Functionally Graded Coatings Research Articles

Vibrational Analysis of Composite Conical-Cylindrical Shells with Functionally Graded Coatings in Thermal Environments.

This article studies the vibrational behavior of composite conical-cylindrical shells (CCSs) with functionally graded coatings (FGCs) in thermal environments using the first-order shear deformation theory. Firstly, the equivalent material parameters, fundamental frequency, and resonant displacement responses of the CCSs with FGCs are derived using the mixture principle, complex modulus method, and transfer function approach. Then, detailed thermal vibration tests are performed on CCS structures with and without coatings to assess the reliability of the proposed model, revealing that the current model accurately forecasts the thermal vibration behavior of the CCSs with FGCs. Finally, the effect of key parameters on the vibrational properties of the CCSs with FGCs is investigated. The results demonstrate that increasing the functionally graded index, coating thickness, and Young's modulus ratio can greatly enhance the vibration suppression capability of the structure.

Hot Corrosion Behavior of Plasma-Sprayed Gd2Zr2O7/YSZ Functionally Graded Thermal Barrier Coatings

The development of advanced thermal barrier coating (TBC) materials with better hot corrosion resistance, phase stability, and residual stresses is an emerging research area in the aerospace industry. In the present study, four kinds of TBCs, namely, single-layer yttria-stabilized zirconia (YSZ), single-layer gadolinium zirconate (GZ), bilayer gadolinium zirconate/yttria-stabilized zirconia (YSZ/GZ), and a multilayer functionally graded coating (FGC) of YSZ and GZ, were deposited on NiCrAlY bond-coated nickel-based superalloy (Inconel 718) substrates using the atmospheric plasma spray technique. The hot corrosion behavior of the coatings was tested by applying a mixture of Na2SO4 and V2O5 onto the surface of TBC, followed by isothermal heat treatment at 1273 K for 50 h. The characterization of the corroded samples was performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) to identify physical and chemical changes in the coatings. GIXRD was used to analyze the residual stresses of the coatings. Residual stress in the FGC coating was found to be −15.2 ± 10.6 MPa. The wear resistance of TBCs is studied using a linear reciprocating tribometer, and the results indicate that gadolinium zirconate-based TBCs showed better performance when deposited in bilayer and multilayered functionally graded TBC systems. The wear rate of as-coated FGC coatings was determined to be 2.90 × 10−4 mm3/Nm, which is lower than the conventional YSZ coating.

Wear-resistant ZrN/Ti functionally graded coating prepared on TA18 alloy by double glow plasma alloying

Despite being an excellent structural material, α-Ti alloy is with low shear deformation resistance, resulting in susceptibility to friction damage during service, and limits its further application. A wear-resistant ZrN/Ti functionally graded coating (FGC) was prepared on TA18 alloy by double glow plasma alloying technique in this paper. The morphology, phase structure, mechanical properties and wear behavior of ZrN/Ti FGCs were studied and analyzed. The results proved that a gradient structure formed in the coating: the topmost layer was ZrN ceramic phase, in the transition layer were ZrN, TiN, and Ti2N phases, and in the inner layer was α-Ti phase. The coating was dense and metallurgically bonded with the substrate. It exhibited better load bearing capacity and fracture toughness. Upon conclusion of wear tests under different loads, only slight damage was observed on the coating. The main wear form of the coating was delamination wear, and the ZrN layer on the topmost surface was not worn through. More specifically, the Zr pre-diffusion treatment was used to increase the content of ZrN in the transition layer, reducing the unfavorable Ti nitride phase formed. Therefore, the stress concentration caused by the difference in elastic modulus between the different phases was relieved. Thus, ZrN/Ti FGC with continuous properties and good wear resistance was successfully synthesized.

Characterization, wear behavior and biocompatibility of HA/Ti composite and functionally graded coatings deposited on Ti−6Al−4V substrate by mechanical coating technique

Characterization, wear behavior, and biocompatibility of hydroxyapatite (HA)/Ti composite coating and functionally graded coatings (FGCs) deposited on a Ti−6Al−4V substrate by a mechanical coating technique were compared. The composite coating was produced using the initial powder mixture of 50 wt.% Ti + 50 wt.% HA, while the FGC was deposited by two separate layers containing 75 wt.% Ti + 25 wt.% HA and 25 wt.% Ti + 75 wt.% HA, respectively. The XRD results exhibited that no phase changed during the deposition process. Moreover, due to the presence of Ti particles in the primary powder mixtures, a continuous interface with a superior bond was formed between the coating and the substrate, especially in the samples coated by the FGCs. The wear test results of the non-graded composite coating showed a coefficient of friction (COF) of about 0.37 with an abrupt change at the interface, while the COF of the FGCs increased from 0.32 to 0.5 with a gentle change at the interface. The SEM images of the worn surfaces indicated the dominance of the abrasive wear mechanism. The biocompatibility evaluations revealed better cell attachment and proliferation of the FGC-coated samples.

Influence of Functionally Graded Protective Coating on the Temperature in a Braking System.

A mathematical model of heat generation due to friction in a disc-pad braking system was developed with consideration of a thermal barrier coating (TBC) on the friction surface of the disc. The coating was made of functionally graded material (FGM). The three-element geometrical scheme of the system consisted of two homogeneous half-spaces (pad and disc) and a functionally graded coating (FGC) deposited on the friction surface of the disc. It was assumed that the frictional heat generated on the coating-pad contact surface was absorbed to the insides of friction elements along the normal to this surface. Thermal contact of friction between the coating and the pad as well as the heat contact between the coating and the substrate were perfect. On the basis of such assumptions, the thermal friction problem was formulated, and its exact solution was obtained for constant and linearly descending specific friction power over time. For the first case, the asymptotic solutions for small and large values of time were also found. A numerical analysis was performed on an example of the system containing a metal ceramic (FMC-11) pad, sliding on the surface of a FGC (ZrO2-Ti-6Al-4V) applied on a cast iron (ChNMKh) disc. It was established that the application of a TBC made of FGM on the surface of a disc could effectively reduce the level of temperature achieved during braking.

Thermal Fracture of Functionally Graded Coatings with Systems of Cracks: Application of a Model Based on the Rule of Mixtures

This paper is devoted to the problem of the thermal fracture of a functionally graded coating (FGC) on a homogeneous substrate (H), i.e., FGC/H structures. The FGC/H structure was subjected to thermo-mechanical loadings. Systems of interacting cracks were located in the FGC. Typical cracks in such structures include edge cracks, internal cracks, and edge/internal cracks. The material properties and fracture toughness of the FGC were modeled by formulas based on the rule of mixtures. The FGC comprised two constituents, a ceramic on the top and a metal as a homogeneous substrate, with their volume fractions determined by a power law function with the power coefficient λ as the gradation parameter for the FGC. For this study, the method of singular integral equations was used, and the integral equations were solved numerically by the mechanical quadrature method based on the Chebyshev polynomials. Attention was mainly paid to the determination of critical loads and energy release rates for the systems of interacting cracks in the FGCs in order to find ways to increase the fracture resistance of FGC/H structures. As an illustrative example, a system of three edge cracks in the FGC was considered. The crack shielding effect was demonstrated for this system of cracks. Additionally, it was shown that the gradation parameter λ had a great effect on the fracture characteristics. Thus, the proposed model provided a sound basis for the optimization of FGCs in order to improve the fracture resistance of FGC/H structures.

Functionally Gradient Material Fabrication Based on Cr, Ti, Fe, Ni, Co, Cu Metal Layers via Spark Plasma Sintering

The paper presents a method of obtaining functionally graded material (FGM) of heterogeneous (layered) type based on joined metals Cr-Ti-Fe-Co-Ni-Cu using spark plasma sintering (SPS) technology. The structure, elemental and phase composition of FGM obtained on the basis of joined metals with different values of the temperature coefficient of linear expansion (CTLE) were studied by SEM, EDS and XRD methods with regard to the phase states of the alloy system. Based on the Vickers microhardness data, the evaluation of the mechanical characteristics of FGM in the whole sample body and locally at the contact boundaries of the joined metals was carried out. The results of the study are new and represent a potential for FGM, as well as functionally graded coatings (FGC), which have special physical, chemical and mechanical properties and are highly demanded for the manufacture of structures and products for industrial applications.

Hot indentation of a FGM-coated thermoelastic half-space by a conical punch: Approximated analytical solution of the contact problem

Hot indentation of a thermoelastic coated half-space by a rigid punch is considered. The coating is assumed to be functionally-graded with all group of properties arbitrary varying with depth. The deformation of the half-space caused by the simultaneous mechanical and thermal loading is considered. The punch is subjected to a normal centrally applied force and preliminary heated to a uniformly distributed temperature. The contact problem is reduced to the solution of a system of dual integral equations over two unknown functions: Hankel transform of normal stresses and heat flux in the contact area. Heat flux is obtained using the bilateral asymptotic method. This method was sufficiently generalized to take into account additional function describing the displacements of the surface caused by thermal heating and obtain contact stresses. The solution is obtained in an analytical form which clearly demonstrates the dependence of the contact characteristics from the initial physical parameters of the problem. Numerical analysis is provided to illustrate the effect of the presence of the coating and its thickness on the results. Differences between the homogeneous and functionally-graded coatings is illustrated.

Buckling Behavior of Sandwich Cylindrical Shells Covered by Functionally Graded Coatings with Clamped Boundary Conditions under Hydrostatic Pressure.

The buckling behavior of sandwich shells with functionally graded (FG) coatings operating under different external pressures was generally investigated under simply supported boundary conditions. Since it is very difficult to determine the approximation functions satisfying clamped boundary conditions and to solve the basic equations analytically within the framework of first order shear deformation theory (FOST), the number of publications on this subject is very limited. An analytical solution to the buckling problem of FG-coated cylindrical shells under clamped boundary conditions subjected to uniform hydrostatic pressure within the FOST framework is presented for the first time. By mathematical modeling of the FG coatings, the constitutive relations and basic equations of sandwich cylindrical shells within the FOST framework are obtained. Analytical solutions of the basic equations in the framework of the Donnell shell theory, obtained using the Galerkin method, is carried out using new approximation functions that satisfy clamped boundary conditions. Finally, the influences of FG models and volume fractions on the hydrostatic buckling pressure within the FOST and classical shell theory (CT) frameworks are investigated in detail.

Adaptive Control of the Atmospheric Plasma Spray Process for Functionally Graded Thermal Barrier Coatings

Functionally graded coatings (FGCs) have a material composition continuously varying through the thickness but uniform in the surface parallel to the coated substrate. When used as a thermal barrier on a metallic substrate, the coating composition varies from an almost pure metal near the substrate to a pure ceramic adjacent to the outer surface exposed to a hot environment. Challenging issues in producing high quality FGCs in the presence of external disturbances with an atmospheric plasma spray process (APSP) include controlling the mean temperature, the mean axial velocity, and the positions of the constituent material particles when they arrive at the substrate to be coated. The unavoidable disturbances include fluctuations in the arc voltage and clogging of the powder in the delivery system. For a two-constituent coating, this work proposes using three modified robust model reference adaptive controllers based on the σ-modified laws and low frequency learning. One controller adjusts the current and flow rates of argon and hydrogen into the torch. The other two controllers adjust the distance of the two powder injector ports from the plasma jet axis and the average injection velocity of each powder. It is shown through numerical experiments that the three controllers implemented in an APSP consistently produce high-quality FGCs.

Analytical solution of stability and vibration problem of clamped cylindrical shells containing functionally graded layers within shear deformation theory

The aim of the study is to present a new approach to the analytical solution of the vibration and stability problem of clamped sandwich cylindrical shells (SCSs) covered by functionally graded (FG) coatings under an axial compressive load in the framework of shear deformation theory (ST). After modeling the micro and macro mechanical properties of FG coated SCSs with various configurations, the constitutive relations and basic equations are derived depending on the stress, deflection and two angles of rotation functions using modified Donnell type theory. An attempt is made to analytically solve the fundamental differential equations for SCSs covered by FG coatings using novel approximation functions satisfying the clamped boundary conditions. Three different shear stress functions (SSFs) such as parabolic shear stress function (Par-SSF), cosine-hyperbolic shear stress function (Cos-Hyp-SSF) and uniform shear stress function (U-SSF) are used in the analysis. After confirming the accuracy of the obtained results, the influences of coating profiles, volume fractions and layer arrangement variations on the critical parameters for three different transverse shear stress functions are investigated in detail.

A functionally graded coating from ZrN to Ti: Experimental characterization, erosion behavior and design concept

In functionally graded coatings (FGCs), the elemental composition, or structure, within a coating changes progressively as a function of depth, allowing for the property gradual transition from coating to matrix. One method for preparing FGCs with continuous component distribution is double glow plasma alloying (DGPA) technique. In this work, a sandwich-like ZrN/Ti FGC was fabricated, consisting of a single ZrN ceramic layer on the topmost layer, a ceramic/metal intermediate transition layer, and a metallic Ti layer in the inner layer. The transition layer derived from atom diffusion made the metallurgical bonding for entire coating, which endowed the coating with high adhesion. The erosion tests exhibited that the coating improved the erosion resistance of substrate effectively, which was attributed to the high hardness and elastic work consumption. Besides, the transition layer with coexistence of metal and ceramic phases deflected the radial crack propagation, and thus increased the plastic work consumption. The thermodynamic calculation using two sub-lattice and Miedema models showed that controlling the Zr diffusion thickness over N diffusion thickness can increase the ZrN phase content and reduce the unfavorable TiN and Ti2N phase. The experiment results and design concept described herein for characterizing FGCs can be used to promote the understanding and design of other FGCs.

Displacements of the surface of a piezoelectric half-space with functionally-graded coating and circle electrode on the surface

A mathematical model describing the deformation of a material caused by the reverse piezoelectric effect is proposed. The model is based on an axisymmetric problem of electroelasticity for a half-space with a functionally-graded coating. An electric potential difference is applied across a circle area on the surface (an electrode whose thickness and elastic properties are neglected) and an infinity boundary of the half-space. Arbitrary independent variation of all electroelastic properties in depth of the coating is considered. The coating is assumed to be perfectly bonded to the substrate. The problem is solved using the bilateral asymptotic method in an approximated analytical form effective for any value of the relative thickness of the coating. Approximated analytical expressions containing finite quadratures are obtained for the distribution of radial and normal displacements and electric potential on the surface under the electrode and outside of it. Numerical calculations are provided for the distribution of radial and normal displacements and electric potential for two typical examples of functionally-graded coatings in a wide range of relative thickness values of the coating. Convergence of the results to those for a homogeneous non-coated half-space is obtained for small and large values of the relative coating thickness. Special attention is paid to the comparison of the results with those for a non-coated half-space. The redistribution of the electromechanical characteristics caused by the presence of the coating is most noticeably observed near the boundary of the electrode especially for thin and intermediate thickness coatings (in comparison with the size of the electrode).

A critical review on functionally graded coatings: Methods, properties, and challenges

Surface coating on metal substrates has remained a difficult challenge for researchers due to the conflicting requirements for different properties. In recent years, due to their mechanical, thermal, electrical, and tribological properties in many advanced engineering applications, functionally graded coatings (FGCs) have become fascinating materials for researchers worldwide to obtain coatings with specific requirements. FGCs are a novel type of traditional composites in which phases are not equally distributed to form a smooth gradient structure; thus, gradient coatings have shown a new research path. Therefore, this paper critically reviews the different FGC processing techniques by describing the fundamental problems and potential of these processes based on existing literature. Additionally, this research presents the most important potential applications and future challenges for gradient coatings. Consequently, the results expected from the paper are considered a guideline that enables the interested in this field to make informed decisions.

Упрощенное аналитическое решение контактной задачи о вдавливании конического штампа в полупространство с покрытием

The present paper is devoted to the construction of the approximate solution of the contact problem on a conical punch indentation into an elastic isotropic half-space with coating. The solution is suitable for both homogeneous (when elastic moduli are constant) and multilayered or functionally-graded coatings (when elastic moduli change with depth). The solution was obtained using the bilateral asymptotic method in the analytical form. The kernel trans-form approximation was obtained as the ratio of two quadratic functions and contains only one parameter. Thus, the scheme of the approximate analytical solution was constructed in a significantly simplified manner in comparison with the general case in which the product of fractional-quadratic functions is used. Such an approach allowed us to obtain explicit analytical expressions for the distribution of contact stresses and the force-displacement dependences in a simplified form, convenient for engineering calculations. The influence of the parameter characterizing the relative Young's modulus of the coating on the characteristics of contact interaction was analyzed. The accuracy of the obtained solution was studied depending on the ratio of the elastic moduli of the coating and the substrate using a series of homogeneous coatings as an example. Particular attention was paid to the investigation of the indentation stiffness - the most important characteristic used in experimental researches. The results of the work can be used to describe an experiment on nanoindentation of materials with coatings using either a conical or a Berkovich indenter.

Reciprocating Sliding Wear of Cu, Cu-SiC Functionally Graded Coating on Electrical Contact

The present work evaluates the coefficient of friction (CoF), electrical resistivity, and electrical contact resistance (ECR) of the electrodeposited single-layered Cu-SiC nanocomposite coating and five-layered Cu, Cu-SiC functionally graded coating (FGC). Both the coatings have a similar thickness (60 µm) and same composition at the top surface (7 vol.% reinforced SiC nanoparticles), while the FGC has a gradient of composition and microstructure throughout the thickness. The Cu, Cu-SiC FGC has two layers of Cu-SiC with a decrement in the content of SiC nanoparticles from 7 to 2 vol.% followed by three Cu layers with an increasing crystallite size towards the substrate. The electrical resistivity of the Cu, Cu-SiC FGC is measured by the four-wire resistance measurement method and the value is observed to be 50% less than the conventional nanocomposite coating. A linear reciprocating sliding wear test is carried out at 2, 5 and 8 N load at a constant frequency and stroke length of 10 Hz and 2 mm, respectively. The monitored value of CoF is significantly less for the Cu, Cu-SiC FGC than the single-layered coating at 2 and 5 N loads and is nearly equal at 8 N load. It is observed that before wear, the ECR values of both the coatings are higher than the uncoated Cu and after wear the ECR value of Cu, Cu-SiC FGC is the lowest.

A theoretical model for the study of thermal fracture of functionally graded thermal barrier coatings with a system of edge and internal cracks

The problem of fracture of functionally graded coatings (FGCs) on a homogeneous substrate (a semi-infinite medium) is investigated under the influence of thermal and/or mechanical loads (e.g. a heat flux, residual thermal stresses caused by cooling-heating, tension). These loads reflect the most important cases, which arise during the exploitation of FGC structures. The FGC contains pre-existing systems of cracks, such as edge, internal and interface cracks. The mathematical description of the model is based on singular integral equations. The properties of the FGC are continuous functions of the thickness coordinate. Furthermore, the non-homogeneity of the functionally graded material is revealed in the form of corresponding inhomogeneous traction distributions on the surfaces of cracks. This method is approximate and used with the assumption, that the gradation of material properties of the FGC with the depth of the layer is not abrupt. The influence of residual stresses caused by temperature changes on ΔT (e.g. cooling from operating temperatures) is investigated in detail. Different crack patterns (which are reported in experiments and available in the literature) are studied by carrying out numerical experiments with respect to stress intensity factors and fracture angles (a deviation of cracks from the initial direction of propagation). The proposed model in combination with a detailed parametric analysis can help to optimize FGCs in order to improve the fracture resistance of FGC/homogeneous structures.

Analysis of interacting cracks in functionally graded thermal barrier coatings

The problem of thermal fracture of a structure consisting of a functionally graded coating (FGC) on a homogeneous substrate (FGC/H) subjected to a thermal loading is investigated. The main focus is laid on the application of fracture criteria for an FGC and the determination of critical mechanical and thermal loads. The application of fracture criteria requires knowledge of the fracture toughness near the crack tips. Thus, it is assumed that the fracture toughness of an FGC, as well as other material properties, continuously varies through the thickness of the coating. Besides, having a system of cracks, it is important to determine which crack is most dangerous with respect to possible propagation. The proposed model, combined with a detailed parametric analysis, provides a reliable basis for optimizing FGCs in order to improve the fracture resistance of FGC/homogeneous systems operating under high temperatures. An illustrative example for a real combination of materials (ceramic/metal)/metal is provided, which demonstrates that it is important to consider the variation of fracture toughness in FGCs in determining critical loads at which cracks can lead to complete structural fracture.

Corrosion behavior of functionally graded and self-healing nanostructured TiO2–Al2O3 - Benzotriazole coatings deposited on AA 2024-T3 by the sol-gel method

The corrosion behavior of titania-alumina-Benzotriazole and functionally graded (FG) coatings deposited on AA 2024-T3 is investigated. The FG coatings are doped with different amounts of Benzotriazole (BTA) as an inhibitor and the Al2O3–TiO2-BTA coating has a similar amount of BTA in the layers. The coatings are deposited by the sol-gel and dip-coating techniques and the morphology, structure, and corrosion resistance are investigated systematically. The amorphous TiO2–Al2O3-BTA and FG coatings with thicknesses of 2.3 and 2.6 μm show corrosion resistance of 105,672 Ω Cm2 and 252,731Ω. Cm2, respectively, after immersion in 3.5% NaCl for 96 h. The difference is caused by controllable release of BTA from the FG coatings. The results show that the FG coatings are superior to conventional nanostructured TiO2–Al2O3-BTA coatings from the perspective of corrosion protection.

Cu, Cu-SiC functionally graded coating for protection against corrosion and wear

This work presents the performance of the Copper (Cu) based functionally graded coatings (FGCs) when subjected to corrosion in the 3.5 wt% NaCl aqueous solution and sliding wear under the different loads (2, 5, 8 and 10 N). The Cu FGC, with three layers of Cu (each 20 μm thick), has a gradual decrement in the crystallite size from bottom to the top surface. The Cu, Cu-SiC FGC consists of three-layered Cu FGC (each Cu layer 12 μm thick) followed by two layers of Cu-SiC nanocomposite (each 12 μm thick) with a steep increase in the amount of reinforced SiC nanoparticles from 2 to 7 vol% towards the top. A comparison is made among the equally thick (60 μm) electrodeposited FGCs, i.e., Cu FGC and Cu, Cu-SiC FGC, single-layered Cu coating with the finest microstructure (smallest crystallite size) and Cu-SiC nanocomposite coating with 7 vol% SiC. A drastic reduction in the corrosion and wear rate is observed in the Cu and Cu, Cu-SiC FGCs when compared with the same of single-layered Cu and Cu-SiC nanocomposite coatings, respectively. It is also observed that the Cu, Cu-SiC FGC has higher corrosion resistance than the Cu FGC. The Cu, Cu-SiC FGC is found to be more wear resistant than the Cu FGC at high load, while the latter shows lower specific wear rate at low loads.