Physical Properties

    Graphite comes in many different grades and classes of both naturally occurring and manufactured forms. The

following discussion of the mechanical and strength properties is taken as general and/or specific cases. In the

discussion of physical properties of the material, there are specific criteria which map the unique characteristics of 

the material and make for a basis of comparison between it and other materials.

Young’s Modulus

        Young’s Modulus is the relationship between the stress (force per unit area) and strain (change in length due

to force) that a material can tolerate before reaching its yield, or fracture point. Determination of the graphite

modulus is usually made by observations of testing done on polycrystalline rods (Ubbelohde, 41). These rods are

tested by the sudden application of tensile stresses at some specific temperature (Ubbelohde, 41). In the case of

graphite, as the temperature rises, the modulus increases by values of 50, and even up to 100 per cent its room

temperature value until a temperature of about 2500 C is reached (Ubbelohde, 42). The change in the modulus over

this temperature interval is not solely increase however; the modulus has a range of decrease between 1200 C and

2000 C, and then a maximum around 2500 C. Beyond this point, the modulus decreases for higher temperatures

(Ubbelohde, 42). It should be noted that this is a general case for polycrystalline graphite.

        Other types of graphite of certain stocks have different moduli at these temperature ranges and at different

layer grain orientations. Single crystal, short time breaking strength of graphite is shown here in a comparison with

other high temperature materials (Ubbelohde, 11). We see here the comparison of the failure stress of graphite in a

four point stress test, versus a tensile stress test and note their similarity (Walker, 188). For all graphite samples,

the modulus is the greatest with forces acting parallel to the basal planes. Along this plane, the modulus is

approximately 130 X 10⁶ p.s.i. , which makes it "mechanically one of the strongest bonds in nature." (Reynolds,51).

Frictional Behavior

        The hardness of graphite is something that differs greatly with planar orientation. Along directions parallel to

the basal planes, it is rated at one or two on the Mohs scale, while the basal plane is rated as high as nine on the

Mohs scale (diamond being 10 on the scale) (Ubbelohde, 43). We see from this that on one orientation, the material

is very soft, and at a perpendicular orientation the material is extremely hard. One of the properties that graphite is

known for is its lubrication properties, which is due to the properties of its cleavage (Ubbelohde, 43).

        Easy movement of basal planes due to weak interplanar bonding contributes to the low effort with which the

structure is cleaved. Cleavage parallel to basal planes is not the sole mechanism for low friction however. We find

that the sliding of the planes over one another also depends on surface films that normally attach themselves to

graphite, as well as the movement of defects and dislocations in the lattice (Ubbelohde, 43). This may be

demonstrated by the procedure where graphite is heated in a vacuum to remove surface films and the observed

coefficient of friction in shearing forces increases from approximately 0.15 to 0.5 (Ubbelohde, 44). As well as

removing surface films, this heating removes atom defect sites. Upon introduction of this graphite to oxygen and

water vapor (such as in air) the coefficient goes immediately down to around 0.3 (Ubbelohde, 44).