JOURNAL OF MATERIALS SCIENCE 27 (1992) 4089-4096 Solution effect on the slow crack growth resistance of dilute sialons at high temperatures I. TANAKA, G. PEZZOTTI, T. OKAMOTO, K. NIIHARA The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567, Japan High purity Si3N4 powder was sintered with only 0.5-1.0 wt% AIN by hot isostatic pressing (HIP) and its delayed failure behaviour at around 1400~ has been studied with interest to determine the role of AI atoms. AI atoms are found completely dissolved in the Si3N4 matrix as dilute sialon solid solutions by detailed analyses of phase composition. Flaw sensitivity increased considerably with the AIN addition, which was ascribed to the decrease in slow crack growth (SCG) resistance. Static-load lifetime under the same stress was remarkably decreased by the addition, which showed good agreement with that predicted from the experimentally obtained SCG resistance. Internal friction behaviour as well as microstructural investigation found no intergranuIar mechanisms for the SCG enhancement. Solution effect of AI and 0 atoms in the [3-Si3N 4 matrix and resulting virtual defect of covalent bonding was proposed to be their origin. 1. Introduction Silicon nitride based ceramics are one of the most promising materials for structural applications at high temperatures, such as gas-turbine engines. Usually the aid of oxide additives for 5 to 20 volume % is neces- sary for their densification by liquid phase sintering process. The liquid remains as glassy or secondary- crystalline phases after the sintering and the nature of these intergranular phases often determine the high temperature properties of the sintered materials. Engineering of these phases has been, therefore, con- sidered as the most important process for the design of Si3N 4 materials [1, 2]. On the other hand, little 'attention has been paid to the qualities of the matrix SiaN 4 grains. Especially when the interaction between crystalline defects and solute atoms is strong, solution effects must be signific- ant, as is often the case in metallic alloys. Some solute atoms may modify the electronic structure behind the mechanical properties of the matrix material con- siderably. But, few works have explicitly examined these effects in Si3N4, so far, probably because of the lack of a pure Si3N 4 material which can be used as a reference material for such investigations. As shown in our previous reports [3-6], high purity Si3N 4 powder can be fully densified without additives by hot isostatic pressing, and the sintered body shows super- ior delayed failure resistance up to 1400 ~ compared with other Si3N 4 materials sintered with various kinds of additives. Its composition and microstructures are very simple: it can, therefore, be considered as a refer- ence material for sintered Si3N 4. In the present study, influences of Al-addition on high temperature mechanical properties have been pursued, because A1 and O can be double-substituted to Si and N in ]3-Si3N 4 and form 13-sialon solid solu- tion [7, 8]. It is also important, because AI20 3 and A1N are often used as sintering aids for Si3N~. The Si3N 4 powder containing SiO 2 impurity of 2.4 wt % was mixed with 0.5 to 1.0 wt % of an A1N powder, and the solid solutions were made through reaction be- tween A1N, Si3N 4 and SiO 2. The effect of A120 3 addition for the same A1 concentration was also exam- ined for comparison. After studying the phase com- position and microstructures, effects of A1 and O atoms on slow crack growth (SCG) behaviour at high temperatures were carefully evaluated and their roles were discussed from the atomistic point of view. 2. Experimental procedure 2.1. Sample preparation Commercial high purity Si3N 4 powder (El0, Ube) used throughout the present work was identical to that used in our previous studies [3-6]. It contains oxygen, 1.3wt% (SiO2: 2.4wt%) and less than 100 wt p.p.m, of other impurities. Detailed impurity contents obtained by inductively-coupled radio- frequency plasma emission spectrometer (ICP) were shown in a previous report [4]. An A1N powder (Type F, Tokuyama-Soda) containing oxygen 0.9 wt% and cation impurities < 100 wt p.p.m, was mixed with the Si3N4 powder, 0.5 and 1.0 wt %, corresponding to 0022-2461 1992 Chapman & Hall 4089