As-deposited Coatings Research Articles

Direct preparation of nanostructured Ni coatings on aluminium alloy 6061 by cathode plasma electrolytic deposition

A pure nanostructured Ni coating was directly prepared by continuous cathode plasma electrolytic deposition (CPED) on aluminium alloy 6061. The microstructures and compositions of the as-deposited coatings were studied by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD), followed by hardness and adhesion evaluations. Due to the plasma micro-arc and rapid quench, the Ni coatings prepared by CPED exhibit typically molten morphologies with nanocrystalline structures, which are capable of increasing the hardness values of the coatings. In addition, the adhesion abilities of the as-prepared coatings could be enhanced by metallurgical bonding between the coatings and the substrate. Moreover, the continuous CPED method is a high-efficiency approach to fabricate coatings with large areas.

The influence of heat treatment on the microstructure, surface topography and selected properties of PEEK coatings electrophoretically deposited on the Ti-6Al-4V alloy

The aim of this work was to fabricate high temperature PEEK 708 coatings on the Ti-6Al-4V alloy substrate using electrophoretic deposition and post-deposition heat treatment, as well as to determine the influence of the heat treatment process on the structure, microstructure, surface topography and selected properties of the coated alloy. The design of the EPD parameters (voltage and time) enabled the deposition of homogeneous coatings. The heat treatment of the as-deposited coatings, carried out above the polymer melting point, changed the polymer morphology from a granular coating into a dense and continuous one. Depending on the cooling rate, the coating exhibited an amorphous or semi-crystalline structure. The semi-crystalline coating consisted of numerous spherulites and had a well-developed surface topography. In contrast, the amorphous coatings were smooth, with lines consisting of individual crystallites. The polymer structure and microstructure significantly influenced the micromechanical properties and scratch resistance of the coatings. Semi-crystalline coatings had higher hardness and Young's modulus as well as better scratch resistance. Tribological properties were investigated using ball-on-disc tests in dry sliding contact with an alumina ball at room temperature and at elevated temperatures, i.e. 150 °C and 260 °C. It was found that the semi-crystalline coatings showed a significantly higher wear resistance and lower coefficient of friction at all the investigated temperatures compared to the amorphous coatings. The resistance to motion and the wear of the coatings increased with the ambient temperature of the friction pair. Electrochemical studies showed that both coatings improved the Ti-6Al-4V alloy's corrosion resistance in a NaCl solution. The improvement was more significant in the case of amorphous coatings.

Nickel-chromium (Ni–Cr) coatings deposited by magnetron sputtering for accident tolerant nuclear fuel claddings

Nickel-chromium coatings were deposited on Zr1Nb alloy using magnetron sputtering systems with «hot» Ni and cooled Cr targets. The effect of coating composition on high-temperature oxidation resistance and hydrogen uptake of Zr1Nb was studied. Hydrogen uptake of the alloy was measured in situ under gas-phase hydrogenation at 633 K. High-temperature oxidation was performed in air atmosphere at 1173–1373 K for 20 min. It was shown that the coating with high Ni content (83 at.%) drastically increases hydrogen uptake of the Zr1Nb alloy and demonstrates low oxidation resistance even at 1173 K. The coatings with Cr content ≥45 at.% have low hydrogen permeability which reduces the rate of hydrogen uptake of the alloy. The oxidation resistance of the NiCr coatings increases with Cr content in the as-deposited coatings. The pure Cr coating exhibits the best oxidation resistance: only 8 μm-thick oxide layer was observed. There is also found the intensive diffusion of nickel into the alloy during high-temperature oxidation of the samples coated by NiCr films with 55 and 17 at.% Ni. The as-deposited NiCr coatings are less brittle than the pure Cr coating, but their mechanical properties degrade stronger than for the Cr coating after the oxidation test.

Monolayered versus multilayered electroless NiP coatings: Impact of the plating approach on the microstructure, mechanical and corrosion properties of the coatings

Electroless nickel-phosphorous (NiP) coatings were produced on low carbon steel substrates for a total plating time of 3 h. Different preparation modalities were pursued. Multilayered coatings were produced by stacking three layers of the same composition by successive electroless plating with rinsing steps in between. On the other hand, coatings termed ‘monolayered’ for the sake of comparison were deposited by one step electroless process, with and without undergoing bath replenishment of the electrolyte during plating. All the samples were subjected to thermal annealing at 400 °C for 1 h under argon atmosphere.The results show that the multilayer approach prevents crack propagation in the as-deposited coatings because the interfaces between layers block the advance of defects. Bath replenishment during monolayered coatings production creates pseudo-interfaces similar to those of the multilayered case but they are ineffective in terms of corrosion protection. Un-replenishment of the electrolyte promotes a change in the coating's microstructure from lamellar to columnar which severely worsens their performance. Upon annealing, the presence of interfaces, along with the recrystallization of the metallic matrix, promotes an upgrading of the corrosion performance of the multi-layered coatings. The corrosion products spread laterally at the interface where they stockpile. At a certain point, the accumulation of these by-products provokes the exfoliation of the outermost layer exposing the layer underneath to the corrosive media, thereby delaying the advancement of the corrosion attack. The results of this study highlight the importance of the plating approach selection, as well as the need for proper electrolyte maintenance during the production of high-performance electroless coatings.

The influence of phosphorus content on the microstructure and specific capacitance of etched electroless Ni-P coatings

Acidic and alkaline electroless bath Ni-P coatings with varying P contents (4.7–8.1 wt%) were deposited on Ti substrate. These as-deposited Ni-P coatings had no apparent difference in specific capacitance. The deposition rate of coating increased with the increase in NaH2PO2·H2O content regardless of the bath type, while the alkaline bath yielded a lower phosphorus content. After the Ni-P coatings were etched by a HNO3 solution, dealloying first occurred in the boundaries between the nodules and then, in the inside of the nodules. Finally, the nodules were etched into numerous nanoparticles with a size of 25–35 nm. An increased specific surface area, due to porous structures, is the major contribution for high specific capacitance. The phosphorus content significantly affects the corrosion resistance of Ni-P coatings and, thus, plays an important role in deciding the microstructure and specific capacitance of etched Ni-P coatings. The specific capacitance of etched Ni-P coatings is a function of phosphorus content and etching time at a fixed etching condition, and an optimum value exists. The highest value achieved is dependent on phosphorus content. The mechanism of the etched Ni-P coatings used as the electrode of the supercapacitor also was discussed further.

The Influence of Nitrogen Partial Pressure on the Composition, Microstructure, and Mechanical Characteristics of Ti1 –x–yAlxTаyN Coatings Obtained by Reactive Magnetron Sputtering

The evolution of the elemental composition, surface morphology, microstructure, and mechanical properties of Ti1 –x–yAlxTаyN coatings deposited by reactive magnetron sputtering has been studied when the ratio of the partial pressure of nitrogen to the total pressure of reactive gas mixture was increased from 0.1 to 0.5. It is established that most significant changes in the relative content of Ti, Al, and Ta in as-deposited coatings take place when this ratio is varied from 0.1 to 0.3. The observed changes in the elemental composition of Ti1 –x–yAlxTаyN coatings do not influence their phase composition, but lead to changes in the microstructure and mechanical characteristics.

Hot corrosion behavior of wire-arc sprayed NiCrB coatings

A series of NiCrB coating was deposited using the twin wire arc spraying technique with a newly designed NiCrB (B: 0–4 wt%) cored wire, aiming to protect the boiler tubes used in the waste-to-energy plant. The hot corrosion behavior of the coatings was investigated with the molten salt (Na2SO4 + 10% NaCl, wt%) at 800 °C for 200 h using the thermogravimetric technique. The SEM/EDS and XRD results showed that the addition of element B alleviated the oxidation tendency in the as-deposited coatings. Sufficient free stage Cr atoms diffused to coating surface timely to form protective scale, which inhibited further rapid corrosion, owing to low oxides in the coating. In addition, the corrosion resistance was influenced by the B content of the coating. In the present study, the coating with 1–2% wt% B had the best hot corrosion performance.

Development of cold spray chromium coatings for improved accident tolerant zirconium-alloy cladding

The development of a cold spray process for the deposition of chromium (Cr) coatings on zirconium-alloys is presented with the goal of improving the accident tolerance of light water reactor (LWR) fuel cladding tubes. The cold spray parameters and feedstock powders were varied to attain the desired coating properties such as thickness, microstructure, and oxidation resistance, on both Zircaloy-4 flat specimens and Optimized ZIRLO™ cladding tubes. The coated samples were tested at temperatures up to 1300 °C in air to investigate the oxidation performance and inter-diffusion between the Cr coatings and the underlying zirconium-alloy substrate. To simulate the performance of the coatings under normal LWR operating conditions, the coated samples were also tested in a steam autoclave at 400 °C and 10.3 MPa. Microstructures, phases, and hardnesses of the feedstock powders and as-deposited coatings were examined, and oxidation and inter-diffusion profiles were quantified in post-oxidation test samples. Overall, cold sprayed Cr coatings show significant promise for enhancing the accident tolerance of zirconium-alloy fuel cladding in LWRs both in terms of performance and cost-effective manufacturability.

Characterization and corrosion behaviors of TiN/TiAlN multilayer coatings by ion source enhanced hybrid arc ion plating

TiN/TiAlN multilayer coatings with various bilayer numbers were deposited on AISI 410 martensitic stainless steel using ion source enhanced hybrid arc ion plating system. The influences of different bilayer numbers on structure, mechanical properties and corrosion behaviors of TiN/TiAlN multilayer coatings were investigated comparatively. The as-deposited coatings were predominantly composed of fcc-TiN and solid solution (Ti, Al) N phases with (111) preferred orientation, and the growing tendency towards preferential orientation decreased as the increase of bilayer numbers. With increasing bilayer numbers, microhardness of the coatings was enhanced gradually for the increased interfaces and refined crystals, reaching a maximum of 3800 HK0.025 at the coating with 12 bilayers. Multilayer coatings exhibited an abrasive wear mechanism against WC pair, and possessed an excellent tribological performance in comparison to MSS substrate. According to electrochemical results, the coating with bilayer number of 12 presented a high free corrosion potential combined with the lowest corrosion current density, highest polarization resistance, highest protective efficiency and largest charge transfer resistance than that of other bilayer numbers owing to the increased layered interfaces and high structural density.

Mechanical properties and microstructural stability of CuTa/Cu composite coatings

In this study, the structure and mechanical properties of as-deposited and 823.15 K annealed CuTa coatings were studied with the purpose to evaluate the microstructural and mechanical thermal stability as a function of the Ta content. CuTa coatings with a Ta content from 0 to 100% were prepared by magnetron co-sputtering. Their structure and mechanical properties were characterized by grazing angle X-ray diffraction (XRD), scanning and transmission electron microscopy (TEM), and nanoindentation. The XRD results show that a Ta-rich CuTa glass metal is formed in the coatings with 15–67 at.% Ta along with nanocrystalline Cu and such microstructure was retained after vacuum annealing at 823.15 K. TEM analyses of selected samples showed the formation of crystalline Cu islands in an amorphous CuTa matrix in both as-deposited and annealed coatings. The coatings presented a 3D distribution of Cu nanocrystals within an amorphous CuTa matrix which might reduce the brittleness of the glassy CuTa matrix. A significant increase in hardness values from 0.9 GPa for pure Cu to 11.9 GPa for the sample with 98 at.% Ta is observed. It has been shown that the composite coatings preserved the microstructure and were mechanically stable after vacuum annealing.

On the oxidation behaviour of cathodic arc evaporated Al[sbnd]Cr and Al[sbnd]Cr[sbnd]O coatings

This study reports on the temperature-induced, ambient-air-exposure related structural and morphological evolution of various AlCr and AlCrO thin films prepared by cathodic arc evaporation with 20 sccm Ar, 50 sccm O2, or 100 sccm O2 per active source (p.a.s. Detailed cross-sectional scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies are conducted on as-deposited coatings and samples that have been oxidised for 3 h at 900, 1050, and 1200 °C.The high affinity of Al to O promotes the formation of essentially pure α-Al2O3 during oxidation starting between 900 and 1050 °C of the coatings prepared with 20 sccm Ar or 50 sccm O2 p.a.s. and cathodes with Al content ≥50 at.%. However, the AlCrO coatings prepared with 100 sccm O2 p.a.s. form solid solution α-(Al,Cr)2O3 oxides with Al/Cr ratios according to the used AlCr cathodes upon thermal exposure to ambient air.The phase composition (prior to oxidation) strongly determines the oxide phase formation. A sufficient intermixing of Al, Cr, and O (with about 60 at.% O) is needed to form α-(Al,Cr)2O3 oxides, otherwise the high affinity of Al to O would rather lead to the formation of Al2O3.

Evaluation of the adhesion and failure mechanism of the hard CrN coatings on different substrates

The CrN coatings were performed on 316 L austenitic stainless steel, skh51 high speed steel, and YG8 cemented carbide using plasma-enhanced magnetron sputtering system. The adhesion and failure mechanism of the hard CrN coatings on different kinds of substrates were investigated. The morphology, microstructure, crystallinity, as well as nano-hardness and Young' modulus of the as-deposited coatings were investigated by scanning electron microscopy, X-ray diffraction, and nano-indentation testing system, respectively. In particularly, the adhesion and failure mechanism of different coating-substrate systems were evaluated by scratch and indentation tests. Apparently, the adhesion and failure mechanism of the CrN coatings were significantly influenced by the species of substrates. The results of the scratch test indicated that the increase of adhesion strength (Lc = 190 N) of the CrN coating to YG8 was associated with the formation of the obvious indented coating-substrate interface, and the higher modulus of the substrate, which provided a better load-carrying capacity, whereas the CrN coating deposited on 316 L in contrast. In Rockwell indentation test, the main failure mechanism of the CrN coatings on substrate with high modulus were mainly brittle fracture, whereas the CrN coatings deposited on soft substrate was failed in ring cracks, which was attributed to the asynchronous recovery between the substrate and the coating.

Влияние парциального давления азота на состав, микроструктуру и механические характеристики покрытий Ti-=SUB=-1-x-y-=/SUB=-Al-=SUB=-x-=/SUB=-Tа-=SUB=-y-=/SUB=-N, полученных методом реактивного магнетронного распыления

AbstractThe evolution of the elemental composition, surface morphology, microstructure, and mechanical properties of Ti_1 –_ x _–_ y Al_ x Tа_ y N coatings deposited by reactive magnetron sputtering has been studied when the ratio of the partial pressure of nitrogen to the total pressure of reactive gas mixture was increased from 0.1 to 0.5. It is established that most significant changes in the relative content of Ti, Al, and Ta in as-deposited coatings take place when this ratio is varied from 0.1 to 0.3. The observed changes in the elemental composition of Ti_1 –_ x _–_ y Al_ x Tа_ y N coatings do not influence their phase composition, but lead to changes in the microstructure and mechanical characteristics.

Degradation of boron nitride interfacial coatings fabricated by chemical vapor infiltration on SiC fibers under ambient air/room temperature conditions

Hexagonal boron nitride (h-BN) interfacial coatings were deposited on SiC fibers by chemical vapor infiltration (CVI) and their degradation behavior under ambient air/room temperature conditions was studied with time. Degradation of the interfacial coatings with time was investigated by characterizing the morphology and microstructure of these materials with scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). Thermogravimetry coupled with differential thermal analysis (TG-DTA) and X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical reactions and phase transitions taking place in the BN coatings. The results showed that the as-deposited BN interfacial coatings fabricated by CVI were compact and well bonded to the SiC fibers. BN coatings remained relatively stable under ambient air/room temperature conditions for 50 h, while severe degradation was observed after 500 h of exposure. The degradation of BN interfacial coatings was mainly caused by two factors, namely, reaction with atmospheric air to produce boric oxide and amorphization of the hexagonal structure. The degradation observed under ambient air/room temperature might be due to incomplete crystallinity of BN interfacial coatings. Presence of water vapor may accelerate degradation of the coatings. The results of this degradation test can be used as a reference for the storage of BN coatings fabricated by CVI.

Sliding wear characteristics of as-deposited and heat-treated electroless Ni-P coatings against AISI E52100 steel ball

In the present experimental study, the wear characteristics of as-deposited and heat-treated, nickel phosphorus (Ni-P) coatings were studied against bearing steel ball (AISI E52100) at ambient dry conditions using linear reciprocating tribometer. The Ni-P coatings were deposited on En24 grade steel using electroless acidic bath and subjected to heat treatment at 400 °C for 1 h under air atmosphere. The phase structure and surface morphology of the coatings were characterized using x-ray diffraction (XRD) analysis and scanning electron microscopy (SEM), respectively. Grain size of the coatings was analysed using Scherrer technique. Heat-treated coatings showed 50% improvement in Vickers hardness compared to as-deposited coatings, which is attributed to the formation of Ni3P phase. The stable friction coefficient (∼0.21) and low specific wear rate (4.49E-05 mm3/N-m) were observed for heat-treated coatings in comparison to those of as-deposited coatings. Wear tracks were characterized using SEM and energy dispersive spectroscopy (EDS) in order to understand the wear mechanisms.

In-situ XRD studies of arc evaporated Al-Cr-O coatings during oxidation

The structural evolution of cathodic arc evaporated Al-Cr-O coatings with hugely different Al, Cr, and O contents was investigated by in-situ XRD measurements upon annealing in oxygen up to 1200 °C. Different chemical compositions are achieved by arc evaporation of Al0.90Cr0.10, Al0.75Cr0.25, Al0.70Cr0.30, Al0.50Cr0.50, or Al0.25Cr0.75 cathodes and using either 20 standard cubic centimetres per minute (sccm) Ar, 50 sccm O2, or 100 sccm O2 per active source (p.a.s.). Generally, the crystallinity of the as-deposited coatings decreases with increasing O2 flow rate during arc evaporation, while their Al/Cr ratio increases significantly (with respect to the cathode composition). The coatings prepared from Al-rich cathodes show first indications for the formation of corundum structured α-(Al,Cr)2O3 phases at ~900 °C during annealing in O2 atmosphere. Those, prepared from the Cr-rich cathode (Al0.25Cr0.75) with 20 sccm Ar or 50 sccm O2 p.a.s., exhibit the formation of α-(Al,Cr)2O3 phases already at 700–800 °C. Their corresponding coatings prepared with the highest oxygen flow rate of 100 sccm p.a.s., exhibit a considerable phase fraction of α-(Al,Cr)2O3 already in the as-deposited state.Upon oxidation at 1200 °C, most coatings are basically composed of α-oxides, with a wide range of their Al/Cr-ratio. The coatings prepared with 100 sccm O2 p.a.s., however, show the formation of α-(Al,Cr)2O3 with a rather narrow Al/Cr-ratio, which is due to the sufficient mixing of Al, Cr, and O species already in the as-deposited state. Furthermore, selective oxidation (preference for Al2O3) leads in most cases to the formation of Cr, which is detectable up to 1200 °C.Between 950 and 1050 °C, the formation of θ-(Al,Cr)2O3 could be detected for cathodes with Al ≥ 50 at.% prepared with 20 sccm Ar or 50 sccm O2 p.a.s. Coatings prepared with Al0.75Cr0.25, Al0.70Cr0.30, and Al0.50Cr0.50 cathodes and 100 sccm O2 p.a.s. show the formation of θ-(Al,Cr)2O3 already in the as-deposited state. α-(Al,Cr)2O3 could only be detected for as-deposited coatings when preparing them with Al0.25Cr0.75 and 100 sccm O2 p.a.s.

EFFECT OF HEAT TREATMENT ON MICROSTRUCTURE AND CORROSION RESISTANCE OF Ni-B-W-Mo COATING DEPOSITED BY ELECTROLESS METHOD

The present work reports the deposition of a quaternary Ni-B-W-Mo coating on AISI 1040 medium carbon steel and its characterization. Quaternary deposits are obtained by suitably modifying existing electroless Ni-B bath. Composition of the as-deposited coating is analyzed by energy dispersive X-ray spectroscopy. The structural aspects of the as-deposited and coatings heat treated at 300[Formula: see text]C, 350[Formula: see text]C, 400[Formula: see text]C, 450[Formula: see text]C and 500[Formula: see text]C are determined using X-ray diffraction technique. Surface of the as-deposited and heat-treated coatings is examined using a scanning electron microscope. Very high W deposition could be observed when sodium molybdate is present in the borohydride-based bath along with sodium tungstate. The coatings in their as-deposited condition are amorphous while crystallization takes place on heat treatment. A nodulated surface morphology of the deposits is also observed. Vickers’ microhardness and crystallite size measurement reveal inclusion of W and Mo results in enhanced thermal stability of the coatings. Solid solution strengthening of the electroless coatings by W and Mo is also observed. The applicability of kinetic strength theory to the hardening of the coatings on heat treatment is also investigated. Corrosion resistance of Ni-B-W-Mo coatings and effect of heat treatment on the same are also determined by electrochemical techniques.

Microstructure and Properties of Graphene Oxide-doped TiO2 Coating on Titanium by Micro Arc Oxidation

Micro arc oxidation (MAO) coatings doped with graphene oxide (GO) were prepared on pure titanium by adding GO and sodium dodecyl benzene sulfonate (SDBS) into a sodium silicate solution. The as-deposited coatings were comparatively analyzed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The binding forces of the MAO, MAO+GO and MAO+GO+SDBS three coatings were measured by a scratch tester. The mechanical property of the three coatings was analyzed using the nano-indentation technique. The corrosion resistance of the coatings was tested by the electrochemical system in 3.5% NaCl solution. The photocatalytic activity of the prepared samples was evaluated by determining the degradation of methylene blue (MB) solution. The results showed that compared to the MAO coating, the morphologies and phase compositions of MAO+GO and MAO+GO+SDBS composite coatings were significantly different. These two composite coatings all had superior photocatalytic activity. Especially, the MAO+GO composite coating still had enhanced binding force and excellent corrosion resistance. Furthermore, the relationship between the microstructure and the properties of these three MAO coatings was analyzed.

Effect of thermal treatment on structure, phase and mechanical properties of hydroxyapatite thin films grown by RF magnetron sputtering

Hydroxyapatite (HA) thin films with a Ca/P ratio of 1.69 and approximately 500 nm in thickness were deposited by radio frequency magnetron sputtering on Ti-6Al-4 V substrates at 100 °C and a working pressure of 0.5 Pa. The obtained coatings were thermally treated at 400 and 700 °C in argon atmosphere and characterized in terms of elemental and phase composition, morphology, structure and mechanical properties by energy dispersive x-ray spectroscopy, scanning electron microscopy, atomic force microscopy, micro-Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction and nanoindentation. It was found that the thermal treatments increase the crystallinity although induce residual tensile stresses formation in the HA coatings, which are greater at high temperatures. Nevertheless, enhances in the mechanical properties: hardness (H), elastic modulus (E) and adhesion strength for the heat treated at 400 °C samples compared to the as-deposited coatings were obtained. Values of 7.45 ± 0.6 GPa (H), 115 ± 10 GPa (E) and a critical load of 175 ± 4 mN were measured for those thermally treated samples at 400 °C.

Effect of transition layer on the performance of hydroxyapatite/titanium nitride coating developed on Ti-6Al-4V alloy by magnetron sputtering

A gradient transition multilayer hydroxyapatite/titanium nitride (HA/TiN) coating was prepared on the Ti-6Al-4V alloy by magnetron sputtering. The composition, surface topography, microstructure, adhesion strength and electrochemical properties of the as-deposited coatings were characterized by SEM/EDS, AFM, XRD, FT-IR and electrochemical workstation. The experimental results showed that the single TiN coating deposited at a partial pressure of nitrogen (N2) of 0.08 Pa had the best internal stress and tribological performance, and its volume loss was only 0.89% of that of Ti-6Al-4V alloy. The introduction of the TiN transition layer greatly improved the wear resistance of the Ti-6Al-4V alloy, and the adhesion strength of the HA layer to the substrate increased from 6.50 ± 0.5 N to 11.70 ± 1.2 N, an increase of 56%. The HA/TiN coating surface consisted of uniform hemispherical particles with dense structure and invisible defects (micro-cracks and pores). For the HA surface layer, the crystal structure and active hydroxyl (-OH) group was restored after heat treatment. Potentiodynamic polarization experiments indicated that the HA/TiN coating achieved the lowest corrosion current density and the most positive corrosion potential compared to the single TiN layer and Ti-6Al-4V alloy. In summary, it can be conclude that the gradient transition layer can well improve the mechanical properties and electrochemical behavior of the titanium alloy, and largely ensuring the stability of the surface bioactive coating.