Functionally graded materials: review on manufacturing by Liquid and gas based techniques

New materials called functionally graded materials (FGMs) have qualities that gradually alter in relation to their dimensions. This collection of materials represents a significant advancement over earlier composite material. FGM is made up of two or more components that work together to produce the qualities necessary for the intended use. This article provides an overview of the various FGM classifications, fabrication techniques (gas based and liquid based), and applications. The problems associated with the manufacturing of FGM are addressed in the study. This paper also reviews the present state of knowledge in the areas of selection of material, manufacturing process, characterization studies, and modelling of FGM. The potential applications of the FGM, with their advantages and disadvantages, are discussed in this paper. The proposed new primary paths for FGMs research are based on the categories that have been presented and the most recent improvements in analysis and production methods.


Introduction
Sectors like automobiles, aerospace, electronics, and many others require features that cannot be achieved by conventional engineering processes. These sectors require materials with properties like resistance to chemicals, and they should withstand loads even at higher levels. It should have a hard surface, which eventually prevents wear [1]. To achieve this properties surface treatment are usually employed for obtaining desired qualities but it has some disadvantages like adhesion with the surface. Another method available is the alloying of elements, but it has some restrictions like being incompatible with some materials. Alloying may also involve two materials with different temperatures, which makes it unsuitable in some cases [2]. Another method for achieving the same is composite materials, but there are also problems like alloying materials [3][4][5]. When the composite material is laminated, the characteristics of composites are uniformly distributed throughout the material, resulting in uniform product behaviour [6,7].
Understanding materials and working with them has always been essential to our technological advancement. Today's engineers and scientists understand how crucial it is to use novel materials for both financial and environmental reasons [8]. Functionally graded materials (FGM) are sophisticated engineered materials that have a spatial gradation in content or structure that allows for specialized properties [9]. To do this, detailed graded compositions, microstructures, and features are provided. Nature is not a stranger to FGM. Bamboo, a type of functionally graded material, has found applications in the field of construction in recent years [10]. However, the absence of proper manufacturing methods limited the growth of functionally graded materials. In the year 1984, Japan started using functionally graded materials. The limited fabrication methods that were available at the time, however, caused a delay in the future development of graded structure materials [6]. For the research and application of thermal barrier materials, the phrase 'functionally graded material' was Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
first used in Japan in 1984 [11]. In present the capacity to produce FGM which is suitable for advanced applications have initiated the research among the researchers [12].
Under constant and modest composition alteration, FGMs are routinely created with a precise spatial distribution of constituent phases such as metals, ceramics, and polymers. The ability to generate customizable morphologies and structural properties such as physical and mechanical gradients in defined directions is the fundamental advantage of FGMs over traditional composites. There are numerous methods for obtaining the compositional gradient illustrated in Fig. This comprises gas-based, liquid-phase, and solid-phase technologies that can be utilized to produce tailored qualities physically or chemically. Gas-based methods for fabricating FGM include chemical-vapor-deposition (CVD), plasma spraying, ion plating, ion mixing and additive manufacturing [13]. In vapour-phase processes like CVD and PVD, the deposited product can have a gradient composition that is determined by the ratio of phases in the mixture and the production regulating system. Liquid phase technologies, such as plasma spray, are particularly popular for coating applications due to their production flexibility, high deposition rate, and complicated form. The composition gradient in the electrodeposition process is strongly reliant on electrochemical factors and electrolytic solution choices [14][15][16][17][18][19].
The fundamental processes in the fabrication of FGM are the grading of spatially nonhomogeneous materials and the structure settling on the substrate. Different processing techniques have various properties that have a considerable impact on the final attributes of FGM. Manufacturing process modelling was also given considerable attention. Process simulations may enable the prediction of ideal processing parameters for FGMs in the future, minimising the large amount of experimental work required to generate a graded material free of flaws. The manufacturing section is explored and provided in detail in the next section [20][21][22][23][24][25].
The possibility of manufacturing FGM, which has better mechanical, electrical, and thermal characteristics, makes it a suitable material for the fabrication of a number of products in several sectors. In present state the polymer based FGM are gaining popularity is gaining interest among the researchers. Polymer-based FGM is finding applications in several fields, but there are some limitations in obtaining its full potential. The following section discusses the gas and liquid based FGM manufacturing and the processes involved in them. Applications, advantages and disadvantages are also discussed in this paper.

Manufacturing methods of FGM's
The below section discusses the manufacturing method employed in the production of FGMs. FGM is produced using a variety of well-known fabrication methods and many new ones as well. Many articles that describe the specifics of various production techniques and explain their technical aspects, benefits, limits, implementations, and research directions may be found in the literature.

Chemical vapour deposition/infiltration
Chemical Vapor Deposition (CVD) has been used in industry since a patent published in 1893 by de Lodyguine [15]. Schematic diagram of the CVD process is shown in figure 1. The extraction of soot from the partial oxidation of firewood is the oldest method for CVD deposition. CVD is considered to be one of the promising for production of semiconductor for industries based on electronics is CVD at the year 1970. Apart from the electronics industry, it also finds applications in the manufacturing of turbine blades and solar cells [26]. During the process of CVD, the energy sources utilized are plasma and light. Hydride and chloride are the most commonly used gases during the CVD process. The rate of deposition can be varied by varying the temperature applied during the deposition, the chemical composition of gases, and the gas ratio. These methods have a number of benefits, including the ability to fabricate composites with near-net shapes at low temperatures. Metals evaporate in the case of physical vapour deposition, depositing onto the substrate [23,[27][28][29][30][31].
During the CVD process, if the gas concentration and rate of flow of gas are modified, there is a change in microstructure, thickness of the material formed, and chemical composition. Wang et al [31] looked into the effect of modifying the rate of gas during the manufacturing of functionally graded ZnO materials used in semiconductors. In addition, the authors created one-and two-dimensional nanowires, concluding that, unlike 1D nanowires, 2D functionally graded ZnO growth was not constrained to a single direction. The CVD process can be used to deposit material on even tiny surfaces; it is not just restricted to one specific field.
By utilizing the CVD process, the FGM materials consisting of SiC/C were manufactured by Sasaki and Hirai [32]. The output parameters evaluated were barrier and thermal resistance. For SiC functionally graded materials, residual stresses led to vertical cracks, which were mostly caused by heat. They also came to the conclusion that FGM with a specific thickness can be manufactured using chemical infiltration processes. FGM material composed of carbon fiber and SiC was manufactured by Kawase et al [33]. Carbon fiber was densified using resol-type resin, which was utilized for liquid-phase saturation. When the same material was quenched from 0 to 1000°C, there was no evidence of adhesive characteristics. During the processing of FGM using the CVD process, if the parameters like gas flow rate and concentration of gas are varied, there will be a variation in morphological characteristics, material thickness, and composition of the material [34]. By the 1970s, CVD had established itself as a prominent fabrication process for semiconductors and electric circuit board covering. However, this approach required the use of costly and highly hazardous precursor gases. As a result, this approach was used to create enhanced FGCs for turbine blades, and various other purposes where mechanical and tribological qualities were critical [26].

Surface reaction process
Steel materials are usually subjected to surface reaction techniques like nitriding and carburizing [35]. Zhecheva et al [36] fabricated the development of a nitride layer over the aluminum alloys. From the results, it was concluded that when the process parameters like temperature and time are altered, there is a change in microstructure and new phases are formed. Chen et al successfully created a functionally graded WC-Co/Ni composite composed of numerous WC/Co layers, transitional layers (the WC gradient), and Ni layers. Guo et al [37] investigated the workings of carbon diffusion bonding in the WC-Co system as well as the dynamics of metal binder synthesis during the process of liquid transfer. They used a carburising process to produce a cobalt gradient in WC-Co composites since they placed a lot of focus on the end product's wear resistance. To maximize the benefits of the carburising process, it is crucial to comprehend its mechanisms. Variations in processes like time, temperature, and composition of materials were evaluated in studies [38,39].

Thermal spray
The thermal spray process (TSP) has a long history that goes back more than a century. The earliest known references to the thermal spray method come from M.U. Schoop's validated patents, which date from 1882 to 1889 and are from Zurich, Switzerland. In 1908, Schoop and others also co-patented the electric arc spray, which made it possible to spray more material. However, the technology did not significantly advance until after World War II, when powder spraying and plasma spray were created and introduced [40][41][42]. Since then, these processes have seen many improvements, but their fundamental operating principles have not changed The types of thermal spray process are shown in figure 2. Detonation gun spray technique was utilized for producing an FGM layer with a finely mixed microstructure of metals and ceramics and a desirable compositional gradient in the thickness direction. The microhardness and XRD tests demonstrated that the ceramics and metals combined in the FGM layer retained their unique characteristics without significant oxidation or phase transition. Thermal shock testing utilizing a burner rig tester confirmed the projected improvement in thermal shock resistance due to the realisation of a functionally graded layer between the ceramic and metal coating layers in thermal barrier coatings systems [43].
Modern instrumentation and procedures, as well as the need for high-temperature coatings in the aviation and defence sectors, have resulted in a significant paradigm shift in this subject. Thermal spray is now primarily used for surface treatment of materials to produce super alloys, high levels of erosion and wear resistance in working tools, electromagnetic interfaces, and chemical and thermal barrier coatings. Coating materials consist of oxides of metals, carbides, and materials made from ceramics [44].
One of the most important uses of FGMs that is widely accepted is as a thermal barrier coating, which is usually made by spraying. TBCs are used in many ways, like superheating parts of petrol turbine engines and putting tungsten coatings on dampers in jet engines, to improve fatigue resistance, creep resistance, and fuel efficiency. When creating thermal barrier coatings, the difference in thermal expansion between the base material and the coating is a major issue. Coating materials have been replaced with coatings that are graded by how well they do their job. Plasma spray has a relatively high rate of solidification (106 C s −1 ) when it comes to making things. Functionally graded TBCs have a far smaller differential in thermal expansion coefficients between the intermediate layer and the metal substrate than non-graded TBCs, and the coating has the least amount of stress [45].

Liquid phase process 2.2.1. Gel-casting process
The ability to produce components at a faster pace with enhanced wettability and the ability to produce complex parts make the gel casting process an attractive process for manufacturing FGM. For the manufacturing of complex parts, the gel casting process is also known as colloidal processing. It has advantages like a higher yield rate and a lower machining cost. Studies about gel casting were started in the Oak Ridge laboratory in 1990. Gel casting process is shown in figure 3. Gel casting is also utilized for creating composites made of ceramics and metal. During the process, the ceramic particles are mixed with a liquid phase that acts as a cross-linker, and a catalyst is also added to it. Once it is mixed, it is transferred into the mold and allowed to cool [46,47].
When still wet, the component that has gelled and has a consistent consistency is removed from the mold. Finally, under carefully controlled conditions, the wet gelled part is dried to form a dried green body. It is possible to machine-dry the green body [48]. Sintering and binder removal happen similarly to other ceramic processes. Many studies on the gel casting technique have been authored by several researchers in the recent past on the subject of FGMs, with the objective of manufacturing elevated and effective products [49,50]. In the past few decades, numerous studies in the subject of FGM have been written that have concentrated on the gel casting technique in order to produce high-quality and effective products [51].

Tape casting
The tape casting method is suitable for large-scale manufacturing of multi-layered and ceramic substrates [52]. Powders, plasticizers, and binders are mixed in a solvent, which forms a slurry during the tape casting process. The obtained slurry is applied to the layer of belt that moves under the edge of the blade and deposits into green tape with a uniform thickness. The liquid gets evaporated, forming a dense and flexible dry tape that is then sliced according to size requirements. The layers are combined, and it is sintered once it gets assembled. Health hazards of the tape casting process have drawn a lot of attention lately [53]. Acikbas et al [54] performed tape casting of SiAlON ceramics. The outcomes of their research revealed that there is an increase in hardness when the amount of SiAlON is increased. Microstructural studies show that if the process parameters are changed, there will be a noticeable change in the structure formed. Shulong Liu et al fabricated FGM consisting of W/Cu by the process of tape casting. The manufactured FGM has a more homogeneous structure and a finer surface. Individual compositions were made into monolithic tapes for characterization by combining many layers by hot pressing. Schematic diagram of the tape casting process is shown in figure 4. In today's modern society, various research groups are striving to fabricate FGMs with high performance by tape casting process, in response to the need for advanced products and other important situations, and there are also many publications published in this field ten years ago [1].

Centrifugal casting
Until the year 1980, for manufacturing large components like bearings for cylinders, gears, and steel roll mills, Fabrication of metal matrix composites by the centrifugal casting method led to the fabrication of FGM in late 1990. When two materials are added during the centrifugal casting process, there is a difference in their densities. Factors like speed of rotation and centrifugal force influence how the second materials differ from the first material. The disadvantage of making intricate pieces by centrifugal casting can be overcome by the investment centrifugal casting process [55,56]. The procedure of centrifugal casting is depicted in figure 5.
Less dense particles settle at the inner surface during the centrifugal casting process, whereas more dense particles clump together at the outer surface. To manufacture aluminum tubes, particles like titanium aluminide and magnesium silicide must be suitably strengthened at both the inner and exterior surfaces. The factors to be considered during the centrifugal casting process are: (i) cylindrical mold, (ii) forces during pouring, (iii) power transmission, (iv) temperature during processing, (v) rate of cooling, (vi) particle size, (vii) density of material to be poured, and (viii) rotating mold speed. The above-mentioned parameters must be selected properly, which ensures that the parts. To ensure that the parts are produced with better properties, the aforementioned factors must be carefully chosen. Although the aforementioned factors can be changed, they must be carefully chosen for superior qualities. Finding the optimal combination of process parameters when developing a casting process typically results in higher costs and a longer production cycle. Real-time radiography and computer modelling are therefore used in the method's design to reduce costs, speed up production, and improve the quality of the parts made [57][58][59][60]. The approaches like the centrifugal solid particle approach and the centrifugal in situ method can be employed for the manufacture of FGM. If centrifugal solid particle separation is considered, the dispersed phase must be in liquid form, while for the centrifugal in situ method, the centrifugal force is utilized throughout the process. In other words, there are two types of centrifugal casting processes according to the variation in the master alloy and processing temperatures. Centrifugal in situ is a technique that may be used during the solidification step if the processing temperature is greater than the master alloy temperature [61]. Furthermore, there are two drawbacks to this process, which include the capacity to manufacture only cylindrical-shaped FGMs and a limitation on the type of gradient that may be produced [60].

Electrophoretic deposition
Anodic electrodeposition, cathodic electrodeposition, electrophoretic coating, electrophoretic painting, and electrophoretic coating are among the few that make up the above-mentioned electrophoretic deposition. At first, the concept of electrophoretic deposition was invented by Russian scientists. Initially, thoria particles are deposited on a platinum cathode, which behaves like an emitter for electrons. The concept of electrophoretic deposition, which came into existence in 1980 and was mainly used for tools made of ceramics, is used to produce the ceramic basic tools. The schematic diagram of the process is shown in figure The need for simple equipment, a fast-processing time, and accurate shape formation are the main advantages of this method. The setup cost is very low, and the products can be easily manufactured into net shapes. It is a quick-forming process since the deposition rate might be on the order of hundreds of microns per minute. The key restriction is that the deposit on the electrode should be removable after deposition. Another restriction that could apply is that the counter electrode should be made to ensure that the targeted area of the deposition electrode has a consistent electric field [62][63][64].
By carefully altering the suspension composition as a function of time, the EPD approach enables the processing of FGM with a continuous composition gradient. This is possible because the suspension's contents at the time of deposition are strongly linked to the deposit's composition throughout the EPD process. As a result, in a continuous shaping phase, a controlled gradient can be generated by modifying the solids loading, for example, by injecting a different type of powder. In a number of systems, EPD has been shown to be useful in the generation of FGMs. The counter electrode must be properly constructed in order to provide a consistent electric field at the surface of the deposition electrode with complex geometries. Electrical field simulations using finite element analysis must be performed to aid in the design of the best counter electrodes. Many studies have been conducted to investigate the EPD of thin zeolite films. Water as well as chemical solvents such as acetylacetone (AcAc) and isopropyl alcohol (IPA) have been studied [65][66][67][68][69]. By altering the process parameters during the electrophoretic process parameters, the variation in end properties can be achieved [64] (figure 6).

Directional solidification
Directional solidification is applied to several materials, like metallic or ceramic materials [70]. It is mainly used for producing single crystals, which can be applied for high-tech applications. FGM was produced from organic polymer mixtures by Koide et al [71] using non-equilibrium self-organization mechanisms and a uniaxial heat gradient. The above-mentioned methodology was used for the production of 'low molecular weight polymer/ additive system (poly (e-caprolactone) (PCL)/4,4-thiodiphenol (TDP)'. There is an increase in bonding when there is a decrease in temperature. The outcomes of the DSC test revealed that there is crystallization of PEO even at lower temperatures, while PCL has enough time for directed solidification to separate from PEO.

Electrochemical processing
Electrochemical processing can be carried out by changing the porous preforms and finally adding it to the resulting product. The metal-metal or metal-ceramic based composites can be produced by this method. By modifying the factors like density of current, type of material selected, different types of electrodes, materials conductivity several types of materials can be fabricated. When the electrodes are varied several profiles of the material with various shapes can be produced [71].

Sedimentation
The most commonly used method for manufacturing particle-reinforced composites is sedimentation. This is mainly applicable for the manufacturing of particle-reinforced composites as there is a disparity in densities between the filler and matrix as well as Several manufacturing processes involve flotation and sedimentation, particularly when producing particle-reinforced composites based on the disparity in densities between the constituent particles and the matrix. Several types of products can be prepared by considering the size and shape of the particles [72]. Carmine Lucignano and Fabrizio Quadrini [73] manufactured functionally graded materials through the process of sedimentation. The matrix selected for the study is polyester, and the fillers selected are glass and silica. Two types of samples-filled and unfilled polyester samples-were prepared. The manufactured composites were characterized by flat indentation tests. From the experimental curves, characteristics like indentation, slope, and load were extracted.

Chemical solution deposition
Due to the numerous advantages chemical solution deposition method is one of the well-established methods for production of functional oxide films. In the year 1980s the first complex oxide films were produced by chemical solution deposition. During the chemical solution deposition process, the liquid percussor is dissolved in an organic solvent. This process is low cost and frequently used one for thin films. The parameters considered for the percussors are (i) solubility of products in the solvent, (ii) resistance of percussor solution, (iii) rheology of solution, and (iv) wettability of the surface. Because of its less cost, control of composition, and ability to be quickly applied to FGM films, the chemical solution deposition (CSD) method is the preferred one for manufacturing of PZT type ferroelectric type films [74,75].

Laser deposition
The laser deposition method deposits ceramic powder on the surface of a substrate using a high-intensity laser. Figure 1 depicts a schematic diagram of laser deposition. It is appropriate for materials in which the melting temperatures of the filler and matrix differ. Laser deposition can be used to deliver items to a precise location and can readily construct dense structures.
Wilson and Shin [75] reinforced titanium carbide powder on nickel materials and studied the laser deposition characteristics. It was concluded that the addition of titanium carbide resulted in a transition of morphology from columnar to equiaxed, and there was also refinement of structure. They also reported that when the amount of titanium carbide is increased, there is an increase in wear resistance ( figure 7).

Conclusion
FGM has attracted researchers to carry out their work as it has become a new study in the field of material science. FGM finds its application in several sectors, like defence, medical fields, energy sectors, and many other fields. This work gave an overview of the manufacturing of FGM by gas-based and liquid-based techniques. Though there are several works related to FGM, there are still some gaps to be rectified, and they are listed below.
(i) In order to avoid environmental hazards, FGM environmental issues must be researched. In reality, the problem is highly complex and difficult, needing study of more general and complex circumstances.
(ii) FGM has many uses in the military, the auto industry, the energy industry, and other fields, but it is most important in medicine. FGM finds applications in the defense sector, automobile sectors, and many other fields, but it plays a vital role in the field of medicine. FGM finds the application of implantation in the dental field. Thus, a vast study has to be carried out for the implementation of FGM in the medical field.
(iii) Additionally, there is a lot of material that focuses at a specific topic, such as the performance of FGM or how it is manufactured, but there is a need to see it in its entirety. This implies the need for an investigation of FGM's performance, the ways its parts are displayed, or how it is created, but looking at all of these things at the same time is difficult. As a result, a broad-design system is required so that researchers may create models, analyse them, and create FGMs with complex geometry.
(iv) Appropriate modelling techniques for 2D and 3D FGM materials must be developed.
(v) Despite these accomplishments, there are still some accomplishments mentioned below are there for future development of FGM materials which are (a) mass production and upscaling of manufacturing processes, (b) reduced cost of production and (c) quality of the product produced.
(vi) This work can be expanded by investigating numerous additional FGM production methods and comparing them to each other to investigate their merits and downsides.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).