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aluminum borate is an interesting material for applications requiring high hardness. It can be obtained in two ways. One is to melt alumina and boric acid, which then grows whiskers that can be drawn into filaments by cooling. The other way is to heat alumina and boron oxide in the gas phase to induce their decomposition into borate whiskers. The present paper describes the first successful synthesis of dense aluminum borate using this latter method. The thermal and sintering behavior as well as some technological properties of this material are described.

X-ray diffraction studies of the milled powder reveal that it can be sinterred to give alumina-boric oxide ceramics with low porosity, and sintering at high temperatures yields a refractory castable. These materials have the potential to serve as a binder for high alumina and spinel refractory castables.

The sintering behavior of the attrition milled aluminum borate powder is studied by the techniques of thermogravimetric analysis (TMA) and differential thermogravimetric analysis (dTG). TMA curves show that sintering starts at about 900°C and continues to 1200°C with little dimensional change, after which the shrinkage rate decreases from -3.5 to -2.7 (10 3/degC) to 1350°C. After this temperature, sintering ceases because of the decomposition of the borate into alumina and boron oxide.

The dTG and TMA data also permit evaluation of the densification of the aluminum borate. Compared with the starting powder, the sintered material has a higher bulk density. The evaluated Vickers hardness values of 4-7 GPa are encouraging for structural applications. The microstructure of the sintered material reveals that most of the grains are sub-angular to angular with a moderate degree of sphericity. The microstructures developed with soaking for 5 and 10 hours at 1300°C reveal that the rod grains are transformed into more rounded ones.

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