Characterization of nickel-titanium based thin films crystallized by fast and ultra-fast annealing methods
Motemani Sharabiani, Yahya
Date of Issue2012
School of Mechanical and Aerospace Engineering
Titanium-nickel based thin films can be used as actuators in micro-electro-mechanical systems (MEMS) in such applications as valves, pumps, grippers and sensors. These films are typically prepared using magnetron sputtering. The resultant films are amorphous if deposited at room temperature and annealing is necessary as actuation occurs in the crystalline state by a martensitic phase transformation. Consequently, understanding the nature of the annealing process is critical for microstructural control and optimization of MEMS actuators. The first part of the study is concerned with describing and correlating the evolution of structural, surfacial and nano-scale mechanical properties of amorphous Ti-Ni-Cu thin films crystallized by rapid thermal annealing (RTA), where the treatment takes place under a protected atmosphere by electromagnetic irradiation with fast heating rate (< 100 oC/sec). Long range order of amorphous Ti-Ni-Cu thin films was achieved in a few seconds (> 60 sec), with the optimum crystallization temperature of 480oC. The fast crystallization process allows the precise control of microstructure in a lower thermal budget (product of processing temperature and time). With increasing the annealing time (up to 180 sec), roughness increased dramatically, and was far more prominent than in films prepared by conventional thermal annealing (CTA). Although RTA is energy efficient due to the shorter annealing time, the film roughness is less ideal than CTA, which may prove limiting in specific applications. Using X-ray absorption spectroscopy (XAS), it was found that the RTA (180 sec) and CTA (1 hr) films possessed the longest range order. The evolution of the nano-scale mechanical properties of the RTA films during annealing was also studied, vii and demonstrated the suitability of nanoindentation as a simple and reliable tool to study crystalline domain formation in Ti-Ni-Cu thin films. In the second part of this study, the martensitic transformation, crystallization and thermal cycling behavior of Ti-Ni-Hf thin films were analyzed as a function of composition using a high-throughput array of nanocalorimeters. Amorphous as-deposited thin-film samples were crystallized by local heating at a rate of approximately 1.6×104 oC/sec, resulting in an ultra-fine (18 ± 5 nm) grain structure. The (Ti,Hf)-rich Ti-Ni-Hf alloy compositions with Ni contents below ~ 49.3 at.% exhibited the martensite-austenite transformation. The transformation temperature increases linearly with increasing Hf content in the range 14-20 at.% at a rate comparable to that observed for bulk materials, but with reduced martensite-austenite transformation temperature. The decrease in transformation temperature is caused by the fine nano-scale structure in Ti-Ni-Hf thin samples. The response of Ti-Ni-Hf thin films to high-temperature cycling (22ºC< T< 850ºC) changes with Ni concentration. For Ni ≤ 47 at.%, the transformation temperature increases during high-temperature cycling due to precipitation of (Ti1-x,Hfx)2Ni and the resulting enrichment of Hf in the surrounding matrix, while for Ni ≥ 47.7 at.% the transformation is gradually suppressed due to the lack of undercooling as a result of a reduction in transformation temperature. Low-temperature cycling (22ºC< T< 450ºC) resulted in a decrease and stabilization of the transformation temperature. Relaxation of internal stresses by dislocations generated during thermal cycling is suggested as the active mechanism. Thermal cycling stability of the films was improved compared to previous studies on bulk Ti-Ni-Hf. This was attributed to the very small grain size (18 ± 5 nm) of the samples. Alloys with superior thermal cycling stability; i.e. Ti39.6Ni46.1Hf14.3 and Ti42.3Ni42.5Hf15.2 are viii identified. The ability to control the transformation temperature through multiple thermal cycling has been demonstrated. A significantly improved level of time efficiency is achieved using an ultra-fast analysis of Ti-Ni-Hf sample library in a high-throughput manner.