R.C. Bradt

Effect of air oxidation at 873 K on the mechanical properties of a carbon-carbon composite

Abstract A carbon-carbon composite, composed of semi-random chopped PAN fibers in a resin char and CVD carbon matrix, was oxidized in air at 873 K to burn-offs up to 55%. Composite reactivity increased with increasing burn-off up to about 10% and thereafter remained constant at 1.5%/h. Oxidation had a catastrophic effect on mechanical properties. For example, at 20% burn-off, Youngs modulus, flexural strength, work-of-fracture, and fracture toughness were reduced by: 75, 64, 57 and 61%, respectively. Decrease in flexural strength is attributed primarily to a decrease in fracture toughness rather than an increase in flaw size.

The fracture toughness of glassy carbons at elevated temperatures

Abstract The fracture toughness of three different glassy carbons heat treated at 1000, 2000 and 3000°C was measured at temperatures between 25 and 750°C. Youngs elastic modulus varies inversely with measurement temperatures, whereas fracture toughness varies directly. Fracture characteristics of the glassy carbons are compared with those of float glass and graphites.

Oxidation effects on toughness and slow crack growth in polycrystalline graphites

Abstract The fracture toughness ( K Ic ) and slow crack growth behavior of four fine-grained polycrystalline graphites, oxidized to 5, 10 and 20% weight losses, were measured in air at room temperature. Exponential decreases in the elastic moduli as well as decreases in the fracture surface energy contributed to lowering K Ic . Oxidation generally shifted the stress intensity-crack velocity ( K I - V I ) diagram to lower stress intensity values, and decreased the slope, or N -value. Scanning electron microscope fractography revealed that a combination of filler particle and binder phase degradation with increasing oxidation was responsible for the decreased toughness and changes in the ( K I - V I ) characteristics. Oxidation conditions were shown to significantly affect the magnitude of decreases in the physical, elastic and mechanical properties of these graphites.

Subcritical crack growth in fine grained polycrystalline graphites

Abstract Subcritical crack growth was measured in four fine grained polycrystalline graphites in air at room temperature. Filler and textural effects were observed at stress intensity factors (KI) near the critical stress intensity (KIc). Scanning electron microscope fractography permitted correlation with processing conditions and microstructure. The results are compared with those for other carbon materials.

Gasification induced densification of form coke

Abstract Shrinkage of form coke during its gasification in CO 2 in the kinetically limited regime (zone 1) is found. This shrinkage is attributed to preferential gasification of crosslinks between small, poorly aligned carbon crystallites, thereby allowing their improved alignment to proceed during gasification.

Subcritical crack growth of glassy carbon in water

Abstract The subcritical crack growth characteristics of glassy carbons in water were measured and are correlated with their elastic moduli. It is observed that the susceptibility to fatigue varies inversely with the elastic modulus, which is directly related to the crack growth parameter N . Crack propagation characteristics of glassy carbons are compared with those of silicate glasses and amorphous metals.

Oxidation effects on CTE and thermal shock fracture initiation in polycrystalline graphites

The coefficient of thermal expansion (CTE) was measured as a function of oxidation for three commercial fine-grained graphites derived from petroleum cokes and coal tar pitches and fabricated by extrusion, undirectional molding, and isostatic molding. The CTE was observed to vary with the crystallite size and the preferred orientation and to decrease as much as 20% with increasing oxidation. This CTE decrease was attributed to an increase of the accommodation by Mrozowski cracks enlarged by the oxidation process. Effects on thermal shock fracture initiation were examined by estimating changes in the thermal shock resistance parameter, R. It is concluded that in spite of the continuous decrease in CTE, changes in R with oxidation are not continuous for these graphites. The complexity is a consequence of the different extents to which graphite oxidation affects CTE, strength and the Youngs elastic modulus.

Mechanical properties of formcoke after reaction in CO2 and COCO2 environments

The mechanical properties of a special formcoke were studied after reaction in CO2 and in COCO2 mixtures. The effects of iron- and potassium-catalyzed gasification were also investigated. The Youngs elastic modulus, fracture toughness and fracture surface energy were all observed to decrease with increasing amounts of reaction of the formcoke. These properties were observed to be strongly correlated to the final bulk density rather than to the particular reaction conditions.