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Kumara) and S.V. Kamat Defence Metallurgical Research Laboratory, DRDO, Hyderabad 500 058, India

S.K. Sharma and S. Mohan Department of Instrumentation, Indian Institute of Science, Bangalore 560 012, India (Received 16 July 2011; accepted 17 January 2012)

Thin films of Ni-49 at.%Ti were deposited by DC magnetron sputtering on silicon substrates at 300 °C. The as-deposited amorphous films were annealed at a vacuum of 10
6 mbar at various temperatures between 300 and 650 °C to study the effect of annealing on microstructure and mechanical properties. The as-deposited films showed partial crystallization on annealing at 500 °C. At 500 °C, a distinct oxidation layer, rich in titanium but depleted in Ni, was seen on the film surface. A gradual increase in thickness and number of layers of various oxide stoichiometries as well as growth of triangular shaped reaction zones were seen with increase in annealing temperature up to 650 °C. Nanoindentation studies showed that the film hardness values increase with increase in annealing temperature up to 600 °C and subsequently decrease at 650 °C. The results were explained on the basis of the change in microstructure as a result of oxidation on annealing.

I. INTRODUCTION

Shape memory alloy (SMA) thin films based on nickeltitanium (NiTi) alloys of near equiatomic compositions have received considerable attention because of their potential for applications in microelectromechanical systems (MEMS) devices such as microactuators, microvalves and microgrippers.1–4 These shape memory thin films exhibit higher work density and larger recoverable strain as compared with other competing materials such as electrostatic, electromagnetic and piezoelectric materials.5 However, to use a NiTi film in microactuator applications, a precise control over its composition, structure and microstructure (hence its transformation temperature) and a good mechanical response is a prerequisite. In NiTi alloys, the shape memory effect arises due to the reversible phase transformation from high-temperature austenite (B2) phase to low-temperature martensite (B199) phase. Thus NiTi films have to be in crystalline form to show the shape memory effect. However, NiTi films produced by sputter deposition at low ambient conditions are usually amorphous in nature.6 Therefore, these films have to be annealed above their crystallization temperature to crystallize them.7,8 The effect of annealing temperature on the phase transformation behavior of NiTi alloy is very well established in the literature.9–11 Annealing treatment often results in oxidation of these films especially if the vacuum is Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.20 J. Mater. Res., Vol. 27, No. 7, Apr 14, 2012

http://journals.cambridge.org

II. EXPERIMENTAL A. Film deposition

Thin films of NiTi were deposited by DC magnetron sputtering from a NiTi alloy target (48:52: Ni:Ti at.%) on (100)-oriented Si wafer substrate heated to 300 °C.

a)

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not very high. The high-temperature oxidation behavior of eq