Investigation of the formability of thin nanoclay containing polyelectrolyte films on NiTi-substrates in humid environments
Final Report Abstract
Thin polyelectrolyte films are potentially suitable coatings to reduce the release of Ni from shape memory materials in biomedical applications. Given the typically loading conditions encountered in these devices, such coatings need to be able to tolerate large monotonic and/or cyclic strains without defect formation. The current study has focused on the mechanical properties of ultrathin PAA/PAH (polyacrylic acid/polyallylamine hydrochloride) polyelectrolyte films deposited by a layer-by-layer technique on Nickel—Titanium (NiTi) substrates. In order to allow for a systematic study of grain size and orientation effects, both polycrystalline and single crystals substrates were employed and tested in different environments including simulated body fluids. While single tensile strains up to 5% revealed the amazing strain to failure of the applied coating, cyclic strains resulted in defect formation within the polyelectrolytes at smaller strain amplitudes. To provide a better understanding of the mechanisms that are determining the defect formation, macroscopic and microscopic defect localizations were analyzed by digital image correlation and electron back-scattered diffraction techniques. In case of the single crystals, defect formation could be attributed to phase boundary related topographic evolution. By contrast, in thin polyelectrolyte films deposited on polycrystalline samples, cracks and delaminations emerged predominantly in the vicinity of critical grain boundaries, i.e. those with a large difference in orientation-dependent transformation strain. Hence, through an appropriate texturing of the substrate, the load on the coating can be minimized. More importantly, however, are the changes in coating characteristics possible through an appropriate modification of the coating itself. In the current research both thermal curing of the films and nanoclay modifications were studied. The differences in formability of unmodified, thermally cured, and nanoclayreinforced polyelectrolyte films could be related to their chemical characteristics and their swelling behavior in saline media. Thermal curing treatments decreased the formability of the polyelectrolyte films remarkably because of the formation of less mobile covalently bonded chains. By contrast, nanoclay-reinforced, highly electrostatically crosslinked films demonstrated superior formability on both substrates employed. In the optimized system, although an elevated topographic formation was observed along the grain boundaries, 300 cycles with a maximum strain of 3% did not induce any microscopically traceable defects. The results obtained clearly demonstrate that modified polyelectrolytes can tolerate large monotonic and/or cyclic strains without crack formation. It appears that coatings based on these systems could help to reduce nickel release from long-time implants in the future. Clearly, there are still open issue that need to be addressed. A case in point is the behavior under multiaxial loading conditions, as these are typically encountered in actual devices such as stents, and work is underway to address this topic. Provided that the coatings withstand these more severe loading conditions as well, it is reasonable to assume that the research will have a significant impact for future applications of shape memory alloys in the biomedical field. So far, the research had focused on coatings on NiTi shape memory alloys. Obviously, this approach is not limited to this class of materials. Recently, TWIP steels have attracted much attention in the automotive industry as they provide for an excellent combination of high strength and ductility. Given the high manganese content in these steels, coatings are needed to prevent corrosion. An ideal coating can be applied to a steel coil and would not form defects during shape setting of the product. Typically, such forming processes involve local strains that are even larger than those employed in the current study. Developing coatings for this type of application is certainly a challenge. The results obtained in the current research indicates, however, that modified polyelectrolytes appear to be a promising venue to follow.
Publications
- Formability of Thermally Cured and of Nanoclay-reinforced Polyelectrolyte Films on NiTi-substrates. J. Mater. Sci.
J. Lackmann, T. Niendorf, M. Maxisch, R. Regenspurger, G. Grundmeier, H.J. Maier
(See online at https://doi.org/10.1007/s10853-011-5782-3) - Defect Formation in Thin Polyelectrolyte Films on Polycrystalline NiTi Substrates. J. Mech. Behavior of Biomed. Mater., 3, 2010, pp. 436-445
J. Lackmann, R. Regenspurger, M. Maxisch, G. Grundmeier, H.J. Maier
- Single Molecule Desorption Studies on Immobilized Nanoclay Particle Surfaces Langmuir, 26, 2010, pp. 8155-8160
B. Özkaya, O. Özcan, P. Thissen, G. Grundmeier
- High-resolution In-situ Characterization of the Surface Evolution of a Polycrystalline NiTi SMA-alloy Under Pseudoelastic Deformation. Mater. Characterization, 62, 2011, pp. 298-303
J. Lackmann, T. Niendorf, M. Maxisch, G. Grundmeier, H.J. Maier
- In situ Characterization of Martensite Variant Formation in Nickel-titanium Shape Memory Alloy Under Biaxial Loading. Scripta Mater., 65, 2011, pp. 915-918
T. Niendorf , J. Lackmann, B. Gorny, H.J. Maier
- PM-IRRAS Studies of the Adsorption and Stability of Organophosphonate Monolayers on Passivated NiTi Surfaces. Applied Surface Sci., 257, 2011, pp. 2011-2018
M. Maxisch, C. Ebbert, B. Torun, N. Fink, T. de los Arcos, J. Lackmann, H.J. Maier, G. Grundmeier
