Effect of powder treatment on the crystallization behaviour and phase evolution of Al2O3–High ZrO2 nanocomposites
Abstract
Al2O3–ZrO2 composites containing nominally equal volume fraction of Al2O3 and ZrO2 have been synthesized through combined gel-precipitation technique. Subsequently the gels were subjected to three different post gel processing treatments like ultrasonication, ultrasonication followed by water washing and ultrasonication followed by alcohol washing. It was observed that while in unwashed samples crystallization took place at low temperature, crystallization was delayed in the washed gels. The phase transition of ZrO2 in the calcined gels followed the sequence; amorphous fi cubic ZrO2 fi tetragonal ZrO2 fi monoclinic ZrO2. On the other hand, phase transition in alumina followed the sequence amorphous to c-Al2O3, the transition taking place at 650 _C. No a-Al2O3 could be detected even after calcination at 950 _C. However, all the sintered samples had a-Al2O3. In spite of high linear shrinkage (19–21%) during sintering, the sintered sample had density of only above 70% for all the four varieties of the powders. However, in spite of the low sintered density of the pellets, 31% tetragonal zirconia could be retained after sintering at 1400 _C and it reduced to about 16% at 1600 _C.
Introduction
The high elastic modulus of alumina makes it a suitable matrix to incorporate ZrO2 particles for providing a tough ceramics. The increment in toughness mainly results from the transformation toughening and/or micro crack toughening effect [1, 2]. The enhancement in toughness is limited by the coarsening effect of ZrO2 particles during sintering of the composites. A smaller intial particle size and a uniform distribution of fine ZrO2 particles in tetragonal form is the desirable microstructure for toughened Al2O3–ZrO2 composites [3, 4]. The extent of toughening achieved in these composites depends on size of Al2O3 and ZrO2 particles, volume fraction of ZrO2 retained in the tetragonal phase, which in turn will determine the volume fraction of tetragonal ZrO2 available for transformation. A finer particle size distribution of ZrO2 not only helps to achieve a uniform distribution of ZrO2 particles, but it also ensures that ZrO2 remains mostly in tetragonal phase provided the critical size barrier for the t-phase retention is not crossed [5]. Efforts to achieve a finer ZrO2 and Al2O3 particle size have been made possible by use of sol–gel and other and process [6, 7]. However, in these studies the volume fraction of ZrO2 particles was usually varied upto 30 vol% with very few reports on equal volume fraction of alumina and zirconia [8]. Higher volume fraction of ZrO2 is expected to increase the fracture toughness of the composites due to higher volume fraction of tetragonal ZrO2 available for stress-induced transformation. However, an increase in ZrO2 volume fraction tends to cause inhomogeneous distribution of ZrO2, higher grain size and consequently a lowering amount of retained tetragonal ZrO2 content and low


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