ADVANCED MATERIALS & PROCESSES | MARCH 2023 32 free component can be further processed by heat treating, coating, electropolishing, or other typical post-processing. PROPERTIES AND BENEFITS PVD materials typically have finer grains, higher tensile properties, higher purity, improved biocompatibility, smooth finishes, and higher fatigue and corrosion resistance than their wrought counterparts. For example, grain sizes in PVD Nitinol are typically an order of magnitude smaller with inclusion sizes two orders lower than traditional wrought Nitinol. Figure 3 compares tensile curves of binary Nitinol produced by PVD and wrought processes, and includes data for a PVD NiTiCo alloy as well. Because of the stable substrate, these intermediate materials can be grown at their net shape, for example at the expanded diameter of a finished stent. Micron-to-millimeter sized features can be created repeatably due to the support from the underlying substrate. The fine finishes allow for incorporation of engineered surface features. An example of these feature sets is shown in Fig. 4. The substrate support also allows for long device fabrication without cantilever effects. This reduces FEATURE post-processing and retains the higher materials properties. This process allows designers new freedoms that are not practical or possible using current reductive wrought material processing. Ternary alloys can add inherent radiopacity or even higher strength. Predictable feature sizes and finishes enable integration of additional functionality. Since the PVD process does not use drawing dies, or other wearable tooling, it is easy to produce ternary Nitinol alloy materials that are difficult to work using reductive technology. Alloys can be quickly fabricated by using doping or co-sputtering[5] during the deposition process, allowing for rapid alloy development. The PVD process is run by an industrial computer and does not require operator intervention, other than to load and unload a magazine of substrates. Final microstructure, properties, and dimensions are controlled by the deposition software recipe and can be easily manipulated. The minimal labor content and reduced processing steps makes PVD devices cost competitive with wrought processing over some range of sizes. Since the only prerequisite for a new size or configuration is the substrates, new sizes and shapes can be made very quickly, speeding product development timelines. Intricate substrates can even be produced utilizing 3D printing. FORMATS AND APPLICATIONS There are various formats of PVD films available and many more that are possible, given the need. Flat sheet discs and cylindrical tubing are the most commonly available formats. Non-cylindrical or 3D net shape films are less common. Layered films of various materials are also less common. For instance, a tri- layer sandwich film of Nitinol-Tantalum-Nitinol shows excellent full device radiopacity without the need for adding radiopaque markers. Flat sheets of 4 to 6-in. diameters are produced using planar chambers, while cylindrical chambers can produce tubing from 1 to 10 mm up to 12-in. in length, up to a dozen or more per batch on a carousel. Planar chambers can produce cylindrical films as well, using substrates spinning on their long axes during sputtering. In the range of a few microns to 150 µm thickness, the devices are normally cost-competitive with traditional processing. Applications where the fabrication is not possible or practical traditionally, may be economically viable using a PVD-based approach. Other sizes are possible by means of reduced batch throughput or by developing custom chambers. For example, Nitinol PVD tubing up to 25 mm has been deposited. Films of superelastic (SE) or shape memory (SM) Nitinol, SE NiTiCo, SE NiTiPt, SM NiTiCu, 316 SST, Mg and Fe resorbable alloys, NiFe, NiTiMn, CuNiMg, and elemental films of Al, Ti, Fig. 3 — Tensile curves at 37°C: Blue represents wrought binary Nitinol device properties; red indicates binary PVD Nitinol; and green is a PVD NiTiCo alloy. Fig. 4 — PVD demonstration sample showing micron and millimeter sized features with engineered surface texture. Sample diameter is 6.2 mm for scale. 6
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