Some important elements relate to ceramics

Some important elements relate to ceramics
In this section several elements of engineering importance are discussed: Carbon, silicon, and boron. We encounter these materials on occasion in subsequent chapters. Although they are not ceramic materials according to our definition, they sometimes compete for applications with ceramics. And they have important applications of their own. Basic data on these elements are presented in Table 9.6
Table 9.6. Some Basic Date and Properties of Carbon,
Silicon and Boron.
Boron Silicon Carbon Property
B
5
2.34
36860 F
(20300 C)
9.3 Si
14
242
25700 F
(14100 C)
7 C
6
225
6740 Fa
(3727 C)
10h
I0 Symbol
Atomic number
Specific
Melting Temperature

Hardness (Mohs scale)


aCarbon sublimes vaporizes tather than mch.
bCarbon in from of diamond.
cCarbon in from of graphic typical value given.
1. Carbon
Carbon occurs in two alternative Forms of engineering and commercial importance: graphite and diamond. They compete with ceramics in various applications: in situations where its refractory properties are important, and diamond in industrial applica¬tions where hardness is the critical factor (such as cutting and grinding tools).
2. Graphite
Graphite has a high content of crystalline carbon in [lie form of layers Bonding between atoms in the layers is covalent and therefore strong, but the parallel layers are bonded to each other by weak Van der Waals forces. This structure makes graphite very anisotropic; strength and other properties vary significantly with direction. This explains why graphite can be used both as a lubricant and as a fiber in advanced com¬posite materials. In powder form, graphite possesses very low frictional due to the ease with which it shears between the layers, in this form. graphite is valued as a lubricant. In fiber form, graphite is oriented in the hexagonal planar direction to produce a filament material of very high strength and elastic modulus. These graphite fibers are used in structural composites ranging from tennis rackets to fighter aircraft Components.
Graphite exhibits certain high-temperature properties that are both useful and unusual. It is resistant to thermal shock and its strength actually increases with temperature. Tensile strength at room temperature is about 15.000 Ib/in.2 (100 :MPa). but increases to about twice this value at 45300 F ( 25000 C) [3]. Theoretical density of car¬bon is 2.25 g/cm3, but apparent density of bulk graphite is lower due to porosity (around 1.7 g/cm3 ). This is increased through compacting and heating. It is electrically con¬ductive, but its conductivity is not as high as most metals.
A disadvantage of graphite is that it oxidizes in air above around 9000 F ( 5000 C). In a reducing atmosphere it can he used up to around 54000 F ( 30000 C), only a few hundred degrees below its sublimation point of 67400 F (37270 C).
The traditional form of graphite is polycrystalline with with a certain amount of amor¬phous carbon in the mixture. Graphite crystals are often oriented (to a limited degree) in the commercial production process to enhance properties in a preferred direction for the application. Also, strength is improved by reducing grain size (similar to ceramics).