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Mitsuishi, M. Song, and K. Furuya Nanomaterials Laboratory, National Institute for Materials Science, Tsukuba, Ibaraki 305 0003, Japan

K. Chattopadhyay Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India (Received 20 August 2003; accepted 22 December 2003)

Thin films with a nominal composition close to Ti62.5Si37.5 were deposited on NaCl substrate at room temperature by pulsed laser ablation to study the evolution of the intermetallic compound Ti5Si3 using a combination of high-resolution and in situ transmission electron microscopy. The as-deposited amorphous films contain Ti-rich clusters, which influence the phase evolution and the decomposition behavior of the amorphous film. These clusters influence the nucleation of a metastable fcc Ti solid solution (ao ⳱ 0.433 nm) with composition richer in Ti than Ti62.5Si37.5 as the first phase to crystallize at 773 K. The Ti5Si3 nanocrystals form later, and even at 1073 K they coexist with fine fcc Ti-rich nanocrystals. Subsequent Ar+ ion-milling of the crystallized film results in a loss of silicon. The composition change leads to the dissolution of the Ti5Si3 nanocrystals and evolution of a new metastable Ti-rich fcc phase (ao⳱ 0.408 nm).

I. INTRODUCTION

II. EXPERIMENTAL

In recent years, the intermetallic compound Ti5Si3 has attracted attention as a potential candidate for application as a high-temperature protective coating. It has a very high melting point (2403 K) and offers excellent oxidation resistance at high temperature. Among the several physical-vapor-deposition techniques that can be used to obtain silicide thin film, laser ablation technique seems most attractive primarily because of greater control on film stoichiometry.1,2 An earlier study has, however, reported complex crystallization behavior of the as-deposited amorphous film.3 According to this study, a metastable nanocrystalline fcc phase appears prior to the appearance of nanocrystalline Ti5Si3. Thin films deposited by sputtering did not report such a metastable phase.4 The current work, therefore, aims at understanding the mechanism of nanocrystallization of the as-deposited amorphous film through a detailed transmission electron microscopic study and explores its stability on heating and Ar+ ion irradiation. DOI: 10.1557/JMR.2004.0145 1118

http://journals.cambridge.org

J. Mater. Res., Vol. 19, No. 4, Apr 2004 Downloaded: 24 Mar 2015

The target alloy was prepared by melting high-purity Ti-sponge together with an appropriate amount of highpurity Si in a vacuum arc-melting furnace. The nominal alloy composition obtained was close to Ti62.5Si37.5, which corresponds to the intermetallic compound Ti5Si3. Thin films were deposited on freshly cleaved single-crystal NaCl substrates by pulsed laser ablation of this alloy target inside a custom-built vacuum ablation chamber with externally driven substrate and target rotation facilities at the Indian Institute of Science. A Spectra-Physics DCR-11 Nd-YAG laser with Q-switched pulsing facility was used. The laser pulse width w