Determination of mechanical properties of very thin films is rather difficult task as all of currently using testing techniques have some weakness. This master’s thesis deals with microcompressive test and bulge test. Finite element simulations of the two methods were carried out in order to better understanding of experimental record. Microcompression combines the sample preparation with the use of focused ion beam (FIB) with a compression test carried out using nanoindenter. Cylindrical specimens (pillars) were prepared from Al film deposited on Si substrate using FIB. Experimentally measured data on pillars needs correction to obtain undistorted material properties of Al thin film. A necessary correction using FE modeling is suggested in the thesis. Second part of the work is focused on modeling of bulge test. Pressure is applied on freestanding SiNx film while deflection of the film is measured. Stress state in the film is biaxial making determination of mechanical properties of the film more complicated. The goal is to present how to model the whole problem. In addition, preparation of the specimens was simulated to estimate residual stress in the film. The paper contributes to a better characterization of very thin surface layers and determination of their mechanical properties.

The mechanical properties of crystalline metals are strongly affected when the sample size is limited to the micron or sub-micron scale. At these scales, the mechanical properties are enhanced far beyond classical predictions. Besides, the surface to volume ratio significantly increases. Therefore surfaces and interfaces play a big role in the mechanical properties of these micro-samples. The effect of different interfaces on the mechanical properties of micro-samples is not yet well understood. The aim of this project is to characterize, understand, and predict the effect of confinement on deformation mechanisms at micro-scale. In this study, micro-pillars were fabricated by Focused Ion Beam (FIB). Micro-pillars were homogeneously coated with thin films by magnetron sputtering and cathodic arc deposition. The mechanical properties of carbon-coated-, chromium coated-, naked-, annealed- and non-annealed micro-pillars were measured. Afterwards, the results of micro-compression tests and Automated Crystal Orientation Mapping on Transmission electron microscopy (ACOM TEM) were compared and led to some surprising new findings.Dislocations are blocked by amorphous- and even crystalline coating in the deformed samples. Parallel slip systems were detected in the chromium layer and the copper micro-pillar. Even though the chromium layer has parallel slip systems, dislocation pile-up at the interface was found after deformation. The most significant finding in this study concerns the back stress of the dislocation pile-up, which affects the dislocation sources and causes an increase of the flow stress to generate new dislocations from these sources. Thermal annealing increases the strength and flow stress of FIB fabricated micro samples. The annealing treatment restores the lattice that was damaged by the FIB fabrication process. A higher stress is required to initiate the dislocation nucleation in a pristine lattice. Techniques of fabrication and investigation were developed to study the role of confinement and interfaces on the mechanical properties of materials at micro scale. Mechanisms of deformation were unraveled and a better understanding of the key parameters was reached. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

Three testing methods are proposed to characterize properties of fuel cell materials that affect the mechanical durability of proton exchange membranes (PEMs). The first two methods involved measuring the in-plane biaxial strength of PEMs and the biaxial hygrothermal stresses that occur in PEMs during hygrothermal cycles. The third method investigated the nonuniform thickness and compressibility of gas diffusion media which can lead to concentrated compressive stresses in the PEM in the through-plane direction. Fatigue and creep to leak tests using multi-cell pressure-loaded blister fixtures were conducted to obtain the lifetimes of PEMs before reaching a threshold value of gas leakage. These tests are believed to be more relevant than quasi-static uniaxial tensile to rupture tests because of the introduction of biaxial cyclic and sustained loading and the use of gas leakage as the failure criterion. They also have advantages over relative humidity cycling test because of the controllable mechanical loading. Nafion® NRE-211 membrane was tested at three different temperatures and the time-temperature superposition principle was used to construct a stress-lifetime master curve. Tested at 90°C, extruded Ion Power® N111-IP membrane was found to have longer lifetime than Gore™-Select® 57 and Nafion NRE-211 membranes under the same blister pressure profiles. Bimaterial specimens fabricated by bonding a piece of PEM to a substrate material were used to measure the hygral stresses, compressive and tensile, in the PEM during relative humidity cycles. The substrate material and its thickness were carefully chosen so that stresses in the PEM could be obtained directly from the curvature of the bimaterial specimen without knowing the constitutive properties of the PEM. Three commercial PEMs were tested at 80°C by cycling the relative humidity between 90% and 0% and by drying the membrane to 0%RH after submersion in liquid water. Stress histories for all three membranes show strong time-dependencies and Nafion® NRE-211 exhibited the largest tensile stress upon drying. Besides in-plane stresses, hard spots in gas diffusion media (GDM) can locally overcompress PEMs in the out-of-plane direction and cause electrical shorting. In this study, GDM samples sealed with an impermeable Kapton® film on the surface were compressed with uniform air pressure and the nonuniform displacement field was measured with a three-dimensional digital image correlation technique. Hard spots as a result of the nonuniform thickness and compressibility of the GDM were found and their severities as stress risers are evident. Locally, a nominal platen compression (similar to bipolar plate land compression) of 0.68 MPa can lead to compressive stress as large as 2.30 MPa in various hard spots that are in the order of 100s µm to 1 mm in size. / Ph. D.