Sharp Indenters

Microindentation hardness normally is measured by static penetration of the specimen with a standard indenter at a known force. After loading with a sharp indenter a residual surface impression is left on the flat test specimen. An adequate measure of the material hardness may be computed by dividing the peak contact load, P, by the projected area of impression1. The hardness, so defined, may be considered as an indicator of the irreversible deformation processes which characterize the material. The strain boundaries for plastic deformation, below the indenter are sensibly dependent, as we shall show below, on microstructural factors (crystal size and perfection, degree of crystallinity, etc). Indentation during a hardness test deforms only a small volumen element of the specimen (V 1011 nm3) (non destructive test). The rest acts as a constraint. Thus the contact stress between the indenter and the specimen is much greater than the compressive yield stress of the specimen (a factor of 3 higher). 

Fig. 1. Contact geometry for a sharp indenter at (A) zero load, (B) maximum load and (C) complete unload. The residual penetration depth after load removal is given by h...

The nano indentation technique can also be used to measure fracture toughness at small scales, by using the radial crack, which occurs when brittle materials are indented by a sharp indenter [72,73]. Lawn et al. [74] have shown that a simple relationship exists between the fracture toughness, Ag, and the lengths of the radial cracks, c, in the form of ... 

Lawn et al. (1975, 1978), and Lawn and Marshall (1978) distinguish two types of indenter whose action on the tested surface differs significantly (1) a blunt indenter (e.g., a hard ball) distinguished by an ideal elastic contact, so that the crack initiation is controlled by previously present defects (usually on the sample surface), and (2) a sharp indenter (e.g., a cone or pyramid) distinguished by partially plastic contact, so that the original defects start to grow as the result of the indentation process itself. In practice, the contact situations can therefore be seen as intermediate between the two cases. Within this area all typical indenters used for hardness measurement are contained. 

A general model for sharp indenters (Fig. 6.2.8) with the geometry of a Vickers indenter and with critical state considered, is proposed by Marshall... 

Similar tests of sections deformed under the action of a Vickers indenter were conducted by Lawn et al. (1980) who used transmission and interference microscopy for this purpose. Tests with Si, Ge, SiC and A1203 crystals revealed a similar, spherical pattern of stress curves for sharp cracking and sharp indenter. Chandhri -et al. (1980) confirmed the test results of... 

Fig. 6.2.6. Diagrammatic representation of median, lateral and chevron cracks formed by scribing with a sharp indenter. (After Misra and Finnie, 1979)...

Since the strain arising from an ideal sharp indenter cannot be wholly elastic (as is the case with a blunt indenter), a number of new specific features of failure of a particular material may arise, especially in the early stages of crack formation under the influence of surface penetration at low loads. It is reasonable to suppose at the same time that as the crack region extends widely below the contact zone, the influence of indenter geometry should become significant. 

Fig. 6.2.8. Median crack parameters under sharp indenters, general model. (After Lawn et al., 1976)...

A comparison of the results obtained from these two systems (Table 6.2.1) indicates that according to Hagan the critical loadings of sharp indenters are placed at a lower level. 

Optical properties of organic conductors also reflect the appearance of the energy gap, 2A, in the electronic energy spectrum of low-dimensional solids. The approximate value for the total gap from the g-mode line shapes can be estimated by comparing the IR spectra of the organic conductor, measured for the frequencies above and below the energy gap sharp absorption bands are produced at the frequencies w < 2A, whereas for to > 2A sharp indentations occur [87,88]. 

Fig. 1. Sharp indenter in contact with specimen surface.

D. T. Marx and L. Riester, "Mechanical properties of carbon-carbon composite components determined using nanoindentation," Carbon(UK), 37[11] 1679-1684.1999. Diss, J. Lamon, L. Carpentier, J. L. Loubet, andP. Kapsa, "Sharp indentation behavior of carbon/carbon composites and varieties of carbon," Carbon, 40[14] 2567-2579.2002. M. Savage, "Carbon-Carbon Composites," Kluwer Academic Publishers. (1993). 

The elastic moduli of the as-sintered porous LSCF cathode film samples were measured using a NanoTest nanoindentation platform (Micromaterials, UK) with a spherical diamond indenter tip of 50pm diameter. Compared with sharp indenters like Berkovich tips, benefits of using spherical tips include less sensitivity to surface condition. At least 20 measurements were conducted in different locations for each sample in order to measure the variability of the mechanical response of the sample. Prior to nanoindentation tests, the NanoTest platform was precisely calibrated using a standard sihca sample to establish the system frame compliance. 

Figure 12.5 Contact damage (cracks) in the surface of ceramics, (a, b) Cracks caused by a blunt and (c, d) cracks caused by a sharp indenter. (a) Hertzian ring crack in silicon carbide (b) Crack caused in operation by the...

Special attention is required when selecting the correct indenter tip. Sharp indenters such as the Berkovich tip indenter have been used by most researchers to measure the hardness and Young s modulus. However, the assumption of the transition from elastic to plastic behavior of the material is not permissible with a sharp-tipped indenter because these indenters create a nominally constant plastic strain impression. With a spherical tip, on the other hand, the depth of penetration increases as the contact stress increases therefore, the response of the elastic to plastic transition and the contact stress—strain property of a material can be determined (He and Swain, 2007). 

The indentation test is one of the simplest ways to measure mechanical properties of a material. The micromechanical behavior of polymers and the correlation with microstrnctnre and morphology have been widely investigated over the past two decades (23). Conventional microindentation instruments are based on the optical measnrement of the residual impression produced by a sharp indenter penetrating the specimen surface under a given load at a known rate. Microhardness is obtained by dividing the peak load by the contact area of impression. From a macroscopic point of view, hardness is directly correlated to the yield stress of the material, ie, the minimnm stress at which permanent strain is produced when the stress is snbseqnently removed. 

Figure 5.4. (a) The simplified stress profile assumed to be developed below the apex of the sharp indenter. (b) Median crack, half length c nucleated at region of maximum stress on boundary between plastic deformation and normal, elastically strained material. 

SUMMARY OF SEQUENCE OF EVENTS BENEATH A SHARP INDENTER... 

It is necessary to make a distinction based on the shape of the indenter used to produce the cracks because the position and pattern of cracks are dominated by surface flaws for blunt indenters, while such flaws are not so important when sharp indenters are used. Therefore to study crack generation and subsequent propagation in ceramic systems and not the distribution and behavior of pre-existing flaws, work is best restricted to sharp indenters of the Vickers or Knoop type. Conversely, blunt indenters such as the Brinell ball give some indication of flaw distribution on surfaces. 

A sharp indenter of the Vickers type at load P produces a plastic impression with characteristic surface dimensions 2a given by equation (1.6), repeated as equation (5.1) in the form that has load in kilograms... 

Sakai and co-workers have investigated the GCs produced by different densifi-cation processes using a Vickers indenter. The indentation hysteresis behavior was found on curves of indentation load versus indentation depth for all indentation loads from 10 to 500 N. For GCs, it was also observed that ring/cone cracks were induced by Vickers indentation instead of the median/radial crack systems more usually observed in brittle materials under a sharp indenter. A superior feature of GCs compared to ordinary brittle ceramics is their resistance to strength degradation by contact with hard particles as demonstrated by anomalous ring/cone crack formation. 

K. Zeng and D. Rowcliffe, Analysis of Penetration Curves Produced by Sharp Indentation on Ceramic Materials, Phil. Mag. A, 74 (5) 1107 1116 (1996). 

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