A Review of Lubrication of Sliding- and Rolling-Element Electrical Contacts in Vacuum.

The operation of sliding- or rolling-element electrical contacts in vacuum presents a problem because of the occurrence of high noise levels, high coefficients of friction and excessive wear rates due to the absence of beneficial surface films. Graphite cannot be successfully used in vacuum because its lubricating ability is dependent upon surface contamination. The use of molybdenum disulfide MoS2, although a good vacuum lubricant, is questionable because of its high bulk resistivity and semiconducting characteristics. Niobium diselenide, which has a lower resistivity than MoS2, is also a good vacuum lubricant although its wear rate is somewhat higher than that of MoS2 when run against the same base material. Rolling-element slip rings have also been used successfully in high vacuum. Under the severe conditions encountered in rotating anode X-ray tubes, the use of thin silver films as lubricants have resulted in long useful lifetimes. MoS2, utilized as a component of the bearing cage material, has also achieved some success under more moderate conditions. Dielectric cage materials, however, are not useful because of the buildup of an insulating film on the surfaces of the bearing components. The use of high vapor pressure organic materials, as electric contact lubricants in a space environment, is questionable because of the possibility of undesirable polymer formation and the low radiation tolerance of such materials. Thin metallic films are seen as a promising method of lubrication. Three methods of thin film deposition and the endurance lives of these films obtained from friction experiments are discussed. On the basis of the reviewed experimental data, some conclusions are made regarding the performance of the various electric contact materials and lubricants in vacuum.