Effect of sintering additives on the oxidation behavior of Si3N4 ceramics at 1300 °C

This paper describes the results of systematic investigation of the oxidation behavior of Si3N4 based ceramics. The tests were carried out at 1300 °C for 2000 h in a high-temperature dry air environment. The Si3N4 specimens tested include the following: (a) S-1: Si3N4 added 8 mass% Y2O3, (b) S-2: Si3N4/SiC added 8 mass% Y2O3, (c) S-3: Si3N4 added 5 mass% Y2O3 and 3 mass% Al2O3, (d) S-4: Si3N4/SiC added 5 mass% Y2O3 and 3 mass% Al2O3. Several interesting conclusions were obtained as follows: (1) the thicknesses of oxidized layer of S-3 and S-4 were much thicker than S-1 and S-2, (2) oxidation kinetics of S-1 and S-2 obeyed a parabolic law on the whole, while those of S-3 and S-4 had a break, (3) the yttrium (Y) concentration under the oxidized layer decreased significantly. The Y-decreased zone was defined as a diffused layer. The thicknesses of the diffused layers of S-3 and S-4 samples were very large. (4) Primarily, the crystalline phases in the oxidized layer were SiO2 and Y2Si2O7. (5) The effect of SiC composition on the oxidation behavior was small.