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

Abstract

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 Young's elastic modulus.