Cellular Ceramics / 4

4.1 Mechanical Properties 309

There can also be trade-offs in performance due to other microstructural factors in different fabrication methods. Thus, two effects have been identified in the commercial fabrication of open-cell ceramic foams by ceramic slip coating of open-cell polyurethane (PU) foams. First, such replication typically results in hollow ceramic struts due to the incomplete penetration of the PU foam structure by the ceramic slip and subsequent PU removal on firing the ceramic. The hollow struts that result from this fabrication generally do not reduce properties and can actually be of some benefit owing to limited reductions in density that give somewhat better property to mass ratios. However, another common strut effect in such ceramic foam fabrication, namely cracks in the ceramic struts due to differences in drying shrinkage and thermal expansion of the PU and the ceramic, lowers the resultant properties.

Other effects more specific to fabrication with a given material can occur, as illustrated by the unexpected benefits found by Lachman et al [1, 43, 50] in their engineering production of cordierite auto exhaust catalyst monoliths by extrusion. They selected clay as one of the main raw materials for both its lower cost and its easier extrudability. Extrusion resulted in considerable alignment of the clay particles, which in turn resulted in preferred orientation of the cordierite formed from them during firing. This preferred orientation of the resultant cordierite resulted in improved thermal stress/shock resistance of the cordierite honeycomb.

4.1.4.3

Porosity–Property Trade-Offs

Many important applications of cellular and related materials require various aspects of pore structures that place greater limits on the mechanical properties of these materials [1]. Thus most catalyst applications require high surface areas, but the highest surface areas are obtained with bodies made of lightly sintered fine powders, which results in most mechanical properties being lower and thus requires compromises in pore structure. Similarly, many applications require substantial flow or permeability through the porous structure. This is favored by higher P values and tubular versus equiaxial pores, which in turn may require some compromises in pore structure to limit reductions in both mechanical behavior and surface area. Thus, many applications are best met by controlled amounts of differing porosities in a given body

The above differences in different porosity effects are important reasons for better characterization of pore structures, as well for modeling effects of more than one type of porosity in a given body. Some characterization of dual porosities in a body is already done to some extent where effects of extruded tubular holes or foam cells and the pores in the extruded honeycomb walls or foam cell walls/struts are handled separately. Some more general combination of effects of different porosities in the same body has been demonstrated in MSA models [1], and it may be feasible in some other models, but further development is needed.

310Part 4 Properties

4.1.5

Summary

Substantial progress has been made in test methodology, data generation, and modeling and understanding of porosity effects on mechanical properties of cellular ceramics. Thus, some property measurements must be made with compliant test surfaces, and others require a sufficient number of pore cells across the test specimens. However, there is need for a broader range of property measurements, such as effects of various specimen surface finishes on strengths. Valuable methods of porosity characterization are available, in particular stereology, but are often used little or not at all, although they are sorely needed for more comprehensive characterization.

In modeling, which is a major factor in understanding mechanical behavior, substantial progress has also been made. Four modeling approaches – power law, Gib- son–Ashby (G–A), Minimum Solid Area (MSA), and computer modeling – were considered in this chapter. All have some value, and use of two or more models for comparison and building a data base is recommended.

There has been considerable modeling of mechanical properties (and characterizing the porosity) of ceramic honeycomb structures due to development of tubular monoliths for catalytic, heat-exchanger, and filter applications. Though complicated to varying extents by anisotropic property behavior of honeycombs, their modeling is somewhat better developed, since much of it is amenable to 2D modeling.

Good progress has also been made in the study of 3D foam cellular structures, in part due to their development in other materials. However, noted cautions with the power-law modeling approach for 3D pore structures need to be addressed. G–A- type and MSA models are both valuable due to their explicit focus on microstructural factors. The power law and MSA modeling approaches, which cover a broader porosity range, should provide useful comparison with G–A models (which focus on cellular structures and have a substantial data base for such structures). More attention to broader ranges of types and amounts of porosity than commonly found in cellular bodies will also be useful, including extrapolations to PC and P = 0, to better understand the behavior of cellular bodies. MSA models covering many of the structures of interest can aid in addressing transitions in pore character and properties between less and more porous bodies with generic types of porosity. Computer modeling holds substantial promise but needs to better address the issue of effectiveness of obtaining and validating 3D results. MSA modeling with computers may be particularly advantageous in modeling effects of combinations of porosity in bodies. Ultimately, computer modeling may be the only route to analyzing the behavior of more complex porous bodies with variations in amount, character, or both of the porosity, and is likely to be a valuable tool in evaluating trade-offs between important properties with significantly different porosity dependences.