The heating of B 4 C-YTZP (where YTZP denotes yttriastabilized zirconia polycrystals) mixtures, under an argon atmosphere, generates B 4 C-ZrB 2 composites, because of a low-temperature (<1500°C) carbide-oxide reaction. Composites derived from mixtures that include >15% YTZP are better sintered than monolithic B 4 C that has been fired under the same conditions. Firing to ϳ2160°C (1 h dwell) generates specimens with a bulk density of >91% of the theoretical density (TD) for cases where the initial mixture includes >15% YTZP. Mixtures that include 30% YTZP allow a fired density of >97.5% TD to be attained. The behavior of the B 4 C-YTZP system is similar to that of the B 4 C-TiO 2 system. Dense B 4 C-ZrB 2 composites attain a hardness (Vickers) of 30 -33 GPa.

The densification of an MgAl2O4 spinel powder prepared by flame spray pyrolysis was examined in order to determine whether it is suitable for fabricating transparent parts using a sinter/HIP approach. It was found that the powder exhibits excellent sinterability. Firing pressed powder compacts to 1650°C/2 h in air increases the bulk density to 99.9% TD. Similar densification, within the precision range of He picnometry, but a higher level of light transmission, is obtained by firing for 80 h at 1400°C. By HIPing, both specimens, sintered at 1650° and 1400°C, are made transparent but the level of transparency attained is higher in the case of specimens presintered according to the latter conditions. Such specimens (2 mm thick) exhibit, after HIPing at 1500°C/3 h (Ar, 200 MPa), a real in‐line transmission (RIT) of 63% (635 nm), an average grain size of 2.2 μm, and a hardness of 13.2 GPa. After HIPing at 1700°C, the RIT attains a value of 77%, while the average grain increases to 17 μm and the Vickers hardness slightly decreases to 12.8 GPa.

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Transparent, polycrystalline MgAl2O4 ceramics have been fabricated by sintering optimized-configuration, powder compacts formed from a material prepared by flame spray pyrolysis. Transparent parts (80% light transmission at 2 mm thickness) could be obtained after pressureless sintering in air at 1280°C/3 h, followed by hot isostatic pressing at 1320°C/3 h/200 MPa.The average grain size was 0.45 μ m. Such materials exhibit property combinations suitable for transparent armor applications.©2009 The Ceramic Society of Japan. All rights reserved.Key-words : Transparent spinel, Ceramics, Sintering, Optical materials [Received June 12, 2009; Accepted August 20, 2009] Polycrystalline MgAl2O4 spinel is one of the most suitable materials for transparent armor panels strike-face fabrication. 1)-4)For such an application, besides a high optical real in-line transmission (RIT), a submicron grain size (GS) is desired. This is so because a fine microstructure facilitates projectile erosion by causing an optimal ceramic fragmentation pattern and increasing hardness (compared to that of large grains spinel)Most of the fabrication technologies previously developed, allow the obtainment of spinel parts exhibiting excellent optical transmission but associated with coarse microstructure.3),4),7),8)Only a few workers focused on the obtainment of low grains size. For instance, remarkably fine microstructure (GS in the 1-5 μ m range) was reported by Gazza and, respectively, Tsukuma. 9),10) Krell et al. were able to further refine, to the submicron region, the microstructure, and obtain, as a result, transparent spinel plates (TSP) showing improved performance in ballistic tests. 1),5),6) Their technology, though, is presented in the literature only in general terms, which do not allow reproduction of the results. For instance, 6) it requires the use of sintering additives, the nature and amount of which were not disclosed.It was recently determined by us 11) that spinel powders prepared by flame spray pyrolysis exhibit an unusually high sinterability. Transparent parts could be derived from such powders by sintering at relatively low temperature (1400°C/80 h, in air) followed by hot isostatic pressing (HIP) at 1500°C. The microstructure of the specimens was quite fine (average grains size ~ 8 μ m), but not at the level optimal for ballistic applications.The objective of this work was to examine whether it is possible to further reduce the sintering temperature of FSP spinel powder compacts, by improvement of the green-body configuration, to a level at which submicron grains size can be obtained while the optical transmission is kept at ≥ 78% in the visible (VIS) range.A stoichiometric MgAl2O4 spinel powder (Nanocerox, Ann Arbor, MI, USA), prepared by flame spray pyrolysis, having a specific surface area A = 30 m 2 /g and including as main impurities 25 ppm of Cl + 20 ppm Si + 10 ppm Ca was used. The powder is made of spheroidal basic particles (individual crystallites or non-porous clusters of them) 20 to 50 nm in size, assembled i...

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