The kinetics of oxidation of ZrC is related to the nature of the oxidation products and microstructure of the oxide scale formed. In general, oxidation accompanied by the formation of a protective scale is associated with low temperatures. The oxide scale is reported to be protective and its growth to be para­bolic with time, that is, controlled by diffusion of oxygen through the cubic ZrO2 scale.134,137,140,143,152

The cubic ZrO2 structure is hypothesized to facilitate adherence of the scale to the cubic ZrC, as evidenced by high-resolution TEM of oxidized single-crystal ZrC with adherent cubic ZrO2 crystallites face having parallel lattice fringes.144

At high temperatures (and high Po2 usually), oxi­dation is reported to be linear with time, controlled by the reaction at the oxide-carbide inter — face.134,147,152 This process is facilitated by porosity in the oxide, rupture due to CO/CO2 gas pressure, or cracking/delamination due to thermal expansion

mismatch between the oxide and carbide,136,140,144,153

and interruptions in the scale allow oxygen easy access to the ZrC surface. Preferential attack of ZrC along the grain boundaries by ZrO2, leading to inter­granular embrittlement, strengthens the case against protective oxidation of ZrC.154,155

In the case of a two-layer scale on single-crystal ZrC097, the inner carbon-rich scale grew paraboli­cally, leveling out at a critical thickness (2-4 pm), above which the outer carbon-poor scale grew line­arly with time.144,146 The inner scale was crack-free, dense, and adherent, while the outer scale was char­acterized by cracks and porosity. The authors proposed that scale cracking continually exposed fresh surfaces to oxygen, leading to an overall linear oxidation rate.

Oxidation behavior also depends on the pressures of volatile ZrOx species, implicated in active oxida­tion. According to thermodynamic calculations by Maitre and Lefort156 and Minato and Fukuda157 of equilibrium pressures in the Zr-C-O and Zr-C-O — He systems, the maximum pressure of ZrO(g) at 1600 K was predicted to be less than 10~9 Pa, indicat­ing negligible active oxidation.

2.13.6.1.2 Oxidation by water vapor

One study of oxidation by water vapor is reported.134 At 723-843 K, with PO2 > 0.5 kPa (i. e., sufficient for surface saturation with adsorbed oxygen) and Ph2o 21-42 kPa, water vapor did not appreciably oxidize ZrC but did accelerate the oxidation rate in the presence of O2(g).

2.13.6.1.3 Summary and outlook

Clearly, oxidation resistance of monolithic ZrC is compromised at temperatures above about 700 K, and its use at higher temperatures is restricted to inert or reducing atmospheres. To enable the predic­tion of structural component performance under acci­dental oxidizing conditions, it is important to understand the effects of oxidation on mechanical properties. For instance, strength and toughness deg­radation must be characterized for ZrC partially oxi­dized to ZrO2 and ZrC^Oj,, and failure mechanisms must be identified, such as grain boundary attack or scale rupture. Oxidation to ZrO2 will also lower ther­mal conductivity, which is relevant to applications where the high thermal conductivity of ZrC is exploited, but heat transport in ZrC^O,, must also be established.