Effects of dynamic indentation on the mechanical response of materials
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N.R. Moody Sandia National Laboratories, Livermore, California 94551-0969
W.W. Gerberich Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received 17 September 2007; accepted 11 January 2008)
Dynamic indentation techniques are often used to determine mechanical properties as a function of depth by continuously measuring the stiffness of a material. The dynamics are used by superimposing an oscillation on top of the monotonic loading. Of interest was how the oscillation affects the measured mechanical properties when compared to a quasi-static indent run at the same loading conditions as a dynamic. Single crystals of nickel and NaCl as well as a polycrystalline nickel sample and amorphous fused quartz and polycarbonate have all been studied. With respect to dynamic oscillations, the result is a decrease of the load at the same displacement and thus lower measured hardness values of the ductile crystalline materials. It has also been found that the first 100 nm of displacement are the most affected by the oscillating tip, an important length scale for testing thin films, nanopillars, and nanoparticles.
I. INTRODUCTION
Two main loading functions are being utilized with nanoindentation experiments,1 one using quasi-static loading and the other dynamic loading. During quasistatic loading, the load is monotonically increased utilizing either the load-controlled or displacement controlled (N/s or nm/s) mode. With dynamic loading, a small oscillation is superimposed on top of the quasi-static loading. Dynamic loading is more commonly known as the continuous stiffness method (CSM) and is commercially available on most nanoindenters. It is called CSM because with the harmonic oscillation the contact stiffness can be continuously measured during the entire loading segment of the indent and provides stiffness and thus mechanical properties as a function of displacement, namely elastic modulus and hardness. Using a small force or displacement amplitude, with a frequency ranging between 1 and 300 Hz,2 a sinusoidal driving force is superimposed on the load. The resulting displacement ␦ will have the same frequency of oscillation and may have a phase difference leading to ␦ = ␦oei共t+兲 .
(1)
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0205 1604
J. Mater. Res., Vol. 23, No. 6, Jun 2008
It should be noted that the displacements will be affected by the dynamics of the instrument as well as the indenter– specimen interaction.2 Dynamic loading is particularly useful when indenting thin films or multilayers of materials where the properties can change with each layer.3 The harmonic oscillation can also be used when indenting polymers and viscoelastic materials. These materials can dissipate energy during deformation and remain undeformed after stress removal.2 To determine the viscoelastic properties such as storage and loss modulus using indentation techniques, it is best to use dynamic loading. The inde
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