Compressive deformation properties microstructure in the superplastic Y-TZP

The success of traditional glass industry depends on the flexibility in sha ping the products by using the viscous flow of glass. The metal industry al so utilizes various type of plastic forming, for example, forging, extrusio n, and drawing. On the other hand, ceramics are very hard and brittle mater ials that fracture with almost no plastic deformation. The ceramic componen ts have been usually fabricated by powder processing and by precise grindin g after the sintering. Fulrath, Rice, and Nishikawa performed their pioneering works on deformatio n processing of ceramics in 1960s. The concept of "superplasticity" appeare d 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 Ev ans. However, it remained as a dream of ceramists to realize the superplast ic 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 polycryst als (Y-TZP) by Wakai and a series of subsequent reports triggered the inten sive research on ceramics superplasticity internationally: "Superplasticity of Yttria-Stabilized Tetragonal ZrO2 Polycrystals," Advanced Ceramic Mater ials, 1, 259 (1986), "Superplasticity of TZP/Al2O3 Composite," Advanced Cer amic Materials, 3, 71 (1988), "A Superplastic Covalent Crystal Composite," Nature, 344, 421 (1990). Wakai and coworkers demonstrated that the superplasticity is a common natur e of micro grain ceramics that could be observed not only in zirconia, but also in composites, hydroxyapatite, silicon nitride and silicon carbide. Th e superplasticity can be applied to forming components (superplastic formin g), strengthening and toughening (superplastic forging), shaping components concurrent with densification (superplastic sinter forging), and superplas tic bonding. Furthermore the superplastic deformation plays an important ro le in stress assisted densification processes such as hot isostatic pressin g and hot pressing. The researches on ceramics superplasticity in the last decade of 20th century are summarized in several reviews; Jimenez-Melendo a nd Dominguez-Rodriguez, J. Am. Ceram. Sec., 81, 2761 (1998) on zirconia, Wa kai, 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 s uperplasticity 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 poin ted out that the major mechanism of superplasticity in zirconia was the gra in boundary sliding of submicron grains.