The High Temperature Oxidation of Refractory Metals and Carbides in Molecular and Atomic Oxygen

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 = 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 primary 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.
In Objective 1, it was observed that the transition metals investigated all undergo breakaway oxidation caused by cracking in the grown oxides. This breakaway transition from a solid-state diffusion-limited oxidation mechanism to a gas-phase diffusion mechanism leads to a much higher oxidation rate post-breakaway. In general, the carbides form a porous oxide and do not experience breakaway oxidation. Despite the porous oxide, the recession rate of the carbides is slower than the post-breakaway recession rate of the metals; therefore, the carbides are the preferred material for longer-term oxidation resistance. The effects of carbon on the oxidation mechanism are determined to be: 1) reduction in oxygen solubility in the underlying material, 2) CO(g) formation creates a porous oxide, driving gas-phase diffusion but preventing significant cracking in the oxide, and 3) increasing the melting point and stiffness of the underlying material.
In Objective 2, a microplasma resistive heating system (MRHS) was constructed capable of dissociating oxygen with up to 40% efficiency and reaching sample temperatures up to 2400°C. In Objective 3, this new MRHS was used to evaluate the oxidation of the same transition metals and carbides from Objective 1. It was found that atomic oxygen drives more rapid oxidation when oxygen transport is governed by gas-phase diffusion. Oxidation rates are unchanged in situations where solid-state diffusion is the rate-limiting mechanism. Additionally, the increased reactivity of atomic oxygen can affect the microstructure and morphology of the grown oxides by changing layer thicknesses, enhancing grain boundary oxidation, and embrittling the underlying material.