Submicrometer-grained aluminas

At the present stage of knowledge, submicrometer-grained aluminas are in most cases only translucent or, at the best, are transparent with the linear transmission of... 

Godlinski D, Krmtz M, Grathwohl G (2002) Transparent alumina with submicrometer grains by float packing and sintering. J Am Ceram Soc 85 2449-2456... 

Therefore, the development of new hard materials requires the preparation of phases with small microplastic deformability of the crystal lattice, and reduced grain sizes in the sintered polycrystals, and highly perfect microstructures. For example, sintered alumina ceramics which have a submicrometer microstructure and associate this small grain size with a macrohardness of 20-24 GPa (compared with 17-19 GPa in conventionally sintered corundum) were obtained only when new processing approaches became available, which could reduce the flaw population [1]. 

With Eq. (3/3a), this deereasing influence of the load in more fine-grained microstructures is expressed by smaller values of the ratio (5e/5i)o- Table 2 displays fitting parameters for the experimental data of Fig. 4 for smaller grain sizes, increasing asymptotic hardness values H o are associated with decreasing parameters ( e/ i)o- la submicrometer alumina microstructures, the increase of the microplastic deformability (the decrease of the hardness) becomes smaller and smaller and approaches zero already at small indent sizes of 10-20 pm (see Fig. 4), and which characterizes the extension of the plastic zone approaches the initial deformability at rather small loads of about 1N. 

Figure 6. Influence of the surface preparation on the hardness of sintered alumina ceramics with different grain sizes. In the submicrometer range, grinding results in a higher hardness than...

A. Krell and P. Blank, Grain size dependence of hardness in dense submicrometer alumina, /. Am. Ceram. Soc. 1995, 78, 1118-1120. 

Figure 5 shows investigations of the fretting wear of alumina depending on the environmental conditions, submicrometer ceramics may exhibit a wear rate which is an order of magnitude lower than measured for conventional AI2O3. It is important to understand that this improvement is not only a simple consequence of the increased hardness, but is also associated with reduced grain pull-out in the wear track of the submicrometer material (Fig. 6) [11]. 

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