Robert Pompe

The oxidation behaviour of a porous Si2N2OZrO2 composite material

Abstract The oxidation behaviour of a low-cost, porous Si2N2O-ZrO2 composite material produced without the use of sintering aid was investigated in air between 1000 and 1520 °C. The extent of internal oxidation was found to be dependant on the oxidation temperature. At temperatures ≤ 1300 °C considerable internal oxidation of the material occurs with concommitant large weight gains. The material exhibits excellent oxidation resistance at high temperatures (>1300 °C) and the higher the oxidation temperature the lower the weight gain. A very thin layer of oxide primarily consisting of ZrSiO4 forms on the internal pore walls which, together with the surface oxide layer, protects the material from further oxidation. At 1300 °C the stresses induced in the matrix by the volume expansion of the oxidation products cause severe cracking of the material on cooling from the annealing temperature. Following oxidation exposures at temperatures ≥1300 °C the unstabilised ZrO2 phase in the matrix is inhibited from undergoing the t-m transformation. This is most probably due to the dissolution of nitrogen in ZrO2 resulting in the stabilised, non-transformable t′-ZrO2 phase.

Wet and salt corrosion of a porous Si2N2O-ZrO2 composite material

Abstract The hot corrosion behaviour of a low-cost, porous Si 2 N 2 O-ZrO 2 composite material, fabricated without the use of sintering additives has been studied in pure oxygen, humid oxygen (10, 15 and 20 vol% H 2 O) and salt-containing environment (25 ± 5 ppm (in volume) NaCl). The corrosion experiments were carried out at 1000, 1200 and 1400 °C. For salt corrosion, a special test rig was used to simulate salt-containing combustion environments. The extent of internal oxidation has been found to be sensitive to the oxidation temperature. High oxidation resistance has been observed at temperatures > 1200 °C. Wet salt corrosion at temperatures above 1200 °C shows no increased attack on the material.

On the oxidation kinetics of an Si{sub 2}N{sub 2}O-ZrO{sub 2} composite material

The oxidation kinetics of a porous Si{sub 2}N{sub 2}O-ZrO{sub 2} composite material are investigated. Samples were oxidized at different temperatures between 1,000 and 1,450 C. Transmission electron microscopy was used for investigations of the formed oxide layer. Samples of pure Si{sub 2}N{sub 2}O and reaction couples of ZrO{sub 2}/SiO{sub 2} were prepared by a thin-film technique and heat-treated at 1,250 and 1,350 C, respectively. The thickness of the formed reaction layers was determined by spectroscopic ellipsometry. The formation of amorphous silica on Si{sub 2}N{sub 2}O was found to be faster than on Si, and this may have significant impact on the oxidation rate of the composite material. At 1,350 C, ZrSiO{sub 4} starts to form considerably in the ZrO{sub 2}/SiO{sub 2} reaction couple and may retard further oxidation in the composite material. At 1,350 C, ZrSiO{sub 4} starts to form considerably in the ZrO{sub 2}/SiO{sub 2} reaction couple and may retard further oxidation in the composite material. This process is favored at higher temperatures.

Development of Silicon Nitride Based Matrices

A feasible way to produce SiC long fibre reinforced Si3N4 is by vacuum infiltration of Si3N4 slurry followed by reaction bonding of the matrix. To minimize degradation of the fibres the nitridation reaction has to take place at moderate temperatures. In the present work a number of Si3N4-matrices processed by a modified RBSN technique and mixed with ZrO2-additives to optimize the nitridation cycle will be discussed. The results show that ZrO2 has a remarkably accelerating effect on the nitridation. It is thus possible to decrease the exposure of SiC fibres to high temperatures.