Development of PVD coating technology for the hott

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The development of tool PVD coating technology

1 Introduction

at the end of the 1960s, the technology of depositing tin and tic hard coatings on the surface of cemented carbide tools by chemical vapor deposition (CVD) was widely used in the industry. These hard coatings can generally increase the tool life by more than 4 times, greatly improving the cutting performance of the tool and improving the cutting efficiency. However, because the coating temperature of CVD method is above 1000 ℃, it is not suitable for the coating of high-speed steel tools. In the early 1980s, the coating of high-speed steel tools and cemented carbide tools was successfully achieved by physical vapor deposition (PVD). Since then, a variety of new coating methods and coating materials continue to appear. At present, the research and development of tool coating technology is in the ascendant

2 tool PVD coating methods

at present, there are six commonly used tool PVD coating methods: low voltage electron beam evaporation (lvee), cathode arc deposition (CAD), triode high voltage electron beam evaporation (thvee), unbalanced magnetron sputtering (UMS), ion beam assisted deposition (IAD) and dynamic ion beam mixing (DIM). The principles of these coating methods are similar, that is, through the gas phase reaction process, the metal atoms (or gas atoms or ions introduced into the reaction chamber) evaporated or splashed will react in the gas phase, so as to deposit the required compounds on the tool surface. The main differences of these coating methods lie in the different gasification methods of the deposited materials (or by evaporation or sputtering), as well as the different methods of generating plasma (resulting in the different number of ions, electrons and neutrons in the plasma), resulting in differences in film forming speed and film quality. Lvee method can effectively ionize evaporated atoms, and the ionization rate can reach 50%. The CAD method uses sparks generated by the arc to evaporate materials from the target surface, and can effectively ionize and evaporate atoms and reaction gases, with an ionization rate of up to 90%. High ionization rate can promote gas-phase reaction and form a dense coating with strong adhesion to the substrate. However, CVD method is easy to produce metal droplets (called macro particles) with a diameter of 1 ~ 15 m. These macro particles are embedded in the growth film, which will damage the surface finish of the coating. A higher deposition rate can be obtained by arc filtration, but the ionization efficiency is low due to the small ionization cross section of high-voltage electron beam. Thevee law can overcome this shortcoming, and the ionization rate can be controlled by changing other process parameters. The deposition speed of UMS method is very fast, which can produce a very dense film with strong adhesion. However, the composition of the film is difficult to control due to the different saturated evaporation pressure of each component of the multi-component sputtering target. Multi target magnetron sputtering system can simultaneously sputter several different target source materials, which can effectively control the chemical composition of the film. Magnetron sputtering can keep the substrate temperature below 200 ℃. IAD method is a widely used tool coating method, which can effectively deposit various hard coatings. Before deposition, the substrate needs to be sputtered to remove the oxide layer on the substrate surface, which can improve the quality of the coating and make the bonding between the film and the substrate more solid. Dim method uses low-energy sputtering source to bombard, sputter and deposit the target source material, and then uses high-energy ion implanter to inject and mix the substrate, so as to obtain a coating with higher film substrate bonding strength than other coating methods. The AES study of the film shows that there is a mixed layer between the substrate and the film, which improves the bonding strength between the substrate and the film

3 development trend of tool PVD coating technology

at present, there are two development trends of tool PVD coating technology, one is that the coating is getting harder and harder; Second, the coating is getting softer and softer. The hard coating of the tool is mainly the compound of group Ⅳ a, Ⅴ a, Ⅵ a metal elements and C, N, O and other elements in the periodic table of elements; The soft coating of cutting tools is mainly chalcogenide compounds such as MoS2 and WS2

1) ordinary hard coating

the first coating used on high-speed steel tools is tin, which is a simple binary coating. In most cases, this coating makes cutting tools have good wear resistance

On the one hand, the cutting performance of the tool requires high bonding strength between the coating and the substrate, on the other hand, it requires low chemical activity between the coating material and the substrate material. Simple coatings such as tin have been difficult to meet this requirement. If the compounds constituting the coating have good mutual solubility, the complex coating formed thereby can have the best performance. By sputtering or evaporating the titanium target and introducing different proportions of nitrogen and acetylene gas, simple coatings such as tin and tic and complex coatings such as Ti (CN), Ti (CN) 2 and Ti (cn) 3 can be obtained. The American premium rock wool spray insulation coating has excellent thermal insulation performance, class a non combustible fire protection performance, excellent sound absorption and noise reduction performance Advanced spraying construction technology and green, healthy and environmental protection products. Table 1 lists the different coating types and the working gas components that obtained the coating. Table 1 coating type and working gas composition

coating type working gas composition total pressure (PA) discharge current (a) bias voltage (V) Ti (CN) n2=100%

c2h2=25%1.8 × 10-1i1=i2=i3=100v=-150ti (CN) 2n2=75%

c2h2=50%ti (CN) 3n2=50%



Figure 1 shows the durability comparison of simple coating tin, tic and complex coating Ti (CN). It can be seen from the figure that complex coating has better durability than simple coating. If mixed multi-layer coating is used, better coating performance can be obtained. The method is to deposit a layer of TiC on the surface of the tool substrate to make the coating have better bonding strength with the substrate, then deposit TiCN coatings with different proportions of C and N on it, and finally deposit a layer of tin or TiCN to produce beautiful colors. In addition to tin, tic, TiCN and other coatings, the more commonly used coating is aluminum titanium nitride coating. This coating was first developed and used in Europe. At the initial stage, ti0.75al0.25n was selected, and now Ti0.5Al0.5N is preferred. The latter can increase the oxidation temperature of the coating to 700> ℃. At the same time, ti0.5al0.5n> will be produced on the surface of the coating when heated in air. Therefore, it is necessary to verify the test speed and verify a layer of amorphous aluminum oxide (Al2O3) film, Thus, the coating can be protected. In some high-speed cutting occasions, due to the role of the protective layer, the working performance of ti0.5al0.5n> coated tools is better than that of tin or TiCN coated tools

Figure 1 Comparison of the persistence index of tin,

tic and Ti (CN) coatings measured by kalotester device

in tool hard coatings, ZrN, TiZrN diamond-like carbon film coating (DLC) and metal carbon film coating also have their own application scope, and their application scope is constantly expanding. DLC is mainly used for processing non-ferrous alloys. As a tool coating material, TiZrN has partially replaced tin. Metal carbon film coating has been used in Europe, but it is still in the experimental stage. In the foreseeable future, tin, TiCN and Ti0.5Al0.5N will still dominate the field of PVD tool coating

2) new hard coatings

in recent years, four new coatings with higher hardness have appeared in the field of tool PVD coatings, namely cubic boron nitride (CBN) coating, carbon nitride (CNx) coating, polycrystalline nitride superlattice coating and aluminum oxide (Al2O3) coating

cbn coating is the structure and operation method of concrete pressure testing machine. The hardness is 5200kgf/mm2, second only to diamond. Therefore, CBN coated tools can effectively cut quenched steel and other difficult to machine alloys. CBN films have been successfully synthesized by many researchers. The key to success is the adoption of IAD technology. At present, researchers have proposed two theories to explain the importance of ion bombardment during the growth of CBN films: Kester and Missier believe that the kinetic energy of ion bombardment is transferred to the growth film, thus promoting the formation of cubic structure of boron nitride; McKenzie and others believe that the stress caused by ion bombardment in the film must be repaired in time to promote the formation of cubic structure of boron nitride. However, once the thickness of CBN film exceeds 2000, the stresses in the film will make the film stratified, and it is these stresses that limit the thickness of CBN film. How to synthesize CBN films with a thickness of more than 2000 is a difficult problem to be solved in the future

if carbon nitride (CNx) coating can form b-c3n4 structure, its hardness can be theoretically calculated to be higher than that of diamond. At present, although there are reports of synthesizing carbon nitride, b-c3n4 film has not been successfully deposited. Generally, the nitrogen atoms of carbon nitride crystal obtained are insufficient or only amorphous carbon nitride can be obtained, and the nitrogen content in CNx obtained is in the range of 0.1 ≤ x ≤ 1. Transmission electron microscope studies show that the bulk carbon nitride film is amorphous, and there are nanocrystalline regions in the amorphous matrix. These nanocrystals may be the required C3N4 compound, but they need to be confirmed by analytical techniques. The hardness of these amorphous films ranges from 1500 to 7000kgf/mm2, and the hardness values are concentrated between 1500 and 2500kgf/mm2

nitride superlattice coating is a very promising new PVD tool coating. When the repetition period of the smallest double-layer lattice in the multilayer superlattice is 5 ~ 10nm, the hardness and strength of the coating will be significantly improved. The initial research work shows that the maximum hardness of single crystal nitride superlattice tin/vn coating can reach 5600kgf/mm2, while the hardness of tin/nbn coating can reach 5100kgf/mm2, which is much higher than that of uniform single crystal coatings tin, VN and NbN (1700 ~ 2300kgf/mm2). Although single crystal superlattice coating is of great scientific significance, the superlattice coating obtained on cutting tools (such as M2 high-speed steel cutting tools) is polycrystalline. The hardness of polycrystalline tin/nbn and tin/vn superlattice coatings is 5200kgf/mm2 and 5600kgf/mm2 respectively. The high hardness of polycrystalline superlattice coatings shows that they are well suitable for grinding. Researchers believe that the high hardness of polycrystalline superlattice coating is mainly due to the difficulty of dislocation movement within or between layers. When the coating is very thin, if there is a large difference in the dislocation energy between the layers (the difference in dislocation energy represents the difference in the shear modulus of the two materials), the dislocation movement between the layers is quite difficult, that is, the energy of dislocation movement determines the hardness of the superlattice coating. There is an optimal period for the superlattice coating, which makes the coating have the maximum hardness. For tin/nbn and tin/vn coatings, the optimal period is in the range of 4 ~ 8nm

aluminum oxide (al1o3) PVD coating is mainly deposited by RF (R.F) diode sputtering Al2O3 target or sputtering Al target in ar/o atmosphere. r. F power source can sputter non-conductive materials and prevent arc discharge on the target. Because the deposition rate of Al2O3 is very low and the coating is amorphous, this coating cannot be used for tool coating. At present, it has been reported that crystalline g-Al2O3 can be deposited by plasma assisted ECR process under the conditions of substrate temperature 400 ℃ and substrate bias -140v. Crystalline Al2O3 can be used as a tool coating with stable performance

3) soft coating

the high hardness of coating is the main goal pursued in the research and development of coating technology in the past. However, not all materials are suitable for processing with hard coated tools. For example, many high-strength aluminum alloys, titanium alloys or precious metal materials used in the aerospace industry are not suitable for processing with hard coated tools. At present, such materials are still mainly processed with uncoated high-speed steel or hard alloy tools. The development of tool soft coating can better solve the processing problem of this kind of material. The main components of the tool soft coating are chalcogenides (such as MoS2, WS2, etc.). High speed steel tools coated with MoS2 show excellent performance in machining high-strength aluminum and titanium alloys, and can obtain excellent machining surface roughness.

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