Composite materials are a growing area of interest for construction and aerospace applications as they can exhibit the same (or even superior) physical properties as traditional materials while being lighter and more cost effective to produce. Advanced composite materials require engineering at ever-smaller scales, necessitating characterization and analysis with high-resolution tools such as scanning electron microscopes (SEM).
Dr. Sarvi Ghaffari is a postdoctoral fellow at the University of Texas at Arlington (UTA) Advanced Material and Structure Lab (AMSL) whose work focuses on experimental nano-mechanics as well as material design and manufacturing. We talked with her about the composite materials research being conducted at the AMSL and the role that SEM instrumentation plays in advancing aircraft material research.
Dr. Sarvi Ghaffari uses the Thermo Scientific Quattro ESEM for her research on aircraft materials.
Advancing Materials (AM): Could you tell us about your research?
Dr. Ghaffari: The main focus of our research is composite materials, in particular carbon-fiber-reinforced polymer matrix composites. Composite materials are quickly replacing conventional materials as primary structural components in aerospace applications due to their high strength-to-weight ratio and their fatigue and corrosion resistance characteristics.
Our research focuses on developing novel high-performing composite material systems to potentially enable lightweight rotor and airframe structures.
AM: What makes a good carbon-fiber-reinforced polymer composite?
Dr. Ghaffari: Fundamentally, a composite material consists of two main components: the matrix, which acts as the binding material, and the reinforcement which provide the strength and rigidity to the material. Tensile strength of a composite material increases with the strength/modulus of the reinforcing fiber (the modulus is a measure of how stiff a fiber is). Therefore, by increasing the modulus of the fiber, the tensile strength of the composite can be improved. However, compressive strength of composite material is about 50 to 60% of its tensile strength. So, if we want to reduce the weight of aircraft structures, their compression strength needs to be improved. For our project, we’ve been trying to develop a new high-performing material by increasing the compression strength of high-modulus carbon fibers.
Experimentally, we successfully produced a new material system that has a compression strength similar to intermediate modulus composites while having a 30% greater axial modulus, which was amazing!
Carbon-fiber-reinforced polymer composite tested and imaged on the Thermo Scientific Quattro ESEM with a nano-indenter.
AM: Do you see this material impacting the industry?
Dr. Ghaffari: Yes. There has been strong demand from the industry to enable lightweight aircraft structures. That’s the goal of our experimental and modeling efforts—to accelerate the development of novel, high-performing materials for lightweight aircraft structure.
AM: What tools and instruments do you use in your research?
Dr. Ghaffari: The main instrument we use is a Thermo Scientific Quattro ESEM with a nano-indenter, a kind of nano-mechanical load frame. We also use a high-resolution micro-focus X-ray CT system for large structures and in-situ mechanical testing, as well as digital image correlation (DIC) facilities ranging from high-resolution to ultra-high-speed. We have advanced material testing systems for static, fatigue and impact loads, thermal analysis and rheology, and manufacturing facilities.
AM: Did you always rely heavily on SEM?
Dr. Ghaffari: No. Initially, our efforts were purely experimental. We developed a new material with adequate compression strength performance, and all the ingredients of the new material system were commercially available for production, but the process wasn’t scalable. We wanted to move toward computational material design to explore diverse material design options, instead of relying on traditional time-consuming trial and error iterations. However, there are no accurate physics-based models available that can capture all the behaviors of a composite material.
The main challenge with models is meeting their input data requirements. To make an accurate, high-fidelity model, we would need to characterize the properties of all ingredients in the composite. Otherwise we would just be making assumptions about a novel material’s properties—which could be correct, but we wouldn’t be able to verify accuracy.
Therefore, we turned to SEM with a nano-indenter because it allowed us to characterize all essential material input properties for the microstructural model in situ. We learned that the Quattro ESEM was well-suited to our needs and have been using it ever since. The Quattro lets us observe composite material behavior throughout a whole experiment and really understand what is happening to it as the material is manipulated. Simultaneous in-situ observation of the whole failure mode along with mechanical responses allows us to explain the sensitivity and complexity of a material’s response to stress or restraint.
AM: You’re fairly well-versed in SEM. Had you done electron microscopy before or is it something you’ve managed to pick up quickly while working on this project?
Dr. Ghaffari: In the past, we tried to use SEM for a couple of projects and didn’t obtain adequate images. Because of the non-conductive nature of epoxy in the matrix, we had to coat our sample with conductive material for imaging, which was inconvenient and impacted image quality. Once we tried the Quattro, we managed to get a perfect image without any real training or special sample preparation. The system was so robust and automated that we started using it in about a week. It was great! It was a tool we could use without any real skill. As long as you know the basics, I think you can achieve a lot very quickly with the Quattro ESEM.
AM: What impact do you think SEM will have on the field in general?
Dr. Ghaffari: The ability to observe the whole failure phenomenon at a small scale opens a window to a new world. In the advanced materials field, behaviors happening on the micro- or nano-scale can’t be directly observed without SEM. As a result, it’s difficult to replicate experimental designs since we’d just be working off assumptions. By observing everything as it happens with our Quattro, we can support our assumptions with real-world data. Ultimately, this means we can better understand what is actually happening and, in this case, apply that to new material design for aircraft structure.
AM: How could SEM technology improve even further to better support your work?
Dr. Ghaffari: Resolution could always be improved, of course, but otherwise I don’t have any issues with the technology. We are no longer struggling with charging issues that resulted from conductive and non-conductive materials being next to each other in the composite; that’s not an issue with the Quattro and is a huge improvement compared to other SEMs on the market. In the Quattro ESEM, we’re at a point where we don’t even need to use low-vacuum to get rid of charging effects. For my purposes, the instrument is working wonderfully!
I’ve tried quite a few different microscopes and can confidently say that Thermo Fisher’s Quattro ESEM is the most robust system that anyone can get.
Alex Ilitchev, PhD, is a Science Writer at Thermo Fisher Scientific. This interview has been edited for length and clarity.