Nanoscratch Characterization of GaN Epilayers on c- and a-Axis Sapphire Substrates

In this study, we used metal organic chemical vapor deposition to form gallium nitride (GaN) epilayers on c- and a-axis sapphire substrates and then used the nanoscratch technique and atomic force microscopy (AFM) to determine the nanotribological behavior and deformation characteristics of the GaN epilayers, respectively. The AFM morphological studies revealed that pile-up phenomena occurred on both sides of the scratches formed on the GaN epilayers. It is suggested that cracking dominates in the case of GaN epilayers while ploughing during the process of scratching; the appearances of the scratched surfaces were significantly different for the GaN epilayers on the c- and a-axis sapphire substrates. In addition, compared to the c-axis substrate, we obtained higher values of the coefficient of friction (μ) and deeper penetration of the scratches on the GaN a-axis sapphire sample when we set the ramped force at 4,000 μN. This discrepancy suggests that GaN epilayers grown on c-axis sapphire have higher shear resistances than those formed on a-axis sapphire. The occurrence of pile-up events indicates that the generation and motion of individual dislocation, which we measured under the sites of critical brittle transitions of the scratch track, resulted in ductile and/or brittle properties as a result of the deformed and strain-hardened lattice structure

Complex shear modulus of hydrogels using a dynamic nanoindentation method

The micromechanical properties of soft tissues and materials are of considerable interest for biomedical applications. Nanoindentation is a powerful technique for determining localized material properties of biological tissues and has been used widely for hard tissue and material characterization. However, the technique is much more challenging when utilized for soft tissues due to their compliance as well as due to the limitations of commercial instruments which were originally developed for stiff, engineering materials. This study explores the use of a dynamic indentation method with a cylindrical punch (100 μm diameter) to characterize gelatin gel and low molecular weight hydrogels. A Keysight Technologies DCM II actuator is used with the Continuous Stiffness Measurement (CSM) to determine the complex shear modulus of these gels. The method overcomes surface detection issues with standard quasi-static nanoindentation as a change in phase angle can be used to accurately detect the sample surface. The data collected in this study are found to be comparable with macroscopic rheology and demonstrates the utility of the method for characterization of hydrogels