Oxidation of aluminum oxynitride-boron nitride (AlON-BN) composite prepared by reaction sintering

AlON-BN composite was prepared by heating a mixed powder compact with Al2O3-AlN-20 mass% BN-3 mass% Y2O3 (Al2O3/AlN = 2.9 in molar ratio) at 1850 degreesC for 2 h in N-2. The relative density of the heated compact was 70%. Oxidation characteristics of AlON-BN composite were investigated over the temperature range of 1000-1500 degreesC in air. AlON and BN were oxidized to form Al2O3 and B2O3 at the initial stage, respectively, The reaction between the formed B2O3 and Al2O3 yielded Al4B2O9 and Al18B4O33 Below 1200 degreesC, oxygen penetrated into the composite through the open pores and oxidation occurred since the surface layer was porous owing to an insufficient amount of the formed B2O3 liquid. At 1100 and 1200 degreesC, volume expansion accompanied by the formation of A118B1033 caused the occurrence of cracks in the composite. At 1300 degreesC, a relatively dense layer with B2O3 liquid surrounded the composite and suppressed the penetration of oxygen into it. At 1500 degreesC, oxidation was promoted to cause the deformation of the composite.