Citation:
J. Rao, T. Roberts, K. Lawson, J. Nicholls, Nickel titanium and nickel titanium hafnium shape memory alloy thin films, Surface and Coatings Technology, Volume 204, Issue 15, 25 April 2010, Pages 2331-2336
Abstract:
Shape memory alloy (SMA) 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 as amorphous. A high temperature, 1100 à °C anneal followed by a low
temperature, 550 à °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,
(Ap) at around 105 à °C and an exothermic peak martensite phase transformation,
(Mp) at 65 à °C, resulting in a hysteresis of 40 à °C. For a NiTi coating
containing 65 at.% Ti the hysteresis remained unchanged at 40 à °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 â  8 to
â  11 J/g for the martensite phase. The NiTiHf coating shows SMA behaviour for
a film containing 30 at.% Hf. DSC reveals an â  Râ  phase transition in this
film. It is understood that this phase is present in films that have high
internal stresses and is understood to nucleate near Ti3Ni4 precipitates. Phase
transformation temperatures occur at 98 à °C and 149 à °C during heating and occur
at 99 à °C during cooling. Phase transformation enthalpies range between 2 and 3
J/g for the austenite phase and â  7 J/g for the martensite phase. A scratch
tester equipped with a 5 mm spherical tip has been utilised with loads ranging
from 1 to 5 N to determine the recovery properties of the films. The results in
this study conclude that NiTi films containing 65 at.% Ti deform 3 times more
than films containing 52 at.% Ti. For NiTiHf thin films, increasing the Hf
composition from 2 at.% to 30 at.%, doubled the deformation measur