Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 99

Unless formed under highly controlled and, often, expensive conditions, metals and ceramics do not consist of a single crystal. Instead, they are composed of a large number of small crystals, called grains, of size approximately a fraction of a micron to several tens of microns (Figure 3.22). The material is described as polycrystalline. At the two‐dimensional interface where two grains meet, called the grain boundary, the atoms do not pack in an ordered crystalline arrangement because the atomic planes in adjacent grains have different orientations. Consequently, the grain boundary is generally considered a two‐dimensional or planar defect in crystalline solids. However, it is not an infinitesimally thin plane. Electron microscopy has shown that the grain boundary in pure solids is 0.5–1.0 nm wide and, in this region, the atoms are more loosely packed when compared to the atoms within the crystal itself. The presence of grain boundaries have a strong influence on several properties of polycrystalline materials. As the atoms are more loosely packed, grain boundaries provide an easier path for atoms to migrate through the material. Grain boundaries also obstruct the motion of dislocations due to the looser atomic packing and the change in lattice orientation, a property that is used in controlling the strength and ductility of metals (Chapter 6).

Figure 3.22 Illustration of (a) the boundary region between two grains and (b) part of a polycrystalline solid. The atomic planes in adjacent grains have different spatial orientation.

The atoms at a free surface have a different atomic environment and, often, a different composition than those within the solid itself. Consequently, the surface of a material can also be considered a two‐dimensional or planar defect. As the surface of a biomaterial has a strong influence on its performance in vivo, the surface characteristics of solids relevant to their use as biomaterials are discussed in detail in Chapter 5.