Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 61

Covalent‐bonded ceramics are characterized by intrinsic properties similar in nature to those described for ionic‐bonded ceramics (Section 2.6) but as the covalent bond can be stronger than the ionic bond, many covalent‐bonded ceramics have better mechanical properties, such as strength, elastic modulus and hardness, and a higher melting point.

Covalent bonding appears in diamond, a crystalline form of carbon, often regarded as a prototypical example of covalent‐bonded ceramics. The atoms bonded to a given atom point to the corners of tetrahedron due to sp³ hybridization of the electron orbitals of the carbon atoms, giving a directional form of bonding (Chapter 3). Covalent bonding is the dominant type of bonding in many other ceramics, including carbides such as silicon carbide (SiC), and nitrides such as silicon nitride (Si₃N₄), and in the network structure of glasses (Chapter 3). It contributes to the bonding in some oxides such as Al₂O₃ and high melting point metals such as tungsten and tantalum.

Covalent‐bonded ceramics find some use as biomaterials. Examples include silicon nitride used as implants in spinal repair, a few nondegradable glasses in some medical devices and a few bioactive glasses used in healing bone defects and skin wounds (Chapter 7). Carbon materials such as diamond and graphite are often classified as ceramics but their properties are more relevant to industrial applications (Chapter 3). Pyrolytic carbon is an important material in the manufacture of heart valves whereas diamond‐like carbon (DLC) has been investigated as coatings for articulating bearings in hip and knee implants. More recently discovered allotropes of carbon such as fullerenes, carbon nanotubes, and graphene, have been the subject of an enormous amount of investigations for use in technological applications and are now receiving considerable interest for potential biomedical applications.