**3. Classification of FG ceramics**

Future applications will demand materials that have extraordinary mechanical, electronic and thermal properties which can tolerate different conditions and yet are easily available at a reasonable price. As a result, it becomes necessary to reinforce at least one ceramic material in the functionally graded structure. FGM-based ceramic reinforcement is able to withstand high temperature environments due to the higher thermal resistance of the ceramic constituents and their attractive properties. Functionally graded ceramic compositions can be classified into:

#### **3.1. Ceramic/metal**

**1. Introduction**

to achieve superior levels of performance.

2 Advances in Functionally Graded Materials and Structures

**2. Origin of FG ceramics concept**

The result of scientific progresses in materials science and the continuing developments of modern industry, have given rise to the continual demand for advanced materials that can satisfy the necessary advanced properties and qualities. This requirement for advanced materials with specific properties brought about the gradual transformation of materials from their basic states(monolithic) to composites. Recent advances in engineering and the process‐ ing of materials have led to a new class of materials called Functionally Graded Materials (FGMs). These represent a second generation of composite materials and have been designed

FGMs are a type of composite material and are classified by their graded structure. Specifically, an FGM typically consists of a composite material with a spatially varying property and is designed to optimize performance through the distribution of that property. It could be a gradual change in chemical properties, structure, grain size, texturization level, density and other physical properties from layer to layer. FGMs have a graded interface rather than a sharp interface between the two dissimilar materials. Using a material with, for example, a graded chemical composition, minimizes the differences in that property from one material to another. No obvious change may take place in their chemical composition if the gradient is smooth enough, and if the transition is smooth, the mismatches in the property from one point in the material to another will be limited. Therefore, the ideal FGM has no sharp interfaces. Moreover, there will be no single location that is inherently weaker than the rest of the composite.

The aim of the production of FGMs is the elimination of the macroscopic boundary in materials in which the material's mechanical, physical and chemical properties change continuously and have no discontinuities within the material. Thus, these materials exhibit superior mechanical

In the past, the composition of FGMs typically included at least one metal phase. Recently, great attention has been devoted to ceramic-ceramic and glass-ceramic systems due to their attractive properties. Ceramic materials are designed to withstand a variety of severe in-service conditions, including high temperatures, corrosive liquids and gases, abrasion, and mechan‐ ical and thermal induced stresses. In this chapter, special attention will be given to the new advances in Functionally Graded Ceramics (FGCs), their processing and applications.

The FGCs concept originated in Japan in 1984 during the space plane project of Niino and coworkers [1] in the form of a proposed thermal barrier material capable of withstanding a surface temperature of 2000K and a temperature gradient of 1000K in a cross-section of <10 mm. It is difficult to find a single material able to withstand such severe conditions. The researchers used the FGM concept to manufacture the body of a space plane using material with high refractoriness and mechanical properties resulting from gradually changing

properties when compared to basic (monolithic) and composite materials.

Due to the appearance of new industries that require high temperature and aggressive media, it became important to insert at least one ceramic material phase in any advanced FGM due to its attractive properties. In this type of FGC, the desirable properties of both metals and ceramics are combined. For example, we can use the high thermal conductivity and toughness of metals as an internal surface and combine it with the greater hardness and thermal insulation of ceramics as an external surface, thereby enabling the material to withstand high temperature environments. Examples of this type are the **(Ti-TiB2) FGC** that is used as an armor material [3] and (Ni/Al2O3) **FGCs** which are used as lightweight armor materials with high ballistic efficiency [4].

In addition, ceramic/metal FGCs can be designed to reduce thermal stresses and to take advantage of both the heat and corrosion resistances of ceramics, and the mechanical strength, toughness, good machinability and bonding capability of metals — without severe internal thermal stresses.

#### **3.2. Ceramic/ ceramic and glass/ ceramic**

By exploiting the myriad possibilities inherent in the ceramic/ceramic FGCs concept, it is anticipated that the properties of materials will be optimized and new uses for them will be discovered. Examples of these FGCs are **alumina/zirconia**, a material used in biomedical and structural applications, **mullite/alumina,** which is used as a protective coating for **SiC** components in corrosive environments [2, 5]. **Zirconia-mullite/alumina** FGCs can be used as refractory materials in high temperature applications, as well as being suitable for engineering and tribological applications [6, 7].

#### **3.3. Ceramic/ polymer**

An example of this type of FGC is the **boron carbide/polymer** FGC. Due to its light weight and flexibility, the BC/polymer FGC is used in lightweight armor and wears related applica‐ tions [8]. The feature of this FGC is that the ceramic with graded porosity is fully dense on the front surface changing to open porosity on the back surface. The polymer is then infiltrated into the porous side of the ceramic plate to provide a lightweight energy-absorbing backing. A ballistic fiber weave, such as Kevlar, could also be embedded in the polymer to provide constraint and enhanced ballistic protection.

Ceramic/ polymer FGCs could also find applications in reducing the wear of automotive components. Additionally, they are used in many industrial applications requiring materials that are resistant to wear, corrosion, and erosion in hostile environments. Also, this type of FGC can be used in nuclear applications, such as the manufacture, handling and storage of plutonium materials [8].

Recently, the introduction of porosity in ceramic/polymer FGCs has broadened the scope of their application in the fields of biomedicine and tissue engineering [9, 10]. Due to the large surface area, high porosity, low thermal conductivity and high-temperature resistance of the porous ceramics, they were widely used in many fields, such as functioning as supports for ceramic filters, as artificial bones, high temperature insulators, and active cooling parts.
