Aqueous tape casting of silicon nitride Bernd Bitterlich, Ju¨rgen G. Heinrich* Technical University Clausthal, Institute for Nonmetallic Materials, Department for Engineering Ceramics, 38678 Clausthal-Zellerfeld, Germany Received 8 June 2001; received in revised form 19 December 2001; accepted 6 January 2002 Abstract Slurries consisting of a low cost silicon nitride powder, sintering aids yttria and alumina, dispersants, binders, defoamers and water as a solvent were optimised for tape casting by electroacoustic and viscosity measurements and by casting experiments. The slurries exhibit shear-thinning behaviour due to the highly shear-thinning binder emulsion. Crack free tapes with a maximum thickness of approx. 250 mm and a binder content of 13–15 wt.% could be obtained. The green tapes with a high flexibility and green strength could be laminated easily by compression at room temperature. After sintering a dense microstructure developed. A significant shrinkage anisotropy parallel and perpendicular to the cast direction was observed. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Dispersion; Electrokinetics; Rheology; Si 3 N 4 ; Suspensions; Tape casting 1. Introduction Silicon nitride is a candidate material especially for engine applications because of an interesting combina- tion of thermal and mechanical properties. Although the reliability of silicon nitride has been proven by serial application mass production is restricted because com- ponents are still too costly compared to metallic com- ponents. Cost reduction can be achieved by using a low- cost powder, 1 by continuous sintering 2 and by using the tape casting technology for certain geometries e.g. sub- strates. Silicon nitride substrates can be fabricated much thinner than e.g. aluminium nitride parts because of the higher mechanical strength. 3 Tape casting is a widespread and cost effective method to produce flat, large area components with high quality. 4 Thin tapes are used mostly in the electro- nic industry. 5 Furthermore it is possible to build up multi-layered parts with complex geometries 6 or improved properties such as higher fracture resistance, 7 better thermal shock behaviour 8 or continuous transi- tion between properties. There are many publications dealing with tape casting of oxides but only few reports are available on the preparation of tapes made from nonoxide powders such as SiC or Si 3 N 4 , respectively. 6,9 In most publications organic solvents are used to dis- perse the powders. Water as a solvent has the advantage of being non toxic, non flammable, easy available and cheap, 10 but the process becomes more sensitive to casting parameters. 11 Silicon nitride reacts with water developing an oxidized surface. 12 However, this increase of the oxygen content due to hydrolysation is negligible because of the addition of oxidic sintering additives. A well deagglomerated suspension is necessary to get a high green density. 13 Silicon nitride can be well dis- persed in water in the basic pH range without organic additives. 14 But in a mixture with the most common sintering aids yttria and alumina coagulation may occur between the different powder particles. Therefore, it was the aim of this work to obtain a stable aqueous disper- sion of silicon nitride including the sintering aids with the aid of dispersants. Based on these results tape cast- ing slurries were prepared by adding a binder emulsion and a defoamer. The slurry composition was optimised with regard to casting suitability and tape properties. 2. Experimental procedure 2.1. Materials A low cost silicon nitride powder with high b-content (Silzot 1 HQ, SKW Trostberg Metallchemie, Germany) 0955-2219/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(02)00029-8 Journal of the European Ceramic Society 22 (2002) 2427–2434 www.elsevier.com/locate/jeurceramsoc * Corresponding author. E-mail address: heinrich@naw.tu-clausthal.de (J.G. Heinrich).