Journal of the American Ceramic Society, Vol.102, No.10, 6349-6356, 2019
Formation of (Al2OC)(1-)(x)(AlN)(x) solid solution starting from Al-Si-Al2O3 powder matrix at 1300 degrees C in flowing nitrogen
(Al2OC)(1-)(x)(AlN)(x) solid solution-reinforced Si-Al2O3 composite was successfully synthesized by designed heating of the Al-Si-Al2O3 composite to 580 degrees C and held for 8 hours, followed by heating to 1300 degrees C at a rate of 12 degrees C/h in flowing nitrogen. The reaction mechanism is as follows: after the Al-Si-Al2O3 composite is heated to 580 degrees C and held for 8 hours, an AlN cladding is formed on the surface of the Al powder, thus the composite is preconverted into (Al-AlN cladding structure)-Si-Al2O3 system. With increasing temperature, the AlN cladding ruptures and the reactive Al(l) flows out. The Al(l) preferentially undergoes active oxidation to form metastable Al2O(g), which lowers P-O2 inside the composite and inhibits the active oxidation of Si. Moreover, ultrafine carbon is produced by the pyrolysis of the phenolic resin binder. Both metastable Al2O(g) and ultrafine carbon are highly reactive. Therefore, under the induction of AlN and N-2, (Al2OC)(1-)(x)(AlN)(x) solid solution is formed by the reaction Al2O(g)+AlN(s)+C(s)->(Al2OC)1-x(AlN)x(s), which easily occurs at a relatively low temperature. In the presence of a large amount of Al2O(g), the P-O2 in the composite does not satisfy the condition required for both Si nitridation and active oxidation, so the free Si remains stable in the composite, forming a metal-non-oxide-oxide composite. The cold crushing strength of the composites is up to 305 MPa, and the composites do not show hydration after 20 months of storage in the environment.
Keywords:(AlN)(x)(Al2OC)(1-)(x) solid solution;Al2O(g);formation mechanism;metal-non-oxide-oxide composite