Functionally Graded Materials<br />Functionally graded materials (FGMs) are new advanced multifunctional composites where the volume fractions of the reinforcement phase(s) or dispersions vary discretely [1]. FGMs are characterized by spatially varied microstructures created by non-uniform distribution of the reinforcement phase with different properties, size and shape as well as by interchanging the role of reinforcement and matrix material in continuous manner [2]. In other words, FGM is usually a combination of two materials or phases that show a gradual transition of properties from one side of sample to the other. This gradual transition allows the creation of superior and multiple properties without any mechanically weak interface [3]. In general, there are two approaches to fabricate FGMs. The first one is to eliminate the interface of coated or joined-type composites. The second one is to induce non-uniform distributions of reinforcement particles in an originally homogeneous composite, creating multiple functions inside the material [3].<br />FGMs are ideal candidates for applications involving severe thermal gradients, varying from thermal structures in advanced aircraft and aerospace engines to computer circuit boards. These materials have been introduced to take advantage of ideal behavior of its constituents, e.g. heat and corrosion resistance of ceramics together with mechanical strength and toughness of metals [4]. Many processing methods have been proposed to fabricate FGMs, such as chemical vapor deposition, the plasma spray technique and various powder metallurgy techniques. The centrifugal method has been introduced recently and has attracted a lot of attention due to its unique merits [5]. The manufacturing of FGMs can be divided into two steps :<br />1. Building up a structure with gradient properties.<br />2. Transformation of the designed structure into graded bulk material.<br />The gradation process is usually classified in three main groups: constructive, homogenizing, and segregating processes. Stepwise buildup of a graded material from precursor materials is the basic constitutive process. In homogenizing processes, a sharp interface between two materials is converted into a gradient by material transport (i.e. diffusion). Segregating processes start with a macroscopically homogeneous material which is converted into graded material by material transport caused by an external field (gravitational or electric field). The primary advantage of the homogenizing and segregating processes is a possibility to produce a continuous gradient.<br /><br />د. مروان عباس مظلوم