THE EFFECT OF Si CONTENT, FIRING TEMPERATURE, AND UREA ADDITIVE ON THE PROPERTIES OF NITRIDE-BONDED SiC REFRACTORIES A. A. Nourbakhsh, 1 F. Golestani-fard, 2 and H. R. Rezaie 2 Translated from Novye Ogneupory, No. 10, pp. 68 – 72, October 2007. Original article submitted February 17, 2007. The microstructure/properties relationship of nitride-bonded SiC refractories has been studied in the presence of Si and urea additives. The samples were prepared by mixing, shaping, and firing at 1450 to 1600 o C in a con- trolled atmosphere. Physical properties were evaluated by measuring density, porosity, and cold crushing strength (CCS). XRD, SEM, and TEM were employed to study the phase and microstructural evolution. The increase of Si enhanced the bond formation, which contributed to density and strength improvement. The firing temperature had the same effect up to 1550 o C. At 1600 o C, however, the density and CCS de- creased, which were attributed to nitride phase decomposition. The urea addition improved the mechanical properties and increased the depth of nitridation. INTRODUCTION Silicon Carbide based refractories are valuable for their excellent thermomechanical and thermochemical properties [1], and are widely used in the ferrous, non-ferrous, and ce- ramic industries [2]. Their properties are highly dependent on microstructure and bonding system [3]. It has been well established that a nitride bond could provide improved high-temperature properties [4].The behavior of the nitride- bonded refractories, however, is efficiently related to the amount of silicon and additives [5]. Albano has studied the effect of the Si amount on the mechanical strength of Si 3 N 4 - bonded SiC refractories and has shown a linear relation between the bending strength of the composite and its den- sity [6]. It has been claimed that the most effective method to im- prove densification of Si 3 N 4 bodies is the addition of oxide and non-oxide additives. Single and mixed additives such as Al 2 O 3, MgO, Y 2 O 3, Y 2 O 3 +ZrO 2, or Al 2 O 3 +Y 2 O 3 are useful to enhance the mechanical properties at high temperatures [7 – 9]. The evolution of the bond system in the presence of dif- ferent amounts of Si and urea additives is the aim of this study. Also the diffusion depth of nitrogen in the presence of urea additive in nitride-bonded SiC refractories has been in- vestigated. EXPERIMENTAL PROCEDURE The a-SiC powder with particle size below 1.25 mm was supplied by ESK-GmbH. The Si powder was metallurgical grade with average particle size of 15 μm (Elkem Co.). Urea fine powder was analytical grade (Merck 12007). The com- Refractories and Industrial Ceramics Vol. 48, No. 5, 2007 383 1083-4877/07/4805-0383 © 2007 Springer Science+Business Media, Inc. 1 Research Center of Azar Refractories Company, Iran. 2 Department of materials and metallurgical engineering, Iran Uni- versity of science and technology, Iran. TABLE 1. Batch Composition of Samples (wt.%) Samples A B C D E F G H I J Additives: Urea – – – – – 0.5 1 1.5 2 3 Si 10 15 17.5 20 25 25 25 25 25 25 Binder 3 3 3 3 3 3 3 3 3 3