Indentation testing

In indentation testing a hard indenter made of either diamond, sapphire or steel and machined to a specific geometry - either a square based pyramid (Vickers indenter) or a spherical ball (Brinell indenter) - is pressed under load into the surface of a material either in the form of a crystal or compacted specimen. Although well recognized as a method for measuring hardness (see later) it may also be used to measure Young's modulus except that in this case it is the recovered depth after removal of the load that is important (Fig. 7). The test may be either static (Ridgway etal., 1970; Duncan-Hewitt and Weatherly, 1989a,b) or dynamic involving a pendulum (Hiestand etal., 1971).

For crystals a Vickers indenter is generally used and in this respect Duncan-Hewitt and Weatherly (1989a,b) have used a Leitz-Wetzler Mini-

fig. 6 Young's modulus versus cohesive energy density (CED) for various drugs and excipients; ----theoretical line, equation 14 (taken from Roberts eta/., 1991).

fig. 6 Young's modulus versus cohesive energy density (CED) for various drugs and excipients; ----theoretical line, equation 14 (taken from Roberts eta/., 1991).

Fig. 7 Loading geometry and recovery of indent for (a) Brinell and (b) Vickers hardness testers used to measure Young's modulus. F, applied load. For definitions of other terms, see text.

load hardness tester, applying a load of 147 mN. Single crystals were preselected and were mounted in plasticine with the surface normal to the indentation direction. Load was applied over 15 s and was maintained for 10 s. In the case of sucrose crystals, hardness anisotropy of the various faces was demonstrated (see later).

The equation used by Duncan-Hewitt and Weatherly (1989b) involves measuring the recovered depth of indentation and the length of the Vickers diagonal and substituting in the equation of Breval and Macmillan (1985):

2 Zr

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