As mentioned above, the structure of pyrolytic carbon is essentially isotropic and so are its properties. These are summarized in Table 7.3.
Hardness: Being composed of minute crystallites with essentially random orientation, isotropic pyrolytic carbon lacks the easy interlayer slippage which is characteristic of the well-oriented laminar or columnar structures of pyrolytic graphite. As a result, it is considerably harder. This makes it easy to polish and the material can be given a high gloss. The wear resistance is usually superior to that of the columnar and laminar deposits of vitreous carbon.
Mechanical properties: Isotropic pyrolytic carbon is stronger than the oriented pyrolytic graphites and vitreous carbon and can ba considered as a reasonable structural material, comparing favorably with some of the more brittle ceramic materials.
Table 7.3: Properties of isotropic pyrolytic carbon at 25C.
| Density, g/cm3 | 2.1 |
| Vickers DPH hardness, kg/mm2 | 240-370 |
| Flexural stength, Mpa | 350 |
| Young’s modulus, Gpa | 28 |
| Strain to failure, % | 1.2 |
| DPH: diamond pyramid hardness | |
Gas permeability: Isotropic pyrolytic carbon provides a better barrier to gases than the more crystalline pyrolytic graphites. The permeability, K, is as follows:
Isotropic pyrolytic carbon: 10-6 to 10-15 cm2.s
Pyrolytic graphite: 10-2 to about 10 cm2.s