Fabrication of gradient porous b-SiAlON ceramics via a camphene-based freeze casting process Zhaoping Hou n , Feng Ye, Limeng Liu, Qiang Liu School of Materials Science and Engineering, Harbin Institute of Technology, Building C3, Room 515, Science Park, Herbin 150001, China article info Article history: Received 4 May 2012 Received in revised form 20 August 2012 Accepted 21 August 2012 Available online 28 August 2012 Keywords: b-Sialon Freeze casting Porosity Microstructure Mechanical properties abstract Gradient porous b-SiAlON ceramics were prepared by unidirectional freezing of camphene-based suspensions on a cold substrate at 0 1C and subsequent sintering at 1800 1C for 2 h. Effects of solid loading content in the suspensions on porosities, pore structures and strengths of the porous ceramics were investigated. The results showed that a gradient microstructure consisting of aligned channels, dendritic and equiaxed pores from the bottom to the top of the sample was created by the freezing process. Both the microstructure observation and the pore size distribution indicated that with the increase of the solid content from 10 vol% to 30 vol%, the porosity decreased from 71.3% to 36.1% and the pore size decreased from 20 mm to 5 mm. As a result, the compressive strength increased significantly from 49 MPa to 313 MPa. The sample fabricated from the slurry with 10 vol% solid loading content showed a large strain for failure up to 9%. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Porous ceramics have attracted considerable attention as a new class of materials with a wide range of applications under high temperature and corrosive environments, such as gas filters, catalysis supports, high-temperature thermal insulations, light weight structural components and molten-metal filtrations [1]. Among these porous ceramics, SiAlON ceramics have aroused an increasing interest as promising engineering materials for higher temperature applications owing to their outstanding mechanical properties, superior thermal stability, high creep resistance and good oxidation/corrosion performance [2,3]. Porous SiAlON cera- mics are conventionally fabricated based on a straightforward processing route by means of partial sintering of powder mixtures which leads to the pore formation. However, the conventional route suffers from inherent limitations. On one hand, this method results in a relatively low porosity than 60%, higher porosity up to 90% is impossible. On the other hand, it is challenging for it to control pore structures, such as porosity, pore size and shape, because the pores are homogeneously buried in the microstruc- ture as a result of the straightforward processing route. The freeze casting recently emerged as a good candidate for fabrication of porous ceramics with controlled pore microstructure [4]. The typical process consists of freezing material suspension followed by sublimated and subsequent sintering to densify the walls, provides a scaffold with a unique porous microstructure which replicates the shapes of the connected frozen solvent crystal. As shapes of the frozen solvent crystal depend on the types of solvent, freezing rate and the freezing direction, the microstructure in obtained ceramics is highly tuned by varying process parameters. b-SiAlON is a solid solution of b-Si 3 N 4 with the general formula of Si 6 z Al z O z N 8 z . b-SiAlON (Si 6 z Al z O z N 8 z ) is com- monly produced by reaction sintering of a or b-Si 3 N 4 , Al 2 O 3 , and AlN as the starting powders. Due to the fast hydrolysis of AlN when in contact with water [5], the starting powders have to be processed in certain organic solvents. Camphene (C 10 H 16 ) has been shown to be a favorable sublimable vehicle in the prepara- tion of porous ceramic by freezing suspensions [6]. The striking advantage of camphene is the melting point of approximately 45 1C which allows more flexibilities in the process as it can be frozen and easily sublimed at room temperature. Most papers concentrate on the fabrication of homogeneous pore structure [7,8]. In terms of ceramics with graded structures produced by camphene-based freeze casting, only few studies were reported [9]. For special application, such as biocompatible [10], filtration [11]and thermal shock resistant structure [12], ceramics with graded pore structure were desired. The purpose of the present work is to propose unidirectional freeze casting as a route for fabricating porous b-SiAlON with interconnected and graded pore microstructure and to determine the important variables that control the pore microstructure characteristics and mechanical properties of the final product. 2. Experimental procedure The composition of the investigation was Si 4 Al 2 O 2 N 6, i.e. z ¼ 2 in the formula of Si 6 z Al z O z N 8 z for b-SiAlON. a-Si 3 N 4 (5 wt%b, Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.08.094 n Corresponding author. Tel/fax: þ86 451 86413921. E-mail address: houzhaoping@yahoo.cn (Z. Hou). Materials Science & Engineering A 558 (2012) 742–746