Biomedical Engineering Reference
In-Depth Information
5.3.4 Grain structure
As mentioned previously, the term 'grain' refers to a crystal in a polycrystalline
material. Normally these grains are randomly oriented, but they can also
have a preferred orientation, referred to as 'texture'. Where two grains meet,
there is atomic mismatch, creating a grain boundary (Fig. 5.4b). Boundaries
are areas of high energy, making them more chemically reactive than their
surroundings. they also serve to improve mechanical properties by inhibiting
dislocation motion. heat treatments cause grains to grow in size, with the
driving force being the reduction in boundary energy. however, mechanical
properties are increased most when grains are smaller, as there are more grain
boundaries to stop the propagation of dislocations. Additionally, fine-grained
ceramics tend to have lower porosity since pores are removed by the easy
vacancy transport that can occur along grain boundaries. Lower porosity, of
course, means higher strength.
A smaller grain size improves many different mechanical properties. Surface
wear has been found to improve in ceramics with fine grain size (Wang
et
al.
, 2005) as well as strength and fracture toughness in ceramics and metals
(DeWith
et al.
, 1981; takaki
et al.
, 2001). The shape of grains also has an
influence on the mechanical properties, as discussed in more detail by (Lee
and Rainforth, 1994). At the same time, however, smaller grain size and
increased grain boundary area has been found to increase corrosion of metal
implant materials and increase dissolution of bioceramics
in vivo
(Placko
et
al.
, 1998; Porter
et al.
, 2003).
Grain size can be measured from micrographs showing the grain structure.
in some cases a sample may have to be etched with acid, which preferentially
attacks grain boundaries, to reveal the grain structure. By drawing a line across
the image and counting the number of times it intersects a grain boundary,
the grain size can then be calculated by comparing this result with the scale
bar.
5.3.5 Polymer structure and properties
Starting from the smallest structural level, characteristics like molecular weight,
tacticity, chain configuration, degree of polymerisation and crosslinking can
affect the behaviour of polymers. Properties such as crystallinity, melting
temperature and glass-transition temperature are determined by these
molecular-level characteristics. these, in turn, affect mechanical properties
and
in vivo
response.
Molecular-level structural characteristics
During the polymerisation process, single, repeating units (mers) combine to
form long chains of varying lengths. the molecular weight then represents
