AWS WHC-5.09:2015

Ceramics
Scope : Ceramics are inorganic, nonmetallic materials that are manufactured from powders and formed to a desired shape and then heated to high temperature, with or without the application of external pressure, to achieve a final densified product. These materials are separable into two broad categories: traditional ceramics and advanced ceramics.1, 2 Traditional ceramics include the clay products from centuries past, and also the refractories, silicate glasses, and cements of the present. They are most commonly made from inexpensive, readily available, and naturally occurring minerals. Traditional ceramics are typically low-density (high-porosity) materials and are commonly used in applications in which joining by cementing is practical. Advanced ceramics, by comparison, are made from chemically processed (synthesized) powders in which properties such as particle size, distribution, and chemical purity are closely controlled. Exceptional mechanical properties have been developed within this advanced group of ceramics. This subset, often referred to as structural ceramics, includes monolithic materials such as alumina (aluminum oxide [Al2O3]), zirconia (zirconium oxide [ZrO2]), silicon carbide (SiC), silicon nitride (Si3N4), and silicon-aluminum oxy nitrides (sialons), and also ceramic composites, such as alumina that contains SiC whiskers or SiC that contains titanium diboride (TiB2) particles. Advanced ceramics also include alumina-zirconia composites, and a class of ultrahigh-temperature ceramics (UHTCs). Concerning the joining of ceramics, much of the work described in this chapter involves silicon nitrides. The manufacture of structural ceramics is carefully managed to ensure that chemical composition is controlledand that high density (relatively low-porosity content) is achieved. Intensive academic and industrial research since the mid-1990s has been devoted to understanding how the intergranular films in silicon-nitride ceramics affect mechanical properties. Silicon-nitride ceramics have been developed that have reproducible strength, toughness, and a high Weibull modulus for use in some structural applications, such as bearings for hard drives, ball bearings, roller bearings, components of automobile engines, and gas turbines. Technological interest in structural ceramics is directly related to the unique properties of these materials when compared to those of metals. Many ceramics are characterized by high strength, not only at room temperature but also at elevated temperatures. Silicon carbide, for example, maintains a tensile strength in excess of 200 MPa (29 ksi) at 1530°C (2800°F), the melting point of iron. Other ceramics, such as Si3N4and certain ceramic composites, also maintain highstrength at high temperatures. Additional properties are excellent wear resistance, high hardness values, excellent resistance to corrosion and oxidation, low thermal expansion, high electrical resistivity, and a high strength-to-weight ratio. These attributes make ceramics excellent alternatives for applications usually reserved for metallic alloys. Table 9.1 shows comparisons of typical properties of several structural ceramics to those of several metals. Structural ceramic materials are used to manufacture cutting tools, bearings, machine-tool components, dies, pump seals, high-temperature heat exchangers, and a variety of components used in internal combustion and turbine engines. The successful use of ceramics as structural components has promoted advances in ceramics joining technology, especially ceramics-to-metal joining. Effective joining methods for ceramics have a direct impact on the use of ceramics in mass-produced components. One of the most important objectives of ongoing ceramics-joining research is to determine the means for the economic fabrication of complex, multiplecomponent structures that will pass stringent quality tests. Ceramics joining is challenging because of limitations imposed by the manufacturing processes used to create the ceramic materials, and also because of the properties of the finished ceramics. For example, the deformation that would be needed for a complex shape made from a densified ceramic material is practically impossible, because most ceramics are brittle, even at elevated temperatures. In some procedure development programs, complex ceramic components, such as those used in engines designed to perform at elevated temperatures, are made as monoliths by difficult processing techniques or by extensive machining of densified billets. While these methods of component manufacturing are acceptable for special purposes, they are not practical for mass production because of the high cost. The difficulty of machining ceramics adds to costs, but by reducing the complexity of individual components, by designing simpler forms and joining them to create the complex component, significant savings are achieved. Effective methods of joining ceramics can eliminate machining altogether in some cases, and can also contribute to improving the reliability of ceramic structures. Because most ceramics are brittle materials, they are very sensitive to flaws resulting from the quality of the raw materials used in production and to the characteristics of various processing techniques, for example, machining. A single flaw can cause the rejection of a ceramic component or, if undetected, can cause failure of the component. Rather than dealing with complicated monolithic workpieces, it is easier to inspect and detect flaws in simple shapes of components before they are joined to form complex structures. The development of structural ceramics has increased the use of these materials in many critical engineering applications; however, the electronics industry uses the largest quantity of advanced ceramics. Although the development of materials such as zirconia, silicon nitride, and silicon carbide has been vigorously pursued, aluminum oxide still is the most widely used structural ceramic.