inside the material. From the viewpoint of reducing residual tensile stress and improving material strength, the thermal expansion coefficient of the substrate and the coating should be closer, and the difference in elastic modulus is small; but the toughening from residual stress and toughening of microcracks improve the fracture toughness of the material. From the point of view, it is necessary to have proper residual stress. TiAlN coated tool bases are generally selected from high-speed steel or hard alloys, such as YG6, YG8, ultra-fine grain WC-Co cemented carbide, etc., and ceramics can also be used as the matrix.

Interfacial bonding strength is an important factor affecting the performance of coated tools. In addition to selecting matching materials, various methods can be employed in the process to improve the bonding strength of the TiAlN film layer to substrates such as high speed steel and cemented carbide. For example, pre-sputter cleaning before film formation to remove impurities that are not conducive to the bonding of the coating to the substrate; bombardment with high-energy particles during film formation to improve the interdiffusion ability of the inter-membrane components and the reactivity of the atoms on the surface of the film, reducing defects The introduction of intermediate transition layers such as TiAl, Ti, etc., because the bonding strength between the transition layer and the matrix and the transition layer and the TiAlN film layer is significantly higher than the bonding strength of the TiAlN film layer and the substrate, thus having a transition layer of TiAlN The critical load of the film layer is significantly increased, and the thickness of the transition layer can significantly affect the bonding strength between the film layer and the substrate. When the transition layer is further thickened, the probability of the transition layer being sheared increases, causing the critical load to slowly decrease. The interface bond strength can be determined by measuring the critical load by an automatic scratch tester.

2) Coating process parameters

In general, the PVD process can obtain a thinner coating than the CVD process, and the bonding with the substrate is relatively strong. At the same time, the deposition temperature of the PVD coating is low, and the bending strength of the tool material is not higher than 600 ° C0. influences. A variety of PVD methods can be used to prepare TiAlN coated tools, such as multi-arc ion plating, arc ion plating, hollow cathode ion plating, magnetron sputtering ion plating, and the like. There are some differences in the properties of the coated tools obtained by choosing different coating methods. At present, the main parameters for characterizing TiAlN coatings are: rAl/Ti and rM/N. These two parameters can fully reflect the proportion of Ti, Al and N atoms in the coating. The main factors affecting the performance of TiAlN coatings are interfacial bonding strength, Al content in the coating, and composition and structure of the coating.

·Multi-arc ion plating

The coating properties of TiAlN coated tools prepared by multi-arc ion plating are related to the target metal composition and process parameters. In general, the uniformity of a coating prepared using a TiAl composite target is superior to that of a coating prepared by separating Ti, Al targets. Increasing the aluminum content can greatly increase the film-based bonding force. However, for high Al content TiAl composite targets, there are still some difficulties in the preparation process. The composition of TiAlN coating is restricted by process factors such as partial pressure, cathode arc current and substrate bias: as the partial pressure of N2 increases, rAl/Ti increases and rM/N decreases; with the increase of cathode arc current Large, rAl/Ti decreases and rM/N increases in the coating; as the substrate bias increases, rAl/Ti and rM/N decrease (see Figure 1).

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Figure 1 Effect of basic bias on the composition of TiAlN coating

· Hollow cathode ion plating

The TiAlN film layer is deposited by a hollow cathode ion plating method. The main process parameters of the coating are: main arc current, baking temperature, coating time, hydrogen flow rate and vacuum degree during coating. The flow rate of N2 affects the hardness and interfacial adhesion of the tool. As shown in Fig. 2, as the flow rate of N2 increases, the film-based bonding force and the micro-hardness of the film layer increase, and begin to decrease after reaching the maximum value. Therefore, there is an optimum value for the film-based bonding force and the film layer. The microhardness is optimal. The TiAlN film prepared by hollow cathode ion plating method is continuous, smooth and dense. The bond strength of the film base is higher than that between the TiN film and the substrate, but there are still some differences compared with other processes. .

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Figure 2 Microlayer hardness and interfacial adhesion of the film with N2 flow

·Energy pulse plasma gun

The high-energy pulsed eliminator gun independently developed by the Institute of Physics of the Chinese Academy of Sciences can simultaneously consider the advantages of various vacuum-based surface treatment technologies such as ion implantation, physical vapor deposition and plasma nitridation, and can improve the ceramic coating cutting tool. Wear performance. There are many factors affecting the deposition of thin films, including the discharge voltage between the internal and external electrodes of the pulse plasma gun (gun pressure Vgun), the number of pulsed plasma bombardment samples, the pulse electromagnetic inlet valve voltage, the deposition chamber vacuum, the working gas reaction pressure, and the plasma. The distance between the gun and the sample, as well as the temperature of the sample, etc., where the number of pulses and the gun pressure are the two most important parameters. The material of Tsinghua University used a high energy density pulsed plasma gun to deposit TiAlN film with high hardness, wear resistance and strong film-bonding strength on the cemented carbide tool substrate at room temperature. The mechanical properties of the film are shown in Table 2.

Table 2 Mechanical properties of coatings prepared by high energy density pulsed plasma gun

Coating material Nano hardness
(GPa)
Young's modulus (GPa) Nano scratch critical load (mN)
TiN 27 450 90
TiCN 50 550 110
TiAlN 38 650 100

·Magnetron sputtering ion plating

Magnetron sputtering ion plating ion bombardment of the film layer by an ion source is a high-temperature deposition process, and the workpiece is generally heated to 300 ° C ~ 500 ° C. The process overcomes the defects of low magnetron sputtering deposition rate, low ionization rate and strong matrix thermal effect, and can significantly improve the film-based bonding strength. The unbalanced power supply for unbalanced magnetron sputtering ion plating diffuses the plasma confined near the target surface to the vicinity of the workpiece to improve the performance of the system. Unbalanced magnetron sputtering has been widely used at home and abroad for its excellent performance.

Process parameters that affect coating performance are gas partial pressure, bias and vacuum in the deposition chamber, temperature, and the like. Gas partial pressure refers to the ratio of working gas to reactive gas. When the TiAlN coating is prepared, the working gas is Ar, and reducing the partial pressure of Ar is advantageous for increasing the coating rate, the film-based bonding strength and the film-forming density. As the partial pressure of Ar decreases, the working gas increases from the partial pressure. N2 not only reacts with the layer atoms deposited on the workpiece to form a compound film, but also reacts with the target to form a compound on the target surface, which causes target poisoning. Therefore, it is important to determine the two optimal N2 partial pressure values. . The IF AC magnetron sputtering eliminates the "anode disappearance" effect and the "cathode poisoning" problem, greatly improving the stability of the magnetron sputtering operation.

3 TiAlN coated tool application

The TiAlN coating has better mechanical and physical properties than the coatings of TiC, TiN, TiCN, etc., and can be combined with other coatings to form a multi-layer composite coating. In the TiAlN coating, there is a high Al concentration, and a very thin amorphous Al2O3 is formed on the surface of the coating during the cutting process to form a hard inert protective film, which is very suitable for high-speed machining. Dry or semi-dry cutting is a trend. In order to achieve dry cutting, the tool coating must have two important functions: it can act as a thermal barrier between the tool and the workpiece to reduce the thermal stress acting on the tool base; it can act as a solid lubricant. To reduce the cutting friction and the adhesion of the chips to the tool. TiAlN coating is a high performance coating that better meets the above requirements.

1) Milling AlSiH13/JIS SKD61 hardened die steel (52HRC)

TiAlN coated milling cutter is the most commonly used tool for high speed milling hardened die steel. The data comparison of high-speed milling of AlSiH13/JIS SKD61 hardened die steel (52HRC) by VC-MD type six-tooth TiAlN milling cutter is shown in Table 3.

Table 3 Comparison of milling lengths of TiAlN milling cutters with different speeds and different cooling modes

Milling speed
(m/min)
Milling length under different cooling modes (m)
Air cooled Dry cutting Add cutting fluid
157 300 300 200
314 300 150 50
471 300 200 50
628 150 120 -

High-speed milling of AlSiH13/JIS SKD61 die steel (52HRC) with VC-MD model TiAlN coated milling cutter, TiN coated milling cutter and uncoated milling cutter, the wear of the flank around the tool after machining length of 50m is shown in the figure. 3 (feeding speed: 0.10mm / tooth; axial depth of cut 10mm, radial depth of cut 0.5mm; down milling; air cooling).

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Figure 3 TiAlN coated milling cutter and other tool wear comparison

It can be seen from Fig. 3 that the TiN coated milling cutter and the uncoated milling cutter have a large amount of wear at V=200 m/min, and severe wear occurs when V continues to increase. The wear curve of the TiAlN coated milling cutter has a smaller slope and a flatter trend; the slopes of the wear curves of the other two tools are larger. It shows that the wear amount of TiAlN coating changes little with the increase of cutting speed, which is very suitable for high speed cutting. When TiAlN coated tools are used to mill die steel at high speed, the main form of wear is micro-flaking, with a small amount of scratches. The higher wear resistance of TiAlN is mainly due to its lower coefficient of friction and its extremely low chemical affinity. The surface of the wear scar was slightly enriched in Al and O, and it was not found that the surface of the wear scar had Fe adhesion because it had a small tendency to adhere to steel.

2) Milling titanium alloy Ti-6Al-4V

When the TiAlN research coating milling cutter is used to mill the Ti-6Al-4V at low speed, the tool wear is small and the wear curve is flat. As the cutting speed continues to increase, the amount of tool wear increases slowly. However, when the cutting speed exceeds 1000 m/min, the amount of tool wear increases rapidly (see Figure 4). Cutting heat and mechanical vibration are important factors in tool failure. The high temperature generated by intermittent cutting of the titanium alloy material causes a large temperature difference between the cutting edge of the insert and other parts, resulting in cracks in the cutting edge, and the crack propagation will cause the cutting edge to break and the blade to break.

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Figure 4 Milling speed and flank wear curve

3) Milling alloy structural steel 42CrMo4V

The coating thickness (typically 2 to 18 μm) has an important influence on tool performance. The thickness and number of layers of the coating mainly depend on the working conditions, and the more the number of layers, the better the performance. The thickness of the film layer is excessively brittle due to being in a high stress state, resulting in a decrease in service life. For intermittent cutting with large impact force, rapid cooling and heating of the tool, the thin coating is better than the thick coating in temperature change, and the stress is small and crack is not easy. Therefore, the life of the thin coating blade during dry cutting can be extend.

The results of PVD (Ti46Al54)N coated cemented carbide milling cutter (coating thickness 3~10μm, hardness 1400~2400HV0.0015) milling alloy structural steel 42CrMo4V show that mechanical properties and hardness have an impact on cutting performance, especially It is for a thin coating. However, for coatings with a thickness of 8 to 10 μm, the strength and hardness have little effect on the cutting performance (see Figure 5).

Jiagong machine grid
Figure 5 Milling cutter performance of different thickness and hardness

When there is a certain interface bonding strength, the coating thickness of the tool has a great influence on the wear of the rear blade surface. Increasing the thickness of the coating is beneficial to improve the wear resistance, and the wear mainly depends on the thickness of the coating itself. When the TiAlN cemented carbide milling cutter mills 42CrMo4V at V=200m/min, the relationship between the coating thickness and the flank wear is shown in Fig. 6. It can be seen from the figure that when the thickness of the coating is 3-6 μm, the influence of the thickness variation on the flank wear is not obvious, and the coating having a thickness of 8-10 μm has a significantly enhanced resistance to the flank wear.

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Fig. 6 The flank wear of milling cutters with different coating thicknesses

At present, the ideal working speed of TiAlN coated tools is 183 ~ 244 m / min, the spindle speed is 20,000 ~ 40,000r / min or higher, and the cutting depth is shallow. Under such working conditions, the production efficiency is high, the spindle is subjected to less pressure, the surface is smooth, and the finer chips are beneficial to take away more cutting heat. When cutting nickel-base alloys at high speeds, the temperature at the cutting edge may be as high as 800 ° C. Dry cutting with TiAlN is particularly advantageous for nickel-based alloys.

The wear form of the coated tool during use is mainly characterized by abrasive wear and adhesive wear at medium speed, diffusion wear and adhesive wear at high speed, and the final failure form is mostly blade micro-peeling. The mechanism is that in the coating, Cracks are generated and expanded between the coating and inside the substrate. There is a clear interface between the coating and the substrate and between the layers, which is the origin of the stress in the film, resulting in peeling and breakage of the coating.

4 Conclusion

TiAlN coated tools have a broad application prospect in high-speed cutting due to their excellent cutting performance. The popularization of TiAlN coated tools will greatly improve tool durability, reduce machining assistance time, reduce cutting and processing costs, and improve enterprise economic efficiency. Since it is difficult to meet the requirements of improving the comprehensive mechanical properties of the tool by a single coating material, it is necessary to optimize the coating process and improve the composition and metallographic structure of the substrate while improving the coating and the high performance and high reliability coating equipment. The bond between the substrate, the coating and the coating and the resistance to plastic deformation of the substrate. The current research direction is: to improve the content of aluminum; improve the composition of the coating, add C (or Si) as a binder to strengthen the microstructure of the TiAlN coating to improve its performance; improve the structure of the coating Multilayer, gradient and nanocrystallization are also trends in TiAlN coatings.

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