THE OXIDATION BEHAVIOR OF A POROUS SI2N2O-ZRO2 COMPOSITE-MATERIAL

The oxidation behaviour of a low-cost, porous Si2N2O-ZrO2 composite ma terial produced without the use of sintering aid was investigated in a ir between 1000 and 1520 degrees C. The extent of internal oxidation w as found to be dependant on the oxidation temperature At temperatures less than or equal to 1300 degrees C considerable internal oxidation o f the material occurs with concommitant large weight gains. The materi al exhibits excellent oxidation resistance at high temperatures (>1300 degrees C) and the higher the oxidation temperature the lower the wei ght gain. A very thin layer of oxide primarily consisting of ZrSiO4 fo rms on the internal pore walls which, together with the surface oxide layer, protects the material from further oxidation. At 1300 degrees C the stresses induced in the matrix by the volume expansion of the oxi dation products cause severe cracking of the material on: cooling from the annealing temperature. Following oxidation exposures at temperatu res greater than or equal to 1300 degrees C the unstabilised ZrO2 phas e in the matrix is inhibited from undergoing the t-m transformation. T his is most probably due to the dissolution of nitrogen' in ZrO2 resul ting in the stabilised non-transformable t'-ZrO2 phase.