Yvonne B. Gerbig

Comparison of nanoscale measurements of strain and stress using electron back scattered diffraction and confocal Raman microscopy

Stresses in Si as small as 10 MPa have been measured using electron backscattered diffraction (EBSD) and confocal Raman microscopy (CRM) with spatial resolutions of 10 nm and 100 nm, respectively. In both techniques, data were collected across wedge indentations in (001) Si. EBSD measured the stress and strain tensors and CRM measured the uniaxial stress. The results agreed very well except close to the indentation, where the surface-sensitive EBSD results indicated larger stresses. Results converged when the CRM laser excitation wavelength was reduced, probing smaller depths. The stress profiles are consistent with the inverse-square power law predicted by Eshelby analysis.

The role of tooth enamel mechanical properties in primate dietary adaptation

Primate teeth adapt to the physical properties of foods in a variety of ways including changes in occlusal morphology, enamel thickness, and overall size. We conducted a comparative study of extant primates to examine whether their teeth also adapt to foods through variation in the mechanical properties of the enamel. Nanoindentation techniques were used to map profiles of elastic modulus and hardness across tooth sections from the enamel-dentin junction to the outer enamel surface in a broad sample of primates including apes, Old World monkeys, New World monkeys, and lemurs. The measured data profiles feature considerable overlap among species, indicating a high degree of commonality in mechanical properties. These results suggest that differences in the load-bearing capacity of primate molar teeth are more a function of morphology-particularly tooth size and enamel thickness-than of underlying mechanical properties.

High resolution surface morphology measurements using EBSD cross-correlation techniques and AFM

The surface morphology surrounding wedge indentations in (001) Si has been measured using electron backscattered diffraction (EBSD) and atomic force microscopy (AFM). EBSD measurement of the lattice displacement field relative to a strain-free reference location allowed the surface uplift to be measured by summation of lattice rotations about the indentation axis. AFM was used in intermittent contact mode to determine surface morphology. The height profiles across the indentations for the two techniques agreed within 1 nm. Elastic uplift theory is used to model the data.

Assessing strain mapping by electron backscatter diffraction and confocal Raman microscopy using wedge-indented Si

The accuracy of electron backscatter diffraction (EBSD) and confocal Raman microscopy (CRM) for small-scale strain mapping are assessed using the multi-axial strain field surrounding a wedge indentation in Si as a test vehicle. The strain field is modeled using finite element analysis (FEA) that is adapted to the near-indentation surface profile measured by atomic force microscopy (AFM). The assessment consists of (1) direct experimental comparisons of strain and deformation and (2) comparisons in which the modeled strain field is used as an intermediate step. Direct experimental methods (1) consist of comparisons of surface elevation and gradient measured by AFM and EBSD and of Raman shifts measured and predicted by CRM and EBSD, respectively. Comparisons that utilize the combined FEA-AFM model (2) consist of predictions of distortion, strain, and rotation for comparison with EBSD measurements and predictions of Raman shift for comparison with CRM measurements. For both EBSD and CRM, convolution of measurements in depth-varying strain fields is considered. The interconnected comparisons suggest that EBSD was able to provide an accurate assessment of the wedge indentation deformation field to within the precision of the measurements, approximately 2×10(-4) in strain. CRM was similarly precise, but was limited in accuracy to several times this value.

Accurate spring constant calibration for very stiff atomic force microscopy cantilevers

There are many atomic force microscopy (AFM) applications that rely on quantifying the force between the AFM cantilever tip and the sample. The AFM does not explicitly measure force, however, so in such cases knowledge of the cantilever stiffness is required. In most cases, the forces of interest are very small, thus compliant cantilevers are used. A number of methods have been developed that are well suited to measuring low stiffness values. However, in some cases a cantilever with much greater stiffness is required. Thus, a direct, traceable method for calibrating very stiff (approximately 200 N/m) cantilevers is presented here. The method uses an instrumented and calibrated nanoindenter to determine the stiffness of a reference cantilever. This reference cantilever is then used to measure the stiffness of a number of AFM test cantilevers. This method is shown to have much smaller uncertainty than previously proposed methods. An example application to fracture testing of nanoscale silicon beam specimens is included.

Indentation device for in situ Raman spectroscopic and optical studies

Instrumented indentation is a widely used technique to study the mechanical behavior of materials at small length scales. Mechanical tests of bulk materials, microscopic, and spectroscopic studies may be conducted to complement indentation and enable the determination of the kinetics and physics involved in the mechanical deformation of materials at the crystallographic and molecular level, e.g., strain build-up in crystal lattices, phase transformations, and changes in crystallinity or orientation. However, many of these phenomena occurring during indentation can only be observed in their entirety and analyzed in depth under in situ conditions. This paper describes the design, calibration, and operation of an indentation device that is coupled with a Raman microscope to conduct in situ spectroscopic and optical analysis of mechanically deformed regions of Raman-active, transparent bulk material, thin films or fibers under contact loading. The capabilities of the presented device are demonstrated by in situ studies of the indentation-induced phase transformations of Si thin films and modifications of molecular conformations in high density polyethylene films.

Two-dimensional strain-mapping by electron backscatter diffraction and confocal Raman spectroscopy

The strain field surrounding a spherical indentation in silicon is mapped in two dimensions (2-D) using electron backscatter diffraction (EBSD) cross-correlation and confocal Raman spectroscopy techniques. The 200 mN indentation created a 4 μm diameter residual contact impression in the silicon (001) surface. Maps about 50 μm × 50 μm area with 128 pixels × 128 pixels were generated in several hours, extending, by comparison, assessment of the accuracy of both techniques to mapping multiaxial strain states in 2-D. EBSD measurements showed a residual strain field dominated by in-surface normal and shear strains, with alternating tensile and compressive lobes extending about three to four indentation diameters from the contact and exhibiting two-fold symmetry. Raman measurements showed a residual Raman shift field, dominated by positive shifts, also extending about three to four indentation diameters from the contact but exhibiting four-fold symmetry. The 2-D EBSD results, in combination with a mechanical-sp...

High Confidence Level Calibration for AFM Based Fracture Testing of Nanobeams

When designing micro- or nanoelectromechanical systems, (MEMS and NEMS), it is important to consider whether structural elements will withstand loads experienced during operation. Fracture behavior at length scales present in MEMS and NEMS is much different than at macro- and mesoscopic scales. Due to a smaller probability of crystal defects and a high surface to volume ratio, fracture is controlled by surface characteristics rather than volumetric ones. Prior measurements using doubly clamped Si beams loaded with an atomic force microscope (AFM) showed that fracture of Si nanobeams is highly affected by surface roughness (Alan T et al., Appl Phys Lett 89:091901, 2006) and oxidation (Alan T et al., Appl Phys Lett 89:231905, 2006). In experiments of this type, calibration of the system, particularly the AFM cantilever stiffness, is critical to the accuracy of both the force and displacement results. A new set of experiments are underway in which the tests are performed by adapting a direct, traceable method for calibrating the AFM cantilever stiffness (Ying ZC et al., Rev Sci Instrum 78:063708, 2007). The improved calibration should not only improve the accuracy of the strength results but will allow linear stiffness measurements of the sample to be used to back out sample thickness, a key parameter in interpretation of the data.

In situ Analysis of Materials Under Mechanical Stress: A Novel Instrument for Simultaneous Nanoindentation and Raman Spectroscopy

An instrument that allows the simultaneous measurement of the mechanical deformation and Raman spectra of materials under contact loading is described. The instrument consists of a custom nanoindenter that operates on a laser-scanning Raman microscope.