Indentations, deformation around

The plastic deformation patterns can be revealed by etch-pit and/or X-ray scattering studies of indentations in crystals. These show that the deformation around indentations (in crystals) consists of heterogeneous rosettes which are qualitatively different from the homogeneous deformation fields expected from the deformation of a continuum (Chaudhri, 2004). This is, of course, because plastic deformation itself is (a) an atomically heterogeneous process mediated by the motion of dislocations and (b) mesoscopically heterogeneous because dislocation motion occurs in bands of plastic shear (Figure 2.2). In other words, plastic deformation is discontinuous at not one, but two, levels of the states of aggregation in solids. It is by no means continuous. And, it is by no means time independent it is a flow process. 

SEM images of the deformation around an indentation in a 90 im thick MWCNT-AI203 composite prepared by in situ formation of CNTs within the regular and very well aligned pores of an alumina membrane, (a) array of the shear bands formed, (b) close up view of lateral buckling or collapse of the CNTs in one shear band. ... 

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

Glide bands are observed around hardness indentations in lysozyme so dislocations (with large displacements) are associated with its deformation. 

Therefore, choosing a suitable load range for brittle crystals we can induce a plastic deformation in the crystal, seen as a starlike rosette around the indentation site, immediately after measurement or after etching the surface. Detailed tests and theoretical studies relating to fall in strength in brittle surfaces by the action of sharp and blunt indenters were conducted by Lawn et al. (1975, 1976). 

Gragert and Meyer (Fig. 6.2.1) and Boyarskaya (Fig. 6.2.2) by observation of surface deformations induced by indentation with a tungsten carbide ball and by scratch. The observations were carried out using secondary electron beam and in cathodoluminescence. They demonstrated on MgO and LiF crystals the occurrence of cracks around the impression of the ball similar to those induced by a Vickers indenter, and also the occurrence of a concentration of screw and edge dislocations in the area of the cracks. 

Surface destructive alterations around the action site of the indenter are used to determine the degree of brittleness of a material (Section 6.3). They are the outcome of a destructive deformation of the crystal induced by multidirectional shear forces released in the crystal structure. The shear stresses liberated in static indenter tests are significantly in excess of those registered under other methods, being of the order of 50 MPa (Yushkin,... 

Palmquist (1957) was the first to make use of the deformation developing around the Vickers pyramid indentation as an aid in the interpretation of results. The Palmquist test consists of determining the resistance of brittle materials to propagation of the cracks appearing at the corners of the Vickers pyramid mapping on a polished surface. The measured value, defined as Palmquist toughness, is given by the formula... 

Deformation induced by indentation. The deformation induced in single crystals of San Carlos (Arizona, USA) olivine around the in-denter in a Vickers microhardness test has been studied by Gaboriaud... 

In summarizing, it can be concluded that the microhardness of elongational flow injection moulded PE is influenced by a local double mechanical contribution (a) a plastic deformation of crystal lamellae under the indenter, and (b) an elastic recovery of shish-fibrils parallel to the injection direction after load removal. Further, the Shish-crystals are preferentially formed when high orientation occurs, i.e. at zones near the centre of the mould and at an optimum processing temperature Tp around 145-150 °C. Below this temperature overall orientation decreases due to a wall-sliding mechanism of the mbber-like molten polymer. 

Extensive TEM studies by Page et al. [65] delivered all previous low-temperature electron microscopy results to the consistent view that (i) silicon becomes amorphous in response to the high contact stresses under a hardness indenter and (ii) limited dislocation arrays are generated around the deformed volume at contact loads exceeding some threshold value. The authors also argued that the dislocation arrays might occur as a means of accommodating the displacements from the densification transformation, rather than as a primary response to the indenter intrusion. 

Scanning electron microscopy revealed the formation of debris around the indentation contact area in diamond [196] (Fig. 41a). This correlates with the behavior of silicon and germanium under contact loading, where the formation of plastic extrusions around indentations is believed to be indicative of the pressure-induced metallization (see Section 2.4). The formation of ductile extrusions was reported along the edges of the Vickers impression in diamond and around the deformed top of the diamond indenter [196] (Fig. 41), suggesting that similar transformations occurred in both the indenter and the crystal. 

High-temperature creep deformation of synthetic forsterite crystals, as studied by Darot and Gueguen [162], occurred by (010) [100] sUp, which is apparently the preferred slip system. This same slip system was detected around Vickers indents produced at temperatures of 600 °C and above by Gaboriaud et al. (163, 164], who used natural olivine single crystals from San Carlos, Arizona (110) [001] slip was also activated. There have been many other extensive deformation studies of the olivine minerals (e.g., see Kohlstedt and Ricoult [165] and Poumellec and Jaoul [166]). 

The use of depth-sensing instrumented indentation to study the inelastic deformation response of thin metal films on substrates remains a work in progress. A key underlying challenge here is to estimate the connection between the projected contact area Ac and the indenter depth of penetration h for particular geometrical configurations, constraint conditions and material parameters by properly accounting for the manner in which the material flows around the indenter. References to relevant literature on this topic can be found in Section 6.8.2 as well as in reports by Ohver and Pharr (1992), Nix (1997) and Dao et al. (2001). 

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