A perspective of microplasma oxidation (MPO) and vapor deposition coatings in surface engineering of aluminum alloys

Over the past years, great achievements have been made in the development of coating technologies for surface improvement of aluminum alloys. Despite these achievements, the role in the market strongly depends on the ability of surface coating technology under technical and economic considerations to meet the increased demands for heavy tribological applications of aluminum alloys. Microplasma oxidation (MPO) technology has recently been studied as a novel and effective means to provide thick and hard ceramic coating with improved properties such as excellent load-bearing and wear resistance properties on aluminum alloys. The present work covers the evaluation of the performances of current single and duplex coatings combining MPO, physical vapor deposition (PVD), and plasma assisted chemical vapor deposition (PACVD) coatings on aluminum alloys. It suggests that the MPO coating is a promising candidate for design engineers to apply aluminum alloys to heavy load-bearing applications. The prospective future for the research on MPO coatings is introduced as well.

Compounded Surface Modification of ZK60 Mg Alloy by High Current Pulsed Electron Beam+Micro-plasma Oxidation

In this study, compounded surface modification technology-high current pulsed electron beam （HCPEB） ＋ micro-plasma oxidation （MPO） was applied to treat ZK60 Mg alloys. The characteristics of the microstructure of ZK60 Mg alloy after single MPO and HCPEB＋MPO compounded treatment were investigated by SEM. The results showed that the density of the ceramic layer of HCPEB＋MPO-treated ZK60 Mg alloy was improved and defects were reduced compared to that under MPO treatment alone. Surface modified layer of ZK60 Mg alloys treated by HCPEB＋MPO was divided into three zones, namely the top loose ceramic zone, middle compact zone and inside HCPEB-induced melted zone. Corrosion resistance of ZK60 Mg alloy before and after the compounded surface modification was measured in a solution of 3.5% NaCl by potentiodynamic polarization curves. It was found that the corrosion current density of ZK60 Mg alloys could be reduced by about three orders of magnitude, from 311μA/cm^2 of the original sample to 0.2μA/cm^2 of the HCPEB＋MPO-treated sample. This indicates the great application potential of the HCPEB＋MPO compounded surface modification technology in improving the corrosion resistance of ZK60 Mg alloys in the future.

Effects of cathodic component of current on porosity and hardness characteristics of micro plasma oxidation(MPO) coatings on aluminum alloy

Micro plasma oxidation(MPO) has recently been investigated as a novel, rapid and effective means to provide modified surfaces with improved properties of load bearing and wear resistance on light alloys particularly aluminum alloys. MPO is a multifactor-controlled process, these factors must be controlled to produce high quality coatings. The main research emphasis in MPO coating development over the past years seems to be the attainment of higher hardness levels and thick coatings. The porosity of MPO coating is the most complex phenomenon affecting the distribution, levels and the measurements of the hardness; and it is controlled by suitable selection of important parameters such as the electrical conditions. Ceramics coatings were synthesized on Al substrate by MPO to examine the effects of adding a cathodic phase alternated with anodic-cathodic current on the porosity and hardness characteristics of coatings by X-ray diffraction(XRD), scanning electron microscopy(SEM), and microhardness tester. The coatings produced by the combined mode are more dense and less porous than that by the anodic-cathodic mode. (Microhardness) test shows that the coatings produced by the combined mode exhibit both the highest hardness, and less reduction percentage in hardness with increasing the coatings thickness. These improvements become more significant for the polished and thicker coatings.

Compound Ceramic Coatings on Ti-6Al-4V by Micro-Plasma Oxidation in NaAlO_2 Solution

Micro-plasma oxidation (MPO) technique is a new technique by which compound ceramic coating can be grown in situ on Al, Ti, Mg and many other valve-metals. Compound ceramic coatings on Ti-6Al-4V alloy were prepared for different time by pulsed bi-polar MPO in NaAlO2 solution. The phase composition, morphology and the element distribution of the coatings were studied by XRD, SEM and JEOL SUPERPROBE 733 electric probe, respectively. Electrochemical Impedance Spectra (EIS) of the coatings were measured to study the structure character of the coatings. Through the proper EIS interpreting software, the "equivalent circuit" of the coatings was established, and the fitting values of equivalent element were calculated. The coating is mainly composed of Al2TiO5, a-A12O3 and rutile TiO2. The content of Ti element in the coating is less than that of substrate; its distribution is relatively uniform, and Ti in the outer layer is less than that in the inner layer. The content of Al in the coating is more than that of substrate, and its distribution is not even: Al in the middle part is more than that on both sides of the coating; And the EIS analysis has illustrated the double-layer structure of the coatings, and the outside layer is loosen and the inner layer compact; with the increase of the oxidizing time, the surface roughness of the coatings, and the porosity of the outer layer of the coating are increased while the compactness of the inner layer of the coatings are improved.

Compound Ceramic Coatings Grown by Micro-plasma Oxidation on Ti-6Al-4V Alloy

Compound ceramics coatings on the Ti-6Al-4V alloy were prepared by the direct current micro-plasma oxidation （MPO） in NaAlO2 solution. The composition and morphology of the coatings were studied with the X-ray diffraction （XRD） and the scanning electron microscopy （SEM）, respectively. Inductively coupled plasma atomic emission spectrometer technique was used to analyze the solution features of Ti-6Al-4V alloy in the process of preparation. The results reveal that Al2TiO5 forms in the coatings at the initial stages of MPO reaction, and its content changes rapidly with the reaction continuing： after 20 min, the ceramics coatings are composed of α-Al2O3, 7-Al2O3 and Al2TiO5, but after 40 min, its main composition is of α-Al2O3. The content of Ti in the solution will increase when the MPO time extends, and as will Al in the anode area until, after 30 min, it reaches the maximum and keeps constant from then on. Both substrata of Ti and Al in the electrolyte join the MPO reaction at the initial stage, where the formation of Al2TiO5 happens; but as the MPO reaction prolongs, more and more Al in the electrolyte will take part in the reaction, leading to the appearance of a large amount ofAl2O3.

Preparation of micro/nano-structured ceramic coatings on Ti6Al4V alloy by plasma electrolytic oxidation process

In order to improve the osseointegration and antibacterial activity of titanium alloys,micro/nano-structured ceramic coatings doped with antibacterial element F were prepared by plasma electrolytic oxidation(PEO)process on Ti6Al4V alloy in NaF electrolyte.The influence of NaF concentration(0.15-0.50 mol/L)on the PEO process,microstructure,phase composition,corrosion resistance and thickness of the coatings was investigated using scanning/transmission electron microscopy,energy dispersive spectroscopy,atomic force microscopy,X-ray diffractometer,and potentiodynamic polarization.The results demonstrated that Ti6Al4V alloy had low PEO voltage(less than 200 V)in NaF electrolyte,which decreased further as the NaF concentration increased.A micro/nano-structured coating with 10-15μm pits and 200-800 nm pores was formed in NaF electrolyte;the morphology was different from the typical pancake structure obtained with other electrolytes.The coating formed in NaF electrolyte had low surface roughness and was thin(<4μm).The NaF concentration had a small effect on the phase transition from metastable anatase phase to stable rutile phase,but greatly affected the corrosion resistance.In general,as the NaF concentration increased,the surface roughness,phase(anatase and rutile)contents,corrosion resistance,and thickness of the coating first increased and then decreased,reaching the maximum values at 0.25 mol/L NaF.

Micro and nano-enabled approaches to improve the performance of plasma electrolytic oxidation coated magnesium alloys

Magnesium(Mg)and its alloys have become a hot research topic in various industries owing to the specific physical and chemical properties.However,high corrosion rate is considered the key lifetime-limiting.Plasma electrolytic oxidation(PEO)method is a simple strategy to deposit an oxide layer on the surface of light metals such as magnesium alloys,to control corrosion rate and promote some other properties,depending on their performances.Nevertheless,their features including their micropore size,distribution,and interconnectivity,and microcracks have not been improved to an acceptable level to support long-term performances of the magnesium-based substrates.Studies have introduced micro/nano-enabled approaches to enhance various properties of PEO coatings such as corrosion resistance,tribological properties,self-healing ability,bioactivity,biocompatibility,antibacterial properties,or catalytic performances.These strategies consist of incorporating of micro and nanoparticles into the PEO layers to produce multi-functional surfaces or the formation of multi-layered coatings to cover the defects of PEO coatings.In this perspective,the present paper aims to overview various nano/micro-enabled strategies to promote the properties of PEO coatings on magnesium alloys.The main focus is given to the functional changes that occurred in response to the incorporation of various types of nano/micro-structures into the PEO coatings on magnesium alloys.

Salt spray corrosion test of micro-plasma oxidation ceramic coatings on Ti alloy

Ceramic coatings were prepared on Ti-6Al-4V alloy in NaAlO2 solution by micro-plasma oxidation （MPO）. The salt spray tests of the coated samples and the substrates were carded out in a salt spray test machine. The phase composition and surface morphology of the coatings were investigated by XRD and SEAM. Severe corrosion occurred on the substrate surface, while there were no obvious corrosion phenomena on the coated samples. The coatings were composed of Al2TiO5 and a little α-Al2O3 and mille TiO2, and the salt spray test did not change the composition of the coatings. The weight loss rate of the coatings decreased with increasing MPO time because of the increase in density and thickness of the coatings. The surface morphology of the coatings was influenced by salt spray corrosion test. Among the coated samples, the coating prepared for 2 h has the best corrosion resistance under salt spray test.

Catalytic technology for water treatment by micro arc oxidation on Ti-AI alloy

The feasibility of the formation of a liquid plasma catalysis system through micro arc oxidation（MAO） under AC power with titanium-aluminum alloy electrodes was investigated.In the decolorization of organic dyeing wastewater simulated with Rhodamine B,Ti-Al alloy electrodes were superior over Ti electrodes and Al electrodes.The optimal molar percentage of Ti in alloy electrodes was 70%and the optimal decolorization rate was up to 88.9%if the additive suitable for Al was added into the solution to be treated.The decolorization rates were the same in the case of the alloy-alloy electrodes and alloy-Al electrodes.The proportion of the effects of plasma,TiO2 catalyzer during MAO and H2O2 after MAO in decolorization has been obtained.With the catalysis of TiO2 formed on the electrodes,the reaction rate was improved by a maximum of 95%and the decolorization rate was improved by a maximum of 71.6%.Based on the spectral analysis,the plasma catalysis mechanism has been studied.

Micro Arc Oxidation of Wire Arc Sprayed Al-Mg6, Al-Si12, Al Coatings on Low Alloyed Steel

Micro arc oxidation of wire arc sprayed Al-Mg6, Al-Si12 and Pure Al coatings on low carbon steel has been performed. The coatings have been analyzed using optic microscope, scanning electron microscopy, X-ray diffraction and surface roughness tester. At the same time, voltage and current regimes are investigated during the process. Then after MAO process, uniform Al2O3 ceramic coatings have been deposited on surface of Al-Mg6, Al-Si12 and Al coated steel. The ceramic coatings are mainly composed of Al2O3 phase. The compound coatings show high hardness and significant improvement of corrosion resistance property.

铝合金在水玻璃-KOH-NaAlO_2体系中的微等离子体氧化

研究了水玻璃 KOH NaAlO2 体系对LY12铝合金微等离子体氧化过程的影响 ,考察了不同的NaAlO2浓度下 ,氧化过程电压时间曲线的变化特点 ,测定了膜层的硬度、击穿电压。运用XRD ,SEM ,EPMA研究了膜层的相组成、形貌及元素分布 ,发现膜层由莫来石相和γ Al2 O3 相组成 ,具有疏松层和紧密层两层结构 ,铝元素含量沿着从基体到膜层的方向逐渐减少 ,而硅元素含量却逐渐增加。

磷酸盐对钛合金微等离子体氧化陶瓷膜结构和耐蚀性的影响

研究了3种磷酸盐分别作为电解质交流微等离子体氧化TC4钛合金所制得的陶瓷膜的耐蚀性,其中3号磷酸盐的效果最好.并在铝酸钠体系和硅酸钠体系下,进一步研究了3号磷酸盐对微等离子体氧化陶瓷膜的结构、形貌、成分和耐蚀性能的影响:磷在陶瓷膜中不形成晶相,而是以非晶相物质沉积在膜层中;铝酸钠体系,磷元素由膜内层向外层逐渐减少;硅酸钠体系,磷元素在膜内、外两层含量高,中间处含量低;两体系下,3号磷酸盐的用量适当增加,陶瓷膜耐点蚀性提高.

重铬酸钾对钛合金表面微等离子体氧化陶瓷膜结构和耐蚀性的影响

在钛合金表面用微等离子体氧化能产生一层陶瓷膜。将重铬酸钾引入磷酸盐电解液将使钛合金微等离子体氧化过程的槽电压升高 ,经对所得微等离子体氧化陶瓷膜表面和截面进行扫描电镜 (SEM )形貌观察 ,发现重铬酸钾的加入使陶瓷膜的致密性增加。XRD分析表明 ,不同电解液中所得膜层都是以锐钛矿型TiO2 为主晶相 ,同时还含有少量的金红石型TiO2 ,重铬酸钾的加入使锐钛矿型TiO2 的含量增加。对膜层进行电偶电流和循环伏安测试表明 ,重铬酸钾的加入使所得的膜层对金属的接触腐蚀降低 。

微等离子体氧化Al_2O_3陶瓷膜的组织结构与形成过程

The alumina ceramic coatings were prepared on aluminum alloy using micro-plasma oxidation. The structure and morphologies of ceramic coatings were studied by X-ray diffraction (XRD) and scanning electron microscope (SEM), and then its formation process was investigated. The results of XRD reveal that with increasing oxidation time the content of decreases, and in which the content of al reaction occurs between the aluminum from the substrate and the oxygen and other ions from the electrolyte. The reaction products then are propelled away from the inner-wall of the discharging channels. Finally, these products agglomerate in the inner-wall of the discharging channels and on the surface near the discharging channels to produce the ceramic coatings. The change of ceramic coatings thickness with increasing oxidation time divides into two stages, and the final thickness is formed in first stage. With increasing current density the final thickness of ceramic coatings increases.

纯铝及其合金的微等离子体氧化成膜特征

研究了纯Al及其合金LC9、LY12在偏铝酸钠溶液中微等离子体氧化成膜的特点。通过X射线衍射,SEM和EMPA等分析了陶瓷膜层的相组成及表面、截面形貌和膜层截面的元素含量分布,利用测厚仪测量了陶瓷膜层的厚度。结果显示:Al膜层由αAl2O3相组成,LC9膜层由γAl2O3相组成,LY12膜层由γAl2O3和αAl2O3两相组成,且膜层都由致密层和疏松层构成;膜层的厚度随时间都近似呈线性变化;在Al和LY12膜层中,由内向外铝元素的含量逐渐增加;在LC9膜层中,铝元素的分布略呈两侧高中间低;在LC9和LY12膜层中,镁元素分布比较均匀;在LC9膜层中,含有很少量的锌元素,而铜元素含量很不明显;在LY12膜层中含有很少量的铜元素。

微等离子体氧化制备TiO_2薄膜的结构特性

利用微等离子体氧化方法在纯Ti金属表面生长出一层厚度达20 mm的陶瓷氧化膜,并用X射线衍射、扫描电镜初步研究了氧化膜的组织结构和表面形貌。在不同电解质溶液条件下,得到3种不同类型结构组成的氧化膜:单一TiO2的锐钛型结构、TiO2锐钛型+金红石型的混合相结构、单一TiO2金红石结构。TiO2单一的锐钛型结构的氧化膜薄膜比较粗糙,TiO2金红石结构的氧化膜表面比较平滑,而混合相结构陶瓷膜的表面层为疏松的锐钛型结构TiO2,亚表层为金红石结构的TiO2。

(NaPO_3)_6-NaAlO_2体系铝合金微等离子体氧化研究

研究了六聚偏磷酸钠 -偏铝酸钠溶液中 ,L Y12铝合金微等离子体氧化过程、陶瓷膜的形成特点以及偏铝酸钠的浓度对氧化过程的影响。通过

微弧氧化α-Al_2O_3陶瓷膜生长过程分析

在交流条件下利用微等离子体氧化技术合成了氧化铝陶瓷涂层。XRD分析结果显示,涂层中-Al2O3相与- Al2O3相的含量随反应时间而变化。氧化2.5 h后可以得到由单一的-Al2O3相组成的涂层。陶瓷涂层厚度的生长速率大约为0.7 m/min;在135 min之后,涂层的厚度基本不变。氧化最初的15 min之内,涂层的表面密度仅为0.088 mg/cm2,然后表面密度增大。在氧化15 min^60 min之内,表面密度(y)与氧化时间呈直线关系。在氧化60 min之后,y的增长速率逐渐减小。

CNTs-SiC/Al_2O_3-TiO_2复合涂层的制备及其性能

采用微弧等离子喷涂技术制备了CNTs-SiC/Al2O3-TiO2复合涂层,借助SEM、X射线衍射、仪热分析仪和网络分析仪对CNTs-SiC/Al2O3-TiO2涂层的组织结构、高温氧化性能、电磁特性进行了测试分析.结果表明:多功能微弧等离子喷涂枪内中心轴向送粉方式制备的CNTs-SiC/Al2O3-TiO2复合涂层的组织结构致密、孔隙率低,SiC和CNTs的物相也保留下来.CNTs-SiC/Al2O3-TiO2涂层的高温氧化性能有所提高,涂层在35～700℃升温阶段失重率为0.71wt%,700℃恒温氧化60min后失重率为0.41wt%.随着频率的增加,CNTs-SiC/Al2O3-TiO2复合粉末的介电常数的实部从17.3下降到10.3,而虚部在6.3～2.9之间,具有频散效应.制备的CNTs-SiC/Al2O3-TiO2复合涂层在一定的范围内随着涂层厚度的增加,吸波能力显著提高,其谐振频率不断向低频移动.

钛表面原位生长钛酸锶钡陶瓷膜

用微等离子体氧化法在钛金属基体上制备钛酸锶钡(BST)陶瓷膜。讨论了Ba/Sr的摩尔比对陶瓷膜相组成的影响,并利用X射线衍射(XRD)、扫描电镜(SEM)和电子探针(EPMA)等测试手段对陶瓷膜的相组成、形貌特征和元素分布进行了分析。结果表明,在所选定的工艺参数条件下可得到钛酸锶钡陶瓷膜,膜层由接近于纯相的钛酸锶钡构成,表面相对较为平整,各元素在陶瓷膜层中分布均匀,无梯度变化。