The Use of Silver in Self-Lubricating Coatings for Extreme Temperatures

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Abstract
The advantages and disadvantages of elemental silver as a tribological material are discussed. The relatively high melting point of 961°C, softness, marked plasticity, and thermochemical stability of silver combine to make this metal useful in thin film solid-lubricant coatings over a wide temperature range. Disadvantages of silver during sliding, except when used as a thin film, are shown to be gross plowing due to plastic deformation under load with associated high friction and excessive transfer to counterface surfaces. This transfer generates an irregular surface topography with consequent undesirable changes in bearing clearance distribution. This paper describes research to overcome these disadvantages of elemental silver. A comparison is made of the tribological behavior of pure silver with that of silver formulated with other metals and high-temperature solid lubricants. The composite materials are prepared by codepositing the powdered components with an airbrush followed by furnace heat treatment or by plasma-spraying. Composite coatings were formulated which are shown to the self-lubricating over repeated temperature cycles from low temperature to about 900°C.