Application of solid lubricants in cutting lubrication
1 Do you know about pressure testing machine and constant stress pressure testing machine? Introduction
friction and wear are a major obstacle in the process of material processing. They damage tools, increase power consumption, and the worn abrasive particles pollute the processing materials. In the process of cutting, the front and rear surfaces of the tool constantly rub violently with chips and workpiece, and the contact area is in a state of high temperature and high pressure. The friction and wear on the tool will cause the passivation failure of the tool and make the cutting impossible. The severe friction on the workpiece will deteriorate the quality of the machined surface. In order to reduce the friction and wear during cutting, the commonly used method is to lubricate in cutting. The main function of lubrication is to improve the friction lubrication state in the cutting process and reduce the cutting temperature, so as to prolong the tool life and improve the machining surface quality. As early as the 19th century, solid lubricants have been used in low-speed machines. Due to their excellent friction performance, high bearing capacity and wear resistance, good wide temperature characteristics and timeliness, they can be used in places where lubricating grease cannot be used, in harsh environments and in occasions where maintenance is not required. Due to the excellent characteristics of solid lubricants, solid lubricants are also widely used in cutting to improve the lubrication of friction. There are three main methods to lubricate cutting: ① using cutting fluid with lubrication function; ② Apply lubricating coating on the tool surface; ③ Use self-lubricating tools. In cutting fluid, solid lubricant only plays an auxiliary role in the form of additives to improve the performance of cutting fluid; In the latter two lubrication methods, solid lubricant is the main factor to play the role of lubrication. There are many kinds of solid lubricants, including graphite with layered crystal structure, MoS2, WS2, MoSe2, wse2, BN, etc; There are soft metals, such as Ag, Pb, Sn, in, etc; There are metal oxides, fluorides, phosphates, etc. Among these solid lubricants, some are only suitable for lubrication in the atmosphere, such as graphite, BN, etc; There are suitable for vacuum lubrication, such as MoS2, WS2, Pb, etc.
2. Solid lubrication mechanism
if the hard metal slides on the soft metal surface, under the action of load, the hard metal is pressed into the soft metal, so that the real contact area increases, and the friction also increases, which will lead to furrow phenomenon. If hard metals slip on the surface of hard metals, although the contact area between hard metals will not increase, the yield strength of hard metals is large, and the friction force will also increase. Due to the temperature rise of the friction surface, it is easy to bite. In both cases, the friction coefficient is large.
If a layer of thin film with low shear strength is coated on the surface of hard metal substrate, the contact area between friction pairs will not increase, but the shear strength will be greatly reduced, and the friction force and friction coefficient will also be greatly reduced, which plays the role of solid lubrication. However, if this film is applied to the surface of soft metal, the film cannot play a lubricating role
therefore, a thin film with low shear strength adhered to the friction surface between hard metals can play a role in friction reduction and lubrication. If the film is filled with solid lubricant, the substance can be called solid lubricant. This extremely thin film is called solid lubricating film.
3. Application of solid lubricants in cutting fluids
cutting (grinding) machining usually uses liquid lubrication, which is based on lubricating oil, emulsion and water-based synthetic cutting fluid, and adds various additives to solve the problem of lubrication and cooling in cutting. Under fluid lubrication, reducing the viscosity of lubricating oil can reduce the friction energy loss, but under boundary lubrication conditions, the most effective way to reduce friction is to select appropriate lubricating oil additives extreme pressure antiwear agents and friction modifiers (FM). There are two kinds of friction modifiers: one is chemical friction modifiers, most of which are polar or oil-soluble macromolecular compounds containing active elements (sulfur, phosphorus and chlorine); The other is the so-called mechanical friction improvers, which are mainly non oil active solid particles suspended in oil, such as graphite, disulfide phase (MoS2), polytetrafluoroethylene (PTFE) powder and melamine cyanuric acid complex (MCA), which are the most commonly used solid lubricant additives
in production practice, people have gradually found that solid lubricants sometimes play a better role in cutting than conventional liquid lubrication. Generally, solid lubricants added to cutting fluid include graphite, MoS2 and red lead powder (Pb3O4). Especially when molybdenum disulfide (MoS2) is added to oil-based or water-based cutting fluid, the effect is more remarkable. When cutting ductile materials, cutting oil containing colloidal graphite or MoS2 can be used; When cutting brittle materials, emulsions containing colloidal graphite can be used. For example, 0.5% graphite (or molybdenum disulfide), 10% - 12% sodium sulfonate by weight, and the rest is emulsified oil composed of lubricating oil (plus a small amount of passivator). The concentration is diluted with water below 1%, which can be used for cutting and processing of superhard metals, and this fluid has good effects in various metal cutting and processing.
Add paste composed of lead soap, colloidal graphite or molybdenum disulfide into lubricating oil, which can be used for cutting conditions with runout or elastic vibration, or for processing with low cutting speed. When in use, it can be directly applied on the tool, or it can be used as emulsion. As long as molybdenum disulfide forms 1 on the metal surface μ M thick lubricating film has high adhesion and cutting capacity. If it is mixed with stearic acid and paraffin to make ointment and applied to the surface of the tool, the service life of the tool can be greatly improved. If tapping on 30CrMnSi material, one tap can only tap 200 threaded holes without solid lubrication, while the tap lubricated with molybdenum disulfide ointment can process 1000 threaded holes.
in grinding, mixing abrasive and solid lubricant can generally reduce the friction coefficient and friction heat and improve the grinding speed. It is found that the wear of the grinding wheel is dependent on the heat resistance of the binder when grinding steel materials with the grinding wheel. Once the friction resistance, the cause of heating, is reduced, the feed amount and speed can be increased. If aromatic polyester (POB) is used as binder to manufacture artificial diamond abrasive tools, and tungsten disulfide or graphite fluoride is added, the feed amount and feed speed can be increased.
in the following cases, solid lubrication can be used instead of liquid lubrication.
(1) occasions where lubricating grease cannot be used: under special working conditions, when the performance of general lubricating grease cannot adapt, solid lubricant can be used for lubrication. When liquid lubrication cannot be used in metal cutting (grinding) and pressure processing, solid lubricants can also be used for lubrication.
(2) in order to prevent the lubricating grease from being polluted, in the environment where the lubricating grease is easy to be polluted or washed away by other liquids (such as water, sea water, etc.), in the humid environment, in the environment containing impurities such as mud, sand, dust, etc., liquid lubrication cannot be used, and solid lubricant can be used for lubrication
4. The application of solid lubricants in coated tools
coating the tools is one of the focuses of Ticron at present. It is one of the important ways to use polycarbonate plastic in electric vehicles with high tool performance. In the past decade, the tool coating technology has achieved rapid development, and the coating process is becoming more and more mature. Coated blades account for 14% of the total output of cemented carbide and ceramic blades in Japan
coated tools can be divided into two categories: one is "hard" coated tools, such as tic and Al2O3 coated tools, whose main advantages and disadvantages are high surface hardness and good wear resistance; The other is "soft" coated tools, such as MoS2, WS2, tas2, mos2/mo, mos2/ti and ws2/w coated tools. The friction coefficient with workpiece material is very low (about 0.01), which can reduce bonding, cutting force and cutting temperature. At present, the commonly used tool coating materials are mainly various hard nitrides, oxides, carbides or boride ceramics, which have high hardness, excellent heat resistance, oxidation resistance and corrosion resistance. In 1992, someone first proposed the method of depositing soft solid lubricating film on the tool surface to improve tool life and productivity. Because the coating material has low formation free energy, stable chemical properties and low chemical affinity with the workpiece material, it can effectively play an anti bonding role between the tool chip and tool tool interface. Under the condition of high temperature in the cutting area, tic, tin and other coating materials can also be oxidized to form some soft TiO2 films. Al2O3 also has considerable fluidity above 800 ℃ and can play the role of extreme pressure lubrication. From the tribological point of view, these two aspects make the coating material play a good role as a solid lubricant, which not only enhances the ability of the tool to resist adhesive wear, but also is beneficial to reducing the cutting force, cutting temperature and improving the machining quality.
solid lubricant is bonded to the tool surface by coating and coating methods to form a solid lubricant film, which can play a lubricating effect. The solid lubricant with layered structure has strong bonding ability with the friction surface, and its own layers have low shear strength. In the cutting process, the solid lubrication film on the tool surface will be transferred to the workpiece material surface to form a transfer film, so that the friction occurs between the transfer film and the lubrication film in the cutting process. Even if the friction occurs inside the solid lubricating film, the purpose of reducing the friction coefficient and tool wear can be achieved
Jing Yang et al. Studied the friction characteristics of tialn-mos2/tialn hard lubricating film. The research shows that depositing tialn-mos2/tialn hard film on the drill bit (W6Mo5Cr4V2 fried dough twist drill, hardness HRC63) can make it have excellent wear resistance, friction reduction and high temperature resistance in the drilling process (drilling material is 38crni3mova, quenching and tempering, hardness hrc42, drill head speed is 600r/min, drilling depth is 20mm, feed rate is 0.13mm/r), Thus, the service life of cutting tools that affect the stable supply of products is greatly improved. The wear life of TiAlN hard film is nearly twice that of tin film. Fox et al. Studied the friction and lubrication characteristics of mos2/ti and graphite/crc lubricating coatings. The research shows that under the condition of reciprocating friction test, mos2/ti lubricating coating has a typical friction trend (load 100N), and the friction coefficient is small and stable. Through the comparative drilling test on 20mm thick jiss50c steel (rotating speed 30m/min, drilling depth 18mm, feed rate 0.12mm/r), under the condition of dry friction, the service life of the drill bit coated with a layer of mos2/ti lubricating coating on the hard coating is greatly improved. The service life of the drill bit coated with mos2/ti and tin composite coating is 2.1 times longer than that of tin alone, and that of the drill bit coated with mos2/ti and TiAlN composite coating is 2.8 times longer than that of TiAlN coating alone
you Rong Liu et al. Studied the cutting performance and wear mechanism of MoS2 coated ceramic tools. Comparative cutting tests were carried out with uncoated and MoS2 coated tools (Si3N4 and si3n4/mos2, TiCN and ticn/mos2, wc/co and wc/co MoS2) (cutting speed 103 ~ 208m/min, feed rate 0.1mm/r, cutting depth 0.25mm, workpiece 1045 carbon steel and 302 stainless steel). The test results show that when cutting 1045 carbon steel, the tool life of Si3N4 and TiCN coated with MoS2 is 50% longer than that of uncoated tools; When cutting 302 stainless steel, the service life of wc/co tools coated with MoS2 is more than 140% longer than that without coating. When machining with ceramic tools, bond wear is the main factor
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