Microstructure evolution during sintering of self-propagating high-temperature synthesis produced ZrB 2 powder
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A transmission electron microscopy investigation on the microstructure evolution of the sintered compact of zirconium diboride was carried out. Grain boundary phases, structure, and morphology of the grains and defect structures were studied to understand the sintering behavior of these borides. The impurities, such as Fe, were found to stay at the triple junction point as (Fe, Zr)2B. Different types of defect structures, such as dislocations, twins, and faceted growth, were observed. No diffusion of Fe into ZrB2 could be observed, but the Zr diffusion into Fe was established.
I. INTRODUCTION
Zirconium diboride is a potential advanced ceramic material for use at high temperatures requiring resistance to wear, oxidation, and corrosion.1,2 It has a very good thermal and electrical conductivity. Sintering of zirconium diboride is difficult due to its high melting temperature and covalent nature. Most often zirconium diboride powder is sintered in the temperature range 2000–2200 °C by hot pressing. Pressureless sintering studies are limited. Some reports are available on pressureless sintering where suitable metal powders have been used.3,4 In recent years, the self-propagating hightemperature synthesis (SHS) process has found applications for preparing advanced materials and is considered very important for fundamental as well as technological applications. Advantages and fundamental and technological aspects of SHS have been reviewed in the literature.5,6 Owing to higher heating and cooling rates during processing, it is possible that more defects are introduced in the powder that makes them more sinterable. Transmission electron microscopy (TEM) investigation on the sintered SHS produced zirconium diboride powder is important to understand the sintering behavior and also for its applications. Very few reports are available on TEM investigation of sintered zirconium/titanium diboride.7 In this investigation, zirconium diboride powder used was prepared by the SHS process using raw materials oxides of zirconium and boron. The process of powder preparation, SEM studies, and sintering kinetic studies of a pressureless sintered compact have been published elsewhere.8 The TEM studies on the sintered samples were carried out, and the results are reported. II. EXPERIMENTAL
Zirconium diboride was synthesized by the SHS process using oxides or salts of zirconium, boric acid, and magnesium or a compound of magnesium. Zirconium J. Mater. Res., Vol. 17, No. 11, Nov 2002
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diboride powder was pressed into 5-mm-thick pellets having a diameter 15 mm. Poly(vinyl alcohol) was used as binder during pelletization. The samples were kept in a graphite resistance furnace flushed with argon gas (IOLAR 1) after pumping out the air by a rotary pump. The samples were sintered at 1800 °C for 1 h under flowing argon gas followed by cooling at the rate of 15 °C/ min to 1300 °C and then furnace cooled to room temperature. The phase analysis of the sintered compact and the starting powders w
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