Journal of the European Ceramic Society 17 (1997) 1901-1909 PII: SO955-2219(97)00087-3 Kinetics of Weight Changes and Developments During Oxidation Sintered j?-SIALONS R. Ramesh, P. Byrne, S. Hampshire and M. J. Pomeroy Materials Research Centre, University of Limerick, Ireland zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG Abstract The oxidation behaviour of yttria denscjied p-sialon materials (Si~6_Z~AIZ0,N~s_,~) with z-values of 0.2, 0.5, 1.0, 1.5, 2.0 and 3.0 at 1350°C is reported for times up to 256 h. The sialons can be grouped into two categories with respect to oxidation kinetics and predominant crystalline phases formed in the surface layers. For the z= O-2 to I.0 materials, changes in oxidation parabolic rate constant occur after 128 h and yttrium disilicate or cristobalite are the major crystalline oxidation products within the surface layers. For the higher z-value materials, no changes in parabolic oxidation rate are observed and mullite is the predominant phase formed. Examination of cross-sectioned specimens of the z =0.2 and 3.0 materials indicates changes in morphology consistent with those reported for the surface layers. The kinetics and mechanism of oxidation and morpholo- gical development of the oxide scale are discussed in terms of additive d$iision efects. 0 1997 Elsevier Science Limited. Resume’ Le comportement a l’oxydation de materiaux p- SiAlON (Si(6_Zl Al, 0, NC,_,,), denstjies par ajout dbxy de d’yttrium et ayant des valeurs de z de 0.2 ,0..5, l-0, 1.5, 2.0 et 3.0, a dte’ suivi a 1350°C pour des temps aliant jusqu ‘a 256 h. Ces materiaux peuvent etre group& en deux categories suivant les cinetiques dbxy dation et la nature des phases cristallines for- mees en surface. Pour des valeurs de z comprises entre 0.2 et 1.0, des evolutions de la constante para- bolique de la vitesse d’oxydation apparaissent apres 128 h de traitement. Les principaux produits dbxy- dation a l’interieur de la couche formee sont le sili- cate djrttrium ou la cristobalite. Pour les materiaux ayan t des valeurs de z > 1, aucun changement de la constante parabolique d’oxydation n’est observe, et 0 1997 Elsevier Science Limited Printed in Great Britain. All rights reserved 0955~2219/97/$17.00 + 0.00 Morphological of Pressureless la mullite est la phase pre’dominante. L‘observation en coupe de SiAlON de z=O.2 et 3.0 indique des evolutions de morphologie compatibles avec celles observees en surface. Les mtcanismes dbxydation et le developpement microstructural de la couche dbxydes sont trait& en terme d’eflets de d@ sion. 1 Introduction Silicon nitride-based ceramics are candidate mate- rials for high temperature structural applications because of their excellent high temperature strength. However, for long-term operation their oxidation behaviour is of major importance. It is well known that the oxidation resistance for nitro- gen ceramics densified with the aid of sintering additives depends on the residual secondary phases formed at the grain boundaries.1,2 These intergra- nular phases give rise to a polyphase scale during oxidation, which in general is not passive and det- rimental to oxidation resistance. In the case of /?- sialons, the z-value defining the fi-sialon phase (Si~6_,~Al,0,N,s_,~)has a significant effect on the amount and the viscosity of the intergranular phase formed and thereby on the nature of crys- talline phases formed in oxide scales during oxida- tion.3-6 For example, Schlichting,5 and Persson and Nygen6 have observed that materials with z- values between 1.0 and 2.0 oxidise more rapidly than materials with higher Al-substitution levels. Intervals between the z-values employed in these studies were, however, large and thus no definite effect of z-value can be assessed. Discontinuities in weight gain-time relationships have been observed7 and these have been related to changes in oxide scale morphology. Clearly, therefore, it is impor- tant to follow the chronological development of oxide scale morphology as well as increases in weight gain. Furthermore, most of the studies zyxwvutsrq 1901