Indentation toughness

Alternative geometries and test methods to determine the fracture toughness of ceramics and hard materials have been developed over the last decades Herzian indentation [88], the double torsion test [89], and compact tension tests [90 92]. 

The methods which have been developed to determine hardness vary in indentor geometry and definition of the contact area. In Brinell s method, a hard sphere of diameter D is indented into the softer sample, and the Brinell hardness is calculated from the diameter of the circular indent which is left in the surface after removing the load  

It is the Vickers test which is most commonly used to specify the hardness of hard materials, with results often quoted in kgmm , but more correctly in SI units, GPa. 

The repeated measurement of the size of indents, and the interpretation of indent geometry for the purposes of calculation, may be tedious, and operator bias is almost unavoidable. The edge of the impression is not always well defined, and misleading edge effects may be associated with anisotropic plasticity or plastic recovery. Faceted and elongated grains, or other microstructural features, together with the limitations of contrast and resolution in the optical microscope, complicate the interpretation, while the shape of the indent may differ in different materials so-called pin-cushion or barreled indents, associated with different constitutive relations and frictional shear on the faces of the indentor in contact with the plastic zone [3]. Mismeasurement of indent size is a major source of scatter in the experimental data and the relative errors in the results of different operators. 

Four microcrack morphologies may develop in a brittle solid after formation of the plastic zone beneath the indent median cracks, Fig. 9a, radial cracks (a halfpenny shaped elliptical crack Fig. 9b, much larger than the median crack), lateral cracks (in a plane normal to the median and radial cracks). Fig. 9c, and Palmqvist 

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Peters C. T., 1979, The relationship between Palmqvist indentation toughness and bulk fracture toughness for some WC-Co cemented carbides, J. Mater. Sci., 14, 1619-1623. 

Figure 8. Typical plastic deformation and cracking after indentation toughness test. Reproduced with permission of.(3).

Figure 9. Cracks formed during an indentation toughness test (a) median cracks (b) radial cracks (c) lateral cracks and (d) Palmqvist cracks. Reproduced with permission of.(3).

Given the variety of cracking mechanisms, both the stress field under load and the residual stress field are clearly very complex [101]. Difficulties of stress analysis have led to some inconsistency in the formulae derived to calculate indentation toughness. Rather than review the large number of published equations [102-107], a... 

Material designation Indent diagonal (2a) (pm) Crack length (2c) (pm) Ha Indentation toughness (MPa m )... 

Figure 10.6. (a) Indentation nanohardness of silver/chromium multilayers and single films of the constituent metals, as a function of depth affected by plastic deformation, (b) Charpy impact energies, a measure of fracture toughness, of three materials, as a function of test temperature they are mild steel, ultrahigh-carbon steel and a composite of the two kinds of steel (courtesy Dr. J. Wadsworth) (Fig. 10.6(b) is from Kum et at. (1983)). 

Since the early 1980s, the study of mechanical properties of materials on the nanometre scale has received much attention, as these properties are size dependent. The nanoindentation and nanoscratch are the important techniques for probing mechanical properties of materials in small volumes. Indentation load-displacement data contain a wealth of information. From the load-displacement data, many mechanical properties such as hardness and elastic modulus can be determined. The nanoindenter has also been used to measure the fracture toughness and fatigue properties of ul-... 

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 ... 

Flowever, in order to probe the fracture toughness of thin films or small volumes using ultra low load indentation, it is necessary to use special indenters with cracking thresholds lower than those observed with the Vickers or Berkovich indenters (for Vickers and Berkovich indenters, cracking... 

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... 

Palmquist S., 1962, Occurrence of crack formation during Vickers indentation as a measure of the toughness of hard metals, Arch. Eisenhuettenwes., 33 (9) 629-633. 

There are two main techniques used to measure the fracture toughness of ceramics fracture stress and hardness indentation. The former measures the load to fracture of a pre-cracked specimen using a single edge notched beam (SENB) or a chevron notched beam (CNB) sample. The main drawback of this technique is ensuring that the crack tip is atomically sharp. The second method uses the crack formed at the corners of the indentation produced during a Vickers indentation hardness test. This technique is rapid and relatively inexpensive. However, the toughness values measured are those of the surface, unlike the values obtained by fracture of the pre-cracked beams which are a measure of the bulk material properties. 

Figure 5.14 shows the depth-profile variation of hardness (Hv) and fracture toughness (KiC) of the hybrid sample. Fracture toughness is evidently higher near the surface due to the abundant presence of both AT and mullite. As the depth increases, the abundance of both phases and thus the fracture toughness decreases. Hardness is lower near the surface due to the abundant presence of much softer AT, but increases with depth towards the alumina region. It should be noted that since measurements were made on a polished cross-sectioned sample, the indentation data could not be collected at distances less than 0.5 mm from the surface. Hence, hardness and fracture toughness values on the surface were measured separately. 

Cottom, B.A., Mayo, M.J., Fracture toughness of nanocrystalline Zr02-3 mol% Y203 determined by Vickers indentation, Scripts Mater., 1996, 34(5) 809. 

Table 10.9 Effect of indentation load on microhardness and fracture toughness...

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