Molded graphite are usually characterized by the following properties:
- Apparent density
- Electrical resistivity
- Young’s modulus
- Coefficient of thermal expansion (CET)
- Flexural strength
The test methods for these and other useful properties are listed in Table 5.3. When appropriate, the tests are performed both “with the grain” and “perpendicular to the grain”.
The hardness is generally measured with a Shore model C-2 Scleroscope with a specially calibrated diamond-tip hammer.
Table 5.3. Test methods for molded graphite:
Hardness ANSI/NEMA CB1-1977
ASTM C-886
Density, porosity ANSI/NEMA CB1-1977
ANSI C-64.1
ASTM C559-79
ASTM C 2080a
ASTM C819-77
Electrical resistivity ANSI/NEMA CB1-1977
ASTM C611-69(76)
Flexural strength ANSI/NEMA CB1-1977
ANSI C-64.1
ASTM C651-70(77)
Compressive strength ASTM C695-75
Tensile strength ASTM C749-73(79)
Thermal expansion ASTM E228-71(79)
Modulus of elasticity ASTM 111-61(78)
Ash content ANSI/NEMA CB1-1977
ASTM C4561-69(79)
Moisture content ASTM C562-69(79)
Testing for flexural strength is critical and the results can vary widely depending on the sample size and geometry and on the test method. For instance, the flexural strength of a square cross-section of a molded graphite, tested in four-point test fixture, can be 40% lower than the measured strength of the same material with a circular cross-section tested in a three-point loading fixture. For that reason, comparing the strength of various molded graphites can be meaningless, if the test methods are different.
The thermal coefficient of expansion of graphite is not a fixed value but varies with temperature. For that reason, the CTE should always be quoted with the range of temperature of the test. When testing above 350C, it is necessary to do so in an inert atmosphere to avoid oxidation of the graphite.
The properties of molded graphite are reviewed in the following sections. The values given are taken from the open literature and manufacturer’s data sheets and must be viewed accordingly.