Stability of friction fade-out at polymer-like carbon films slid by ZrO 2 pins under alcohol-vapored hydrogen gas environment

Friction tests using ZrO₂ (Y-PSZ: yttria partially stabilized zirconia) pin slid against polymer-like carbon film of bilayer and multilayer structures under H₂ gas environment are conducted. It will be shown that friction coefficients of the level of 0.0001 (friction fade-out) is stably realized by adding alcohol vapors to H₂ gas during run-in stage, then by stepping up the load from 19.8 N to 63.7 N after run-in stage. Four kinds of vapors of alcohol aqueous solutions are tested using bilayer samples, and ethanol-vapored H₂ gas shows the longest friction fade-out duration. Polymer-like carbon/diamond-like carbon multilayer sample shows long-life friction fade-out of 4 h, and it will be shown that the friction trace of 4 h reflects wear process of the first layer of polymer-like carbon and the second layer of diamond-like carbon. ZrO₂ surface is observed by an optical microscope and scanning electron microscopy and measured by surface profiler after friction fade-out test, and it is shown that flat contact area at the central region has many blisters and crimps, and is surrounded by peripheral bumps. It is also shown that the sliding marks are seen only at the top of crimps at the central region. Raman measurements indicate that short-chain carbons are predominant at blisters and ring carbons of small clusters are predominant at bumps. With these observations friction fade-out mechanism is discussed.