Superelasticity of free-standing NiTi films depending on the oxygen impurity of the used targets
✍ Scribed by C. Zamponi; H. Rumpf; B. Wehner; J. Frenzel; E. Quandt
- Publisher
- John Wiley and Sons
- Year
- 2004
- Tongue
- English
- Weight
- 246 KB
- Volume
- 35
- Category
- Article
- ISSN
- 0933-5137
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✦ Synopsis
Abstract
Off‐stoichiometric NiTi sputter targets were manufactured by ingot metallurgy in order to produce Ni‐rich NiTi thin films. Crystalline samples, which undergo a martensitic transformation, were fabricated by magnetron sputtering and subsequent thermal treatment. The sacrificial layer technique was applied to obtain free‐standing NiTi films. The stoichiometry of the sputtered film is different to the stoichiometry of the target material. Sputtering targets with a Ti content higher than 49.5 at.% have a typical Ti‐loss rate in the order of 4.3 at.%, whereas at more Ni‐rich targets a Ti loss rate of only 0.5–1.5 at.% is observed. Due to this Ti loss rate target composites of Ti‐53.1Ni‐46.9 at.% and Ti‐53.2Ni‐46.8 at.% were taken. The NiTi films obtained from these two targets were compared. The main difference between the two targets is the impurity content of oxygen and carbon. Auger electron spectroscopy measurements were performed to determine qualitatively the content of oxygen in the free‐standing NiTi films. Transformation temperatures and hysteresis properties of the martensitic transformation were characterised by differential scanning calorimetry. Free‐standing films obtained from the Ti‐53.1Ni‐46.9 at.% and Ti‐53.2Ni‐46.8 at.% sputtering targets revealed superelastic properties at 45 °C as demonstrated by tensile testing. Tensile tests were taken at various temperatures of 40 °C, 45 °C, 50 °C, 55 °C and 60 °C, revealing e.g. a maximum superelastic strain of approximately 5 % at 45 °C.