|Ph.D Student||Zlotnikov Igor|
|Subject||Bioinspired Nanolaminates: Synthesis by Deposition of|
Zirconium Oxide on LbL Organic Layers and
|Department||Department of Materials Science and Engineering||Supervisors||Professor Elazar Gutmanas|
|Dr. Irena Gotman|
|Full Thesis text|
The present research is motivated by the remarkable mechanical properties of natural nanostructured composites, specifically nacre, whose highly regular “brick-and-mortar” structure is composed of alternating layers of aragonite sheets separated by thin layers of organic material. The excellent combination of stiffness, strength and impact resistance of nacre has long been inspiring researchers to reproduce nature’s achievements and to synthesize structural materials using the mechanical design principles found in nature. While the reproduction of the highly complex and, in a way, enigmatic structure of nacre is beyond the current capabilities of materials processing techniques, the length-scale and some morphological aspects of this structure can be imitated by a simplified nanolaminate model comprised of alternating ceramic and organic nanolayers. The general design concept in fabrication of such nanolaminates is to replace the relatively weak inorganic constituent found in nacre (aragonite) with more advanced engineered ceramics (e.g. zirconium oxide), with the goal of producing structural composite materials with high mechanical properties.
This work reports the first successful attempt to grow robust multi-layer zirconium oxide (ZrO2)-organic nano-assemblies on Si wafers. 8 to 28 nm thick organic laminae were synthesized employing Layer-by-Layer (LbL) deposition of alternating polyanion and polycation sublayers - a method that allows the fabrication of organic films with accurately controlled and practically unlimited thickness. Two processing routes were used to grow 10 to 170 nm thick ZrO2 layers on LbL organic films: ‘biomimetic’ deposition from aqueous zirconium sulfate solution or magnetron sputtering.
Solution deposited zirconium oxide was amorphous and porous and exhibited a low nanohardness, on a par with that of the organic layer. Sputtered zirconium oxide layers consisted of tetragonal-ZrO2 and monoclinic-ZrO2, and exhibited a high nanohardness approaching that of dense polycrystalline zirconia. It has been demonstrated by nanoindentation measurements that the presence of thin organic layers between ZrO2 ceramic layers decreases the hardness and elastic modulus of the latter and increases plastic energy dissipation (toughness). This effect was weak for the soft and porous solution-deposited zirconia, and was much more pronounced for the hard and dense sputtered zirconia. The higher toughness of sputtered ZrO2/organic nanolaminates compared to pure zirconia was indirectly confirmed by the results of Scratch Test with massive cracking obtained in the zirconia layer vs. practically no damage in the nanolayered structure. Numerical modeling of ZrO2/organic nanolaminated structure behavior during nanoindentation procedure has been performed and good agreement between the model predictions and experimental results has been obtained.