Recent advances in the manufacturing processes of functionally graded materials: a review

4.1 PM process

The PM process is a very feasible and promising route for FGM fabrication. The steps followed for FGM production using the PM process is given in Figure 1.

Figure 1:

Flowchart of the powder metallurgy process.

Initially, selection of material combination was done, followed by the design of the most conducive distribution of composition; stacking of premixed powder in a continuous or a stepwise manner according to the predefined composition distribution; then compaction of stacked powder by CIP; and finally the prepared compact is consolidated in a sintering furnace.

In order to reduce the defects like small porosity and small cracks during processing by the PM method, two main factors should always be kept in mind. First is the type of method used for preparation of graded green compact, and second is sintering of the prepared compact in such a way that the temperature should be uniform throughout the compact.

Using the PM technique, it is possible to create different types of gradients, e.g. porosity gradient or pore size gradient, gradient in chemical composition, and volume content of phases. Mainly two types of gradients are important from the application point of view: stepwise gradient and continuous gradient.

Several methods are available to obtain graded green compact. For continuous gradation, the available methods are wet powder spray forming, wet filtration process, centrifugal process, vibrating stacking process, slurry dipping process, and sequential slip casting. It is easier to produce a step-by-step gradation comparable to continuous gradation. For stabilization of prepared green bodies for all the above-mentioned processes, binders are necessary; apart from this, a de-waxing process, which is the same as metal injection molding, is also required [14]. Uniform sintering is also a considerable factor because it is a challenging task for different mixing ratios of constituent materials. Sintering imbalance due to non-uniform sintering is the prime reason for warping, frustum formation, splitting, cracking, etc. These problems may be avoided by size control of powder particles with the addition of activating element (in most cases, transition metals such as iron and copper) and by pressure sintering, such as hot pressing and hot isostatic pressing (HIP) [14], etc.

Based on this gradient type, the PM method can be further classified as discussed below.

4.1.2 Processing route to create the continuous gradient

The centrifugal powder forming method is used to prepare a continuous gradient. In this method, powder of different compositions is continuous supplied into the distributor plate of the rotating cylinder. After a time on the plate, powder particles strike the inner wall of the cylinder and simultaneously an organic binder is sprayed to form a body of sufficient strength. The shape of the parts is the only limitation – a cylindrical shape is required. Gravity sedimentation is used to create a pore size gradient, in which particles of different velocities or different densities cause sedimentation in the column, which creates the pore size gradient [14].

The electrophoretic deposition (EPD) method is also used to create a continuous gradient. This is a low-cost technique and capable of producing complex geometry. It is also possible to create stepped gradient FGM. This was discovered by G.M. Bose in 1740 in a liquid siphon experiment. It consists of two processes: one is electrophoresis and the other is deposition. Electrophoresis is the study of the effect of electricity on the suspension, which shows the effect of an electric field on the motion of suspension particles. Sedimentation of suspension particles is called deposition. For EPD, it is necessary that particles of suspension should respond to the applied electric field, and that particles should be stable – either electrostatically or electrosterically stable. Table 2 shows efforts made by different researchers for the development of FGM with the EPD technique.

Table 2:

List of different authors who used EPD for fabrication of FGM.

Sr. no.	Material combination	Author	Remarks
1	YSZ/Al2O3	Sarkar et al. [20]	Using the EPD process, green compact with composition gradient through the thickness was formed.
2	Al2O3/YSZ, Al2O3/MoSi2	Sarkar et al. [21]	Effect of suspension stability was examined with the help of the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory.
3	WC/Co	Put et al. [22]	FGM with continuous Co gradient was successfully created.
4	Al2O3-Y-TZP/Al2O3	Kaya [23]	Tubular FGM was prepared with a tough inner surface and a hard outer surface.
5	Al2O3/ZrO2	Anne et al. [24]	Fabricated disc showing considerable increase (almost double) in strength due to a graded structure was prepared.
6	Al2O3-ZrO2	Hvizdos et al. [25]	Al2O3-ZrO2 FGM was prepared by the EPD process using suspension of n-butylamine with acetone.
7	Hydroxyapatite CaSiO3-chitosan	Pang et al. [26]	Material for biomedical application was developed by room temperature processing.
8	Al2O3/ZrO2	Mehrali et al. [27]	Using a non-aqueous stable suspension in the EPD process, a composition gradient was created with improved hardness.
9	Al2O3/SiC/ZrO2	Askari et al. [28]	Material for artificial joint was prepared with a hard outer surface.
10	NiO-YSZ	Zarabian et al. [29]	Voltage decay EPD was used to fabricate an electrode for solid oxide fuel cell.

Graded composition is created by the difference in the electrophoretic response of different powders under an applied electric field. Several theories have been proposed to explain the mechanism of EPD: the DLVO theory (named after the Russian scientists B. Derjaguin and L. Landau, and the Dutch scientists E. Verwey and J. Overbeek) is one of them, which explains the stability of colloidal suspension by considering the concept of attractive and repulsive energy. Sarkar et al. [20] prepared FGM with a continuous gradient of Al₂O₃/Y₂O₃-stabilized zirconia (YSZ) using a suspension made from Al₂O₃ and ZrO₂ in ethanol and acetic acid. After EPD and sintering, the prepared FGM hardness varied from 16 to 24 HV and the fracture toughness ranged from 2 to 10 MPa-m^1/2. Sarkar et al. [21] explained the EPD mechanism in the background of the DLVO theory and lyosphere distortion/thinning. The authors fabricated Al₂O₃/YSZ, MoSi₂/Al₂O₃, YSZ/Ni, and Al₂O₃/Ni by using the EPD technique. The starting materials were YSZ, Al₂O₃, MoSi₂, and Ni, and after deposition of powder, sintering was done in air at 1525°C. The EPD method is very effective to synthesize FGMs with stepped gradient as well as continuous-profile FGMs. Börner et al. [17] used this method to fabricate Al₂O₃-ZrO₂ FGM with grain size gradient along with the gradient of the zirconia concentration. This gradient can be controlled accurately by controlling several factors, such as deposition current density, flow rate, suspension concentration, etc. It is difficult to create WC-Co FGM due to the fast migration rate of cobalt, which results in the homogenization of previously prepared layers. Put et al. prepared fully dense and closed porosity WC-Co FGMs by preparing acetone and hard metal suspension containing 4–17% cobalt by EPD with pressureless sintering at 1290–1340°C and liquid phase sintering for 1 h at 1400°C [22]. Tubular-shaped Al₂O₃-Y-TZP/Al₂O₃ FGM was prepared with continuous gradient, resulting in improved hardness, improved fracture toughness, and small grain size.

The scanning electron microscope (SEM) micrograph in Figure 2 represents the microstructure at different points along the thickness from the center, from Figure 2A having high fracture toughness to a pure alumina layer having high hardness (Figure 2D). Also, a significant change in microstructure was observed. In Figure 2A, Al₂O₃ is shown to have a fine grain size, and when the amount of Al₂O₃ is increased, the grain size is also increased [23].

Figure 2:

SEM micrographs of Al₂O₃-Y-TZP graded layer where the dark phase is alumina and the light phase is TZP. (A) Al₂O₃ with 71 vol.% TZP. (B) Al₂O₃ with 35 vol.% TZP. (C) Al₂O₃ with 13 vol.% TZP. (D) Pure alumina surface layer. The effect of different percentages of TZP of grain size is clearly observed [23].

Al₂O₃/ZrO₂ discs fabricated by EPD followed by sintering and HIP showed increase in strength because of the compressive surface residual thermal stresses generated in the Al₂O₃ surface layer. Due to the gradient in composition, the strength also increased from 288 to 513 MPa, which is very difficult to fabricate because of thermal expansion mismatch [24]. Al₂O₃-ZrO₂ FGM was prepared by the EPD process using a suspension of n-butylamine with acetone. The prepared FGM had fine grains as well as improved hardness (15–25 GPa), as compared with pure Al₂O₃. The transverse crack propagation resistance also increased under impact loading due to this composition gradient [25].

The EPD method is more suitable for the development of biomaterials. In this method, processing at room temperature is possible. EPD has been used for the development of hydroxyapatite (HA)-CaSiO₃ (CS)-chitosan composite coatings with excellent corrosion resistance for biomedical applications at room temperature. With this method, HA and CS are co-deposited to form an HA-CS coating; other materials can also be co-deposited with this method to make this composite coating more functionalized. Figure 3 shows the cross sections of the HA-CS layers prepared with different thicknesses along with pure chitosan for layer separation [26].

Figure 3:

(A, B) Cross section of the prepared FGM having a layer of different materials in the form of laminates, where hydroxyapatite (H) and chitosan (Ch) form different layers [26].

For improvement in the mechanical properties, Al₂O₃-ZrO₂ composite was prepared by using a suspension prepared in 2-butanone, n-butylamine, and cellulose nitrate solution, which resulted in improved hardness (15–25 GPa) as compared to pure alumina [27]. Al₂O₃/SiC/ZrO₂ FGM for bio-implants (artificial joints) were developed from EPD followed by the pressureless sintering process applied as pre-sintering. Then, to densify the deposit and for hardness improvement, HIP was performed. By incorporating SiC in Al₂O₃-ZrO₂ composite, an outer layer with compressive stress and an inner layer with tensile stress were developed [28]. Because of its flexibility to control the thickness and morphology of the deposit layer, it was used in the development of electrode for solid oxide fuel cells, and NiO/YSZ FGMs were prepared with varying NiO concentrations and porosities [29]. Despite all of the above-mentioned advancements in the EPD process, there is still scope for improvement in terms of processing cost and time reduction. In addition to that, one may concentrate on the development of a mechanism to generate an accurate and precise gradient thickness and composition distribution as required.

Slip casting (pressure-infiltration technique) is another method to prepare FGMs with continuous gradient, in which there is one mold whose material is porous in nature and is filled with slurry. The slurry is made of desired material that is fed through a cavity provided with the mold. After filling, the liquid part of the slurry is absorbed by the mold via capillary action, and the material particles whose size is larger than the size of pores remain in the mold. For producing hollow casting, the remaining slurry is drained from the mold after sufficient thickness is achieved. Sometimes, it is accompanied by pressure infiltration (liquid or gas). With this method, it is possible to obtain a gradient in porosity and in composition. The mold can comprise plaster of Paris, gypsum mold, alumina mold, porous resin mold, acrylic mold, etc.

Gypsum mold is commonly used in slip casting; however, it has some shortcomings such as chances of CaSO₄ contamination and low productivity because of the time consumed in drying. Jung and Choi [30] prepared TZP (tetragonal zirconia polycrystals) SUS (stainless steel 304) FGM with gypsum mold and alumina mold to investigate the property changes, and found that alumina mold had no contamination on the surface and a comparatively easy-to-control layer thickness. Thus, alumina mold is superior to gypsum mold. Figure 4 shows the microstructure of SUS-TZP FGM sintered by using a tube furnace with an Ar-H₂ environment. H₂ is used to avoid erosion of interfaces of layers.

Figure 4:

Microstructure of SUS-TZP FGM having 11 layers prepared in alumina mold [30].

Yan et al. [31] utilized the behavior of the constituent of suspension under applied magnetic field and prepared Ni-ZrO₂ FGM by using slip casting along with gradient magnetic fields applied by the Maxwell coil system. Continuous gradient can be created along the field direction. Katayama et al. used this method to prepare Al₂O₃-W FGM by slip casting using acrylic mold, and the prepared FGM was used in high-intensity discharge lamps as a sealing and conducting component that has a translucent alumina envelope [32].

The above-mentioned methods are the initial steps in the PM process for preparation of green compact. After powder preparation, powder is mixed for desired composition, and stacked or pressed for consolidation by cold pressing or hot pressing before sintering of green compact. Sintering is the process in which powder after compaction is heated in a furnace below its melting point, which causes diffusion of atoms and results in a bonding that takes place by diffusion of atoms to form a dense compact with close adherence between powder particles [33]. Table 3 shows the development in the PM technique by listing different authors’ work of FGM fabrication along with the material combination chosen.

Table 3:

List of different authors who used powder metallurgy for fabrication of FGM.

Sr no.	Material combination	Authors	Remarks
1	PSZ/stainless steel	Kawasaki and Watanabe [34]	TBC was prepared. The responsible mechanism for crack formation, deflection, and spallation was explained by considering the effect of stress distributions in the specimen.
2	HA-Ti	Chenglin et al. [35]	The prepared HA-Ti FGM is a prominent material as a bone implant because of its improved bending strength and toughness.
3	ZrO2-NiCr	Zhu et al. [36]	FGM was prepared with improved mechanical properties and a macro interface between layers was eliminated.
4	Mullite/Mo	Jin et al. [37]	A multilayered mullite/Mo composite was prepared with high thermal shock resistance.
5	Pb-Sn-Te	Gelbstein et al. [38]	Annealing of the sintered product was performed to remove the sintering defects of thermoelectric material.
6	Al/SiC and Ni/Al2O3	Bhattacharyya et al. [39]	It was found that in the prepared FGM with increased number of layers, thermal shock resistance and fatigue strength were also increased.
7	Ni and Al2O3	Yang et al. [40]	Defects like distortion and bending that occurred due to non-uniform shrinkage during sintering led to cracking. To prevent this defect, a mathematical model was proposed to uniformly distribute the shrinkage throughout the layers.
8	HA-SS316L-CNT	Hussain et al. [41]	HA-SS316L-CNT defect-free FGM prepared successfully by pressureless sintering showed suitability as a bio-implant.
9	Ni- Al2O3	Bykov et al. [42]	For the creation of gradient, generation of temperature gradient was found to be very effective, so millimeter-wave heating was used to prepare the metal-ceramic or metal-ceramic-metal combination.

In an oxidation environment, spalling appears in ceramic coating. When TBC undergoes thermal cycling, large thermal stresses are developed. Due to the difference of thermal expansion between ceramic coatings, a metal bond coat and substrate thermal stress are generated. With the replacement of a homogeneous composite with FGM coating, thermal shock cracking can be avoided. For preparation of TBC, green compact that is prepared by stacking of layers of different compositions after CIP undergoes pressureless sintering and hot pressing [34].

With the PM route, microstructure and properties can be effectively controlled; hence, this technique was used in preparation of multilayered mullite/Mo FGM for TBC. Due to the thermal residual stress generated after the sintering process, mullite/Mo FGM became more thermal shock resistant. It is clear from optical micrographs (Figure 5) that polished samples have a macroscopically heterogeneous structure without any defect at the interlayer, and delamination is also absent because of the composition gradient [37].

Figure 5:

Optical micrograph of a cross section of mullite-Mo FGM [37].

The PM process is not good enough for fabrication of thermoelectric materials due to defects like atomic defect, local strains, and alteration in carrier concentration. Functionally graded Pb-Sn-Te materials that underwent annealing after sintering showed improved thermoelectric properties along with dense structure and good carrier concentration. It is explained that the electronic transport properties of Pb-Te and Pb-Sn-Te depend on the processing condition, and due to the PM conditions atomic defect and local strains are generated. To reduce or eliminate this defect, annealing has been done to stabilize its thermoelectric property [38]. The PM process is very effective in eliminating ceramic/metal interfaces; hence, ZrO₂-NiCr FGM was prepared by CIP and sintering at 1400°C by different powder composition and processing. From microscopic observations, it was found that there is an absence of a macroscopic ceramic/metal interface along with a stepwise change in chemical composition [36].

The developed HA-Ti FGMs have improved bending strength, fracture toughness, and absent macroscopic interphase. The dark phase (Figure 6) represents HA and the white phase is titanium, and change in composition from HA to Ti is stepwise and a clear interface is absent. Fracture toughness and bending strength improved considerably with increasing Ti volume fraction (Figure 7) [35].

Figure 6:

Microstructure of a cross section of HA-Ti FGM having a continuous composition gradient where TI percentage continuously increases from (A) to (E) [35].

Figure 7:

Effect of the volume fraction of HA on Young’s modulus, Vickers hardness, and fracture toughness of HA-Ti FGM [35].

It is very difficult to make FGMs with carbon nanotubes (CNTs); however, with the PM process, its fabrication is possible. FGM with HA, stainless-steel 316L (SS316L), and CNT composition gradient were prepared using the PM technique. Application of uniaxial compression along with pressureless sintering resulted in increased hardness, fracture toughness, biocompatibility, and bioactivity [41]. It is a well-known fact that powder metallurgy is a relatively simple FGM fabrication process in which composition can be accurately controlled. In case of multilayered materials, there is a chance of distortion and cracking due to different shrinkage properties. This non-uniform shrinkage can be prevented by the prediction of shrinkage and preparation of uniform shrinkage gradient. Thus, Yang et al. has proposed one mathematical expression to establish the relation between material composition and dimensional shrinkage, and Ni-Al₂O₃ multilayer samples have been prepared with a uniform shrinkage gradient [40].

It is a challenge to prepare materials having different sintering behaviors such as metal-ceramic. Because of differences in the temperature range of densification, it is better to generate a temperature gradient at the time of sintering. Various methods have been proposed to do this, including pulse electric discharge consolidation, field-assisted sintering technique, and SPS. In this category, microwave heating provides several advantages like separation of thermal source from sink, resulting in ease in the creation of a desirable temperature gradient. Along with this, each material have different dielectric properties, so their capacity to absorb microwave is also different and results in selective heating. Thus, Bykov et al. prepared bulk multilayer graded Ni-Al₂O₃ samples with millimeter-wave heating to avoid cracks and delamination of layers. Figure 8 shows the cross section of polished samples sintered in an inhomogeneous temperature field [42].

Figure 8:

Microstructure of Al₂O₃-Ni FGM having layers with different compositions [42].

Al/SiC FGM has been prepared, and resulted in noticeable improvement in flexural strength, thermal fatigue behavior, and thermal shock resistance with an increase in the number of layers [39]. The concept of FGM is successfully utilized in dental implants with help of the effective combination of mechanical and biocompatible properties. This property combination is helpful in solving several implant failure problems such as lack of biomechanical bonding, insufficient mechanical strength, etc. Other concerns with this concept are the cost of production (which is very high) and the desired shape of the implant. Still, many techniques have evolved that are capable of overcoming these problems [43]. HA/Ti is a suitable material combination for use as a bone implant, in which HA provides biocompatibility and Ti provides mechanical strength. However, the sintering temperatures of both are different, so there are chances that coating of HA on Ti will peel off. To prepare uniform biocompatible bioceramics, functionally graded HA is fabricated by press forming and sintering with a gradient of nano-/micrograin. Functionally graded HA with nanograins have a rough surface that provides biocompatibility, and micrograins provide better mechanical properties [44]. An HA/316L gradient was prepared by a pressureless sintering process, and it was found that nanosized HA gives more densification and there is interdiffusion of chromium from the 316L to the HA interface. Nanograined FGMs show the best hardness and corrosion resistance as compared to micrograined FGMs due to interfacial reaction [45]. For improvement of biocompatibility and mechanical properties of HA, it is prepared in the form of CNT- and 316L-reinforced HA FGM; however, due to the difference in thermal expansion coefficient, after sintering, residual thermal stresses are generated and result in cracking. Thus, with the optimum combination of sintering parameters and binder content, this problem can be controlled and the reinforcement plays a critical role here [46]. SS316L/HA and SS316L/CS (calcium silicate) were prepared using solid-state sintering, and it was found that SS316L/CS had better mechanical properties as well as load-bearing capacity as compared to SS316L/HA with increasing sintering temperature [47]. Al₂O₃-ZrO₂ gradient FGM was prepared and found to be very useful, with improved impact resistance by delay in crack propagation and improved layer integrity [48]. The main advantage of the PM technique is the elimination of macroscopic interface, atomic defect, cracks, ability to prepare FGM with continuous or stepwise gradient, as well as higher composition uniformity along with the absence of segregation and second-phase carbides. The precipitates that form after processing are also more uniformly distributed. Despite all these research works and advancement in this method, still some disadvantages remain, such as the high cost of the material and bulk production, etc. These are the main associated problems that require further research [49].

4.2 Self-propagating high-temperature synthesis

An effective means of surface hardening of steel components is SHS, which produces refractory compounds and functional coatings by the solidification of liquid reaction products. SHS was discovered in 1967 and has been used in practice to produce various powders, refractory compounds, and functional coatings.

The use of SHS is economically attractive, as it does not require expensive and complex high-temperature heating systems. Although the process occurs at 1500–4000°C, the temperature is attained by the internal energy of the reacting system. At these temperatures, the time required for synthesis is reduced from a few hours to a few minutes or even seconds, and consequently the products are of high purity. The initial components are metals, non-metals, and metal oxides in most cases. The reaction of these components in extreme conditions (high temperature and pressure) yields new compounds and phases. By considering the above-mentioned advantages, many studies for the application and improvement of this process have been done. Table 4 shows the development by listing different authors who used SHS for fabrication of FGM along with the material combinations.

The MoSi₂/Al₂O₃ combination is very useful for application in electrical devices because MoSi₂ has good electrical property and Al₂O₃ has excellent mechanical property. It is a very good combination for the preparation of a multilayered composite by the SHS technique. Tape casting is used to prepare layers of different compositions. Then, these layers are stacked in sequence and processed with SHS under a weak load to densify the product. Preservation and restoration of the gradient depend on the amount of liquid, which has to be controlled precisely. To control the amount of liquid, the maximum temperature reached during the reaction should be close to the melting temperature of the reaction product. Another way is to add inert powder so that the heat dissipated is utilized by the product. Variation in electrical resistivity has been obtained with >10 orders of magnitude through the composite [50]. It is also suitable for biomaterial preparation due to the ease in developing a porosity gradient. Addition of blowing agent in the mixture helps create a porosity gradient. SHS was used to create functionally graded porous Al-Ti metallic foam, and a strong dependence of microstructure on the gasifying agent was found by mixing nanoscale titanium and nanoscale aluminum. Nanoscale powder has higher surface energy, reduced melting temperatures, and distinctive absorption properties that result in higher ignition sensitivity [51]. Combustion synthesis was combined with centrifugal casting to prepare FGM from a high melting temperature or refractory material (intermetallics, ceramic, and composite material). Along with this, it is also possible to prepare carbide and boride. Hence, ZrC-ZrB₂ FGM was prepared for aerospace application by SHS, using different compositions of Zr powders, CrO₂ powders, and Al powders as raw materials. It was found that with increases in the amount of CrO₃+Al, the grain size also continuously increases [52]. From the above-mentioned work, it is clear that the SHS process consumes less energy, and is a simple process capable of producing a high-purity product. However, some time for preheating of reactant mixture is required to increase the ignition capability. Moreover, sometimes, the relatively longer time of heating of the reactants as compared to the combustion time results in compound formation by a solid-state reaction, which causes inhibition of ignition.

4.3 Spark plasma sintering

SPS is a sintering technique with some modification over the conventional sintering technique, with direct heating of compacted powder instead of separate heating in the furnace; that uses pulsed DC current that passes through the die and powder compact, causing internal heating; and with simultaneous use of temperature and pressure, resulting in a high heating rate along with a high sintering rate. Because of internal heating, only a few minutes are sufficient to complete the sintering process; the holding time is very small at the sintering temperature, so that it completes quickly.

SPS is very effective in the fabrication of FGMs, that is why nowadays research on fabrication with the SPS technique obtained momentum. Figure 9 shows the setup of the SPS system with various components. There are two movable punches for applying pressure during heating. The DC power supply is used for heating when it passes through graphite punch and causes internal heating of the powder. A vacuum and cooling chamber is present to provide inert atmosphere and cooling. The prepared powder is placed into the graphite die, which is closed by punching and this assembly is placed into the SPS chamber under the required atmosphere condition (vacuum/argon, etc.). Finally, with the help of a control unit, the product is prepared in less time. Many researches have been done in the field of SPS because of its effectiveness in fabrication, and improved material properties were obtained as compared to the conventional sintering process. Table 5 lists the work done by different researchers on the SPS technique.

Figure 9:

System configurations.

SPS is also used in the fabrication of biomaterials and metal-ceramic combination for high fracture toughness, bending strength, and wear resistance. The prepared biomaterial is applicable in joint prostheses, as bone implant material without decomposition. The compliant pad for thermoelectric energy conversion is prepared from nickel and aluminum by SPS with an optimum composition distribution, and densification can be optimized.

It is also useful for the reduction of residual stress due to the particle size distribution and thermal expansion mismatch of different layers. Al₂O₃/Ti₃SiC₂ has been prepared for armor application with combined toughness, electrical conductivity, and increase in the Ti₃SiC₂ electrical conductivity; however, decreases in hardness have been reported. Figure 10 shows the optical micrograph of a polished sample in which interfaces between different layers are clearly shown, which clears the heterogeneous nature of the prepared composite due to the gradient distribution of composition [53].

Figure 10:

Optical micrograph of polished Al₂O₃/Ti₃SiC₂ FGM [53].

SPS has the capability to sinter product at lower temperatures as compared to conventional methods, because of which it is possible to prepare stable TiN/HAP FGM for bone implants. It was found to be very stable for implants with uniform hardness, improved flexural strength, and compression strength. Also, new bone is generated without inflammation. The prepared TiN/HAP FGM has a Brinell hardness of around 60 throughout the composite. The flexural strength of the prepared FGM with a sintering temperature of 1100°C and 1200°C was 65.4 MPa and 71.3 MPa, respectively, whereas the compression strength increased to >100 MPa as shown in Figure 11 [54].

Figure 11:

Brinell hardness of each part of TiN/HAP FGM [54].

Hot pressing, microwave sintering, and combustion synthesis are also capable of sintering at fast rates; however, there are chances of generation of crack, whereas in the case of SPS it may be avoided because of internal heating. Ti-TiB-TiB₂ FGM with intermetallic compound has been fabricated at a relatively decreased sintering temperature and in less holding time, and the developed FGM has three types of layers: Ti, Ti+TiB, and Ti+TiB+TiB₂. It is a very useful combination for defense purposes. If it is prepared by any other process such as hot pressing, microwave sintering, or combustion synthesis, then there are chances of the occurrence of cracking problems and the formation of undesirable phases [55]. B₄C-Al cermets were prepared successfully by SPS for B₄C consolidation, and there was a need for high pressure and temperature. Initially, the porous compact of B₄C is made by SPS. Then, this compact is infiltrated by pure molten aluminum and heated to remove moisture, and finally left in a vacuum furnace to cool down. Figure 12 shows the difference in hardness before and after infiltration; the increase in hardness is because of the reinforced aluminum [60].

Figure 12:

Variations of hardness of the FGM with and without aluminum [60].

With this technique, granules of different kinds having different melting temperatures can be sintered in a vacuum at a relatively lower sintering temperature and sintering time. ZrO₂/AISI316L FGM prepared for joint prostheses had good fracture toughness, wear resistance, and biotribological properties. FGMs were prepared by SPS from ZrO₂ and AISI316L granules (10–50 vol.%) having two to six layers. The fracture toughness and wear resistance of FGM were improved due to the reinforced metallic AISI316L phases in the ZrO₂ matrix, which created obstacles to crack propagation [56]. Functionally graded silicon nitride (Si₃N₄) with continuous variations in composition and grain size has been fabricated in mass production. Three FGMs were prepared at 1550°C, 1600°C, and 1650°C, and continuous gradients in α/β phase content as well as in grain sizes were obtained [57]. In FGM, there is a problem of radial cracking in the Al₂O₃-rich region, regardless of the number of layers or the gradient compositional profile, which is solved by a short sintering time and low sintering temperature. Crack-free and dense SUS316L/Al₂O₃ FGM is prepared with a multilayer system to reduce thermal stress along with YSZ as an intermediate layer. YSZ is added as a toughening phase to avoid radial cracking of the graded layer [58]. Another advantage of a short sintering time along with a low sintering temperature is controlled migration rate. It is difficult to prepare FGM with conventional sintering due to the Co migration problem, and this can be solved by the SPS technique. It is possible to achieve a small grain size due to the reduced sintering times at high temperature, and undesired phases are also minimized and other phases (ductile weldable part) are joined to the cemented carbide [59]. A ZrO₂/Ti gradient was successfully prepared by SPS, along with ZrO₂ and Ti; oxide of Ti was also formed and resulted in improved nanohardness. Simultaneously generated thermal residual stresses were also minimized with optimum combination of processing parameters and composition distribution [61]. A four-layered FGM with TiB-Ti gradient was prepared by the SPS process, and it showed a well-bonded interface layer with higher microhardness and resulted in improved fracture toughness and bending strength [62]. Using the finite element method, the temperature and stress distribution in FGM after the SPS process were predicted successfully, and the obtained results were useful to understand the process mechanism and optimization of the operating variable [63]. Three layered W-Cu FGM successfully prepared by SPS at 1050°C at 40 MPa and thermal conductivity, hardness as well as relative density increased considerably [64].

Despite all the advantages, there are some limitations of the SPS process. These include size limitation of the prepared sample and the very high cost of the machine. For mass production of the sample, more interventions are required.

4.4 Friction stir processing

FSP is another FGM-making process that follows a solid-state route. The concept of FSP comes from the process of friction stir welding (FSW) technology. The basic idea and principle behind both processes are the same; however, the goal of the two processes is different. FSW is a metal-joining process and FSP is a process of modifying the microstructure of a workpiece. In FSP, there is also one metallic tool provided with a shoulder and threaded pin. This pin plunges into the groove on the surface of the plate on which reinforcement is desired. This tool rotates continuously about its axis and plunges into the groove with a proper tool tilt angle along with the material to be reinforced, and then traverses along a predecided path. The frictional heat generated due to pin motion causes plastic deformation, and due to pin movement the material flows around the pin and is then forged and consolidated under hydrostatic pressure [65], [66].

The advantages that distinguish FSP from other processes are accurate control, homogeneity, densification, microstructural refinement, and variable depth of the processed zone. Moreover, FSP is environment friendly, consumes less energy, and does not change the size and shape of the processed component [65], [66].

FSP is mostly applied in aluminum, and the first ever application of this process was in the fabrication of AA5083/SiC FGM. SiC was reinforced successfully in the aluminum matrix by the FSP process [65]. This FSP process is mainly suited for aluminum-based alloys; however, it is also applicable for bronze, Cu alloy, Mg alloy, tool alloy, and Zr alloy. This method is very suitable for micro-/nanoparticle-reinforced composite and microstructure modification of casting. Hangai et al. used this FSP for producing functionally graded foam [67]. Functionally graded aluminum (FG) foam was fabricated with gradient in pore density by friction stir processing method and found that functionally graded foam have capability to control their deformation location [68], and then Miranda et al. used FSP along with a consumable tool technique in which there is some modification in the FSP process such as pre-preparation of U-shaped groove in which reinforcement has been placed [69].

Recently, Salehi et al. used this technique for the production of bulk Al-SiC functionally graded nanocomposites. Initially, the groove was prepared on a 6061 aluminum plate; then, this groove was filled with SiC nanoparticles and FSP was performed by using a tool with a pin of 6-mm length. Subsequently, the FSP was re-applied by using a tool with a pin of 3.2-mm length. The SiC content varied from 18% to 0% along the length, and correspondingly the microhardness varied with the highest hardness being 160 HV, which was 3.2 times higher than that of the SiC-depleted zone.

Figure 13 shows the macrograph of the cross section of a prepared FGM sample. It is clear from Figure 13 that the size of the rotating pin is a limiting factor for the dimension of the composite zone. Accordingly, three layers can be clearly observed from Figure 13, where the dark region is an SiC-rich zone; then, its content decreases up to the SiC-depleted zone [70].

Figure 13:

Cross section of Al-SiC FGM prepared by the FSP process [70].

Use of finer particles or increase in SiC content during FSP results in the reduction of interparticle spacing. Figure 14 shows the variation of microhardness with the depth of an FGM sample. With the help of the strengthening mechanism of nanocomposite, it is possible to give the best explanation for linear correlation.

Figure 14:

Variations of microhardness with change in distance from the top surface [70].

FSP can also be used to produce polymeric composite materials. It can also play a very important role in the materials used in the energy sector, e.g. materials used in fusion reactors, including vanadium alloy joining and processing. Several shortcomings remain in FSP technology. The main problems that occur during the processing of reinforced composite materials is tool wear, fatigue, and reduced strength of the prepared joint [66]. An A1050/A6061gradient foam was prepared by FSW in which porosity was uniformly distributed similar to an aluminum foam. From a compression test, it was determined that the deformation behavior can be controlled by the position of the base material in the foam that resulted in improvement [67].

Beyond having the capacity to sinter high-quality products, the sample size and sample quantity are limitations. Moreover, the cost of processing and machinery is very high, and further research is needed for the commercialization of this process.

4.5 Cast-decant-cast process

This method was first used in 2005 in the University College Dublin (Ireland) by Scanlan and other materialists [71], for lightweight, wear-resistant functionally graded alloys, to obtain a smooth change in properties (microstructure/composition). At that time, the other ways of processing FGMs are expensive because the cost of production equipment is high. Mainly two routes are available: powder metallurgy and centrifugal casting. The limitation of powder metallurgy is the high cost of powder, whereas with the centrifugal method basic shape can be produced. With this cast-decant-cast process, FGM can be produced with contradictory material properties (lightweight-wear resistance, wear resistance-machinability, hardness-toughness) with cost-effectiveness, e.g. iron cylinder liner in aluminum cylinder. The first use was cited by Scanlan [71] for an aluminum silicon alloy. Three variations are invented for the cast-decant-cast process with the same end product and one common step in all, i.e. decanting: the first alloy (alloy A) is poured into the mold, and when this alloy is solidified on the mold wall or core and sufficient thickness is obtained, then the unsolidified part is decanted; then, another alloy (alloy B) is poured into the mold. Three available techniques are turnover, internal, and low-pressure decanting.

4.5.1 Turnover decanting

Figure 15 depicts the apparatus for the turnover decanting process. In this, the materials to be mixed are placed initially and then melted into the furnace separately. For example, to create an FGM of two materials, melts A and B are used. Melt A from the furnace is poured into the mold and when the desired thickness is produced, the mold is inverted with the given arrangement and then melt B is poured.

Figure 15:

Apparatus used in the turnover decanting process.

4.5.2 Internal decanting process

Figure 16 shows the schematic diagram of the internal decanting process apparatus, where there is a reservoir for the collection of unutilized melt and drainage for the passing of melt to the reservoir. Two carbon rods prevent the melt from draining into the reservoir. First, melt A is poured from the furnace and then solidified. When the desired thickness of the solidified melt is produced, the carbon rods are pulled upward to open the drainage channel, then melt B is poured onto the mold from the furnace. (Here, melts A and B are used to create the FGM.) [72].

Figure 16:

Schematic of the internal decanting process apparatus.

4.5.3 Low-pressure technique

The schematic of the apparatus is shown in Figure 17. One crucible is partitioned into two parts to carry two different materials, and a crucible lead is present to make it air tight. Moreover, to prevent cross contamination, a barrel valve is provided. Initially, two different materials are put into two parts and then the crucible is placed into the furnace. When the melt forms there, then the melt from one of the partitions is forced through ceramic tube 1 using pressurized nitrogen into the mold, and at that time valve 2 is closed. After the melt has solidified and the desired amount of thickness is achieved, then valve 1 is opened and the remaining melt is decanted. Then, valve 1 is closed and the melt from the second partition is forced through the second ceramic tube with pressurized nitrogen into the mold. The main advantage of this process is its cost-effectiveness because the equipment used are standard foundry equipment, and also there is flexibility about the thickness of the gradient because the thickness of the gradient layer depends on the thickness of the first solidified layer. When the second melt is poured, then it is superheated to a temperature that will melt the first solidified layer, and a thicker gradient layer is achieved [72].

Figure 17:

Schematic of the low-pressure casting technique.

This method also works with the Al-Mg, Al-Li, and Cu-Sn [72]. A smooth gradient between the A390/A356 alloys was produced by this casting process, with different decanting times and different superheating temperatures of the second melt, and the thickness of the gradient layer and wear resistance were tested.

A perfect and firm interface was obtained at 860°C. Figure 18A shows the microstructure at 860°C. In this gradient, the primary Si content is clearly shown. With 35 s of decanting time, the obtained width of the transition zone was higher than that obtained with other times, as shown in Figure 18B. The hardness of the sample varied from 120 HV to 75 HV with variation in the Si content, and the highest hardness was found at the transition zone because of its finer microstructure due to a higher solidification rate. The wear resistance of the surface was 35% because the wear resistance of A390 was higher [72].

Figure 18:

(A) SEM microstructure of the prepared sample at 860°C. (B) SEM micrograph of the sample prepared at 35-s decanting time [72].

Although this process proved to be successful in formation of the gradient, very few research works are available in this area and the available studies are on a combination of two or three materials. There is a limitation on the thickness of the layer that can be grown before decanting.

4.6 Laser direct metal deposition (LDMD)

Sandia National Laboratories have done several works for direct preparation of prototypes and patterns. Direct preparation of fully dense materials from a computer-aided design (CAD) solid model is only possible with the laser engineered net shaping (LENS) method that is developed by Sandia. The same concept is utilized in traditional laser-initiated rapid prototyping (RP) technologies such as stereolithography and selective laser sintering. In these techniques, CAD data are used for fabrication of the physical part, and then metal particles are supplied continuously on the laser beam for deposition over the substrate. This metal particle coming under a focused laser is melted, and this melt is deposited over the substrate continuously. Layers are deposited consecutively to produce a three-dimensional part [73], [74].

The concept of this LENS technology is the same as the RP process that is used in the creation of casting patterns and prototypes from plastics. In both cases, a CAD model is prepared in the form of a slice or a thin layer in the orthogonal direction. This slice data provide a path for laser movement. The first outline is generated, and then the cross section is filled by using the rastering technique. This process is continuous until the part is completely finished. The distinguishing feature of LENS over RP processes is that it is possible to make components directly from structural metals. Thus, it is possible to use it in near-net-shaped prototype development as well as in fabrication of injection mold tools and other metal parts [73], [74], [75].

The laser beam is focused on the substrate, which is made up of the same material that will be deposited. Because of the focused laser, a melt pool is created over the substrate on which metal powder is deposited. The thin cross section is created by continuous movement of the substrate. This process is continuously repeated until the part is completely finished. To obtain good finishing and product quality, advanced powder delivery nozzles and powder feeders have been developed.

From the literature, it was found that by using an automatically generated code, it is not possible to produce a gradient perpendicular to the wall of the tube, and there is a need for low temperature annealing treatment to relieve internal stresses and to obtain high hardness and a good shaped component with high metallurgical quality by this process. Thin wall structures of Stainless Steel 316L and Inconel 718 FGM were fabricated, and the powder mass flow rate and laser power variation affected the performance of the prepared material. Due to the presence of a hard niobium carbide (NbC) and Fe₂Nb, the hardness and wear resistance increased. It was also found that with increases in the laser power and powder mass flow rate, the tensile strength decreased [74]. A gradient of SS304L and IN625 nickel alloy was prepared by direct energy deposition, and by comparison of the gradient zone with a single material zone, it was found that hardness was reduced with the introduction of IN625. The different phases formed were compared with the results of the simulation software CALPHAD, and found to be in accordance with experimentally found results and to show the formation of secondary phases [75].

This method offers several advantages, such as minimum dilution of base metal, possibility to prepare a near net shape component, low distortion of workpiece, and potential for automization of the process. However, there are still some points on which more research is required, such as the low-dimensional accuracy of the product as well as the rough surface finish acquired by product that needs further machining or polishing.

4.7 Plasma transferred arc centrifugal cladding (PTACC)

PTACC is a technique that is used for cladding by iron because of its high temperature and high density of energy. It has gained application in the field of coating on the internal wall of cylinder by using iron-based FGM. Plasma transferred arc (PTA) is a high-energy heat source, with qualities like high efficiency, no need for pretreatment, availability of a high heat source, ease of operation, and synchronized powder feeding. Therefore, it is very useful for cladding to steel.

Figure 19 shows the diagram of the PTACC process. The substrate cylinder is fixed on a centrifugal machine where the rotation axis is horizontal, and the plasma torch travels axially inside the cylinder. PTACC combines the advantage of the PTA technique and centrifugal casting, because with the help of centrifugal casting it is possible to create a thick coating layer inside the cylinder. Lu et al. used this technique for coating of the cylinder inner wall by Fe-Cr-M-C. In this, the powders were first dried and form a blend; thereafter, during cladding, the substrate cylinder was fixed on the machine that was rotating this cylinder about a horizontal axis, and the plasma torch traveled inside the horizontal cylinder in axial direction. The wear resistance improved by 18 times due to graded coating as compared with the cylinder substrate, as in Figure 20. The improvement in wear resistance of the coating was because of the uniformly distributed carbide, which resulted in high hardness [73].

Figure 19:

Apparatus of PTAC.

Figure 20:

Variations of microhardness across the graded coating [73].

Figure 21 shows the backscattered electron and secondary electron images of the coating at the inner, intermediate, and outer layers. In the microhardness test, hardness varied throughout the thickness. In the inner layer, the microhardness ranged from 1062 HV_0.5 to 1480 HV_0.5; in the middle layer, it ranged from 797 HV_0.5 to 891 HV_0.5; and in the outer layer, it was around 800 HV_0.5, as shown in Figure 22 [73].

Figure 21:

Microstructure of the (A) inner, (B) intermediate, and (C) outer layers of the coating. (D) Outer layer microstructure at high magnification [73].

Figure 22:

Variation of microhardness across the graded coating [73].

Although it is effective in cladding, PTACC cannot be fully utilized for bulk FGM fabrication. Moreover, more expertise is required for effective control of processing parameters.

4.8 Chemical vapor deposition (CVD)

Thermal spraying is a technique by which a structure is coated with melted materials through a spraying process. Relating the process with FGM preparation, the melted materials will be the functionally varied materials that are used to make the coating layers. In this process, the coating precursor is heated either electrically or chemically. Two well-developed processes utilizing the vapor processing route are CVD and PVD (e.g. evaporation, sputtering, and ion plating). PVD techniques are simple processes and normally used by physicists. They utilize high-temperature vacuum evaporation or plasma sputter bombardment in PVD coating.

This method is used to produce highly pure materials without any external defect by depositing the desired material at the atomic level in a very controlled manner at the atomic/nanostructure level. The gradient is produced in CVD by the chemical reaction of gaseous reactant on the heated substrate, and the final product is chemically stable in nature. With this method, it is possible to produce single or multilayered, nanostructured functionally graded coating materials with accurately controlled dimension and structure. It has properties like low processing temperatures, non-line-of-sight-deposition capability, etc. CVD has a wide range of application in the field semiconductors for microelectronics, optoelectronics, energy conversion devices, metallic films (e.g. Mo, Al, Au, Cu, Pt) for microelectronics and protective coatings, fiber production (e.g. SiC monofilament fibers), and fiber coating. CVD is mainly used for producing coatings and films. With the vapor processing route, highly pure coating along with atomic and nanometer level structural control at low temperatures can be obtained, and this is very helpful in the microelectronics and opto-electronic industries [76].

There are several issues related to the CVD process, such as involvement of complex chemistry processes. To solve this problem, on-line monitoring and diagnostic tools have been used. With the help of process modeling of thermodynamics, the kinetics and mass transport of the CVD process chemistry and fluid dynamics in the CVD process can be understood. Moreover, the use of a single chemical precursor source has minimized the CVD process parameters that need to be controlled [37]. CVD is a very effective method for coating with a variety of combinations of material without any defect. So, in the present day, many studies have been done with this CVD technique. Figure 23 shows the mechanism of CVD in which atoms of a reactant gas is absorbed by the surface, which deposit as a surface coating.

Figure 23:

Mechanism of CVD.

Table 6 shows the works done by different researchers for the development of the CVD technique by listing the different authors and material combinations used.

This method proved to be very useful for coatings; it is possible to create a coating on materials, which is difficult for other methods such as coating on carbon fiber. The reasons behind this difficulty are that metals cannot completely wet carbon fiber, and the chemical compatibility between the matrix and the reinforcement is poor. The physical compatibility of the carbon fiber with the metal matrix is also poor due to the different thermal expansion coefficients and interfacial residual stresses. There are chances of oxidation of the carbon fiber at the time of composite processing, which results in decreased mechanical properties. With the help of the coating deposited on carbon fiber, this problem can be solved. Coating of carbon fiber with C-Si makes it compatible as reinforcement for aluminum matrix, and increases its oxidation resistance and oxidation weight loss [76].

The CVD process has proved to be very useful because of its ability to precisely control the composition and microstructure of coating materials. Carbon fiber reinforced carbon (C/C) composites are well suited for thermostructural applications; however, they are very reactive to atmospheric oxygen and hydrogen at high temperatures. Thus, they can be effectively protected by SiC coating; however, the difference of the thermal expansion coefficient causes cracking problems. Thus, it is difficult with other coating methods. To solve this problem for using SiC as a C/C composite coating, there is a need for a C/SiC FGM interlayer between these two that can be deposited by CVD. With CVD, it is easy to control its composition and microstructure, and to obtain optimum coating thickness, and the number of layers and compositional profiles can be obtained. In Figure 24, it is clearly observed from the image of the cross section that the C/SiC FGM has dense microstructures and the absence of delamination between the layers [77].

Figure 24:

Image of the polished C/C composites coated with C/SiC FGM and SiC layers along the cross section [77].

The new approach of electron cyclotron resonance CVD (ECR-CVD) has been developed and used in growing C:H film in a steady state. With this technique, it is possible to obtain an independent control of the ion energy in depositions along with high-density plasma and low substrate temperature simultaneously. Printed circuit board (PCB) microrouters were coated with Ti/TiN/TiCN/a-C:H thin film by a hybrid PVD-ECR-CVD coating system. Coating causes reduction in friction between the microrouting surface and the trench sidewall, which results in improvement of quality, efficiency, and lifetime of microrouters, and reduces the time consumed in new microrouter replacement on PCBs. It also reduces the manufacturing cost and increases productivity [78]. CVD has a capability to prepare crack and porosity defect-free coating with precise microstuctural control. It proved to be very useful in environmental barrier coating for advanced high-temperature operating turbines. Hence, SiC substrates have been coated with uniform, crystalline, and highly dense mullite (3Al₂O₃.2SiO₂) environmental barrier coatings by CVD [79].

Tungsten when deposited by the CVD technique has the advantage of producing a highly pure and dense large-sized product with better flexibility and better coverage along the surface as compared to other coating techniques. It is also possible to perform in situ repair of the damaged tungsten armor. W/Cu FGM has been used as an interlayer between W-CVD and CuCrZr. When this CVD-W combines with W/Cu FGM, it acquires excellent high-temperature material properties. Tungsten was deposited by CVD method on the W/Cu FGM, and this tungsten-coated surface was brazed to CuCrZr heat sink. In Figure 25, perfect gradation from CVD-W to W/Cu FGM and then CuCrZr heat sink is observed. In the micrograph, W, W-10Cu, W-32Cu, W-60Cu, and CuCrZr are shown. These solutions can reduce the cost of fabrication and environmental pollution [80], [81].

Figure 25:

Micrograph of the cross section of W/Cu FGM prepared by CVD [80].

CVD has proved to be very useful for FGM fabrication; however, it has some limitations that require further research for generalization. For example, the precursor used should be volatile at room temperature, the deposition of film should be preformed at high temperature, and thermal stresses are generated in materials of different thermal expansion coefficients.

4.9 Plasma spraying

Plasma spraying is the most promising technology for ceramic and metallic coatings in many industrial applications for making surfaces hard and wear-resistant, useful in bio-implants, thermal-barrier, and chemical-resistant coatings, etc. In this process, ceramic or metallic powder is melted by the application of high-energy plasma flame. This melted powder is deposited on the substrate with the help of gas pressure. This process is less time consuming, and large surface areas can be prepared in a single pass, in comparison to other processes such as hot and cold pressing and then sintering of tiles. Table 7 shows the development in the plasma spray technique, by listing different studies on fabrication of FGM and material combination used by the authors.

Plasma spraying has sufficient flexibility and low cost as compared to other coating methods. When mechanical or thermal stresses are applied to plasma-sprayed TBCs, the poor bond strength between the coating and substrate causes problems. To improve its performance, plasma spray has been used to prepare FGMs. With increase in the number of layers, its bond strength has been improved. The ZrO₂-NiCrAIY FGM coating was fabricated in which residual stress decreased and bond strength increased considerably. HIP treatment and tempering treatment was performed on the prepared FGM and resulted in improved bond strength due to densification of the microstructure, interdiffusion between layers, and the reduction in the residual thermal stresses.

Figure 26 shows the microhardness distribution in duplex coating and FGM coating, and a significant difference between both types of coatings are represented. In FGM coating, the bond strength between the coating and the substrate is improved because of reduction in the difference of elastic modulus between the ceramic and metal layers [82].

Figure 26:

Variations of microhardness in coating [82].

YSZ/NiCoCrAlY FGM coating was prepared by plasma spraying and resulted in chemical homogeneity. It also had a uniform density as well as improved bond strength, and had no delamination [83]. YSZ and NiCoCrAlY powders (8 wt.%) were used for coating preparation in YSZ/NiCoCrAlY, and different NiCoCrAlY contents were mixed with YSZ, ball milled and spheroidized, and finally plasma sprayed. It was reported that the coefficient of thermal expansion, thermal conductivity, heating temperature, and diffusivity increased considerably with increase in NiCoCrAlY content, which resulted in increased resistance of thermal fatigue (five times as compared to duplex coating) and improvement of the oxidation resistance (up to 100 h) [84]. In a similar way, FGM was prepared by plasma spraying YSZ/NiCoCrAlY on the aluminum substrate, and it was reported that with an increase in the content of YSZ, the elastic modulus and bending strength of functionally graded coating decreased [85].

A plasma sprayed product does not have a clear interface boundary, which means that the composition varies smoothly over the length. HA/Ti-6Al-4V coating was prepared and it was found that with change in composition, the mechanical and microstructural properties also changed progressively. In Figure 27, the dark-gray phase is HA and the bright phases are Ti-6Al-4V. Between the HA phase and the Ti-6Al-4V phase, some light-gray phases are present, which are the mixtures of HA and Ti-6Al-4V. It is clear from the microstructure that the composition varies in a continuous manner, so Young’s modulus and fracture toughness vary in an anisotropic manner [86].

Figure 27:

Functionally graded HA/Ti-6Al-4V coating [86].

Al₂O₃/ZrO₂ TBC prepared via plasma spraying showed superior mechanical properties and oxidation resistance. Al₂O₃ was introduced as the interlayer in the TBC to improve bonding strength, hardness, and fracture toughness. The results showed that the thickness of Al₂O₃ should be less to obtain good mechanical properties [87]. Glass-alumina functionally graded coatings were prepared on alumina substrate to solve problems like defective microstructure and interface between the graded coating, by proper thickness control and thermal treatment of the prepared coating. The alumina substrate resulted in improvement in the performances of the glass-alumina FGM [88]. Onto Ti-64 substrates, TiO₂-HA functionally graded coatings were deposited. For the desired result, spraying parameters were optimized. The prepared coating had dense coating and less residual porosity. With post-treatment, the defect can be reduced [89]. A ZrO₂/Al₂O₃ composite system was developed for high functionally graded TBC with the help of magnetoplasma compressor of compact geometry and a novel gas tunnel-type plasma source, which exhibited improved hardness and oxidation resistance [90]. Functionally graded TBC based on LaMgAl₁₁O₁₉ (LaMA)/YSZ was designed and prepared by air plasma spraying, and the presence of LaMA resulted in improved hardness and wear resistance [91].

The advantage of plasma sintering over conventional sintering is the extremely high heat transfer rate to the sample, which may result in rapid sintering (in minutes) with minimal grain growth. In this process, electricity is used as an energy source, which influences the process economics unfavorably. A further consideration is that plasma processes have more process parameters to control than traditional processes, and therefore requires a higher degree of automation in the process control. However, still there is lack of research on a solid engineering base for certain types of large-scale installations. Furthermore, in this process, another problem is the occurrence of relatively high energy losses.

4.10 Centrifugal casting

Centrifugal casting is the most viable and reliable method to produce FGMs. In this method, molten metal is poured into the mold under the effect of gravity force. This gravity force generated due to the rotation of the mold along with centrifugal force acts on the mold assembly due to the rotational or spinning motion. Because of these forces, mold filling is very good with the advantages of effective control on the microstructure and, hence, enhanced mechanical properties. For fabrication of a perfect FGM, the centrifugal casting method can be modified by three methods: centrifugal method, centrifugal slurry method, and centrifugal pressurization method.

In a centrifugal casting method for FGM generation, the mold is filled with homogeneous molten metal having dispersed intermetallic or ceramic particles. When the molt metal starts rotating, centrifugal force is applied to the molten metal; however, due to the density difference, the amount of centrifugal force on solid particles and molten metal is also different, which is the main reason behind the gradient formation.

In the centrifugal slurry method, there are two types of particles: one is high-velocity or high-density or large-sized particles and another is low-velocity or low-density or small-sized particles. Due to these different characteristics of the particles, the amount of centrifugal force is different, which allows the control of the migration rate of these particles. After sedimentation is completed, the liquid part is removed from the slurry, so that it will not become any part of the prepared FGM. Thereafter, the prepared green body will undergo sintering by SPS or other sintering methods, and finally FGM with continuous gradient will be prepared.

In the centrifugal pressurization method, centrifugal force is used to generate pressure. Initially, a powder mixture of matrix metal A and reinforcement metal B is placed inside the rotating mold, then the matrix metal in the melted form is poured into the rotating mold where the powder mixture A + B is already present. Now, molten metal A penetrates into the interparticle space of particles A and B; at the same time, this molten metal will melt the particles of powder A. After all these processes, finally FGM is obtained with reinforced particle B. It is very effective for in situ and ex situ fabrication of FGM. Many studies have been done for fabrication of FGM by the centrifugal casting method. Figure 28 shows the mechanism of centrifugal casting in which molten metal is poured into the rotating mold. Table 8 shows the development in the centrifugal casting technique by listing different research works for fabrication of FGM and the material combination used by the authors.

Figure 28:

Centrifugal casting process.

AI-A1₃Ni FGM was produced by the centrifugal casting method and resulted in decrease in fracture load and fracture stress with an increase in the volume fraction of Al₃Ni. Fractographic study was also performed, and it was found that on the fractured surface, the cleavage fracture of the Al₃Ni phase was surrounded by dimple fractures of the aluminum phase [92]. The developed Al-Mg₂Si reinforced in situ composite resulted in poor ultimate tensile strength (UTS) and elongation due to Mg₂Si clusters, and increased hardness. As in Figure 29, in the inside wall, the Mg₂Si particles were finer sized and clustered; in the outside wall, there was a higher volume fraction of Mg₂Si; and in middle layer, Mg₂Si particles were also present. Hardness varied with the presence of Mg₂Si particle distribution [93].

Figure 29:

Microstructures at different positions of the centrifugal cast Al-Mg₂Si specimen: (A) inside the wall, (B) outside the wall, and (C) the middle area [93].

Wear-resistant, light Al-Al₃Ti FGM was prepared, which showed significantly higher wear resistance than pure aluminum ingot [94]. Zn-AI-Si in situ FGMs were fabricated with a high volume fraction of Si particles in the inner layer, which resulted in the highest hardness and wear resistance. In Figure 30A, the primary Si particle is shown. Figure 30B shows the transition zone segregation of the primary Si particle. Figure 30C is similar to the inner layer, but polyhedral primary silicon particles are absent and needle-like eutectic silicon particles are observed. Finally, in Figure 30D, large primary Si particles are shown [95].

Figure 30:

Microstructure of the composite: (A) inner layer, (B) transitional layer, (C) middle layer, and (D) outer layer [95].

Al A359/SiC FGM was fabricated and a significant effect of casting rotational speeds and elevated cooling rates were found. Figure 31 depicts the microstructure of the prepared FGM at different points along the thickness. In the outer layer, the volume fraction of SiC was high as compared to the middle and inner layers, which resulted in higher wear resistance [96].

Figure 31:

Microstructure of the composite: (A) outer layer, (B) middle layer, and (C) inner layer [96].

Al-Cu-Fe ternary in situ FGM ring phases were formed, which consisted of four different phases, namely Al, Al₂Cu, Al₇Cu₂Fe, and Al₁₃Fe₄ [96]. Al-Al₃Ni in situ intermetallic composites were prepared using different Al-Ni alloys. The high volume fraction of Al₃Ni in the outer periphery of the fabricated Al-Al₃Ni FGM resulted in a gradient with the size and shape of the Al₃Ni particle [97].

The fabricated Al-SiC FGM showed increase of SiC content and decrease of the FGM composite wear coefficient. Figure 32 shows the microstructure at different mold rotation speeds [99].

Figure 32:

Optical micrographs of the FGMs processed under 1500 rpm (A, B) and under 2000 rpm (C, D) [99].

Although this processing method is very effective for FGM preparation, some areas that need modification remain: the inner diameter of the sample cannot be controlled, not all alloys can be processed, and there is a shape limitation (the shape should be symmetrical or cylindrical).