Millennial Special Leading Papers on Ceramics in the 20^th Century: the Best of JCerSJ Compressive Deformation Properties and Microstructure in the Superplastic Y-TZP

Abstract

The success of traditional glass industry depends on the flexibility in shaping the products by using the viscous flow of glass. The metal industry also utilizes various type of plastic forming, for example, forging, extrusion, and drawing. On the other hand, ceramics are very hard and brittle materials that fracture with almost no plastic deformation. The ceramic components have been usually fabricated by powder processing and by precise grinding after the sintering.
Fulrath, Rice, and Nishikawa performed their pioneering works on deformation processing of ceramics in 1960s. The concept of “superplasticity” appeared in metallurgical field in those days, and attracted the interests of many materials scientists. The superplasticity refers to extraordinarily large elongations of polycrystalline solids at elevated temperatures. The search for superplasticity in ceramics has been made by Morgan, Bradt, Raj, and Evans. However, it remained as a dream of ceramists to realize the superplastic elongation as an intrinsic nature of polycrystalline solids without the help of viscous flow of intergranular glass phase.
The finding of superplasticity of Y₂O₃-stabilized tetragonal ZrO₂ polycrystals (Y-TZP) by Wakai and a series of subsequent reports triggered the intensive research on ceramics superplasticity internationally: “Superplasticity of Yttria-Stabilized Tetragonal ZrO₂ Polycrystals, ” Advanced Ceramic Materials, 1, 259 (1986), “Superplasticity of TZP/Al₂O₃ Composite, ” Advanced Ceramic Materials, 3, 71 (1988), “A Superplastic Covalent Crystal Composite, ” Nature, 344, 421 (1990).
Wakai and coworkers demonstrated that the superplasticity is a common nature of micro grain ceramics that could be observed not only in zirconia, but also in composites, hydroxyapatite, silicon nitride and silicon carbide. The superplasticity can be applied to forming components (superplastic forming), strengthening and toughening (superplastic forging), shaping components concurrent with densification (superplastic sinter forging), and superplastic bonding. Furthermore the superplastic deformation plays an important role in stress assisted densification processes such as hot isostatic pressing and hot pressing. The researches on ceramics superplasticity in the last decade of 20th century are summarized in several reviews; Jimenez-Melendo and Dominguez-Rodriguez, J. Am. Ceram. Soc., 81, 2761 (1998) on zirconia, Wakai, Kondo, and Shinoda, Current Opinion in Solid State & Materials Science, 4, 461 (1999) on silicon nitride and silicon carbide.
The paper, which is selected in this issue, is one of the early papers on superplasticity of zirconia. The relationship between flow stress and strain rate was necessary for superplastic foming in compression. The flow stress was a function of grain size, and it showed that the grain refinement was essential for superplastic deformation. This was the first paper which pointed out that the major mechanism of superplasticity in zirconia was the grain boundary sliding of submicron grains.