Alumina Ceramics with Fine Grains

PCA ceramics with submicron grain size and high in-line transmittance have found applications in metal halide lamps [18]. The in-line transmittance in the visible range could reach 70 % of that of sapphire, which is almost the theoretical limit for alumina ceramics with that range of grain sizes [19]. In addition, mechanical 

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Alumina based fibers are subject to creep, even at low temperatures, This phenomenon is exacerbated by the fine grain size of the fibers [20] [54] [67] [70] [73] [79-80]. At 1200°C and with an applied stress of 70 MPa, the strain rate for the fine-grained Nextel 610 a-alumina fiber is higher than that of Fiber FP with a coarser microstructure. Both strain rates are about one order of magnitude higher than that for a bulk alumina ceramic with a grain size of 1.2 jm. The aeep of Nextel 610 is already measurable at a temperature as low as 900 C under an applied stress of 200-500 MPa [70]. 

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).

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. 

The majority of ceramics are multiphased materials that comprise both ciystallized and vitreous phases. Porcelain thus consists of silicate glass reinforced by acicular crystals of crystallized muUite, but we can also observe millimetric ciystal agglomerates with a very porous microstracture (iron and steel refiactoiy materials), or fine grained polycrystals (< 10 pm) without vitreous phases and with very low porosity (hip prosthesis in alumina or zirconia). It should be reiterated that, in addition to the chemical nature of the compound(s) in question, it is the microstracture of the material (size and shape of the grains, rate and type of porosity, distribution of the phases) that controls the properties. 

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