Effect of frequency and duty cycle on corrosion behavior of pulsed nanocrystalline plasma electrolytic carbonitrided CP-Ti

Potentiodynamic polarization and electrochemical impedance spectroscopy were employed to test carbonitrided CP-Ti, treated by a relatively new method called pulsed-plasma electrolytic carbonitriding. The results show excellent corrosion resistance for modified CP-Ti. The effect of frequency and duty cycle of pulsed current were investigated. It was found that pulse frequency and duty cycle affect the size and porosity of nanocrystalline carbonitrides and by controlling these effective parameters surface modification can render the CP-Ti material extremely corrosion resistant.     

Study of bipolar pulsed plasma electrolytic carbonitriding on nanostructure of compound layer for a gamma Ti-Al alloy

The surface hardening of a gamma Ti-Al alloy by using bipolar pulsed nanocrystalline plasma electrolytic carbonitriding has been studied in this investigation. Coating process was performed on a triethanolaminebased electrolyte by a cooling bath. The nanostructure of the obtained compound layer was examined with the figure analysis of the scanning electron microscopy (SEM) nanographs. The effects of the process variables, i.e., frequency, temperature of the electrolyte, applied voltage and treatment time, have been experimentally studied. Statistical methods were used to achieve the optimum size of the nanocrystals. Finally, the contribution percentage of the effective factors of the pulsed current was revealed, and the confirmation run showed the validity of the obtained results.     

Study of bipolar pulsed plasma electrolytic carbonitriding on nanostructure of compound layer for a gamma Ti-Al alloy

The surface hardening of a gamma Ti-Al alloy by using bipolar pulsed nanocrystalline plasma electrolytic carbonitriding has been studied in this investigation. Coating process was performed on a triethanolamine-based electrolyte by a cooling bath. The nanostructure of the obtained compound layer was examined with the figure analysis of the scanning electron microscopy (SEM) nanographs. The effects of the process variables, i.e., frequency, temperature of the electrolyte, applied voltage and treatment time, have been experimentally studied. Statistical methods were used to achieve the optimum size of the nanocrystals. Finally, the contribution percentage of the effective factors of the pulsed current was revealed, and the confirmation run showed the validity of the obtained results.     

Pulsed nanocrystalline plasma electrolytic boriding as a novel method for corrosion protection of CP-Ti (Part 1: Different frequency and duty cycle)

Potentiodynamic polarization and electrochemical impedance spectroscopy were employed to test borided CP-Ti, treated by a relatively new method called pulsed plasma electrolytic boriding. The results show excellent corrosion resistance for modified CP-Ti. The effect of frequency and duty cycle of pulsed current was investigated. It was found that pulse frequency and duty cycle affect the size and porosity of nanocrystalline borides and by controlling these effective parameters, surface modification can render the CP-Ti material extremely corrosion resistant as a biomaterial.     

Formation of nanocrystalline layers by surface severe plastic deformation and pulsed plasma electrolytic carburizing

Surfaces of various kinds of metallic materials spheres were treated by nanocrystalline surface severe plastic deformation and then pulsed nanocrystalline plasma electrolytic carburizing to study nanocrystalline substrate effect on formation and nano-hardness of hard nanocrystalline layer. The surface layers of the metallic materials developed by the nanocrystalline surface severe plastic deformation were characterized by means of high resolution scanning electron microscope. Nearly equiaxed nanocrystals with grain sizes ranging from 15 to 90 nm were observed in the near surface regions of all metallic materials, which are low carbon steel and commercially pure titanium. The effect of substrate nanocrystallization on growth kinetics and hardness of formed nanocrystalline carbide layer was studied with the means of figure analysis and nanohardness tests. Figure analysis show the length to diameter ratio and distribution curve of nanocrystals and it has been found that the achieved properties of hard layer (growth rate, nano-hardness, nanostructure...) are related to these factors. It was also clarified that these techniques and surface nanocrystallization can be easily achieved in most of metallic materials. Results indicate that the resultant hardened carburized layers exhibited excellent hardness profile. Investigation of the layer characteristics showed strong dependence followed from the treatment experimental parameters as well as the shape of nanocrystals.     

Neural networks prediction of different frequencies effects on corrosion resistance obtained from pulsed nanocrystalline plasma electrolytic carburizing

This paper deals with the surface modification of commercially pure titanium by using pulsed nanocrystalline plasma electrolytic carburizing. In order to fully characterize the complex underlying mechanism of this process and evaluate the effects of a thorough range of frequencies, a prediction model is developed using a hybrid of Neural Networks and Genetic Algorithms (GA). Process variables, i.e. time, frequency and corrosion resistance of nanocrystalline carbides, have been experimentally studied. Corrosion resistances were measured by PDS technique for different coated samples. A portion of this dataset is used to train the prediction model, while the rest is set aside to test its predictive performance. This hybrid Neural Networks model uses GA to achieve its optimal architecture for prediction. Finally, it is concluded that the proposed model has an excellent prediction capability of final corrosion resistance of nanocrystalline carbides in the various range of frequencies by comparing the results with the experimental data.     

Influence of rf-power on the plasma carbonitriding of titanium

The present work reports on the effect of input plasma processing power in the range of 350–650 W on the microstructure and mechanical properties of plasma nitrided Ti. The plasma processing time was 20 min and a gas mixture of 15% C₂H₂ and 85% N₂ was used. The characteristics of the carbonitrided layer have been investigated by microhardness measurements, surface roughness measurements, optical microscopy, and X-ray diffraction. The measured surface hardness values of the compound layer shows a maximum of 2050 HV0.1 for the sample treated at a plasma power of 550 W. The thickness of the carbonitrided layer continuously increases as the plasma power increases. Moreover, the highest carbonitriding rate of 3.52 μm²/s was observed when the input plasma power was adjusted at 600 W. This high carbonitriding rate of treated titanium samples is ascribed to the high concentration of active carbon and nitrogen species in the plasma atmosphere and the formed microcracks in the near surface of the sample during the plasma processing.     

纳米Cu粉末的SPS烧结与晶粒长大行为的研究

为了研究金属材料在放电等离子烧结(SPS)过程中晶粒的长大行为和激活能的变化情况,利用SEM、FESEM、TEM等技术分析测定了纳米Cu粉坯体在SPS过程中组织形貌和晶粒尺寸的变化情况.研究表明:特定的工艺和烧结制度下,应用SPS技术可以得到均匀、致密的组织;脉冲电流的作用使晶粒表面大大活化,晶粒长大激活能大大降低,材料在迅速烧结的同时,晶粒也迅速长大.     

Duplex treatment of 304 AISI stainless steel using rf plasma nitriding and carbonitriding

Surface of 304 AISI austenitic stainless steel has been modified using duplex treatment technique of nitriding and carbonitriding. A thick modified nitrided layer, of approximately 20 µm, has been achieved when rf inductively coupled plasma was adjusted at 450 W for processing time of only 10 min. After performing the nitrided layer, the nitrided samples were carbonitrided using the same technique at different acetylene partial pressure ratios ranges from 10% to 70%, the balance was pure nitrogen. Different amount of nitrogen and carbon species are diffused underneath the surface through the nitrided layer during carbonitriding process and are found to be gas composition dependent. The treated samples were characterized by glow discharge optical spectroscopy, X-ray diffractometry, scanning electron microscopy and Vickers microhardness tester. The microstructure of the duplex treated layer indicates the formation of γ?-Fe₄N, Fe₃C, CrN and nitrogen-expanded austenite (γ_N). The thickness of the duplex treated layer increases with increasing the acetylene partial pressure ratio. The surface microhardness of duplex treated samples has been found to be gas composition dependent and increased by 1.29 fold in comparison to the nitrided sample.     

适合复合共沉积用的液体微胶囊制备研究

采用水相分离法制备出粒径在2～10μm之间,适合复合电沉积用的液体微胶囊.通过微胶囊壁材的力学性能、水溶解性等研究,发现聚乙烯醇(PVA)和明胶的强度、弹性及镀液中的稳定性等可以满足复合电沉积对微胶囊壁材的要求.探讨了微胶囊壁材浓度、囊芯与囊壁比、搅拌、凝聚荆等微胶囊制备工艺对微胶囊的影响.制备出不同芯材(润滑油、有机硅树脂、缓蚀剂、憎水剂等)液体微胶囊和含有机硅树脂的液体微胶囊复合镀铜层.     

An inverse problem of carbonitriding

The problem of determining diffusion coefficients of carbon and nitrogen during carbonitriding has been stated and solved.M. V. Lomonosov Moscow State University, Russia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 63, No. 3, pp. 462–467, October, 1992.     

A universal electrolyte for the plasma electrolytic oxidation of aluminum and magnesium alloys

The plasma electrolytic oxidation (PEO) of aluminum and magnesium alloys is carried out in electrolytes which contain the same reactants, but in fundamentally different concentrations. In this research the possibility of the PEO of aluminum and magnesium alloys in a universal electrolyte is studied. The two most commonly encountered alloys, namely, aluminum alloy 5052 and magnesium alloy AZ91D, are chosen. The oxide layers obtained are studied using SEM, EDX, XRD, and a microhardness tester. The corrosion properties are determined using a potentiostat. The effect of variation of the silicate concentration in the electrolyte on the growth kinetics of the coating and its qualitative characteristics is discussed. It is shown that at Na₂SiO₃·5H₂O concentrations in the electrolyte ranging from 3.2 to 32 g l^− 1, the rate of growth of the oxide layer increases from 15 to 55 μm h^− 1 with significant variation of the phase composition of the coating. The greatest hardness of an oxide ceramic layer was obtained in the outer sublayer on the magnesium alloys (874.7 HV₁₀) and in the inner sublayer on the aluminum alloys (1123 HV₁₀). The most favorable combination of physical and chemical properties for both alloys is obtained in an electrolyte containing 12.72 g l^− 1 Na₂SiO₃·5H₂O.     

Enhancement of coating characteristics via improved plasma electrolytic oxidation set-up

The plasma electrolytic oxidation process involves high-temperature operations and exposed to corrosive chemicals. Thus it requires stringent experimental set-up to produce uniform and high-quality coatings. This study proposes an enhancement to the current plasma electrolytic oxidation experimental set up to improve: 1) electrode holder; 2) cathode-anode electrodes, and 3) water-cooling system. In this paper, the focus is dedicated to the first two items. For the first item, the redesigned electrode holder is developed using 3-dimensional software. For the second item, results due to cathode via stainless container as well as carbon electrode are discussed. The performance of the new set-up plasma electrolytic oxidation is compared with the old one by analysing the microstructure and the mechanical properties of titanium dioxide coatings formed on a titanium aluminium vanadium alloy substrate using both set-ups. Surface morphology shows that the coating produced using the new set-up is thicker, denser and has lower porosity as compared to the coating deposited using the old set-up. The mechanical properties of hardness and adhesive strengths of the coating are also improved in the new set-up. Thus, the use of this set-up is recommended for improved coating performance to produce a uniform coating having enhanced mechanical performance.     

Ceramic coatings of LA141 alloy formed by plasma electrolytic oxidation for corrosion protection

Superlight Mg-Li alloy is a promising structural materials in aerospace, automobile, and electronics because of its excellent properties such as low density, high ductility, superior strength-to-weight ratio, and good damping ability. The fabrication of compact plasma electrolytic oxidation coatings with excellent corrosion resistance is valuable for the widespread application of Mg-Li alloy. Here we present a ceramic coating on the surface of Mg-14Li-1Al (LA141) alloy for corrosion protection via plasma electrolytic oxidation (PEO) in an alkaline silicate electrolyte with tungstate as an additive. X-ray photoelectron spectroscopy and thin film-X-ray diffraction analysis of coatings show that the surface coating is mainly comprised of Mg(2)SiO(4), MgO and WO(3). Scanning electron microscopy observations have revealed that the dense and compact coating formed in the presence of tungstate has less structural imperfections in comparison to the control one fabricated without use of tungstate. The effect of oxidation time on the morphology and phase composition of coatings is also examined in detail.     

Preparation of nanocrystalline YSZ powders by the plasma technique

A plasma synthesis method has been devised to produce nanosize YSZ powders with various yttria contents. The powders are synthesized by introducing a mixture of coarse-grained zirconia and yttria into an r.f. inductively coupled plasma flame. The average particle size of the as-prepared powders is in the range 20–40 nm and the specific surface area is 18–50 m²g^–1. The phase and granulometric composition of the produced powders depend on the degree of evaporation of raw powders, reagent concentration in the gas flow and quenching rate, and on the content of Y₂O₃. Up to 5.5 mol% yttria, the major phase of nanosize powders is tetragonal ZrO₂, mostly as the non-transformable (t) form. For yttria contents higher than 6 mol%, the major phase is cubic ZrO₂.     

Densification mechanisms in spark plasma sintering of nanocrystalline ceramics

Effect of the particle size on the possible electric discharge during the SPS was examined. Nanoparticle compacts enable accumulation of high electric charge, and discharge under conventional voltages used for the SPS. The critical particle size for the electric discharge is both morphological and material dependent. The early stages of densification of the nanocrystalline powder compact proceed either by the plastic deformation or grain-rotation coalescence and sliding, aided by softening of the particle surfaces. The active densification mechanism depends on the changes both in the mechanical and electrical properties with temperature. Densification of 11 nm nc-MgO particles with low yield stress proceeds by plastic deformation already at 700 °C. However, densification of 34 nm nc-YAG particles with high yield stress proceeds by nano-grain rotation aided by particle surface softening. Densification at the final stages of SPS is associated with diffusional processes, where curvature driven grain growth predominates.     

Stability of plasma electrolytic oxidation coating on titanium in artificial saliva

Bioactive PEO coating on titanium with high Ca/P ratio was fabricated and characterized with respect to its morphology, composition and microstructure. Long-term electrochemical stability of the coating and Ti⁴⁺ ion release was evaluated in artificial saliva. Influence of the lactic acid and fluoride ions on corrosion protection mechanism of the coated titanium was assessed using AC and DC electrochemical tests. The PEO-treated titanium maintained high passivity in the broad range of potentials up to 2.5 V (Ag/AgCl) for up to 8 weeks of immersion in unmodified saliva and exhibited Ti⁴⁺ ion release <0.002 µg cm^?2 days^?1. The high corrosion resistance of the coating is determined by diffusion of reacting species through the coating and resistance of the inner dense part of the coating adjacent to the substrate. Acidification of saliva in the absence of fluoride ions does not affect the surface passivity, but the presence of 0.1 % of fluoride ions at pH ≤4.0 causes loss of adhesion of the coating due to inwards migration of fluoride ions and their adsorption at the substrate/coating interface in the presence of polarisation.     

Preparation and apatite layer formation of plasma electrolytic oxidation film on titanium for biomedical application

Porous and rough titania film containing Ca and P on titanium was prepared by plasma electrolytic oxidation. The film was composed of titania and a little CaTiO₃. Apatite layer was formed on the titania film when the film was immersed in the simulated body fluid, indicating the potencial capability for formation of the plasma electrolytic oxidation film on titanium.