Optical Metal Oxide Multilayers with Optimal Transparency and Mechanical Properties

The research in this grant was motivated by the fact that optical nanomultilayers (NMs) are promising candidates for materials that are simultaneously transparent and durable, however, additional understanding of the microstructural variations and deformation behavior as a function of optical optimization is needed before fully realizing their multifunctional potential. Specifically, optical multilayer systems had yet to be explored for the relationship between transparency and mechanical properties. To address this challenge, we designed, synthesized, and characterized systems of metal oxide nanoscale multilayers to leverage nanoscale features to enhance optical properties, while optimizing mechanical performance. The objective in this work was to explore the role of microstructure, interfaces, and composition in optically optimized NMs and then to relate these characteristics back to multilayer optical and mechanical behavior, and as a result, provide a crucial step towards discerning, then leveraging, this interplay of properties. In the first part of this grant, an approach for investigating these relationships was developed in the form of investigating a metal/ceramic AlN/Ag optical NM system to explore the relationship between transmittance and deformation behavior. This work demonstrated the success of a combinatorial approach of synthesis (via magnetron sputtering), optical predictions, and mechanical behavior investigations in optical NMs. From there, this approach was extended to ceramic optical NMs capable of achieving high (>95%) transmittance, which elucidated the role of microstructural variations as a function of introducing aperiodic layer thicknesses for improved optical behavior. This was achieved by synthesizing AlN/SiO2 NMs with repeated bilayer and optically optimized layer thicknesses across a range of total interfaces. It was found that, in addition to achieving high experimental transparency (%T200-1100nm=94-96%), substantial shifts in grain morphology and compositional transitions occurred when designing configurations for maximized transmittance. Furthermore, nanoindentation and residual stress results for the varying NM configurations highlight a correlation between high optical transparency configurations and the observed physical attributes. Such correlations laid the groundwork for an understanding of these interfacial and microstructural changes concerning optical behavior to facilitate the introduction of additional functionalities to optical NMs. However, global insight into optical and mechanical properties across multiple material compositions is necessary and a first undertaking of this was conducted in the final portion of the NSTRF grant, which explored the effect of maximizing optical properties on microstructural and multifunctional properties across a range of common crystalline/amorphous (C/A) and amorphous/amorphous (A/A) optical NM systems. The three ceramic nanomultilayer compositions (AlN/SiO2, AlN/Al2O3, TiO2/SiO2) demonstrated high experimental transmittance ranging from 94-99% and verified that tuned layer thicknesses and aperiodicity generated microstructural variations both within the layers and at the interfaces across all material systems. Moreover, optically optimized designs in C/A (AlN/SiO2, AlN/Al2O3) multilayers led to deviations within each system in the residual stresses and nanoindentation hardness that was not observed in the A/A (TiO2/SiO2) multilayers. By capturing the changes that arise with optimized optical properties, these shifts can be correlated to other multilayer properties. These findings lay the groundwork for elucidating how transparent optical multilayer systems can be tailored with optimal layer thicknesses and the effect this has on multifunctional performance, all in the effort to enable the design and synthesis of new high-performance, long-lasting optical materials. Promising areas of future exploration has also been identified for NASA, in addition to communication of all results to date.