Effect of Microstructure on the Tribological and Mechanical Properties of CuO-Doped 3Y-TZP Ceramics Shen Ran, Louis Winnubst, w and Dave H. A. Blank Inorganic Materials Science, Faculty of Science and Technology and MESA 1 Institute for Nanotechnology, University of Twente, 7500 AE Enschede, the Netherlands Henry R. Pasaribu, Jan-Willem Sloetjes, and Dik J. Schipper Surface Technology and Tribology Section, Faculty of Engineering Technology, University of Twente, 7500 AE Enschede, the Netherlands Dense 8 mol% CuO-doped 3Y-TZP ceramics were prepared by pressureless sintering for 8 h at 15001 and 15501C, respectively. Transmission electron spectroscopy revealed that the ceramic sintered at 15001C exhibits grain boundaries free of any amorphous phase, while crystalline copper-oxide grains were found in the zirconia matrix, whereas the sample sintered at 15501C contains a Cu-rich amorphous grain boundary layer. The tribological behavior of these materials was tested under dry-sliding conditions using a pin-on-disk tribometer. The material sintered at 15001C showed self-lubrication resulting in a low co- efficient of friction (f) of 0.2–0.3 and a low specific wear rate (k)  10 6 mm 3 . (N . m) 1 . In contrast, the material sintered at 15501C showed poor tribological behavior (f 5 0.8–0.9; k  10 6 mm 3 . (N . m) 1 under the same conditions. The difference in the tribological behavior of these two materials was inter- preted on the basis of mechanical properties and microstructural characteristics. I. Introduction A DVANCED materials suitable for unlubricated tribo-engineering applications are increasingly becoming attractive as the usage of liquid lubricants is becoming more and more undesirable for economic and environmental reasons. These tribological material systems should have both a sustainably low coefficient of friction, fr0.2, and a low specific wear rate, kr10 6 mm 3  (N  m) 1 . 1 The specific properties of ceramics, like high hardness, chem- ical inertness, and high thermal stability, make these materials increasingly attractive for engineering applications, especially as materials for tribological systems. Extensive researches on the tribological properties of various ceramics systems have been car- ried out in the past decades. 2–7 As indicated by these researches, high wear resistance can be obtained with ceramic tribosystems under dry contact situations. However, the coefficient of friction of these dry-sliding ceramic tribosystems generally varies in the range of 0.5–1.0, which is unacceptable for real applications. Ideas of self-lubricating composites were introduced by Alex- eyeve and Jahanmir 8 in order to reduce friction and wear in dry- sliding situations. A self-lubricating composite is essentially a material containing relatively small amounts of a soft solid phase uniformly dispersed in a matrix of a hard phase. During sliding of this composite against a hard counter body, it is ex- pected that the soft phase in the surface zone will be deformed and squeezed out over the surface by the counter body to form a soft and relatively smooth film between the contacting surfaces. This interfacial soft film can reduce wear and friction of the sliding pair as a (solid) lubricant. An advantage provided by self-lubricating composites is the fact that the friction-reducing film can be created continuously during sliding so that a low coefficient of friction can be sustainably achieved. Baumann and Zum Gahr 9 used solid lubricants like graphite or MoS 2 to protect materials from corrosion and wear at high temperatures. Although here the addition of up to 30 vol% of graphite lowered the friction of Yttria-doped tetragonal zirconia polycrystalline (Y-TZP) ce- ramics, it increased wear significantly. Liu 10 introduced soft met- als, such as Pb or Ag, in orderly mico-pores of several cermets, which led to a decrease in coefficient of friction of 60%–65%. Most references used metals or non-oxide systems as solid lubri- cants for self-lubrication. For zirconia–yttria systems, a self-lubri- cating mechanism was observed when relatively large amounts (30 wt%) of Ag or CaF 2 were added to the oxide matrix. 11 Wang et al. 12 used copper powder, a copper film, or a CuO film on par- tially stabilized zirconia (PSZ) to reduce the wear and friction of PSZ. The self-lubricating mechanism was also implemented in our group by Kerkwijk et al. 13 by adding various soft oxides like CuO, MgO, MnO 2 , ZnO, or B 2 O 3 as a second phase in alumina or zir- conia ceramics. Tribology tests under dry-sliding conditions re- vealed that among these soft oxides, only the addition of CuO could significantly reduce the coefficient of friction. Pasaribu et al. 14 investigated the friction behavior of various amounts of CuO-doped alumina and zirconia ceramics, showing again that addition of CuO results in a remarkable reduction in the friction of alumina and zirconia under certain dry-sliding conditions. In the present work, two self-lubricating ceramic composites of 8 mol% (5 wt%) CuO-doped 3Y-TZP (3 mol% Yttria-doped tetragonal polycrystals) were prepared from commercial start- ing powders by conventional mixing and isostatic pressing, fol- lowed by pressureless sintering at 15001 and 15501C, respectively. Tribology tests were carried out on a pin-on-disk tribometer under dry-sliding conditions, using an alumina ball as the counter material. Microstructure characteristics were investigated in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron spectro- scopy (TEM). Mechanical properties were studied by four-point bending and single–edge–notched–beam (SENB) techniques. The significant different tribological behavior of these two materials will be interpreted on the basis of mechanical proper- ties and microstructural characteristics. II. Experimental Procedure (1) Sample Preparation Ceramic disks of Y-TZP doped with 8 mol% CuO were pre- pared from 3Y-TZP (3 mol% Yttria-doped tetragonal zirconia K.-H. Zum Gahr—contributing editor This project was sponsored by the Dutch Technology Foundation (STW). w Author to whom correspondence should be addressed. e-mail: a.j.a.winnubst@ utwente.nl Manuscript No. 22805. Received February 13, 2007; approved April 19, 2007. J ournal J. Am. Ceram. Soc., 90 [9] 2747–2752 (2007) DOI: 10.1111/j.1551-2916.2007.01823.x r 2007 The American Ceramic Society 2747