Lubricious-surface-silicon-nitride rings for high-temperature tribological applications

Use of lubricious, high-temperature-capable ceramic-piston-rings may reduce frictional losses in engines. Silicon nitride (Si3N4) rings with a functional gradient of titanium nitride (TiN) towards the ring outside diameter (OD) were fabricated by centrifugal spinning of a ceramic slurry followed by nitriding. A 10 volume percent (v/o) TiN-silicon slurry resulted in a greater concentration of TiN (and higher microhardness) near the ring OD (compared to 20 v/o TiN). Unlubricated pin-on-disk tests of a 10 v/o TiN/Si3N4 pin vs. a 30 v/o TiC/Si3N4 disk in air exhibited coefficients of friction (COF) of about 0.65 that were independent of temperature (20-800 degrees C). Wear tests were also performed in which boron nitride (BN) powder was added between the ceramic pin and ceramic disk materials, which subsequently reduced COF to <0.2 for temperatures less than or equal to 300 degrees C.