An investigation into the mechanical and tribological properties of plasma electrolytic oxidation and hard-anodized coatings on 6082 aluminum alloy

A ceramic coating on AA6082 aluminum alloy prepared by plasma electrolytic oxidation (PEO) has been studied and compared against a sulphuric acid hard-anodized coating on the same alloy. Surface morphology and microstructures of the coatings have been examined by scanning electron microscopy. X-ray diffraction is used to determine the phase composition of the coatings. The adhesion strength of the coatings has been evaluated using a scratch test method. The coating's mechanical properties such elastic modulus and hardness data have been generated using a dynamic ultra-microhardness tester. Sliding wear tests with different loading rates are performed on the coatings in order to assess their wear resistance. Test results show that the PEO treated samples exhibit significantly better mechanical properties compared to hard anodized samples. The elastic modulus and hardness of the PEO coating are 2-3 times greater than of the hard anodized coating and subsequently, an improved wear resistance of the PEO coating has been achieved. The mechanical properties of the coatings and their relations to their tribological performance are discussed.     

Fabrication and luminescence properties of europium oxysulfide/(ZrO<Subscript>2</Subscript> + TiO<Subscript>2</Subscript>)/Ti composites

Combining plasma electrolytic oxidation and extract pyrolysis, we have produced composite oxide coatings on titanium, which exhibit bright luminescence in the red spectral region. The present results and data in the literature suggest that combining these approaches is potentially attractive for the ability to produce composite coatings with various properties on the surface of valve metals and alloys.     

Formation and bioactivity of porous and nanostructured TiO2/beta-TCP coating on titanium

Titanium and its alloys have been widely used as hard tissue implants due to their excellent mechanical properties and biocompatibility. However, their near bio-inertness and metallic ion release are still the problems with clinical uses. In this paper, porous and nanostructured TiO2/beta-tricalcium phosphate (beta-TCP) composite coatings were prepared on titanium substrates by plasma electrolytic oxidation (PEO) in a Ca and P-containing electrolyte. The influence of PEO electric current density on phase composition and bioactivity of the coatings were studied. X-ray diffraction, scanning electron microscopy and Fourier transfer infrared spectroscopy were utilized to characterize the phase composition and microstructure of the coatings. Simulated body fluid immersion tests were employed on the coatings to evaluate their bioactivity. The results reveal that TiO2/beta-TCP composite coating with pores size less than 10 microm and grains of 50-100 nm in size was prepared. The electric current density of PEO is an important factor in the formation of the composite coating. The TiO2/beta-TCP composite coating shows good bioactivity, which are attributed to the incorporation of beta-TCP.     

Coatings of Aluminide Intermetallic Compounds on Steel Utilizing a Hybrid Technique of Spraying and IR-Laser Fusion

Titanium aluminide coatings were produced using a hybrid technique of arc-spraying followed by IR-laser fusion in an argon atmosphere. A titanium coating free of oxides was deposited onto a low-alloy steel by DC-arc spraying in argon. Optimal laser irradiation conditions and the amount of preplaced aluminum powder on the sprayed titanium were determined to obtain a composite coating of TiAl₃+Al of 150 urn thickness. Metallurgical and mechanical properties were examined using acoustic emission. The oxidation resistance of the coating was excellent up to 1173 K because of a protective alumina layer. Growth of the TiAl₃interlayer by diffusion of aluminum into titanium improved the corrosion resistance. The intermetallic coadng showed microcracking at ambient temperature, but possessed capability for filling and healing of cracks with alumina and titanium nitride during high-temperature exposures. However, at temperatures higher than 1200 K, the oxidation performance decreased by diffusion of iron into the coating     

钛合金微弧氧化一步制备含石墨的减摩涂层

为了进一步提高钛合金表面微弧氧化陶瓷涂层的摩擦磨损性能,在石墨分散的Na2CO3-Na2SiO3-KOH电解液溶液中一步制备了含自润滑微粒的微弧氧化复合涂层.利用X射线衍射仪(XRD)、X射线光电子能谱仪(XPS)和扫描电子显微镜(SEM)研究了未添加和添加石墨微弧氧化涂层的相组成和微结构,采用往复式球-盘试验机评价了两种涂层的摩擦学性能.结果表明:加入到电解液中的石墨在微弧氧化过程中进入到涂层中,从而得到含有固体润滑微粒的复合涂层;在干摩擦条件下,含石墨的微弧氧化涂层相比于不含石墨的涂层具有更小的摩擦系数.     

Study of nanocrystalline plasma electrolytic carbonitriding for CP-Ti

Commercially pure titanium is subjected to carbonitriding according to a new procedure called plasma electrolytic carbonitriding. The influence of voltage and time on the thickness of carbonitrided layers is investigated. The kinetics of growth of the layers is analyzed according to their thickness and microhardness. The coatings formed on the titanium substrate are multilayer. They are formed by a combination of TiN, TiO₂, and the α-phase in the form of inclusions of high concentrations of nitrogen and carbon in a close-packed hexagonal crystal lattice of titanium. All layers are very hard with hardnesses varying from 1100 (Vickers-Knoop hardness) for the TiN layer and 900 for the α-phase to 250 for the nonmodified surface. There exist strong metallic bonds between the layers of the coating. The time dependence of the form t ^1/2 is established for the diffusion of nitrogen and carbon in the outer layers. The results of wear tests reveal that the mass losses of carbonitrided samples become 4–17 times lower than for the intact samples. Thus, the procedure of plasma electrolytic carbonitriding enables one to get highly efficient stable coatings on titanium. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 6, pp. 36–42, November–December, 2007.     

反应等离子喷涂TiN/Ti3O复相陶瓷涂层

采用等离子反应合成技术,制备出了TiN/Ti3O复相陶瓷涂层,并分析了复相涂层的组织及其性能.研究结果表明:复相涂层主要由TiN相组成,并含有少量的钛的氧化物;复相涂层具有典型的层状组织结构,且层与层之间结合较好;制备的复相涂层的韧性得到明显提高,其韧性优于等离子喷涂Al2O3陶瓷涂层;特别是复相涂层具有优于M2钢的耐磨性.     

In vitro fatigue behavior of surface oxidized Ti35Zr10Nb biomedical alloy

New compositions of titanium alloys with low Young's modulus as well as multiple surface biofunctionalities are under intense research focus for biomedical applications due to the proven ability of titanium for enhancing implant integration. This study presents the effect of plasma electrolytic oxidation coating on the fatigue response of a novel β-Ti35Zr10Nb alloy tested under physiological conditions (Hanks' solution at 37 °C). The electrolytic oxidation was conducted in calcium acetate/calcium glycerophosphate electrolyte that allowed incorporation of Ca and P within the oxide layer with potential benefits for bone apposition. The fatigue results revealed that the presence of oxide layers decreased significantly the fatigue strength of Ti35Zr10Nb alloy relative to uncoated condition. The specific coating morphology featuring interconnected micropores, microcracks, a scalloped coating/substrate interface, as well as the ceramic nature of the oxide layer was identified as the main factors responsible for the lower fatigue performance of the coated titanium alloy.     

Nanostructured Al2O3–TiO2 coatings for high-temperature protection of titanium alloy during ablation

Plasma-sprayed nanostructured Al₂O₃–13 wt.%TiO₂ coatings were successfully fabricated on titanium alloys (Ti–6Al–4V) using as-prepared feedstock. Ablation experiments for the titanium alloy samples with or without a coating were carried out using a Metco 9MB plasma gun. The microstructure, phase constituents and mechanical properties of the titanium alloys before and after ablation were investigated by scanning electron microscope (SEM), X-ray diffractometer (XRD) and Vickers hardness tester. The surface morphologies, cross-sectional microstructure and hardness of titanium alloys with coatings are similar before and after ablation. In contrast, the microstructure and mechanical properties of the titanium alloy without coating are significantly changed after ablation. The surface coating is found to serve as a protective coating during ablation.     

Influence of zirconia on biomimetic apatite formation in pure titanium coated via plasma electrolytic oxidation

This work demonstrated the effect of zirconia incorporation on the formation of biomimetic apatite in pure titanium coated by plasma electrolytic oxidation (PEO) method. To incorporate zirconia particles into the oxide layer, electrochemical coating was carried out under AC condition in an electrolyte containing zirconia powder. After PEO coatings, zirconia particles were distributed uniformly throughout the titanium oxide layer while the size and distribution of micro-pores remained unchanged when compared to titanium coated by PEO in an electrolyte without zirconia. It was found that a number of fine zirconia particles played an important role in triggering the occurrence of biomimetic apatite on top of the PEO-coated titanium in a simulated body fluid solution. This was mainly attributed to increased surface roughness of the oxide layer as well as inherent activation of zirconia particles to form biomimetic apatite.     

等离子体电解氧化制备生物活性多孔纳米TiO2涂层

采用等离子体电解氧化(PEO)技术,在纯钛基体上制备了具有多孔和纳米结构并含有不同钙磷含量的氧化钛涂层.采用扫描电子显微镜和X射线衍射仪研究了涂层的显微结构和相组成.采用模拟体液浸泡实验评价涂层的生物活性.研究结果表明:涂层主要由锐钛矿和金红石相组成,涂层表面孔径小于10μm,晶粒直径约10～100nm,且涂层表面晶粒、涂层厚度、表面粗糙度和涂层中的钙磷元素含量均随着PEO过程中电流密度的增大而增加.PEO涂层在模拟体液中浸泡14 d即能够诱导类骨磷灰石在其表面沉积,显示出良好的生物活性.涂层中的钙磷含量和结晶形态对其生物活性起关键作用.     

Microstructure and thermal behaviour of plasma sprayed zirconia/alumina composite coating

In thermal barrier coatings (TBC), failure occurs near or at the interface between the metallic bondcoat and topcoat. On high temperature conditions, an oxide scale which is named thermally grown oxide (TGO) occurs along the bond/topcoat interface. For diminishing the creation of TGO, a dense coating with low residual stress and thermal stress buffer layer was preferable. High hardness ceramic coatings could be obtained by gas tunnel type plasma spraying, and the deposited coating had superior property in comparison with those deposited by conventional type plasma spray method. In this study, the gas tunnel type plasma spraying system was utilized to produce a zirconia/alumina functionally graded thermal barrier coating and discussed its physical and mechanical properties, thermal behavior and high temperature oxidation resistance of the coating are discussed. Consequently, the proposed system exhibited superior mechanical properties and oxidation resistance at the expenses of a slightly lower thermal insulating effect. This interlayer is preferred in order to minimize the detrimental effect of the phase transformation of gamma-Al2O3 to alpha-Al2O3.     

Incorporation of carbon nanotubes into micro-coatings film formed on aluminum alloy via plasma electrolytic oxidation

The influence of the incorporation of carbon nanotubes (CNTs) on the coating structure of aluminum alloy processed by plasma electrolytic oxidation (PEO) has been studied as a function of the current density. A series of PEO coatings was applied in a silicate-electrolyte containing CNTs at three current densities, such as 50, 100, and 150 mA/cm². As the current density increased, the responding voltage also increased due to a gradual increment in the amount of CNTs incorporated uniformly into the coating film. In addition, a number of CNTs were observed mainly near micro-pores formed in the coating film by plasma explosive arcs, resulting in a fairly uniform coating structure with a low density of micro-pores. This phenomenon was discussed based on the electrophoretic activity of CNTs.     

Uniform porous and functionally graded porous titanium fabricated via space holder technique with spark plasma sintering for biomedical applications

Porous materials with low stiffness and high strength are sought as implant materials to prevent stress shielding and fracture during in vivo use. This study proposes a powder metallurgy-based space holder technique to fabricate porous titanium with mechanical performance suitable for implant materials. Mixed powders of titanium and sodium chloride were sintered at low temperature using spark plasma sintering, and then the sodium chloride was dissolved in water. As a result, uniform porous titanium (UP-Ti) with a wide range of microstructures: porosity from 26% to 80% and average pore size from 75 μm to 475 μm was successfully fabricated. Also, functionally graded porous titanium (FGP-Ti) was successfully fabricated, in which porous titanium with high porosity and dense titanium were placed at the inside and surface, respectively. The stiffness of UP-Ti was comparable to that of natural bones, but its strength was lower than that of natural bones, which would be insufficient for use as an implant. In contrast, the mechanical performance of FGP-Ti was improved, compared with UP-Ti with the porosity comparable to the average porosity of FGP-Ti: its strength was higher than that of natural bones and its stiffness was comparable to that of natural bones. These results imply that porous titanium, especially functionally graded porous titanium, is a candidate metal for implants used to replace heavily loaded natural bone.     

Single‐layer and multi‐layer wear‐resistant coatings

In this paper the formation of wear‐resistant coatings based on titanium and aluminum compounds using vacuum arc discharge and molecular nitrogen as a working gas is discussed. The experiments were carried out using an installation containing two independent evaporators and a system for attenuation and purification of the plasma flow. To obtain a high‐quality coating, it is necessary to ensure the equality of the ion flux densities coming to the substrate. The results of the experiments show that by changing the bias voltage on the substrate it is possible to adjust the content of elements in the coating and thus to control its parameters. Multi‐layer coatings have better performance characteristics, but require an improved degree of purification of the plasma flow from the droplet fraction.     

Synthesis of a two-phase nanopowder from prototype human synovial fluid and the use of the nanopowder for the preparation of coatings on titanium plates

Using prototype human synovial fluid, we have synthesized a calcium phosphate nanopowder consisting of two crystalline phases (whitlockite and carbonate-containing hydroxyapatite). The nanopowder was used to produce calcium phosphate (monetite) coatings on titanium substrates by microarc oxidation. It has been shown that, with increasing microarc oxidation voltage (from 200 to 300 V), the mass, thickness, roughness parameter, and Ca : P ratio of the coatings increase linearly. Increasing the voltage to 300 V and the calcium phosphate coating growth time to 10 min allows a single-phase coating up to 100 μm in thickness, consisting of monetite nanocrystals, to be produced on a metallic surface and its adhesion strength to be improved.     

纳米石墨/聚氨酯复合涂层的制备及耐腐蚀性能

为研制高性能电力金具防护涂层,首先,以羟基丙烯酸树脂和异氰酸酯为主要成膜物质,纳米石墨为填料,制备了不同纳米石墨含量的纳米石墨/聚氨酯复合涂料;然后,将涂料喷涂在电力热镀锌钢上,固化干燥后得到纳米石墨/聚氨酯复合涂层;最后,测试了纳米石墨/聚氨酯复合涂层的力学性能和耐磨性,并采用模拟酸雨试验、中性盐雾试验及电化学阻抗谱（EIS）研究了纳米石墨/聚氨酯复合涂层的耐腐蚀性能。结果表明:添加纳米石墨后,涂层与热镀锌钢的附着力有所提高,纳米石墨含量为2.0wt%的纳米石墨/聚氨酯复合涂层的耐磨性比未添加纳米石墨的空白涂层提高了92%,并且涂层中纳米石墨的分布较均匀,表现出良好的耐腐蚀性能。 所得结论表明在涂层中添加适量的纳米石墨可以提高涂层的耐磨性和耐腐蚀性能,进而可将涂层用于电力金具的表面防护。      

YSZ-Ti3AlC2热障涂层及其高温自愈合行为

采用Ti₃AlC₂作为新型自愈合剂, 利用大气等离子喷涂将混合均匀的YSZ-Ti₃AlC₂粉体制成厚涂层。为观测高温下涂层氧化及裂纹的自愈合行为, 通过外加载荷的方式在涂层表面预制裂纹, 并将样品置于1050℃空气气氛中进行热处理。通过分析涂层制备、热处理前后的物相和形貌演变发现：涂层中的部分Ti₃AlC₂在喷涂后分解为TiC, 热处理后涂层表面形成外层为TiO₂, 内层为TiO₂和Al₂O₃混合物的双层结构。在自愈合过程中, 裂纹内的愈合剂氧化生成Al₂O₃与低密度的TiO₂, 随着扩散控制的氧化反应不断进行, 氧化物逐渐积累并填补裂纹。此外, 在TiO₂生成的同时引起的体积膨胀使裂纹周围产生一定的压应力, 强化愈合效果, 最终完全愈合裂纹。     

Plasma Sprayed Copper-Nickel-Indium Coatings on Titanium Alloys

Copper-nickel-indium coatings were deposited on the surface of titanium alloy specimens by plasma spraying. The structure and properties of the coatings were studied thoroughly using optical and scanning electron microscopy. The mechanical behavior of the Cu-Ni-In coated titanium substrates was examined. The coating was found to decrease the tensile strength of the titanium alloy substrate. The elastic modulus and the ultimate tensile strength of the coated system were found to decrease with increasing coating thickness. The residual stresses of the coated system were also calculated.     

Modification of NiCoCrAlY with Pt: Part I. Effect of Pt depositing location and cyclic oxidation performance

Pt layers of 5?μm in thickness were electroplated before or after depositing NiCoCrAlY coating by arc ion plating (AIP) aiming for identifying the effect of Pt enriching position on microstructure and cyclic oxidation behavior of Pt modified NiCoCrAlY coatings. Al-rich zones formed at the same position of Pt-rich zones for both modified coatings due to uphill diffusion of Al driven by Pt. Cyclic oxidation tests at 1000 and 1100?°C indicated that oxidation resistance of NiCoCrAlY was improved by Pt modification via different mechanisms: at surface, Pt-rich zone promoted selective oxidation of Al to form α-Al₂O₃, whilst at coating/substrate interface Pt-rich zone acted as effective diffusion barrier for titanium. Roles of Pt played in enhancing the oxidation performance of various Pt-modified NiCoCrAlY coating were investigated.