THE EFFECT OF PROLONGED EXPOSURE TO 750-DEGREES-C AIR ON THE TRIBOLOGICAL PERFORMANCE OF PM212 - SELF-LUBRICATING COMPOSITE-MATERIAL

The effect of prolonged exposure to 750 degrees C air on the tribological performance and dimensional stability of PM212, a high temperature, self-lubricating composite, is studied. PM212, by weight, contains 70% metal-bonded Cr3C2, 15% BaF2/CaF2 eutectic and 15% silver. Rub blocks were fabricated from PM212 by cold isostatic pressure followed by sintering. Prior to tribo-testing, the rub blocks were exposed to 750 degrees C air for periods ranging from 100 to 1000 hours. Then, the rub blocks were slid against nickel-based superalloy disks in a double-rub-block tribometer in air under a 66N load at temperatures from 25 degrees to 750 degrees C with a sliding velocity of 0.36 m/s. Unexposed rub blocks were tested for baseline comparison. Friction coefficients ranged from 0.24 to 0.37 for the unexposed rub blocks and from 0.32 to 0.56 for the exposed ones. Wear for both the composite blocks and superalloy disks was typically in the moderate to low range of 10(-5) to 10(-6) mm(3)/N-m. Friction and wear data were similar for the rub blocks exposed for 100, 500. and 1000 hours. Prolonged exposure to 750 degrees C air increased friction and war of the PM212 rub blocks at room temperature, but their triboperformance remained unaffected at higher temperatures, probably due to the formation of lubricious metal oxides. Dimensional stability of the composite was studied by exposing specimens of varying thicknesses for 500 hours in air at 750 degrees C. Block thicknesses were found to increase with increased exposure time until steady state was reached after 100 hours of exposure, probably due to oxidation.