Nickel titanium and nickel titanium hafnium shape memory alloy thin films

Abstract

Shape Memory Alloys (SMAs) coatings of NiTi and NiTiHf have been deposited onto Si substrates using pulse DC sputtering. Coatings of NiTi with compositions containing 45 to 65 at% Ti have been fabricated by co-sputtering NiTi with Ti. NiTiHf coatings with Hf compositions ranging from 2 to 30 at% Hf have been fabricated by co-sputtering NiTi with Hf. XRD results reveal the as-deposited coatings are amorphous. A high temperature, 1100 o C anneal followed by a low temperature, 550 o C anneal was employed to crystallise the coatings. The XRD then shows the coatings to be martensitic at room temperature. Two sets of samples were produced for characterisation; one set was used for indentation studies and the other set used to prepare freestanding films required for differential scanning calorimetry, (DSC) studies. Using the DSC, a NiTi coating containing 52 at% Ti shows an endothermic austenite peak phase transformation, (A p) at around 105 o C and an exothermic peak martensite phase transformation, (M p) at 65 o C, resulting in a hysteresis of 40 o C. For a NiTi coating containing 65 at% Ti the hysteresis remained unchanged at 40 o C, but there was a decrease in the phase transformation enthalpies when compared with the coatings containing 52 at% Ti. Calculated phase transformation enthalpies in the NiTi coatings ranged from 6 to 13 J/g for the austenite phase and-8 to-11 J/g for the martensite phase.

Simple, scalable active cells for articulated robot structures

“…Nitinol has fundamental limitation in terms of thermodynamic efficiency [18], however efforts to miniaturize [19], [20], [21] and give microstructure [22], [23], [24] to Nitinol makes it a feasible method of actuation for scaling, as opposed to magnetic or electrostatic actuation methods which have fundamental challenges at very small scales. The choice of Nitinol also drives many of the design decisions such as the mechanical design of the cell, spring selection for a return force for the one-way SMA spring coils, and cell interconnects and mating, which are addressed in the following.…”

Active cells for redundant and configurable articulated structures

“…The choice of SMA Nitinol as the primary method of actuation, coupled with the goal of a minimalist approach to engineering the structure of the active cells, sets forth specific design challenges. Nitinol has fundamental limitation in terms of thermodynamic efficiency [20]; however several efforts in both miniaturization [21][22][23] and micro-machining or laser etching [24][25][26] has shown Nitinol to be feasible for making scalable actuators, as opposed to magnetic or electrostatic actuation methods that have fundamental challenges at very small scales. The choice of Nitinol also drives many of the design decisions, such as the mechanical design of the cell, spring selection for a return force for the one-way SMA spring coils, and cell interconnects and mating, which are addressed in the following.…”

Fabrication and Characterization of Freestanding NiTi Based Thin Film Materials for Shape Memory Micro-actuator Applications

“…Another benefit of the micro-system technology approach is that samples from different alloys can be fabricated and structured with almost identical process parameters, without the need of cold or hot work or forging which often lead to difficulties in the processing of brittle materials. Thus, beside extensive work on binary NiTi [13][14][15][16][17][18], sputtered films from NiTiCu [19][20][21][22], NiTiHf [23][24][25], NiTiPd [5,[26][27][28][29][30], and NiTiZr [23,31] have been investigated. Like NiTiCu bulk material, sputtered NiTiCu films reveal excellent functional stability during cyclic loading.…”