Functionally Graded Materials Coatings Research Articles

Functionally Graded Material Coatings (FGMC) – A Review

The coatings are substances that are applied over the surface of an object to prevent it from wear, tarnish and corrosion. Coatings maintain the properties of base metals (thermal, optical and electrical) over a long duration of time. The coatings are generally characterized by the properties like good adhesion, low porosity and substrate compatibility that include geometry and temperature. Coatings often suffer from problems like variable thermal expansion coefficient compared to their base metals. To overcome the variable thermal expansion issue, the functionally graded material (FGM) layers may be imposed. Present work aims to study the FGM coatings capable of enhancing the mechanical, thermal and tribological properties of base metals by providing scratch resistance. An attempt has been made to present a comprehensive review of high-velocity-oxy-fuel, plasma spray and other techniques adopted by industries and other groups. The effects of applying FGM coatings on the operating conditions and the performance of the base metals are also covered in detail.

Contact mechanics of two elastic spheres reinforced by functionally graded materials (FGM) thin coatings

This paper examines the normal frictionless point contact between two dissimilar elastic spheres reinforced by FGM coatings with arbitrarily varied shear modulus and Poisson's ratio. Both the coating thicknesses and contact area are small compared to the radiuses of the two spheres. The material inhomogeneity of the FGM coating is only along the radial direction. The contact problem is formulated and reduced to a nonlinear Fredholm integral equation of the second kind. A multi-layer elastic half space model is used to develop the relationship between the contact stress and the vertical displacements of the FGM coated spheres. In this model, the FGM coating is divided into large number of perfectly bonded dissimilar thin sub-layers. Each sub-layer is homogeneous and has constant values of shear modulus and Poisson's ratio. Analytical solutions are derived for the contact force and stress. New numerical results are presented to show the effects of graded shear modulus and graded Poisson's ratio on the contact responses. It is shown that the contact force, stress and area can be significantly influenced by the graded shear modulus, and the effects of the graded Poisson's ratio are not negligible.

Thermal radiation and conduction in functionally graded thermal barrier coatings. Part I: Experimental study on radiative properties

Functionally graded thermal barrier coatings (FGTBCs) are functional composites with multilayered structures whose mechanical properties are better than those of conventional ceramic thermal barrier coatings (TBCs). Although many theoretical and experimental studies concerning both the mechanical properties and heat transfer including heat conduction and thermal radiation of TBCs have been conducted, little attention was paid to heat transfer process, especially the coupled conduction-radiation heat transfer, inside FGTBCs. This two-part paper systematically studied the heat transfer inside YSZ/NiCoCrAlY FGTBCs. In Part I, we investigated their radiative properties through experimental measurements. In Part II, we presented a study on the thermal conductivity of FGTBCs and established a coupled heat transfer model. In this paper, more than thirty yttria-stabilized zirconia (YSZ)/NiCoCrAlY duplex TBC and multilayered FGTBC samples with different multilayer structures, porosities and thicknesses were fabricated via air plasma spraying. The transmittance and reflectance spectra within the 0.3–15μm wavelength range were experimentally obtained by UV–visible spectrometry and Fourier transform infrared spectroscopy. A scanning electron microscope analysis was carried out to characterize the microstructures of FGTBCs. Additionally, we studied each layer of the FGTBCs individually; the four-flux method was employed to obtain the radiative properties based on our experimental measurements. Functionally graded materials (FGM) show larger absorption coefficients and smaller scattering coefficients compared to pure YSZ. The FGTBCs show lower reflectance than YSZ TBCs. We also study the influence of the number of layers on reflectance. The results indicate that FGTBCs have worse thermal insulation ability against radiative flux tconventional ceramic TBCs of the same thickness. This paper provides practical guidance to improve the design of YSZ/NiCoCrAlY FGTBCs and other FGM coatings to reach a balance between good thermal insulation and mechanical strength.

Frictionally excited thermoelastic dynamic instability of functionally graded materials

The perturbation method is applied to investigate the frictionally excited thermoelastic dynamic instability (TEDI) of a functionally graded material (FGM) coating in half-plane sliding against a homogeneous half-plane. We assume that the thermoelastic properties of the FGM vary exponentially with thickness. We also examine the effects of the gradient index, sliding speed, and friction coefficient on the TEDI for various material combinations. The transverse normal stress for two different coating structures is calculated. Furthermore, the frictional sliding stability of two different coating structures is analyzed. The obtained results show that use of FGM coatings can improve the TEDI of this sliding system and reduce the possibility of interfacial failure by controlling the interfacial tensile stress.

The coupled thermoelastic instability of FGM coatings with arbitrarily varying properties: in-plane sliding

The hot spot caused by frictional heat and thermal contact resistance is one of the most important reasons for hot-cracks and brake failure in brake systems. This paper simulates the brake system as a functionally graded material (FGM) coated half-plane sliding against a homogeneous half-plane. The motivation of using the FGM coating is to improve the coupled thermoelastic instability (TEI) of the brake system due to the thermal contact resistance and frictional heat. The thermoelastic properties of the FGM coating are assumed to vary arbitrarily along the thickness direction. The homogeneous multi-layered model is employed to simulate arbitrary properties of the coating. The perturbation method and transfer matrix method are used to derive the characteristic equation of the coupled TEI problem. The effects of the thermal contact resistance, friction coefficient, heat generation factor and different gradient types of the FGM coating on the stability boundaries are discussed in detail. The results show that the stability boundary is sensitive to the varying thermal parameters of the FGM coating, and an appropriate gradient type can adjust the coupled TEI of the sliding system.

Improving the performance of cementless knee prosthesis coating through functionally graded material

AbstractSurfaces are the primary place of contact a biomaterial and its host organism. Surface treatment or coating provides means to overcome the problem which appeared after knee replacement surgery. There are two main problems: stress shielding and subsequent bone remodeling cause bone resorption around the implant especially at the proximal part of the knee under the tibia tray. The other problem is stem tip pain. Therefore, the aim of this investigation is to find a new design of coating material of a cementless total knee replacement using functionally graded material (FGM). In particular, the objective of this research is to find the optimal material compositions as well as the optimal gradation direction. It has been found that using both vertical and horizontal FGM coatings increased the von Mises stress at the proximal part of the tibial cancellous bone. The von Mises stress in cancellous epiphyseal bone was increased by 77 % using vertical and horizontal FGM coating compared to HAP coating. It has been determined that this new coating design will improve the performance of a cementless tibia tray and will increase the life of the knee prosthesis.

Elastic analysis of thick-walled pressurized spherical vessels coated with functionally graded materials

In recent years, functionally graded material (FGM) has been widely explored in coating technology amongst both academic and industry communities. FGM coatings are suitable substitutes for many typical conventional coatings which are susceptible to cracking, debonding and eventual functional failure due to the mismatch of material properties at the coating/substrate interface. In this study, a thick spherical pressure vessel with an inner FGM coating subjected to internal and external hydrostatic pressure is analyzed within the context of three-dimensional elasticity theory. Young’s modulus of the coating is assumed to vary linearly or exponentially through the thickness, while Poisson’s ratio is considered as constant. A comparative numerical study of FGM versus homogeneous coating is conducted for the case of vessel under internal pressure, and the dependence of stress and displacement fields on the type of coating is examined and discussed.

Development of a semi automated dual feed unit to produce FGM coatings using the HVOF thermal spray process

The application of functionally graded materials (FGMs) is quite difficult, but thermal spray processes like Plasma spray have demonstrated their unique potential in producing graded deposits, where researchers have used twin powder feed systems to mix different proportions of powders. FGMs vary in composition and/or microstructure from one boundary (substrate) to another (top service surface), and innovative characteristics result from the gradient from metals to ceramics or non-metallic to metals. The present study investigates an innovative modification of a high velocity oxy-fuel (HVOF) thermal spray process to produce functionally graded thick coatings. In order to deposit thick coatings, certain problems have to be overcome. Graded coatings enable gradual variation of the coating composition and/or microstructure, which offers the possibility of reducing residual stress build-up with in coatings. In order to spray such a coating, modification to a commercial powder feed hopper was required to enable it to deposit two powders simultaneously which allows deposition of different layers of coating with changing chemical compositions, without interruption to the spraying process. Various concepts for this modification were identified and one design was selected, having been validated through use of a process model, developed using ANSYS Flotran finite element analysis. In the current research the mixing of different proportions of powders were controlled by a computer using LabVIEW software and hardware, which allowed the control and repeatability of the microstructure when producing functionally graded coatings.

Fabrication of <i>In Situ</i> TiC Reinforement Ti Matirx Wear-Resistant and its Wearing Performance at 500°C

TiC reinforced titanium matrix functionally graded materials （FGM） has been produced by processes of laser metal deposition through changing the powder feed rate of Ti and Cr3C2 powder. The OM, SEM, EDS methods were used to analyze the components and microstructure of the coatings. Microhardness and wearing resistance at 500°Ctemperature of the FGM coating were examined by microhardness tester and wear tester respectively. The results show that FGM coating reinforced by in-situ TiC apparently improved hardness of Ti alloy; the microhardness can reach HV1100, and present gradient distribution along deposition direction. Dry sliding wear properties of these FGM coatings have been compared with substrate materials wearing. The observed wearing mechanisms are summarized and related to detailed microstructural observations. The results show the wear resistance of the coating can be improved by 2.8 times.

Application of Laser Metal Deposition for Fabrication of Titanium Matrix Wear-Resistant Coating and its Wearing Performance

TiC reinforced titanium matrix functionally graded materials （FGM） has been produced by processes of laser metal deposition through changing the powder feed rate of Ti and Cr3C2 powder. The OM, SEM, EDS methods were used to analyze the components and microstructure of the coatings. Microhardness and wearing resistance at room temperature of the FGM coating were examined by microhardness tester and wear tester respectively. The results show that FGM coating reinforced by in-situ TiC apparently improved hardness of Ti alloy; the microhardness can reach HV1100, and present gradient distribution along deposition direction. Dry sliding wear properties of these FGM coatings have been compared with substrate materials wearing. The observed wearing mechanisms are summarized and related to detailed microstructural observations. The results show the wear resistance of the coating can be improved by 46.6 times.

A Mathematical Analysis of Thermoelastic Characteristics of a Rotating Circular Disk with an FGM Coating at the Outer Surface

A thin circular rotating disk having a concentric hole and a functionally graded material (FGM) coating at the outer surface is considered with a view to analyzing the thermoelastic characteristics due to a thermal load and rotation of the disk. The FGM coating is assumed to have exponentially varying Young's modulus, coefficient of thermal expansion (CTE), and density in the radial direction of the disk. The Poisson's ratio is assumed to be constant throughout the disk. The incompatible eigenstrain developed in the disk owing to the nonuniform CTE and variation of temperature is taken into consideration. Using the two-dimensional thermoelastic theories, the two-dimensional plane stress axisymmetric problem is formulated as a second order differential equation. A finite element model is developed using the variational approach and Ritz method to obtain the numerical solution of the differential equation. The validity of the finite element model is justified for a rotating circular disk of homogeneous material by comparing the finite element results with analytical solution obtained by Timoshenko. Then the finite element model is applied to the problem of an Al disk with an Al2O3/Al FGM coating at its outer surface. The numerical results of the thermoelastic field demonstrate that the temperature distribution profile, angular speed of the disk, and FGM coating thickness are the crucial factors to be considered in controlling the thermoelastic characteristics of a rotating disk with an FGM coating.

Electrophoretic deposition of composite hydroxyapatite–chitosan–heparin coatings

Electrophoretic deposition method has been developed for the fabrication of organic–inorganic composite coatings for biomedical applications. The coatings were obtained as multilayer and functionally graded materials (FGM). Needle-shaped hydroxyapatite (HA) particles were prepared by a wet chemical method and used for the fabrication of chitosan–HA layers. The HA particles in the chitosan matrix showed preferred orientation of c-axis parallel to the substrate surface. Multilayer coatings were obtained, which contained pure chitosan layers and composite HA–chitosan layers. The thickness of the multilayer coatings was in the range of 2–100 μm. The thickness of the individual layers can be varied by the variation of deposition conditions. The feasibility of co-deposition of chitosan and heparin has been demonstrated. The proposed mechanism of heparin deposition is based on the use of non-stoichiometric chitosan–heparin complexes. The deposition yield and coating composition have been studied at various deposition conditions. The addition of heparin to chitosan solutions resulted in increasing deposition rate. Composite chitosan–heparin layers were used for the surface modification of HA–chitosan coatings. Obtained results pave the way for the electrophoretic fabrication of novel FGM coatings for biomedical implants with improved blood compatibility. The coatings were studied by X-ray diffraction analysis, thermogravimetric and differential thermal analysis, electron microscopy, and Fourier transform infrared spectroscopy.

Fretting contact with finite friction of a functionally graded coating with arbitrarily varying elastic modulus Part 1: Normal loading

The two-dimensional normal contact of a functionally graded coated half-space by a rigid cylindrical punch under the action of a monotonically increasing normal load is considered. Friction with a finite coefficient is assumed between the contact surfaces. The whole contact region is composed of an inner stick region and two outer slip regions. The linear multilayered model is used to model functionally graded materials (FGMs) with arbitrarily varying shear modulus and constant Poisson's ratio under plane strain deformation, i.e. the FGM is divided into several sublayers and in each sublayer the shear modulus is assumed to be a linear function while Poisson's ratio is a constant. With the use of the transfer matrix method and Fourier integral transform technique, the problem is reduced to a set of Cauchy singular integral equations. An iterative method is developed to determine the stick-slip region. Normal and tangential tractions in the whole contact region are calculated. It is found that the stick region depends on the gradient of the functionally graded coating as well as on Poisson's ratio and the friction coefficient. The results also show that appropriate gradual variation in the shear modulus can significantly alter the contact traction. This may lead to suppression of Hertzian cracking at the edges of the contact region and thus modify the contact damage. Therefore, it is believed that FGM coatings would have potential applications in improving the resistance to contact damage at the contact surfaces.

Optimization of Material Composition of FGM Coating under Steady Heat Flux Loading by Micro-Genetic Algorithms

This paper studies the optimization problem of composition distribution of functionally graded material (FGM) coating subjected to steady heat flux loading. The investigation object of the paper is an infinite plate substrate with FGM coating in the surface. The materials are heated at the ceramic surface (upside) with a steady high-intensity heat flux input, and cooled at the metal surface (underside) with flowing liquid nitrogen. The thermal stress distribution and the temperature distribution are obtained by formulation. For optimization, the design variables are the thickness of each interlayer and the volume fraction distribution of the coating. The objective function is the danger coefficient and the restricted condition is the total thickness of FGM plate and heat insulation coefficient. In the paper, three different optimization schemes are considered and compared. The µGA and related parameters are discussed in detail. Optimizing the thermal stress distribution and minimizing the danger coefficient are carried out by µGA. The optimization results of composition distribution are gained, and the results show the optimum composition distribution can distinctly reduce the danger coefficient.

Three dimensional fracture analysis of FGM coatings under thermomechanical loading

The main objective of this study is to examine the three dimensional surface crack problems in functionally graded coatings subjected to mode I mechanical or transient thermal loading. The surface cracks are assumed to have a semi-elliptical crack front profile of arbitrary aspect ratio. The cracks are embedded in the functionally graded material (FGM) coating which is perfectly bonded to a homogeneous substrate. A three dimensional finite element method is used to solve the thermal and structural problems. Collapsed 20-node isoparametric elements are utilized to simulate the strain singularity around the crack front. The stress intensity factors are computed by using the displacement correlation technique. Four different coating types are considered in the analyses which have homogeneous, ceramic-rich (CR), metal-rich (MR) and linear variation (LN) material composition profiles. In the mechanical loading problems, the composite medium is assumed to be subjected to fixed-grip tension or three point bending. In the thermal analysis, a transient residual stress problem is considered. The stress intensity factors calculated for FGM plates are in good agreement with the previously published results on three dimensional surface cracks. The new results provided show that maximum stress intensity factors computed during transient thermal loading period for the FGM coatings are lower than those of the homogeneous ceramic ones.

Experimental study on heat insulation performance of functionally graded metal/ceramic coatings and their fracture behavior at high surface temperatures

The objective of this research was to study heat insulation performance and fracture behavior of thermal barrier-type functionally graded material (FGM) coatings under high heat flux. The FGM coatings with thicknesses varying from 0.75 to 2.1 mm were designed and deposited onto a steel substrate by plasma spraying. With the high heat flux of 0.68 MW m −2, the thickest specimen exhibits a high surface temperature of 1690 °C and the corresponding temperature difference between the top surface and the bottom surface of the coating is high up to 760 °C. The results of thermal cycling test with a surface temperature of 1400 °C (during heating) showed that there existed a great difference in fracture location between different specimens. Fracture behavior of the coatings at surface temperatures of 1500, 1600 and 1700 °C was also investigated. The coating thickness and surface temperature were found to influence the number and the length of cracks. The specimen with the coating thickness of 1.1 mm is the most susceptible to local spallation at the top ceramic coat, while the thickest FGM coating is the most susceptible to cracking at the interface close to the bond coat. Heating time also had an effect on cracking behavior. These results provided some necessary information for appropriate design of FGM coatings to sustain higher temperatures and gradients than those in today's applications.

Residual Stress Distribution in Graded PSZ/Inconel Composite Deposited by Particle Collision

Interfacial strength distribution and thermal residual stresses in multi-layered or compositionally graded NiCrAlY/ZrO2 coatings are analyzed. These coatings are fabricated by detonation gun spraying using mechanically alloyed, plasma-spheroidized composite powders. The problems in design of functionally graded materials (FGMs) are outlined and their modeling approaches are reviewed. Due to the concentrational or structural gradients in FGMs, the normal approximations and models, used for traditional composites, are not directly applicable to graded materials. The goal is to show the efficiency of the simplest models to provide the most accurate estimates of the properties and even to make simple elasto-plastic analysis of FGM components without vast computations by finite element methods with an arbitrary non-linear distribution of phases and corresponding properties is presented. Results showed that bonding strengths increased gradually with increase of the composition of metals in the FGM coatings. The FGM coating was more stable on the mechanical properties than normal duplex composites. And, the maximum compressive radial stress is found to be at or near the surface of the specimen where surface cracking may be generated. The maximum axial stress is at the edge of the specimen where spallation may occur. The maximum shear stress is also at or close to the edge.

Cracking in coating–substrate composites with multi-layered and FGM coatings

This investigation evaluates, by finite element method, the stress intensity factors (SIF) of cracked multi-layered and functionally graded material (FGM) coatings of a coating–substrate composite, due to the action of uniform normal stress on the crack surfaces. The substrate is assumed to be homogeneous material, while the coating consists of multi-layered media or sigmoid FGMs. The sigmoid FGM is a kind of FGM in which the material properties of the coating are governed by two power-law functions of volume fractions such that the functions of the material property represent sigmoid distributions in the thickness direction, simply called S-FGM in this paper. For the multi-layered coatings, one, two, and four-layered homogeneous coatings with stepwise changing volume fractions are considered. The primary problem addressed herein is the appearance of a crack in the coating surface and its expansion into the substrate along the direction perpendicular to the interface between the coating and the substrate. The results show that if the coating is stiffer than the substrate, a crack in a one-layered coating is much more susceptible to propagation into the substrate than a crack in the two- or four-layered coating. But crack growth can be effectively averted by using an S-FGM coating. However, if the coating is softer than the substrate, the S-FGM coating behaves like a bridge to connect the soft coating and the stiff substrate, and facilitates the expansion of the crack expanding into the substrate. Whereas the one-layered coating can more effectively prevent the crack from propagating into the substrate than can the two- or four-layered coating. The investigation also indicates that the material gradations of S-FGMs influence SIFs obviously only when the crack tip is inside the coating that is stiffer than the substrate. As the crack extends through the coating and into the substrate, the material gradation of the S-FGM coating and the material mismatch of the multi-layered coating slightly bear on the values of SIF.

Inverse problems of material distributions for prescribed apparent fracture toughness in FGM coatings around a circular hole in infinite elastic media

This study is concerned with the inverse problem of calculating material distributions intending to realize prescribed apparent fracture toughness in functionally graded material (FGM) coatings around a circular hole in infinite elastic media. The incompatible eigenstrain induced in the FGM coatings after cooling from the sintering temperature, due to mismatch in the coefficients of thermal expansion, is taken into consideration. An approximation method of determining stress intensity factors is introduced for a crack in the FGM coatings in which the FGM coatings are homogenized simulating the nonhomogeneous material properties by a distribution of equivalent eigenstrain. A radial edge crack emanating from the circular hole in the homogenized coatings is considered for the case of a uniform pressure applied to the surfaces of the hole and the crack. The stress intensity factors determined for the crack in the homogenized coatings represent the approximate values of the stress intensity factors for the same crack in the FGM coatings, and are used in the inverse problem of calculating material distributions in the FGM coatings intending to realize prescribed apparent fracture toughness in the coatings. Numerical results are obtained for a TiC/Al 2O 3 FGM coating, which reveal that the apparent fracture toughness in FGM coatings around a circular hole in infinite elastic media can be controlled within possible limits by choosing an appropriate material distribution profile in the coatings.

Friction and wear properties of WC-Co and Mo-Mo2C based functionally graded materials

Wear resistant coatings based on functionally graded materials (FGMs) applied on industrial machinery components can reduce weight, increase adhesion strength, decrease internal stresses and improve the resistance against propagation of surface defects. Macroscale FGMs offer a new method of surface engineering to produce tailored tribological properties. In order to fully exploit the FGM concepts, an efficient fabrication with advanced process control assuring the stability of the resulting properties is desirable. The fundamental understanding of wear damage modes of each layer, as well as the development of predictive models for through-thickness behavior can increase the industrial applicability of graded coatings.In the present study, two industrially relevant FGM coatings were investigated: high-velocity-oxygen-fuel (HVOF) deposited WC-Co/stainless steel and plasma sprayed Mo-Mo2C/stainless steel FGMs. For both deposition processes, high degree of automation was achieved and linearly graded coatings were successfully prepared. Sliding friction and abrasive wear responses were evaluated through thickness and damage mechanisms controlling the coefficient of friction and wear rates were described. A correlation between composition, microstructure characteristics and damage modes was established. The enhancement of Mo-Mo2C coating properties through FGM approach with stainless steel was reported.