Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 86

applied, the type and grading of the abrasive used, the rubbing speed, and the
temperature induced in the material during the process. The stresses initiated by
abrasion can cause different types of defects to form in the material: dislocations,
strain hardening, twinning, microfractures, fissures, and the dispersion of gaps,
cavities, etc.
There may also be thermal effects that result in the localized re-fusion of the
material, its transformation, and a phase migration or loss. Lastly, chemical changes
due to pollution, matter transport, or chemical-element diffusion may occur.

2.2.1 Techniques Involving Cutting by Means of Mechanical Abrasion:
Sawing and Grinding
Cutting techniques are directional abrasions that lead to the separation of the sam-
ple into two or more parts. This is the case for sawing, cutting with a cutting tool,
ultrasonic grinding, and electrical-discharge machining (EDM). The stresses initi-
ated by abrasion cause cracks at different depths in the material, up to complete
cutting.
Sawing can be performed with a disk or wire, of which either the whole or outside
(internal or external) contains abrasive grains of different types (diamond, carbide,
etc.) and varying gradings. The use of a lubricant (water, oil, etc.) helps to eliminate
abraded grains and limit temperature rise during the process.
Ultrasonic cutting is performed using a hollow cutting tool in the necessary
shape. This tool is attached to an ultrasonic generator. Abrasive grains in solution
are placed on the sample surface, and the tool is moved into contact with the mate-
rial. The tool is vibrated (and therefore the abrasive grains are vibrated as well), and
the application of pressure helps to cut the material.
The experimental conditions chosen depend on the characteristics of the tool
used (wire saw, wheel saw, cutting tool with or without ultrasound, etc.) and the
nature of the sample.
Cutting is a preliminary preparation technique inducing defects that must be
eliminated by the thin slice preparation technique. However, the more defects intro-
duced in the sample at this level, the more difficult it is to eliminate them during
final preparation.

2.2.2 Abrasive Techniques: Mechanical Polishing, Dimpling, and Tripod
Polishing
The process of abrasion uses abrasive grains of decreasing grain sizes down to below
0.025 m, in order to minimize material surface roughness.

Abrasion by mechanical polishing generally uses polishing disks containing car-
bide grains (SiC, ZrC, etc.) of decreasing size (e.g., 60, 30, 12, 7

µ

m) that are
embedded in the support. Carbide abrasion is followed by diamond abrasion, either
using diamond pastes or diamond sprays, also of decreasing grain sizes (6, 3, 1,
0.25, 0.1

µ

µ m).