Jeanette Persson

The oxidation kinetics of β-sialon ceramics

Abstract The oxidation behaviour of β-sialons (Si6 − zAlzOzN8 − z) with the composition z = 0·5 and 3·8, prepared without and with an addition of 3 and 6 wt% yttria, has been investigated isothermally during 20 h in a TG unit at temperatures between 1200 and 1450 C. A comparison is also given of the oxidation behaviour of β-sialons prepared from two Si3N4 powders with different amounts of impurities; and finally the oxidation behaviour of single-phase β-sialons with z = 1,2·5 and 3·8 is compared with that of Si3N4. Only a few of the obtained oxidation curves followed the parabolic rate law during the entire experiment. Most of the curves could, however, be interpreted with the modified rate law. ( δw A 0 )=a arctan(bt) 1 2 +c(t) 1 2 +d either exhaustively or in combination with the parabolic rate law. The parabolic rate constants and activation energies for the oxidation process could be determined and were found to increase with increasing amount of intergranular phase and with increasing amounts of impurities. The parabolic rate constants were also found to be lower for the β-sialons prepared without yttria than for ‘pure’ Si3N4, but the activation energy was about the same. The oxidation kinetics of the materials with z = 0·5 and 3 wt% yttria could not be conclusively determined, but the evolution of the microstructure of the oxide scale showed that its crystallization had an important influence on the oxidation rate.

Parabolic-non-parabolic oxidation kinetics of Si3N4

Abstract The isothermal oxidation behaviour of Si3N4, HIP sintered without additives, has been investigated thermogravimetrically in the temperature range 1250–1500°C. The formed oxide scales were found to be partly crystalline, and the obtained weight gain curves followed the parabolic rate law after a certain time, t0, but not during the entire oxidation experiment. In the time interval t Δw A 0 = a arctan , which is developed with the assumption that the cross-section area available for oxygen diffusion decreases during the experiment, due to a crystallization process and to formation of nitrogen bubbles within the oxide scale. The rate constants, Kp were calculated from the constants in the arctan function, and the activation energy could be determined to be 310±25 kJ/mol. The oxygen diffusion rates found are in fair agreement with those obtained for amorphous silica.

Oxidation behaviour and mechanical properties of β- and mixed α-β-sialons sintered with additions of Y2O3 and Nd2O3

Sialon ceramics have been prepared from two different Si3N4 powders obtained from different commercial sources, using Y2O3 and/or Nd2O3 as sintering aid. Although both powders were high-purity and high-quality silicon nitrides, surprisingly different phase compositions were obtained for the same overall starting compositions upon sintering at 1825°C. The phase compositions, densities, microstructures and mechanical properties of the samples obtained from the two powders have been compared. The samples prepared from one of the powders were oxidized at 1350°C for 20 h in dry oxygen in a TG unit. The oxidation curves obtained have been interpreted within the framework of a new rate law. The new rate law covers the observation that the oxidation behaviour of Si3N4-based ceramics is often non-parabolic, probably as a result of crystallization processes and the formation of cracks and/or bubbles within the oxide scale or at the scale/matrix interface during oxidation.

Non-Parabolic Oxidation Kinetics of Advanced Ceramics

Non-parabolic kinetics have been frequently observed for the oxidation of Si-based structural ceramics. To account for the non-parabolic behaviour, a new model has been developed, the mathematical form of which is Δw/A o =a aictan(bt+b)1/2 + c(t b ′ b −1)1/2 where Δw/A o is the measured weight gain per unit surface area, t is the time and a, b, b’ and c are constants. It is shown that this law yields apparently consistent parabolic rate constants, K p, and activation energies, E a, when applied to data obtained from the oxidation of fully dense polycrystalline Si3N4, Si2N2O and sialon ceramics.

Oxidation behaviour of Si3 N4 -based ceramics, studied by the thermogravimetric method

Abstract The oxidation curves of three different Si3N4-based ceramics are presented. In most cases, the obtained weight gain curves do not follow the parabolic rate law (ΔW/Ao)2 = Kpt + B. If crystallization occurs in the formed oxide scale during the experiment, the amorphous cross section area, A, through which oxygen most easily diffuses will decrease with time. With a function A(t), which describes the time dependence of the decrease of A, incorporated into the parabolic rate law one obtains a new rate law which reads ΔW/Ao = a★arctan√bt +c√t. It is shown that all oxidation curves can be explained within the framework of this rate law in combination with the parabolic rate law.

Oxidation Kinetics of Si2N2O Ceramics

Ceramics based on silicon nitride have been considered promising candidates for high temperature engineering applications due to their excellent refractory properties combined with high strength an ...