A4 Degradation by Oxidation at High Temperature

Topic

The formation of an oxide layer—the phenomenon described by this model—occurs in non-oxide ceramics exposed simultaneously to an oxidizing atmosphere and high temperatures. The combined action of these two conditions triggers the growth of an oxide layer on the ceramic's surface; neither condition alone is sufficient to initiate this growth.

This oxide layer growth does not proceed at a constant rate over time but follows a parabolic law: as the oxide layer thickens, it acts as a barrier between the oxidizing atmosphere and the unoxidized ceramic, causing the growth rate to slow down progressively—meaning it no longer scales linearly with elapsed time but instead decelerates from its initial speed.

The rate of this parabolic growth is determined by the parabolic oxidation constant, which in turn depends on temperature according to an Arrhenius relationship. Even a slight increase in temperature causes a disproportionately large increase in this constant, thereby accelerating the growth rate of the oxide layer.

Thus, the formation of an oxide layer on a non-oxide ceramic—the process initiating this model—depends on a complete chain of factors: the oxidizing atmosphere and high temperature that trigger it; the parabolic law that causes the growth to self-retard as the layer thickens; and the parabolic oxidation constant which, being temperature-sensitive according to the Arrhenius equation, dictates the overall speed of the process.

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