Colette O'Meara

The rate-controlling processes in the oxidation of HIPped Si3N4 with and without sintering additives

Abstract Various possible rate-controlling processes in the oxidation of HIPped Si 3 N 4 with 4 wt% Y 2 O 3 and without sintering additives have been studied using TGA, XRD, FTIR, SEM and TEM/STEM methods. It is proposed that, in the temperature range 1000 to 1450°C, the oxidation kinetics of the studied Si 3 N 4 materials is mainly determined by O 2 diffusivity in the heterogeneous oxide scales and possibly also by the chemical reactivity of Si 3 N 4 materials with O 2 . It is shown that the effective O 2 diffusion coefficient in the glassy oxide scales decreases with an increasing volume of devitrified phases and changes with the structural form of the oxide scales. This may explain why experimental oxidation kinetic curves usually deviate from the conventional parabolic growth law.

Formation, crystallisation and oxidation of selected glasses in the YSiAlON system

Abstract Oxynitride glasses were prepared which correspond to the compositions of the intergranular phase in Si3N4 ceramics fabricated with additions of Y2O3 and Al2O3 as sintering aids. Transparent glasses were obtained after melting for 10 h at a firing temperature of 1450°C. Two distinct melting temperatures of 1310°C and 1375°C were obtained for all compositions within the oxynitride glass-forming region. For all N-containing compositions the glass softening point (Mg) was 975°C which is an increase of ≈ 110°C over that of the corresponding oxide glass. Devitrification of the glasses in N2 required 12 h at 1200°C and the phases present after crystallisation included Y2Si2O7, Si2N2O and 3Y2O3.5Al2O3 (YAG). The oxynitride glasses oxidised rapidly in air at temperatures in excess of the softening point. Porous oxide ‘scales’ developed from the evolution of nitrogen was during oxidation and the main crystalline phases that formed in the scale were Y2Si2O7 and 3Al2O3.2SiO2 (mullite).

Oxidation of pressureless sintered Si2N2O materials

Abstract The oxidation behaviour of pressureless sintered Si2N2O materials prepared from both amorphous and crystalline starting powders has been examined. The materials exhibited excellent oxidation resistance at temperatures of up to 1350°C. Thin protective oxide scales formed which had a duplex morphology after long exposures to air at high temperatures. Substantial crystallisation of the intergranular glass phase with formation of Y2Si2O7 occurred during oxidation at 1200°C and 1350°C. Catastrophic oxidation occurred at temperatures ≥ 1400°C. This behaviour is enhanced by an oxidation-induced shift in the composition of the material to a liquid-forming region in the Y-Si-Al-O-N system.

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.

The effect of yttria additions on the composition of O′-sialons prepared by pressureless sintering

Abstract The compositional range of O′-sialons, Si 2−x Al x N 2−x O 1+x′ has been investigated for different preparative conditions. Samples made by pressureless sintering with and without additions of yttria, covering a compositional range corresponding to 0·04 x 20% in samples prepared with additions of yttria as a sintering aid. The amount of yttrium in O′-sialon crystals was found to be very low,

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.