Introduction to Materials - Sample Preparation Topic for Transmission Electron Microscopy - page 10

2.3.2 Rigidity: From Hard to Soft
The hardness of a material characterizes its resistance to penetration. It also char-
acterizes the intensity of the atomic bonds, structure, and crystallization. It may
be measured by the impression of an indenter of known geometry (tempered steel
billet, square-based diamond pyramid, or tempered diamond cone) applied to the
surface of a material. The smaller the indentation left behind, the harder the mate-
rial. On the other hand, a material that is not hard is therefore soft; in other words,
it is easily deformable, penetrable, or abraded. While hardness is not a simple prop-
erty to define, its measurement gives a good idea of the mechanical properties of a
material. There are several hardness scales based on the Vickers, Brinell, Rockwell,
and Shore measurement tests. The well-known Mohs scale is based on resistance to
abrasion: Each body scratches the material below it and is, in turn, scratched by the
material above it. This makes it easy to gauge the hardness of a material. The scale
is composed of 10 materials ranging from talc to diamond, with 1 denoting minimal
hardness and 10 indicating maximum hardness.
For informational purposes, Table 2.1 shows examples for all of the types of
materials, which are expressed using the Mohs scale.
Metals have variable hardnesses ranging from very soft (copper) to very hard
(certain metal alloys). Semiconductors generally have average hardnesses. Minerals
have variable hardnesses. Ceramics are generally harder than the preceding classes.
Polymers have widely ranging hardnesses.
Animal biological organic matter, tied to its high proportion of water content, is
soft to very soft. Plant organic material, less hydrated and composed of molecules
that are often oriented (cellulose and lignin), is harder.
2.3.3 Tensile Strength: Ductility-Brittleness
A material's brittleness derives from the fact that it cannot deform under a stress
without breaking due to the rigidity of the crystal lattice. Generally, the most duc-
tile materials are found among metals and alloys. On the other hand, ceramics have
ionic bonds and exhibit brittle rupture. For example, materials such as quartz min-
erals or the ceramics Al 2 O 3 and SiO 2 exhibit brittle behavior. Between the two
types of materials, semiconductors (whose crystal structure is formed by covalent
and therefore directional bonds) have intermediate properties with regard to plas-
tic deformation. For informational purposes, Table 2.2 shows examples of materials
that are brittle, of average brittleness, resistant, or ductile.
Despite the presence of strong bonds, polymers are deformable. This is due
to chemical compositions that are made up of light atomic elements (C, H, and
O), an amorphous structure, and the flexibility of molecular chains. The hardness
of polymers can vary widely at room temperature, depending on the glass tran-
sition temperature. They have a high elasticity, meaning that they can withstand
deformations and may return to their original shape. The same principle applies to
non-mineralized organic biological matter; however, this often crystallized matter is
brittle.