PS300 is a self-lubricating solid coating material for use in sliding contacts at temperatures up to 800°C. PS300 is a composite of metal-bonded chromium oxide with barium fluoride/calcium fluoride eutectic and silver as solid lubricant additives. The "PS" in the name of this and other self-lubricating, high-temperature composite materials that have been reported in NASA Tech Briefs signifies that the material is applied to a substrate by plasma spraying of a powder blend of its constituents.
PS300 is similar to a previously developed material of this type, called "PS200." The main difference between the two materials is that instead of metal-bonded chromium oxide, the major constituent of PS200 is metal-bonded chromium carbide (see table). In either material, the major constituent serves as a tough, wear-resistant matrix. The additives provide lubrication by virtue of their low shear strengths: In particular, silver lubricates at low temperatures; at higher temperatures, where silver is too soft to support an appreciable load, the fluoride eutectic phase softens and behaves plastically.
The big disadvantage of PS200 is high cost. The metal-bonded chromium carbide is a highly processed, expensive constituent, and the presence of chromium carbide makes it necessary to resort to costly diamond grinding to polish the coating to the requisite finish prior to use. Moreover, at temperatures above 800 °C in air, the chromium carbide oxidizes, with a resultant increase in friction and loss of resistance to wear.
The metal-bonded chromium oxide in PS300 costs less than does the metal-bonded chromium carbide in PS200. Metal-bonded chromium oxide can be ground and polished by silicon carbide tools, eliminating the need for diamond finishing. Inasmuch as the chromium in PS300 is already in an oxidized state, the coating has potential for better tribological performance in air at high temperatures. Yet another advantage of PS300 over PS200 is that at high temperatures, chromium oxide is a good solid lubricant. Finally, in comparison with PS200, PS300 exhibits less of a tendency to clog spray-gun ports and can thus be plasma-sprayed more easily.
PS300 and PS200 have been tested under identical conditions on a pin-on-disk tribometer. In tests at temperatures up to 650°C in air, the two materials exhibited comparable friction and wear properties.
This work was done by C. DellaCorte and B. J. Edmonds of Lewis Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Materials category,or circle no. 152 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
Mail Stop 7 -3
21000 Brookpark Road
Refer to LEW-16413.