Synthesis of TiN/Si 3 N 4 composite powders by mechanically activated annealing
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iN/Si3N4 composite powders were obtained by a process that combines the mechanical activation of titanium and silicon powders at room temperature through high-energy milling with an isothermal annealing in a nitrogen atmosphere to complete the synthetic reaction. Mechanical activation has allowed us to complete the synthesis at 1350 °C only. The –Si3N4 content in the final powder tends to increase as the milling time is prolonged. The microstructure of the TiN/Si3N4 composite powders has a bimodal character composed of TiN and –Si3N4 grains and ␣-Si3N4 nanowires. Diameters of the nanowires range from 10 to 70 nm.
I. INTRODUCTION
Silicon-nitride–based ceramics are excellent candidates for high-temperature structural applications because of a good combination of mechanical, thermal, and thermochemical properties, such as high temperature strength, oxidation resistance, thermal shock resistance, low density, and low coefficient of thermal expansion.1 Application of the monolithic material is generally limited by a low toughness, which produces lifetime and reliability problems that are difficult to solve. The ␣–Si3N4 phase is the generally preferred raw material for manufacturing compacted bodies because of the favorable microstructural characteristics obtained during the ␣– to –Si3N4 transformation at high temperatures. However, as has been shown, increasing the fracture toughness of the monolithic ceramic requires a bimodal microstructure composed of fine matrix grains and a few large, elongated –Si3N4 grains.2 On the other hand, the form of whiskers has gained increasing importance due to the fact that Si3N4 whiskers can be used as reinforcement in ceramic–matrix composites having high toughness.3–5 More recently, the synthesis of Si3N4 nanowires has received extensive attention for its interesting properties (high strength with good flexibility) and potential applications in the fields of nanoelectronics and nanomechanics.6–8 From a technological point of view, silicon-nitride– based ceramics need to possess a precisely tailored set of properties for demanding applications. In this sense, the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0107 864
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 4, Apr 2005 Downloaded: 17 Mar 2015
addition of secondary phases (fibers, whiskers, or particles) can induce specific functions as well as electrophysical, chemical, and other properties to be specifically modified.9–12 The addition of the second phase reinforcement does not reduce the essential physical property advantages of the ceramic matrix. Much attention is devoted to particulate composites due to the high cost of fibers and whiskers and process problems associated with their fabrication. Silicon carbide–silicon nitride composites have received a great deal of attention because of their potential high strength and toughness.13 More recently, the development of composites with an electrically conductive secondary phase such as TiC, TiN, and ZrN has offere
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