Materials scientists have long recognized that grain boundaries and interfaces can limit a material’s bulk properties, especially at high temperatures. Significant research effort has gone into understanding their nature with an eye to engineering them to minimize deleterious affects, and with the advent of tools capable of characterizing materials in the nanoscale range, direct observation of interface phenomena has become possible.
Technion-Israel Institute of Technology researchers, Mor Baram and ACerS members Dominique Chatain (CNRS, France) and Wayne Kaplan, shed some new light on interfaces with a study of nanoscale intergranular films published last month in Science (doi 10.1126/science.1201596). Kaplan is dean of the Department of Materials Engineering at the Technion.
Intergranular films about 1 nm thick are found at grain boundaries, phase boundaries, and on free surfaces, however, it has not been known whether the IGFs are an equilibrium interface effect, a wetting film (a bulk phase), or a transient effect related to impurities, mass transport, etc. If the IGFs (also called complexions) are in a state of equilibrium, then it should be possible to map them onto bulk phase diagrams as tie-lines describing the 2-D state of grain boundaries and interfaces, opening the door to engineering their properties.
In the study, basal plane sapphire substrates were partially coated with anorthite glass droplets and then coated with gold films. Anorthite (CaO-2SiO2-Al2O3) was used because its constituent elements are often found in alumina grain boundary IGFs. On annealing, the gold films broke into particles dispersed across the substrate. By looking at the interfaces in an aberration-corrected, high resolution TEM, the authors were able to show that the IGF was not a bulk wetting film. Analyzing the dihedral and contact angles showed that the interface energy at the gold-sapphire interface was reduced when an IGF was present.
Another ACerS member, Martin Harmer, offered some perspectives on the Technion group’s work (“The Phase Behavior of Interfaces,” doi 10.1126/science.1204204). Harmer, director of Lehigh University’s Center for Advanced Materials and Nanotechnology, notes that IGFs are thermodynamically stabilized by the interface between grains, although they are not bulk phases. From the perspective of engineering the grain boundary, it is interesting that IGFs with different structures can coexist in equilibrium within the same material because, as Harmer observes, if the properties of the different IGF structures vary much, the effect on material behavior could be significant.
As Harmer explains, a better understanding of what’s happening at interfaces could explain phenomena like abnormal grain growth or embrittlement brought on by diffusion of impurities into the grain boundaries. Even better would be to add constituents to engineer the composition of grain boundaries and IGFs for improved high temperature properties, thus extending component service life.
It seems to me that an improved understanding of what is happening at interfaces will grow in importance as devices and systems shrink in size. I’m thinking about things like micro-electrical-mechanical systems or even nano-electrical-mechanical systems, ever-smaller electronic devices, implanted medical devices, etc., where any loss of integrity to the grain boundaries or material interfaces can become catastrophic quickly. On a larger scale (component-wise, that is), engineering the interfaces could yield better coatings, welds, composites, etc.
Aside from the interesting technical implications of Baram, et al‘s work, this paper represents a good example of the practical importance of well-executed basic science research. But, that would be preaching to the choir, wouldn’t it?