Improved densification by nano-sized sintering aids for Si 3 N 4
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Improved densification by nano-sized sintering aids for Si3N4 Liwu Wang,a) Wolfgang M. Sigmund,b) Sukumar Roy,c) and Fritz Aldinger Max-Planck-Institut fu¨r Metallforschung and Institut fu¨r Nichtmetallische Anorganische Materialien, Universita¨t Stuttgart, Pulvermetallurgisches Laboratorium, Heisenbergstr. 5, 70569 Stuttgart, Germany (Received 10 May 1999; accepted 28 September 1999)
The densification of Si3N4 with nano-sized sintering aids that were in situ incorporated by a combustion process was studied in comparison with that of sintering aids mixed by ball milling. The combustion process directly produces amorphous and nano-sized Y–Al oxides within the Si3N4 powder. X-ray diffraction results indicate that amorphous Y–Al oxides begin to crystallize into Y3Al5O12 at about 600 °C. Additionally the nano-sized sintering aids are more homogeneously distributed and thereby promote the formation of eutectic melts at lower temperatures during liquid-phase sintering. Therefore, the densification process of Si3N4 during liquid-phase sintering is strongly accelerated. The microstructure of as-sintered parts from combusted powder seems more dense and homogeneous.
I. INTRODUCTION
Si3N4 has excellent overall high-temperature properties comprising good chemical resistance, high strengthto-weight ratio, good wear and thermal shock resistance, and therefore is a potential candidate for advanced structural and functional applications.1,2 However, the densification of Si3N4 following the classical solid-state sintering techniques is not feasible due to the existence of strong covalent bonds between silicon and nitrogen atoms and hence low self-diffusivity of Si3N4. Liquidphase-assisted sintering is almost universally used as a method for densifying Si3N4-based ceramics to overcome the relatively low solid-state diffusivities of Si and N at temperatures below the decomposition temperature of Si3N4 (1920 °C at 1 bar N2).2 In order to densify Si3N4 this way, appropriate sintering aids such as Y2O3, Al2O3, and MgO are generally used to promote liquid-phase sintering via particle rearrangement and solutionreprecipitation processes.3 High strength and fracture toughness Si3N4 materials are sintered by using mixtures of Y2O3 and Al2O3 as additives. These oxides, in conjunction with the existing SiO2 layer on the Si3N4 particle surface, form a liquid phase according to the ternary system SiO 2 –Y 2 O 3 –Al 2 O 3 at temperatures above
a)
Address all correspondence to this author. e-mail: [email protected] b) Present address: Dept. of Materials Science and Engineering, University of Florida, Gainesville, Florida. c) Ceramic Technological Institute, Bharat Heavy Electricals Limited, Bangalore 560 012, India. 4562
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J. Mater. Res., Vol. 14, No. 12, Dec 1999 Downloaded: 11 Jul 2014
1400 °C, and lead to the densification of the materials. The earlier the formation of the liquid phase the better will the densification be. The degree of homogeneity of
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