1. Effects of Passivation Layer on Material Properties

The mechanical behavior of metallic thin films can be changed when they are layered with other thin materials due to the existence of interface. Therefore, understanding the performance of metallic thin films with and without a passivation layer is important. To compare the mechanical behavior of freestanding and passivated thin films, we utilized a custom-built tensile tester which has a stroke of 250μm with a displacement resolution of 10nm and a load resolution 9.7μN. Freestanding specimens were fabricated with MEMS process, and 2D MoS2 passivation layers were transferred on water. Stress-strain curves showed a significant increase in elongation for metallic films with passivation layer, which is attributed to the delay in the strain localization due to constraint imposed by ultra-thin passivation layer.

​​Tensile test of Au thin film
​(a) Custom built in-situ SEM tensile tester, (b) optical microscope images of freestanding Au thin films with and without passivation layer, and (c) true stress-strain curve of Au thin films with and without passivation layer. 
2. Tensile Test using a Nano-indenter

In micro-scale, mechanical properties, such as yield stress and strength, differ from those in macro-scale. In order to measure the mechanical properties of micro-scale materials, we built a measurement system based on nanoindenter. We fabricated tensile bars with a feature size less than 10 μm that are suitable to research its tensile properties in micro-scale. Tensile bars can be fabricated in many ways, such as MEMS, femto-second laser, FIB. The advantage of this measurement technique is that it allows us to perform more measurements over time than typical tensile testing methods.

​Tensile test of a tensile bar
​(a) Nanoindentor setup, (b) nanoindentor mounted on SEM stage, and (c) schematic of tensile testing.
 3. Beam Deflection Experiment

​Our research aims to investigate mechanical behavior of freestanding submicron thin films using various experimental techniques. In cantilever & membrane deflection test, nanoindenter is employed to apply load and to measure deflection of the samples. From these deflection tests, mechanical properties related with bending or stretching can be obtained based on the sample geometry. When films are deposited with a composition spread, the membrane deflection technique can be very useful in conducting high-throughput combinatorial experiments on high entropy alloys.
(Top left) Fabrication process of cantilever beams through MEMS fabrication techniques and FIB milling. (Bottom right) The scanning electron microscope (SEM) images below show the membrane deflection experiment. (Right) Composition spread over a single chip consisting arrays of freestanding membranes. 
​Finite element analysis of
​the membrane deflection experiment
​After three plastic hinges are created, the membrane is stretched and it can be analyzed as a tensile test.  The plastic strain is almost the same over the specimen.