Original Research Paper Sintering behaviour of silicon nitride powders produced by carbothermal reduction and nitridation Nuray Karakus ß a,⇑ , A. Osman Kurt a , Cihangir Duran b , Cem Öztürk c , H. Özkan Toplan a a Sakarya University, Engineering Faculty, Dept. of Metallurgy and Materials Engineering, 54187 Sakarya, Turkiye b Gebze Institute of Technology, Dept. of Materials Science and Engineering, 41400, Gebze-Kocaeli, Turkey c MSE Technology Ltd., Sultan Orhan Mah. Hasköy Industrial Site, 7 Blok No. 6, Gebze-Kocaeli, Turkiye article info Article history: Received 7 June 2012 Received in revised form 3 December 2012 Accepted 26 December 2012 Available online 11 January 2013 Keywords: Sintering Tape casting Si 3 N 4 Densification Cold isostatic press abstract In this study, the sintering behaviour of silicon nitride (Si 3 N 4 ) powders (having in situ form sintering aids/ self-sintering additives) produced directly by the carbothermal reduction and nitridation (CRN) process is reported. The sintering of as-synthesised a-phase Si 3 N 4 powders was studied, and the results were com- pared with a commercial powder. The a-Si 3 N 4 powders, as-received contains magnesium, yttrium or lith- ium–yttrium-based oxides that were shaped with cold isostatic pressing and tape casting techniques. The compacts and tape casted samples are then pressureless-sintered at 1650–1750 °C for up to 2 h. After sin- tering, the density and the amount of b-phase formation were examined in relation to the sintering tem- perature and time. The highest density value of 3.20 g cm 3 was obtained after only 30 min of pressureless sintering (at 1700 °C) of Si 3 N 4 powders produced by CRN from silica initially containing 5 wt.% Y 2 O 3 . Silicon nitride powders produced by the CRN process performed similarly or even better than results from the pressureless sintering process compared with the commercial one. Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. 1. Introduction Silicon nitride (Si 3 N 4 )-based structural ceramics have been ex- plored since the late 1960s, primarily for use in high temperature and structural applications such as heat engines [1]. Taking into ac- count their unique combination of properties, Si 3 N 4 and related materials have become the most thoroughly characterised non- oxide ceramics, with a wide range of applications including heat exchangers, turbine and automotive engine components [2], valves and cam roller followers for gasoline and diesel engines [1,3]. The widespread use of Si 3 N 4 ceramics has been limited by their low mechanical stability, their difficulty in machining, and the high manufacturing costs of their components [4], especially for fabri- cating parts with complex shapes. The manufacturing cost of such ceramics might be reduced using low-cost (but high-grade) start- ing powders such as powders from the carbothermal reduction and nitridation (CRN) process [5,6] and easy and economical pro- duction techniques, like pressureless sintering [7]. In addition, the colloidal processing has been reported as a cost-effective route to prepare ceramics of complicated shape with improved stability [8]. Various colloidal methods have been attempted to prepare bulk Si 3 N 4 samples, including tape casting [9], slip casting [10] and gel casting [1]. Silicon nitride is characterised by covalent bonds and very low atomic self-diffusion. Thus, liquid-forming sintering aids, such as MgO, Y 2 O 3 , Al 2 O 3 and rare earth oxides added singularly or in com- bination, must be used to achieve the theoretical density [8–11]. Liquid-phase sintering of Si 3 N 4 is conventionally considered to be a multi-stage process that includes (i) particle rearrangement aided by the lubricant action of the liquid, (ii) dissolution and crys- tallization of very fine grains and (iii) dissolution and re-crystalli- zation of coarse grains [12,13]. Thus, the sintering behaviour of Si 3 N 4 , which includes densification, a-to-b phase transformation, and grain growth, is influenced substantially by the amount and chemistry of the liquid phase [12]. The most common sintering methods used to consolidate Si 3 N 4 -based ceramics are as follows: (i) reaction bonding (RBSN) [14], (ii) hot pressing (HPSN), (iii) hot isostatic pressing (HIPSN) [15], (iv) sintering (SSN) [16], (v) gas pressure sintering (GPSN) [17] and (vi) sintering reaction bonding (SRBSN) [18]. Regardless of the areas of its application, the main concern in the sintering of Si 3 N 4 is to achieve the desired densities using an economical method. Pressureless sintering (SSN), in this respect, is a favourable method to be chosen because it is an easy and a suitable technique for continuous mass production. How- ever, it is rather difficult to achieve high densities using this meth- od without using very high amount of sintering aids, which has the drawback of creating a glassy intergranular phase [19,20]. 0921-8831/$ - see front matter Ó 2012 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2012.12.004 ⇑ Corresponding author. E-mail address: nurayc@sakarya.edu.tr (N. Karakus ß). Advanced Powder Technology 24 (2013) 697–702 Contents lists available at SciVerse ScienceDirect Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt