MariAnne Sullivan

Evaluating Pile-Up and Sink-In During Nanoindentation of Thin Films

Determining mechanical properties is crucial when selecting materials for thin film applications. Nanoindentation has been used as an experimental method because of its ease in testing small-scale applications, such as thin films. When the film material properties are complicated by substrate effects, more analysis is necessary to capture the correct information using this technology. In this work, we discuss the differences in pile-up when changing film and substrate. We aim to describe these trends quantitatively through material properties. First, we show our model is an improved method to the nanoindentation technique, and is vital in testing films on substrates. Next, we have measured pile-up and sink-in with a technique including scanning electron microscopy images to better understand the effects of these phenomena on material properties. Also, we have examined sink-in similarly based on projected area measurements with an emphasis on yielding properties. These evaluations lead to further information of film properties that were unknown through nanoindentation in previous tests, which in turn is crucial for thin film applications like electronics and coatings.

Controlling Abalone Shell Architecture with Temperature

Biomimetics is a growing field, and abalone shells have become a recently studied topic because of their amazing strength properties. Their shell is composed of calcium carbonate, and it is formed by the mollusk in a unique structure of interlocking tablets called nacre, also known as mother of pearl. There has been much interest in the interactions between tablets, but there are also structural growth lines that occur between layers of nacre that have been unnoticed by other researchers. With these additional non-nacre layers, the material’s strength is increased. We show how abalones can be cultured with changes in temperature to adjust shell and nacre growth. Then, they are tested using nanoindentation for qualitative mechanical data. In all, we aim to utilize the abalone nacre architecture based on these composite layers for improved strength applications such as protective armor.

Evaluating Indent Pile-Up with Gold Films on Non-Plastically Deforming Substrates

This work focused on ascertaining the effect of pile-up during indentation of thin films on substrates. Conventional understanding has postulated that differences in contact area resulting from pile-up or sink-in significantly alter the extraction of material properties. In this work, the specific case of pile-up with compliant, plastically deforming films on stiff, non-plastically deforming substrates was studied. Several literature methods to assess pile-up were leveraged, and a new technique was developed and validated to quantify projected pile-up. Indentation testing was performed on gold films of multiple thicknesses on several ceramic-based substrates. The results indicated that the degree of pile-up was solely a function of indent depth into the film. Pile-up was not influenced by film thickness or substrate elastic modulus. In other words, the pile-up development was insensitive to the presence of the substrate and how it contributes to the composite’s elastic properties. In such case, if the elastic response of the film/substrate composite was independent of the degree of pile-up, then elastic data acquired from unloading did not require a contact area correction. The findings are confirmed using the Zhou–Prorok model for extracting film elastic properties for both gold and platinum films.

Newly Discovered Pile Up Effects During Nanoindentation

This work focuses on clearly defining the effects of pile up during nanoindentation of thin films deposited on substrates. Thin film behavior is important to understand in order to prevent failure in nano- and microscale mechanical devices utilized in computers or cell phones. During nanoindentation tests, phenomena such as sink-in or pile-up can occur depending on the mismatch of elastic moduli and Poisson’s ratios. This, in turn, alters the projection of the indent on the sample. While others have tried to measure and account for the pile up through changes in contact area, we have found that the pile up area does not affect the extracted elastic mechanical properties of the film or substrate materials. By depositing different thicknesses of gold on various plastically deforming substrates, pile up trends are visualized. Accounting for pile up is not necessary, as demonstrated through experimental data matched with models and images from scanning electron microscopy. These findings will help future experiments to correctly calculate elastic mechanical properties that have pile up issues.

New Insight into the Toughening Mechanisms of Nacre

Many living organisms form biogenic minerals, or biominerals, which are composite materials containing an organic matrix and nano- or micro-scale minerals assembled in a hierarchical architecture. These biogenic composites possess excellent mechanical properties in comparison to their abiogenic architectures (on the order of 3,000 times greater), which make them attractive for mechanically protective applications. One biogenic material that has garnered a lot of attention is Nacre, or “Mother of Pearl,” found in many Mollusk shells. The Nacre architecture has been well studied the past decade, however little work has focused on the fact that the Nacre composite is also itself a component of another composite architecture in the shells. In between thick layers of Nacre is a thin layer of an organic matrix that marks the seasonal growth patterns of the shells, analogous to tree rings. No work has focused on how these two layers interact to determine mechanical properties, which are likely as important as the tablet sliding itself. Determining this relationship would have a great impact on designing composite architectures that can improve the performance of mechanically protective armor.