Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 87

Dimpling uses the same diamond pastes as those used for mechanical polishing.
The final polishing can be done using colloidal silica or another colloid of a very
small grain size.
The tripod technique uses supports in which diamond (alumina, etc.) grains are
embedded within the support and are therefore immobile. This enables better quality
unidirectional polishing.
Even when the best polishing conditions are chosen, such as those used in the
tripod technique, there are always residual microcracks at the last level of mechani-
cal polishing. These are to be eliminated, if the type of material so allows, through
the use of an additional ionic or chemical technique to produce a thin slice lacking
defects visible in the TEM.
2.3 Rupture Principles
Unlike abrasion, rupture does not involve any loss of sample matter. It involves a
fracture induced within the sample, resulting in the sample separating into two or
more parts. In the case we are interested in, a tool induces the crack.
Rupture is heavily influenced by the presence of internal defects such as micro-
cracks, pores, inclusions of fragile particles, material heterogeneity, and notches
(microcracks resulting from manufacturing or design defects).
Fracture involves three phases: a crack-initiation phase, a crack-propagation
phase, and a sample-rupture phase under the effect of stored elastic energy.
Through geometric focusing, the crack causes a high concentration of stress at its
end that is capable of breaking the atomic bonds. Rupture mechanics show that this
phenomenon depends on two magnitudes: the stress applied and the dimensions of
the microcracks. These two magnitudes are combined in the stress intensity factor.
For a single material, the critical stress can vary depending on the dimensions of the
microcracks present in the material. The rupture occurs when the critical value is
reached.
Depending on the preparation technique, the crack initiation is what will vary;
the propagation of the crack and the rupture develop naturally from the application
of a stress.
In the case of crushing, the start of the crack is produced by the crushing of
the material between a mortar and pestle. In the case of cleavage and sectioning
by ultramicrotomy, the start of the crack is produced by introducing a tip (or knife
blade) into the edge of the sample.
In the case of a freeze fracture performed with a double cupel, the start of the
fracture is caused by opening the cupel.
2.3.1 Techniques Involving Fracture: Crushing, Wedge Cleavage,
Ultramicrotomy, and Freeze Fracture
Crushing applies to brittle materials or materials made brittle by cooling. It involves
applying a stress powerful enough to cause the material to rupture in random