Development and application of the hottest TiAlN c

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Development and application of TiAlN coated tools

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

1 introduction

coating the tool is one of the important ways to improve the tool performance. With the emergence of coated tools, the cutting performance of tools has made a major breakthrough. Coated cutting tools can improve machining efficiency and accuracy, prolong the service life of cutting tools and reduce machining costs

since the 1970s, tool coating technology has made rapid development, and the coating process is becoming more and more mature. The proportion of coated blades used in western industrial developed countries in indexable blades has increased from 26% in 1978 to 50 ~ 60% in 1985. About 80% of the cutting tools used in new CNC machine tools use coated tools. The proportion of coated blades of Sweden Sandvik company and American kena metal company has reached more than 80 ~ 85%. The proportion of coated carbide blades used in CNC machine tools in the United States is 80%, and coated blades used in turning in Sweden and Germany account for more than 70%. From 1981 to 1985, the output of cutting tools in the former Soviet Union increased by 16%, that of cemented carbide cutting tools increased by 29%, and that of coated cutting tools increased by five times. Coated tools have become an important symbol of modern tools, and will be one of the most important tool varieties in the field of NC machining in the future

As a new coating material, TiAlN coating has the excellent characteristics of high hardness, high oxidation temperature, good thermal hardness, strong adhesion, low friction coefficient and low thermal conductivity. It is especially suitable for high-speed cutting of high alloy steel, stainless steel, Qin alloy, nickel alloy and other materials. When high wear resistance is required, 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 and others first published the research results on TiAlN coating in 1985, people have paid great attention to its excellent high temperature resistance Jinan new era Gold Testing Instrument Co., Ltd. based on the principle of large manufacturers and standards, oxidation capacity and good service performance, and TiAlN film has been successfully prepared by various PVD methods. Due to the different preparation methods of TiAlN coatings, the reported TiAlN coatings also have different properties. Table 1 shows the main performance comparison of several commonly used coatings (data from Balzers coating Co., Ltd.). It can be seen from the table that TiAlN coating has high hardness and oxidation temperature. With the multilayering and nanocrystallization of the coating, the performance of TiAlN coating is gradually improving. TiAlN has good chemical stability and strong anti-oxidation and wear resistance. Its hardness is 3400 ~ 3600hv), and its wear resistance is only lower than that of diamond-like carbon film. It is the most respected superhard coating in the international tool industry at present. When the Al content exceeds 50%, it is called AlTiN in order to distinguish it from TiAlN. The hardness of AlTiN coating increases with the increase of aluminum content, and the aluminum content can exceed 65%; When the aluminum content increases, it will also form a softer AlN phase, which will reduce its hardness, but the aluminum content is not the only factor affecting the hardness. The hardness of AlTiN can reach 4500hv. At present, tialn/al2o3 multilayer PVD coating has also been successfully studied. The coating hardness of this tool can reach 4000hv, the number of coatings is 400 (thickness 5nm), and the cutting performance is better than that of tic/l2o3/tin coated tools. Table 1 Comparison of main properties of several coatings

microhardness of coating materials

(HV) modulus of elasticity

(GPA) maximum applicable

temperature (℃) coating structure tin single layer TiCN single layer TiAlN nanostructure

2 TiAlN coating process

in addition to coating materials, the cutting performance of coated tools also depends to a large extent on the technical level of the coating process, the bonding strength between the coating and the substrate, the coating and its interface microstructure The preferred orientation, the thickness of each single coating and the total thickness are all important factors that determine the performance of coated tools. The selection of substrate and coating materials and the suggestion of the Inspection Bureau of coating process parameters that plastic exports need to avoid new risks are described below

1) selection of substrate and coating material

for coated tool materials, the physical matching degree between coating and substrate and different coatings affects the interface stress load transfer and material properties, especially the matching degree of thermal expansion coefficient and the difference of elastic modulus between layers have the greatest impact on material properties. Due to the different degree of thermal expansion mismatch and elastic modulus, the residual stress field with different size and distribution will be formed in the material. From the point of view of reducing residual tensile stress and improving material strength, the thermal expansion coefficient of matrix and coating should be closer, and the difference of elastic modulus should be smaller; However, from the perspective of residual stress toughening, microcrack toughening and improving the fracture toughness of materials, the existence of appropriate residual stress is necessary. TiAlN coated tool matrix generally adopts high-speed steel or cemented carbide, such as YG6, YG8, ultra-fine grain WC Co cemented carbide, etc., and ceramics can also be selected 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 also be adopted in the process to improve the bonding strength of TiAlN film with high-speed steel, cemented carbide and other substrates. For example, pre sputter cleaning shall be carried out before film formation to remove impurities that are not conducive to the bonding between the coating and the substrate; In the process of film formation, high-energy particles are bombarded to improve the mutual diffusion ability of components between the film substrate and the reaction activity of atoms on the film surface, so as to reduce the generation of defects; Introducing intermediate transition layers such as TiAl and Ti, because the bonding strength between transition layer and matrix and between transition layer and TiAlN film is significantly higher than that between TiAlN film and matrix, the critical load of TiAlN film with transition layer is significantly increased, and the thickness of transition layer can significantly affect the bonding strength between film and matrix. When the transition layer is further thickened, the probability of the transition layer being sheared increases, resulting in a slow decline in the critical load. The interfacial bonding strength can be determined by measuring the critical load with an automatic scratch tester

2) coating process parameters

generally speaking, the PVD process can obtain a thinner coating than the CVD process, and it is firmly bonded to the substrate. At the same time, the deposition temperature of the PVD coating is low, and it has no effect on the bending strength of the tool material below 600 ℃. 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, etc. The performance of coated tools obtained by different coating methods in the experimental system of Z drop hammer impact tester is different. At present, the main parameters characterizing the composition of TiAlN coating are: ral/ti and rm/n, which can comprehensively reflect the proportion of Ti, Al and N atoms in the coating. The main factors affecting the properties of TiAlN coating are interface bonding strength, Al content in the coating, and the composition and structure of the coating

· multi arc ion plating

the coating performance of TiAlN coated tools prepared by multi arc ion plating method is related to the metal composition of the target and process parameters. Generally speaking, the uniformity of the coating prepared with TiAl composite target is better than that prepared with separated Ti and Al targets. Increasing the content of aluminum can greatly improve the adhesion of film base. However, for TiAl composite targets with high Al content, there are still some difficulties in the preparation process. The composition of TiAlN coating is restricted by process factors such as partial voltage, cathode arc current and substrate bias voltage: with the increase of N2 partial voltage, ral/ti increases and rm/n decreases; With the increase of cathode arc current, ral/ti in the coating decreases and rm/n increases; With the increase of matrix bias, ral/ti and rm/n decrease (see Figure 1)

Fig. 1 Effect of basic bias voltage on TiAlN coating composition

· hollow cathode ion plating

TiAlN film is deposited by hollow cathode ion plating. The main process parameters of coating include: main arc current, baking temperature, coating time, hydrogen flow and vacuum degree during coating. The flow rate of N2 affects the hardness and interface adhesion of the tool. As shown in Figure 2, with the increase of N2 flow, the film base bonding force and the microhardness of the film show an upward trend, and begin to decline after reaching the maximum value. Therefore, there is an optimal value to optimize the film base bonding force and the microhardness of the film. The TiAlN film prepared by hollow cathode ion plating method is continuous, smooth and compact, and the adhesion between the film substrate is higher than that between the tin film and the substrate, but there are still some differences compared with other processes

Fig. 2 Changes of film microhardness and interface adhesion with N2 flow

· energy pulse plasma gun

the high-energy pulse isolift gun independently developed by the Institute of physics of the Chinese Academy of Sciences can simultaneously take into account the advantages of various true space based surface treatment technologies such as ion implantation, physical vapor deposition and plasma nitridation, and can improve the wear performance of ceramic coated cutting tools. There are many technological factors that affect the deposition of thin films, including the discharge voltage (gun pressure vgun) between the inner and outer electrodes of the pulse plasma gun, the number of times the sample is bombarded by the pulse plasma, the voltage of the pulse electromagnetic inlet valve, the vacuum of the deposition chamber, the reaction pressure of the working gas, the distance between the plasma gun and the sample, and the sample temperature, among which the number of pulses and the gun pressure are the most important parameters. The Department of materials of Tsinghua University deposited TiAlN thin films with high hardness, wear resistance and strong adhesion to the film base on the cemented carbide tool substrate at room temperature with a high-energy density pulse plasma gun. The mechanical property parameters of the film are shown in Table 2. Table 2 mechanical property parameters of coatings prepared by high energy density pulsed plasma gun

nano hardness of coating materials

(GPA) Young's modulus (GPA) nano scratch critical

Load (MN) tinticntialn

· magnetron sputtering ion plating

magnetron sputtering ion plating is a high-temperature deposition process, in which the workpiece is generally heated to 300 ℃ ~ 500 ℃. This process overcomes the defects of low deposition rate, low ionization rate and strong substrate thermal effect of magnetron sputtering, and can significantly improve the bonding strength of film substrate. Unbalanced magnetron sputtering ion plating uses unbalanced power supply to diffuse the plasma confined near the target surface to the workpiece, so as to improve the performance of the system. Unbalanced magnetron sputtering has been widely used at home and abroad for its excellent performance

the process parameters that affect the performance of the coating include gas partial pressure, bias voltage, vacuum degree and temperature of the deposition chamber, etc. Gas partial pressure refers to the ratio of working gas to reaction gas. When preparing TiAlN coating, the working gas is ar. reducing the partial pressure of AR is conducive to improving the coating rate, film substrate bonding strength and film density. With the decrease of AR partial pressure, the working gas increases from partial pressure. N2 not only reacts with the film atoms deposited on the workpiece to form a compound film, but also reacts with the target to form a compound on the target surface, resulting in target poisoning. Therefore, it is very important to determine two optimal N2 partial pressure values. Medium frequency AC magnetron sputtering eliminates the "anode disappearance" effect and "cathode poisoning" problems, and greatly improves the stability of magnetron sputtering operation

3 application of TiAlN coated tools

tialn coating has better mechanical and physical properties than tic, tin, TiCN and other coatings, and can cooperate with other coatings to form a multi-component multi-layer composite coating. There is high Al concentration in TiAlN coating, and the coating surface

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