Grain size control and phase transformations in nanocrystalline ZrO(2)-Al(2)O(3)

An effort has been made to develop nanocrystalline ZrO{dollar}sb2{dollar}-{dollar}rm Alsb2Osb3{dollar} powders that exhibit grain size and phase stability during one thermal cycle from room temperature to 1100-1200{dollar}spcirc{dollar}C for potential use as thermal barrier coating materials. For this use, the tetragonal phase of ZrO{dollar}sb2{dollar} must be maintained. Tetragonal ZrO{dollar}sb2{dollar} can be prevented from transforming to the monoclinic form by maintaining the grain size below a critical value. {dollar}rm Alsb2Osb3{dollar} was intended to provide this grain size control due to its immiscibility with ZrO{dollar}sb2.{dollar} Several sol-gel and precipitation methods of producing the powders were compared, along with two different forms of high energy mixing. The powders were subsequently calcined and heat treated in order to assess their ability to maintain the desired phase distribution during thermal cycling. The powders were characterized by x-ray diffraction and transmission electron microscopy. The method producing the greatest fraction of tetragonal ZrO{dollar}sb2{dollar} with the least amount of added {dollar}rm Alsb2Osb3{dollar} was that in which a commercial colloidal solution of ZrO{dollar}sb2{dollar} was mixed with an aluminum nitrate solution. The critical grain size of ZrO{dollar}sb2{dollar} in this system was 30 nm. The grain size was controlled not by a pinning mechanism as is often seen in conventional, high {dollar}rm Alsb2Osb3, Alsb2Osb3{dollar}-ZrO{dollar}sb2{dollar} ceramics, but instead by mutual constraint of surrounding grains aided by sluggish grain boundary diffusion. The grain growth kinetics in all the phases tended to be slower than in micron sized materials, and a range of grain growth exponents from n = 1 to n = 30 were determined for the various phases. Transformation kinetics in ZrO{dollar}sb2{dollar} followed classic Avrami behavior. {dollar}rm Alsb2Osb3{dollar} phase transformation kinetics were not specifically determined, however, {dollar}gamma{dollar}-{dollar}rm Alsb2Osb3{dollar} was identified at temperatures well beyond its usual stability, which is possibly a grain size effect.