Alumina-zirconia composite is known to be a material harder and stronger than either alumina or zirconia alone if a certain amount of zirconia is dispersed within the alumina matrix. The composite exist as a two phase material. Various theories have been formulated to explain the increase in strength. Among the more common theories are crack-tip transformation toughening and stress induced transformation toughening. These two examples have been widely investigated. Although zirconia is known to increase the strength and toughness in this composite, preparation route of these composite also complement to the increase in the mechanical properties of this composite. Among these are: compacting pressure of the green compacts, particle size of initial powders, sintering temperature and sintering time and addition of other compound/elements to stabilized and enhancing the sintering properties of the composites. The elastic moduli of Al2O3-ZrO3 composite were studied in relations to their processing parameters like compacting pressures, volume fraction of Al2O3 and ZrO3, sintering time and the addition of MgO as a sintering aid in achieving a higher elastic moduli were investigated. The relations between ultrasonically determined moduli, pore volume fraction of the composite, thermal expansion and fracture stress of the Al2O3-ZrO2 composite were investigated. The elastic properties of Al2O3-ZrO2 composite were determined from ultrasonic velocity measurements and were found to be dependent upon amount of the ZrO2 phase, the compacting pressure of the green ceramic and sintering time. The velocity increases upto a maximum for about 3wt% of unstabilized ZrO2 dispersed in Al2O3 and decreases monotonically thereafter. The increase in moduli, as shown by an increased in velocity, is attributed to phase transformation of the unstabilized ZrO2 from tetragonal to a monoclinic phase, which presumably leads to a toughening and strengthening effect and also, due to the action of ZrO2 in stopping grain growth of Al2O3 during densification. The excessive shear strain, induced by the tetragonal monoclinic transformation phase with increase ZrO2 content causes microcracks to appear in the composite. This reduces the elastic moduli of the composite. The thermal expansion of the composite was also investigated and found not to change very much between 1 to 5wt% of ZrO2 but shows the transformation of ZrO2 phases particularly at high ZrO2 content. Diametrical compressive fracture stresses of the composite samples were carried in an instron machine and initial results shows an empirical linear relation with ultrasonically determined elastic modulus. ASAP determined sintered crystal sizes of the composites show that samples with higher fracture stress have on average smaller crystal sizes. It was found that for the highest modulus, a high compacting pressure, small particle size of powders and long sintering time give the best result while the highest diametrical fracture stress require a low compacting pressure. The addition of about 1wt% MgO causes an overall increase in moduli by a factor of 1.5%. The MgO is known to add in sintering of the composites. The variation in moduli with various wt% ZrO2 remains unchanged as above.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 4, pages 2283 - 2290 Publisher:Ashgate Publishing Company
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