Abstract
The problems of lubrication and wear in vacuum are mostly caused by the low pressure and the lack of oxygen. Important considerations are: (1) inability to maintain oxide films; (2) evaporation of lubricants and (3) poor heat transfer. The usefulness of the adhesion concept of friction is demonstrated by correlation of cleavage and adhesion studies with sliding friction. Behaviour of solids in these areas of study is significantly influenced by crystal structure and atomic bonding forces. Better definition of surface, lubricating films and slider material on the basis of atomic arrangement in crystals and molecules is required for continued progress.
Metals and alloys with hexagonal crystal structure and a c/a stacking ratio approaching ideal (1·633) have useful friction and wear properties. Crystal structures of inorganic lubricants are also important. Adhesion between surfaces in sliding contact may result in junctions formed from (1) diffusion bonding (i.e. solid solubility), (2) chemical bonding and (3) mechanical bonding. Material transfer occurs during sliding with most material combinations by one or more of these adhesion processes. Properties such as the proper hexagonal structure or contamination films that cause subsequent shearing to occur in the plane and region of the original interface will minimize adhesive wear. Friction responds to factors such as crystal structure that influence the shear strength of metal junctions.
Methods for maintaining useful surface contamination and obtaining proper crystalline structure are the most promising areas for study in achieving efficient operation of sliding surfaces in vacuum. The problems as well as materials for their solutions need to be defined on the basis of atomic structure.
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