"In some ceramic composites," says Tony evans of Harvard University, "the toughness is now at least as good as aluminum alloys."

   In the world of materials, those known as ceramics are as close as one might get to functional perfection. They can be made hard and durable, resistant to heat and abrasion, as strong as metals—if not stronger—but able to withstand considerably higher temperatures. Their shortcomings, while few, have been profound. Ceramics can be brittle, and they can be expensive. For the past ten years researchers have been working furiously to solve both problems, particularly by developing ceramic composites.

   At the forefront of ceramics research in recent years has been Harvard's Anthony B. Evans. His prominence in materials science was underscored in the fall of 1995 by his showing in this publication's listing of highly cited authors in the field (see Science Watch, 6[9]:1-2; 8, October 1995). Between 1990 and 1994, Evans's 19 publications garnered 624 citations—some 200 more than the next-highest author in the total-citations ranking. Since 1982, Evans has published 13 papers that have subsequently collected more than 100 citations each. All focus on the properties and behavior of advanced structural materials. In particular, Evans's research has led to revelations regarding the structure of ceramic-matrix composites and the mechanics of toughening brittle materials.

   Born in Wales in 1942, Evans has followed, by his own account, "a rather circuitous career path." He did both his undergraduate and doctoral work at Imperial College in London, obtaining his Ph.D. in metallurgy in 1967. He was a project leader at the Harwell Laboratory and then the National Bureau of Standards before moving on to the Rockwell International Science Center, where he originated a ceramics program—motivated, he says, "to some extent by the space shuttle tile program." In 1978, he moved on to the materials science and mineral engineering department at the University of California at Berkeley, and then in 1985 started a materials department and the High Performance Composites Center at UC Santa Barbara. And most recently, two years ago, he became the Gordon McKay Professor of Materials Engineering at Harvard. Speaking with correspondent Gary Taubes from his office at Harvard, Evans gave Science Watch his views on the world of high-performance ceramics.

            Let's begin at the beginning. Can you give us a brief history of the field of ceramics research to give us a context?

   Evans: Ceramics really started with refractories for steel furnaces and for glass-making furnaces. It also simultaneously got started by making abrasive powders for grinding and sandpaper around the turn of the century. Some would regard glass as a ceramic, so in one sense the glass industry parallels the ceramics industry. In many respects, ceramics had been a low-tech industry. But starting around the late 1950s, various groups began to explore the idea that high-tech ceramics, if they were carefully developed, would have important applications in many diverse technologies. For the next 20 years, until about 1970, progress was fairly slow and empirical. New materials were identified, along with ways of making them, and gradually the level of sophistication went up—motivated, to a significant extent, by the glass industry. Corning, for example, was successful in bringing science to glass technology. In the 1970s, developments in ceramics for electronics and in engine components required a much higher level of sophistication—a better understanding of the material—and that's when the effort really burgeoned.

            So what exactly is a ceramic composite, and how is one made?

   Evans: The original definition of ceramic was "something made by going to very high temperatures." It comes from the Greek keramos, which means "hot." But that's not a sufficient definition. Oxides, carbides, and nitrides, in particular, are ceramics. The Japanese have been very effective in producing ceramic fibers—carbide and oxide fibers. These are very thin strands, like thin strands of nylon, thinner than hair. For a ceramic composite, the fibers are first woven into a cloth, using some sophisticated weaving technologies that came from the textile industry. Then layers of the cloth are built up and compressed slightly. This produces a very dense packing of the fibers. The next step is to infiltrate the interstices between the layers with a very, very fine ceramic powder. The last step involves various heat treatments to impart some strength to the powder that's been infiltrated and to tailor the interface between the fibers and the ceramic powder.

            What key technologies had to come into place to permit a better understanding of the materials?

   Evans: One technology relates to the ability to control the chemistry of the powder material, and to understand the sintering process by which it densifies. A second concerns design: how is a composite material designed to withstand the loads and temperatures? That requires mechanical engineering principles, such as fracture mechanics. Those two technologies had to come together and meet in the middle to facilitate the development of ceramics and ceramic composites.

            What is it about ceramic composites that makes them so potentially revolutionary?

   Evans: They're rather remarkable materials. You can do things like drive a nail through the material without affecting its load-bearing capacity. That's also the case with metals. But imagine trying to do the same thing with window glass, or porcelain, which are more traditional ceramics. They would just fragment into pieces. So what's been accomplished is to make ceramic composites behave, from the mechanical engineering point of view, just like metals. But they have much higher melting temperatures and they're lightweight. Aluminum is lightweight, too, but it cannot go to very high temperatures. The combination makes ceramic composites special.

            Can you give us some comparisons?

   Evans: Well, aluminum can be used up to about 200 degrees C, steel up to about 600 degrees C, nickel alloys up to about 1,000 degrees C. But a ceramic composite can be used to about 1,300 degrees C. And furthermore, steel and nickel are very dense, very heavy materials. A ceramic composite has a density comparable to aluminum. So you can imagine a lot of applications in aerospace, for example, where high temperature combined with light weight is very important. Not all the applications have emerged yet.

            What challenges are you dealing with in developing ceramic composites?

   Evans: There are three main challenges. One is that ceramics by themselves are quite brittle materials, so it is essential to find ways of making them tough. In a sense, that challenge has been met. Much of the cited work concerns the understanding of how to make brittle ceramics into tough ceramic composites.
   There is a common thread in all of this work. The start is the understanding of why metals are tough. The toughness is there because dislocations in the metal can be made to move around under stress. This dissipates energy. So now we conceive of different mechanisms that have the same effect in ceramics; that is, dissipating energy inside the material when it is stressed. For instance, microscopic, internal interfaces that can slip over each other and dissipate energy by friction.
   However, it is necessary to understand the details—through the mechanisms and the models, with the experiments to validate the models. With such insight, the means for making the materials and demonstrating very high toughness have been demonstrated. In some ceramic composites, for instance, the toughness is now at least as good as aluminum alloys.

            And the second challenge?

   Evans: When we introduce the mechanisms that make ceramics tough, it has to be ensured that the high-temperature durability will be maintained over long periods of time. But, the very mechanisms conceived to increase the toughness may degrade over time. There are still challenges being met in that area. The most important problem is that in any atmosphere containing oxygen, the oxygen tends to infiltrate the material and gradually eliminate the toughening.

            Do materials scientists have ideas about how to solve this?

   Evans: We have leads. In particular, a collaboration with chemists is really important, because we need to begin with materials that are inherently stable in an oxidizing environment.

            And the last challenge?

   Evans: Cost. As academics we were a bit naive. We didn't appreciate the importance of manufacturing cost. Sometimes we developed the science and the engineering without thinking of the technological implementation of the material. Now we've realized that for an idea to be put into practice, the material needs to be made at a reasonable cost.

            What does that require at this point?

   Evans: It entails going right back to the beginning, back to the drawing board. For example, some of the manufacturing methods for the materials are based on inherently expensive processes, such as vapor deposition, in which the raw material ingredients are expensive, the manufacturing cycle times are long, and the temperatures used for manufacturing are high. That leads to an overall manufacturing approach which is inherently expensive.

            Is that what you're working on now at Harvard?

   Evans: Yes. I became aware that many of the concepts that were put together as good science never made it to application, because we never really understood the connections between design, manufacturing, and cost. So I am trying to integrate these factors wherever I can. Applications of the materials are only slowly evolving because they still await manufacturing cost-reduction initiatives.

            How far do costs have to drop?

   Evans: A part of my work now is to try to achieve insights that help formulate goals for manufacturing costs. This will enable new materials to be put into advanced systems. I'm doing this not only for the same materials I've worked on for the past 15 years—ceramics and their composites—but for thin films, coatings, and metal structures.

            So it's not a question of coming up with new materials, but of figuring out how to make the existing materials cost effective?

   Evans: Exactly. Not everybody will agree with me, but the statement widely conveyed by all the industry people with whom I work, and I believe it, is that they have plenty of new materials sitting on the shelf—hundreds of them—but implementation has not been achieved because they don't have a good way of predicting manufacturing cost and relating cost to the performance.

            Do you think you can do that?

   Evans: I'm going to give it a try. I think it's a worthwhile way to spend the remaining years of my career. I may fail, I may succeed—but I will try.