Mechanical Engineering
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Item Leveraging Porous Silicon Carbide to Create Simultaneously Low Stiffness and High Frequency AFM Microcantilevers(2014) Barkley, Sarice; Solares, Santiago; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many operative modes of the atomic force microscope (AFM) are optimized by using cantilever probes that have both a low force constant and a high resonance frequency. Due to fabrication limitations, however, this ideal cannot be achieved without resorting to sizes incompatible with standard AFM instrumentation. This project proposes that cantilevers made from electrochemically etched porous silicon carbide (SiC) enjoy reduced force constants without significantly sacrificing frequency or size. The study includes prototype fabrication, as well as parametric experiments on the etching recipe and suggestions to improve the process. Analysis of the mechanical properties of the prototypes proves that introducing porosity to the structure greatly reduces the force constant (porous k = 0.27 bulk k) while only slightly reducing the resonance frequency (porous f0 = 0.86 bulk f0).Item SURFACE CHARACTERIZATION OF VISCOELASTIC MATERIALS THROUGH SPECTRAL INTERMITTENT CONTACT ATOMIC FORCE MICROSCOPY(2012) Williams, Jeffrey Charles; Solares, Santiago D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ability to recover material properties at the atomic scale has been the ongoing objective of the Atomic Force Microscope (AFM). More specifically, the most popular operation of the probe with this microscope (Intermittent Contact AFM) has not yet been able to resolve material properties of viscoelastic samples. By using the force and position time signals of the AFM and the constitutive equations for linear viscoelasticity, a method is developed by which such material properties are extracted in real-time scanning. A parametric study is then performed by simulating surface and AFM system conditions to understand the limits under which the method can accurately be performed in experiment. Suggestions are made to help experimentalists optimize the method to cater to the range of viscoelastic materials being measured and the results are related to measured material properties in literature. The method is found to be accurate for a wide range of viscoelastic materials.Item Quantitative Prediction of Tip-Sample Repulsive Forces and Sample Deformation in Tapping-Mode Frequency and Force Modulation Atomic Force Microscopy(2008-08-27) Crone, Joshua C; Solares, Santiago D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ability to predict sample deformation and the resultant interaction forces is a vital component to preventing sample damage and acquiring accurate height traces in atomic force microscopy (AFM). By using the recently developed frequency and force modulation (FFM) control scheme, a prediction method is developed by coupling previously developed analytical work with numerical integration of the equation of motion for the AFM tip. By selecting a zero resonance frequency shift, the sample deformation is found to depend only on those parameters defining the tip-sample interaction forces. The results are represented graphically and through a multiple regression model so that the user can predict the tip penetration and maximum repulsive force with knowledge of the maximum attractive force and steepness of the repulsive regime in the tip-sample interaction force curve. The prediction model is shown to be accurate for a wide range of imaging conditions.Item NEW METROLOGICAL TECHNIQUES FOR MECHANICAL CHARACTERIZATION AT THE MICROSCALE AND NANOSCALE(2004-12-20) jin, huiqing; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)New metrological techniques have been developed for mechanical characterization at the microscale and nanoscale as follows: (1) Development of a control system and integrated imaging capability at the microscale and nanoscale for a new biaxial microtensile tester, (2) a new method for characterizing nonlinearity in AFM imaging using Digital Image Correlation (DIC), and (3) development of pointwise DIC technique. In the biaxial microtensile tester, loading of specimen is induced through the opposing motion of dual picomotor linear actuators in orthogonal directions with a displacement resolution of less than 30 nm. Using an optical microscope, in situ digital images are obtained and analyzed with DIC to determine the full field displacements at the microscale over an Area of Interest (AOI) in order to characterize the biaxial performance of the microtensile tester. An objective AFM has been integrated into the biaxial microtensile tester to obtain in situ digital images of topographic microstructural features at the nanoscale. These topographic images can then be converted to gray scale images with textures that are suitable for DIC to calculate full field displacements at the nanoscale. This measurement capability is demonstrated on a sputtered nanocrystalline copper film subjected to uniaxial loading in the microtensile tester. Since image quality is critical to the accuracy of the nanoscale DIC measurements, a new method was developed to calibrate the errors induced by the nonlinearity of AFM scanning. In this new method, the DIC technique was applied to AFM images of sputtered nanocrystalline NiTi films to calculate the displacement errors caused by the probe offset that must be eliminated from the apparent displacement field. The conventional DIC technique assumes a zero-order or first order approximation of the variation in displacement fields (i.e., displacement gradients) relative to the center of a subset of the image. In the case of displacement fields associated with the microstructure of a material, the displacement gradients can vary discontinuously, which violates the assumed nature of the displacement gradients in the conventional DIC. Therefore, a pointwise DIC technique has been developed to calculate displacements independently at each pixel location, eliminating the constraints imposed by the subset on the calculated displacements. Because of the potentially large number of unknown displacement variables that need to be determined using this approach, an efficient Genetic Algorithm (GA) optimization algorithm with a Differential Evolution (DE) method was investigated for optimizing the correlation function. To guarantee uniqueness of the optimized displacement field, the correlation function was modified using intensity gradients that had to be transformed from an Eulerian to Lagrangian reference frame using displacement gradients. The theoretical development of pointwise DIC is discussed in detail using ideal sinusoidal images, and its validation using real images is also presented.