The High Temperature Oxidation of Refractory Metals and Carbides in Molecular and Atomic Oxygen
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Department of Materials Science and Engineering
To: All Interested Faculty, Students and Research Scientists
Announcing: Ph.D. Dissertation Defense by Connor Stephens
Date: Monday, March 31, 2025
Time: 10:00am – 12:00pm
Location: Wilsdorf 200
Committee: Dr. Jon Ihlefeld (UVA MSE, Chair)
Dr. Prasanna Balachandran (UVA MSE)
Dr. Chloe Dedic (UVA MAE)
Dr. Lavina Backman (Naval Research Laboratory)
Dr. Elizabeth Opila (UVA MSE, Advisor)
Title: The High Temperature Oxidation of Refractory Metals and Carbides in Molecular and Atomic Oxygen
Abstract:
Leading-edge components on hypersonic vehicles, such as nose caps and wing tips, will experience temperatures greater than 2000°C in highly oxidizing environments. Additionally, shockwaves and high temperatures created during high Mach flight cause molecular oxygen to dissociate into atomic oxygen (AO). Component materials must be able to withstand these extremely oxidizing conditions while also maintaining their mechanical properties. State-of-the-art thermal protection systems are often Si-based due to the excellent protective capabilities of SiO2 scales against high temperature oxidation. However, these material systems are limited to temperatures less than ~1723°C because of active oxidation to form SiO(g) and SiO2 melting. Therefore, new materials systems are needed that are oxidation resistant and stable at higher temperatures than Si-based materials.
The transition metals, M, and metal carbides, MC (M = Ti, Zr, Hf, Ta) form oxides which have melting points greater than 1723°C. The carbides are considered candidate materials for leading-edge hypersonic components because they have high melting temperatures (>3000°C). However, they are often expensive and challenging to manufacture. The metals, on the other hand, are much more cost-effective and simpler to manufacture, but have significantly lower melting temperatures. Both the metals and carbides have poor oxidation resistance at high temperatures and the mechanisms which drive oxidation are not well understood. Despite the significant body of work regarding the oxidation of these materials, the role of carbon in the oxidation process has not been well documented. It is generally believed that carbides oxidize more rapidly than metals because the CO(g) generated during oxidation creates a porous oxide network which allows rapid oxygen ingress to the underlying material. However, there have been no direct comparisons between the oxidation behavior of the metals and carbides in identical experimental conditions which would isolate the role of carbon on the oxidation kinetics and mechanisms.
Additionally, the presence of AO in a high temperature oxidizing environment could significantly affect oxidation rates by lowering the energy barrier required for oxygen to react with materials. However, few studies have been conducted which focus on the effects of AO on oxidation. Primarily, this is due to the prohibitively expensive operating costs of the facilities traditionally used to generate AO in high temperatures, such as arc-jets and plasmatrons, which can cost upwards of $150K/day. These technologies also do not allow for the separation of AO effects from high temperature, pressure, or flow velocity. A new technique is needed to isolate the effects of AO on oxidation at high temperatures.
This work has three objectives: 1) conduct identical oxidation experiments of transition metals, M, and metal carbides, MC (M = Zr, Hf, Ta), which isolate the effects of carbon on oxidation and allow for kinetic and mechanistic comparisons to be drawn; 2) construct a new resistive heating system for ultra-high temperature oxidation experiments that is equipped with a DC microplasma for generating AO which can be used to isolate AO’s effects on oxidation; and 3) conduct oxidation experiments of the same materials in objective 1 in ordinary molecular oxygen and in AO-containing environments to determine the effects, if any, of AO on oxidation kinetics and mechanisms.
All interested persons are invited to attend.