Effect of cubic boron nitride grain size in the reinforcing of a-Sialon ceramics sintered via SPS

Utilising SPS, fully dense a-Sialon ceramics were obtained during sintering at temperatures as low as 1550. °C. This enabled the full densification a-Sialon composites with 10 and 100. mm cubic boron nitride (cBN) grains. The detailed microstructural analysis of these materials with SEM and Raman spectroscopy revealed different cBN to hexagonal BN (hBN) conversion mechanisms. The smaller cBN grains only show a conversion into hBN on their surface whereas the large grains show not only conversion on the surface but also conversion at the internal grain boundaries between the twins detected inside the cBN grains. The increasing degree of formation of hBN results in decreasing hardness and increasing fracture toughness. Therefore the properties of materials with 100. mm cBN grains are more sensitive to the sintering temperature.