Effect of preparation route on the properties of slip-casted Al 2 O 3 /YAG composites F. Sommer a, * , F. Kern a , H.F. El-Maghraby b , M. Abou El-Ezz c , M. Awaad b , R. Gadow a , S.M. Naga b a Institut fu ¨r Fertigungstechnologie Keramischer Bauteile, Universita ¨t Stuttgart, (Institute for Manufacturing Technologies of Ceramic Components and Composites, University of Stuttgart) Allmandring 7b, D-70569 Stuttgart, Germany b Ceramics Department, National Research Centre (NRC), 12622 Dokki, Giza, Egypt c Graduate School of excellence for advanced Manufacturing Engineering, Stuttgart University, Allmandring 7b, D-70569, Germany Received 13 December 2011; received in revised form 20 February 2012; accepted 23 February 2012 Available online 3 March 2012 Abstract Alumina composites containing 5, 10 and 20 vol.% YAG were produced by a slip-casting process. Two different routes for adding the YAG phase were chosen. Either by mixing of alumina with previously produced YAG powder or by coating of an adequate amount of yttria to form YAG precipitates during the calcining step. The mechanical properties of the sintered samples were measured and compared. Independent from the preparation route Al 2 O 3 /10 vol.% YAG composites showed best mechanical properties. Significant differences in the densification behavior appeared between the two different manufacturing technologies. The presence of intermediately formed phases like YAM and YAP during the powder preparation step of coated composites is probably the reason why mixed composites showed higher mechanical properties than coated composites. # 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Slip casting; B. Microstructure-final; B. Nanocomposites; C. Mechanical properties; Al 2 O 3 –YAG 1. Introduction In recent years, considerable efforts have been made to develop high performance structural materials which can be used at high temperature with superior mechanical properties [1]. For several decades non-oxide ceramics such as SiC and Si 3 N 4 have been developed as promising competitors to replace Ni-based superalloy. The room temperature brittleness, the high temperature instability and especially the serious oxidation above 1500 8C extremely restrict their application. Oxide/oxide composites are being considered for long-term high temperature operations in an air environment. These composites require thermo-chemical compatibility with their application environment as well as thermo-mechanical compatibility with each other [2]. Creep behavior and mechanical properties of alumina ceramics can be improved by dispersing yttrium aluminum garnet (YAG) inclusion in the alumina matrix [3,4]. It was reported that the bending strength of YAG/Al 2 O 3 composites at room temperature – around 400 MPa – can be maintained almost up to the melting point of 2093 8C when the displacement speed is high [5–9]. Accord- ingly, YAG/Al 2 O 3 has received much attention in high temperature applications. The synthesis process of YAG/Al 2 O 3 composites has an obvious impact on the powder characteristics. In other words, powders synthesized by different methods have different sintering behavior. Some articles study YAG/Al 2 O 3 composites prepared by the directional eutectic crystallization [9–12]. On the other hand, a great deal of research has been made to synthesize ultra fine powders with particle size less than 100 nm. Nano-powders have significantly enhanced sintering rates and decreased sintering temperatures compared to micrometer–sized particles due to higher surface area [13–16]. Sol–gel technique is a promising method for the preparation of nano-sized inorganic powders with homogeneous mixture of several components at a molecular level [17,18]. It was used for the preparation of YAG/Al 2 O 3 composite fibers [19]. Lach et al. [20] elaborated a new technique for Al 2 O 3 /YAG composites’ preparation covering a broad range of YAG contents from 5 to 30 vol. %. The technique is based on precipitation of yttria www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 38 (2012) 4819–4826 * Corresponding author. Tel.: +49 711 68568234; fax: +49 711 68568301. E-mail address: frank.sommer@ifkb.uni-stuttgart.de (F. Sommer). 0272-8842/$36.00 # 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2012.02.070