Effect of potassium fluoride in electrolytic solution on the structure and properties of microarc oxidation coatings on magnesium alloy

Oxide coatings were produced on AM60B magnesium alloy substrate making use of microarc oxidation (MAO) technique. The effect of KF addition in the Na₂SiO₃-KOH electrolytic solution on the microarc oxidation process and the structure, composition, and properties of the oxide coatings was investigated. It was found that the addition of KF into the Na₂SiO₃-KOH electrolytic solution caused increase in the electrolyte conductivity and decrease in the work voltage and final voltage in the MAO process. Subsequently, the pore diameter and surface roughness of the microarc oxidation coating were decreased by the addition of KF, while the coating compactness was increased. At the same time, the phase compositions of the coatings also varied after the addition of KF in the electrolytic solution, owing to the participation of KF in the reaction and its incorporation into the oxide coating. Moreover, the coating formed in the electrolytic solution with KF had a higher surface hardness and better wear-resistance than that formed in the solution without KF, which was attributed to the changes in the spark discharge characteristics and the compositions and structures of the oxide coatings after the addition of KF.     

Preparation of ceramic coating on Ti substrate by plasma electrolytic oxidation in different electrolytes and evaluation of its corrosion resistance: Part II

The aim of this work is to discuss the growth characteristics and corrosion behavior of the prepared ceramic coatings on titanium by plasma electrolytic oxidation (PEO) technique in different electrolytes. PEO process was carried out on titanium under constant voltage regime using a pulse power supply. Three kinds of electrolytes, phosphate, silicate and borate based solutions, were used to evaluate the influence of electrolyte composition on the structure, surface morphology, phase composition and corrosion behavior of prepared ceramic oxide films (titania). The phase composition of the coatings was investigated by X-ray diffraction. Scanning electron microscopy was employed to evaluate the growth and surface morphology of coatings. Elements of coatings were investigated with energy dispersive spectrometer. Corrosion behavior of the coatings was also examined by potentiodynamic polarization and electrochemical impedance spectroscopy. The spark voltage of oxide films had a significant effect on the surface morphology, size and homogeneity of micro-pores, thickness and corrosion properties of coatings.     

Preparation and properties of titanium oxide film on NiTi alloy by micro-arc oxidation

Titanium oxide ceramic coatings were prepared by micro-arc oxidation (MAO) in galvanostatic regime on biomedical NiTi alloy in H₃PO₄ electrolyte using DC power supply. The surface of the coating exhibited a typical MAO porous and rough structure. The XPS analysis indicated that the coatings were mainly consisted of O, Ti, P, and a little amount of Ni, and the concentration of Ni was greatly reduced compared to that of the NiTi substrate. The TF-XRD analysis revealed that MAO coating was composed of amorphous titanium oxide. The coatings were tightly adhesive to the substrates with the bonding strength more than 45 MPa, which was suitable for medical applications. The curves of potentiodynamic porlarization indicated that the corrosion resistance of NiTi alloy was significantly improved due to titanium oxide formation on NiTi alloy by MAO.     

Growth characteristics of plasma electrolytic oxidation ceramic coatings on Ti-6Al-4V alloy

The aim of this work is to discuss the growth characteristics of the ceramic coatings on Ti alloy by plasma electrolytic oxidation (PEO) technique. Ceramic coatings were prepared on Ti alloy by plasma electrolytic oxidation in different electrolyte solutions under different pulse modes. The composition and the structure of the coatings were investigated by X-ray diffraction and scanning electron microscopy (SEM), respectively. The amount of the dissolved titanium into the electrolytes during PEO process was measured by inductively coupled plasma-atomic emission spectrometer (ICP-AES). The structure and the composition of the coatings were related to the mode of the spark discharge during PEO process. (a) Under the pulsed single-polar mode: In Na₃PO₄ solution, the spark discharge was mainly due to the breakdown of the oxide film, and the coatings prepared were porous and mainly structured by the Ti from the substrate. In K₄ZrF₆-H₃PO₄ and NaAlO₂-Na₃PO₄ solutions, the main mode of the spark discharge was the breakdown of the oxide film at the initial stage, and then changed into the breakdown of the vapor envelope, and the coatings were rough and thick, and mainly structured by the elements from the electrolyte. (b) Under the pulsed bi-polar mode in NaAlO₂-Na₃PO₄ solution, the spark discharge may be mainly due to the breakdown of the oxide film, the coatings prepared were dense in inner layer and loose in outer layer, and structured by the elements from both the substrate and the electrolyte. Besides, the ICP-AES analyses showed that the amount of the dissolved titanium in the electrolyte during PEO process was more under the breakdown of the oxide film than under the breakdown of the vapor envelope, which was consistent with the changes of the structure of the coatings. Cathode pulse in the pulsed bi-polar mode increased the amount of the dissolved titanium in the electrolyte, compared with the pulsed single-polar one.     

Characterization of calcium containing plasma electrolytic oxidation coatings on AM50 magnesium alloy

An attempt was made to produce calcium containing plasma electrolytic oxidation (PEO) coatings on AM50 magnesium alloy using an alkaline electrolyte. This study was performed in three alkaline electrolytes containing calcium hydroxide and sodium phosphate with three different mass ratios viz., 1:2.5, 1:5 and 1:7.5. All the three coatings produced were found to contain Ca and P in appreciable amounts. The concentration of P was found to be higher in the coatings obtained in the electrolytes with higher concentration of phosphate ions. Even though all the three coatings were found to be constituted with magnesium oxide and magnesium phosphate phases, X-ray diffraction analyses revealed that the phase composition was influenced by the phosphate ion concentration/conductivity of the electrolyte. Further, the PEO coating obtained in the 1:7.5 ratio electrolyte was found to contain di-calcium phosphate (monetite) and calcium peroxide phases, which were absent in the other two coatings. Potentiodynamic polarization studies performed in 0.1 M NaCl solution showed that the coatings obtained from the 1:5 ratio electrolyte possessed a superior corrosion resistance, which is attributed to the combined effect of thickness, compactness and phase/chemical composition of this coating.     

Analysis of the interface between a pulsed laser deposited calcium phosphate coating and a titanium alloy substrate

Calcium phosphate coatings deposited on titanium alloy are intended to add a bioactive surface to medical implants. This work presents the characterisation of the interface between Ti-6Al-4V and a crystalline calcium phosphate coating obtained by pulsed laser deposition, with a KrF excimer laser, at 575 °C and under a 45 Pa water-vapour atmosphere. The coating–substrate system was studied by secondary-ion mass spectrometry, scanning electron microscopy, X-ray diffractometry, Raman spectroscopy and X-ray photoelectron spectroscopy. The results show that the deposition process promotes the interdiffusion of substrate elements into the coating and coating elements into the substrate oxide layer. Thus, a graded layer of mixed calcium phosphate and amorphous titanium oxide is formed. For the substrate, a hydroxyapatite coating acts more as a barrier for oxygen incoming from a gas than as an oxygen source during deposition. Moreover, oxygen diffusion into the substrate occurs. Thus, the content of oxygen of this oxide layer diminishes with depth. When the oxygen concentration is low enough it is incorporated in solid solution in the titanium alloy . PACS 81.15.Fg; 68.55.-a; 87.68.+z     

Chemical treatment of TiO2-based coatings formed by plasma electrolytic oxidation in electrolyte containing nano-HA, calcium salts and phosphates for biomedical applications

TiO₂-based coatings were formed on titanium alloy by plasma electrolytic oxidation (PEO) in an electrolyte containing nano-HA, calcium salts and phosphates. Bioactive surface was formed after chemical treatment (NaOH aqueous solution) of the PEO coating. The surface of the PEO coating was mainly composed of Ti, O, Ca and P showing anatase and rutile; while that of the chemically treated PEO (CT-PEO) coating mainly contains Ti, O, Ca and Na showing anatase, rutile and amorphous phase. And the chemically treated surface exhibits dissolution of P and introduction of Na during the chemical treatment process. The chemical treatment has no effect on the chemical states of Ca and Ti of the PEO coating. In addition, the surface constituents of the CT-PEO coating show a uniform distribution near its surface with increasing depth. When incubated in a simulated body fluid for 7 and 14 days, the PEO coating does not exhibit apatite-forming ability; however, apatite was successfully deposited on the CT-PEO coating after 7 days probably due to the formation of hydroxyl functionalized surface, enhancing the heterogeneous nucleation of apatite. The addition of nano-HA in the electrolyte has effects on the surface character and apatite-forming ability of the PEO coating; however, it has no obvious influence on those of the CT-PEO coatings.     

Preparation of silicate tungsten bronzes on aluminum by plasma electrolytic oxidation process in 12-tungstosilicic acid

The growth of silicate tungsten bronzes on aluminum by plasma electrolytic oxidation in 12-tungstosilicic acid is experimentally investigated and discussed. Real time imaging and optical emission spectroscopy characterization of plasma electrolytic oxidation show that spatial density of microdischarges is the highest in the early stage of the process, while the percentage of oxide coating area covered by active discharge sites decreases slowly with time. Emission spectrum of microdischarges has several intensive band peaks originating either from aluminum electrode or from the electrolyte. Surface roughness of obtained oxide coatings increases with prolonged time of plasma electrolytic oxidation, as their microhardness decreases. Raman spectroscopy and energy dispersive X-ray spectroscopy are employed to confirm that the outer layer of oxide coatings formed during the plasma electrolytic oxidation process is silicate tungsten bronzes.     

钛铝合金高温氧化机理电子理论研究

为了从电子层面揭示钛铝合金高温氧化的物理本质,采用递归法与Castep相结合的方式, 计算了原子埋置能、亲和能、结合能等电子结构参数,探索合金氧化机理.研究表明: 氧在钛中有较大固溶度,氧原子可以在钛表面的基体内聚集,逐步向深层扩散. 氧与钛具备较强的亲和力,能形成钛的氧化膜.钛基体中铝原子间具有相互吸引力, 能形成铝的原子团簇.铝原子团簇中的钛原子间相互排斥与铝形成化合物. 铝、钛与氧的亲和能相近,不易发生铝的优先氧化,而是同时生成钛的氧化物和铝的氧化物. Al₂O₃比TiO₂的结合能略低,因而更加稳定,铝在TiO₂中有较大的固溶度, 能替换其中的钛形成更稳定的Al₂O₃氧化物.     

Characteristics of grain growth of microarc oxidation coatings on pure titanium

Grainy titania coatings are prepared by microarc oxidation on pure titanium （TA2） substrates in a Na2SiO3NaF electrolytic solution. The coating thickness is measured by an optical microscope with a CCD camera. Scanning electron microscope （SEM） and x-ray diffraction （XRD） are employed to characterize the microstructure and phase composition of coatings. The results show that the coating thickness increases linearly as the treatment time increases. The coatings are mainly composed of anatase and rutile （TiO2）. With the increase of treatment time, the predominant phase composition varies from anatase to rutile, which indicates that phase transformation of anatase into rutile occurs in the oxidizing process. Meanwhile, the size of grains existing on the coating surface increases and thus the surface becomes much coarser.     

Fretting wear behaviour of microarc oxidation coatings formed on titanium alloy against steel in unlubrication and oil lubrication

Ceramic coatings were formed on Ti6Al4V alloy surface by microarc oxidation (MAO) in a Na₂SiO₃ system solution. Unlubricated, smear oil and oil bath lubricated fretting tests were performed on MAO coatings against 52100 steel on a fretting wear tester. Microstructural investigation of the worn surfaces was performed and the wear mechanisms were studied. The results show that the coatings are mainly composed of rutile and a small amount of anatase TiO₂, both in nano grain structure. Friction coefficient of microarc oxidation coatings under oil bath lubrication was significantly reduced, favorable stable at 0.15, which indicates that the coatings with oil lubricated lowered the shear and adhesive stresses between contact surfaces, consequently alleviating the possibility of initiation and propagation of cracks in the inner layer of the coating or titanium alloy substrate.     

Characterization of oxide coatings formed on tantalum by plasma electrolytic oxidation in 12-tungstosilicic acid

Oxide coatings were formed on tantalum by plasma electrolytic oxidation (PEO) process in 12-tungstosilicic acid. The PEO process can be divided into three stages with respect to change of the voltage-time response. The contribution of electron current density in total current density during anodization results in the transformation of the slope of voltage-time curve. The surface morphology, chemical and phase composition of oxide coatings were investigated by AFM, SEM-EDX, XRD and Raman spectroscopy. Oxide coating morphology is strongly dependent of PEO time. The elemental components of PEO coatings are Ta, O, Si and W. The oxide coatings are partly crystallized and mainly composed of WO₃, Ta₂O₅ and SiO₂. Raman spectroscopy showed that the outer layer of oxide coatings formed during the PEO process is silicate tungsten bronze.     

Laser decoration of precious metals

A technique for the laser coloration of precious metals is described that is based on the oxidation of a titanium film deposited on the surface of a metal. When laser radiation acts on the film, it is heated and oxidizes. Depending on the radiation parameters, the resulting oxide films have different thicknesses and, due to light interference, they acquire different colors. The visible color of the surface depends on the angle of viewing after imaging. The aim of this work is to identify the color palette of a gold plate’s surface with a thin film of titanium deposited on it. The titanium film is oxidized via fiber laser irradiation with a wavelength of 1.064 μm. Samples of color palettes are examined spectrophotometrically, and the chemical and mechanical stability of the resulting oxide coatings are tested.     

Improvement of corrosion properties of microarc oxidation coating on magnesium alloy by optimizing current density parameters

The microstructure, composition and corrosion performance of oxide coatings formed on AM60B alloy using microarc oxidation techniques at different waveforms of applied current densities were investigated within this study. It is found that the use of optimizing current density waveforms, i.e. decaying freely current density in the later stage and stepped decreasing current density, significantly improved the microstructure of oxide coatings compared with the constant current density mode, which are connected with changes in behaviors of spark discharges on the surface in oxidation process. The optimal waveform of current density is showed to be decaying freely current density in the later stage, which results in sealing the originally formed large micropores. The optimisation of the microstructure results in a significant improvement of the corrosion resistance of oxide coating.     

Thermally oxidised rutile-TiO2 coating on stainless steel for tribological properties and corrosion resistance enhancement

In the present work, a novel process has been developed to improve the tribological and corrosion properties of austenitic stainless steels. Efforts have been made to deposit titanium coatings onto AISI 316L stainless steel by magnetron sputtering, and then to partially convert the titanium coatings to titanium oxide by thermal oxidation. The resultant coating has a layered structure, comprising of rutile-TiO₂ layer at the top, an oxygen and nitrogen dissolved α-Ti layer in the middle and a diffuse-type interface. Such a hybrid coating system showed good adhesion with the substrate, improved corrosion resistance, and significantly enhanced surface hardness and tribological properties of the stainless steel in terms of much reduced friction coefficient and increased wear resistance.     

Mechanism of formation of structure and phase composition in coatings produced by microarc oxidation on titanium alloys

The physicochemical properties of coatings produced by microarc oxidation on titanium alloys are investigated. The dependence of their structural and phase state on types of base alloys and electrolytic compositions is studied. The applicability of the titanium substrate-calcium phosphate coating system as a material for medical applications is substantiated. Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 6, pp. 72–76, June 2000.     

Oxidation of vanadium nitride and titanium nitride coatings

The oxidation of vanadium nitride (VN) and titanium nitride (TiN) coatings in ultra-high vacuum has been investigated in situ by X-ray photoelectron spectroscopy. On the VN coatings mixed oxide layers containing V³⁺ and V⁴⁺ species form at elevated temperatures (?600°C) and at high oxygen exposures, which cover completely the VN surface. Under similar oxidation conditions the TiN surface oxidises partially to a mixture of TiO₂ and Ti oxynitride (TiO_xN_y) phases. This oxidation behaviour has been correlated to the tribological properties of the VN and TiN coatings investigated recently.     

Effect of two-step functionalization of Ti by chemical processes on protein adsorption

Titanium and its alloys are widely used for orthopedic and dental implants because of their superior mechanical properties, low modulus, excellent corrosion resistance and good biocompatibility. However, it takes several months for titanium implants and bone tissue to reach integration. Hence, there is growing interest in shortening the process of osseointegration and thereby reducing surgical restrictions. Various surface modifications have been applied to form a bioactive titanium oxide layer on the metal surface, which is known to accelerate osseointegration.The present work shows that titanium dioxide (TiO₂) layers formed on titanium substrates by etching in a solution of sodium hydroxide (NaOH) or hydrogen peroxide/phosphoric acid (H₃PO₄/H₂O₂, with a volume ratio of 1:1) are highly suitable pre-treatments for apatite-like coating deposition. Using a two-step procedure (etching in an alkaline or acidic solution followed by soaking in Hanks’ medium), biomimetic calcium phosphate coatings were deposited on porous TiO₂ layers. The combined effects of surface topography and chemistry on the formation of the calcium phosphate layer are presented. The topography of the TiO₂ layers was characterized using HR-SEM and AFM techniques. The nucleation and growth of calcium phosphate (Ca-P) coatings deposited on TiO₂ porous layers from Hanks’ solution was investigated using HR-SEM microscopy. AES, XPS and FTIR surface analytical techniques were used to characterize the titanium dioxide layers before and after deposition of the calcium phosphate coatings, as well as after the process of protein adsorption. To evaluate the potential use of such materials for biomedical applications, the adsorption of serum albumin, the most abundant protein in the blood, was studied on such surfaces.     

Thermal behavior and catalytic activity in naphthalene destruction of Ce-, Zr- and Mn-containing oxide layers on titanium

The present paper is devoted to studies of the composition and surface structure, including those after annealing at high temperatures, and catalytic activity in the reaction of naphthalene destruction of Ce-, Zr- and Mn-containing oxide layers on titanium obtained by means of the plasma electrolytic oxidation (PEO) method. The composition and structure of the obtained systems were investigated using the methods of X-ray phase and energy dispersive analysis and scanning electron microscopy (SEM). It was demonstrated that Ce- and Zr- containing structures had relatively high thermal stability: their element and phase compositions and surface structure underwent virtually no changes after annealing in the temperature range 600-800 °C. Annealing of Ce- and Zr-containing coatings in the temperature range 850-900 °C resulted in substantial changes of their surface composition and structure: a relatively homogeneous and porous surface becomes coated by large pole-like crystals. The catalytic studies showed rather high activity of Ce- and Zr-containing coatings in the reaction of naphthalene destruction at temperatures up to 850 °C. Mn-containing structures of the type MnO_x + SiO₂ + TiO₂/Ti have a well-developed surface coated by “nano-whiskers”. The phase composition and surface structure of manganese-containing layers changes dramatically in the course of thermal treatment. After annealing above 600 °C nano-whiskers vanish with formation of molten structures on the surface. The Mn-containing oxide systems demonstrated lower conversion degrees than the Ce- and Zr-containing coatings, which can be attributed to substantial surface modification and formation of molten manganese silicates at high temperatures.     

Hydrogen accumulation and distribution during the saturation of a VT1-0 titanium alloy by an electrolytic method and from a gas atmosphere

The accumulation, distribution, and thermally stimulated release of hydrogen in a VT1-0 titanium alloy during electrolytic saturation and gas-phase saturation are studied. After electrolytic saturation, a 0.4-μm-thick surface layer consisting of δ hydrides with a binding energy of 108 kJ/mol forms in the alloy. The hydride dissociation after electrolytic saturation in heating occurs in the temperature range 320–370°C. After saturation from a gas atmosphere, δ hydrides with a binding energy of 102 kJ/mol form throughout the alloy volume. The dissociation of the hydrides formed during gas-phase saturation in heating occurs in the temperature range 520–530°C. A further increase in the temperature is accompanied by the transformation of titanium from the α into the β modification. At 690–720°C, the phase transformation is completed, and another hydrogen desorption peak appears in a thermally stimulated hydrogen desorption spectrum.