Little Things and Big Pictures: ongoing research and new materials research projects at PurdueDr. Alexander H. King - ASM Fellow - Purdue University Material Dept. Head
We will be looking at some of my personal research "Little Things" studied with electron microscopy, and then at some of the new initiatives happening in material engineering at Purdue University "Big Pictures".
Since very early in my life I have had a passion for the study of the unseen "little things". It is quite natural that I have found the study "Interfaces" (the infinitely divisible junction between two materials) to be my primary research focus. The tools for such research are the "electron microscope", particularly the TEM (transmission electron microscope) and the computer for modeling.
[Dr. King's presentation includes numerous SEM and TEM photomicrographs which cannot be shown here. We will simply highlight some of the conclusions he has reached as a result of his studies. - webmaster]
A very interesting study is the deposition of Zirconia on polished stainless steel. Many would say that a "splat" of Zirconia would not stick to a non-roughened stainless surface without a bond coat but actually the opposite is true. The interface between Zirconia spats and polished stainless is actually stronger than with a prepared surface. At the interface, four layers exist. From outside to base metal these are: the Zirconia, an interfacial oxide, resolidified substrate, and base metal (substrate). There is some lateral cracking due to thermal expansion mismatch. This can be controlled and predicted with the parameters: CTEs (thermal expansions), temperature of substrate, and fracture toughnesses. When the "splat" of Zirconia is applied to a roughened stainless surface, an "ingot" structure is formed where the Zirconia penetrated a cavity in the roughened stainless. Like all "ingots" there is a shrinkage cavity with forms on cooling, but in this case the cavity forms on the outside of the cast structure (breaking the bond with the stainless) rather than at the top center (as in gravity casting). Because of this broken bond, the splats on the roughened surface are much more weakly bonded than those on the polished surface.
Another interesting finding in the world of interfaces is demonstrated when ZrO (zirconia) splats are bonded to prior ZrO splats. Looking at the "structure" of such a bond, it appears that "columnar crystals" exist as a uniform entity right through the interface yet contradicting this; is the fact that the interface is a weak area and the structure will predictably break right at the interface when stressed. Detailed study has revealed that exactly at the interface there is a mixture of crystalline and amorphous ZrO which allows the crystalline growth to continue (without grain boundary) through the interface, but this is accomplished with only small points of contact within the tiny amorphous/crystalline layer. The same effect can be demonstrated with the epitaxial formation of SiC on Si at low temperature.
A final observation in the realm of "small things" is the study of "triple junctions". From the observations of a Grad student, it was observed that individual grains can actually "rotate" within a crystalline structure. The process is slow, but can be shown to involve diffusion at the corners of the grains (specifically the "triple junctions"). Bismuth diffusing into Cu can be used as an illustration of this concept. Dr. King has developed a mathematical model for this phenomenon.
In the area of Big thing at Purdue there are several new programs:
TAP; is the Technology Assistance Program available to Indiana businesses with needs in the area of materials problems.
A "new undergraduate curriculum" focuses initially on "practical" production and use of materials and then progresses into "theory" instead of the traditional "reverse" approach.
For "senior projects" students work with industrial partners to solve real problems. This involves considerable commitment on the part of the industrial partner, but the benefits are usually quite high.
New facilities are planned for Miltidisciplinary Engineering (including materials) and for Nanoscale Fabrication.
Under an Electron Microscopy Management Plan, "electron microscopes" will be selected for unique "state-of-the-art" capabilities for use by all science/engineering disciplines, rather than maintaining "redundant" equally capable units dedicated to individual disciplines.
A Materials Consortium is being formed to coordinate all schools with involvement in materials.