Introduction to Materials - Sample Preparation Topic for Transmission Electron Microscopy - page 9

in TEM sample preparation, given the fact that one must go frommacroscopic forms
to microscopic dimensions by cutting the initial material and/or performing different
types of mechanical polishing. The fundamental properties of materials are their
rigidity, tensile strength, and plasticity. Material hardness ranges from hard to soft; it
is measured by the Mohs scale, which, among other things, characterizes resistance
to indentation through pressure. Hardness is a function of the rigidity of the crystal
network or the degree of reticulation that exists between the atoms or molecules
making up a material. It may be supplemented by the notion of elasticity, measured
by Young's modulus. Elasticity is characterized by the material's ability to return
to its original shape and volume after a stress is applied. Tensile strength is the
evaluation of the material resistance, ranging from brittle to ductile. A material's
ductility is its ability to deform under a given stress without breaking. Thus, the
breaking point is defined as the resistance to the propagation of cracks before the
material breaks. A material is brittle if it breaks easily. Ductility enables the material
to be shaped. For softer and therefore more ductile materials, we refer to plasticity.
2.3.1 Mechanical Properties and Crystallinity
A material's fragility and ductility first depends on the interatomic bonds. Not all
crystalline materials are ductile: Ductility is a characteristic feature of a deformation
produced by the gliding of atomic planes. It involves the presence of crystal defects
such as dislocations. These dislocations must not only be able to form but also move
easily under the effect of a stress.
In a metallic material , the absence of preferential direction of the bonds between
atoms facilitates the displacement of dislocations and will not result in the definitive
rupture of the bonds when these dislocations are displaced under the effect of the
deformation. The ductility of a metal is even greater if there are a large number of
different slip planes.
Thus, the ductility of metals with face-centered cubic (fcc) structures is greater
than the ductility of metals with body-centered cubic (bcc) structures, which is, in
turn, greater than that of metals with hexagonal close-packed (hcp) structures.
In a material with covalent bonds , which are highly directional, the displacement
of the dislocation generally results in a definitive rupture of the bond between the
atoms and a rupture of the material along the glide plane. Materials with covalent
bonds will be brittle.
In a material with ionic bonds , the bonds are also directional. These materials
will display brittle behavior.
In organic materials (most polymers and biological materials), the structure
is amorphous, and yet they may be ductile. In these materials, ductility may not
result from dislocation displacement, since dislocations may not be present in a
non-crystalline material. Elasticity and ductility in these materials are linked to the
flexibility of molecular chains and their deformable macromolecular configuration.
In soft biological organic matter, the significant presence of water increases the flex-
ibility of the molecular chains and their ability to deform and return to their original
shape.