Microstructure inhomogeneity in Al2O3 sintered bodies formed during the plasma-activated sintering process

Abstract

Much attention [1–9] has recently been focused on a new rapid-sintering method, the so-called plasmaactivated sintering (PAS) [1] or spark plasma sintering (SPS) method [2]. Similar to the hot-press method to some extent, the apparatus used for this technique was derived from an electric-discharge machine for sintering metallic materials [10] but is now employed to consolidate not only metals and alloys but also ceramics, composites, functionally graded materials and so on [3]. The salient characteristics of both PAS and SPS apparatuses are the employment of a direct pulse current for heating and the arrangement of the graphite mold and punches that act as heating elements. Although rapid densification is said to be attributable to the effect of plasma generated by a pulse current applied in both the PAS [1] and SPS [2] process, and although the neck formation between metallic particles observed by Tokita [11] was due to the presence of sparks in the plasma, the literature contains no convincing evidence of the occurrence of plasma when non-metallic materials are sintered by PAS or SPS. Because rapid sintering of such processes has not been sufficiently understood, it has also been called pulse electric-current sintering (PECS) [4, 6], and pulse discharge pressure sintering (PDPS) [7] by different research groups. Several investigations have been made from different standpoints to explore the effect of pulse current on rapid sintering in PAS and/or SPS. AlN ceramic was prepared from high-purity aluminum nitride powders by a PAS apparatus by Risbud and Groza [12]. The clean grain boundaries they observed in this AlN ceramic suggests that the plasma activation step in the PAS process efficiently removes impurities and/or oxide layers prior to rapid densification. On the other hand, Makino [13] measured the electric current passing through an Al2O3 compact in the SPS process and reported that it increased with a rise in temperature, reaching 100 mA at 1000 ◦C. He found that an electric current was able to pass through the Al2O3 compact but that the current was very weak compared with the operating current (generally more than 1000 A) applied in the sintering of Al2O3 powder. In another case of Al2O3 powder sintering by the SPS process [14], temperature distributions in the graphite mold and in the sample indicated that electric current went only through the graphite mold and punches. As revealed by the two findings mentioned above, i.e., a very weak electric current passing through an Al2O3 compact and no electric current going through the Al2O3 compact, respectively, no opportunity seems to be available for a pulse current to generate plasma among particles when Al2O3 powder was sintered by SPS. To clarify this matter, more experimental results are needed to investigate the effect of pulse power and the sintering process of PAS and SPS. The purpose of this letter is to help clarify this rapid sintering process by evaluating microstructures of Al2O3 sintered bodies that form during the PAS sintering process. Approximately 6 g of Al2O3 powder (CB-A05S, 99.6 wt % purity, Showa Denko), without any additives, was poured into a graphite mold (20 mm in diameter)