Sintering Behavior of Nanocrystalline Silicon Carbide Using a Plasma Pressure Compaction System: Master Sintering Curve
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MATERIALS with microstructures on the order of a few hundred nanometers are termed as nanoscale materials. Since the early 1980s, several researchers have investigated the effect of nanoscale microstructures on the properties of metals.[1–5] Kamigatio[6] suggested that nanoscale microstructures could lead to improved strength in mechanical ceramics. Silicon carbide (SiC) is an important structural ceramic that exhibits a unique combination of properties, such as very high hardness, good thermal shock resistance, high wear resistance, and chemical inertness at elevated temperatures. Nanoscale microstructures have been investigated in SiC. Kim et al.[7] developed nanograined SiC based ceramics and showed that these ceramics could be superplastically deformed at 1973 K (1700 C). Nanograined SiC reinforced with Si3N4 exhibits enhanced strength and toughness.[8,9] Apart MANISH G. BOTHARA, Postdoctoral Student, and SUNDAR V. ATRE, Associate Professor, are with the Oregon Nanoscience and Microtechnologies Institute (ONAMI), Oregon State University, Corvallis, OR 97331. Contact e-mail: [email protected] SEONG-JIN PARK, Associate Professor, is with the Pohang University of Science & Technology (POSTECH), Pohang 790-784, Republic of Korea. RANDALL M. GERMAN, Professor, is with San Diego State University, San Diego, CA 92182. T.S. SUDARSHAN, President, and R. RADHAKRISHNAN, Scientist, are with Materials Modification Inc., Fairfax, VA 22031. Manuscript submitted February 9, 2008. Article published online August 11, 2010 3252—VOLUME 41A, DECEMBER 2010
from the improvement in properties, nanograined ceramics led to other benefits. For example, fabrication of nanograined ceramics also involves the use of nanosized starting powders that have a much larger surface area to volume ratio than the larger particles. These finer particles can increase the process efficiency by reducing the sintering temperature and time.[10,11] The most common method of fabricating bulk SiC parts is by sintering. Sintering is a thermal treatment for bonding particles into a coherent, predominantly solid structure via mass transport events that often occur on the atomic scale.[12] The mass transport can occur either by solid-state diffusion of atoms or by motion of atoms through a liquid phase. The self-diffusion of SiC is extremely slow,[13] so various additives are used to assist sintering. Prochazka[14] demonstrated that B and C could be used as solid-state sintering aids for SiC. This was followed by the discovery of liquid-phase sintering using oxide additives such as Al2O3 and Y2O3[15] and oxynitrides such as AlN and Y2O3.[16,17] The oxide or oxynitride additives, along with the SiO2 on the surface of the powder particles, melt at temperatures lower than the usual sintering temperatures of SiC [~2273 K (2000 C)]. The liquid phase formed as a result of the melting of the additives facilitates mass transport to enhance densification, thereby reducing the sintering temperature. Conventionally, SiC is densified by hot pressing (HP) or pressureless sinte
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