TABLE OF CONTENTS Conventional monolithic materials can be divided into three broad categories: metals, ceramics, and polymers. Although there is considerable variability in properties within each category, each group of materials has some characteristic properties that are more distinct for that group. In the case of ceramics one must make a distinction between two forms, bulk and fiber.
Table 2.1 presents a list of properties and a rating of the three groups of materials with regard to each property. The advantage or desirability is ranked as: superior (++), good (+), poor ( ?), and variable (v). Thus, for example, metals are superior with regard to stiffness and hygroscopic sensitivity, but they have high density and are subject to chemical corrosion ( ?). Ceramics in bulk form have low tensile strength and toughness ( ?) but good thermal stability, high hardness, low creep, and high erosion resistance (+). Ceramics in fibrous form behave very differently from those in bulk form and have some unique advantages. They rank highest with regard to tensile strength, stiffness, creep, and thermal stability (++). The biggest advantage that polymers have is their low density (++), but they rank poorly with respect to stiffness, creep, hardness, thermal and dimensional stability, and erosion resistance ( ?). The observations above show that no single material possesses all the advantages for a given application (property) and that it would be highly desirable to combine materials in ways that utilize the best of each...
