The strengthening and toughening mechanismso fh ot-pressed Ce-TZP/Al 2 O 3 ceramics werei nvestigated by X-ray diffraction (XRD), scanning electron microscopy (SEM),and transmission electron microscopy (TEM) methods.T he results showt hat the contributions to the strength and toughness mainly come from the hindrance of the growth of Al 2 O 3 grains by ZrO 2 nanoparticles and the transformation from tetragonal ZrO 2 to monoclinicZrO 2 .Additionally,TEM observations showthat the dislocation structuresf ormed both in the Al 2 O 3 and on the Al 2 O 3 grain boundaryh aveacertain contribution, because the dislocation agglomeration and fixation by ZrO 2 nanoparticles coulddeflect crackingo rstop crackingd evelopment. Meanwhile, it was found that the mechanical properties and microstructures of samples were preferential when ZrO 2 contentsi nt he sample reached 20 pct.

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ExperimentalThe precursor W 0.8 Al 0.2 was prepared as follows. [8] Firstly, the elemental powders of tungsten (-200 mesh, 99.8 % purity) and aluminum (-200 mesh, 99.5 % purity) were weighed out with the atomic ratio W:Al= 4:1. Secondly, the mixed powders were sealed under an argon atmosphere into the WC vial with ball-to-powder weight ratio of 15:1. Finally, materials were ball milled at the rotation speed of 650 rpm for about 8 h, and then the pure W 0.8 Al 0.2 alloy was obtained.The prepared W 0.8 Al 0.2 alloy (< 0.1 lm, 99 % purity) and carbon (< 34 lm, 99 % purity) were ball milled stoichiometric amounts of W 0.8 Al 0.2 and C powder with molar ratio of 1:0.7. The rotation speed was also 650 rpm, and the ball to powder ratio was about 10:1. The total milling time was 20 h.The ball-milled powders were then pressed into a column of 10 mm diameter and 5 mm thick with a compaction pressure of 400 MPa, and then reaction sintered at high pressure (4.5 GPa) and high temperature (1873 K) for 30 min.The materials for various reaction times were examined by X-ray diffraction (XRD) using a Rigaku D/max-&B X-ray diffractometer with Cu K a radiation (k=1.54056 ). The scan speed was 4 /min. The environment scanning electron microscope (ESEM, Philips XL30) and the energy dispersive analysis of X-rays (EDAX) measurements were conducted to investigate the morphology and the quantitative material composition of the prepared powders.The density of the sample was determined by the Archimedes' principle using distilled water. The hardness was determined with a Vickers micro hardness tester (HX-1, China) with a load of 200-gf and dwell time of 15 s.Engineering ceramics are promising materials for tribological applications because of their high hardness, high fractural strength, low density and high stiffness, moreover, excellent inertness and mechanical properties at high temperature. During the last few decades, many scientists and engineers have thoroughly investigated the friction and wear behavior of engineering ceramics, [1,2] especially, the wear behavior of ZrO 2 -containing ceramics. [3] Nevertheless, it has been found that the reports on friction and wear behavior of ZrO 2 -containing ceramics are different yet because of different research methods and experimental environments. [4] Therefore, it is still necessary to investigate the wear behavior of ceramics, especially, the wear failures of ceramics under unlubricated environments since many friction parts on industrial equipments run in unlubricated environments. [5] In the research presented here, the wear behavior of ceriatetragonal zirconia polycrystal / alumina (Ce-TZP/Al 2 O 3 ) nanocomposites with different ZrO 2 contents against bearing steel under unlubricated enviroments was investgated. Based on the morphological observation of worn surface by scanning electron microscope (SEM) and phase structural analysis by Xray diffractometer (XRD), the wear mechanisms of Ce-TZP/ Al 2 O 3 nanocomposites were revealed. Additionally, the influences of ZrO 2 nanopart...