Homogeneous fabrication and densification of zirconia-toughened alumina (ZTA) composite by the surface-induced coating
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This article proposes a new scheme for fabricating homogeneous Al2O3-ZrO2 composite, in which a thermodynamic theory of interfacial electrochemical phenomena is applied. The theory predicts that a heterogeneous A12O3 interface in colloidal dispersion can induce an enhanced concentration of the ionic species needed for a selective formation of the ZrO2 precursor at the Al2O3/aqueous solution interface. Based on this proposition, a homogeneous Al2O3-ZrO2 composite powder was fabricated by a surface-induced coating of the fine ZrO2 precursor on the kinetically stable colloid particles of A12O3. The composite prepared by the surface-induced coating was characterized by a uniform spatial distribution of the dispersed ZrO2 phase and by the absence of large ZrO2 grains formed from hard ZrO2 agglomerates. The composite also showed highly uniform grain size distribution of both the dispersed ZrO2 and the matrix A12O3 phases. The uniform grain size distribution of the matrix phase indicates that the homogeneous coating of the fine ZrO2 particles is effectively pinning the A12O3 grain boundaries.
I. INTRODUCTION
The role of ZrO2 in improving mechanical properties of both dispersion-toughened ceramics (e.g., ZrO2 toughened A12O3; ZTA) and precipitation-toughened ceramics (e.g., MgO-partially stabilized ZrO2; Mg-PSZ) has been extensively investigated. When the ZrO2 inclusions have tetragonal symmetry, the toughening and concomitant strengthening have been attributed to volume and shape changes associated with the ZrO2 particles during the stress-induced tetragonal -» monoclinic (t -» m) transformation, which occurs in the stress field of propagating cracks.1"6 On the other hand, the microcrack nucleation at transformed m-ZrO2 particles in ZTA occurs in the stress field of propagating cracks, leading to increase in fracture toughness comparable to that achieved by the stress-induced t -> m transformation.6"9 The toughening resulting from the stressinduced transformation or the microcrack nucleation in ZTA can be optimized by minimizing the ZrO2 particle size, the size distribution and agglomerates, and by the uniform spatial distribution and location of ZrO2 particles throughout the alumina matrix.510"12 For example, ZrO2 agglomerates form fractures12 by inducing cracklike voids during the sintering stage, and these cracks propagate to the ZrO2 deficient region. Furthermore, ZrO2 grains larger than 1 jam do not provide any transformation toughening.5 Various processing schemes have been attempted to optimize the above requirements in the fabrication of J. Mater. Res., Vol. 5, No. 3, Mar 1990
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ZTA. Among these, colloidal processing routes have definite advantages in producing homogeneous composite systems and tailoring the microstructures.13"15 Fegley et al.14 prepared alumina-zirconia powders by means of a controlled hydrolysis of zirconium propoxide in an alcoholic dispersion of a-Al2O3. For a homogeneous coating of ZrO2 precursor on A12O3 particles by the controlled hyd
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