Indentation techniques, mechanical characterization

The technological importance of thin films in such areas as semiconductor devices and sensors has led to a demand for mechanical property information for these systems. Measuring the elastic modulus for thin films is much harder than the corresponding measurement for bulk samples, since the results obtained by traditional indentation methods are strongly perturbed by the properties of the substrate material. Additionally, the behaviour of the film under conditions of low load, which is necessary for the measurement of thin-film properties, is strongly influenced by surface forces [75]. Since the force microscope is both sensitive to surface forces and has extremely high depth resolution, it shows considerable promise as a technique for the mechanical characterization of thin films. 

Vickers hardness as a principal parameter for the mechanical characterization of materials has been commonly used as a technique to measure the mechanical properties of materials, but the microhardness commonly decreases with applied load, which is known as the indentation size effect (ISE). 

Nanoindentation is nowadays one of the most used methods to measure the mechanical properties of polymers, attracting great attention as a technique to mechanically characterize polymer nanocomposites [137-142]. This technique uses the same principle as microindentation, but with much smaller probe areas and very low loads (on the order of nanonewtons), so as to produce indentations from less than a hundred nanometers to a few micrometers in size and depth [143]. Although it has been vastly used to characterize the mechanical properties, particularly hardness, elastic modulus, yield stress, and fracture toughness, of several polymers [144—152] and shown to be mainly influenced by the testing procedure, penetration depths, and holding time, limited work has been dedicated to the characterization of the mechanical behavior of polymer nanocomposites using this technique. 

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

Pellicle and tea-immersed pellicle were analyzed using nanoDMA (dynamic mechanical analysis) to see if the tannins had an effect on the viscoelasticity of the pellicle. NanoDMA is a technique used to study and characterize mechanical properties in viscoelastic materials. The method is an extension of nanoindentation testing [58, 59], An analysis of the nanoindentation load-depth curve gives the hardness (H) and reduced elastic modulus (E ), provided the area of contact, A, between the indenter tip and the sample is known [ 13]. By... 

Mechanical properties of nanomaterials have been characterized using both tensile test and nanoindentation techniques. Nanoindentation experiments describe the deformation of the volume of material beneath the indenter (interaction volume). The nanoindentation of cellulose composites can be performed by using an AFM. 

A scanning force instrument also allows for the acquisition of force-distance curves to characterize the local mechanical properties of the sample. Well-defined indentation experiments on soft surfaces like swollen hydrogels in aqueous media are possible with the colloidal probe technique. Raw data are assessed, for example, according to the Hertz model, with the assumption... 

Within a range of acceptable loading rate, the linear elastic approximation can accurately provide a framework for interpretation of data obtained from a variety of biophysical techniques - micropipet aspiration [Kuznetsova, 2007], atomic force microscope (AFM) indentation [Titushkin, 2007], and magnetic twisting cytometry [Vliet, 2003] - developed and refined to measure and characterize the cellular mechanical properties. 

In order to realize the potential of these ceramics they must be fabricated, and with such a low decomposition temperature sintering to high strength is proving a problem. Hardness determinations will prove useful in characterizing the mechanical properties of these and other ceramic superconductors, but as yet little has been reported. Table 6.21 contains hardness values obtained for the YBa2Cu307 superconductor at room temperature and at liquid Nj temperatures (77 K). As expected, the Vickers hardness rises to 3.1 GPa at 77 K and some success with sinter additives is achieved because the standard material hardness is raised from 2.2 to 2.5 GPa after sintering in their presence. Indentation hardness techniques have been used to establish the Kic value of 1.1 MPam. ... 

A. L. Yurkov and R. C. Bradt, Load Dependence of Hardness of SIALON Based Ceramics, in Fracture Mechanics of Ceramics, Vol 11, Eds. R. Bradt et al., (Plenum Press, NY, 199S), pp. 369 378. P. M. Sargent, Use of the Indention Size Effect on Microhardness for Materials Characterization, in Microindentation Techniques in Materials Science and Engineering, ASTM STP 889, eds. P. J. Blau and B. R. Lawn, (ASTM, West Conshohocken, PA, 1986), pp. 160 174. 

Characterization of the mechanical properties of these thin silica layers, unreinforced or reinforced, is usually conducted by using the nanoindentation technique [33-37] to determine the hardness (H) of the layer and the elastic modulus ( ) using the Oliver-Pharr method [38]. In these tests, a Berkovich indenter is used and low maximum loads are applied (in the range of mN) to avoid the influence of the mechanical response of the substrate. A complete review of how to calculate different key mechanical parameters ( , H, fracture toughness, residual stresses, and adhesion) of thin sol-gel coatings using nanoindentation tests and scratch testing with nanoindenter equipment can be found in the work of Malzbender et al. [39]. 

Finally a very unusual technique that can be used to characterize polyelectrolyte gels is to study their electromechanical properties. To do this the gel is placed on top of a concentric array of platinum electrodes and then a well-defined pressure distribution is applied to the top of the gel using a spherical indenter. The potential generated due to the applied mechanical force is measured using the electrodes. Polyelectrolyte gels due to their charged species may be able to transduce mechanical forces and deformation applied on them into electrical potentials, these can be measured using this kind of a setup [93]. 

The ability to probe surface mechanical properties with nanometer-scale lateral and vertical resolutions is critical for many emerging applications. For nanomechanical prohing experiments, one usually exploits either AFM or microindentation techniques. AFM enables characterization of mechanical properties of the sample through probe indentation and shear. Mechanical tests include indentation, wear, and force-distance curves measured upon normal and shear loading. ... 

---