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Boundary Lubrication and Lubricants Seiichro Hironaka School of Engineering, Tokyo Institute of Technology

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Boundary Lubrication and Lubricants Seiichro Hironaka School of Engineering, Tokyo Institute of Technology Three Bond Technical News Issued July 1, 1984 9 Boundary Lubrication and Lubricants Seiichro Hironaka School of Engineering, Tokyo Institute of Technology 1. Introduction coefficient [f], viscosity of the lubricating oil [η], load The term tribology has been used for 18 years to [FN], and velocity [V] in the Stribeck curve (Figure 1). refer to the branch of engineering that deals with This curve brilliantly captures the characteristics of friction, wear, and lubrication. Tribology is defined as various lubrication regions, including [I] boundary "the science and technology of interacting surfaces lubrication, [II] elastohydrodynamic lubrication moving relative to each other and associated practical (EHL), and mixed lubrication. problems." Tribology is a major research field in the In hydrodynamic lubrication, the fluid completely mechanical industry-indeed, in any field with even the isolates the friction surfaces [h >> R], and internal slightest connection to machinery. Especially in the fluid friction alone determines tribological industrial community, the recent rapid progress in characteristics. In elastohydrodynamic lubrication [h machinery and acceleration of machine speeds has ≈ R], fluid viscosity, the viscosity-pressure coefficient increased production rates, but at the significant and the elastic coefficient of the solid surface are the added cost of machine damage and energy most dominant factors. In contrast, the boundary consumption. With the solution of tribological lubrication mode is mainly characterized by the problems anticipated to save some 800 billion to 1 following three points: trillion yen per year, tribology is attracting (i) Friction surfaces are in contact at microasperities ever-growing attention. (ii) Hydrodynamic effects of lubricating oil or This issue deals specifically with boundary rheological characteristics of bulk do not lubrication, which addresses the complex problems significantly influence tribological arising from contact between friction surfaces. It also characteristics discusses fundamental topics related to this issue and (iii) Interactions in the contact between friction reviews the role of lubricants in boundary lubrication. surfaces and between friction surfaces and the lubricant (including additives) dominate 2. Boundary lubrication mode tribological characteristics. Based on friction experiments on bearings, Stribeck expressed the relationship between the friction Contents 1. Introduction............................................................ 1 6. Friction modification...............................................6 2. Boundary lubrication mode.................................... 1 3. Wear in boundary lubrication................................. 3 Introducing Pando series: a crowning 4. Formation of lubricating films ................................ 4 achievement in chemical technology ..........................7 5. Solid lubricants ...................................................... 5 Information on the activities of the 3B Group ............8 1 Boundary Elastohydrodynamic Hydrodynamic lubrication lubrication and mixed lubrication lubrication Surface roughness Oil film thickness Figure 1: Stribeck curve and lubrication regions Area of contact between microasperities Lubricating oil Figure 2: Friction surface contact model (A) In other words, since the thickness [h] of the oil microasperities, and the load is supported by a film between the friction surfaces is smaller than much smaller area than the apparent contact area (a their surface roughness [R] in boundary lubrication, × b). Thus, in boundary lubrication where large metallic contact occurs across every portion of the amounts of friction increase the temperatures of surface, as shown in Figure 2. Friction surfaces friction surfaces and cause wear and seizure and contact each other over a true contact area ( Ai, various resulting problems, it is critical to reduce n: number of contact points), which is the sum of friction and wear by preventing metallic contact; the contact areas [Ai] between the discontinuous that is, it is critical to form lubricating films on 2 friction surfaces by applying lubricants (lubricating F = A {αSm + (1 - α) Sι} oils, additives, and solid lubricants). Where A is load bearing area, α is the ratio of As shown in Figure 3 (the cross section of Figure metallic contact, Sm is the shear strength of metal 2), when oil film rupture causes metallic contact, bonding, and Sι is the shear strength of the oil film. friction force [F] is spent on the shearing not only Since Sι is significantly smaller than Sm, of the oil film, but of metal bonding. Thus, F is minimizing α reduces friction. expressed by the following equation: Metal 1 Lubricant film Metal 2 Contact between metals Figure 3: Friction surface contact model (B) 3. Wear in boundary lubrication mutual transfer cause adhesive wear, while The main types of wear in boundary lubrication generated abrasive particles or hard foreign are as follows. particles cause abrasive wear. In contrast, (i) Adhesive wear: Caused by adhesion of interactions between solid [1] or solid [2] and microasperities to friction surfaces. lubricant [3] or atmosphere [4] (for example, (ii) Abrasive wear: Caused by cutting by hard, adsorption and chemical reaction) form a foreign particles or abrasive particles in oil. lubricating film that reduces friction and wear and (iii) Surface fatigue wear: Caused by surface prevents seizure. However, chemical reaction fatigue resulting from repeated stress and products are sometimes removed into the oil by tribological chemical interactions. shearing and dissolution, causing chemical wear. (iv) Chemical wear: Dominated by chemical reactions between friction surfaces and lubricant molecules. In most cases, these types of wear more or less occur side by side. Mechanical conditions, types of lubricants and metal materials, oil temperatures, atmosphere, generation of friction heat, exo-electron emission, and other factors result in a wide range of complex phenomena on friction surfaces, making boundary lubrication problematic. As shown in Figure 4, Solid [1] and solid [2] are in relative motion under given mechanical conditions (load and sliding speed). In the area of contact between the two solids, adhesion and 3 [4] Atmosphere: Gas (oxygen, etc.) Load Water, contaminants, etc. Solid [1] Motion Solid [2] [3] Lubricants: Lubricating oils Additives Wear mechanism [3] and [4] Viscous oil film Adsorption film Formation of lubricating film Tribological chemical reaction product film Chemical reactions Solid lubricant coating, etc. [1] [2] Adhesion, transfer, surface fatigue Abrasion, friction heat Figure 4: Wear under boundary lubrication 4. Formation of lubricating films improvers are suitable for low to medium load Listed below are factors that dominate film conditions but cannot be expected to provide formation for so-called boundary lubricity, in which lubrication under severe, high-load, lubricant forms lubricating films on friction high-temperature conditions. On the other hand, surfaces, reducing friction and wear and preventing antiwear agents (for example, metal seizures. dithiophosphates and phosphoric acid esters) form (i) Formation of viscous oil films reaction films that react with friction surfaces to (ii) Formation of physical or chemical adsorption reduce wear under low to medium loads and films high-temperature conditions. In high-load (iii) Formation of in situ lubricating films conditions, surface asperities penetrate the oil film, (a) Formation of polymer films (for example, causing metallic contact. The subsequent friction polymers) on friction surface spots temperature increase at the friction surfaces and (b) Formation of solid lubricant films strong shear desorb or remove the oil film, (iv) Formation of inorganic reaction product films increasing friction and wear and eventually (v) Formation of solid lubricant coatings resulting in seizures with total loss of lubrication. These lubricating films vary in range of Extreme pressure agents (EP agents) react with the thicknesses from 20 to 30 Å of adsorbed molecules friction (metal) surfaces under this extreme-pressure to micrometer scale reaction films. Oiliness lubrication condition to form a reaction coating with improvers are chemical compounds that have long less shear strength than the metal to prevent seizure. hydrocarbon chains in molecules terminating in The films formed under extreme-pressure strong acidic groups, which physically or lubrication conditions are generally inorganic chemically adsorb to friction surfaces to form reaction films. Iron sulfide and iron oxide coatings lubricating films that prevent metallic contact and created by organic sulfur compounds are typical reduce friction and wear. Long chain fatty acids examples. Coatings by solid lubricants such as such as stearic acids, long chain fatty alcohols, and molybdenum disulfide and graphite are also amine are typical oiliness improvers. Oiliness effective. 4 5. Solid lubricants coating on friction surfaces for solid lubrication Solid lubricants are solids applied to friction even when oils are not desired, or when oils with surfaces to reduce friction and wear and prevent which they are mixed are drained out. surface damage. They may be powders, films, or Molybdenum disulfide and graphite are composite materials. Solid lubricants are roughly frequently used as thermally stable classified as shown in Table 1. They include extreme-pressure agents. (Molybdenum disulfide is substances
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