Sintering of Al2O3–NbC composites using TiO2 and MnO additives: preliminary results

Cemented carbides have been replaced by ceramic composites in the manufacture of cutting tools characterized by high cutting speeds and hot hardness. Liquid-phase sintering of Al2O3–NbC composites using TiO2 and MnO was studied in an attempt to produce dense composites at relatively lower temperatures than those characteristic of pressure-assisted processes. The sintering behavior of the composite was assessed from a dilatometric study carried out under vacuum up to 1800 °C. The formation of crystalline phases was evaluated from X-ray diffraction analyses. Mercury porosimetry and Vickerʹs microhardness tests were also carried out. Liquid phase sintering took place with the formation of a non-volatile liquid over 1300 °C. TiO2 and MnO were effective in densifying Al2O3 to >95% TD at sintering temperatures as low as 1550 °C. Upon cooling, both crystalline and vitreous phases were formed but they did not affect the hardness of the composite (∼18 GPa).