Pure Aluminum Coatings Research Articles

Study on Porosity of Thermal-Sprayed Commercially Pure Aluminum Coating.

Porosity is closely related to the corrosion and wear properties of a coating processed by thermal-spraying technology, and the quantitative characterization of porosity is a crucial part of the research on coating structures. The current image analysis method often uses the mechanical polishing method recommended by ISO to measure a coating porosity. This method has been proved to be an effective method for the characterization of oxide coatings. However, due to the significant differences in the physical and chemical properties between aluminum and oxides, this method may not be suitable for aluminum coatings, and a more appropriate approach needs to be explored. In this paper, the effects of three polishing technologies (mechanical polishing, argon-ion-beam polishing, and electrolytic polishing) on the porosity measurement of pure aluminum coatings were compared and studied. The research results showed that the commonly used mechanical polishing method and more advanced argon-ion-beam polishing method could not completely reveal the pore structure because SiC particles would be embedded in the pure aluminum coatings during mechanical polishing, filling large pores. Although electrolytic polishing technology had advantages in revealing the macroporous structure, it would introduce a microporous structure and oxides, which would affect the measurement of the coating porosity. The composite polishing technology (electrolytic polishing + argon-ion-beam polishing) could perfectly reveal the pore structure in the pure-aluminum coating, and the porosity of arc-sprayed aluminum coating was 9.9%, which was close to the macroscopic true value measured using the weighing method of 10.2%.

High-Performance Pure Aluminum Coatings on Stainless Steels by Cold Spray

Aluminum target material is an important target material and is widely used in preparations of semiconductor films, integrated circuits, display circuits, protective films, decorative films, etc. In this study, pure aluminum coatings were deposited on stainless steel substrates by cold-spray technology as part of an overall project to produce large-size pure aluminum sputtering target materials. The results show that pure aluminum coatings exhibit high adhesive strength (~98 MPa), high deposition efficiency (~95%), and low porosity (~0.3%) on stainless steel substrates. The bonding mechanisms of pure aluminum coatings on stainless steel are a combination of metallurgical and mechanical interlocking. The evolutions of microstructure and mechanical properties of pure aluminum coatings under different heat treatments were also studied. With the increase of heat treatment temperature, it is found that cold-sprayed aluminum coatings become more homogenous in microstructure, the microhardness is reduced, and the adhesive strength seems to be slightly reduced. Overall, this study demonstrates significant advantages of cold-spray technology in depositing high-performance pure aluminum coatings on stainless steels.

Interface Characteristics, Erosion Behavior, and Thermal Shock Resistance of Al–Ta Alloy Coatings Produced by Arc Spraying

The pure aluminum coatings of physical vapor deposition chambers used in current semiconductor processes offer insufficient protection against ion bombardment. In this study, the arc spraying process was used to produce Al–Ta alloy coatings to replace pure aluminum coatings. The microstructure, mechanical properties, and particle erosion properties of these coatings were investigated at 1% and 3% Ta content and compared with those of pure Al. The surface and surface morphologies of the three coatings were similar. However, Ta was uniformly dispersed to form a supersaturated solid solution in the 1Ta and 3Ta coatings. The tensile results indicated that the adhesion of the three coatings was similar. According to particle erosion results, cohesive strength and erosion resistance apparently increased. Moreover, the thermal shock effect caused by the extreme thermal cycle in these chambers causes coating degradation. So, this study compared the thermal shock properties of the Al–Ta alloy coatings with Y2O3 ceramic coating. The 1Ta and 3Ta coatings did not differ considerably from pure Al coating in terms of thermal shock resistance, and all three were higher than that of the Y2O3 coating. The aforementioned results indicate that the produced Al–Ta alloy coatings can offer superior chamber protection ability.

An investigation of plasma sprayed nickel-based and pure aluminum coatings on austenitic stainless steel AISI 304

Atmospheric Plasma Spray (APS) is one of the most leading industrial techniques for protective coating, by improving the performance of parts in the thermal barrier, and wear resistance. Ni-Al alloys are very effective players in the field of design of protective coatings. Accordingly, mixed Al, Ni/Al, and Ni5Al powders were applied on 304stainless steel substrate to develop plasma sprayed coatings. The effect of different compositions on microstructure, microhardness, and porosity was measured. The microstructures of the as-deposited films were characterized utilizing X-ray diffraction (XRD), scanning electron microscopy (SEM), and microhardness measurements. The results showed the formation of two intermetallic compounds, namely: NiAl and Ni3Al. The existence of NiAl is inevitable in all samples, despite the amount of Ni-based alloys in mixtures, or even the atomic percentage of nickel, where the appearance of Ni3Al depends only on increasing the amount of the Ni-based alloy to 50 % percent in mixtures. As regards the steel substrate, the microhardness of the interdiffusion zone of the substrate has been significantly enhanced. Results have shown that the microhardness of the different tested coatings is increased directly with the increment of Ni-based percentage in the coating mixture. The average porosity of the plasma sprayed coatings has proven to be within the normal range.

Cold Spray Aluminum–Alumina Cermet Coatings: Effect of Alumina Content

Deposition behavior and deposition efficiency were investigated for several aluminum–alumina mixture compositions sprayed by cold spray. An increase in deposition efficiency was observed. Three theories postulated in the literature, explaining this increase in deposition efficiency, were investigated and assessed. Through finite element analysis, the interaction between a ceramic particle peening an impacting aluminum particle was found to be a possible mechanism to increase the deposition efficiency of the aluminum particle, but a probability analysis demonstrated that this peening event is too unlikely to contribute to the increment in deposition efficiency observed. The presence of asperities at the substrate and deposited layers was confirmed by a single-layer deposition efficiency measurement and proved to be a major mechanism in the increment of deposition efficiency of the studied mixtures. Finally, oxide removal produced by the impact of ceramic particles on substrate and deposited layers was evaluated as the complement of the other effects and found to also play a major role in increasing the deposition efficiency. It was found that the coatings retained approximately half of the feedstock powder alumina content. Hardness tests have shown a steady increase with the coating alumina content. Dry wear tests have revealed no improvement in wear resistance in samples with an alumina content lower than 22 wt.% compared to pure aluminum coatings. Adhesion strength showed a steady improvement with increasing alumina content in the feedstock powder from 18.5 MPa for pure aluminum coatings to values above 70 MPa for the ones sprayed with the highest feedstock powder alumina content.

Effect of powder size on the long‐term corrosion performance of pure aluminium coatings on mild steel by cold spraying

To achieve the long‐term corrosion protection of mild steel under corrosive environment, pure aluminium coatings were cold sprayed (CS) on mild steel with three different powder sizes, namely within the three following classes 85–100, 60–75 and 40–50 µm. The as‐sprayed samples were exposed to the atmosphere for 11 years. The long‐term corrosion behaviour of the sprayed steels was studied by atmospheric exposure, immersion tests as well as electrochemical measurements in 3.5 wt% sodium chloride (NaCl) solution. It was found that the coatings with coarse powder reduced the long‐term corrosion resistance of the samples. The deteriorated corrosion resistance of the sprayed samples was ascribed to the penetration of the corrosive medium (oxygen, water and electrolyte) through the porosity defects of coatings and with coarse powder. These results provide a guide for the design of cold sprayed corrosion‐resistance pure Al coatings for engineering applications.

Optimization of the Quality Characteristics of Laser-Assisted Cold-Sprayed (LACS) Aluminum Coatings with Taguchi Design of Experiments (DOE)

This study employs Taguchi design of experiments to identify optimal process parameters (deposition temperature, transverse speed, powder feed rate, and operating pressure) associated with quality characteristics (porosity and hardness) of laser-assisted cold-sprayed (LACS) pure aluminum coatings. Optimal quality characteristics of the coatings were obtained by setting deposition temperature at 450°C, transverse speed at 5 mm/s, operating pressure at 30 bar, and powder feed rate at 25 rev/min. This study helps in improving the yield and stability of pure aluminum coatings produced via LACS process while it also reduces its variability which results in high rejection, rework, and warranty costs.

429 低圧および高圧コールドスプレーにより作製した航空機の脚部品用純アルミニウム皮膜の耐食性

429 低圧および高圧コールドスプレーにより作製した航空機の脚部品用純アルミニウム皮膜の耐食性

Effect of Surface Preparation on the Microstructure, Adhesion, and Tensile Properties of Cold-Sprayed Aluminum Coatings on AA2024 Substrates

Commercially pure aluminum coatings (CP-Al) were applied to AA 2024-T351 substrates utilizing the cold spray process using different surface preparation methods and carrier gases; the resulting microstructures and mechanical properties were investigated. Substrate preparation methods were examined to understand the effect of substrate roughness on coating properties, to minimize embedded grit, and to identify the surface preparation method that yielded the best combination of coating properties. Three substrate roughing preparations, glass bead, SiC grit, and alumina grit blast, were examined while utilizing both helium and nitrogen as carrier gases in the cold spray process. Coatings that were oxide free, possessing densities greater than 99% were achieved, with the mean coating porosity ranging 0.1-0.5%. The highest mean adhesion strength was 42 MPa for the nitrogen gas and 20 MPa for the helium gas, both using glass bead surface preparation. For the nitrogen process gas samples, the surface preparation methods that produced high pull strengths correlated to bend test specimens that showed no signs of cracking on surfaces or edges. The overall best combination of mechanical property results was achieved with coatings prepared by glass bead surface roughening using nitrogen as a carrier gas.

Preparation of aluminum coating on Lexan by cold spray

Metallization of polycarbonate is challenging by the conventional polymer metallization technologies. For the first time, this study explored an industrially practical method to deposit dense and thick metallic coating on a polycarbonate substrate. Pure Aluminum coatings were successfully deposited onto Lexan surface using cold spray technology. The powder feeding rate was examined to be a critical processing parameter for forming continuous coating on Lexan, in addition to generally studied spray temperature and pressure. The deposition behavior of Aluminum on the polycarbonate, especially the effect of powder feeding rate, was discussed. The results demonstrate that cold spray is a promisingly economical and practical metallization method for polycarbonate.

Abrasive Wear Performance of Hot-Dipping Al-Mn Alloy Coatings on Q235

The Q235 steel was modified by hot-dip-aluminizing technique, and Al-Mn alloy layer was obtained on the steel surface. Scan electron microscopy (SEM), X-ray diffraction (XRD) and abrasive wear tester were used to investigate the microstructure and wear performance of the Al-Mn alloy coatings. The results show that the surface of the obtained pure aluminum coatings is acicular, while blocks Al-Mn compounds phase exists in Al-Mn alloys layer, and the Al-Mn alloy layer is composed of Al, FeAl3, Fe2Al5 and MnAl6 phases. The experimental results showed that wear weight loss of aluminum coating is more than Q235 steel. However, after manganese added to the aluminum, Al-Mn alloy coating abrasion wear loss of weight far below the Q235 steel and aluminum coating. And weight loss increases along with Mn content decreases. When worn after 100 h, Al-13% wtMn wear alloy coating weight loss of 45% of Q235 steel, aluminum coating of 35%. So the hot-dipping Al-Mn alloy layer has excellent abrasive wear resistance.

Effect of heat treatment on the intermetallic layer of cold sprayed aluminum coatings on magnesium alloy

Dense and thick pure aluminum coatings were deposited on AZ91D-T4 magnesium substrates using the cold spray process. Heat treatments of the as-sprayed samples were carried out at 400°C using different holding times. The feedstock powder, substrate and coating microstructures were examined using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) as well as Vickers microhardness analysis. The results demonstrate that aluminum coatings having dense and uniform microstructure can be deposited successfully using a relatively large feedstock powder. It has been identified that the intermetallics Al3Mg2 (γ phase) and Mg17Al12 (β phase) were formed at the coating/substrate interface during heat treatment. The growth rate of these intermetallics follows the parabolic law and the γ phase has a higher growth rate than the β phase. The thickness of the Mg17Al12 and Al3Mg2 intermetallic layers has reached 83μm and 149μm, respectively. This result is almost 45% higher than what has been reported in the literature so far. This is attributed to the fact that T4 instead of as cast Mg alloy was used as substrate. In the T4 state, the Al concentration in the Mg matrix is higher, and thus intermetallic growth is faster as less enrichment is required to reach the critical level for intermetallic formation in the substrate. The AZ91D-T4 magnesium substrate contains single α phase with fine clusters/GP-zones which is considered beneficial for the intermetallic formation as well as the intimate contact between the coating/substrate interface and the deformed particles within the coating.

Study on corrosion resistance properties of hydrothermal sealed arc sprayed aluminium coating

In this study, traditional hydrothermal sealing (HTS) treatment was for the first time employed to arc sprayed pure aluminium coatings, and proves to be a successful sealing method. Corrosion performances were examined in salt solution immersion test (SSIT) and electrochemical polarisation measurements. The HTS coating is covered with a continuous film of aluminium hydroxide structures, and some deionised water manages to penetrate. Besides, pores in the surface as well as in the inner part get smaller and fewer. In SSIT, the HTS coating has a similar corrosion process to the unsealed one, but corrodes much more slightly. In polarisation curves, the HTS coating exhibits a wide passive region, higher corrosion potential and lower current density than the unsealed coating before immersion. After SSIT, it exhibits small changes in polarisation curves, implying slight corrosion in 90 days immersion and proving much higher corrosion resistance than the unsealed coating in the whole process.

Corrosion resistance of thermally sprayed Al and Al/SiC coatings on Mg

Thermally sprayed Al and Al/SiC composite coatings have been deposited on pure magnesium and their corrosion behaviour in 3.5 wt.% NaCl solution has been studied. Al/SiC composite coatings were fabricated with 9 vol.% reinforcement percentage and with porosities around 3.5%, while the porosity of the as-sprayed pure Al coatings was in the range 2.4 vol.% to 4.4 vol.% and depended more on the spraying distance and numbers of layers deposited. A mechanical compaction was applied to the coatings at room temperature and coatings that were nearly free from pores were fabricated for the Al and Al/SiC coatings. In the electrochemical tests, all the coatings showed higher corrosion potentials (about of 200 mV) than uncoated substrates while current densities were nearly two orders of magnitude lower for the Al/SiC composite coatings and three orders of magnitude lower for the pure aluminium coatings. This protective effect was even better in the compacted coatings, currents densities were between five or six orders of magnitude lower than that of uncoated magnesium.

Properties of Aluminum Deposited by a HVOF Process

Pure aluminum coatings deposited by a high velocity oxyfuel (HVOF) process have been produced and studied. A simple design-of-experiment (DOE) was used to assess the effect of two deposition parameters, the spray distance and oxygen-to-fuel ratio, on relevant coating properties. Porosity, surface roughness, and micro-hardness of the coatings were measured as responses to changes in the DOE parameters. The results indicated that these three properties of the aluminum coatings were normally insensitive to spray distance. Oxygen-to-fuel ratio, by flow, did appear to affect the porosity level of the coatings. Some post-coat processing of the aluminum coatings and minimization of nozzle loading are discussed.

Effects of laser surface remelting on hydrogen permeation resistance of thermally-sprayed pure aluminum coatings

The effects of laser surface remelting on the microstructure and hydrogen permeation resistance of thermally-sprayed aluminum coatings on the surface of austenitic stainless steel have been investigated by means of XRD, SEM and thermal hydrogen charging. The results show that the residual porosity is eliminated and the phases are mainly AlFe(Ni, Cr) and CrFeNi. A metallurgical bond between the coatings and the base metal was formed after laser surface remelting. Also, laser surface remelting can improve the hydrogen permeation resistance of thermally-sprayed pure aluminum coatings to a certain extent.

Life prolongation of far ultraviolet reflecting aluminum coatings by periodic recoating of the oxidized surface

In-orbit operating optical instruments provided with pure aluminum coatings are expected to promote great advances in far ultraviolet (FUV) astronomy. A difficulty to be overcome however, is the oxidizability of aluminum coatings in low earth orbits (LEO), where the residual atmosphere is constituted mainly by atomic oxygen. It is here demonstrated that the lifetime of an aluminum coating oxidized by limited exposures to atomic oxygen can be substantially extended by periodic recoating of the oxidized aluminum surface with new aluminum coatings. Recoating thicknesses of 30 and 60 nm are shown to completely recover the near normal FUV reflectance in the case of a mirror exposed to doses of atomic oxygen which caused decreases in the reflectance of about 13% and 30%, respectively, at 104.8 nm. After ten recoatings the reflectance of the primary aluminum film was completely recovered. Accordingly the lifetime of an aluminum coating placed in a LEO could be increased by an order of magnitude or more.

Ion-beam-assisted deposition of aluminium and aluminium alloy coatings for corrosion protection

Ion-beam-assisted deposition (IBAD) was used for depositing pure aluminium coatings on low carbon steel in order to study the corrosion properties as a function of the energy of the incident argon ions, the ion-to-atom arrival ratio and the angle of ion incidence. With theoretical considerations about the localized corrosion behaviour of substrate-coating systems it is possible to evaluate the porosity, the relative number of coating defects and their mean diameter. Their dependence on the process parameters gives an insight into the initial stages of film growth and shows that the energy deposition in the near-surface region is the most important factor for modifying coating defects and therefore the corrosion properties of the steel-aluminium system. The modification potential of each parameter of IBAD is studied in comparison to pure vapour deposition for the corrosion in acetate buffer. In chloride-containing aqueous solutions the depassivation behaviour of aluminium can be improved by use of alloying elements. Aluminium and magnesium were deposited on inert glass substrates; the magnesium concentration and the bombarding energy were varied. The resulting alloys have equilibrium potentials less noble than pure aluminium and increased passive regions. The passive region decreases with increasing magnesium concentration. From glancing-angle X-ray diffraction measurements it can be seen that structural changes influenced by the ion beam go parallel with changes in the localized corrosion behaviour.

Corrosion properties of aluminum based alloys deposited by ion beam assisted deposition

The replacement of cadmium coatings by other protective measures is an important task because of the environmentally detrimental properties of cadmium. Therefore, aluminum and aluminum alloy coatings containing elements such as silicon or magnesium with more positive or negative positions in the galvanic series in relation to pure aluminum were deposited by ion beam assisted deposition onto glass and low carbon steel. Pure aluminum films were deposited onto low carbon steel in order to study the influence of the ion -to-atom arrival ratio and the angle of ion incidence on the corrosion properties. For examination of the pitting behavior as a function of the concentration of alloying element, quasipotentiostatic current-potential and potentiostatic current-time plots were measured in chlorine-containing acetate buffer. It is shown that these alloys can protect steel substrates under uniform and pitting corrosion conditions considerably better than pure aluminum coatings.

Corrosion behaviour of physical vapour deposition aluminium-based coatings on 2024 aluminium alloy

Pure aluminium and Al-Li coatings applied by physical vapour deposition on 2024 aluminium alloy were tested for their corrosion properties. It was concluded that both the effects of self-dissolution of the coating and cathodic protection should be considered in the assessment. The results favour the use of pure aluminium instead of Al-Li alloy.