**4. Design and processing of FG ceramics**

The processing of advanced ceramics is a complex operation requiring several process control steps to achieve the ultimate product performance in the end. A successful forming technique leads to a ceramic product with an engineered microstructure which is characterized by a small defect size and by a well-distributed homogeneous grain boundary composition in order to achieve optimal performance and a high degree of reliability.

The manufacture of FGCs can be divided into two steps, namely gradation and consolidation. Gradation is the building of the spatially inhomogeneous graded structure, while consolida‐ tion is the transformation of this graded structure into the bulk material. The gradation process is usually classified into three main groups: constitutive, homogenizing, and segregating processes. The stepwise creation of a graded material from precursor materials is the basic constitutive process. In the homogenizing processes, the sharp interface between the two materials is converted to a gradient by material transport i.e. diffusion. In the segregating process, the macroscopically homogeneous material is converted into a graded material by an external gravitational or electric field. The primary advantage of the homogenizing and segregating processes is the production of a continuous gradient. Following this, drying and sintering (or solidification) steps need to be adapted relevant to the particular material selected, and attention has to be paid to the different shrinkage rates during the sintering of FGCs [11].

The manufacturing process is one of the most important areas of FGC research. A large part of the research into FGCs has been dedicated to processing, and a large variety of production methods have been developed for use in the processing of FGCs. Most of the processes of FGC production are based on variations of conventional processing methods, which are already well-established. Methods that are capable of accommodating a gradation step include powder metallurgy [12-14], sheet lamination, chemical vapor deposition and coating processes. In general, the forming methods used include centrifugal casting [15-17], slip casting, tape casting [18], and thermal spraying [19, 20]. Which of these production methods is the most suitable? It depends mainly on the material combination, the type of transition function required, and the geometry of the desired component. However, it was found that powder metallurgy (PM) will be the most suitable method for the manufacture of FGCs in the future. It is believed that the main issue in the implementation of the PM method is the sintering process, which needs to be explored further in order to achieve improvements in the microstructure and mechanical properties of the resulting FGCs [21].

#### **4.1. Powder metallurgy**

discovered. Examples of these FGCs are **alumina/zirconia**, a material used in biomedical and structural applications, **mullite/alumina,** which is used as a protective coating for **SiC** components in corrosive environments [2, 5]. **Zirconia-mullite/alumina** FGCs can be used as refractory materials in high temperature applications, as well as being suitable for engineering

An example of this type of FGC is the **boron carbide/polymer** FGC. Due to its light weight and flexibility, the BC/polymer FGC is used in lightweight armor and wears related applica‐ tions [8]. The feature of this FGC is that the ceramic with graded porosity is fully dense on the front surface changing to open porosity on the back surface. The polymer is then infiltrated into the porous side of the ceramic plate to provide a lightweight energy-absorbing backing. A ballistic fiber weave, such as Kevlar, could also be embedded in the polymer to provide

Ceramic/ polymer FGCs could also find applications in reducing the wear of automotive components. Additionally, they are used in many industrial applications requiring materials that are resistant to wear, corrosion, and erosion in hostile environments. Also, this type of FGC can be used in nuclear applications, such as the manufacture, handling and storage of

Recently, the introduction of porosity in ceramic/polymer FGCs has broadened the scope of their application in the fields of biomedicine and tissue engineering [9, 10]. Due to the large surface area, high porosity, low thermal conductivity and high-temperature resistance of the porous ceramics, they were widely used in many fields, such as functioning as supports for ceramic filters, as artificial bones, high temperature insulators, and active cooling parts.

The processing of advanced ceramics is a complex operation requiring several process control steps to achieve the ultimate product performance in the end. A successful forming technique leads to a ceramic product with an engineered microstructure which is characterized by a small defect size and by a well-distributed homogeneous grain boundary composition in order to

The manufacture of FGCs can be divided into two steps, namely gradation and consolidation. Gradation is the building of the spatially inhomogeneous graded structure, while consolida‐ tion is the transformation of this graded structure into the bulk material. The gradation process is usually classified into three main groups: constitutive, homogenizing, and segregating processes. The stepwise creation of a graded material from precursor materials is the basic constitutive process. In the homogenizing processes, the sharp interface between the two materials is converted to a gradient by material transport i.e. diffusion. In the segregating process, the macroscopically homogeneous material is converted into a graded material by an

and tribological applications [6, 7].

4 Advances in Functionally Graded Materials and Structures

constraint and enhanced ballistic protection.

**4. Design and processing of FG ceramics**

achieve optimal performance and a high degree of reliability.

**3.3. Ceramic/ polymer**

plutonium materials [8].

Powder metallurgy (PM) is one of the most prevalent techniques due to its wide range control of composition, its microstructure and its ability to form a near net shape. It is a cost-effective technique and has the advantages of greater availability of raw materials, simpler processing equipment, lower energy consumption and shorter processing times. In powder processing, the gradient is generally produced by mixing different powders in variable ratios and stacking the powder mixtures in separate layers.

The thickness of the separate layers is typically between 0.2 mm and 1mm. Several techniques have been introduced for powder preparation, such as chemical reactions, electrolytic deposition, grinding or comminution. These techniques permit the mass production of powder form materials and usually offer a controllable size range of the final grain population. In powder processing, the main consideration focuses on the precision in weighing of amounts of individual powders and the dispersion of the mixed powders. These elements will influence the properties of the structure and need to be handled very carefully. In the subsequent processes, the forming operations are performed at room temperature, while sintering is conducted at atmospheric pressure as the elevated temperature used may cause further reactions that may affect the materials [22]. [23] studied the manufacturing method of another constituent, **ZrO2/AISI316L FGCs** for use in joint prostheses. The mechanical and biotribo‐ logical properties of the FGCs were evaluated through studies of their fracture toughness, bending strength, and wear resistance. It was found that FGMs with a layer thickness of less than 1.0 mm showed a low wear resistance. FGCs with a layer thickness of more than 2 mm, therefore, have mechanical and biotribological properties which are suitable for use in joint prostheses. [24] studied the relative density, linear shrinkage and Vickers hardness of each layer of **8YSZ/Ni FGC**. The microstructure and the composition of these components were also studied. The results obtained showed that FGCs produced by spark plasma sintering exhibited a low porosity level and consequently fully dense specimens. There are no macroscopic distinct interfaces in **YSZ/Ni** FGM due to the gradual change in components. Another successful FGC prepared by the PM method is **ZrO2/NiCr** FGC, as studied by [12].

#### **4.2. Hot pressing**

Yittria stabilized zirconia **(YSZ)** and nickel 20 chromium **(NiCr)** are the two materials com‐ bined using **YSZ-NiCr FGC** interlayer via the hot pressing method [25]. At the initial stage of processing, the powdered YSZ and NiCr were mixed in a ball milling machine for 12 hours before being stacked layer-by-layer in a graphite die coated with boron nitride. In this study, the concept of stepwise gradation was applied by arranging the composition of each layer to be a certain desired percentage. The preoccupation of each layer was performed at a lower pressure before stacking the adjacent layer under higher pressure (10 MPa) to ensure an exact compositional distribution within the layers.

A new composition profile of 15 layers with a crack-free joint of the **Si3N4-Al2O3 FGC** was proposed using the hot pressing technique [26]. Bulk SiC/C FGC is another pair successfully manufactured using the hot pressing process. In terms of thermal properties, the hot pressed SiC/C FGC was found to have a high effective thermal conductivity at the interface of the 1 mm SiC layer when compared to the specimens prepared using other methods. No cracks were found in the SiC/C coatings, as a result of the high thermal fatigue behavior of the FGC. The plasma-relevant performance also indicated that the specimen has excellent high temperature erosion resistance [27]. Moreover, hot pressed **hydroxyapatite/Ti (HA/Ti) FGC** showed a strong biocompatibility and a high bonding strength with the bone tissue of rabbits, as investigated by [28]. The study concluded that the HA/Ti FGC has a good potential for use in hard tissue replacement applications as it possesses a high bonding strength which could exceed the 4.73 MPs shear strength of new bone tissues when compared to pure Ti metal. Amongst the successfully manufactured hot pressed FGCs are the novel **TiB2/ZrO2** and **TiB2- SiC/ZrO2 FGCs** which show excellent properties and have been identified for possible use in ultra-high temperature applications [29].

#### **4.3. Cold pressing**

A beam-shaped porous lead zirconia titanate-alumina **(PZT-Al2O3) FGC** actuator that exhibits the theoretically matched electric-mechanical response with a crack-free structure based on the pyrolyzable pore-forming agent (PFA) porosity gradient, has been successfully manufac‐ tured using a cold sintering method [25].

The binder addition is similarly applied in the manufacture of another FGC composed of Ni and **Al2O3** in order to investigate the influence of the particle size used. In this study, the appropriate Ni, Al2O3 and Q-PAC 40 (organic binder) particle sizes were selected, based on the desired microstructure of the corresponding composition. After being mixed together in the blending process, the powder mixtures were cold pressed under 86 MPa pressure. This was followed by pressureless sintering at 1350°C with specific sintering [30]. The titanium/ hydroxyapatite **(HA/Ti)** and other FGC implants with a gradually changing composition in the longitudinal direction of the cylindrical shape were also manufactured via cold isostatic pressing (800 to 1000 MPa) in order to optimize the mechanical and biocompatibility properties of the resultant structures [31]. Figure 1 shows the flow chart outlining the manufacturing process of the cold pressed Al2O3-ZrO2 FGC used in the study [30]. Different elemental consideration under powder characteristic in terms of the addition of the space holder material was investigated on porous Ti-Mg (titanium-magnesium) FGM.

Most researchers working with this technique increasingly intend to use microscale particles in the manufacture of FGCs since nanoparticles need greater precision during processing. Only a small number of limited studies report using nano-sized composition particles [21]. **Co/α-Al2O3 FGC** composed of nano-sized powders was successfully manufactured using a high pressure torsion procedure [32]. This procedure is classified as a PM method, and cold pressing — as the consolidation or sintering process — is performed after compaction. The difference is only in the way of delivering the pressure in the torsional mode.

**Figure 1.** Flow chart detailing the manufacturing process of Al2O3/ZrO2 FGC [30].

#### **4.4. Sintering process**

layer of **8YSZ/Ni FGC**. The microstructure and the composition of these components were also studied. The results obtained showed that FGCs produced by spark plasma sintering exhibited a low porosity level and consequently fully dense specimens. There are no macroscopic distinct interfaces in **YSZ/Ni** FGM due to the gradual change in components. Another successful FGC

Yittria stabilized zirconia **(YSZ)** and nickel 20 chromium **(NiCr)** are the two materials com‐ bined using **YSZ-NiCr FGC** interlayer via the hot pressing method [25]. At the initial stage of processing, the powdered YSZ and NiCr were mixed in a ball milling machine for 12 hours before being stacked layer-by-layer in a graphite die coated with boron nitride. In this study, the concept of stepwise gradation was applied by arranging the composition of each layer to be a certain desired percentage. The preoccupation of each layer was performed at a lower pressure before stacking the adjacent layer under higher pressure (10 MPa) to ensure an exact

A new composition profile of 15 layers with a crack-free joint of the **Si3N4-Al2O3 FGC** was proposed using the hot pressing technique [26]. Bulk SiC/C FGC is another pair successfully manufactured using the hot pressing process. In terms of thermal properties, the hot pressed SiC/C FGC was found to have a high effective thermal conductivity at the interface of the 1 mm SiC layer when compared to the specimens prepared using other methods. No cracks were found in the SiC/C coatings, as a result of the high thermal fatigue behavior of the FGC. The plasma-relevant performance also indicated that the specimen has excellent high temperature erosion resistance [27]. Moreover, hot pressed **hydroxyapatite/Ti (HA/Ti) FGC** showed a strong biocompatibility and a high bonding strength with the bone tissue of rabbits, as investigated by [28]. The study concluded that the HA/Ti FGC has a good potential for use in hard tissue replacement applications as it possesses a high bonding strength which could exceed the 4.73 MPs shear strength of new bone tissues when compared to pure Ti metal. Amongst the successfully manufactured hot pressed FGCs are the novel **TiB2/ZrO2** and **TiB2- SiC/ZrO2 FGCs** which show excellent properties and have been identified for possible use in

A beam-shaped porous lead zirconia titanate-alumina **(PZT-Al2O3) FGC** actuator that exhibits the theoretically matched electric-mechanical response with a crack-free structure based on the pyrolyzable pore-forming agent (PFA) porosity gradient, has been successfully manufac‐

The binder addition is similarly applied in the manufacture of another FGC composed of Ni and **Al2O3** in order to investigate the influence of the particle size used. In this study, the appropriate Ni, Al2O3 and Q-PAC 40 (organic binder) particle sizes were selected, based on the desired microstructure of the corresponding composition. After being mixed together in the blending process, the powder mixtures were cold pressed under 86 MPa pressure. This

prepared by the PM method is **ZrO2/NiCr** FGC, as studied by [12].

compositional distribution within the layers.

6 Advances in Functionally Graded Materials and Structures

ultra-high temperature applications [29].

tured using a cold sintering method [25].

**4.3. Cold pressing**

**4.2. Hot pressing**

The sintering process is performed simultaneously with the compaction process if the FGC is prepared using a hot pressing process. However, in the cold pressing process, the sintering process is performed only after the powders have been compacted. The effectiveness of three different sintering methods, including electric furnace heating, high frequency induction heating, and spark plasma sintering (SPS) were investigated, [33]. SPS is a newly developed process which enables the sintering of high quality materials in short periods by charging the intervals between powder particles with electrical energy. Their systems offer many benefits in terms of ease of operation, low cost, a more uniform and rapid sintering compared to the conventional systems using hot press sintering, hot isostatic pressing or atmospheric furnace processes applied to many advanced materials. Amongst the reported SPS FGCs are WC based materials **(WC/Co, WC/Co/steel, WC/Mo),** and**ZrO2** based composites **(ZrO2/steel, ZrO2/TiAl, ZrO2/Ni), Al2O3/TiAl**, etc. [34]. The influence of **ZrO2** content and sintering temperature on microstructures and mechanical properties of the composites were investigated by [35].

In order to evaluate the sintering performances, one of the parameters that could be investi‐ gated is the porosity. As a result, some sintering models have been developed and analyzed to this end. These studies proved that the amount of porosity is directly related to the rate at which shrinkage occurs [36]. The changes in porosity and shrinkage in the theoretically sintered nickel/alumina **(Ni/Al2O3) FGC** have been studied [37]. This study shows how the porosity reduction model can be used to access the quality of particle-reinforced metal-ceramic FGCs formed by pressureless sintering and to predict the changes that can be achieved in porosity reduction through the engineering of the particle dispersion in the processing of FGCs. The influence of other sintering parameters including time, temperature, sintering atmosphere and the isostatic condensation on the performance of the resulting FGCs, was investigated [38]. During the manufacture of the sintered tool gradient materials — composed of **wolfram carbide** and cobalt — used in the study, the sintering parameters were changed in order to find their optimum values. The sequential concentration of the molding, with layers having an increasing content of carbides and a decreasing concentration of cobalt and sintering, ensures the acquisition of the required properties, including resistance to cracking. Another successful example of pressureless sintering is the functionally graded zirconia-mullite/ alumina ceramics **(ZM/A FGC).** These exhibit a homogenous structure with highly improved and unique properties. The recorded value of each test of tailored FGZM/A was nearly equal to the average of the test values of its non-layered composites. This is good evidence of the strength of the interfacial bonding between subsequent layers of the composite as well as the homogeneity and uniformity of the powders in each layer [6, 7].

#### **4.5. Infiltration process**

Infiltration, or to give it the correct scientific terminology — hydrology —is the process by which fluid on the ground surface precipitates into the soil. This process is governed by the force of either gravity or capillary action. The rate of infiltration depends on soil characteristics such as storage capacity, transmission rate through the soil, and the ease of entry of the fluid.

The infiltration method was introduced in order to prepare certain complex FGCs shape. This manufacturing method needs little or no bulk shrinkage and more rapid reaction kinetics. As the common process for mold shaping is the heating of the powder to a temperature that is higher than the liquid phase, the demand of ensuring there is no bulk shrinkage is quite challenging.

A compositionally graded **Al-SiC FGC** was successfully manufactured using the pressureless infiltration method in the early part of the last decade. This indicated that the thermal con‐ ductivity of the FGC produced increased in a nonlinear manner, while the volume fraction of the ceramic element decreased [39]. An innovative method of infiltration processing using microwave sintering and an environmental barrier coating (EBC) was subsequently developed for the manufacture of **Si3N4 FGC**. This FGC is composed of **α-Si3N4-Yb-silicate** green parts and porous **β-Si3N4** ceramics as the substrates [40]. Figure 2 shows the successful manufacture of **YSZ/SiC FGC** via the infiltration method, as investigated by [41]. In addition, different compositions of porous **Ti/HAP FGCs** were also manufactured using the infiltration techni‐ que. The Young's Modulus of the manufactured FGCs was comparable to human cortical bone in the porosity range of 24 to 34%, [42]. The effect of glass infiltration was investigated on the **CaO-ZrO2-SiO2 system** in the development of **glass/alumina FGCs.** In order to obtain the final compositional gradient which is indicated by blue glass, the glass formulation of the system was doped with cobalt by adding a small molar percentage (0.1 mol %) of CoO. Characterization of the specimens proved that the cobalt-doped glass has interesting mechan‐ ical properties, including a high elastic modulus, good fracture toughness, and an acceptable coefficient of thermal expansion [43].

**Figure 2.** Schematic diagram of the infiltration process of YSZ/SiC FGM [41].

#### **4.6. Centrifugal casting**

intervals between powder particles with electrical energy. Their systems offer many benefits in terms of ease of operation, low cost, a more uniform and rapid sintering compared to the conventional systems using hot press sintering, hot isostatic pressing or atmospheric furnace processes applied to many advanced materials. Amongst the reported SPS FGCs are WC based materials **(WC/Co, WC/Co/steel, WC/Mo),** and**ZrO2** based composites **(ZrO2/steel, ZrO2/TiAl, ZrO2/Ni), Al2O3/TiAl**, etc. [34]. The influence of **ZrO2** content and sintering temperature on microstructures and mechanical properties of the composites were investigated by [35].

8 Advances in Functionally Graded Materials and Structures

In order to evaluate the sintering performances, one of the parameters that could be investi‐ gated is the porosity. As a result, some sintering models have been developed and analyzed to this end. These studies proved that the amount of porosity is directly related to the rate at which shrinkage occurs [36]. The changes in porosity and shrinkage in the theoretically sintered nickel/alumina **(Ni/Al2O3) FGC** have been studied [37]. This study shows how the porosity reduction model can be used to access the quality of particle-reinforced metal-ceramic FGCs formed by pressureless sintering and to predict the changes that can be achieved in porosity reduction through the engineering of the particle dispersion in the processing of FGCs. The influence of other sintering parameters including time, temperature, sintering atmosphere and the isostatic condensation on the performance of the resulting FGCs, was investigated [38]. During the manufacture of the sintered tool gradient materials — composed of **wolfram carbide** and cobalt — used in the study, the sintering parameters were changed in order to find their optimum values. The sequential concentration of the molding, with layers having an increasing content of carbides and a decreasing concentration of cobalt and sintering, ensures the acquisition of the required properties, including resistance to cracking. Another successful example of pressureless sintering is the functionally graded zirconia-mullite/ alumina ceramics **(ZM/A FGC).** These exhibit a homogenous structure with highly improved and unique properties. The recorded value of each test of tailored FGZM/A was nearly equal to the average of the test values of its non-layered composites. This is good evidence of the strength of the interfacial bonding between subsequent layers of the composite as well as the

Infiltration, or to give it the correct scientific terminology — hydrology —is the process by which fluid on the ground surface precipitates into the soil. This process is governed by the force of either gravity or capillary action. The rate of infiltration depends on soil characteristics such as storage capacity, transmission rate through the soil, and the ease of entry of the fluid. The infiltration method was introduced in order to prepare certain complex FGCs shape. This manufacturing method needs little or no bulk shrinkage and more rapid reaction kinetics. As the common process for mold shaping is the heating of the powder to a temperature that is higher than the liquid phase, the demand of ensuring there is no bulk shrinkage is quite

A compositionally graded **Al-SiC FGC** was successfully manufactured using the pressureless infiltration method in the early part of the last decade. This indicated that the thermal con‐ ductivity of the FGC produced increased in a nonlinear manner, while the volume fraction of

homogeneity and uniformity of the powders in each layer [6, 7].

**4.5. Infiltration process**

challenging.

Centrifugal casting is one of the most effective methods used in the processing of FGCs due to its wide range control on composition and microstructure. The microstructure and compo‐ sition gradients in some aluminum based FGCs including **Al/SiC, Al/Shirasu, Al/Al3Ti, Al/ Al3Ni,** and **Al/Al2Cu** combinations have been made by evaluating the dispersion of the different phase particles within the FCM structures manufactured via different centrifugal casting processes [44]. The study found that **Al/SiC, Al/Shirasu** and **Al/Al3Ti FGCs** can be manufactured using the centrifugal solid-particle method, while the centrifugal in-situ method is suitable for the manufacture of Al/Al3Ni and Al/Al2Cu FGMs. The combination of both processing methods is required for **Al/(Al3Ti+Al3Ni)** hybrid FGCs.

The phase compositions of FGCs manufactured using this approach depend strongly on the condition of the centrifugal sedimentation process. Relevant factors include the duration of the process, rotation speed, and solid and dispersive fluid contents [45]. A self-propagating high temperature synthesis reaction is added as one of the steps, followed by centrifugal casting, in the manufacture of TiC-reinforced iron base **(Fe-TiC) FCC**. Observation of the manufactured specimen indicated an increasing trend in the hardness profile from the outer surface to the TiC-rich inner surface. The wear performance of the TiC-rich inner face was found to be better when compared to the particle free outer surface of ferritic steel matrices [46].

The formation of gradient solidification is another aspect that was evaluated in the investiga‐ tion into FGCs manufactured via centrifugation. In this study, **SiC, B4C, SiC- graphite** hybrid, primary silicon, **Mg2Si** and **Al3Ni** reinforced aluminum based FGCs were prepared using centrifugal casting. The densities and the size of the reinforcements were found to be two major factors influencing the formation of the graded microstructure [47].

#### **4.7. Slip casting**

**TZP/SUS304 FGC** was developed using a slip casting technique [48]. The gradual distribution of the chemical composition and microstructure of the manufactured specimens eliminated the macroscopic FGC interface that occurs in a traditional ceramic/metal joint. Another FGC material that was successfully manufactured via the slip casting method is **Al2O3/W FGC**, which has the potential to be used as a conducting and sealing component in high-intensity discharge lamps (HiDLs) [49].

#### **4.8. Thermal spraying**

Thermal spraying has been frequently used to produce FGC coatings. Thermal spraying of FGCs offers the possibility of combining highly refractory phases with low-melting metals, and allows for the direct setting of the gradation profile. [50] studied the heat insulation performance of thermal barrier-type FGC coatings under a high heat flux. The FGC coatings with thicknesses varying from 0.75 to 2.1 mm were designed and deposited onto a steel substrate using plasma spraying. [51] studied and investigated the different properties, microstructure and chemical composition of FG 20 wt.% **MgO-ZrO2/ NiCrAl** thermal barrier coatings that were obtained using the plasma spraying process. Scanning Electron Microscope (SEM) observations of the fractured surface revealed that the intermediate graded layer had the compositional mechanical properties of strength and toughness, due to improvement of the microstructure and relaxation of the residual stress concentration. In another study, the spark plasma technique used in the thermal spraying process was employed in the manufac‐ ture of an FGC composed of Hydroxyapatite **(HAp)** and titanium nitride **(TiN)** [52]. In order to improve the adhesion between the adjacent graded layers of the FGC, a proper bond coat should be introduced. It is thought that by arranging the smooth change of the mismatch between the thermal expansion coefficients of the composition, the delamination within the FGC structure could be addressed. Other FGCs manufactured using this technique are **HAp/TiO2, Yttria stabilized zirconia (YSZ)/mullite** coats deposited on **SiC** substrates [53] and tungsten carbide/cobalt **(WC/Co) FGC** [54].

#### **4.9. Laser cladding**

The phase compositions of FGCs manufactured using this approach depend strongly on the condition of the centrifugal sedimentation process. Relevant factors include the duration of the process, rotation speed, and solid and dispersive fluid contents [45]. A self-propagating high temperature synthesis reaction is added as one of the steps, followed by centrifugal casting, in the manufacture of TiC-reinforced iron base **(Fe-TiC) FCC**. Observation of the manufactured specimen indicated an increasing trend in the hardness profile from the outer surface to the TiC-rich inner surface. The wear performance of the TiC-rich inner face was found to be better when compared to the particle free outer surface of ferritic steel matrices [46].

The formation of gradient solidification is another aspect that was evaluated in the investiga‐ tion into FGCs manufactured via centrifugation. In this study, **SiC, B4C, SiC- graphite** hybrid, primary silicon, **Mg2Si** and **Al3Ni** reinforced aluminum based FGCs were prepared using centrifugal casting. The densities and the size of the reinforcements were found to be two major

**TZP/SUS304 FGC** was developed using a slip casting technique [48]. The gradual distribution of the chemical composition and microstructure of the manufactured specimens eliminated the macroscopic FGC interface that occurs in a traditional ceramic/metal joint. Another FGC material that was successfully manufactured via the slip casting method is **Al2O3/W FGC**, which has the potential to be used as a conducting and sealing component in high-intensity

Thermal spraying has been frequently used to produce FGC coatings. Thermal spraying of FGCs offers the possibility of combining highly refractory phases with low-melting metals, and allows for the direct setting of the gradation profile. [50] studied the heat insulation performance of thermal barrier-type FGC coatings under a high heat flux. The FGC coatings with thicknesses varying from 0.75 to 2.1 mm were designed and deposited onto a steel substrate using plasma spraying. [51] studied and investigated the different properties, microstructure and chemical composition of FG 20 wt.% **MgO-ZrO2/ NiCrAl** thermal barrier coatings that were obtained using the plasma spraying process. Scanning Electron Microscope (SEM) observations of the fractured surface revealed that the intermediate graded layer had the compositional mechanical properties of strength and toughness, due to improvement of the microstructure and relaxation of the residual stress concentration. In another study, the spark plasma technique used in the thermal spraying process was employed in the manufac‐ ture of an FGC composed of Hydroxyapatite **(HAp)** and titanium nitride **(TiN)** [52]. In order to improve the adhesion between the adjacent graded layers of the FGC, a proper bond coat should be introduced. It is thought that by arranging the smooth change of the mismatch between the thermal expansion coefficients of the composition, the delamination within the FGC structure could be addressed. Other FGCs manufactured using this technique are **HAp/TiO2, Yttria stabilized zirconia (YSZ)/mullite** coats deposited on **SiC** substrates [53] and

factors influencing the formation of the graded microstructure [47].

**4.7. Slip casting**

discharge lamps (HiDLs) [49].

10 Advances in Functionally Graded Materials and Structures

tungsten carbide/cobalt **(WC/Co) FGC** [54].

**4.8. Thermal spraying**

In the laser cladding process, two or more dissimilar materials are bonded together using laser intercession. During the process, the material which is in powdered form is injected into the system — which is purpose-built for the cladding process — while the laser, which causes melting to occur, is deposited onto the substrate. Although the technique has become the best method for coating various shapes and has been declared to be the most suitable process for applications with graded material, limitations still exist because the setup of the high technol‐ ogy system processes is very expensive and is unsuitable for mass production as a result of the layer-by-layer process. The Nd:YAG type of laser was also being used in the manufacture via selective laser melting (SLM) of super **nickel alloy and zirconia FGC**, Figure 3. The resulting materials contained an average porosity of 0.34% with a gradual change between the layers, and without any major interface defects [55]. The final **WC-NiSiB alloy FGC** product manufactured by this method was found to be suitable for use in high-temperature tribological applications. The study mentioned that the surface roughness and the geometrical properties of the synthesized FGCs can be controlled by adjusting the heat input during the laser cladding process [56].

**Figure 3.** Experimental setup used for laser assisted processing using an Nd:YAG laser power source [55].

#### **4.10. Vapor deposition method**

Vapor deposition is a process by which materials in the vapor phase are condensed to form a solid material. This process is generally employed to make coatings for the alteration of the properties of the substrates such as mechanical, electrical, thermal, and wear etc. Basically, vapor deposition is classified into two categories, namely chemical vapor deposition (CVD) and physical vapor deposition (PVD). C-based materials that have an excessive chemical sputtering which yields at 600 to 1000 K and exhibits irradiation with enhanced sublimation at >1200 K when exposed to plasma erosion conditions, were successfully manufactured via the CVD method in 2002. The problem of serious C-contamination of the plasma was solved by using chemically deposited SiC coatings on the surface of the C-substrate. C-based FGCs such as **SiC/C, B4C/Cu, SiC/Cu and B4C/C bulk FGC** were also successfully manufactured using this method [57].
