Self Lubricating Metal Matrix Composite
An Austrian SME developed a Self-lubricating Metal Matrix composite. It can be used up to 300 degrees Celsius minimum, in vacuum and with suitable fillers also in humid air. The technology provider was leading in the development of a very specialized process and application in ball-bearings or electrical slip rings was demonstrated. Partners with challenging applications are sought.
Today, mainly polymer-based composites are available for space applications; they can be used up to 250 degrees Celsius . Metal matrix composites with embedded solid lubricating particles (graphite, WS2) have been available for ground applications in wet environments for some time. The SL-MMC-process described here is based on powder metallurgy, and allows embedding MoS2 inside a copper matrix. Filler contents may be as high as 60 volume%. The selection of fillers can be done based on the need of application under vacuum (MoS2), but can also be combined with other fillers for functionality in humid air.
Material data derived in an ESA GSTP project shows that mechanical properties are better than those of conventional powder metallurgic Cu-MMCs. Thermal expansion is close to that of steel, which makes design of bearings for use in a wide temperature range easier (no thermal mismatch). The friction coefficient on material level for a proper combination of fillers was found to be ~0,2 in humid air, under vacuum friction rises from ~0,05 (RT) to ~0,1 (at 300°C). Ball bearing tests up to 250.000 revolutions done by an independent test house showed a proper solid lubrication and low torque (0,002Nm) at 300°C under vacuum. Also in bushes tested for more than 200.000 revolutions, torque was in the range of 0,12Nm. Application of the material in electrical slip-ring was evaluated, too. Hence, the functionality of this material up to 300°c under vacuum was proven.
Innovations & Advantages
The main advantage of this SL-MMC is its lubrication efficiency under high temperature in vacuum or dry environment like inert gases. It combines the advantages of a metal (high thermal conductivity, thermal expansion in the range of steel, radiation insensitive) with the solid lubrication of MoS2 being able to lubricate in dry environment/vacuum and temperature up to more than 300°C.
Current and Potential Domains of Application
For any kind of roller bearings, plain bearings or electrical slip-rings. However, it is a high class material, i.e. the best cost benefit is achieved in high temperature applications up to 300°C (evaluated), up to 400°C envisaged, in dry environment. There are no alternatives commercially available up to now, since temperature is too high for polymers and fluid lubricants, and too low for real high temperature friction materials like ceramics. Thus it might be unique to be used in dry environment in cases where grease replacement of lubrication is targeted, e.g. in aeronautic applications, like turbines and air-bleed systems and in vacuum devices.