Intermediate Coatings Research Articles

Microstructure and wear properties of plasma‐sprayed aluminum–silicon–polyester coatings

AbstractPowder coatings, which are made by plasma‐spraying processes, are being used in industrial applications because of their wear resistance, chemical resistance, and high impact strength even at low service temperatures. These factors increase the importance of plastic and plastic‐based coatings in industrial applications. In this study, an aluminum–silicon–polyester‐based composite coating was applied by plasma‐spraying processes with and without an intermediate bond coat (Ni–Al). The effects of the coating thickness, intermediate bond coat, and processes parameters on the microstructure and wear properties of the coating were studied experimentally. The wear properties of the coatings were determined according to ball‐on‐disk procedure. The microstructures of the coating were examined by optical microscopy and scanning electron microscopy. The results indicated that the plasma‐spraying current and thickness had a strong influence on the wear resistance and microstructural properties of the aluminum–silicon–polyester coating. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3609–3614, 2006

Comparative EIS study of different Zn-based intermediate metallic layers in coil-coated steels

The effect the nature of the intermediate zinc-containing layer in coil-coated steels (CCS) has on their anticorrosive performance was examined by electrochemical impedance spectroscopy (EIS). The zinc-based metallic intermediate coatings considered in this work were namely galvanised (Zn), galfan (Zn–5Al) and aluzinc (Zn–55Al). Corrosion tests were performed by exposing the coated panels to a 3% NaCl aqueous solution in a horizontal flat-cell. EIS data were analysed considering the values of the distinct elements in equivalent electrical circuit models, which allowed the long term evaluation of the coating condition for the different CCS. Changes in the impedance characteristics of the systems were found to occur as a function of the exposure time in all three cases. These results have revealed differences in the barrier characteristics of the various CCS, namely the galvanised system exhibited the shortest time to breakdown, whereas both galfan and aluzinc maintained the system protected for significantly longer immersion periods.

Accelerated tests for the evaluation of the corrosion performance of coil-coated steel sheet: EIS under cathodic polarisation

Cathodic polarisation has been used as an accelerated test method to assess the degradation of coil-coated architectural cladding. Data were recorded via electrochemical impedance spectroscopy (EIS) measurements. Coil-coated steel (CCS) sheet with three different intermediate zinc-based metallic coatings were investigated, namely galvanised, galfan and aluzinc. Corrosion tests were performed by polarizing the painted panels to two cathodic potentials corresponding to either aluminium or magnesium sacrificial anodes while immersed in 3% NaCl aqueous solution in a horizontal flat-cell. The cathodic potential values considered were −1.082 and −1.522 V versus saturated calomel electrode (SCE). Cathodic polarisation conditions were found for which differences in the corrosion resistance of the three systems were detected from the analysis of the EIS data at significantly shorter exposure times than in the case of EIS data measured at their corresponding open circuit potential, thus providing an accelerated test method for determining the quality of the CCS system.

Coil-coated steel: corrosion resistance and adhesion as a function of the composition of the intermediate galvanic layer

The corrosion behaviour of coil-coated steel (CCS) sheets with different intermediate zinc-based coatings was examined by electrochemical impedance spectroscopy (EIS). Corrosion tests were performed in a 0.5 M NaCl aqueous solution in contact with the air, at ambient temperature. The results of pull-off adhesion test are also presented. The impedance data show that the improvement in the protective behaviour of the composite system is connected with the higher resistance values for the samples in which aluminium was added to the galvanized layer, and the best results are shown by CCS with aluzinc (Zn-55Al). These effects have been related to differences in the adhesion of the organic coating to the underlying metal substrate, as determined from both non-exposed samples and samples exposed to the electrolyte solution.

Structure and properties of AlSi/polyester coating

AbstractPowder coatings, which are formed by plasma spray technique, are being used in industrial applications. Resistance of plastics and their based composite materials to chemicals, solvents, atmospheric conditions, and high impact strength even at low service temperature increases the importance of plastic and plastic based coating applications. In this study, aluminum silicon based polyester (AlSi/polyester) coating was applied by plasma spraying technique with and without intermediate bond layer coat (NiAl). The effect of coating thickness, intermediate bond layer coat, and plasma spraying parameters on bond strength of coating were studied experimentally. The bond strengths of the coatings were determined according to the ASTM C‐633–79. Microstructures of the coating were examined by optic microscopy and scanning electron microscopy (SEM), respectively. Obtained results indicated that plasma spraying current rate, coating thickness, and spraying distance were important factors on bond strength of coating. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2437–2444, 2004

In situ observations of the real-time stress-evolution and delamination of thin Ta films on Si(100)

Polycrystalline sputtered Ta coatings on Si(100) substrates were thermally tested for 1 h at 585 °C, while their stress evolution and eventual delamination were observed in situ and in real-time using a recently developed synchrotron X-ray white beam technique. Films deposited at ‘low’ (0.26–0.67 Pa), ‘intermediate’ (0.80–0.93 Pa), and ‘high’ (1.06–2.00 Pa) Ar pressures exhibited severe, moderate, and no blistering, respectively. These results were explained by the attendant stress data, which indicated the development of high, moderate, and low maximum compressive stress in the low, intermediate, and high-pressure coatings, respectively. Additionally, the structure of the films was probed before and after thermal testing using X-ray diffraction in both grazing incidence and θ−2 θ scanning geometries. It was determined that all coatings made a complete conversion to orthorhombic Ta 2O 5 after the 1 h thermal test, regardless of their deposition pressure.

A comparative study of the cyclic thermal oxidation of PVD nickel aluminide coatings

Static components used in the hot sections of gas turbines increasingly have a thermal barrier coating (TBC) of partially yttria-stabilized zirconia (PYSZ) routinely applied to improve their high temperature properties. Developments are also in progress to make TBCs better suited for use on rotating components such as blades. To help protect against high temperature oxidation of such components, an intermediate bond coat is applied, typically of an MCrAlY-type metal alloy. An alternative bond coat material is nickel-aluminum intermetallic alloy. Various processing routes have been studied for both bond coat and TBC deposition. A potentially attractive processing route is to deposit both bond coat and TBC by a physical vapor deposition (PVD) method; this would have the advantage of permitting sequential deposition in the same coating cycle. Whilst much research has been carried out on PVD MCrAlY and PYSZ coatings, relatively little work has been carried out on PVD NiAl, which represents a simpler intermetallic alloy with less critical composition control requirements. Thus, we have investigated NiAl deposition by three ion-assisted PVD coating routes: arc, electron-beam, and sputter ion plating. Coatings were deposited on a nickel-based alloy (Inconel 600) and an AISI 304 stainless steel. The differences in microstructure and phase composition from each deposition method are reported, together with data on the cyclic oxidation performance. The influence of process parameters on coating characteristics and degradation mechanisms is discussed.

Effect of Combined Elastic Coatings and Their Application on the Strength Properties of Silver Solders

We have established that, in the course of formation of vacuum-plasma titanium-nitride coatings on soldered articles, which is accompanied by their heating to temperatures higher than 500°C, the parameters σ0.2 and σs of PSr45 solder decrease. Electroplating of intermediate anticorrosive chromium coatings leads to the saturation of the solder with hydrogen, and, as a result, its plasticity decreases by 25–30%.

The laser drilling of multi-layer aerospace material systems

Due to continuing advances in jet engine efficiency, components are exposed to ever-increasing combustion and exhaust gas temperatures. Air plasma sprayed thermal barrier coatings (TBCs) protect jet engine components from direct exposure to the corrosive temperatures. The TBC is affixed to the superalloy substrate by an intermediate bond coat thus producing a multi-layer material system. A laser drilling technique has been developed, to generate cooling holes in multi-layer systems, for use in the aerospace industry. The cooling holes are required to conform to standards stipulated by the original engine manufacturer (OEM). A Nd:YAG laser was used to study the parameters affecting hole generation in superalloy materials. This paper reports on the investigation into the negative effects of percussion laser drilling on material interfaces, bond strength, and the negative effects on the individual microstructures such as remelt layers and microcracking.

Urethane-Acrylates as Main Components of Lacquers for Protective Coating of Some Materials

The synthesis of urethane-acrylic UV-cured resins intended for application as protective coatings for optical fibers. The utility of these compositions for protective soft, hardness and intermediate coatings of optical fibers was checked and confirmed under manufacturing conditions.

Investigation of plasma spray ceramic coatings on austenitic manganese and mild steels

An investigation was conducted to explore the feasibility of applying plasma spray ceramic (alumina) coatings to Hadfield manganese and mild steels. The results suggest that an intermediate bond coat of Ni–Al alloy is necessary for satisfactory coating adhesion on both the mild and manganese steel substrates. The coating chemistry is altered by large scale diffusion of elements from the substrate to the coating and counterdiffusion of elements from the coating into the substrate, both of which adversely affect coating bond strength and abrasive wear resistance.

Thermal Spreading Resistance in Multilayered Contacts: Applications in Thermal Contact Resistance

Application of highly conductive coatings to contacting surfaces is a commonly employed method to enhance thermal contact conductance. In many applications it is often necessary to apply an intermediate coating such that the conductive coating may be applied to a nonadhering substrate. In these instances, it is desirable to predict the effect that the intermediate and final coatings have on the spreading resistance. A solution for computing the thermal spreading resistance of a planar circular contact on a doubly coated substrate is presented. Also, a model is developed to compute the contact conductance between a bare substrate and a coated substrate. Comparisons are made with data obtained in the literature for which no analytical model was available. Solution of the governing equations and numerical computation of the spreading resistance were obtained using computer algebra systems

Recent advances in aqueous two-component systems for heavy-duty metal protection

Although waterborne coatings have successfully replaced their solventborne counterparts in a variety of areas, their use generally has been limited to applications with relatively modest performance requirements. Recently, advances in technology have allowed for the design of waterborne coatings with performance suitable for more demanding applications. The emergence of two-component waterborne polyurethane coatings exemplifies this trend. These coatings can provide performance similar to that of solventborne 2-pack urethanes, but with much lower VOC. The chemistry of these coatings is discussed, in light of its relevance to some of the technical challenges, e.g., pot life and CO 2-gassing in thick films, unique to these systems. The use of 2-pack waterborne urethane topcoats in combination with low VOC waterborne acrylic/epoxy primers and intermediate coats is also described. Such multiple coat systems offer the promise of long term corrosion protection in severe industrial and marine environments, coupled with very low VOC.

Micromechanics of failure of aluminide coated single crystal Ni superalloy under thermomechanical fatigue

Ni-base single crystal superalloy turbine components show better creep resistance than their polycrystal counterparts and are used as blades and vanes in aircraft and gas turbine engines. In order to achieve higher operating temperatures combined with good environmental performance, a range of coatings and intermediate coatings are being examined. In this respect it has been realized that the thermomechanical fatigue (TMF) response is a major life limiting factor. The aim of this paper is to determine the influence of an aluminide coating on the TMF lives of a CMSX4 single crystal superalloy and to identify the dominant failure mechanism operating under out-of-phase TMF testing conditions. 300--1,050 C, 135{degree} out-of-phase mechanical loading cycles have been chosen to represent the harshest conditions faced at a critical location across on a blade in service. This information is of great significance to aero-space industries and provides a sound basis for predicting the TMF lives of superalloy components.

Laser cladding of Al-Sn alloy on a mild steel

Direct laser cladding of Al alloy on mild steel produces continuous brittle intermetallic FexAly layers at the interface resulting in weak bonding of the clad and the substrate. To overcome this problem, both Ni and Al were investigated as intermediate coatings. Although a Ni coating with a thickness of 70 μm was successful in eliminating the formation of continuous Al-Fe intermetallics, the bonding at the Al alloy/Ni interface was not strong enough to survive a bending test. A pure Al coating was found to be a better barrier layer. In both cases the optimization of processing parameters for Al alloy cladding was realized via microstructural screening and mechanical property testing including hardness, preliminary wearing and bending tests. Operating windows were established in terms of low dilution, freedom from cracks and porosity, coating-substrate bonding and chemical homogeneity. The results of a comparative study indicated that the laser clad layers showed superior wear resistance to the conventional alloy. The morphology of the rapid solidification clad was investigated by means of optical microscopy and scanning electron microscopy. The alloying element distribution was analyzed using electron probe microanalysis. A simplified model based on the calculation of the clad/substrate interface temperature was developed to provide a guideline for choosing the processing parameters for successful cladding.

Topographical characterization and microstructural interface analysis of vacuum-plasma-sprayed titanium and hydroxyapatite coatings on carbon fibre-reinforced poly(etheretherketone).

In the present study, topographical characterization and microstructural interface analysis of vacuum-plasma-sprayed titanium and hydroxyapatite (HA) coatings on carbon fibre-reinforced polyetheretherketone (CF/PEEK) was performed. VPS-Ti coatings with high roughness values (Ra=28.29+/-3.07 microm, Rz=145.35+/-9.88 microm) were obtained. On this titanium, intermediate layer HA coatings of various thicknesses were produced. With increasing coating thickness, roughness values of the HA coatings decreased. A high increase of profile length ratio, Lr, of the VPS-Ti coatings (Lr=1.45) compared to the grit-blasted CF/PEEK substrate (Lr=1.08) was observed. Increasing the HA coating thickness resulted in a reduction of the Lr values similar to the roughness values. Fractal analysis of the obtained roughness profiles revealed that the VPS-Ti coatings showed the highest fractal dimension of D=1.34+/-0.02. Fractal dimension dropped to a value of 1.23-1.25 for all HA coatings. No physical deterioration of the CF/PEEK substrate was observed, indicating that substrate drying and the used VPS process parameter led to the desired coatings on the composite material. Cross-section analysis revealed a good interlocking between the titanium intermediate layer and the PEEK substrate. It is therefore assumed that this interlocking results in suitable mechanical adhesive strength. From the results obtained in this study it is concluded that VPS is a suitable method for manufacturing HA coatings on carbon fibre-reinforced PEEK implant materials.

Urethane‐acrylate compositions cured by UV radiation as intermediate protective covers of optical fibers

AbstractThe present paper deals with studies concerned with an optimization of the synthesis of urethane‐acrylic UV‐cured resins intended for application as protective coatings for optical fibers. Oligoetherdi‐ and triols, toluylene diisocyanate and isophorone diisocyanate, as well as hydroxyethyl methacrylate and hydroxypropyl methacrylate were used for synthesis. Active diluents butyl acrylate, 2‐ethylhexyl acrylate, 1,4‐butanediol diacrylate or their mixtures, and 2,2‐dimethoxy‐2‐phenylacetophenone as a photoinitiator were added to the urethane‐acrylic dimers comprising polymerization‐active double bonds. The compositions were characterized by the determination of density, viscosity, refractive index, gel content, glass transition temperature, Shore hardness, Young's modulus, breaking stress, relative elongation at break, thermal resistance and water vapour diffusion coefficient before and after curing.The utility of these compositions for protective intermediate coatings of optical fibers was checked and confirmed under manufacturing conditions.

EIS study of environmentally friendly coil coating performances

The corrosion protection of metal structures by the application of organic coatings is obtained using complex paint cycles. The metal pretreatment as well as primer, intermediate and top coat are the usual layers composing the protection system. Although chromate pretreatment processes and chromate containing primers are widely used, they require highly toxic chromic acid solutions, with consequent effluent disposal and ecological problems. This fact pushed many research laboratories to develop new chromium-free pretreatments and primer inhibitors. Electrochemical impedance spectroscopy (EIS) proved to be a very useful technique to study the protection properties of the organic coating because of the possibility of evaluating the corrosion process on the metal substrate and to measure the electric and dielectric characteristics of the coating. In this study, new chromium-free pretreatments and new primers containing ecological inhibitors were applied to steel substrates coated using an Al-Zn alloy. The results were compared with those obtained using traditional protection systems. The characterization of the complete system (pretreatment and primer) in sodium sulfate solution clearly showed the different water uptake behavior of the two primers, which highlights the better barrier properties of the one containing phosphate. However the corrosion protection of the primer containing chromates is also due to the inhibitive action of the chromates. Hence such a type of primer better withstands the presence of defects in the coating. Moreover the presence of environmentally friendly pretreatments do not worsen the corrosion protection which appears comparable to that observed when chromatation is employed.

Microstructural changes and phase transformations in a plasma-sprayed zirconia-yttria-titania thermal barrier coating

A thermal barrier coating comprising a zirconia-yttria-titania ceramic coat deposited by plasma spraying on an alloy steel substrate using an NiCrAlCoY intermediate bond coat has been soaked at various temperatures in the range 700–1400 °C and cooled to room temperature in static air. Some thermal treatments were also carried out in an argon atmosphere. Scanning electron microscopy (SEM) studies show that a discontinuous thickness of the bond coat is the reason for the failure of the coating due to thermal treatment. The phase transformations and microstructural changes due to thermal treatment have been investigated using X-ray diffraction and SEM. Thermal treatment did not affect the monoclinic phase of the ZrO 2 content present in the ceramic coat. In the case of thermal treatment in air and argon (below 800 °C) the cubic→tetragonal phase transformation occurred. The appearance of the fracture surfaces of the ceramic coat suggests that interlamellar fracture in the as-sprayed condition changed to intralamellar fracture after thermal treatment below 1200 °C. At higher temperatures the fractures were intergranular in appearance. Reasons for these changes in the appearance of the fracture surfaces are suggested with reference to the microstructure. Microstructural investigations also show some changes in the area of porosity distribution of the ceramic coat. Thermal treatment heals some pores, but other voids appear (cracking of the coating).

Coatings which control adhesive strength in a metal-polymer system

In creating composite materials, glued joints, and also in preparing production equipment for casting and pressing polymers considerable attention should be devoted to choosing the composition of intermediate layers and coatings which control the adhesion reaction between metal and polymer.