Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 85

In ductile materials, the material deformation is first linear (the elastic domain),
then it bends (plastic domain) before decreasing (domain of damage and rupture).
In an amorphous or disordered environment, there is no lattice, and therefore no
dislocations. However, there are plastic deformations. For this type of material, the
irreversible deformations may be considered to correspond to a local restructuring.
When the stresses exerted on one of these areas exceed a threshold value, the atoms
or molecules rearrange themselves locally, resulting in a redistribution of the elastic
stresses in the material as a whole. Other zones in the material will in turn rear-
range themselves, causing a domino effect. Plastic deformation occurs at a constant
volume, a bit like if one slides playing cards over one another (Fig. 5.2) .

Stres s Fig. 5.2 Constant-volume

deformation

Plastic deformation is irreversible. Residual deformation still remains even when
the stress is relaxed.
Ductility refers to a material's ability to deform itself plastically without break-
ing. Rupture occurs when a defect (crack or cavity) induced by the plastic
deformation propagates itself. Therefore, ductility is a material's ability to resist
this propagation. Soft materials, some amorphous thermoplastics, as well as most
non-crystallized biological materials fracture in a ductile manner.

2.2 Abrasion Principle

Mechanical abrasion corresponds to rubbing a tool against a sample to cut it, to
reduce its thickness, or even to polish it. Unlike the fracture mechanism, which
results in sample rupture, the abrasion mechanism is still within the domain of
plastic deformation (Fig. 5.1) . Nevertheless, it creates defects in the sample. This
damage results from the application of a stress, corresponding to irreversible plastic
damage (strain hardening, dislocations, compression, and creation of cracks).
Abrasion always involves the loss of matter from the sample. The mechanical
action of abrasion corresponds to a loss of matter from microcracks or fissures in a
material and is caused by a tool. This type of preparation involves three components:
the material, the abrasive, and the lubricant. The determinant parameters of this
physical action are the stress applied, the rate of erosion, and the nature of the tool.
The abrasion mechanism causes damage at different depths in the material.
Depending on the properties of mechanical material, the damage caused to it may
reach a depth equal to three times the grain size of the abrasive used.
The damage caused is less significant in brittle materials than in ductile mate-
rials. This damage also has a varying degree of intensity depending on the stress