Functionally graded materials obtained by combustion synthesis techniques: A review

Among the large number of different techniques to produce Functionally Graded Materials (FGMs), Combustion Synthesis (CS) is gaining an increasing interest due to the possibility of achieving high-purity products in short processing times (typically of the order of few seconds or less), and with low energy consumption (limited to the ignition step), as well as low cost of the manufacturing equipment required. CS exploits high exothermic reactions between reactants, which, after reaching the ignition temperature, start to form the desired products and the reaction becomes self-sustaining, not requiring any other external energy contribution. Depending on the way of ignition, combustion synthesis can be conducted in the Self-propagating High-temperature Synthesis (SHS) mode or in the Thermal Explosion (TE) mode. In the SHS mode, the reaction is ignited at one end of the reactive sample and it self-propagates in the form of a combustion wave at very high velocities. In the TE mode (also known as reactive sintering or volume combustion synthesis) the whole volume of the sample is heated uniformly in a controlled manner until reaction takes place essentially simultaneously throughout the volume. The use of CS in FGMs manufacturing can benefit from the fast kinetics involved, allowing to create non equilibrium structures or to lead to products less prone to homogenization, thus preserving the gradient structure imparted during the forming step. Among the wide variety of possible ignition techniques, which will be discussed in the present review, recent results obtained by microwave (MW) irradiation will be presented, discussing the advantages of such a heating technique in FGMs manufacturing. MWs, in fact, can enhance the previously mentioned advantages due to their peculiarities of rapid, volumetric and selective heating, the latter particularly relevant when dealing with multi-phase systems. © 2012 by Nova Science Publishers, Inc. All rights reserved.