Phase formation in molybdenum disilicide powders during in-flight induction plasma treatment
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Phase formation in molybdenum disilicide powders during in-flight induction plasma treatment Xiaobao Fan, Takamasa Ishigaki,a) and Yoichiro Sato National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba-shi, Ibaraki 305, Japan (Received 2 August 1996; accepted 30 September 1996)
In-flight modification of MoSi2 powders has been carried out by using an Ar–H2 induction plasma. Reactor pressure, powder feed rate, and plate power level were taken as the experimental parameters to alter the thermal history of the injected powder particles. Metastable hexagonal structure of b –MoSi2 is the major phase observed in the Ar–H2 induction plasma-treated molybdenum disilicide powders, while the stable phase of tetragonal structure of a –MoSi2 usually retains no less than 30 wt. %. Depending on the experimental condition and the deviation from stoichiometry in raw materials, low silicides, Mo5 Si3 and Mo3 Si, and free Si were observed,
I. INTRODUCTION
The intermetallic compound, molybdenum disilicide MoSi2 , was previously proposed as a high temperature corrosion protective coating material for ductile metals, at the turn of this century. Over the past decade, rising interest in the development of MoSi2 stems from its significant potential to meet the demands of advanced high temperature structural applications in the range 1200–1600 ±C, in oxidizing and aggressive environment.1 Advanced materials for use at temperatures up to 1600 ±C must combine strength, creep resistance, fracture toughness, oxidation resistance, and mechanical and microstructural stability. MoSi2 based materials are receiving much attention, because of their unique combination of physical attributes. These may be enumerated as (1) high melting point (2030 ±C), (2) moderate density (6.27 gycm3 ), (3) excellent oxidation resistance, and (4) high modulus at elevated temperatures etc. It is also nontoxic and environmentally benign.1 Due to the intrinsically low ductility, MoSi2 is usually produced from powders consolidated by sintering and/or hot isostatic pressing (HIP) techniques. In the last ten years, a plasma-spraying process has been demonstrated by a number of researchers as a viable technique for producing dense monolithic and composite forms of MoSi2 .2–11 The plasma-spraying process has proved to be able to reduce segregation and residual stresses in MoSi2 that typically limit the formability of such a brittle material. The ductility and tensile strength have been improved.10
a)
Author to whom correspondence should be addressed. J. Mater. Res., Vol. 12, No. 5, May 1997
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The plasma-spraying process is characterized by a high enthalpy heat source in combination with a high quenching rate (up to 106 Kys) for the materials treated. These materials are commonly micro-size (10–100 mm) powder particles. The plasma-spraying process is therefore one of rapid solidification processes (RSP). Powders injected into a plasma are subjected to mod
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