Cyclic oxidation resistance of Ni–Al alloy coatings deposited on steel by a cathodic arc plasma process

The cyclic oxidation resistance of nickel-aluminide coatings deposited on steel using a cathodic arc plasma (CAP) process has been investigated. Our results show that nickel-aluminide films can be successfully deposited on carbon steel and stainless steel substrates by this process; NiAl₃ is the major phase in the deposited films. The thermal cycling behaviour suggests that such coatings can resist oxidation through physical blocking of oxygen, either by the coating itself or by the aluminium oxide scale subsequently formed in-service. Aluminium diffusion inwards to the substrate may also be beneficial to the thermal oxidation resistance. The coating protects stainless steel substrate materials at 500°C by transforming the NiAl₃ phase into NiAl, producing aluminium oxide on the open substrate surface. At 800°C, oxide flaking is suppressed by the trace amounts of nickel or aluminium which have partially diffused into the substrate.     

Substrate effect on the mechanical and tribological properties of arc plasma physical vapour deposition coatings

The present study describes and compares the mechanical and tribological properties of CrN coatings deposited by PVD/CAPD (Cathodic Arc Plasma Deposition) on three different substrates: steel, aluminium alloy and reinforced aluminium alloy. The coating-substrate interfaces were analysed by scanning electron microscopy (SEM). The ultra-microindentation technique was applied to measure coating hardness. Experiments using a tribometer (pin on disc configuration) under lubricated conditions have been performed in order to evaluate the friction and wear properties of the different systems.

The measured coating hardness depends on the indentation depth reached in the ultra-microindentation tests. In this study the coating-substrate system has been characterized, analyzing the hardness variation from the coating to the substrate by different indentation depths, and modelling the experimental curve with a universally approved mathematical model. The CrN–steel system exhibits the best performance, in which the system hardness is close to the CrN coating hardness. The CrN–AMC system performs better than CrN–aluminium but worse than CrN–steel system.     

Surface engineering with light alloys—Hard coatings, thin films, and plasma nitriding

Light alloys have been attracting increasing attention over the past decade, since they can be used to reduce weight and save energy. For many years, light metals such as titanium and aluminum have also been used to synthesize hard compound coatings such as physically vapor deposited (PVD) TiN, (Ti,Al)N, and chemically vapor deposited (CVD) Al₂O₃. The coatings field is developing rapidly. Combining plasma-aided coating and diffusion processes has led to the development of so-called “duplex treatment,” consisting of plasma nitriding and subsequent hard coating. Another interesting development is TiN coating of aluminum vacuum parts, such as pumps, to reduce degassing and make the cleaning of the surfaces easier. Despite the many advantageous properties of light alloys, their surface properties sometimes cause problems. For example, galling may be a severe problem with titanium parts, and plasma nitriding has been applied successfully to combat it. However, due to adherent oxide scale, plasma nitriding of aluminum has proven to be more difficult. In this paper, we discuss some recent trends in the application of plasma-aided coating, thin film deposition, and diffusion processes, and give practical examples of industrial applications.     

Performance evaluation of PVD coatings for high pressure die casting

During high-pressure die-casting (HPDC) of aluminium alloys, there is a tendency for the molten alloy to react with the tool steel die, core pins and inserts. This occurrence within the high pressure die casting (HPDC) industry is referred to as ‘soldering’. It is of concern to high-pressure die casters because of down-time due to the regular removal of the soldered layer and its detrimental affect on die life and casting quality. In this investigation, several physical vapour deposited (PVD) coatings, namely, TiN, CrN and TiCN, were evaluated for their ability to eliminate soldering during HPDC of aluminium alloys. Accelerated semi-industrial trials were carried out in a 250-t Toshiba HPDC machine using a specially designed die made of P20 tool steel with removable core pins. The results from these trials showed that PVD coatings can act as a physical barrier coating preventing any reaction between the molten aluminium alloy and the tool steel. Thus the problem of soldering on such tools as core pins can be eliminated in high HPDC of aluminium alloys. In the accelerated trials, it was found that soldering was replaced by a built-up layer of cast aluminium alloy, which was less detrimental to tool life and reduced machine down-time due to the reduced need for tool polishing. The experimental results were confirmed by conducting in-plant HPDC trials.     

Einfluß einer Faserbeschichtung aus pyro-Carbon auf das elektrochemische Korrosionsverhalten von Kohlenstoffaser/Aluminium-Verbunden

Influence of pyro-carbon fibre coating on the electrochemical corrosion behaviour of carbon fibre/aluminium composites Carbon fibre reinforced aluminium belongs to leightweight materials with crucial potentials for practical application. The electrochemical corrosion of this material was investigated. Expectedly, the reinforcement of the aluminium with 70 vol.-% carbon fibres (Toray T800) leads to a remarkable decrease of the corrosion resistance, which was assessed by means of immersion tests (3.5% NaCl solution), potentiodynamic measurements (3.5% NaCl solution) and after exposure of the MMCs in a climate chamber (DIN EN ISO 6988). The coating of carbon fibres with a pyrolytical carbon layer is useful in terms of providing a sufficient fibre protection during the processing of the MMCs. In that case, chemical interactions between the fibres and the aluminium can be regulated. Furthermore, there is the possibililty to tailor a definite mechanical behaviour of the MMCs due to the application of the pyrolytical carbon layer. Because the utilization of pyrolytical carbon layers on carbon fibres is necessary in some cases, the electrochemical behaviour of MMCs made from coated carbon fibres and aluminium was investigated as well. It was found that the used pyrolytical carbon layer on the T800 fibres caused a further significant degradation of the corrosion resistance. A probable reason for that may be an increased reactivity of the pyrolytical carbon, which was syntheticized during a CVD (chemical vapour deposition) process.     

Self-lubricating Al2O3/PTFE composite coating formation on surface of aluminium alloy

Self-lubricating composite coating is fabricated on the surface of aluminium alloy, which integrates the advantages of wear resistance of microarc oxidation (MAO) ceramic coating and a low friction coefficient of polytetrafluoroethylene (PTFE). Inspection of SEM and EDS analysis indicates that PTFE can be deposited into the MAO coating, and a compact film forms which can fully cover the MAO coating. The yielded coating possesses superior tribological properties. The friction coefficient remains steady with minimal weight loss during long-term sliding.     

Corrosion resistance of lamellar aluminium pigments coated by SiO2 by sol–gel method

The aluminium pigments were coated with SiO₂ by sol–gel method to improve their stability. The effects of formulation factors, such as medium of reaction, adding sequence of catalyst and number of coating, were investigated. The stability of the coated aluminium pigments in acid was examined by measuring the hydrogen generation amount. It was found that the coating layer formation is due to the condensation of tetraethyl orthosilicate (TEOS) to form a dense 3D cross-linked layer on the surface of aluminium. The optimized sequence of adding catalysts would be hydrochloride first, then ammonia. Stability tests confirmed that the aluminium pigments have better corrosion resistance after coating with SiO₂.     

Microstructure and 3D microtomographic characterization of porosity of MAO surface layers formed on aluminium and 2214-T6 alloy

A fine characterization of the microstructure of the MAO coatings formed on aluminium (purity 99,999%) and its alloy 2214-T6 was observed using field emission gun scanning electron microscopy (FEG-SEM) with energy dispersive spectrometry (EDS) and X-ray microtomography (XMT). In addition, the influence of current frequency (pure aluminium) and the precipitates of substrate were investigated (case of 2214-T6 alloy).The MAO surface layers formed on aluminium and its alloy 2214-T6 can be divided into two parts: the outer layer with high porosity (so-call porous layer) and the inner layer with low porosity (so-call dense layer). However, the porosity of the inner layer increases toward the MAO layer/substrate interface. It is found that, both the thickness and the porosity increase when the current frequency decreases. The porosity of the MAO coating can be attributed to discharges formation in the vicinity of the MAO coating/substrate interface.In the particular case of 2214-T6 aluminium alloy (3.9% Cu), with coarse non-valve metal rich precipitates aligned perpendicular to the surface of the work electrode; the transformation of precipitates under discharge effect can provoke the formation of big channels, which go through the MAO surface layer. This last one induces local formation of a layer rich electrolyte species toward the substrate.     

Initial stages of plasma electrolytic oxidation of titanium

The initial stages of oxide growth on titanium are examined in a recently developed commercial alkaline pyrophosphate/aluminate electrolyte of interest for plasma electrolytic oxidation of light metal alloys. Constant current anodizing was employed, with resultant films examined by scanning and transmission electron microscopies and Rutherford backscattering spectroscopy. The initial film is relatively uniform and composed of TiO₂, with low concentrations of aluminium and phosphorus species incorporated from the electrolyte. With increase in voltage the film breaks down locally, and regions of original and modified film develop simultaneously, with the latter occupying more of the surface as the voltage rises. Porous regions due to dielectric breakdown also become increasingly evident. At 240 V, sparking commences, and the surface reveals extensive, relatively uniform porosity, with the coating now containing much enhanced concentrations of aluminium and phosphorus species compared with the coating at lower voltages. The films develop at low efficiency due to generation of oxygen. The oxygen is produced within the original film material and at sites of dielectric breakdown. The former type of film develops a two-layered morphology, with an outer layer of amorphous TiO₂ and an inner layer with numerous fine and course cavities. The cavities are due to the generation of oxygen that may be associated with the formation of anatase in the inner layer.     

Determination of the protective properties of electrodeposited organic epoxy coatings on aluminium and modified aluminium surfaces

The electrochemical and transport properties and thermal stability of epoxy coatings electrodeposited on aluminium and modified aluminium surfaces (anodized, phosphatized and chromatized-phosphatized aluminium) have been investigated during exposure to 3% NaCl solution. From the results obtained from electrochemical impedance spectroscopy (EIS), gravimetric liquid sorption experiment and thermogravimetric analysis (TGA) it can be concluded that electrochemical and transport properties of epoxy coatings on anodized and chromatized-phosphatized aluminium are significantly improved with respect to the same epoxy coatings on aluminium and phosphatized aluminium: higher values of pore resistance and charge-transfer resistance and lower values of coating capacitance and double-layer capacitance, from EIS; smaller values of diffusion coefficient of water through epoxy coating, from sorption measurements and smaller amount of absorbed water inside the coating, from TGA. On the other hand, the somewhat lower thermal stability of these coatings was obtained.     

Influences of ceria and yttrium on properties of D127 surfacing electrode

The coating of D127,a surfacing electrode,was added ceria(CeO_2) and yttrium (Y) for possible enhanced electrode properties.The method of orthogonal test was adopted.The coating with different amount of CeO_2 and Y was pressed.The hardness of surfacing layer was carried on by hardness tester,microstructure was investigated by metallograph,and the content of rare earths was analyzed by X-ray energy dispersive spectrometer.The results show that the microstructures of deposited metal can be effectively made fine,accordingly,the hardness and the wear-resistance of deposited metal are also raised.But the addition amount of the rare-earth should be proper.The deposited metal microstructure is the finest and the most homogeneous when the addition amount of CeO_2 is 2 wt.% and Y 1.2 wt.%,and the hardness of the surfacing layer is the highest.     

Thin films of MoN, WN, and perfluorinated silane deposited from dimethylamido precursors as contamination resistant coatings on micro-injection mold inserts

To enhance their surface properties, micro-injection mold inserts made of electroplated nickel were coated with thin films of molybdenum nitride and tungsten nitride by atomic layer deposition. Alkylimido-alkylamido complexes were used as precursors together with ammonia. In addition, a perfluorinated hydrophobic coating was deposited by gas-phase method from tridecafluoro-1,1,2,2-tetrahydrooctylmethyl-bis(dimethylamido)silane. Injection molding tests were performed with two plastic materials: poly(4-methyl-1-pentene) copolymer TPX™ and polycarbonate Makrolon® DP1-1265. With both plastics, the nickel insert with thin film of molybdenum nitride was clearly more resistant to contamination than the uncoated insert. Also the perfluorosilane coating provided good resistance to contamination. After the 15,000 shot injection molding test, all of the coatings were still attached to the insert. Friction coefficients were determined between the nitride and silane coatings and the plastic materials. The coatings showing good contamination resistance in injection molding also had a low coefficient of friction.     

An improved method for the deposition of corrosion-resistant aluminium coatings for aerospace applications

The gradual reduction in the use of electroplated cadmium as a corrosion barrier for aircraft parts has led to the development of ion- vapour-deposited (IVD) aluminium as an alternative. The most common IVD technique is the McDonnell Douglas “Ivadize” process, but the morphology of such aluminium coatings is usually open and columnar. Consequently, a post-deposition shot peen is used to densify the structure. This has led to very poor control over the thickness and uniformity of the coating, and precision parts cannot be coated by such techniques. The use of multimagnetron ion plating can guarantee close control over the coating dimensions by the precise deposition of fully dense aluminium films which do not require peening. To further improve the properties of the films, a series of aluminium alloys have been deposited to provide enhancement of the corrosion and tribological properties of the films.     

An investigation of the influence of physical vapour deposited aluminium layers on the kinetics of organic coating disbondment on iron

A high-throughput method of investigating organic coating delamination from iron substrates incorporating interfacial thin metallic films of varying thickness is presented. Physical vapour deposited aluminium is demonstrated as a means of limiting underfilm oxygen reduction and slowing rates of corrosion-driven cathodic disbondment. A wedge of graded thickness is deposited on an iron surface and over-coated with a model organic layer. After initiating corrosion by applying corrosive electrolyte to a penetrative defect, rates of corrosion-driven delamination are determined by in situ scanning Kelvin probe measurements, enabling the influence of a range of Al thicknesses to be studied on a single sample.     

Microstructure and properties of TiN/Ni composite coating prepared by plasma transferred arc scanning process

The TiN/Ni composite coatings were deposited on 7005 aluminium alloy by high speed jet electroplating and then processed with plasma transferred arc(PTA) scanning process. The microstructure, microhardness and friction coefficient of PTA scanning treated specimens were investigated. It is shown that the PTA scanning treated specimens have a rapidly solidified microstructure consisting of the uniformly distributed TiN phase and fine Al₃Ni₂ intermetallic phases. The composite coating has an average microhardness of approximately HV 800. The friction coefficient of PTA scanning treated specimens (oscillated at around 0.25) is considerably lower than that of TiN/Ni composite coating (oscillated at around 0.35). The corrosion behavior of the composite coating in 3.5% NaCl solution at room temperature was also determined using a potentiostat system. In comparison with the corrosion potential _corr of −0.753 V for 7005 aluminium alloy, the corrosion potentials for TiN/Ti composite coating and PTA scanning treated specimen are increased by 0.148 V and 0.305 V, respectively. The PTA scanning treated specimen has the lowest corrosion current densityJ_corr as well as the highest corrosion potential _corr, showing an improved corrosion resistance compared with 7005 aluminium alloy.     

Electrostatic Powder Coatings

This paper describes two new electrostatic processes, used for dry painting, plastic coating, talcing, aluminium coating, applying rust inhibitors, applying electrical insulation and vitreous enamelling of sheet steel and castings. These applications are discussed, a wide range of plastic powders being commercially available for use with the processes. In terms of physical chemistry, the main advantages of dry powder spraying are that unfavourable chemical reactions between the liquid vehicle and the solids, or between the liquid and the metal surfaces, can no longer occur, and that the permeability of the final film can be considerably lower, since there are no solvents to boil off during the stoving operation. These factors have recently led to the formulation of interesting materials—particularly some unique epoxide resins. For ideal combination of good anti-corrosion properties, mechanical strength and low cost plastic coatings, there is an optimum thickness of coating. With epoxy coatings, for example, thicknesses of 0.002 to 0.004 in are required. Coatings of this order may be applied automatically by electrostatic powder coating. The process is flexible and coatings of from 0.0005 to 0.04 in can be obtained. For the application of special chemicals, light dustings having a few particlcs/cm² can be applied.     

Oxidation behaviour of PACVD TiBN coating at elevated temperatures

The elevated-temperature oxidation behaviour of a TiBN coating on a plasma-nitrided hot-work tool steel (DIN 1.2367) by means of plasma-assisted chemical vapour deposition (PACVD) was investigated under the condition where a coated die would be preheated prior to being mounted on the press for aluminium extrusion. The TiBN coating was found to possess good resistance to oxidation up to 400 °C. Rapid oxidation started to occur at 450 °C. Radio frequency glow discharge optical emission spectroscopy (rf-GDOES) indicated that the oxidised layer was thickened from 100 nm to 1.0 μm, as the soaking time at 500 °C was prolonged from 2 to 16 h, which was attributed to the high temperature that promoted the penetration of oxygen into the coating. rf-GDOES also showed that boron initially in the coating vanished from the oxidised layer when the temperature was 450 °C or higher. X-ray diffractometry confirmed that the oxidised layer was composed mainly of TiO₂. SEM revealed that the TiO₂ layer was pulverised, leaving many microcracks and cavities, as a result of the losses of boron oxide and nitrogen. The rapid oxidation at above 450 °C was attributed to the pulverised TiO₂ layer that was unable to hinder the diffusion of oxygen into the coating. It is therefore recommended to apply a protective gas during the preheating of the TiBN-coated die for aluminium extrusion. Alternatively, an advanced TiBN coating with enhanced resistance to oxidation must be developed, which will be conducive to its application for aluminium extrusion dies.     

The Corrosion of Metals with Special Reference to Protective Metallic Coatings

This paper describes two new electrostatic processes, used for dry painting, plastic coating, talcing, aluminium coating, applying rust inhibitors, applying electrical insulation and vitreous enamelling of sheet steel and castings. These applications are discussed, a wide range of plastic powders being commercially available for use with the processes. In terms of physical chemistry, the main advantages of dry powder spraying are that unfavourable chemical reactions between the liquid vehicle and the solids, or between the liquid and the metal surfaces, can no longer occur, and that the permeability of the final film can be considerably lower, since there are no solvents to boil off during the stoving operation. These factors have recently led to the formulation of interesting materials—particularly some unique epoxide resins. For ideal combination of good anti-corrosion properties, mechanical strength and low cost plastic coatings, there is an optimum thickness of coating. With epoxy coatings, for example, thicknesses of 0·002 to 0·004 in are required. Coatings of this order may be applied automatically by electrostatic powder coating. The process is flexible and coatings of from 0·0005 to 0·04 in can be obtained. For the application of special chemicals, light dustings having a few particles/cm² can be applied.     

3Cr13不锈钢低温双辉等离子渗铬

对3Cr13不锈钢在700 ℃进行低温等离子渗铬处理,并观察处理后试样的组织,进行了截面成分检测、硬度检测、X射线衍射物相分析以及电化学腐蚀性能测试.结果表明,表面覆盖4～5 μm 的铬沉积层后在沉积层的下边形成了一定区域的脱碳层,表面硬度达728 HV0.05;表层物相主要为Fe、Cr、Cr_(1.36)Fe_(0.52)及Cr_(23)C_6;在2.5%HCl溶液中耐腐蚀性能提高约6倍,抗点蚀能力有一定的增强,提高了耐晶间腐蚀的能力.     

Q235钢氩弧熔覆铁基合金涂层的耐磨性研究

和用氩弧熔覆技术,选择合适的工艺参数,在Q235钢材表面熔覆了铁基合金耐磨涂层.通过金相显微镜和SEM分析了熔覆涂层的显微组织,并测试了涂层的显微硬度和耐磨性.结果表明,在Q235钢表面制备了以马氏体组织和γ-(Fe-Cr-Ni-C)合金固溶体为基体,以(Cr,Fe)7C3、Fe3C、Fe2B等化合物为增强相的合金涂层;涂层的显微硬度可达600 HV;涂层的耐磨性较基体提高近8倍.在低碳钢表面熔覆一层耐磨材料,既保留了低碳钢较高的塑、韧性,又提高了表面层的硬度和耐磨性.