Functionally graded materials (FGMs) are engineered materials that are inhomogeneous and can be purposefully processed to obtain discrete or continuously varying compositions/microstructures over a definable geometrical length. FGMs can be used in a number of applications, such as aircrafts, combustion nozzles, gas turbines, energy conversion cells, biomaterials, etc. The objective of this paper is to review the new developments in production processes and their prospects in the creation of next-generation FGMs. Traditionally, four potential methods were used for fabrications of FGMs, i.e. powder metallurgy, chemical vapor deposition, self-propagating high-temperature synthesis, and plasma spraying. Some of the recently developed methods are the cast-decant-cast process, friction stir processing, and laser-engineered net shaping, which are usually cost-effective and used to make a quiet change in properties. An effective production method for conversion of the concept of gradient into practice is still a challenge for the research community. In this paper, research works toward meeting these challenges will be highlighted, and the future scopes of investigation in this area will be explored.

The present study aims at introducing a newly developed natural fiber called castor oil fiber, termed ricinus communis, as a possible reinforcement in tribo-composites. Unidirectional short castor oil fiber reinforced epoxy resin composites of different fiber lengths with 40% volume fraction were fabricated using hand layup technique. Dry sliding wear tests were performed on a pin-on-disc tribometer based on full factorial design of experiments (DoE) at four fiber lengths (5, 10, 15, and 20 mm), three normal loads (15, 30, and 45 N), and three sliding distances (1,000, 2,000, and 3,000 m). The effect of individual parameters on the amount of wear, interfacial temperature, and coefficient of friction was studied using analysis of variance (ANOVA). The composite with 5 mm fiber length provided the best tribological properties than 10, 15, and 20 mm fiber length composites. The worn surfaces were analyzed under scanning electron microscope. Also, the tribological behavior of the composites was predicted using regression, artificial neural network (ANN)-single hidden layer, and ANN-multi hidden layer models. The confirmatory test results show the reliability of predicted models. ANN with multi hidden layers are found to predict the tribological performance accurately and then followed by ANN with single hidden layer and regression model.

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Functionally graded cemented tungsten carbide belongs to a hybrid Functionally Graded Materials family. However synthesis of functionally graded cemented tungsten carbide is a great challenge due to possibility of “Co migration” and CaF2 vaporization during liquid phase sintering. Hence the present work deals with the development of nanocrystalline materials using ball milling and consolidation of milled material by Spark Plasma Sintering (SPS) for the preparation of functionally graded cemented tungsten carbide. The effects of ball milling and contents of Co, CaF2 and WC on WC/Co/CaF2 powder mixture were analyzed. Grain morphology, particle size, micro-strain, crystallite size, etc. of milled samples were investigated using scanning electron microscopy and X-Ray diffraction techniques. In addition to that, density, microstructure, hardness, fracture toughness and residual stress of consolidated sample are also reported. These tests confirm the formation of nanocrystalline particles and sub-sequent consolidation of functionally graded cemented tungsten carbide with solid lubricant. Hence ball milling with SPS is a prominent and viable process combination for the preparation of functionally graded cemented tungsten carbide.

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