Nanomechanical Characterisation of Graded NiTi Films Fabricated Through Diffusion Modification D. P. Cole*, H. A. Bruck* and A. L. Roytburd † *Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA † Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA ABSTRACT: Ni 47 Ti 53 films of varying thickness were deposited onto Ni 56 Ti 44 substrates. Annealing the films produced compositional gradients through diffusion modification. Nanoindentation mea- surements were used to probe the mechanical properties at various depths into the film. The films exhibited a variation in elastic modulus as a function of indentation depth that depended on the thickness of the film. A self-similarity principle permitted the mechanical properties of the graded NiTi films to be resolved further into substrate, beyond the contact depth of the tip. The observed variation of elastic modulus with indentation depth in the graded NiTi films was considered to be a combined response from changes in microstructure, substrate effects, and mechanically-induced phase transformations. This variation was predicted using an analysis that accounted for the transformation effects occurring under the tip during loading and the graded distribution of mar- tensite volume fraction obtained from diffusion modification. KEY WORDS: functionally graded material, nanoindentation, NiTi films, shape memory alloy Introduction Shape memory alloys (SMAs) have the unique ability to recover inelastic deformation upon being heated. The effect is due to a diffusionless first-order solid- solid martensitic transformation which consists of a low temperature martensite phase and a high tem- perature austenite phase [1–3]. SMA thin films are gaining attention as microactuators capable of high power and high frequency responses. They are being developed for a variety of microelectromechanical systems (MEMS) applications [4–6]. NiTi films are particularly attractive due to their high work density (10 7 Jm )3 ) [7] and biocompatibility [8]. Controlling the transformation of SMA films is currently achieved by changing the composition and microstructure of a single homogenous layer [9–11]. Most devices that implement SMA films as actuators only permit repeatable actuation behaviour by applying a biasing force to a homogeneous film [7, 12]. Films with compositional gradients have the added feature of an intrinsic two-way shape memory effect (SME) that greatly increases its functionality [13–16]. Our focus is to control the transformation characteristics of the films through gradient archi- tecture. Functionally grading the composition across the film thickness produces a membrane with a gradual variation in transformation temperature that exhibits a two-way SME as-fabricated. The shape memory effect in homogenous NiTi films has been studied using nanoindentation. Shaw et al. observed the partial recovery of indents in martensitic NiTi films on the nanoscale [17]. They proposed a zone model scheme that accounted for plastic deformation, martensitic rearrangement and elastic deformation. Ma et al. indented NiTi films in the high-temperature phase and induced the iso- thermal austenite-martensite transformation at a critical mechanical load [18]. However, the nanoin- dentation of SMA films presents several challenges. First, indenting thin films in general yields a response that is due to both the film and substrate properties. This complication is usually avoided by restricting indentation depth to less than 10% of the film thickness. Second, as Ma and coworkers showed, indenting an active material can induce a phase transformation that can also complicate the mea- surement. For example, the austenite phase of NiTi has an elastic modulus approximately 2–3 times that of the martensite phase [19, 20]. Lastly, our research effort deals with a graded material where the com- position and crystal structure varies throughout the film thickness. To fully understand the indentation response, all of the mentioned complications must be addressed. In this work, we demonstrate a new method for fabricating graded SMAs by modifying the surface of a nickel-rich NiTi substrate with a titanium-rich NiTi 232 Ó 2009 Blackwell Publishing Ltd j Strain (2009) 45, 232–237