More papers from the Nanomechanics group
Several papers of the Nanomechanics group have recently been published:
Ghanem, M., Göken, M., Merle, B.  Plane-strain bulge testing of thin films under compressive residual stresses. Surface and Coatings Technology, in press
The Bulge Test is a well-established method for the mechanical testing of freestanding thin films but is generally restricted to the investigation of membranes exhibiting tensile residual stresses. A new procedure based on successively evacuating and pressurizing the membrane from one side is suggested which provides a better estimation of the membrane initial height and extends testing possibilities to compressively stressed or buckled membranes. The robustness of the method is demonstrated by measurements on several material systems, SiO2 and Cu/Nb sputter-deposited onto a thin SiNx substrate. Measurements of the plane-strain modulus of the membranes show good agreement with literature and results of the residual stress levels were reasonable and coincide with the buckled state of the membranes. Moreover, Cu/Nb multilayer membranes show a consistent decrease in the compressive stress level with increasing layer thickness.
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Merle, B., Maier-Kiener, V., Pharr, G.M.  Influence of modulus-to-hardness ratio and harmonic parameters on continuous stiffness measurement during nanoindentation, Acta Materialia, 134, pp.167-176.
Abstract: Dynamic nanoindentation is a popular method for continuously probing the mechanical properties of a sample as a function of depth. It is shown here that special caution should be exercised when testing materials with high modulus-to-hardness ratios (E/H) at fast loading rates, as the choice of harmonic parameters can result in a significant underestimation of the contact stiffness and hence the elastic modulus. The origin of this behavior is traced back to a bias occurring during signal processing by the lock-in amplifier. The possible consequences of flawed measurements are highlighted and a practical method for detecting possible occurrence from the phase angle signal is presented.
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