Solid lubricating materials
Solid lubricating materials refer to specialized materials that utilize the properties of solids to reduce friction and wear between two load-bearing surfaces, including powders, thin films, or certain integral materials. During the lubrication process, solid lubricating materials interact with the friction surface to form a solid lubricating film, reducing friction and wear.
Characteristics of solid lubricating materials
◆Sufficient adhesion force
Solid lubricating materials should firmly adhere to the friction surface and have good film-forming ability. They can form a strong chemical or physical adsorption film with the friction surface to prevent serious welding or material transfer between surfaces with relative friction.
◆ Lower shear strength
When the shear strength is low, it can reduce the friction coefficient, power loss, and temperature rise of the friction pair, and the shear strength should not change significantly with temperature changes. It has a wide range of applications.
◆ Good stability
Including physical thermal stability, chemical thermal stability, and aging stability. Physical thermal stability refers to the condition where, without the participation of active substances, temperature changes do not cause phase transitions or lattice changes, nor do they cause changes in shear strength or friction performance. Chemical thermal stability refers to the absence of strong chemical reactions, corrosion, and other harmful effects caused by temperature changes in various active media. It is non corrosive to bearings and related components, non-toxic to humans and animals, and does not pollute the environment. Time stability refers to the requirement for solid lubricants to be stored for a long time without deterioration, in order to be used for a long time.
◆ High load-bearing capacity
Due to the fact that solid lubricants are often used under harsh working and environmental conditions, they are required to have high load-bearing capacity.
Commonly used solid lubricating materials
◆ Molybdenum disulfide
Its characteristics include low friction, high load-bearing capacity, good thermal stability, strong chemical stability, radiation resistance, and high vacuum resistance.
◆ Graphite
In a frictional state, graphite can slip along the crystal layers and orient along the direction of friction. Graphite has good adhesion ability to surfaces such as steel, chromium, and rubber. Figure 1 shows the crystal structure of graphite.
◆ Fluorinated graphite
Good wear resistance, the distance between layers is much larger than graphite, making it easier for shear to occur between layers. Due to the introduction of fluorine, its performance is superior to graphite or molybdenum disulfide under high temperature, high speed, and high load conditions.
◆ Hexagonal boron nitride
Density is 2.27 g/cm3, melting point is 3100-3300 ℃; Mohs hardness is 2; The friction coefficient is 0.2 in air and 0.3 in vacuum; The thermal stability in air is 700 ℃, while in vacuum it is 1587 ℃; Corrosion resistance and good electrical insulation, with a specific resistance greater than 10-6 Ω· cm; The compressive strength is 170 MPa; The coefficient of thermal expansion in the c-axis direction is 41 × 10-6/℃, while in the d-axis direction it is -2.3 × 10-6/℃; The maximum operating temperature in an oxidizing atmosphere is 900 ℃, while it can reach 2800 ℃ in a non active reducing atmosphere.
◆ Silicon nitride
Belonging to the hexagonal crystal system, it is a ceramic material that does not have the layered structure of graphite or the plastic fluidity of lead oxide. It has high hardness and does not have lubricity in powder state. But the surface of its formed form can exhibit low friction coefficient after appropriate precision processing. After precision machining of the surface of silicon nitride ceramics, the number of micro convex bodies in contact with the friction surface decreases, and the friction coefficient can be significantly reduced. The surface is precision machined to 0.05-0.025 μ At m, the friction coefficient can reach 0.01.
◆ Polytetrafluoroethylene
Has good chemical and thermal stability. At high temperatures, it does not react with concentrated acid, concentrated alkali, or strong oxidants. It can maintain good chemical stability, thermal stability, and lubricity over a wide temperature range and almost all environmental atmospheres. Having anisotropic properties, good orientation can also occur under sliding friction conditions. Its friction coefficient is lower than that of graphite and MoS2. The friction coefficient decreases with the increase of load, and for steel, the friction coefficient is 0.04. Under high load conditions, the friction coefficient will decrease to 0.016.
◆ Nylon
The friction coefficient decreases with the increase of load, and under high load conditions, the friction coefficient can be reduced to 0.1~0.15. It has good wear resistance, especially in environments with a large amount of dust and mud, and its wear resistance is unmatched by other plastics. The disadvantages of nylon are: strong moisture absorption, high water absorption, and poor dimensional stability.
◆ Polyoxymethylene
It is a thermoplastic engineering plastic with good comprehensiveness and color difference, high melting point, high crystallinity, good dimensional stability, excellent impact resistance, water resistance, oil resistance, chemical resistance, and wear resistance. Its friction coefficient and wear are relatively low.
◆ Polyimide
Has excellent friction resistance, wear resistance, and dimensional stability. It still has excellent dielectric properties at high temperatures. But it is not alkali resistant and has high production costs.
◆ Polyhydroxybenzoate
It has properties similar to metals and is the variety with the highest thermal conductivity and thermal stability in air in plastics. It is a self-lubricating material with an extremely low friction coefficient, which can reach 0.005, even lower than when lubricated with lubricating oil or grease.
◆ Soft metal
Soft metals such as tin, lead, magnesium, and indium can be used as solid lubricants. Soft metal can be used alone or in combination with other lubricants. When used in combination with lubricating oil, its friction coefficient can be reduced. Usage of solid lubricating materials.
◆ Making integral parts
Polytetrafluoroethylene, polyacetal, polyoxymethylene, polycarbonate, polyamide, polysulfone, polyimide, chlorinated polyether, polyphenylene sulfide, and polyethylene terephthalate with low friction coefficient, good formability and chemical stability, excellent electrical insulation, and strong impact resistance can be used to make integral components. For example, gears, bearings, guide rails, camshafts, rolling bearing cages, etc.
◆ Produce various covering films
Apply solid lubricant to the friction interface or surface through physical methods, making it a dry film with certain self-lubricating properties. The methods for film formation include sputtering, electrophoretic deposition, plasma spraying, ion plating, electroplating, adhesive bonding, chemical generation, extrusion, impregnation, roll coating, etc.
◆ Making composite or composite materials
The purpose of combination or composite is to obtain a new material with superior performance, namely composite materials. The physical, chemical properties, and shape of composite materials are all different.
◆ As a solid lubricating powder
Adding solid lubricating powder (such as MoS2) in an appropriate amount to lubricating oil or grease can improve the load-bearing capacity of the grease and improve the boundary lubrication state.
Solid lubrication bearings
Basic concepts
By utilizing the self-lubricating properties of solid lubricants, bearings do not require oil maintenance during use. Solid lubricants are used to lubricate inorganic solid substances in mechanical friction parts where lubricating oil or grease cannot be applied. There are many types, the most important of which are graphite, molybdenum disulfide, and talc powder. The powder of tetraboron nitride (melting point 2350 ℃) can also be used as a component of solid lubricants. The particles of solid lubricants have a crystal lattice structure, which may slide over each other and cause lubrication.
Solid lubrication bearings are particularly suitable for use under special working conditions such as oil-free, high temperature, high load, low speed, pollution prevention, corrosion prevention, radiation protection, and the inability to add lubricating oil film when immersed in water or vacuum solution. Solid lubrication bearings are widely used in metallurgical steel rolling equipment, filling equipment, water turbines, gas turbines, instruments and meters, mining machinery, ship machinery, textile machinery, shipbuilding industry, aerospace and other fields, Meanwhile, solid lubrication bearings are increasingly being used in other industrial and agricultural machinery. Figure 2 is an example of a solid lubricated bearing.
Figure 1 Crystal structure of graphite
Figure 2 Solid lubricated bearings
Powder metallurgy solid lubrication bearings
Powder metallurgy solid lubrication bearings refer to metal based solid self-lubricating materials manufactured using powder metallurgy methods. Due to the unique advantages of powder metallurgy in preparation technology, structural composition, and other aspects, powder metallurgy solid lubrication bearings have a uniform structure, almost no segregation, and good thermal processing performance. Especially in terms of wear resistance, it is significantly better than traditional casting materials. It has broad application prospects in many special working conditions.
◆ Classification
Common powder metallurgy solid self-lubricating materials include copper based, iron based, aluminum based, etc. Nickel based, titanium based, etc. are also used in aerospace. Copper itself is a solid lubricant, therefore copper based powder metallurgy solid self-lubricating materials have high anti friction performance, as well as good corrosion resistance and conductivity. The commonly used copper based powder metallurgy solid self-lubricating materials are copper graphite composite materials, as well as bimetallic materials such as copper iron, copper aluminum, and copper nickel. Improving the anti friction performance of iron based materials through alloying and additives can result in higher anti friction performance and load-bearing capacity than copper based materials.
Powder metallurgy copper based graphite self-lubricating materials have been widely used in the manufacturing of friction parts in sewing machinery, pharmaceutical machinery, and food machinery industries due to their advantages of good thermal conductivity, low noise, running in, low friction coefficient, and no pollution. Because the products in these industries have strict requirements for oil pollution, it is urgent to develop new materials that can self lubricate without oil or with little oil to meet the needs of environmentally friendly development and solve the problem of oil pollution of products.
The typical components of copper based self-lubricating materials are copper alloy matrix, solid lubricating phase with anti friction effect, and pore structure. Copper has low strength and requires the addition of alloying elements to form a copper alloy for matrix strengthening to improve the hardness and strength performance of the material. The improvement of self-lubricating friction performance is mainly achieved through the action of solid lubricating phases, commonly used solid lubricating components include graphite, molybdenum disulfide, polytetrafluoroethylene, etc. The new copper based self-lubricating composites are operated under the conditions of no oil or little oil. The requirements for wear resistance and self-lubricating properties of the materials are different from those of traditional self-lubricating materials in terms of structure, composition ratio and preparation methods. In order to obtain powder metallurgy copper based self-lubricating materials with high wear resistance and self-lubricating function, in-depth research on improving self-lubricating performance has become a key research direction.
After adding additives such as graphite and copper to the iron matrix, its friction performance is on par with that of the copper matrix, but its mechanical properties such as tensile strength and bending strength are significantly better than those of the copper matrix.
Aluminum has the advantages of low density, good thermal conductivity, and high corrosion resistance. Therefore, aluminum based powder metallurgy solid self-lubricating materials have attracted great interest as a lightweight material. Aluminum itself has relatively low strength and hardness, and alloying elements need to be added for strengthening. The commonly added elements include copper, magnesium, silicon, etc. Adding these elements can mainly form certain strengthening phases, thereby giving the matrix high strength, high modulus, and corrosion resistance.
Titanium based materials have low density and good sealing properties, and are mainly used to manufacture various components such as bearings and seals for aircraft, with relatively high costs.
Silver based self-lubricating materials have the advantages of low friction coefficient, small and stable contact resistance, and high conductivity. Silver graphite composite materials are widely used in electrical contact parts, brushes, bearings, sliders, contacts, and bushings in chemical solutions. The Ag-MoS2 self-lubricating composite bearing material can be used in special working conditions such as ultra-high vacuum and strong radiation. Ag Ta MoS2 graphite based self-lubricating brush material can be used for space power transmission and signal transmission mechanisms. Other matrix powder metallurgy solid self-lubricating materials, such as nickel based, can be used at temperatures ranging from 500 to 1000 ℃. However, due to the high cost of nickel, its application range is greatly limited.
Introduction to New Technology - Coating Technology
In traditional powder metallurgy self-lubricating materials, direct mixing, pressing and sintering of solid lubricant powder and matrix powder results in poor wettability of certain solid lubricants with the matrix, significant differences in physical and mechanical properties, and large density differences. This leads to problems such as uneven mixing, low interfacial bonding strength, and strong cutting effect on the metal matrix when preparing composite materials using traditional powder metallurgy methods. During the sintering process, solid lubricants also undergo decomposition or oxidation due to high temperature, which reduces the lubrication performance of the material and makes it difficult to improve its service life. These issues limit the application of powder metallurgy solid self-lubricating materials, but coating technology can effectively solve these problems.
Coating technology is the process of coating a solid lubricant with nickel or copper to form a coating powder. Nickel coating is mainly generated through chemical vapor reactions or chemical plating. Copper coating is mainly prepared by supersaturation solution or electroplating. Adding this coating powder to the matrix can increase the density of the solid lubricant, reduce segregation, and the coating layer can protect the solid lubricant from evaporation or oxidation caused by high temperature.
The Institute of Tribology and School of Materials Science at Hefei University of Technology studied the effects of copper and nickel plated graphite powder on the friction, wear, and mechanical properties of copper based solid self-lubricating materials. Adding copper plated graphite powder can make the graphite distribution more uniform, significantly improve the interface bonding between graphite and copper, and the microstructure of composite materials. Its bending strength can be increased by 40% to 60%, significantly improving the performance of composite materials. The copper based material with added graphite undergoes three processes under a certain force, including mild wear, moderate wear, and severe wear. The copper based material added with nickel plated graphite only underwent two processes: slight wear and moderate wear. Nickel plating makes the bonding between graphite and matrix more firm and tight, forming a self-lubricating film that is not easy to peel off and more complete. It also significantly improves the strength, toughness, and high-temperature strength of copper based graphite composites, with little impact on the hardness of the composite materials. Research has shown that adding an appropriate amount of SiO2 reinforcement phase can have a dispersion strengthening effect, slow down the trend of high-temperature strength decline, and improve the high-temperature strength of copper based graphite composites.
Xu Feng, Wang Xuran, and others studied the effect of copper plating on graphite surface on the friction properties of iron-based materials. Research has shown that the coating technology can change the physical and chemical state of the surface of graphite particles, making it easy for graphite particles to mix evenly with the matrix, overcoming the phenomenon of particle segregation, achieving uniform particle distribution, and improving the microstructure of iron-based composite materials, reducing the friction coefficient of the composite material and increasing the friction and wear performance by 20% to 30%.
Professor Li Xibin from Central South University studied the effect of nickel coated MoS2 powder on Ni2Cr high-temperature solid self-lubricating materials: due to the uniform deposition of Ni element around MoS2 particles, the bonding strength between the metal matrix and non-metallic particles is improved, while partially preventing the decomposition of Mo and its reaction with the matrix, thereby improving the mechanical properties of the material, The bending strength and compressive strength of the material increased by about 25% compared to the material with an equal amount of MoS2 added without nickel coating, and the self-lubricating performance of the material was improved, with lower friction coefficient and wear rate, and good wide temperature band tribological performance.
The Iron and Steel Research Institute uses carbonyl method to coat nickel on graphite and then add it to iron-based materials. Due to the mutual solubility of nickel and iron, it can effectively improve the interfacial bonding strength of materials and achieve a uniform structure. It has been proven that powder metallurgy self-lubricating materials prepared by this method can not only improve mechanical properties but also enhance their tribological properties compared to composite materials with an equal amount of uncoated graphite. As the content of nickel coated graphite in the lubricant increases, the mechanical properties of the material continue to decrease, and the friction coefficient also decreases. However, the wear rate shows a trend of first decreasing and then gradually increasing. When the content of nickel coated graphite reaches a certain value, the wear rate decreases to the lowest, which can achieve good comprehensive performance.
◆ Introduction to new materials - High temperature metal based self-lubricating bearings
For anti friction materials that can work at higher temperatures or high temperatures, it is necessary to ensure high thermal strength, oxidation resistance, and good anti friction performance. Only multi-component composite materials can meet these requirements. The solid lubricant in powder metallurgy high-temperature metal based self-lubricating materials is added to composite materials as a structural component. The addition of solid lubricants can improve the anti friction performance of materials, while also affecting their physical and mechanical properties. According to the properties of the lubricant, it can be added to the original mixture in the form of powder, or added to the pores of the material after sintering. Depending on the method of addition, lubricants can interact with the material matrix during sintering during the material preparation process, partially or completely transforming into new forms.
The high-temperature friction and wear performance of powder metallurgy high-temperature metal based solid self-lubricating materials depends on the properties of the matrix structure (strength, oxidation resistance, etc.), solid lubricants (properties, types, sizes, shapes, volume fractions, etc.), and the lubrication characteristics formed during the friction process, including film properties, thickness, coverage, etc. It is generally believed that most materials working at 450-500 ℃ are composed of copper and iron bases; When the working temperature is above 700 ℃, nickel based and cobalt based materials are usually used; When the temperature reaches 1200-1300 ℃, refractory metal based alloys need to be used.
The addition of solid lubricants to the matrix metal powder can significantly improve the wear resistance of the material. Due to the presence of solid lubricants in the material itself, during motion, due to thermal effects and friction, the solid lubricant forms a relatively stable lubricating film on the sliding surface, and relies on its own "self consumption" to continuously supplement and provide solid lubricants, repairing torn or damaged lubricating films, thereby achieving lubrication and friction reduction effects. The hardness and other properties of the base metal no longer significantly affect the tribological properties of this composite material. Under these conditions, the friction and wear of the material are mainly influenced by the properties of the solid lubricant itself.
Nickel and cobalt are commonly used substrates in high-temperature self-lubricating alloys, occupying a particularly important position. Nickel based alloys still have excellent mechanical properties above 500 ℃, and can work at high temperatures and high stresses. Moreover, the nickel surface is easily oxidized to form a NiO layer with good plasticity and adhesion, which is beneficial for reducing wear. At the same time, it is also a high-temperature solid lubricant. The research on nickel based high-temperature self-lubricating alloys has attracted great attention.
Ni Cu solid solution has good corrosion resistance and cold deformation resistance, as well as considerable strength and heat resistance. A high-temperature self-lubricating material made by powder metallurgy re pressing and re firing method with nickel copper as the skeleton and graphite as the solid lubricant, as a high-temperature bearing retainer, can meet various technical requirements for bearings in nuclear reactor drive mechanisms and has been applied in engineering.
Iron based materials using metal fluorides as solid lubricants can work under high loads and temperatures, as well as under non lubricated or vacuum conditions. Therefore, this material can be used for bearings in continuous casting equipment. Iron graphite molybdenum materials can achieve high temperature, lubrication free friction, and have a very low friction coefficient. The precision motor bearings produced can achieve lubrication free friction at a temperature of 400 ℃. The research on powder metallurgy high-temperature metal based solid self-lubricating materials is rapidly developing and has achieved relatively successful applications, widely used in many fields such as aerospace and mechanical industry.
Application examples
◆ Aerospace field
Due to the extremely special nature of the space environment, the bearings of some satellites, spacecraft, and other spacecraft will be affected by high vacuum, high and low temperatures, radiation, and other factors. Conventional oil and grease lubrication is difficult to meet the requirements, but solid lubrication is a good solution to this problem. Solid lubrication bearings have been widely used in the aerospace field abroad, and China has been applying solid lubrication bearings in the aerospace field since the 1970s. Solid lubrication has better chemical stability than grease lubrication, and does not deteriorate due to radiation. The amount of air released under vacuum conditions is very small, making the bearing lubrication system simple and not causing pollution to other adjacent components. Therefore, it is widely used in the aerospace field. Solid lubricating materials have good adaptability to aerospace environments, are more suitable for ultra-high vacuum environments, and have sufficient adaptability to temperatures. The application scenarios of solid lubrication bearings in the aerospace field mainly include: (1) high-temperature applications. When the temperature exceeds 250 ℃, solid lubricating bearings are used, such as magnetron bearings and engineering high-temperature valve bearings (below 400 ℃); (2) Low temperature applications. When the temperature is below -70 ℃, solid lubricated bearings are used. The working temperature can reach -253 ℃, which can meet the requirements of high speed, heavy load, and high reliability; (3) High vacuum applications. (4) Optical applications. To maintain the optical system from contamination and ensure clear images, solid lubricated bearings are used.
Molybdenum disulfide has the best lubrication performance in vacuum and has the characteristic of small torque variation when used in rolling bearings. Therefore, the rolling bearings used in various driving devices of artificial satellites are often sprayed with solid lubricating film of molybdenum disulfide. The multi-channel scanning radiometer is a new remote sensing detection system for meteorological satellites, which integrates high-tech disciplines such as optics, mechanics, electronics, and information processing. It is an important system component of meteorological satellites. The newly developed solid lubrication bearing is the supporting component of the scanning device. Molybdenum disulfide bearings, as key components of the shaft system, are widely used in multiple models of satellites. Typical solid lubrication bearings are used on satellites, which have high quality requirements, long service life tests, and high experimental costs. Figure 3 shows a spacecraft using solid lubricating materials. Figure 4 shows the extraction of solid lubricating materials by Shenqi astronauts from the spacecraft.
Figure 3: Spacecraft using solid lubricating materials
Figure 4: Divine Seven Astronauts Extracting Solid Lubricating Materials from the Cabin
◆ Metallurgical field
Metallurgical equipment is often subjected to harsh environments such as high temperatures, heavy loads, impacts, high levels of dust and moisture, resulting in the failure of lubricating grease. Solid self-lubricating bearings have played a particularly good role in improving the continuous operation rate of modern metallurgical equipment. In the mechanical transmission of steel rolling and smelting equipment in metallurgical enterprises, the use of bearings is quite common. The selection principle is based on factors such as load size, speed, and working environment in the mechanical transmission, while also considering the requirements of maintenance and repair. The selection object is determined by comprehensive comparison of the functional characteristics and usage conditions of the product. By using solid lubricating materials, the average service life can be increased by more than 10 times under the same conditions. Solid lubricating materials exhibit tribological properties of high mechanical strength, good toughness, low friction, wear resistance, and no damage to the dual. The shaft sleeve and isolation ring of the sintering machine trolley (Figure 5) are usually made of copper and lubricated by oil holes at the end of the shaft. In production operation, due to high working temperature and large dust, grease loss, coking, and oil hole blockage are easily caused, which can lead to dry friction and cause significant friction between sliding bodies, resulting in severe wear and tear of the shaft sleeve and neck and scrapping; Or there is jamming between the shaft, sleeve, and pulley; When the card wheel meshes with the sintering machine star wheel, it experiences sliding friction, which generates significant frictional resistance. Local wear of the card wheel and the star wheel tooth plate causes an increase in the driving current of the sintering machine and the deviation of the trolley, which poses great difficulties to production and maintenance. The use of solid lubrication bearings can solve the above problems. Many parts of the billet continuous casting machine in the steelmaking plant (Figure 6) use rotating or sliding friction parts lubricated with a large amount of grease. Due to the high operating temperature of the continuous casting machine, the lubricating grease is often not in place, or it dries and carbonizes too early, resulting in the failure and damage of these components, seriously affecting the operation rate of the continuous casting machine. After efforts, a type of heat-resistant and wear-resistant solid self-lubricating materials and products have been developed, and heat-resistant and wear-resistant solid self-lubricating bearings have been developed. The effect is good in several parts of the billet continuous casting machine.
Conclusion
The continuous pickling production line has a reasonable configuration and advanced technology. Advanced shallow groove turbulent pickling not only shortens pickling time, but also improves pickling efficiency; The 5-stage cascade countercurrent rinsing process adopted has good cleaning effect and low water consumption; The configured online leveling machine and oiling machine have laid a good foundation for the surface quality of hot-rolled pickled commercial coils. In summary, this continuous pickling production line not only has a compact production rhythm, advanced technology, and high product quality, but also follows the principles of energy conservation, consumption reduction, and green environmental protection. Therefore, it is of great significance for the diversification of product structure.
2024 January 1st Week Marginal Product Recommendation:
Three-layer metal-polymer self-lubricating bearings:
Three-layer metal-polymer self-lubricating bearing is lighter and has better mechanical and load performance. In addition, the wear resistance and noise absorbing feature is improved while more variable torque range could be fitted.
https://www.marginalbearings.com/bimetallic-bushes/three-layer-metal-polymer-self-lubricating.html
