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Tribology: from microscopic mysteries to engineering problems
Tribology is mainly the study of interaction, friction, wear and lubrication laws between surfaces and their control technology. Statistics show that friction consumes 1/3 of the world's primary energy, wear causes about 60% of machine parts to fail, and more than 50% of mechanical equipment accidents originate from lubrication failure and excessive wear. Tribology research is an important field related to the development of national economy. In addition, tribology has also played a huge role in the progress of human civilization. In primitive society, flint people make fire by friction (drilling wood to make fire) marked the beginning of human civilization. Sled appeared in 9000 BC, and cars with rolling friction instead of sliding friction appeared in China in 4600 BC, which made human beings get rid of the primitive life mode. In particular, the invention of bearings, which is closely related to tribology, has given birth to the development of modern industry. These are the contribution of tribology to the development of human civilization. With the further development of high-speed, green and far-reaching, many new challenges have been put forward to tribology. How to explore the most essential mechanism of tribology from the micro point of view, and provide new ways, methods and technologies to solve the problems of engineering tribology has become the mainstream of today's friction world.
Development course of tribology
1. Tribology development process
The real quantitative study of friction began in Europe during the Renaissance in the 15th century. Leonardo da Vinci began an experimental study of the friction of solids using stone and wood, measuring the friction between objects on horizontal and inclined planes. Da Vinci also studied the effect of lubricating oil and other media on friction between the friction surfaces. Unfortunately, Leonardo's work on friction was not published at the time. His manuscripts were not discovered until 1967, and his friction-related writings and designs became known.
The first scientific study of friction was made by French physicist Amenton (G. Amontons). In 1699 he wrote: "If it is assumed that the friction of two parallel sliding surfaces increases with the increase of the contact area, it is incorrect. Experiments show that the friction increases with the increase of the load." When reported to the French Academy of Sciences in Amontons, this conclusion aroused surprise and questioning. Euler (L. Euler) in 1750 in the mathematical form of Amontons experimental results, that is, F = fN. This formula is often referred to as Coulomb's law of friction [1] because the phenomenon of friction was systematically studied by the 18th century French physicist Coulomb (C.A. de Coulomb). After a large number of experiments, he established the classical friction law based on the deformation of microprotrusions. British physicist De Sagulis (J.T. Desagaliers) put forward the "adhesion theory", that is, the real cause of friction is the existence of molecular (or atomic) force on the friction surface. In 1939, the Soviet scholar Kraagelsky unified the molecular theory and the concave-convex theory on the basis of the duality of friction, and established the theory of friction molecular mechanics. In the 1850 s, Bowden and Tabor[2] established a relatively perfect adhesive friction theory through systematic experimental research, which is of great significance to modern tribology theory. Since 1986, nano-friction research has emerged, and friction research has entered a new category.
Modern friction research is exploring non-wear friction (wearless friction), near-zero friction or ultra-slip (near-frictionless or superlubricity), atomic-scale friction (atomic-scale friction), and the origin of friction, hoping to fundamentally explore friction mechanisms and methods to control friction.
2. Lubrication theory research
As a core component of tribology, lubrication has also experienced thousands of years of development. As early as the Spring and Autumn period in the "Book of Songs", there is a description of suet lubricated axle. Since the development of lubrication theory, a lubrication theory system from full fluid film to dry contact has been gradually formed, that is, with the thinning of the lubrication film thickness, the lubrication state undergoes the following processes: fluid lubrication [3], elastohydrodynamic lubrication [4], thin film lubrication [5], boundary lubrication [6], and dry friction. In practice, several lubrication states often coexist, collectively referred to as mixed lubrication [7,8].
Fluid lubrication is a lubrication state based on fluid dynamics established in Reynolds 1886 [3]. It lays a theoretical foundation for the design of sliding bearings and promotes the rapid development of the bearing industry.
The concept of boundary lubrication was proposed by Hardy et al. [6] in 1922, that is, the lubrication of the interfacial monomolecular adsorption layer. Boundary lubrication illustrates the importance of the chemical structure of lubricant molecules in the lubrication process. In order to clarify the mechanism of lubrication molecules and the failure mechanism of boundary lubrication, Bowden model, Adamson model, Kingsbury model, Cammeron model and cobblestone (Cobblestone) model have appeared successively, which greatly enriched the theoretical research of boundary lubrication.
The traditional fluid lubrication theory is not suitable for high secondary contact states such as point and line. In 1949 by the Soviet scholars Grubin and Vinogradova[4] put forward the theory of elastohydrodynamic lubrication (referred to as elastohydrodynamic lubrication). However, the real rapid development of elastohydrodynamic lubrication is due to the numerical calculation method developed by Dowson et al. [9,10] using computer technology. In the 1970 s and 1980 s, many tribologists in the world invested in this field of research, and successively developed the complete numerical solution of the outgoing contact problem, the complete numerical solution of the point contact problem, the micro-elastohydrodynamic lubrication, the theory of interface slip and limit shear stress, etc. The theory of elastohydrodynamic lubrication takes into account the elastic deformation of point and line contact areas, the viscous pressure effect and thermal effect of lubricating fluid. Chinese scholars Wen Shizhu, Zhu Dong, Hu Yuanzhong, Yang Peiran, Huang Ping and so on contributed to perfecting the theory of elastohydrodynamic lubrication. Elastohydrodynamic lubrication theory further improves the lubrication theory based on fluid mechanics and elastic mechanics, and provides theoretical guidance for the design of point and line contact bearings such as ball bearings and roller bearings.
However, how the elastohydrodynamic lubrication is transformed into boundary lubrication and the physical nature of the transition state are major remaining problems in lubrication theory. The thin film lubrication proposed by Luo Jianbin, Wen Shizhu and others [5,11] from 1994 to 1996 filled this gap. It is mainly due to the rapid development of nano-measurement technology and nano-rheological technology. Jianbin and others put forward that the induced ordered layer is the main feature of film lubrication, established the transformation relationship between elastic flow lubrication and film lubrication and the failure criterion of film lubrication, and put forward the physical model of film lubrication and the new lubrication state division criterion. Guo Feng et al. [12] have obtained good experimental results on the interfacial slip problem of thin film lubrication. At the same time, in terms of simulation, Hu Yuanzhong [13] used molecular dynamics simulation to study the rheological properties of thin film lubrication, revealing the phenomenon of rapid increase in the density of liquid molecules near the wall, and the simulation results were more consistent with the experimental results.
In the actual operation process, it is often not a lubrication state that exists independently, and there may be several different lubrication states coexist, so there is a mixed lubrication state. It was proposed by German tribologist Stribeck et al. [7,8] based on the change of friction coefficient with speed, viscosity and pressure, and is to describe the state when different lubrication states coexist. Therefore, in different stages of mixed lubrication, the performance difference is very large. One of the key factors that determine the performance of mixed lubrication is the contact rate. Many scholars have contributed to the study of mixed lubrication, including Zheng Xuyun's partial elastohydrodynamic lubrication theory, J. Greenwood's contact model, K.L. Johnson's average film thickness model, Zhu Dong and Hu Yuan's complete numerical solution of mixed lubrication. In addition, the relationship between contact rate and pressure, speed, viscosity, friction elastic modulus and surface comprehensive roughness was established by experimental methods.
Although the theoretical system of lubrication has been completed, the research on lubrication has not been terminated. At present, it has entered the research on the relationship between lubrication performance and molecular structure, super-slip, green lubrication, micro-lubrication, ultra-low viscosity lubrication, and lubrication under harsh working conditions (high speed, high temperature, high pressure, corrosive environment, etc.).
3. Wear and Surface Engineering
Wear is an important branch of tribology, which refers to the process in which the surface material of objects in contact with each other is continuously damaged and the geometric size continues to become smaller in relative motion [14]. Wear accompanied by friction is one of the important contents of tribology research, and it is also a basic form of component failure. The failure caused by wear accounts for 60% ~ 80% of the failure of mechanical parts. Wear is generally caused by mechanical, physical and chemical interactions between objects [15].
Because wear occurs on the surface of the contact surface, the most effective way to reduce the wear of parts is to improve the lubrication effect between the surfaces, reduce the solid contact to reduce wear, on the other hand is to improve the quality of the solid surface layer, improve its wear resistance. This system engineering to obtain the required surface properties is called surface engineering [16], which was first proposed by T. Bell in 1983. Surface engineering technology can be divided into surface modification technology, thin film technology and coating technology three categories [16].
Main Research Directions of Modern Tribology
With the rapid development of high technology, the requirements for tribology are also rapidly increasing, such as near-zero friction, near-zero wear, high temperature lubrication, ultra-low temperature lubrication, and strong corrosive environment lubrication. To break through these new technical requirements, we first need to achieve a breakthrough in the friction mechanism and lubrication mechanism. In addition, the continuous breakthrough of measurement technology has made it possible to discuss friction, lubrication and wear in essence. Thus, tribology has entered the eve of a new explosive breakthrough. At present, the main research focuses on the following aspects.
1. The origin of friction and super-slip
The origin of friction and super-slip [17] is a hot topic of international tribology research. The friction phenomenon is determined by the cross-scale characteristics of the friction material and the interface, and the role of micro-molecules, atomic scale and the mechanical properties and operating conditions of macro-materials jointly determine the tribological behavior. Therefore, a comprehensive revelation of the friction mechanism requires the establishment of a cross-scale physical model. Super-slip is a special state in which the coefficient of friction approaches zero. At present, the lubrication state with friction coefficient less than 0.01 is called super slip [17]. Super-slip can be divided into solid super-slip (including structural super-slip)[18,19] and liquid super-slip [20-22], and its mechanism is closely related to the origin of friction. At present, many scholars in the world have studied the laws and phenomena of friction from the point of view of atoms and molecules, and have made important progress in the super-slip properties of single crystal two-dimensional materials, the ultra-low friction phenomenon of amorphous carbon films, new liquid super-slip systems (phosphoric acid system, biological liquid, acid and polyhydric alcohol mixed solution), friction emission, friction power generation, friction control and so on.
At present, the problems to be solved are as follows:
(1) the origin of friction;
(2) super-slippery mechanism;
(3) wear prediction;
(4) Accurate prediction of friction coefficient;
(5) friction quantum theory and friction spintronics;
(6) Trans-scale (micro/meso/macro) tribology theory;
(7) the evolution of friction-induced material structure;
(8) the mechanism of frictional emission;
(9) Interface slip in lubrication;
(10) Detection of the behavior of thin film lubrication molecules.
2. Bio-tribology and bionic tribology
Biological tribology (bio-tribology) was proposed by Dowson in the 1970 s [23] and focuses on friction and wear problems such as joint lubrication, heart valves, vascular flow, artificial retina, artificial heart, and teeth. Joint lubrication was first discussed in Rgynolds[3], but not studied. The first to study joint lubrication was MacConaill[24], who proposed a hydrodynamic lubrication model for joints based on joint structure. Jones[25] was an early scholar who carried out experimental research on joint lubrication and proved the phenomenon of viscous damping of joint lubrication. The hydrodynamic lubrication theory of joint lubrication has been maintained for decades, and Charnley[26] proved that its lubrication is more inclined to boundary lubrication with real-life joint swing experiments. Unsworth et al. [27] used experiments to prove that joint lubrication can be in three states: boundary lubrication, full-film lubrication or mixed lubrication, depending on the number of operations and the size of the load. In the 1980 s and 1990 s, artificial joints developed rapidly, especially the use of ultra-high-strength polyethylene (UHMWPE)[28] in artificial joints. Its non-toxic, low density, wear resistance, corrosion resistance, and resistance The characteristics of impact and self-lubrication make artificial joints possible. At the beginning of the 21st century, the concept of joint hydration lubrication was put forward, which played a good role in exploring the nature of joint lubrication [29]. In recent years, the development of oral tribology, skin tribology, eye tribology, vascular tribology, pleural tribology, foot tribology, etc., greatly expanded the scope of biological tribology.
Bionic tribology is a field closely related to biological tribology, which has developed rapidly in the past 20 years. It takes learning from nature as the core, carries out research and study on the friction reduction, anti-adhesion, friction enhancement, anti-wear and high-efficiency lubrication mechanism of biological systems, and produces bionic structures and devices, such as self-cleaning surface imitating lotus leaf, gecko adsorption and desorption surface, intestinal detection machine pill, shark drag reduction surface imitating, non-smooth anti-adhesion surface imitating dung beetle body surface, etc.
Current challenges to be addressed include:
(1) Design criteria for long-life hard artificial joint materials;
(2) the modification method of ultra-low friction and wear joint materials;
(3) the mechanism of drug inhibition of toxic side effects of wear particles;
(4) corrosion mechanism of metal nano-wear debris in artificial joints;
(5) the structural relationship between the structure and tribological properties of human natural teeth;
(6) the mechanism of tooth injury and self-repair;
(7) Anisotropic mechanical properties of different skin layers and their influence on friction behavior;
(8) the lubrication mechanism of human soft tissue;
(9) Design criteria for the new generation of artificial joints;
(10) Excavation of new bionic materials with increased friction, high adhesion and easy desorption characteristics.
3. Green Tribology
The concept of green tribology (green tribology) was proposed by Zhang Siwei [30] and Jost presented it at the 4th World Tribology Congress. Green lubrication, that is, environmentally friendly lubrication, is the core of lubricant research for more than 20 years, and it is also one of the mainstream development directions of contemporary tribology [31,32]. With the urgent needs of the development of manufacturing and ecological civilization, environmental pollution has become the focus of attention all over the world. The typical research of green tribology is the development of alternatives to mineral lubricants, such as anti-extreme pressure additives without sulfur, phosphorus, etc., nano-particle additives, water-based lubrication, micro-lubrication, oil and gas mixed lubrication.
Current challenges to be addressed include:
(1) the relationship between molecular structure and lubricating performance;
(2) solid-liquid compound lubrication law;
(3) Green extreme pressure additive;
(4) micro lubrication mechanism;
(5) oil and gas mixed micro-lubrication law and control technology;
(6) Design and preparation of all water-based lubricating molecules;
(7) Efficient lubrication methods for difficult-to-process materials;
(8) Green ionic liquid;
(9) Motion law and characterization of nanoparticles in liquid;
(10) Supramolecular gel lubricant.
4. Tribology under extreme working conditions
Extreme working conditions include high speed, vacuum, high temperature, high pressure, strong oxidation, strong corrosion, etc. Extreme working conditions are involved in high-speed aircraft, deep-sea exploration, high-speed rail, energy and other fields.
In the field of aerospace tribology, the harsh environmental conditions of high speed, vacuum, high temperature and strong oxidation are very prominent, but the performance, life and reliability requirements of the motion pair are very high. In the aerospace field, due to the requirements of the vacuum environment, MoS2, DLC and other solid lubricants are used more. At the same time, the volatile lubricating fluid is also the main body of research and development. For example, the introduction of fluorine, chlorine and other elements modified silicone oil space liquid lubricants [33], as well as liquids with extremely high thermal stability and evaporation loss properties (such as PFPE, x1-P and ionic liquids)[34-36]. PFPE and x1-P were early applied to the lubrication between the head of hard disk and the surface of the disk. From 1998 to 2001, Yang Mingchu and others [35] studied the tribological properties of the x1-P and its lubrication characteristics on the surface of the head of the hard disk. In 2001, Liu Weimin et al. [36] made important progress in the study of the tribological properties of room temperature ionic liquids. In addition, how to solve the problem of lubrication failure under high pressure and high temperature is also very important. The successful development of ZDDP has played a great role in solving the problem of engine lubrication. However, because ZDDP contains phosphorus, sulfur and other environmentally harmful elements, the development of green anti-extreme pressure additives has become a difficult problem.
Marine transport ships, submarines/submersibles, underwater robots, seabed mining equipment, seabed oil and gas development equipment, marine structures and other related facilities are in high salt, high pressure, low temperature, corrosion and biological fouling and other harsh marine environment, the service performance has received great attention, its tribological problems are becoming more and more prominent. It is also an important research task to simulate the harsh marine environmental conditions and study the friction and wear mechanism and characteristics of key components.
In the field of high-speed rail, the speed of operation has been continuously improved. The speed of wheel-rail high-speed rail commercial in China has reached 350 km/h, which is the highest speed in the world. High-speed trains above 500 km/h are currently being studied. Some scholars are also studying maglev trains with speeds above 600 km/h and vacuum pipeline trains with speeds above 1000 km/h. In wheel-rail high-speed rail, the wheel-rail relationship, especially the influence of surface contaminants, train speed, wheel-rail surface roughness, load, etc. on the wheel-rail adhesion characteristics is very important, and the noise outside the high-speed train mainly comes from the wheel-rail noise [37].
The high load, high temperature, particle wear and corrosion environment in the energy equipment make the wear of the workpiece more serious than the general working conditions, so the surface strengthening and anti-friction and wear technology is very important. The failure of friction pairs such as lubrication and sealing in nuclear power and other energy equipment under the action of nuclear radiation is very important. Coal, wind power and other large energy equipment in bearings, gears and other friction pairs of heavy load, temperature and other harsh conditions and high humidity, high salt, high particle pollution and other environmental impact is worth studying.
Current challenges to be addressed include:
(1) Damage behavior and mechanism of multi-field coupling under extreme conditions;
(2) state detection method of tribological system;
(3) Reconstruction of space environment simulation experiments such as space, ultra-high vacuum, high and low temperature, and radiation irradiation;
(4) the correlation between the foundation experiment and the actual lubrication effect and life of the space moving parts;
(5) the design method of wide temperature range lubricating material;
(6) Friction and wear problems in nuclear radiation environment;
(7) ionic liquid tribology;
(8) atomic scale removal mechanism of material surface;
(9) the structural characteristics and microscopic mechanism of surface nanocrystallization;
(10) Rail/wheel wavy wear mechanism and prevention.
5. Intelligent lubrication
The development of intelligent monitoring, micro-nano sensing and feedback control technology of friction pair lays the foundation for the realization of intelligent diagnosis, intelligent repair, adaptive adjustment and intelligent storage of lubrication system. Explore the design and preparation methods of intelligent lubricating materials with repair agent/lubricant storage-release function, and develop intelligent friction pairs with self-repair, self-storage, self-diagnosis and other functions for environmental adaptation (high and low temperature, ocean, desert, etc.).
Current challenges to be addressed include:
(1) Online identification of the state of the active machine tribological system;
(2) Design principle of self-repairing friction pair;
(3) self-storage, self-sensing lubrication capsule;
(4) The design and implementation of self-diagnosis and self-repair friction system.
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Author | Luo Jianbin Xie Guoxin
Tsinghua University
SOURCE | Tribology Frontiers
Reprinted... Journal of Mechanical Engineering.
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