A scanning photoemission microscope (SPEM) to study the interface chemistry of AlTi/C system

Abstract

The interaction of liquid metals with various carbon substrates plays a key role in many areas of materials science from manufacturing to application in the development of advanced materials, such as, composites, ceramics for automotive, aerospace, electronic and thermal management industries [1, 2]. Liquid aluminum normally does not wet carbon materials (vitreous carbon or polycrystalline graphite) at temperatures lower than 1173 K because of an oxidized drop [3, 4]. Deoxidization of Al either by the capillary purification technique or by high temperature annealing in vacuum [5, 6] enhances wettability in the Al/C system due to an interfacial chemical reaction forming harmful aluminum carbide. Suitable alloying of a liquid metal may promote favorable interfacial reactions, which often control the mechanical properties of the end product. The effect of Ti in Al/C systems not only improves the wettability and compatibility [6–8] but also can be used as cast Al-Ti-C grain refiners for aluminum alloys [9]. By combining the spectroscopic chemical information with submicron spatial resolution, “spectromicroscopy” at the Advanced Light Source (ALS) at Berkeley has opened up an entirely new area for materials research using synchrotron radiation based analytical techniques [10–15]. By using the capability of this novel “Scanning Photoemission Microscope (SPEM)”, this paper addresses some key features ranging from chemical reactions at the interfaces in composite materials to electronic states and chemical bonding. The use of SPEM is expected to contribute to a broader understanding of the interfacial bonding in the Al-Ti-C system. In this study, a sessile drop technique [5] is used to deposit AlTi10 alloy on graphite substrates (E 28 type) in a vacuum of 1 mPa. Because of technological applications the AlTi10/graphite couples were obtained at a temperature of 1223 K when the alloy is in a semiliquid state consisting of Al3Ti precipitates dispersed in liquid aluminum matrix. Prior to deposition, all graphite substrates were polished, cleaned in acetone and vacuum annealed at 1223 K for 1 h. The test specimens were cross-sectioned and polished for microscopic examination of interfaces. The working principle of SPEM [16] is based on a zone plate [17], a circular diffraction grating to focus light from a pinhole object to a highly demagnified focus. In our study a focus (or spatial resolution) of 0.2-micron diameter has been achieved [18]. Photoelectrons emitted from the sample surface and interfaces were collected by a standard Omega electron analyzer, and the X-ray beam is rastered across the sample by a piezo driven stage controlling the lateral scanning of the zone plate. By selecting a photoelectron peak and measuring its intensity as a function of peak position, local area chemical state maps were produced. In this case, both photoelectron images and XPS Al(2p), C(1s), Ti(3p) core-level spectra were measured (at ho= 400 eV, base pressure ∼4× 10−10 torr) from the interfaces of AlTi10/E28 graphite. Additional cross-sections of AlTi6 and AlTi10 alloy were also analyzed for binding energy referencing [19, 20]. The binding energy values are referenced to the adventitious C(1s) line at 284.6 eV [21]. An important part of this study is to employ the capabilities of SPEM to find the exact location of TiC and Al4C3 phase formation at the melt/graphite interface. The graphite substrates were preheated to expel any adsorbed moisture in order to increase their surface energies to facilitate the chemical reaction between C and Ti to form TiC from the following reactions: