PVD deposition technology has been practiced for many years as a new surface modification technology, especially vacuum ion coating technology, which has gained great development in recent years and is now widely used in the treatment of tools, molds, piston rings, gears and other components. The coated gears prepared by vacuum ion coating technology can significantly reduce the friction coefficient, improve anti-wear and certain anti-corrosion, and have become the focus and hot spot of research in the field of gear surface strengthening technology.
The common materials used for gears are mainly forged steel, cast steel, cast iron, non-ferrous metals (copper, aluminum) and plastics. Steel is mainly 45 steel, 35SiMn, 40Cr, 40CrNi, 40MnB, 38CrMoAl. Low carbon steel mainly used in 20Cr, 20CrMnTi, 20MnB, 20CrMnTo. Forged steel is more widely used in gears because of its better performance, while cast steel is usually used to manufacture gears with diameter > 400mm and complex structure. Cast iron gears anti-glue and pitting resistance, but the lack of impact and wear resistance, mainly for stable work, power is not low speed or large size and complex shape, can work under the condition of the lack of lubrication , suitable for open transmission. Non-ferrous metals commonly used is tin bronze, aluminum-iron bronze and casting aluminum alloy, commonly used in the manufacture of turbines or gears, but the sliding and anti-friction properties are poor, only for light, medium load and low-speed gears. Non-metallic material gears are mainly used in some fields with special requirements, such as oil-free lubrication and high reliability. The field of conditions such as low pollution, like household appliances, medical equipment, food machinery and textile machinery.
Gear coating materials
Engineering ceramic materials are extremely promising materials with high strength and hardness, especially excellent heat resistance, low thermal conductivity and thermal expansion, high wear resistance and oxidation resistance. A large number of studies have shown that ceramic materials are inherently heat resistant and have low wear on metals. Therefore, the use of ceramic materials instead of metal materials for wear-resistant parts can improve the life of the friction sub, can meet some of the high temperature and high wear-resistant materials, multi-functional and other tough requirements. At present, engineering ceramic materials have been used in the manufacture of engine heat-resistant parts, mechanical transmission in the wear parts, chemical equipment in the corrosion-resistant parts and sealing parts, increasingly show the wide application of ceramic materials prospects.
Developed countries such as Germany, Japan, the United States, the United Kingdom and other countries attach great importance to the development and application of engineering ceramic materials, investing a lot of money and manpower to develop the processing theory and technology of engineering ceramics. Germany has launched a program called “SFB442″, the purpose of which is to use PVD technology to synthesize a suitable film on the surface of the parts to replace the potentially harmful lubricating medium to the environment and human body. P.W. Gold and others in Germany used the funding from SFB442 to apply PVD technology to deposit thin films on the surface of rolling bearings and found that the anti-wear performance of rolling bearings was significantly improved and the films deposited on the surface could completely replace the function of extreme pressure anti-wear additives. Joachim, Franz et al. in Germany used PVD technology to prepare WC/C films demonstrating excellent anti-fatigue properties, higher than those of lubricants containing EP additives, a result that similarly yields the possibility of replacing harmful additives with coatings. E. Lugscheider et al. of the Institute of Materials Science, Technical University of Aachen, Germany, with funding from the DFG (GermanResearch Commission), demonstrated a significant increase in fatigue resistance after depositing appropriate films on 100Cr6 steel using PVD technology. In addition, the United States General Motors has begun in its VolvoS80Turbo type car gear surface deposition film to improve fatigue pitting resistance; the famous Timken company has launched the name ES200 gear surface film; registered trademark MAXIT gear coating has appeared in Germany; registered trademark Graphit-iC and Dymon-iC respectively Gear coatings with the registered trademarks Graphit-iC and Dymon-iC are also available in the UK.
As an important spare parts of mechanical transmission, gears play an important role in industry, so it is of very important practical significance to study the application of ceramic materials on gears. At present, the engineering ceramics applied to the gears are mainly the following.
1、TiN coating layer
1、TiN
Ion coating TiN ceramic layer is one of the most widely used surface modified coatings with high hardness, high adhesion strength, low friction coefficient, good corrosion resistance, etc. It has been widely used in various fields, especially in tool and mold industry. The main reason affecting the application of ceramic coating on gears is the bonding problem between ceramic coating and substrate. Since the working conditions and influencing factors of gears are far more complicated than those of tools and molds, the application of a single TiN coating on gear surface treatment is greatly restricted. Although ceramic coating has the advantages of high hardness, low friction coefficient and corrosion resistance, it is brittle and difficult to obtain a thicker coating, so it needs a high hardness and high strength substrate to support the coating in order to play its characteristics. Therefore, ceramic coating is mostly used for carbide and high-speed steel surface. The gear material is soft compared to the ceramic material, and the difference between the nature of the substrate and the coating is large, so the combination of the coating and the substrate is poor, and the coating is not enough to support the coating, making the coating easy to fall off in the process of use, not only can not play the advantages of the ceramic coating, but the ceramic coating particles that fall off will cause abrasive wear on the gear, speeding up the wear loss of the gear. The current solution is to use composite surface treatment technology to improve the bond between the ceramic and the substrate. Composite surface treatment technology refers to the combination of physical vapor deposition coating and other surface treatment processes or coatings, using two separate surfaces/subsurfaces to modify the surface of the substrate material to obtain composite mechanical properties that cannot be achieved by a single surface treatment process. TiN composite coating deposited by ion nitriding and PVD is one of the most researched composite coatings. The plasma nitriding substrate and TiN ceramic composite coating have a strong bond and the wear resistance is significantly improved.
The optimal thickness of TiN film layer with excellent wear resistance and film base bonding is about 3~4μm. If the thickness of the film layer is less than 2μm, the wear resistance will not be improved significantly. If the thickness of the film layer is more than 5μm, the film base bonding will be decreased.
2、Multi-layer, multi-component TiN coating
With the gradual and widespread application of TiN coatings, there are more and more researches on how to improve and enhance TiN coatings. In recent years, multi-component coatings and multilayer coatings have been developed based on binary TiN coatings, such as Ti-C-N, Ti-C-N-B, Ti-Al-N, Ti-B-N, (Tix,Cr1-x)N, TiN/Al2O3, etc. By adding elements such as Al and Si to TiN coatings, the resistance to high-temperature oxidation and hardness of the coatings can be improved, while adding elements such as B can improve the hardness and adhesion strength of the coatings.
Due to the complexity of the multicomponent composition, there are many controversies in this study. In the study of (Tix,Cr1-x)N multicomponent coatings, there is a big controversy in the research results. Some people believe that (Tix,Cr1-x)N coatings are based on TiN, and Cr can only exist in the form of replacement solid solution in the TiN dot matrix, but not as a separate CrN phase. Other studies show that the number of Cr atoms directly replacing Ti atoms in (Tix,Cr1-x)N coatings is limited, and the remaining Cr exists in the singlet state or forms compounds with N. The experimental results show that the addition of Cr to the coating reduces the surface particle size and increases the hardness, and the hardness of the coating reaches its highest value when the mass percentage of Cr reaches 3l%, but the internal stress of the coating also reaches its maximum value.
3、Other coating layer
In addition to the commonly used TiN coatings, many different engineering ceramics are used for gear surface strengthening.
(1)Y. Terauchi et al. of Japan studied the resistance to frictional wear of titanium carbide or titanium nitride ceramic gears deposited by the vapor deposition method. The gears were carburized and polished to achieve a surface hardness of about HV720 and a surface roughness of 2.4 μm prior to coating, and the ceramic coatings were prepared by chemical vapor deposition (CVD) for titanium carbide and by physical vapor deposition (PVD) for titanium nitride, with a ceramic film thickness of about 2 μm. The frictional wear properties were investigated in the presence of oil and dry friction, respectively. It was found that the galling resistance and scratch resistance of the gear vice were substantially enhanced after coating with ceramic.
(2)Composite coating of chemically coated Ni-P and TiN was prepared by pre-coating Ni-P as a transition layer and then depositing TiN. The study shows that the surface hardness of this composite coating has been improved to a certain extent, and the coating is better bonded with the substrate and has better wear resistance.
(3) WC/C, B4C thin film
M. Murakawa et al., Department of Mechanical Engineering, Japan Institute of Technology, used PVD technology to deposit WC/C thin film on the surface of gears, and its service life was three times that of ordinary quenched and ground gears under oil-free lubrication conditions. Franz J et al. used PVD technology to deposit WC/C and B4C thin film on the surface of FEZ-A and FEZ-C gears, and the experiment showed that the PVD coating significantly reduced the gear friction, made the gear less susceptible to hot gluing or gluing, and improved the load-bearing capacity of the gear.
(4) CrN films
CrN films are similar to TiN films in that they have higher hardness, and CrN films are more resistant to high temperature oxidation than TiN, have better corrosion resistance, lower internal stress than TiN films, and relatively better toughness. Chen Ling et prepared a wear-resistant TiAlCrN/CrN composite film with excellent film-based bonding on the surface of HSS, and also proposed the dislocation stacking theory of multilayer film, if the dislocation energy difference between two layers is large, the dislocation occurring in one layer will be difficult to cross its interface into the other layer, thus forming the dislocation stacking at the interface and playing the role of strengthening the material. Zhong Bin et studied the effect of nitrogen content on the phase structure and frictional wear properties of CrNx films, and the study showed that the Cr2N (211) diffraction peak in the films gradually weakened and the CrN (220) peak gradually enhanced with the increase of N2 content, the large particles on the film surface gradually decreased and the surface tended to be flat. When the N2 aeration was 25 ml/min (target source arc current was 75 A, the deposited CrN film has good surface quality, good hardness and excellent wear resistance when the N2 aeration is 25ml/min (target source arc current is 75A, negative pressure is 100V).
(5) Superhard film
Superhard film is the solid film with hardness greater than 40GPa, excellent wear resistance, high temperature resistance and low friction coefficient and low thermal expansion coefficient, mainly amorphous diamond film and C-N film. Amorphous diamond films have amorphous properties, no long-range ordered structure, and contain a large number of C-C tetrahedral bonds, so they are also called tetrahedral amorphous carbon films. As a kind of amorphous carbon film, diamond-like coating (DLC) has many excellent properties similar to diamond, such as high thermal conductivity, high hardness, high elastic modulus, low coefficient of thermal expansion, good chemical stability, good wear resistance and low friction coefficient. It has been shown that coating diamond-like films on gear surfaces can extend the service life by a factor of 6 and significantly improve the fatigue resistance. C-N films, also known as amorphous carbon-nitrogen films, have a crystal structure similar to that of β-Si3N4 covalent compounds and are also known as β-C3N4. Liu and Cohen et al. performed rigorous theoretical calculations using pseudopotential band calculations from the first-nature principle , confirmed that β-C3N4 has a large binding energy, a stable mechanical structure, at least one sub-stable state can exist, and its elastic modulus is comparable to diamond, with good properties, which can effectively improve the surface hardness and wear resistance of the material and reduce the friction coefficient.
(6) Other alloy wear-resistant coating layer
Some alloy wear-resistant coatings have also been tried to be applied to gears, for example, the deposition of Ni-P-Co alloy layer on the tooth surface of 45# steel gears is an alloy layer to obtain ultra-fine grain organization, which can extend the life up to 1.144~1.533 times. It has also been studied that Cu metal layer and Ni-W alloy coating are applied on the tooth surface of Cu-Cr-P alloy cast iron gear to improve its strength; Ni-W and Ni-Co alloy coating are applied on the tooth surface of HT250 cast iron gear to improve the wear resistance by 4~6 times compared with the uncoated gear.
Post time: Nov-07-2022