The Effects of Processing Parameters and Additives on Sintering Behavior of Molybdenum and its Brittleness

• Main Authors: Hung-Shang Huang, 黃宏勝
• Other Authors: Kuen-Shyang Hwang
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/08048512725695688806

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 91 === Molybdenum is known as a refractory metal and thus is mostly fabricated through the powder metallurgy (P/M) process, in which fine molybdenum powders and high sintering temperatures are frequently used to obtain high sintered densities. However, without lowering the high oxygen content of fine powders, the compact sintered under vacuum usually shows poor ductility. The activation-sintered Ni-doped molybdenum also has severe brittleness problem. These embrittlement limits its application in the industry. The objective of this study was thus to investigate the causes and the solutions of the embrittlement for the both cases. The results can be divided into the following parts： For vacuum sintering, it was found that when 4.3-mm-thick molybdenum compacts were sintered at 1750℃ for 5 hours under a vacuum of 4 Pa, the compacts showed brittle fracturing. However, the ductility increased as the sintering time increased despite of severe coarsening of the grains. This was mainly attributed to the reduction of the MoO2 particles, which were segregated at the triple points and at the grain boundaries. Evidence of ductile fracturing can be found on specimens with 10 hours or longer of the sintering. The oxygen content decreased from 9270 ppm in the green compact to less than 200 ppm. The x-ray photoelectron spectroscopy (XPS) analysis, morphology evolution, weight changes, and the x-ray analysis indicated that the oxide was first present in the form of MoO3 on molybdenum powder surfaces. It was then transformed into MoO2 during heating. Two deoxidation mechanisms were identified：evaporation and decomposition of MoO3 and MoO2, with evaporation being dominant in the early-stage sintering and decomposition being dominant in the later stage. For activated sintering, it was found that when 1.5wt% nickel was added into the molybdenum compacts and sintered at 1300℃ for 1 hour under hydrogen, the densities of compacts increased drastically from 82.1% to 95.5%. However, the compacts showed brittleness with intergranular fracturing. The Auger electron spectrometer (AES) and the field emission transmission electron microscope (FEG-TEM) analyses indicated that a δ-NiMo intermetallic film, about 2 nm thick, was present at the grain boundaries. This compound shows a hardness of HV992 and is intrinsically brittle. It is thus believed that this compound is responsible for the embrittlement of the Ni-doped molybdenum. This finding excludes the possibility that the activated sintering is caused by the nickel layer or amorphous layer at the grain boundaries. The experiment of the Mo/Ni -25wt%Mo diffusion couple showed that the diffusivity of Mo in the δ-NiMo layer is about 30,000 times of the self-diffusivity of molybdenum. This suggests that the thin δ-NiMo layer serves as a fast diffusion path and imparts the activation effect for Ni-doped molybdenum. In addition, the sintering of the Mo-Ni-Cu-Fe system was attempted to improve the ductility of the activated sintering of the Mo-Ni system. It was found that when the Mo-4Ni-1Cu-1Fe compacts were liquid-phase-sintered at 1300℃, the compacts showed ductile fracturing because of disappeared the δ-NiMo intermetallic compound in the liquid area.