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The dispersability of Si and Si3N4 powders in aqueous media was monitored by particle-size distribution, sedimentation behavior, viscosity/rheological studies, and electrokinetic behavior [zeta potential (ZP) analysis] as a function of pH of their slips. The pH values of 4 and 8 for Si and 10 for Si3N4 resulted in optimum dispersion, characterized by minimum in sedimentation height, minimum in viscosity, and maximum in ZP. The optimum slips of Si + Si3N4 mixtures conditioned in the pH range 8 to 10 were slip cast to obtain green compacts having a density in the range of 59% to 66% theoretical value. When nitrided, these compacts yielded nitride-bonded Si3N4 products having a density of 2.06 to 2.28 g cm−3, Si3N4 bonding phase of 20–60%, and 3-point flexural strength values in the range of 50 to 150 MPa. The microstructure consisted of very fine particles as well as fibrous or whiskerlike ␣−Si3N4 binding phase enveloping the matrix Si3N4, in total consisting of 90% ␣−Si3N4 and the rest being ␤−Si3N4 phase.

I. INTRODUCTION

Silicon nitride (Si3N4) ceramics have been recognized as potential candidates for high temperature, wear, and structural applications, e.g., in heat engine parts (diesel and gas turbine), in heat exchangers, in friction and wearresistant components (bearings), and as cutting tools. Si3N4 exhibits superior mechanical (strength and hardness) and chemical (oxidation and corrosion resistance) properties as well as thermal stability at high temperatures.1– 4 Among the two major types of Si3N4 products, namely, dense sintered Si3N4 (obtained by sintering, hot pressing, hot isostatic pressing, etc., of Si3N4 powder), and the porous reaction bonded silicon nitride (RBSN) obtained by reactive nitridation of silicon, the later has attracted the attention of ceramic researchers during the last 3–4 decades because of its near net-shape forming capability as well as its lower production costs.3–6 This process has two main drawbacks because it requires longer nitriding times (20–100 h), especially for thicker samples, and considerably larger amounts of porosity (10–20%), apparently retained in the structure that degrades their strength, toughness, and oxidation resistance at higher temperatures. To overcome these problems, two interesting approaches to Si3N4 product development have been followed. One is the nitridation of Si compacts containing various amounts of Si3N4 as inert filler, which reduces the nitridation time. In another approach, the post sintering of a RBSN product containing appropriate sintering aids has been followed to improve density by minimizing the large volume shrinkage associated with 386

http://journals.cambridge.org

J. Mater. Res., Vol. 17, No. 2, Feb 2002 Downloaded: 21 Jun 2014

the sintering of Si3N4 powder compacts. The later approach also lowers the production costs associated with the machining process.4,7,8 Focusing on the first approach of the nitriding of Si containing Si3N4 as an inert secondary phase, very few investigations are presented in the open literature.7–15 Pompe