Journal of the European Ceramic Society 26 (2006) 223–232 Wear mechanisms associated with the lubrication of zirconia ceramics in various aqueous solutions Mitjan Kalin a,∗ , Goran Draˇ ziˇ c b , Saˇ sa Novak b , Joˇ ze Viˇ zintin a a Center for Tribology and Technical Diagnostics, University of Ljubljana, Bogiˇ siˇ ceva 8, 1000 Ljubljana, Slovenia b Department for Nanostructured Materials, Joˇ zef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia Received 7 July 2004; received in revised form 22 September 2004; accepted 24 October 2004 Available online 7 January 2005 Abstract The wear and friction behaviours of self-mated zirconia ceramics were investigated with sliding tests in various aqueous solutions using a reciprocating ball-on-flat testing geometry. The surface charge on the native surfaces and the wear debris were “controlled” by varying the pH of the aqueous solution over a wide range, i.e., between 1 and 13. The aim of this investigation was to determine the wear mechanisms via analyses of the wear debris using SEM, XRD and TEM. Significantly different properties were observed, depending on the testing conditions in the different aqueous solutions; and the nature of the wear debris and the native surfaces was also found to depend on the testing conditions. Regions with high and low levels of wear and friction were detected. The wear was found to vary by as much as an order of magnitude with the change in pH, and under the same conditions the friction was observed to change by a factor of two. In the first region, i.e., under very acidic conditions, chemical dissolution was the predominant effect, resulting in low wear and low friction. At higher pH values, more complex processes consisting of a hydrothermal transformation causing bulk zirconia fracture and the formation of tribochemically assisted wear-debris layers, which subsequently spalled, resulted in rough surfaces with very high wear and high friction. Furthermore, the electrochemical effects in this region caused a wear peak to appear at the isoelectric point. © 2004 Elsevier Ltd. All rights reserved. Keywords: ZrO 2 ; Wear resistance; Isoelectric point; Lubrication 1. Introduction Different types of engineering ceramics, e.g., Al 2 O 3 , ZrO 2 , Si 3 N 4 and SiC, exhibit strengths and weaknesses un- der various loading and lubrication conditions. With the in- crease in ecological awareness, together with the improve- ments in the performance of ceramics, the interest in using water to lubricate mechanical systems involving ceramics is growing. However, not all ceramics have the same or even suitable properties under water-lubrication conditions. For example, it is well known that silicon nitride and silicon car- bide can provide very low levels of wear and a low coefficient of friction; under certain conditions, even super-low friction ∗ Corresponding author. Tel.: +386 1 4771 462; fax: +386 1 4771 469. E-mail address: mitjan.kalin@ctd.uni-lj.si (M. Kalin). (≈0.002) can be obtained, 1 which has encouraged several re- searchers to investigate these effects in detail. Tribochemical reactions with the formation of SiO 2 layers and extremely smooth surfaces were often observed, depending on the con- ditions. These layers can lead to hydrodynamic effects, stress reduction and improved boundary-lubrication properties. 1–3 However, silicon nitride is currently used mainly in appli- cations that are oil-lubricated, i.e., mainly rolling bearings, because of its non-catastrophic failure mode, but also because of its acceptable oil-lubrication performance under specific conditions and/or carefully selected pre-treatments. 4–6 On the other hand, silicon carbide has been found to have excellent tribological properties when lubricated with water, 2,7,8 and in fact it is frequently used in mechanical seals for pumps. Alumina ceramics also exhibit low wear and moderate friction (≈0.4) when used with water 9–12 or oil 5 lubricants. 0955-2219/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2004.10.026