Functionally Graded Materials J. Lendvai Department of General Physics Loránd Eötvös University H-1117 Budapest, Pázmány P. sétány 1A.
In structural components, service conditions and, hence, required materials performance vary with location. Examples for this range from a kitchen knife, which needs only be hard at its cutting edge elsewhere the material must mainly be strong and tough; through gear teeth in car transmissions the body of which must be tough whereas its surface must be hard and wear resistant; to heat-resistant airframe structures and engine parts of ultra super-sonic planes (beginning with space shuttles). Many high-tech applications of materials fall under this category. It is now well known that abrupt transitions in materials composition and properties within a component often result in sharp local concentrations of stresses. E. g. in ceramic-metal joints or in metal-ceramic composites due to the different material properties of the ceramic and metallic phases, the difference between thermal expansion coefficients of the metal and the ceramic phases results in thermal residual stresses near the metal-ceramic interfaces during and after processing or in the course of applications. These thermal residual stresses can become large enough to fracture the composite during manufacture or in service. It is also known that these stress concentrations are greatly reduced if the transition from one material to the other is made gradual. These two considerations form the logic underlying the concept of functionally graded materials (FGMs). By definition [1] FGMs are used to produce components featuring engineered gradual transitions in microstructure and/or composition, the presence of which is motivated by functional performance requirements that vary with location in the part. FGMs are characterised by a generally non-linear 3D-distribution of phases and corresponding properties. They are distinguished from isotropic materials by gradients of composition, phase distribution, porosity, texture, and related properties such as hardness, density, Young’s modulus etc. Of course, as shown by the examples cited, gradient materials are not new, they exist in the nature (the Earth itself is graded), and humans have extensively utilised either natural or processed materials containing microstructural gradients since the earliest days of craftsmanship and engineering construction. There are examples of graded materials developed long ago, such as case-hardened steel, which are still in common use today. Contemporary examples of these materials serve in technologically significant applications, as, for example, in thermal barrier coatings for gas turbines. Nevertheless, what is new about FGMs is the realisation that gradients can be designed at the microstructural level to tailor materials, most notably composites, for the specific functional and performance requirements of an intended application. With the advent of these new processes and materials, one can envision the development of a novel approach to the fabrication of components for demanding applications, which has been termed “inverse design procedure” [2,3]. Here component design and fabrication are based not on a list of existing materials, but on a choice of available basic material ingredients and material processes, combined with three-dimensional mechanical analysis of graded structures. These two engineering disciplines are thus combined to design synergistically both the component and its processing. This can be considered as another definition of the FGM concept as an approach in engineering rather than a physical entity. The very dynamic activity in the FGM research is reflected by the biannual International Symposia on FGM, the proceedings of the last [4] has been recently published. FGMs can be produced in many different ways, nevertheless two principal classes of production methods have been identified [1] as constructive processes (like powder densification, coating, lamination) and transport based processes (like diffusion, gravitational or centrifugal separation, infiltration, macrosegregation etc.). The first class of processes produces gradients by stacking selectively two or more starting materials, allowing full and potentially automated control of compositional gradients. The second class utilises natural transport phenomena to create compositional and microstructural gradients during production of a component. In certain cases new concepts were also needed for the characterisation of FGMs, like for example the hardness and modulus determination from indentation testing or the determination of fracture characteristics. In this contribution the basic concepts underlying the production processes and characterisation of FGMs will be outlined. Acknowledgement. This work was partially supported by the Hungarian Scientific Research Fund (OTKA) in projects No. T-022976 and T-029701
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