Characterization and Modeling of Mechanical Behavior of Polymers and Composites at Small Scales by Nanoindentation
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Polymer has been used extensively for the final MEMS structures or devices. In order to ensure the design reliability, it is critical to precisely determine the mechanical properties of the polymer-based electronic packaging materials at microscale. In recent years, nanoindentation technique is gradually becoming an effective technique for determining the local mechanical properties at the microscale and nanoscale. In this study, the mechanical properties of SU-8, a photoresist material of great interest to MEMS community, were measured under both micropillar compression and nanoindentation on a film on a substrate by nanoindentation. Measurement results in literature characterizing the mechanical behavior of SU-8, by elastic-plastic analysis of nanoindentation data, have shown to provide incorrect results. In this study, an appropriate viscoelastic analysis of nanoindentation load-displacement data was conducted, the time-average Young’s modulus at a given strain rate was determined to be near 3.6 GPa, which agrees with the reported values in literature obtained from tension and bending, and also correlates reasonably well with data from microcompression. This work indicates that viscoelastic analysis is necessary to extract the valid mechanical properties at nano/microscales for SU-8. The same viscoelastic analysis approach has also been applied to characterize the mechanical properties of a molding compound on a packaged integrated circuit (IC) by spherical nanoindentation using a 50 μm radius diamond tip. The molding compound is a heterogeneous material, consisting of assorted diameters of glass beads embedded in an epoxy. Statistical analysis was conducted to determine the representative volume element (RVE) size for a nanoindentation grid. Nanoindentation was made on the RVE to determine the effective viscoelastic properties. The relaxation functions were converted to temperature-dependent Young’s modulus at a given strain rate at several elevated temperatures. The spatial distribution of the Young’s modulus at a given strain rate was also determined using nanoindentation with a Berkovich tip. In addition to the application on MEMS structure, nanoindentation technique has also been extended to characterize the fiber reinforced polymer matrix composites, which have found increasing applications in such areas as aerospace, automotive, wind farms, offshore drilling, sports, and construction. In this study, Fiber push-in nanoindentation was conducted on a unidirectional carbon fiber reinforced bismaleimide resin composite (IM7/BMI) subjected to environmental degradation to determine the interfacial shear strength. It was found that thermal oxidation at 245oC in air leads to a significant reduction in interfacial shear strength of the IM7/BMI unidirectional composite. Moisture-saturated specimens under thermal shock showed a significant reduction in interfacial shear strength as well. It is thus encouraged to increase the interfacial strength of fiber reinforced polymer matrix composites. In this study, we propose using multiwall carbon nanotube sheet to spiral-wrap around an individual carbon fiber for enhancement of mechanical properties of the fiber/matrix interphase that directly influences the fiber/matrix debond strength and compressive strength of the composite. Different methods were used in experiments to characterize the interfacial shear strengths. All experimental results show consistently a significant improvement in interfacial shear strength by using MWNT scrolled carbon fibers in a composite.