EFFECTS OF MIXING CONDITIONS ON THE PROPE RTIES OF HOT-PRESSED ALUMINUM NITRIDE HEXAGONAL BORON-NITRIDE COMPOSITES

Aluminum nitride-hexagonal boron nitride ceramic composites were sinte red by means of uniaxial hot pressing with changing the mixing conditi ons of raw materials. Mixed powders with a planetary-type milling had a larger surface area than those mixed with a rotary-type milling, whi ch was a consequence of the fragmentation of the h-BN particles. The b ulk density of the powder compact increased with increasing the specif ic surface area of the starting powders independent of the mixing cond itions, The density of the isostatically cold pressed body reached abo ut 74% of the calculated theoretical density. These results were thoug ht to be caused by a bimodal particle-size distribution, suitable for dense compact during mixing, which also reduced the friction among par ticles during forming because of the excellent lubricant action of the B-EN. It was noticed that the maximum density in the sintered materia ls was achieved in correspondence of a specific surface area of the st arting powders. The bulk densities of the sintered materials produced from small specific surface area powders increased with increasing the green density. On the other hand, the sintered density decreased beca use of the presence of fine h-BN particles which impeded the grain gro wth and the densification of AlN with a large specific surface area. I t was considered that the densification of the composites is dominated by the densification of AlN.