Q&A: Bioinspired nacre-like alumina with a bulk-metallic glass-forming alloy as a compliant phase

Amy Wat is a recent doctoral graduate from the Materials Sciences and Engineering department at UC Berkeley. Together with Je In Lee from Seoul National University, she is the first author on a highly collaborative paper about melt-infiltrated ceramic-metal composites published in Nature Communications entitled ‘Bioinspired nacre-like alumina with a bulk-metallic glass-forming alloy as a compliant phase’. Kristina Kareh, the editor who handled the paper, asks her some behind-the-scenes questions about this work.
Q&A: Bioinspired nacre-like alumina with a bulk-metallic glass-forming alloy as a compliant phase

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1. What are the key findings of your paper?

Our main goal when we started this work was to create a tough ceramic. Ceramics are great materials that are stable at high temperatures, but they suffer from brittleness, which makes them unusable in structural applications because the probability of catastrophic failure is very high. We therefore elected to be inspired by nacre,  the mother-of-pearl portion of abalone shells, which is 95 vol% ceramic and 5 vol% biopolymers arranged in a brick-and-mortar structure. This microstructure can be considered as a gold standard for toughness because it imparts three orders of magnitude higher fracture toughness (in energy terms) than its components.

Today, we know that biological structures such as nacre have great specific strength and toughness due to their hierarchical organisation and their complex microstructure. In the case of nacre, the ‘brick’ is hard inorganic ceramic platelets and the ‘mortar’ a soft organic matrix holding the platelets together. Previous work has tried to replicate this organisation using synthetic structures, and it has been successful:  many of the synthetic brick-and-mortar structures that are artificially man-made use a ceramic as a strengthening phase and a polymer to hold the ceramic phase together. One of the reasons for using a polymer as mortar, is that they have an easier time infiltrating highly dense porous structures, especially when there is upwards of 80% ceramic. However, one of the problems of using polymers is that they cannot operate at very high temperatures. While ductile, they are also not intrinsically strong.

The key finding in our work, and why we are very excited about it, is that we have created a brick-and-mortar ceramic which uses metal as the mortar. This is difficult because metals don’t usually wet ceramics, and the higher the ceramic content, the more difficult it is to force the metal into the porous ceramic structure.  This brings us to study a particular metallic material called bulk metallic glass (BMG), which is an alloy with an amorphous atomic structure. Due to the reactive wetting behavior and temperature-sensitive viscosity of the BMG melt, the alumina scaffolds were rapidly infiltrated by the molten BMG in ten minutes without any applied pressure. The resulting material has a fracture toughness that is three times higher than monolithic alumina because it successfully replicates the crack deflection and brick pullout mechanisms found in natural nacre.

2. What was your role in this work?

This work was a highly collaborative work with many different labs across five different countries. My own expertise is in macroscale mechanical properties and casting the ceramic scaffolds. I therefore cast the scaffolds and designed the experiment to study the macroscale properties of our composite such as the toughness, while the metallic infiltration was done by my colleague Je In Lee at Seoul National University. We also collaborated with the Ishikawa lab in Japan, Meyer group in Germany, and the Kiener group in Austria to measure the thermophysical properties and microscale properties of our designed ceramic composite.  We had a lot of assistance from internal and external facilities and many people contributed their expertise. What was really amazing was when Je In Lee visited Berkeley: meeting him in person and being able to discuss our work together face-to-face was invaluable.

3. Everyone has a story: how did you come to do this particular piece of research?

I have always been excited about creating new materials, and how they can be incorporated into new applications. I joined the Ritchie group after the initial brick-and-mortar paper was published, and I really wanted to work on this idea. At that time, a key issue was how to incorporate a metallic phase within the freeze-cast brick-and-mortar materials the group has pioneered. We found that reactive wetting with BMGs provided an interesting solution to the problem, but found that we needed assistance processing and infiltrating a ceramic scaffold with the BMG. We brought in Prof Eun Soo Park and his group from Seoul National University to provide this expertise. Together with Je In, we found a straightforward route to infiltrate the porous alumina, and it seemed like this could spawn a lot of interesting ideas. I was very fortunate, because I joined the Ritchie group at Berkeley National Lab as an undergraduate intern, during what was supposed to be a short undergraduate internship, and I loved the work so much that I decided to stay on as a graduate student. This feels like the very satisfying culmination of a project that was not easy because it had so many moving parts.

4. What was the most critical moment during the study? Any anecdotes you’d like to share with us about challenges with obtaining these results and preparing the manuscript?

Apart from the various ‘oh no!’ moments associated with broken equipment, which I am sure is a story in all projects, one critical moment was when we knew there was something exciting to report: when the material was first created, and that I shared the first scanning electron micrograph of it with my collaborators. It is the feeling that you have reached for what you were striving for, and that you just know that you have something so exciting to talk about, despite the various rate limiting steps during fabrication. For example, one of the challenges for creating these samples are how low the yield can be when trying to form a brick-and-mortar ceramic scaffold. This is caused by the need to press the cast alumina scaffolds to densify the scaffold from the lamellar structure to the brick-and-mortar structure. However, after discussing this issue at length with the Tomsia group, I found that changes in the casting technique can vastly improve the yield of the brick-and-mortar ceramics, which made this project feasible.

This generated a lot of data and a lot of results. One of my main challenges was how to write the paper and distill the results to highlight what we had accomplished. When the experimental parts came together, I had no outline and a very large amount of data.  Due to the various data and results presented in this paper, it was challenging to write about the work in a way that was cohesive and readable. There was a processing story about the key variables that made rapid, pressureless infiltration possible and another story about how different processing conditions leads to changes in the mechanical properties of the material. However, by writing about this work as a single paper, we can highlight the relationship between the processing and the mechanical properties of these materials. This allows the paper to be a short manual on how to use reactive wetting to create bioinspired materials and what considerations to make for its ease of processing and its mechanical properties.

5. What do you hope will be the impact of your research?

I hope that this piece of work will create a more nuanced understanding of interfaces between different phases, especially when creating these sorts of materials. One reason to look for strength and toughness and develop brick-and-mortar architectures is that they are great for high temperature applications. Incorporating a metal allows us to push the high-temperature angle, making it attainable, and also highlight the importance of the interface between the matrix and the ceramic (which is usually overlooked). Increasing the strength and toughness of a composite because of the higher shear strength of the matrix is great, but if you do not account for the stresses between the brick and the mortar, you will not be able to perfectly replicate what is seen in similar natural materials. By highlighting this issue, I hope that other researchers can find new and other ways of creating composite materials that take this aspect into account.     

6. Which is the development that you would really like to see in the next 10 years?

The key to critical structural applications is a good combination of strength, toughness, lightness and high temperature stability. Imparting toughness to brittle materials could lead to more lightweight armour and more lightweight high-temperature turbine blades and engines, which could function at higher temperature and therefore be more efficient. While there have been fascinating insights to how to build bioinspired materials from the bottom-up using 3D printing or biomineralization, are there other methods to build bioinspired materials from the bottom up?  When we create these materials, how do the interfaces react to corrosive environments or at high temperature? These issues are key for building bioinspired materials for aggressive environments.

Another development I would like to see in the next ten years is that more disciplines, such as ceramicists and chemists or polymer scientists, collaborate to create bioinspired structural ceramics. I find that a variety of expertise is essential for solving issues for how to merge two dissimilar materials together to form different hierarchical microstructures and multiple functions that were otherwise not possible. After many decades of developing sophisticated tools to study the materials, I am excited to see bioinspired materials community to pursue new materials that combines multiple capabilities, such as high damage tolerance, self-healing, and well-designed surfaces to control adhesion. There are many other things to consider in bioinspired materials: what is the effect of the orientation of different phases, could other processing techniques lead to the same or to an improved result, could techniques potentially be combined? This is a critical, long-term problem that can only be solved through interdisciplinary research.

7. What would your dream conference be like?

What a good question! I would love a conference that is a mix of all the symposia I enjoy. It would emphasise new biological materials, fracture toughness, strength, interesting processing techniques and materials for difficult environments. A focus on inspiration for new material systems derived from biological materials would also be really exciting: from obtaining the brick-and-mortar architecture in different materials to discussing completely new microstructures…These symposia often inspire me to develop new ideas for my research and create new collaborations to study materials because they introduce me to potential new microstructures to explore, various nuances in nacre I have not considered before, and new methods to create or test samples that other groups have developed.

While I don’t feel strongly about the setup for talks at conference symposia, I wish poster sessions were structured differently. I propose having everyone who has a poster give a 3-minute poster elevator pitch at the beginning of the poster session. This will help attendees figure out which posters interest them, where to go and who to talk to.

8. What is your favourite material?

Ah, a difficult question! I would say that right now, my favourite material is glass. Not metallic glass, but regular glass. It is probably not the material most people wonder about, but it is an incredibly versatile material. It is the backbone of the modern internet and makes all of our telecommunications possible today using optical glass fibres. Glass fibers are also found in deep sea sponges, where they are intricately built and woven together very precisely. It is also in a lot of our electronics as covers for our phones, watches, and tablets. I find it really exciting.

9. If you weren’t a material scientist, what would you like to be (and why)?

I have always been interested in building and making things, specifically things that are strong and that can be used. If I wasn’t a material scientist, I would probably want to be a civil engineer. They work on large-scale projects that can also break or fail, and they also have the possibility of building things that directly benefit people. There is also an adventure aspect to it, where you can travel and learn about other ways to live that require different building needs. In addition, civil engineers deal a lot with concrete, which is also a composite! It is a deceptively complicated material with many interfaces, which also reacts with water which infiltrates the soil. 

10. Where do you go from here?

We have now created a brick-and-mortar composite with a metallic matrix: we could try to look at other ways of manufacturing a similar microstructure using additive manufacturing, for example, or investigate other ceramics, or look into casting the ceramic phase, or tailor the interfacial reaction between the brick and the mortar phases and tie it in with the interfacial properties. All of the above brings the questions: how are bonding and wetting actually achieved, how can we control the solid-liquid and resulting solid-solid interfaces, and are there other biological materials we could reproduce or be inspired by? There are many possibilities in store, and we hope to build on this research using these questions.

Read Amy's paper here.   

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