Oxidation of Refractory Multi Principal Element Alloy Thin Film

I’m back with another update on a project I’ve been pouring my heart into. This time, we’re exploring how a special kind of material- refractory multi-principal element alloy thin films- handles oxidation when heated in air at intermediate temperatures (300°C to 500°C). I’m working on this as part of a research effort to understand how we can make tough, high-performance alloys even more durable in harsh environments. It’s been a fascinating journey, and I’m excited to share what we’ve been up to, what we’ve learned, and why it matters. Let’s get into it!

What’s This Project About?

Imagine you’re designing parts for something that has to withstand heat and air, like components in a jet engine or a power plant. Over time, exposure to oxygen at high temperatures can cause metals to oxidize, forming oxide layers that can either protect the material or lead to its breakdown. Our project focuses on a high-entropy alloy (HEA) called CrMoNbTaW, which is made by mixing chromium, molybdenum, niobium, tantalum, and tungsten in roughly equal amounts. These HEAs are exciting because their complex mix of elements can lead to unique properties, like better resistance to wear and tear.

We’re specifically looking at how these thin films—super thin layers of the alloy deposited on sapphire wafers—behave when heated in air. We’re also testing two versions: one with just the CrMoNbTaW alloy and another with an aluminum (Al) cap layer on top. The goal is to see how adding chromium and an Al cap affects the formation of protective oxide layers, which act like a shield to slow down further oxidation. We’re digging into how these layers form, how they hold up, and how they change the material’s mechanical properties, like hardness and stiffness.

How We Made and Tested the Films

To create these films, we used a technique called DC magnetron co-sputtering, which is like painting with metal atoms. We used an AJA Orion system with individual targets for each metal—Cr, Mo, Nb, Ta, W, and Al for the capped version. After making the films, we annealed them in air at 300°C- 500°C for 20 hours in a preheated furnace, carefully controlling the temperature to within ±3°C. This mimics real-world conditions where materials are exposed to heat and oxygen for long periods. To figure out what happened during annealing, we used a ton of cool tools- X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Nanoindentation, and Thermodynamic Modeling.

Why This Matters

This research is a big deal because it helps us understand how to make HEAs more durable in real-world conditions. By figuring out how Cr alloying and Al capping improve oxidation resistance, we’re learning how to design alloys that can handle heat and oxygen without breaking down. This is huge for applications like aerospace components, gas turbines, or even nuclear reactors, where materials need to stay strong and stable in tough environments.

I love how this project combines so many different techniques- sputtering, microscopy, spectroscopy, and even thermodynamic modeling- to tell a complete story about what’s happening at the atomic and nanoscale levels. It’s like being a detective, piecing together clues from different tools to understand how these materials behave.

What’s Next?

We’re not done yet! Next, we want to dig deeper into how the oxide layers evolve at even higher temperatures and longer annealing times. We’re also curious about relevant bulk HEA systems where there’s an optimal mix that gives us the best of both worlds. Plus, we’re planning to test these materials in more realistic conditions, like cyclic heating and cooling with Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC), to see how they hold up over time.

Thanks for reading this far!