Deoxidation of molybdenum during vacuum sintering
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I. INTRODUCTION
THE high melting point, low thermal-expansion coefficient, and high thermal/electrical conductivity make molybdenum a popular material for power semiconductor components, glass-melting electrodes, and high-temperature structural parts. These molybdenum parts are mostly fabricated through the powder metallurgy (P/M) process, in which fine molybdenum powders and high sintering temperatures are used in order to obtain high sintered densities. Without adequate care, the deformability and weldability of P/Mprocessed molybdenum are, however, less than desirable.[1,2,3] The main reason is that the oxygen content is usually high in the starting powder, so oxides are easily segregated at the grain boundaries during sintering. This causes intergranular brittle fracturing even at room temperatures.[3,4,5] Previous studies suggested that to endure heavy mechanical deformation, such as extrusion, rolling, and forging, the oxygen content must be lower than 200 ppm[3] and even as low as 50 ppm.[4,6] One way to reduce the oxygen content in the sintered molybdenum is to start with clean powders and to sinter them in hydrogen. The obvious advantage of using hydrogen is its capability to reduce molybdenum oxides. Vacuum sintering is another process frequently used, because it has lower operational cost and the deoxidation can take place under a high degree of vacuum. Brewer and Rosenblatt showed that molybdenum and tungsten oxides both have relatively high evaporation rates compared to most other metals.[7] When high vacuum and high temperatures are employed, these metals can be deoxidized through evaporation of their volatile oxides.[8] In addition, the thermodynamics also suggests that molybdenum oxides can be decomposed when the oxygen partial pressure is low enough or when reducing agents, such as carbon, are present. Although the deleterious effect of oxygen on the ductility H.S. HUANG, Graduate Student, and K.S. HWANG, Professor, are with the Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, Republic of China. Contact e-mail: [email protected] Manuscript submitted June 4, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
of molybdenum has been known for some time, little information is available on how and to what extent the sintering parameters will influence the oxygen content. The detailed mechanism of deoxidation of molybdenum compacts under vacuum is also unclear. The first objective of this study was, thus, to investigate the effects of the sintering time, the sintering atmosphere, and the organic lubricant on the mechanical properties of molybdenum compacts. The second objective was to analyze the changes in the content of oxygen, carbon, and nitrogen and the microstructure evolution through different stages of sintering, so that the deoxidation mechanism can be better understood. II. EXPERIMENTAL PROCEDURE Fine molybdenum powder, with an average particle size of 2.5 m, was selected for this study. The characteristics and the morphology of the powder ar
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