Zirconia martensitic transformation

Martensite transformations are not limited just to metals. Some ceramics, like zirconia, have them and even the obscure system of (argon + 40 atom% nitrogen) forms martensite when it is cooled below 30 K. Helical protein crystals in some bacteria undergo a martensitic transformation and the shape change helps the bacteria to burrow into the skins of animals and people ... 

However, there is one very important difference between the martensitic transformation in inherently ductile materials such as iron and brittle materials such as zirconia. As for the latter the von Mises criteria for plastic deformation are not satisfied the deformation process is accommodated by a change in shape as manifest in a serrated surface akin to that shown in Figure 4.5, panel c on a microscopic... 

Behrens, G. (1993) The martensitic transformation in zirconia. PhD dissertation. Case Western Reserve University. 

One of the key features of zirconia lies in its polymorphism. Zirconia exhibits three polymorphs. The monoclinic phase is stable up to 1170°C where it transforms to the tetragonal phase, which is Itself stable up to 2370 C. Above this temperature, zirconia exists as a cubic Cap2 type phase. The reversible m- to t-Zr02 transformation is key to the use of zirconia in ceramics. First, it is reversible but occurs with a thermal hysteresis. Second, it is rapid and takes place by a diffusionless shear process similar to that of a martensitic transformation. Finally, it is dependent on particle size and occurs with a volume change (3 to 5%) [82]. 

The zirconia toughening of various ceramics is of great interest and technological importance. It also undergoes Martensitic transformation. Figure 5.50 shows such a transformation in MgO-partially-stabilized Z1O2 [henceforth Mg-PSZ]. The Martensitic transformation upon cooling is a t -> m transition. 

More on the t-Zr02 m-Zr02 Martensitic transformation in TZP ceramics is found in the work of Yin in his discussions on the thermodynamics, crystallography and kinetics of this transformation. The solid transformation in pure zirconia is ... 

Maneshian, M.H., Banerjee, M.K. Reverse martensitic transformation in alumina - 15 vol % zirconia nanostructured powder synthesized by high energy ball miUing. J. Alloy. Compd. 459, 531-536 (2008)... 

The exceptional mechanical properties of zirconia are mainly based on the phenomenon of transformation toughening (this is described in detail below). The delayed martensitic transformation of the tetragonal high-temperature modification into the monoclinic low-temperature modification by adding stabilizing oxides leads to increased stress intensity factors JCic and also R-curve behavior. Hence, appropriately stabilized zirconia has been dubbed ceramic steel (Garvie et al, 1975). 

There is a slight decrease in flexural strength and toughness of zirconia ceramics exposed to bodily fluids. The reason is associated with the martensitic transformation from the tetragonal to the monoclinic phase. 

It is partially stabilized zirconias (PSZ) that have justified the resounding article ( Ceramic steel ) published in 1975 by Garvie et al. [GAR 75], The title suggests that a ceramic can exhibit the high mechanical performances associated with steel, but also that toughening mechanisms recall those used by steel manufacturers. The t- m transformation of zirconia is a martensitic transformation, in analogy with the transformation used to obtain martensite in tempered steels, and the role of microstructural parameters inZr02 is similar to what is observed in metals. 

G.K. Bansal and A.H. Heuer, On a martensitic phase transformation in zirconia (Zr02)-I. Metallographic evidence, Acta Metall. 20, 1281-1289 (1972). 

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