Effect of Devitrification Temperature on the Microstructure of NiTi Films

Crystalline nickel titanium (NiTi) films are known to possess superelastic properties offering superior advantages in engineering applications where resilient mechanical response is essential. One such application of increasing importance in the biomedical field is the fabrication of micro-needles for transdermal or subdermal fluid transport, including the integration of micro-needles with silicon structures such as bio-MEMS assemblies. Unfortunately, the direct deposition of Ni-Ti alloys onto silicon is sometimes compromised by grossly mismatched thermal expansion coefficients resulting in delamination and spalling of the metallic films. This obstacle can be overcome by depositing the metallic alloy as an amorphous phase followed by a release of the amorphous film, and then a crystallization anneal (also called "devitrification" since it follows from a glassy phase) to develop the microstructure needed to induce the superelastic effect.  

This work seeks to understand the effect of devitrification temperature on the microstructure of crystallized, freestanding amorphous NiTi films and the kinetics of the crystallization transformation. This has been accomplished using transmission electron microscopy (TEM) combined with differential scanning calorimetry (DSC) and x-ray energy dispersive spectrometry (EDS). Results reported here indicate that the free surface of the film transforms differently and results in a different microstructure than the interior of the film. The grains that nucleate at the surface grow into the film in a columnar fashion and transform mostly between the rhombohedral phase and the austenite phase when thermally cycled between-100ºC and 200ºC. The interior of the film crystallizes separately from the columnar grains to form platelets. These platelet grains transform between martensite and austenite when thermally cycled between -100ºC and 200ºC.

Temperatures as low as 390ºC are reported here to partially crystallize amorphous films removed from a substrate, while crystallizing at 400ºC is shown to fully crystallize the amorphous film. The film crystallized at 420ºC for 200 minutes was fully characterized with TEM, electrons energy loss spectroscopy (EELS), EDS and DSC, showing CMOS compatible temperatures can be used to produce fully crystalline NiTi films that undergo phase transformations between martensite, austenite and an intermediate rhombohedral phase.