Vacuum Tribology

OutlineStudy of tribology in vacuum environments emphasizes that it is not only limited to space applications but also there exist numerous applications of vacuum tribology in research as well industrial processes. In this article, central idea is to draw attention of reader towards these applications. In the beginning adhesive nature of contacting surfaces in vacuum environment is described. The dependency of friction coefficient on surface energy and real area on contact is explained. Different fields of applications are summarized in two tables. One of the most important lubricant in vacuum related applications is MoS2. A brief discussion on comparison of graphite and MoS2 lubrication is discussed to get insight of mechanism. High friction coefficient in Vacuum tribologyVacuum tribology is mainly study of wear and friction in vacuum environment. In this environment solid to solid contact is most prominent. It is due to this fact, surface contacts in vacuum environment exhibit strong adhesive bonds and high coefficient of friction. This further leads to high surface damage compare to normal environment where chances of oxide layer protection are more. Adhesion depends on the environment, the surface cleanliness, the real area of contact, properties of solids in contact and that of interface and modes of junction rupture. In ambient conditions, most metals oxidize and form oxide films, typically between 1 and 10 nm thick within a few minutes of exposure of clean surfaces. This oxide films act as low shear strength film and minimize the metalmetal contact at asperity level and thus lead to low friction. On the other hand, in vacuum condition, there is less chance of oxidation and direct contacts exist between metalmetal surfaces. In this case, depending on metallurgical compatibility, the level of adhesion governs the frictional behavior of the metals.The values of the friction coefficient in vacuum were about 10 times higher than values measured in air. This graph show the variation in coefficient of friction with relative vapor pressure in different environments. Note that the friction coefficient is highest and constant for dry nitrogen. Dry nitrogen behaves similar to vacuum environment. The coefficient of friction is an empirical parameter i.e. it has to be measured experimentally and cannot be found through calculations. The coefficient of friction can vary over a wide range: from 0.001 in a lightly loaded rolling bearing to greater than 10 for clean metals sliding against themselves in vacuum. The two materials actually come in contact in vacuum environment. It has been found that coefficient of friction between two surfaces depends on surface and bulk properties of materials. Effect of hardness (H) (one of bulk property) is inversely related to real area of contact (A) i.e. less hard material will offer more real contact area in the zone of contact. This real contact area directly influence coefficient of friction i.e. larger real contact area leads to higher coefficient of friction.Surface energy per unit area (S) (one of surface property) affect coefficient of friction. If S is less, it can be concluded that interfacial bond strength is less thus directly influencing coefficient of friction.Conclusively, coefficient of friction can be treated as function of product of S and A. To reduce friction and to provide lubrication, therefore, the product of the real area of contact A and the surface energy per unit area S must be minimized in vacuum. The effect of this product on coefficient of friction is depicted in this figure. It can be observed from the graph that Aluminium-Aluminium surfaces has higher product of S and A compare to Iron-Iron surfaces thus coefficient of friction of Aluminium-Aluminium surfaces expected to be higher compare to that of Iron-Iron surfaces in vacuum environments.

S*A (nJ)Under ultra-high vacuum environment, coefficient of friction and wear rates of ceramic pair do not reach the very high values as compared to that of clean metals. So it is preferable to use ceramics in vacuum environmentsApplications of vacuum tribology This figure gives an idea about type of lubricant used in different pressure application

In ultrahigh vacuum condition solid lubrication techniques are recommended. Some particular applications of vacuum tribology applications are vacuum products, space mechanisms, satellites, space telescope, space antennae etc.But vacuum tribology is not limited to only space applications. Depending upon the radiations and intensity of vacuum present in surrounding we can classify as follows

Fields of application of vacuum tribology in industrial process

Fields of application of vacuum tribology in research and developmentMethod of lubrication in vacuum environment (MoS2 as a solid lubricant)Temperatures encountered in supersonic aircraft, spacecraft, and certain industrial applications are beyond the useful range of even the synthetic lubricants. This trend to the operation of bearing surfaces at higher temperatures and low pressures (vacuum environment) has led to the development and use of solid lubricants to attain the necessary lubrication. Solid lubricants have at least one very desirable feature-they do not evaporate under the above mentioned conditions.

Graphite and molybdenum disulfide (MoS2) are the predominant materials used as solid lubricant. In the form of dry powder these materials are effective lubricant additives due to their lamellar structure. The lamellas orient parallel to the surface in the direction of motion.

Even between highly loaded stationary surfaces the lamellar structure is able to prevent contact. In the direction of motion the lamellas easily shear over each other resulting in a low friction. Large particles best perform on relative rough surfaces at low speed, finer particle on relative smooth surface and higher speeds.

Graphite is structurally composed of planes of polycyclic carbon atoms that are hexagonal in orientation. The distance of carbon atoms between planes is longer and therefore the bonding is weaker. Graphite is best suited for lubrication in a regular atmosphere. Water vapor is a necessary component for graphite lubrication. The adsorption of water reduces the bonding energy between the hexagonal planes of the graphite to a lower level than the adhesion energy between a substrate and the graphite. Because water vapor is a requirement for lubrication, graphite is not effective in vacuum.

Molybdenum disulfide (MoS2) like graphite, MoS2 has a low friction coefficient, but, unlike graphite, it does not rely on adsorbed vapors or moisture. In fact, adsorbed vapors may actually result in a slight, but insignificant, increase in friction. MoS2 also has greater load-carrying capacity and its manufacturing quality is better controlled. Sliding friction tests of MoS2 using a pin on disc tester at low loads (0.12 N) give friction coefficient values of