Wetting of Al/Sapphire (0001) System: Measurement Effect and Affecting Factors
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I.
INTRODUCTION
EXACT property measurements of interfacial phenomena such as contact angles and surface tensions are important for adhesion and joining applications, including the following: (1) low temperature (i.e., water/ polymer) and (2) high temperature (i.e., molten metal/ ceramic). The low-temperature systems have afforded original experimental approaches, discussed in earlier reviews. They face major technical challenges, however, including the following: (1) repeatability and versatility, (2) image analysis in both hardware and software, and (3) applicability.[1–13] Limited methods have been proposed for high-temperature systems, but challenges increase, especially on surface reactivity under certain conditions.[14] The open literature simplifies reports with contact angles only and often leaves surface tensions unattended. The majority of these reports lack good measurement techniques for both properties. As a result, semiempirical measurement methods are commonly used with variations of ±75 pct, which is comprised of data and the technical theories evolved.[4–13] An understanding of surface adhesion and its technology developments depends on surface science control and a platform of integrated properties, in order to advance further in this area. The objective of this article is to improve understanding of the Al/sapphire (0001) system arising in the measurement of properties under controlled oxygen conditions. To illustrate this, pure Al droplets (1020 JOAQUIN AGUILAR-SANTILLAN, formerly doctoral Student, Department of Materials and Metallurgical Engineering, The University of Alabama, Tuscaloosa AL, is Senior R&D Test Engineer, Intel Corporation, Chandler, AZ. Contact e-mail: [email protected] Manuscript submitted August 18, 2008. Article published online April 29, 2009. 376—VOLUME 40B, JUNE 2009
grade) resting on a polished sapphire (0001) substrate are studied in both phases: (1) wetting and (2) nonwetting. The main outcomes are as follows: (1) a developed measurement technique, stable and capable, (2) data from the literature for comparison of the Al/sapphire (0001) system under similar experimental conditions, and (3) our recommendations and future key studies for the Al-sapphire system.
II.
SURFACE TENSION AND WORK OF ADHESION EFFECTS
The interface relationship of the surface tensions in an equilibrium sessile drop (SD) geometric configuration is usually expressed by Young’s equation:[15] c csl cos / ¼ sv ½1 clv or cos h ¼
csl csv clv
½2
where / is the contact angle, clv the surface tension of the liquid, csv the surface tension of the solid, and csl the surface tension of the interface. With reference to Figure 1, some authors define the contact angle as h, but that definition will not be used here. Another important parameter is the interface adhesion defined by Dupre´[16] as the work of adhesion (Wslv); this is the work or energy per unit area of an interface that is required to reversibly separate a solid/ liquid interface. The process creates both a solid-vapor interfa
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