Hot Superplastic Powder Forging for Transparent nanocrystalline Ceramics

Abstract

Summary:The goal of the program was to fabricate nanocrystalline ceramics from superplastically deforming powder with nanocrystalline microstructures using a new approach. Rather than starting with nano-sized powders, we used powders 2-25 μm in diameter (average 16 μm) having a nanoscale microstructure. The major challenge was to hot press or to hot forge the powders to full density while maintaining the nanostructure. The second goal was to achieve transparent nanocrystalline ceramics. Accomplishments:The first goal of fabricating nanocrystalline ceramics from superplastically deforming powder was accomplished. However, not all features of the microstructure were <100 nm. It was found, for instance, that in the fully dense Al 2 O 3 -ZrO 2 eutectic specimens that a bicontinuous microstructure exists containing <100 nm ZrO 2 particles in a matrix of Al 2 O 3 grains extending over 1-2 μm was observed. Crystallization, growth, phase development and creep during hot pressing and forging were studied for several compositions and so provided some details on development of polycrystalline microstructure from heating quenched ceramics. It was predicted, however, that fully nanostructured microstructures of the Al 2 O 3 -ZrO 2 composition could not be hot pressed at pressures on the order of 100 MPa. The second goal of transparent nanocrystalline ceramics was not achieved because of microstructure coarsening though thin sections of some samples were translucent. Research AccomplishmentsIntroduction. This program studied an alternate and, for some applications improved method of obtaining nanocrystalline ceramics. Typically, nanocrystalline ceramics are fabricated starting with powder of particle diameter on the order of a ten nanometers, with the intent of maintaining that diameter through pressureless sintering, hot pressing or hot forging. Several disadvantages of this method are: (1) Powders are difficult to handle and difficult to avoid agglomeration during the forming operation. (2) Powders, in dry forming operations, hydrate or pick up other impurities because of their high specific surface area. (3) Adsorbed gases, water vapor from hydration and April 2006 Final Report-W. Roger Cannon 2 other gases are trapped during sintering are difficult to eliminate from a powder compact and limit the final density. (4) Inhalation of nano-particulate powders may pose a health hazard.As an alternative, this program studied coarse spherical powders (<25 μm in diameter and average 16 μm) rapidly quenched from the melt, whose microstructure ranged from fully nanocrystalline to fully amorphous. The program, however, mostly concentrated on the Al 2 O 3 -ZrO 2 eutectic powders, which after quenching from the melt were between 30% and 40% amorphous. These powders were then hot pressed and forged to study their densification behavior.Two important aspects of the program were, first, a study of microstructure development from the as-quenched state to various annealing temperatures and then through the hot pressing stage and, second, th...