Acm ma~r. Vol. 46, No. 7, 2369-2379, pp. 1998 6 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved zyxwvutsrqpo Pergamon PII: S1359-6454(97)00393-5 Printed in Great Britain 1359-6454/98 $19.00 + 0.00 THE INFLUENCE OF Ca ON INTERFACE STRUCTURE AND CHEMISTRY IN MELT-INFILTRATED c(-A1203/Al COMPOSITES W. D. KAPLAN Department of Materials Engineering, Technion ~ Israel Institute of Technology, Haifa 32000, Israel Abstract--High resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM) were used to investigate the structure and chemistry of (0001) c+Al20JAl interfaces in melt-infil- trated polycrystalline a-A1203 composites. HRTEM revealed an 0.8 + 0.2 nm wide interfacial region struc- turally d@rent from both Al and c+A1203. AEM of the same interfaces revealed a Ca excess of r = 2.5 + 0.5 Ca/nm*. Since the metal-ceramic interfaces were the basal r-Al203 surfaces at pores before melt-infiltration, it can be concluded that Ca segregates to the basal surface of c(-Al203. Furthermore, the Ca at the free surfaces does not reside on only one cation plane, but is spread over 4 k 1 basal cation layers, and forms an interfacial phase with a nominal composition of Ca0.6A1203. The formation of the interface phase via segregation and reconstruction is discussed, based on microstructural observations of the c+A1203 matrix and possible roles of Ca in its development. The implications of the observations are the potential to control the ceramic-metal interface structure and chemistry via dopants (impurities) in the ceramic. I~I 1998 Acta Metallurgica Inc. 1. INTRODUCTION The properties of composites are often dictated by the nature of the interfaces between dissimilar ma- terials. Since interfaces are critical in modern ma- terial design, many scientific studies have focused on their structure, chemistry, and to some degree their properties. The present work focuses on the structure and chemistry of interfaces in a metal- ceramic composite, formed by melt-infiltration of Al into a porous cc-Alz03 preform. Since the crystal- lography of interfaces in melt-infiltrated composites depends strongly on the ceramic microstructure (grain size, grain shape and porosity), and the microstructure of a-A1203 depends strongly on the processing conditions, analysis of the interface structure and chemistry requires an understanding of the a-A1203 microstructure and the a-AlsO sur- faces at pores, the latter which dictate to a large degree the structure and chemistry of the metal- ceramic interfaces in the composite. The microstructure of sintered ~(-A120~ depends strongly on the presence of dopants and impurities. The role of dopants and impurities in sintering and grain growth of c(-A1203 has been a topic of study for some time (for a recent review of the technologi- cal development of high density c(-A1203 see [l]). The most commonly used dopant in a-A1203 pro- cessing is MgO, due to its ability to promote sinter- ing and prevent exaggerated (discontinuous) grain growth [24]. Common impurities in a-Al203 include Ca and Si, both noted for leading to exaggerated grain growth [5], even at such low concentrations as tens of ppm’s [6.7]. Of particular interest in the present study is the role of Ca in promoting exaggerated grain growth during sintering. Ca is a common impurity in many commercial powders, or can be introduced by the use of non-distilled water during water-based slip processing. As mentioned briefly above, Ca is known to cause exaggerated grain growth during sintering of cc-Alz03. However, the mechunism by which Ca leads to exaggerated grain growth is less understood. If indeed Ca is playing a role in varying the mechanism by which grain boundaries migrate, and thus causing exaggerated grain growth, then Ca may be active at grain boundaries and perhaps the surfaces of pores. A number of studies have clearly shown the segregation of Ca to “general” (high energy) grain boundaries in a-A1203 [8]. Li and Kingery used energy dispersive X-ray spectroscopy (EDS) in a dedicated scanning transmission electron microscope (STEM) to show Ca segregation to “general” grain boundaries in sintered s(-AllO [9], and Cook and Schrott conducted Auger analysis of grain boundaries after intergranular fracture to demonstrate Ca segregation [lo]. It should be noted that the term “segregation” is used here with some caution. In classical metallurgical terms, segregation is used to describe the transport of a specific el- ement(s) to a surface, regardless of the driving force for this process, whether it be that the specific el- ement is above the solid solubility limit in the grain, or if the process is one of equilibrium segregation where the element is transported to the surface in order to reduce the surface energy. Impurity segregation to the free surfaces of CI- A1203 is also an important issue, possibly affecting 2369