The synergistic effects of solid and liquid lubrication on the tribological behavior of transformation-toughened ZrO₂ ceramics

In this study, the authors investigated the synergistic effects of liquid and solid lubrication on the tribological behavior of yttria-stabilized zirconia (ZrO₂) ceramics. Thin silver films 1 to 2 µm thick were produced on ZrO₂ flats by ion-beam-assisted deposition (IBAD) and used as the solid lubricant. A fully formulated synthetic oil (polyolester-based) served as the liquid lubricant. Wear tests were performed on an oscillating-slider wear test machine at temperatures up to 250°C. For the specific test conditions explored, it was found that: (a) without any type of lubrication, the friction coefficients of ZrO₂/ZrO₂ test pairs were on the order of 0.6, and the average wear rates of pins were in the range of 5 × 10^–5 to 10^–4 mm3·N^–1 m^–1, depending on the test temperature; (b) the use of solid-lubricant silver films alone reduced the friction coefficients of sliding pairs by factors of two to four, and the wear rates of pins by three orders of magnitude; (c) the use of liquid lubricants reduced the friction coefficients of ZrO₂/ZrO₂ test pairs by a factor of about five and the wear rates of pins by one to two orders of magnitude; and (d) the concurrent use of silver films and lubricant oil resulted in the virtual elimination of the wear of both the pins and the flats, and in a significant reduction of the friction coefficients at room temperature and at 250°C.