ADVANCED MATERIALS & PROCESSES | OCTOBER 2025 42 SCALABLE MANUFACTURING OF THIN-FILM SHAPE MEMORY ALLOYS FOR ELECTRONICS, AEROSPACE SYSTEMS, AND ROBOTICS The use of industrial-scale physical vapor deposition allows thin-film products to be fabricated at scale and enables new high-tech, miniaturized applications for shape memory alloys. Sabrina Curtis Khanjur R&D, Silver Spring, Maryland Shape memory alloys (SMAs) derive their function- ality from a diffusionless, solid-state transformation between a high-symmetry austenitic phase and a low-symmetry martensitic phase. This reversible transformation, triggered thermally or mechanically, gives rise to two hallmark phenomena: (i) the shape memory effect, in which a deformed martensitic alloy recovers its original shape upon heating; and (ii) superelasticity, where stress- induced martensite reverts immediately upon unloading, allowing recoverable strains of several percent. A widely used SMA is near-equiatomic Nitinol (NiTi), which is valued for its transformation behaviors, biocompatibility, and work density. Alloying additions such as Cu, Hf, or Co enable ternary and quaternary systems, which extend transformation ranges and modify fatigue and hysteresis characteristics. SHAPE MEMORY ALLOY APPLICATIONS Limitations of Bulk Form SMAs. Commercial SMAs are typically produced as wires, torque tubes, or rolled sheets, geometries that simplify processing and allow actuation through tensile contraction, torsion, or bending. However, these bulk formats exhibit inherent limitations: (i) restricted recoverable strains of ~4–8% before fatigue or fracture; (ii) bulky packaging that precludes integration into microscale systems; (iii) predominantly one-dimensional actuation which is poorly suited for multifunctional devices; and (iv) limited compatibility with electronics fabrication processes. As a result, bulk SMAs remain largely confined to niche products such as eyeglass frames, guidewires, vascular stents, and high-force aerospace actuators. Broader adoption into electronics, robotics, and consumer products requires alternative formats capable of leveraging SMA functionality in reduced sizes and greater design flexibility. Thin-film SMAs for Multifunctional Integration. Thin-film SMAs fabricated by physical vapor deposition (PVD), as currently under development by Khanjur R&D, present a transformative format. Films ranging from tens of nanometers up to 100 µm can be deposited with precise stoichiometry, extending alloying options. Wafer-scale integration and lithographic patterning enable direct incorporation onto diverse substrates, providing tunability and flexibility suited for miniaturized systems. Thin-film SMAs can be fabricated as freestanding structures, by release from sacrificial layers, or retained on a substrate for device integration. Over the past three decades[1], thin-film SMAs have been demonstrated in implantable medical devices[2,3], micro-actuators[4,5], elastocaloric cooling[6,7], sensors[8], and stretchable electronics[9,10]. They are commercialized in microfluidics (e.g., Memetis GmbH) and implantable medical products (e.g., Acquandas GmbH). MATERIALS SCIENCE OF SMA THIN FILMS Alloy Systems and Transformation Control. The most widely studied thin-film alloys are TiNi-based, often with ternary or quaternary additions of Cu, Hf, Pd, or Co. These materials can modify the transformation temperatures of NiTi, namely –100° to +300°C. Thermal hysteresis and fatigue life can be improved. Moreover, PVD sputtering enables precise stoichiometry, crucial because slight deviations in Ni:Ti ratio can shift transformation temperatures substantially. Microstructural Characteristics. SMA thin films exhibit fine, uniform grain structures (tens to hundreds of nano- meters) compared to the coarse grains in drawn SMA wires. As-deposited SMA thin films are typically amorphous; post-deposition annealing is required to form the crystal structure that enables functional shape recovery and simultaneously serves as a shape-setting step. Nanocrystalline microstructures improve fatigue resistance, reduce hysteresis, and provide smooth interfaces for multilayer integration. Functional Fatigue and Cycle Life. Studies report TiNiCu[11] and TiNiCuCo[12] SMA thin films sustaining greater than 10 million transformation actuation cycles with minimal degradation. The ultra-low fatigue can present advantages where longevity and reliability are essential. FEATURE 6
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