Effect of Grain Size on Mechanical Properties of 3y-tzp Ceramics

Yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) have proved to be important structural ceramics. High strength and fracture toughness make them attractive candidates for a number of demanding applications. It was established that these advantageous properties are strongly influenced by the ceramic microstructure [1]. The relationships between microstructure and mechanical properties in Y-TZP ceramics have been studied extensively over the past two decades. It is now generally agreed that the transformation toughening effect (stress-induced tetragonal to monoclinic phase transformation) in Y-TZP ceramics is grain size dependent [2 7]. The larger the tetragonal grains the greater propensity to undergo stress-induced transformation to an equilibrium structure, resulting in an enhanced toughness. The maximum toughness lies near the critical grain size, where the tetragonal grains undergo spontaneous tetragonal to monoclinic transformation [7]. It was found that this critical grain size depends on the yttria content [6, 7]. However, the critical grain size, cited for 3Y-TZP (3 mol% Y2O3), which is the most common zirconia ceramic material, varies significantly (from 1 to 6 μm) [3, 8-9]. This could be attributed to different processing procedures that result in different ceramic microstructures. Thus, microstructural parameters like porosity, grain size distribution, yttrium distribution in grains, purity, phase assemblage, etc. need to be taken into account for evaluating the microstructure. In contrast to the fracture toughness the strength of 3Y-TZP reaches its maximum usually at smaller grain size [5, 10]. Increasing size of failure origin coupled with microstructural coarsening was identified as the reason for strength decrease in ceramics with large grains [11]. However, the increase of strength with decreasing grain size cannot be extended to the nanocrystalline range in 3Y-TZP. Eichler et al. [10] determined the strength maximum for 3Y-TZP in the submicron range and noted a clear strength decrease in the nanocrystalline grain size range (~100 nm). In this case, the strength decrease was attributed to the grain size dependence of fracture toughness. A decrease in the fracture toughness was observed as the grain size changed from the submicrometre to the nanometre range [10, 12]. These results validate the assumption that the increase of critical defects during microstructural coarsening is connected with the formation of stress induced transformed regions at the free surface and takes place preferentially in ceramics with grains near the critical size [11]. Nevertheless, special processing of TZP ceramics has been reported, which can partially eliminate the increase of critical defects during coarsening, resulting in almost constant strength, independent of grain size [8, 9]. Original papers