G. N. Babini

A diffusion model for the oxidation of hot pressed Si3N4-Y2O3-SiO2 materials

The parabolic oxidation behaviour of silicon nitride hot pressed from compositions of the Si3N4-Si2N2O-Y2Si2O7 subsystem studied in 98 kPa air and 1273 to 1673 K has been discussed in terms of a diffusion model in which the most relevant parameters are the amount of the grain boundary phase, the width of the diffusion zone and the concentration gradient at the Si3N4/oxide reaction interface. The model accounts for both kinetic (oxidation rate constants) and thermodynamic (apparent activation energy for oxidation) variations with amount and composition of the grain boundary phase and proves suitable for the extension to other additive systems. The apparent activation energy for oxidation ranged from 260 to 623 kJ mol−1 according to composition. It is suggested that a more appropriate evaluation of the thermodynamic parameters of the diffusion process must account for the variation of concentration profiles of the diffusing species with temperature.

Factors influencing structural evolution in the oxide of hot-pressed Si3N4-Y2O3-SiO2 materials

X-ray diffraction, SEM, EDS and WDS studies were performed of the structure and morphology of the oxide developed on hot-pressed compositions of the Si3N4-Si2N2O-Y2Si2O7 sub-system subjected to short-term (∼30 h) oxidation in air atT=1273 to 1673 K. Viscosity of the oxide glassy phase, ionic mobility and oversaturation degree of the atomic species involved were found to be the most important parameters in the control of structural evolution of the oxide. Clear links appear to exist between compositional parameters of the grain-boundary phase of the hot-pressed materials and the evolution of the crystal phases in the oxide. The ΣMi/Y ratio (ΣMi, total amount of metal impurities) in the grain-boundary phase, seems to play a major role in the transport properties of the oxide glassy phase.

Oxidation of silicon nitride hot pressed with Y2O3+MgO

Oxidation tests of silicon nitride hot pressed with combined Y2O3 and MgO additions in the compatibility field Si3N4-Si2N2O-Y2Si2O7, performed at 1193 to 1658 K under 98 kPa air atmosphere for 30 h, result in parabolic oxidation kinetics and three different oxidation regimes. At T<∼1400 (ΔH=120 kJ mol−1) oxygen diffusion is suggested to be the more probable limiting step for oxidation, whereas at higher temperatures, diffusion of additive and impurities appears rate-controlling (ΔH=580 kJ mol−1). The very high oxidation rates and ΔH values (ΔH=960 kJ mol−1) at T>1600 K are possibly associated with a softening of the grain-boundary phase. The existance of silicate solid solutions in the grain-boundary phase which are able to accomodate additive and impurity cations in their structure might explain the very good oxidation resistance of (Y2O3+MgO)-doped HPSN at T<1400 K.

Thermal instability of Si3N4/ZrO2 composites

Heat treatments of hot-pressed Si3N4MgOY2O3ZrO2 composites in an oxidizing atmosphere result in serious material instability at ∼ 800 < T < ∼ 1200 K where an oxygen controlled auto-catalytic oxidation mechanism possibly associated with minor amounts of zirconium oxynitride secondary phases is suggested to cause the catastrophic oxidation and loss of structural integrity of the material. The irreversible increase in the dimensions of the samples subjected to thermal expansion tests possibly derives from cracks generated by the permanent volume increase associated with oxidation of zirconium oxynitride phases.

Stability of Si3N4−Al2O3−ZrO2 composites in oxygen environments

Oxidation of dense Si3N4-Al2O3-ZrO2 and Si3N4-Al2O3 compacts, at 873–1773 K and 98 KPa air atmosphere, results in two different parabolic oxidation regimes. Oxygen diffusion is likely to be the governing step at low temperature (T<∼1623 K (ΔH= ∼100kJ mol−1)), whereas at T> ∼1623 K (ΔH = ∼800kJ mol−1) metal cation diffusion through the grain boundary phase appears limiting. The excellent stability in oxygen environments of the Si3N4-Al2O3-ZrO2 composites compared to other ZrO2-Si3N4 materials derives from (i) absence of easy-to-oxidize Zr-O-N phases; (ii) reduced amount of grain boundary phase, and possibly (iii) decreased solubilization rate of the nitride phases in the high viscous oxide film.

Oxidation of silicon nitride hot pressed with CeO2 and SiO2 additions

Abstract (CeO 2 + SiO 2 )-doped hot pressed silicon nitride shows parabolic oxidation kinetics in dry flowing air at T = 773 to 1673 K. Possible oxidation mechanisms have been proposed. The apparent activation energy for oxidation (∼ 350 kJmol −1 ) suggests migration of impurity cations through the grain boundary phase to the silicon nitride/oxide reaction interface as the probable rate-limiting step. The very low solubility of cerium in silicate melts appears to be an important factor for oxidation. Cristobalite and ceria were the main crystalline oxidation products, the genesis and morphology of CeO 2 crystals being strictly related to oxidation temperature and cooling rate. A previously unknown hexagonal high-temperature form of ceria crystallized in samples subjected to very fast quenching.