8.2 Surface andLlocal Spectroscopy

Abstract

Mechanical properties of solids (elasticity, anelasticity, plasticity) are generally measured onmacroscopic samples. But many phenomena in materials science ask for measurements ofmechanical properties at the surface of a material, or at the interfaces between thin layers deposited onthe surface, or with a high spatial resolution, for example in the cases of multiphased materials,composite materials, phase transitions, lattice softening in shape-memory alloys, precipitation in lightalloys, glass transition of amorphous materials, etc.In the case of multiphased materials, such as nanomaterials, composites, alloys or polymer blends,the location of the dissipative mechanism in one phase has to be done either through modeling or byseparately studying each phase, when possible without changing its behavior. The latter is onlypossible in a limited number of cases due to the interactions between the different phases within amaterial. To give an example, the global behavior of a composite is mostly driven by the stresstransfer properties between reinforcement and matrix, which are controlled by the local mechanicalproperties in the interface region, in particular by the dynamics of the structural defects in this area[1]. It is obviously impossible to prepare a sample only composed of interface regions. Therefore, amethod for locally studying the dynamics of the structural defects will thus help make importantsteps in the understanding and the improvement of such materials.Different techniques have been developed to probe the elastic and anelastic properties of surfaces,interfaces or phases of inhomogeneous materials at the micrometer and the nanometer scales. Thesetechniques are essentially based on Scanning Probe Microscopies (SPM). One of these techniques,which was first developped in the mid-1970's, is the Scanning Acoustic Microscopy (SAM), that ispresented in paragraph 9.5 and which allows one to study the materials properties at the micrometerscale.Amongst the different ways explored to study local mechanical properties of materials, severalgroups have recently used techniques based on Scanning Microscopy (SFM) [2]. For most ofthem, the focus has been placed on elasticity, using the so-called Force Modulation Mode(FMM) at low frequencies [3]. modulation mode generally uses a large amplitude (more than10 nm), low frequency (some kHz), vibration of the sample underneath the scanning forcemicroscope tip. The component of the tip motion at the excitation frequency and the tip mean positionare simultaneously recorded, giving several images of the sample surface. In particular, the in-phaseand out-of-phase components of force modulation mode at room temperatures have been interpretedin terms of stiffness (elasticity) and damping (viscoelasticity) [4,5]. However, it has beenrecently shown [6] that the contrast of force modulation mode is dominated by friction properties,and gives only little information on the elasticity. Consequently, some care has to be taken in theinterpretation of these low-frequency studies. A way to suppress this influence of friction on thecontrast is to use smaller amplitudes (some A) at higher frequencies [7]. Scanning Local-Acceleration Microscopy (SLAM) implements this idea [8]: SLAM is a modification of contact-modescanning force microscopy. Its principle is to vibrate the sample at a frequency just above theresonance of the tip-sample system. In this case, the inertia of the tip prevents it from completelyfollowing the imposed displacement, inducing non-negligible forces and giving rise to elasticdeformation of the sample. Contact stiffness is obtained from the measure of the residualdisplacement of the tip. Mapping the contact stiffness at different temperatures with SLAM [9] hasopened the way towards local mechanical spectroscopy. Some other techniques also use highfrequencies, but with different approaches to image elasticity at room temperature [7,10,11]. Presenthigh-frequency techniques are appropriate to map properties such as stiffness or adhesion at constant