Frontiers of Engineering: Reports on Leading Edge Engineering from the 1996 NAE Symposium on Frontiers of Engineering (1997)

Glenn S. Daehn

Department of Materials Science and Engineering

Ohio State University

Columbus, Ohio

Despite sizable and sustained federal and corporate investment in the science and technology if inorganic composites, there are few important commercial applications of these materials. The reasons for the sustained investments in research are clear. Appropriate composites may provide a mix of properties unavailable in monolithic materials. Also, many engineering devices or structures can benefit from the tailored properties composites may provide. Composites in which both phases are continuous, or interpenetrate, have received special interest (Clarke, 1992), because in these cases the composite may have some of the approximate macroscopic properties of each phase. For example, one phase may provide strength while the other contributes transport properties, such as thermal or electrical conductivity.

But the reasons for limited commercial application of advanced inorganic composites are less clear. Many of the impediments to application have to do with the cost of creating composites and then processing them to meet the dimensional and functional requirements of the component. Also, it simply takes time for any new material to gain commercial acceptance. This work demonstrates a novel method for producing co-continuous composite components that, in principle, should be inexpensive in large-quantity production. The method (Breslin, 1993) is illustrated for alumina-aluminum composites, but this scheme might be applied as well to many other chemical systems.

The system we have the most experience with is based on immersing shaped and formed silica bodies into liquid aluminum at a temperature near

Figure 1

Transmission electron micrograph of a co-continuous alumina-aluminum  composite produced by the immersion of glassy silica in liquid aluminum.  The darker phase is alumina. Source: Reprinted with permission  from Elsevier Science Ltd. (Daehn et al., 1996).

1100°C. The coupling of silica in aluminum is unstable, so a reaction takes place:

When transforming three units of silica to two of alumina, there is a nearly 25 percent reduction in volume of the solid oxide phase. This opens up fine channels in the microstructure, which then are filled with the aluminum alloy surrounding the transforming sample. The typical material microstructure is show in Figure 1. The reaction generally penetrates at a rate of 1–3 mm/hour at 1100°C. Because the remainder of the precursor supports the transforming material, the transformed composite has virtually the same size and shape as the precursor. Since production techniques for shaped silica precursors based on traditional ceramic processing are widely practiced, procedures for fabricating net-shape bodies from advanced composites are widely available and may be quite inexpensive.

Although experiments have given many hints at how the microstructure evolves, we do not fully understand it and cannot yet truly control it. Despite

this, it appears that similar reactions can be used with other chemical systems, provided the following criteria are met: (1) the produced compound is more compact and thermodynamically favored relative to the precursor and (2) the bath wets both materials. Application of these simple criteria could lead to a new class of co-continuous materials based on other chemical systems, including carbides, nitrides, or other compounds.

Since this material is a co-continuous mixture of a metal and a ceramic, it does have an unusual macroscopic mixture of properties. For example, the metal phase gives electrical conductivity and high fracture toughness to a material that is otherwise ceramic-like (i.e., very hard with a high specific stiffness).

Quantitative prediction of properties in these co-continuous materials presents significant difficulties. The process of simply characterizing, describing, and visualizing the geometry of the microstructure represents a significant problem. Usually, such descriptions are taken for granted as a starting point for models that correlate a material's structure and properties. Despite this, very simple descriptions of the microstructure have been shown to give good predictions of the magnitudes and trends in the elastic and plastic deformation of these co-continuous materials (Daehn et al., 1996). It is significant that even though the ceramic phase (which makes up 75 percent of the material) is incapable of plastic deformation, the composite will still exhibit plastic deformation under load. This basic elastic and plastic behavior forms the basis for determining higher-order properties, such as fracture toughness and wear resistance. The relatively small fraction of interconnected metal in the composite dramatically increases its damage tolerance and toughness.

The obvious applications for these materials are those requiring some of the usual properties of ceramic materials (high specific strength and stiffness, high temperature strength, wear resistance, and high hardness), along with some of the properties found in metals (high thermal or electrical conductivity, high toughness, and damage tolerance). Such applications include the following:

• automotive combustion cylinder liners—where high wear resistance is required and improved thermal conductivity and a lower-density, smaller engine mass are desired;
• automotive brake rotors—where wear resistance, low density, and high thermal conductivity are all benefits; and

• electronic packaging—which could benefit from a low coefficient of thermal expansion, high thermal conductivity, and high specific strength.

In other classes of applications, the ability to easily produce net-shape components may be an important attribute. For example, methods of rapidly producing dies for polymer injection molding have been considered. Here, high thermal conductivity and wear resistance would benefit the component in service.

In each of the applications mentioned, there appear to be good technical reasons to use co-continuous composites processed as discussed above. However, at our present stage of developing this idea, it becomes clear that technical merit is only one important aspect of commercializing a new material or technique.

Several individuals have contributed to the work described here. In particular, I wish to acknowledge Hamish Fraser and Michael Breslin, who have contributed to all facets of this work.

Breslin, M. C. 1993. Process for Preparing Ceramic-Metal Composite Bodies. United States Patent 5,214,011.

Clarke, D. R. 1992. Interpenetrating phase composites. Journal of the American Ceramic Society 75:739-759.

Daehn, G. S., B. Starck, L. Xu, K. F. ElFishawy, J. Ringnalda, and H. L. Fraser. 1996. Elastic and plastic behavior of a co-continuous alumina-aluminum composite. Acta Materialia 44(1):249-261.