Effect of MnO on the microstructures, phase stability, and mechanical properties of ceria-partially-stabilized zirconia
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I. INTRODUCTION
Among zirconia ceramics, tetragonal zirconia polycrystals (TZP), partially-stabilized with either Y2O3 (YTZP) or CeO2(Ce-TZP), have emerged as the most promising because of the impressive range of mechanical properties that has been achieved. As recently summarized by Tsukuma and Takahata,1 bend strengths as high as 2.4 GPa have been measured for some Y-TZP ceramics containing 30 vol. % A12O3 dispersions, while fracture toughness in excess of 30 MPa Vm has been reported for Ce-TZP. Apart from their mechanical properties, TZP ceramics are interesting from a materials science point of view because they can be processed with widely differing microstructures and phase stability, and they are now known to exhibit at least two fundamentally different mechanisms of deformation and toughening. Gupta et al.2 were the first to sinter fully-dense, single-phase tetragonal polycrystalline zirconia with Y2O3 as the stabilizing additive. Lange3 followed this effort with his study of the stability of the tetragonal phase as affected by Y2O3 content and grain size and their influence on the fracture toughness via transformation toughening. A number of studies have been conducted since these early efforts (see the review 1948
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article by Tsukuma and Takahata1), particularly by Japanese investigators. Among the notable developments are the significant strengthening effect of dispersed alumina on Y-TZP,1'4 development of the large-grained, single-phase Y-TZP with the addition of TiO2 to achieve higher fracture toughness1 and the in situ growth of needle-like grains of /3-La 2 O 3 11A12O3 in Y-TZP.1 It is now possible to design microstructures in Y-TZP to achieve a desired combination of high strength and fracture toughness. Development of Ce-TZP ceramics is relatively more recent. Tsukuma and Shimada5 and Tsukuma6 were the first to report systematic studies of the effect of sintering temperatures and CeO2 content on the grain size of ZrO2, relative phase contents, and the mechanical properties of Ce-TZP. The same investigators have also since studied the influence of dispersed alumina phase in Ce-TZP.17 As in the case of the Y-TZP system, highest strengths were obtained at 30 vol.% A12O3; however, this strength increase was accompanied by a concurrent decrease in fracture toughness relative to the base Ce-TZP. A number of different Ce-TZP ceramics containing AI2O3 and several proprietary secondary additives that exhibit high strengths and fracture toughness have
J. Mater. Res., Vol. 5, No. 9, Sep 1990
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J. S. Wang ef a/.; Effect of MnO on ceria-partially-stabilized zirconia
been developed by Virkar and Matsumoto.8'9 An interesting observation in their studies was ferroelastic domain switching during grinding of Ce-TZP.89 Ferroelastic domain switching has also been proposed as a toughening mechanism in TZP ceramics sintered in the cubic phase equilibrium temperature range and then cooled through the tetragonal phase temperature regim
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