Tailoring microstructure, mechanical and tribological properties of NiTi thin films by controlling in-situ annealing temperature Soroush Momeni a, ⁎, Johannes Biskupek b , Wolfgang Tillmann a, ⁎ a Institute of Materials Engineering, Technical University of Dortmund, Leonhard-Euler-Str 2, 44227 Dortmund, Germany b Electron Microscopy Department of Materials Science, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany abstract article info Article history: Received 15 March 2016 Received in revised form 19 February 2017 Accepted 22 February 2017 Available online 24 February 2017 Magnetron sputtered near equiatomic NiTi thin films were deposited on Si (100) and hot work tool steel sub- strates. The deposited thin films were in-situ annealed at four different temperatures viz., 80 °C, 305 °C, 425 °C, and 525 °C. The effect of the in-situ annealing temperature on the microstructure of the film, the morphology, as well as mechanical and tribological properties was studied using X-ray diffraction, synchrotron diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, ball-on-disc, scratch test, and three di- mensional optical microscopy. The obtained results revealed how the variation of in-situ annealing temperature affects the crystallization, microstructure evolution, as well as mechanical and tribological properties of NiTi thin films. © 2017 Published by Elsevier B.V. Keywords: Nickel titanium Shape memory alloy Thin films Wear Adhesion Microstructure Tool steel 1. Introduction NiTi shape memory alloy (SMA) thin films with an equiatomic or near equiatomic composition ratio are an intermetallic compound of nickel and titanium. These thin films have long been classified as smart materials or ingredients of smart systems, due to their unique properties such as shape memory effect (SME) and superelasticity (SE). These unique properties are consequences of a reversible phase transformation capability from austenite to martensite phase and vice versa. Shape memory effect refers to the ability of NiTi to return to its original shape upon heating after an apparent plastic deformation. Superelasticity, however, is the ability of this alloy to return to its origi- nal shape upon unloading after being significantly strained. NiTi thin films must be well crystallized to demonstrate SME and SE. The crystallization of these thin films can be conducted by employing either a post-annealing treatment (annealing after deposition) or an in-situ annealing technique (annealing during deposition) [1]. Shape memory effect of NiTi thin films make them suitable materials for microelectromechanical systems (MEMS) [2] and thermomechanical data storage technology [3]. NiTi coatings are biocompatible [4] and can be used to deposit corrosion [5] and cavitation [6] resistant coating systems. The emerging thin film technology has further expanded the use of these coatings for medi- cal devices [7]. For the last two decades, NiTi coatings were mainly investigated as reliable thin films within these broad fields of appli- cation. There are considerable number of high quality researches that investigates the deposition [8], processing optimization [9], mi- crostructure [10], morphology [11], mechanical properties [12], phase transformation behavior [13], and crystallization [14] of these thin films. One issue which has not been properly investigated yet is the tribological performance of NiTi thin films, particularly their wear resistance and adhesion strength to the substrates. Tribological performance of NiTi thin films is an important issue of concern. For example, the adhesion of surface micro-machined coatings to the underlying substrate can adversely affect efficiency and applicability of the thin films in MEMS [15]. Despite this impor- tance, only few attempts have been made to analyze the wear and adhesion of NiTi thin films which were post-annealed after deposi- tion [16,17]. The authors of this paper have recently reported the in- fluence of the in-situ and post-annealing technique on the microstructure, phase transformation and tribological behavior of NiTi thin films [18]. It was found out that the in-situ annealed coatings have a remarkably better wear resistance and adhesion strength to the substrates when compared to post-annealed coatings. Due to the efficiency of in-situ annealing technique, it is worthwhile to further study the influence of in-situ annealing temperature on microstructure, mechanical and tribological perfor- mance of NiTi coatings. Thin Solid Films 628 (2017) 13–21 ⁎ Corresponding authors. E-mail addresses: soroush.momeni@tu-dortmund.de (S. Momeni), johannes.biskupek@uni-ulm.de (J. Biskupek), wolfgang.tillmann@udo.edu (W. Tillmann). http://dx.doi.org/10.1016/j.tsf.2017.02.052 0040-6090/© 2017 Published by Elsevier B.V. Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf