# The Impact of Cyclic Oxidation in Dissociated Air on the Mechanical Properties of Freeze-Cast ZrB2/MoSi2 Ceramics

## TL;DR

This study examines how ZrB2/MoSi2 ceramics withstand high-temperature oxidation, finding that they are suitable for reusable thermal shields below 1800 K.

## Contribution

The paper provides new insights into the oxidation behavior and mechanical degradation of ZrB2/MoSi2 ceramics under cyclic high-temperature conditions.

## Key findings

- At 1800 K, ZrB2/MoSi2 forms a protective silica-based oxide layer with semi-parabolic oxidation kinetics.
- At 2150 K, the material shows continuous linear oxidation and significant mechanical degradation.
- Nanoindentation revealed that oxidation weakens the material, suggesting a temperature limit for reusability.

## Abstract

Creating reusable thermal shields would decrease our carbon footprint by eliminating the need for the reapplication of single-use ablative alternatives. Our previous investigations identified ultra-high-temperature ZrB2 with 20 vol.% MoSi2 ceramics as a promising candidate for the fabrication of reusable thermal shields. Therefore, in this study, this material was exposed to cyclic oxidation at 1800 and 2150 K in dissociated air in order to investigate how it might withstand multiple terrestrial re-entries. At 1800 K, we observed semi-parabolic oxidation kinetics with the growth of a protective oxide layer, the silica-based composition of which was determined using XRD and SEM (coupled with EDS). More dramatic damage was observed at 2150 K, with continuous linear oxidation kinetics seen. Cross-section hardness measurements using nanoindentation revealed that the oxidized part of the samples was the source of their mechanical weakness, suggesting that the material should be used below 1800 K to ensure its reusability.