What is physical vapor deposition (PVD)
Physical vapor Deposition (PVD) is a process of depositing the coated material to the surface of the workpiece by Physical method under vacuum conditions. When making PVD process, the heating temperature of workpiece is generally under 600 ℃, which for the use of high speed steel, alloy mold machining steel and other steel mould has important significance. At present, there are three commonly used physical vapor deposition methods, that is, vacuum evaporation, sputtering and ion plating. At present, physical vapor deposition technology can not only deposit metal film, alloy film, but also deposit compound, ceramic, semiconductor, polymer film, etc.
1. Vacuum evaporation
Under the pressure of 1.33x10-3 to 1.33 x10-4pa under the vacuum condition, vacuum evaporation is made by heating the sedimentary material with such heat sources as electron beam, and the evaporated atoms or molecules directly form sedimentary layer on the surface of the injected workpiece under the pressure of 1.33x10-3 to 1.33 x10-4pa. However, direct evaporation of refractory metallic carbides and nitride is difficult and tends to break down compounds. For this purpose, reaction evaporation by introducing chemical process was developed. For example, titanium metal was vaporized by electron gun, and a small amount of reactive gases such as methane and acetylene were introduced into the evaporation space to enable titanium atoms and reactive gas atoms to react on the surface of the workpiece and deposit TiC coating.
Vacuum evaporation is mainly used for coating the surface of optical elements such as lens and reflector, various electronic components and plastic injection products.
2. Sputtering coating
Sputter coating is a physical vapor deposition method without evaporation technology. During the plating process, the workroom is vacuumed and filled with hydrogen gas as the working gas, and its pressure is kept at 0.13-1.33pa. The depositing material is used as the target (cathode) and the negative pressure is added at hundreds to thousands of volts. The workpiece is taken as the anode, and the filament on both sides has negative pressure (-30-100v). Heating filament to about 1700 ℃, the filament emit electrons to make hydrogen happen glow discharge, produce hydrogen ions H +, H + is accelerated by bombarding target material, make the atoms or molecules sputtering target material burst into the workpiece surface, formation of sediments.
Sputtering can be used to deposit various conductive materials, including high-melting metals and compounds. If TiC is used as the target material, the TiC coating can be deposited directly on the workpiece. Of course, the metal Ti can also be used as the target, and then the reactive gas can be imported to conduct reactive sputtering. The sputtering coating is uniform but with slow deposition speed, and it is not suitable for coating of more than 105mm thickness. Sputtering can make the substrate temperature increases to 500-600 ℃, therefore, only applicable to this temperature is the secondary hardening steel mould processing.
3. The ion plating
Ion plating is to make the gas or vaporized matter ionized under the vacuum condition by discharge of the gas, and to evaporate the vaporized matter or its reactants on the workpiece by bombarding the gas ions or evapotranspiration ions. By combining glow discharge, plasma and vacuum evaporation technology, ion plating can not only improve the performance of coating, but also extend the application range of coating technology.
In addition to the advantages of vacuum sputtering, ion plating also has the advantages of strong adhesion of film layer, good diffraction and extensive coating materials. Using ion plating technology, for example, can be in metal, plastic, ceramic, glass, paper and other non-metallic materials, coating with different performance of single plating, alloy plating, compound coating, and various kinds of composite coating, and the sedimentary speed (up to 755 m/min), cleaning before plating process is simple, no pollution to the environment, as a result, both at home and abroad in recent years has been rapid development.
Ionizing a metal or alloy vapor by means of a glow discharge of an inert gas. Ion plating involves the heating, evaporation and deposition of the coating material (such as TiN, TiC).
Evaporation coating material atoms after glow, a small number of ionization, and flew to the workpiece under the action of electric field, with the energy of the thousands of electron volts shoot to the surface, can into the matrix around a few nanometer depth, thus greatly improve the adhesion strength of the coating, and evaporation materials without ionization atomic deposition film directly on the parts. The sputtering of inert gas ions and coating material ions on the surface of the workpiece can also remove contaminants on the surface of the workpiece, thus improving the binding force.
If the reactive gas is introduced into the evaporation space, a metal compound coating can be deposited on the surface of the workpiece, which is called reactive ion plating. Due to the adoption of plasma activation, the workpiece only needs to be coated at a low temperature or even at room temperature to fully guarantee the dimensional accuracy and surface roughness of the workpiece. Therefore, the final process can be arranged after the workpiece is hardened or tempered. Such as sedimentary TiN or TiC, the basal body temperature can choose within the range 150-600 ℃, at high temperature coating of high hardness, bonding force with substrate is high also. Substrate temperature can choose according to matrix material and its tempering temperature, such as the substrate for high speed steel, can choose 560 ℃, and in this way, for the quenching, tempering and processing to the size of the high precision mould processing, need not worry about the matrix to reduce the hardness and deformation problems. In addition, the deposition speed of ion plating is faster than that of other gas-phase deposition methods, and it usually takes only a few minutes to obtain the TiC or TiN coating of 10mm thick.
TiN or TiC coatings deposited by PVD can be compared with those by CVD and have the following characteristics:
(1) the upper and lower molds are processed with high-precision metal molds, and it is quite effective to use PVD superhard compound plating to strengthen the surface;
(2) the PVD coating effect will be lost on rough mold surface;
(3) PVD coating is more effective for static loading;
(4) the precision before and after PVD plating does not change, and there is no need to process again;
(5) PVD coating has superior wear resistance and high corrosion resistance.
For example, when TiN is coated with high-speed steel punches used for making screws, the service life is 3-5 times longer than that of uncoated punches. TiN is coated on precision blanking die of automobile parts. When the thickness of the blanked steel plate is 1-3mm, the service life is extended 5-6 times, but when the thickness of the steel plate increases to 5-8mm, the effect is lost due to the shedding of the TiN layer. The corrosion resistance of TiN can be increased by 5-6 times, and the wear resistance can be increased at the same time.
The basic principle of physical vapor deposition can be divided into three process steps:
(1) gasification of the plating material: even if the plating material vaporizes, or is sputtering, that is, the gasification source through the plating material.
(2) migration of atoms, molecules or ions in the plating material: a variety of reactions are generated after the atoms, molecules or ions have been collided by the gasification source.
(3) atoms, molecules or ions are deposited on the substrate.
Understanding PVD physical vapor deposition techniques
Physical gas-phase deposition technology has simple process, improved environment, pollution-free, less consumables, uniform and compact film formation, and strong binding force with the matrix. The technology is widely used in the fields of aerospace, electronics, optics, machinery, construction, light industry, metallurgy, materials and so on. It can prepare film layers with wear resistance, corrosion resistance, decoration, electrical conductivity, insulation, optical conductivity, piezoelectricity, magnetism, lubrication, superconductivity and other properties.
Gas-phase deposition is a technology to form functional film on the surface of the matrix. It is to use the physical or (and) chemical reactions of materials in the gas phase to deposit single-layer or multi-layer, single-substance or compound film on the surface of the product, thus enabling the product surface to obtain various excellent properties required.
As a surface coating method, the basic steps of gas deposition -> transport -> deposition are required. Its main feature is that no matter the original material to be plated is solid, liquid or gas, it must be converted into gas-phase form during transport, and finally the surface of the workpiece is deposited and condensed into solid film.
The vapor deposition is mainly divided into two categories:
Initially, TiCI gas and NH gas were obtained by slightly heating the volatile liquid TiCI, and were introduced into the high temperature reaction chamber. These reaction gases were decomposed, and then the thermodynamic chemical reaction was conducted on the high temperature solid surface to generate TiN and HCI. HCI was extracted, and TiN was deposited on the solid surface to form a rigid solid film. ChemicalVaporDeposition (CVD) is a process of chemical reactions on solid surfaces and formation of non-volatile solid sediments via volatile compounds and gaseous substances containing the constituents of thin-film elements.
At the same time, people put another kind of vapor deposition, through high temperature heating of metal or metal compounds evaporated into gas phase, or through electronic, plasma, the photon energy can charge particles such as metal or compound target sputtering out corresponding atoms, ions, molecules (gas), deposit into a solid film on a solid surface, which has nothing to do with the material of chemical reactions (decomposition or combined), known as physical vapor deposition (PhysicalVaporDeposition, PVD).
With the development and application of gas-phase deposition technology, the two types of gas-phase deposition have their own new technical content. The two types of gas-phase deposition are interlaced with each other, and they are intertwined with each other. For example, plasma and ion beam are introduced into the evaporation and sputtering of traditional physical gas deposition technology to participate in the film deposition process. Meanwhile, the reactive gas can also be injected into the solid surface for chemical reaction to generate a new synthetic product, which is called reactive plating. One example is the synthesis of TiN through the reaction gas N2 in sputter titanium (Ti) plasma. This means that physical vapor deposition can also contain chemical reactions. Another example, in indoor ventilation with methane reaction, with the aid of w target cathode arc discharge, the Ar, w under the action of plasma methane decomposition, and the solid surface to recombine the carbon bonds, mixed w diamond-like carbon anti-friction film, people used to place on the deposition process is still classified as chemical vapor deposition, but it is in a typical physical vapor deposition technology, metal cathode arc ion plating. In addition, people put the plasma, ion beam technology is introduced into the traditional chemical vapor deposition process, chemical reaction has not fully follow the traditional thermodynamics principle, because the plasma has higher chemical activity, can be in the traditional chemical reaction thermodynamics that are lower than the temperature of reaction, the method known as the plasma assisted chemical vapor deposition (PlasmaAssistedChemicalVaporDeposition, hereinafter referred to as PACVD; Some data are called plasma enhanced chemical vapor deposition (PECVD), which gives more physical meaning to chemical vapor deposition.
In today's discussion of the differences between chemical and physical gas-phase deposition, I am afraid that only the differences in the morphology of coating materials are left: the former USES volatile compounds or gaseous substances, while the latter USES solid (or liquid) substances. This distinction seems to have lost the essence of its original definition.
We still according to the existing habit, mainly in the plating material form to distinguish the difference between a chemical vapor deposition, physical vapor deposition, the solid (liquid) plating material through high temperature and evaporation, sputtering, electron beam, plasma, ion beam, laser beam and arc, and other forms of energy produced by gas atoms, molecules, ions (gas, plasma) for transport, condensation of solid deposition on the surface (including chemical reactions with other gas phase reaction substances generated reaction products), to generate solid phase membrane process known as physical vapor deposition.
PVD technology appeared in the prepared films with high hardness, low friction coefficient, good wear resistance and chemical stability. The successful application in the field of high-speed steel cutting tools has attracted great attention from the manufacturing industry around the world. While developing high-performance and highly reliable coating equipment, more in-depth application research has also been carried out in cemented carbide and ceramic cutting tools. Compared with the CVD process, PVD process temperature is low, under 600 ℃ when the bending strength of cutting tool materials; The internal stress state of the film is compressive stress, which is more suitable for the coating of cemented carbide precision complicated tools. PVD process has no adverse effect on environment and is in line with the development direction of modern green manufacturing. Currently, PVD coating technology has been widely used in coating treatment of carbide end milling cutter, drill bit, step drill, oil drilling, reamer, tap, indexable milling cutter sheet, turning blade, special-shaped cutter, welding cutter, etc.
PVD technology not only improved the bonding strength of thin film and tool matrix materials, but also developed the coating composition from the first generation of TiN to multi-composite coating such as TiC, TiCN, ZrN, CrN, MoS2, TiAlN, TiAlCN, tin-aln, CNx, DLC and ta-c.
Enhanced magnetic cathode arc: The cathode arc technique is to separate the target into an ion state through low voltage and high current under the vacuum condition, so as to complete the deposition of the thin film material. The enhanced magnetic cathode arc USES the combined action of electromagnetic field to control the arc of target surface effectively, which makes the material's ionization rate higher and the film's performance better.
Filter cathode arc: Filtered cathodic arc (FCA) electromagnetic filtration system, equipped with high efficient ion source can be produced by the macroscopic particles in plasma and ion mass filter clean, after magnetic filtration of sedimentary particles ionization rate was 100%, and it can filter out particles, so the preparation of the film is very compact and smooth, with good corrosion resistance, and the adhesion strength of the body is very strong.
Magnetron sputtering: In a vacuum environment, the target is bombarded with ionized inert gas ions through the combined action of voltage and magnetic field, causing the target to be ejected in the form of ions, atoms or molecules and deposited on the substrate to form a thin film. Conductor and non-conductor materials can be sputtering as target materials according to different ionization power sources used.
Ion beam DLC: Hydrocarbon gas is separated into plasma in the ion source, and carbon ions are released by the ion source under the combined action of electromagnetic field. Ion beam energy is controlled by adjusting the voltage applied to the plasma. The hydrocarbon ion beam is introduced to the substrate and the deposition rate is proportional to the ionic current density. The star arc coating ion beam source adopts high voltage, so the ion energy is larger, which makes the film and substrate bond well. The larger ion current makes the deposition of DLC film faster. The main advantage of ion beam technology is that it can deposit ultra-thin and multi-layer structure, the process control precision can reach several angstrom, and it can reduce the defect caused by the particle pollution in the process to the minimum.
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