In the end of 20th century people could easily achieve grain sizes in the sub-micron range (<1 micron) in zirconia ceramics, but it wasn't sufficient; there was a strong driving force to refine the grain size to nano range (< 100 nm) of course for obtaining certain advantages, mostly enhanced structural and physical properties. Beginning of this century observed a difference in mechanical behaviour in nano-grained tetragonal zirconia from the sub µm grained one. This drove scientist to study this mechanical behaviour of nano zirconia whether it is tetragonal or monoclinic. Now the problem associated with density. It was not so difficult to achieve a grain size which is in nm range but with a density as high as 99% of the theoretical density, it was difficult. Currently there are few techniques which satisfies both requirements. Spark plasma sintering (SPS), Microwave sintering, Sinter-forging, HIPing, also the brand new one is crystallization from glass!In the first two processes control grain size is very limited and the sintering mechanism is still not completely understood. In SPS there is a huge current of the order of 1 kA and more is passed through a powder compact and it claimed to form plasma which sinters the material to full density very fast with a very small grain size, but with very little control. In the microwave process, both electric and magnetic field interact with the material, and the absorbed energy actually leads to very fast sintering, again microwave, though, is in industries now, and still is feeding significant no. of researchers around the world. This leaves us with Sinter-forging, an excellent processing technique which was first opted by couple of groups, Rehemans’ and Raj’s, in mid 80s. At high temperatures without any wall constraint if a compact under load is sintered, the sintering rate is enhanced by couple of factors; one is the stress induced diffusion becoming faster and disappearance of big pores by plastic deformation. In zirconia it was observed that instead of the applied stress actually the plastic strain controls the densification. Now here the advantage is that, one can independently play around with two parameters; stress and temperature and you can achieve grain size really in the nano range in a fully dense material. The pictures below are from sinter-forging experiments. One needs to monitor the transverse strain also along with the axial strain and in these experiments, for this specific experiment I used a digital camera in my I.I.Sc lab. These images were taken at 1423 K. Extreme right one is from the final stage of sinter-forging. You may notice that there is very little strain in the transverse direction compared to the axial direction, which indicates densification. I did not put the scale which is a “crime” but take it from me; the height of the specimen is 3.5 mm.
So now we have a nano-grained material, but what is the size? It is difficult to push the grain size of the bulk dense body below 50 nm, this is one shortcoming of sinter-forging. So here comes glass. The idea is like this: you take these oxides to very high temperature and cool it with a cooling rate of the order of 103 to 104 K/min and they may form glass or some amorphous phase, and now if this glass is taken above the transition temperature, the crystallite starts nucleating and the body becomes a dense nanocrsystalline body with a crystallite size may be as low as 15 nm. This processing technique is difficult and there are critical issues involved with density, still it gives you the grain size you want. What else, if I have such material (from 15 -100 nm) in hand, a through mechanical property study is possible. Then I know serendipity will knock my door any day and I will jump and shout…………:D
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