Sintering alumina ceramics

Figure 1. The Vickers hardness (testing load 1 kg) of fine-grained sintered alumina ceramics prepared by a sol/gel approach starting with boehmite (7-seeding).

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]. 

Figure 4. Load dependence of the Vickers hardness of sapphire (same data as in Fig. 3) and of finegrained sintered alumina ceramics (relative density >99%, purity >99.7% AI2O3) [16],...

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...

Figure 1. Relicts of hard powder granules (from spray drying) in pressureless sintered alumina ceramics (solid state sintering).

Figure 2. Microstructure with typical cluster of micropores in a submicrometer sintered alumina ceramic produced from a boehmite sol with corundum (a-Al203) seeds <0.2 pm.

The grain size-hardness relationship in Fig. 3, published first in 1995 [9], has opened new prospects for improved tool materials. Until that time it had not been clear whether reducing the grain size of sintered alumina ceramics would cause a technically significant increase in hardness in the USA it was accepted that the hardness increases down to crystallite sizes of 2-4 xm, whereas smaller grain sizes do not contribute to an improved hardness [10]. But even people with a different opinion did not have a chance to test it because more fine grained microstructures could not be prepared until the early nineties. 

Flocculated powders have ramified clusters, which do not stack well in the green form which is then not dense enough. A sintered alumina ceramic will not become fully dense after sintering if it starts out from a green form made near the isoelectric pH. 

Natural rocks and ceramic bodies alike are usually porous to some extent. Figure 5.18 after Kingery and Pappis (1956) presents the impact abrasion resistance of sintered alumina relative to sintering temperature. 

Until recent times, Rockwell hardness testers, equipped with a diamond cone with included angle of 120°, were used for hardness determination of sintered microcrystalline ceramics of alundum type (alumina ceramics) or of steatitic type. Tests conducted by Palushkin (1961) showed (Table 5.4)... 

The thermal shock behaviour of hot-pressed alumina, hot-pressed and sintered alumina reinforced with iron particles (reprinted from Journal of the European Ceramic Society, 19, Aldridge and Yeomans, The thermal shock behaviour of ductile particle toughened alumina composites, 1769-1775, copyright 1999, with permission from Elsevier). 

Maensiri, S., Roberts, S.G. (2002), Thermal shock resistance of sintered alumina/silicon carbide nanocomposites evaluated by indentation techniques , J. Am. Ceram. Soc., 85(8), 1971-1978. 

Glass-ceramics Porcelain Sintered alumina AI2O3 crystal... 

The typical and most widely employed representative of oxide ceramics is sintered alumina on which it is convenient to demonstrate the characteristic features of this group of materials. 

Figure 2. (a) The surface of an alumina ceramic from which all porosity has been removed during sintering the microstructure consists of grains and the boundaries between them, (b) The sintering of silicon results in the formation of a continuous network of solid material (white) and porosity (black). The microstructural change is not accompanied by any densification. (Reprinted from R. J. Brook, ed., Concise Encyclopedia of Advanced Ceramic Materials, eopyright ( 1991, p. 4, with kind permission from Elsevier Science Ltd.)... 

Preparation of Solid State Electrolytes 17.3.7.1. -Alumina Ceramic Electrolytes 17.3.7.1.3. Sintering (Densification) and Annealing. 

D. Hayes, D. W. Budworth and J.P. Roberts, Selective permeation of gases through dense sintered alumina. Trans. Br. Ceram. Soc., 60 (1961) 494-504. 

---