Gradient Coatings Research Articles

Comparative study of the structure and properties of homogeneous and gradient Ni-Cr-Al coatings

This paper compares the structure and properties of homogeneous and gradient coatings in the Ni-Cr-Al system obtained by detonation spraying. According to X-ray analysis, only the CrNi3 phase appears in the homogeneous coating, and CrNi3 , Al, and NiAl phases appear on the gradient coating. The elements distribution graphs show that a small amount of aluminum is distributed over the depth of the homogeneous coating and in the gradient coating. The distribution of aluminum in the gradient Ni-Cr-Al coating gradually increases from the depth to the surface, and a high amount of aluminum forms in the coating surface. According to EDS analysis, a small amount of aluminium is distributed on the surface of the homogeneous coating and a large amount on the gradient coating. In the gradient coating, aluminium gradually increases from the substrate surface to the coating surface. Also, the gradient coating has a higher hardness than the homogeneous coating.

Design Analysis and Test Verification of Double‐Layer Gradient Coating Reinforced Concrete Flexural Strength

The residual compressive stress of concrete members can offset the tensile stress caused by some external load and hinder the generation and expansion of surface cracks to enhance the flexural strength of concrete. In order to carry out this strengthening method, the single‐layer and double‐layer gradient coatings mixed by sulphate aluminium cement with different amounts of expansion agent on the surface of concrete specimens were discussed, and the theoretical calculation formula of surface compressive stress caused by this was deduced. Combined with experimental data, the influence of surface compressive stress on flexural strength of concrete was studied. The results demonstrate that the greater the surface compressive stress generated in the coating, the better the effect of improving the flexural strength of concrete. To obtain sufficient surface compressive stress, it is recommended that the cross‐sectional product ratio of the substrate to the coating is more than 80; the higher the elastic modulus of the coating, the greater the surface compressive stress; the smaller the shrinkage rate of the coating, the greater the surface compressive stress. The improvement effect of double‐layer gradient coating on the flexural strength of concrete is better than that of single‐layer coating. Compared with the reference specimen without coating, the improvement rates of double‐layer gradient coating on the early and late flexural strength of concrete are 57.7% and 45.7%, respectively.

Structural levels of process modeling the formation and wear of high-strength thermal spray coatings

Modeling of physical processes at different structural levels provides an effective solution to a number of physicochemical problems connected with production and application of thermal spray coatings. The use of structure simulation methods allows one to optimize the structure of coatings for particular service conditions. A bidirectional scale of structural levels for modeling the properties of strengthening thermal spray coatings on machine parts is considered. Moving along the scale in a positive direction, we can distinguish the following structural levels: groups of several sprayed particles that form separate microareas of a coating, extended reinforcing elements in the form of wires, rods or grid, transitional interlayer zones, zones between the coating and the substrate, individual layers obtained in one pass, thick transition zones in gradient coatings with a variable composition across the thickness, blocks of several monolayers in multilayer coatings, island elements in discrete coatings, macroelements of the block structure of coatings with controlled distribution of microcracks, individual macrostripes of the coating, continuous single-layer or multi-layer coating, structural coating. Moving from the zero level in the opposite direction, we have the following structural elements: composite particles of conglomerated grains, cladding shells, zones of different phase composition, individual domains of sprayed particles, individual elements sprayed particles and groups of grains, grains, dendrites, shear zones and slip systems, fragments of grains, packets of martensite laths, disclination cells, loops and dipoles, bands in the substructure, kink bands, microtwins, disclination clusters, dislocation pile-ups and tangles, shear bands, dislocation walls, slip systems, linear defects in the form of dislocations, ledges at grain boundary and crowdions, kinks and jogs formed due to intersection of dislocations, point defects in the real structures of solids, crystal structure, electronic structure of matter, electronic structure of individual atoms.

Preparation and Characterization of NiCr/NiCr-Al2O3/Al2O3 Multilayer Gradient Coatings by Gas Detonation Spraying

This paper investigates the influence of the technological parameters of detonation spraying on the phase composition of NiCr- and Al2O3-based coatings. It was determined that the phase composition of Al2O3 coatings during detonation spraying strongly depends on the barrel filling volume with the gas mixture. The acetylene–oxygen mixture, which is the most frequently used fuel in the detonation spraying of powder materials, was used as a fuel gas. To obtain a ceramic layer based on Al2O3, spraying was performed at an acetylene–oxygen O2/C2H2 mixture ratio of 1.856; the volume of filling of the detonation gun barrel with an explosive gas mixture was 63%. To obtain a NiCr-based metallic layer, spraying was performed at the O2/C2H2 ratio of 1.063; the volume of filling of the detonation gun barrel with an explosive gas mixture was 54%. Based on a study of the effect of the detonation spraying mode on the phase composition of NiCr and Al2O3 coatings, NiCr/NiCr-Al2O3/Al2O3-based multilayer coatings were obtained. Mixtures of NiCr/Al2O3 powders with different component ratios were used to obtain multilayer gradient coatings. The structural-phase composition, mechanical and tribological properties of multilayer gradient metal–ceramic coatings in which the content of the ceramic phase changes smoothly along the depth were experimentally investigated. Three-, five- and six-layer gradient coatings were obtained by alternating metallic (NiCr) and ceramic (Al2O3) layers. The phase composition of all coatings was found to correspond to the removal of information from a depth of 20–30 μm. It was determined that the five-layer gradient coating, consisting of the lower metal layer (NiCr), the upper ceramic layer (Al2O3) and the transition layer of the mechanical mixture of metal and ceramics, is characterized by significantly higher hardness (15.9 GPa), wear resistance and adhesion strength.

Bipolar electrodeposition of gradient polypyrrole films as a catalyst matrix for anodic ethanol oxidation

Lateral gradient coatings displaying various compositions and microstructures along a single surface are desirable for material screening. Here we report a successful application of bipolar electrochemistry to deposit lateral gradient polypyrrole films on a graphite strip substrate. The gradient film shows varying morphologies and a wettability gradient along the length of the bipolar electrode, with superabsorbent property in the middle portion of the film. The film was use as a catalyst matrix for ethanol oxidation in alkaline electrolyte. Aqueous Ni(NO3)2 solution as the catalyst precursor was absorbed into the film. Area-step cyclic voltammetry was used to evaluate the electrocatalytic activity of various local parts along the gradient film to ethanol oxidation. The local surface with the highest catalytic activity was screened out and the excellent catalytic activity is attributed to the outstanding properties of this zone in both water absorption and electrical conductivity. The nickel oxide catalyst can be highly dispersed in the nanoporous network polymer matrix, due to the uniform penetration of the aqueous precursor solution into the superabsorbent film. XPS and EIS analyses support the catalytic mechanism based on the redox transition between NiOOH and Ni(OH)2. This work indicates that loading aqueous catalyst precursors into superabsorbent conducting polymer films may be a promising method for the preparation of electrocatalysts.

Effect of shot peening pretreatment on the high-temperature tribological behaviours of a TaN coating prepared via double-cathode glow plasma alloying

Gradient coatings with excellent physical and mechanical properties show great potential for use in engineering applications. In this study, a TaN gradient-modified layer was prepared on TA15 titanium alloy by double-cathode glow plasma alloying (DGPA) with a gradient structure pretreated by rotationally accelerated shot peening (RASP). The effects of the RASP pretreatment at different speeds on the mechanical and tribological properties and microstructure of the TaN coatings were investigated. The results indicated that the crystal size was refined to the nanometre scale (41.8–71.4 nm) by RASP pretreatment, and a gradient structure layer (30–55 μm) formed, which facilitated the adsorption and inward diffusion of Ta and N atoms. The pretreated TaN-modified layer contained an interdiffusion layer with a thin deposition layer consisting of Ta 2 N and TaN. Furthermore, the pretreated TaN-modified layer maintained good wear resistance in ball-on-disc testing at 700 K, and the wear rate was only 36.9% of that without pretreatment, which can be attributed to the high hardness, elastic modulus , toughness and thermal stability of the modified layer. This work provides insights relevant to the development of a mechanical-diffusion composite process to produce titanium alloy coatings with enhanced mechanical properties and wear resistance at elevated temperature. • TaN modified layer with a gradient change of composition was prepared on titanium alloy using RASP and DGPA. • The refined surface promotes the adsorption and diffusion of Ta and N atoms to obtain a thicker gradient-modified layer. • RASP pretreatment can improve the wear resistance of TaN modified layers at high temperatures.

Improvement of Erosion-Corrosion Behavior of AISI 420 Stainless Steel by Ion-Assisted Deposition ZrN Coatings

In this paper, a pragmatic technique has been developed to evaluate the erosion-corrosion behavior of three kinds of ZrN coatings (i.e., monolayer, multilayer, and gradient layers) which were deposited on AISI 420 martensitic stainless steel using an ion-assisted deposition technology. Among them, the monolayer coating refers to the coating with no change in composition and structure, the multilayer coating refers to the coating with alternating change of Zr/ZrN, and the gradient coating refers to the ZrN coating by increasing N2 partial pressure gradually. The morphology, composition, and microhardness of these ZrN coatings were examined by means of integrating the scanning electron microscopy (SEM), X-ray diffraction (XRD), and Knoop hardness measurements, while anodic polarization tests and salt fog spray tests in a simulated industrial environment have been performed to evaluate and identify the corrosion mechanisms of these coatings. The surface microhardness and corrosion resistance of the AISI420 martensitic stainless steel is found to be significantly improved by depositing the ion-assisted deposition ZrN coatings. The study indicates that the erosion-corrosion behavior in the slurry is the result of the synergistic effect of small-angle erosion and acid solution corrosion. Three ZrN coatings hinder the slurry erosion-corrosion behavior from two aspects (i.e., erosion resistance of small-angle particles as well as corrosion resistance of the substrate), thereby significantly improving the erosion-corrosion resistance of AISI 420 stainless steel. In addition, the ZrN gradient coatings show a much better erosion-corrosion resistance than that of the monolayer/multilayer ZrN coating because they have excellent crack resistance, bearing capacity, and electrochemical performance.

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.

Microstructure and properties of WC/diamond/Co-based gradient composite coatings on high-speed steel fabricated by laser cladding

Improving the wear resistance and corrosion resistance of high-speed steel (HSS), WC/diamond/Co-based gradient composite coatings were produced on HSS substrates by laser cladding with different composition powder mixture (Co-Cr alloy powder, 80Co-Cr alloy powder+20WC, 53Co-Cr alloy powder+40WC+7diamond, wt%). The macromorphology, microstructures, and phase composition were characterized by optical microscope (OM), scanning electron microscopy (SEM) equipped with energy-dispersive spectrometry (EDS), and X-ray diffraction (XRD) techniques. The microhardness, wear resistance, and corrosion resistance of the gradient coatings were also investigated, respectively. The results indicate that the prepared WC/diamond/Co-based gradient composite cladding layer has a fine morphology on the cross sections and a gradient transition of the grain size has been achieved. The microhardness result presents gradient distribution along the depth of the coating. The microhardness is strengthened due to the dispersions of M7C3 (M is Fe, Cr), Co3C, CrCo, Cr3C2, and Fe3C in the composite coating, and the highest microhardness of 1342 HV0.2 can be detected in the cladding layer. The friction coefficient values of the coatings range from 0.27 to 0.40, which is much lower than that of the substrate (0.50–0.60). Furthermore, the wear loss of coatings (1.1 mg) decreases by more than 3 times comparing with that of the substrate (3.5 mg). The polarization resistance results show that the cladding layer has excellent corrosion resistance with polarization resistance that can reach the value of 236488.1 Ω·cm2. The gradient transition of the mechanical properties and chemical metallurgical combination between particle (WC, diamond) and adhesive phase can be obtained in laser cladding, which improves the wear resistance and corrosion resistance of the HSS surface.

Scanning electron beam alloying of composite gradient coatings

The surface WC/Ni-Co based alloying treatment of 30CrMnSi steel was carried out using scanning electron beam technology, and the tissue distribution after alloying were tested and investigated. The results show that the cross-section of the alloyed specimen consists of three parts: the alloy layer, the heat affected zone and the substrate. The thickness of the alloy layer was increased by 18.3 μm compared to the thickness of the pre-coated layer. The alloy layer is organised in a composite gradient, consisting of martensite and ledeburite with carbides.

Study on the ZrN/Ag2O Micro–Nano Gradient Composite Structure Constructed on Pure Ti for Biomaterials

Titanium and titanium alloys have been extensively utilized in biomedical implants due to their excellent comprehensive mechanical properties and biocompatibility. In this study, a ZrN/Ag2O micro–nano gradient composite structure was prepared on the surface of pure Ti by multi-arc ion plating (MAIP) technique and metal vapor vacuum arc (MEVVA) ion implantation technology. This study indicated that a dramatic improvement in performance in the surface hardness (~1800 HV0.1) was attributed to the presence of the ceramic phase (ZrN) with high hardness included in composite structure. The relatively low wear rate of gradient composite structure confirmed its excellent performance in abrasion resistance and the abrasion mechanism of gradient composite structure was mainly abrasive wear. After the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests, because of the synergy effect of ZrN micron coating and Ag2O nanoparticles, the ZrN/Ag2O gradient coatings indicated the highest free corrosion potential (Ecorr) and lowest corrosion current density (icorr) in Ringer’s solution, and the polarization resistances of multilayer coatings were greater than that of the substrate, exhibiting positive effects on retarding localized corrosion tendency. Additionally, the suitable dose of ZrN/Ag2O gradient composite coating can obtain antibacterial ability, which exerts no significant cytotoxicity and even excellent cytocompatibility over a longer service process. Furthermore, this study is conducive to design and develop for multifunctional coatings of implant materials.

Reaction synthesis of gradient coatings by annealing of three-layer Ti–Ni–Ti nanolaminate magnetron sputtered on the TiNi substrate

In this work, Ti-Ni-Ti nanolaminate was deposited on a TiNi substrate by magnetron sputtering. Subsequent synthesis in Ar at temperatures of 500, 700 and 900 °C was carried out. To investigate the effect of the reaction synthesis temperature for obtaining a dense thin intermetallic biofunctional coating, a comparative analysis of the morphology, structure, phase composition, mechanical properties and wettability of the coatings was carried out. It was found that crystallization processes have a different impact on the amorphous Ti and Ni layers depending on temperature. In all cases, multilayer gradient crystalline coatings were formed that bound to the substrate by the diffusion zone. The reaction synthesis temperatures of 500 and 700 °C are insufficient for reaction completion and complete transformation; therefore, the obtained thin loose layers are unstable under stresses caused by the substrate. Reaction synthesis in Ti – Ni – Ti nanolaminate at 900 °С formed a dense two-layer gradient coating 2 µm thick, which reliably protects the matrix from further diffusion of interstitial impurities and segregation of Ni atoms onto the surface. An analysis of the nano-indentation deformation diagrams showed that the coatings do not prevent the viscoelastic deformation of the TiNi substrate.

Microstructure and Ablation Behavior of W/ZrC/SiC Coating on C/C Composites Prepared by Reactive Melt Infiltration and Atmospheric Plasma Spraying

A W/ZrC/SiC multilayer coating is prepared on carbon/carbon (C/C) composites by reactive melt infiltration (RMI) and atmospheric plasma spray (APS). The morphology and microstructure of the coating are characterized by scanning electron microscopy coupled with energy dispersive spectrometry (SEM‐EDS), X‐ray diffraction (XRD), and focused ion beam (FIB) and transmission electron microscopy (TEM). Microstructure analysis reveals that the gradient coatings exhibit smooth surface morphology, well‐defined interfaces, uniform thickness, and a highly dense architecture. After 60 s of ablation, the linear ablation rates (LARs) and mass ablation rates (MARs) of the coated composites stand at 2.58 × 10−3 mm s−1 and 5.25 × 10−3 g s−1, respectively. In general, the reason for the great ablation resistance of W/ZrC/SiC‐coated C/C composites is the comprehensive effect of the multilayer coating. The outer W coating is used as the first barrier layer to resist most thermal shocks. Then, the molten eutectics SiO2–ZrO2 formed in the ZrC/SiC layer at service temperature can fill the pores and act as a second diffusion barrier for oxygen and heat.

In-situ growing amorphous carbon film with attractive mechanical and tribological adaptability on PEEK via continuous plasma-induced process

The a-C:H films with plasma-induced in-situ transition layer were fabricated on Polyether ether ketone (PEEK) substrates by continuous acetylene plasma treatment, using plasma-enhanced chemical vapor deposition (PECVD) method. Microstructure, chemical bonding state, mechanical and tribological properties of the as-fabricated a-C:H and Cr/a-C:H films were systematically investigated. Based on these, a self-consistent in-situ growth mechanism is discussed in detail. Results show that the a-C:H films with plasma-induced in-situ transition layer endows the treated PEEK substrates high hardness, toughness and adhesion strength. In addition, the interaction of in-situ transition layer and amorphous enhanced phase show better load-bearing capacity in the scratch and tribological test. The failure mechanism of scratch test is proposed and wear mechanisms in vacuum, air and seawater environment are analyzed. Importantly, this novel plasma-induced in-situ transition layer structure is a further development of the concept of gradient coatings in its application to polymers.

Structure and Tribological Properties of Ni–Cr–Al-Based Gradient Coating Prepared by Detonation Spraying

In this paper, Ni–Cr–Al coatings were deposited using the detonation spraying method. The aim was to investigate how technological parameters influence coating structure formation, phase composition and tribological performances. We observed that the degree to which the barrel is filled with an O2/C2H2 gas mixture strongly influences the chemical composition of manufactured coatings. High degrees of barrel filling led to a decrease in aluminum content in the coating. Filling degrees of 40% and 50% produced sprayed coatings in which only Ni–Cr phases could be found. When the filling degree was reduced up to 25%, Ni–Al phases began to form in the sprayed coatings. Gradient Ni–Cr–Al coatings were produced by gradually reducing the filling degree from 50% to 25%. These coatings are characterized by Ni–Cr near the substrate level with Ni–Cr and Ni–Al phases at higher levels. The results obtained confirm that gradient Ni–Cr–Al coatings exhibit high hardness as well as good wear resistance.

Electrochemical and Tribological Characteristics of CrNx Composition Gradient Coating in Aqueous Solution

Electrochemical and Tribological Characteristics of CrNx Composition Gradient Coating in Aqueous Solution

Development and Application of Thin Wide-Band Screening Composite Materials

The paper is dedicated to the development of effective composite coatings with the use of carbon fillers of different morphology, their research, and application in the broadband frequency range. Electromagnetic loss studies were performed according to international standards ASTM D4935, IEEE-STD-299, and the US Department of Defence standard MIL-STD 461F. The impact of hybrid carbon nanomaterial "graphene/nanotubes" on the electrophysical properties of the composite material has been analyzed. As a result, the research laboratory technologies of production of composite coating on water and non-water (alcohol) basis are developed based on the carbon fillers of various morphology and also magnetite. The shielding properties of most of the created composites are estimated in the frequency range from 50 MHz to 30 GHz. The state enterprise “All-Ukrainian center for standardization, metrology, certification, and consumers’ rights protection” (here and after “Ukrmetrteststandart”) conducted comparative tests of the developed coating (in the form of paint) with a protective coating # 842 MG Chemicals (Burlington, Ontario, Canada) based on silver microparticles. Developed water-based composites can be used for interior decoration, in the formation of electromagnetic screens, thin gradient coatings to protect people from electromagnetic radiation in the microwave range.

Obtaining functional gradient coatings based on Al2O3 by detonation spraying

The article deals with the phase composition and hardness of Al2O3 coatings obtained by detonation spraying. It was found that a decrease in the delay time between shots is leading to an increase in the hardness and elastic module of Al2O3 coatings. It was found based on X-ray diffraction analysis that the main reason for the increase in hardness with a decreasing in the delay time between shots is associated with increases in the volume fraction of α- Al2O3 phase. A high content of the more ductile γ-Al2O3 phase at the substrate-coating interface leads to an increase in adhesion characteristics, and a high content of the α-Al2O3 phase on the coating surface provides high hardness and wear resistance. The studies of X-ray diffraction presented that the highest phase content is achieved when the coatings are formed with a delay time between shots of 0.25 s. It was found that increase in the volume fraction of the α-Al2O3 phase is caused by the secondary recrystallization γ → α, which occurs due to the heating of particles during coating formation, i.e. due to increase in temperature above 1100 ºС in single spots of the coating when they are put each other.

Microstructural characteristics and oxidation behavior of the modified MCrAlX coatings: A critical review

Currently, the service temperature of the MCrAlX coating (M = Ni, Co, or NiCo; X = minor elements such as Y, Ce, Si, Ta) either as an overlay or as a bondcoat in some critical engineering parts for gas turbine and aero-engine applications is more than 1000 °C. Therefore, the improvement of thermal stability and service lifetime of the MCrAlX coatings are the key factors that need to be emphasized. Among them, the high-temperature oxidation resistance is one of the key properties of MCrAlX coatings. The key modification processes can be employed to improve the structural properties and oxidation behavior of MCrAlX coatings. These methods mostly include techniques such as vacuum heat treatment, nano-crystallization, pre-oxidation, alloying with the reactive element or rare earth elements, dispersion of oxide particles, laser treatment, spark plasma sintering, hot isostatic pressing, and using multilayered/graded coating systems. In recent decades, investigations on the increasing of the oxidation resistance of MCrAlY coatings were mostly focused on the modification of these coatings using reactive elements, laser treatment, and development of advanced multilayered or gradient coatings. Recent investigation exemplified that reactive elements (or their oxides( as well as nano-crystallization can increase high-temperature corrosion and oxidation resistance of MCrAlX coatings. This enhancement may be associated with the structure refinement, grain boundary purification, and adhesion improvement of the Al 2 O 3 oxide scale on the coating surface. However, there are conflicting opinions regarding the effect of reactive elements on the oxidation behavior of the MCrAlY coatings. The controversial findings in the literature indicated that the dominant mechanism of the enhancement of oxidation resistance for the reactive element or rare earth element modified MCrAlY coatings is still unclear and complicated. Therefore, a basic conception of the mechanism of the oxide scale growth in MCrAlY coating under short- and long-term oxidation is still far from obvious. The present work is a review with critical viewpoints in the case of the improvement of MCrAlY coatings and investigation of their structural characteristics and oxidation behavior, focused on different types of modification techniques. • Different modification techniques can be used to improve the structural characteristics of the MCrAlX coatings. • Reactive element modified MCrAlX coatings have higher oxidation resistance than conventional coatings. • The enhancement of oxidation properties may be attributed to the higher oxide scale adhesion in modified MCrAlX coating.

Application of variational calculus for optimization of optic gradient coatings

The design optimization for n-graded optic coatings based on variational calculus is discussed. In order to exemplify the approach, the Lagrange problem for minimizing refraction is stated and solved. The refraction law is generalized for alteration of the continuous refraction index. To solve problems of reflection minimization, an integro-differential equation of retarding potentials is proposed to use as a condition to the nonholonomic Lagrange problem.