Al Coatings Research Articles

Effect of Hf addition on initial microstructure of (Ni, Pt)Al coating and inhibiting the detrimental sulfur during high temperature exposure at 1150 °C

Single-phase (Ni, Pt)Al coatings with and without Hf addition were prepared via electroplating a Pt/Pt-Hf layer and subsequent gaseous phase aluminisation. The initial microstructure and oxidation behaviour of two coatings were investigated comparatively. The results show that finer grains were obtained in Hf-doped Pt-plating layer and as-received (Ni, Pt)Al coating, which contributes to better high temperature performance and oxide scale peeling resistance. Additionally, Hf can effectively retard the θ-Al2O3 transformed to α-Al2O3 during the early oxidation and enhanced oxidation resistance during the stable oxidation. The effect of Hf on inhibiting detrimental sulfur in acid-(Ni, Pt)Al during high-temperature exposure were intensively discussed.

Study on the corrosion electrochemistry behavior and wear resistance of the arc thermal sprayed Zn–Al alloy coating

The mechanisms of deposition and solidification processes of molten droplets after collision with the substrate during arc spraying were investigated by Fluent simulation. Subsequently, the Al, ZnAl alloys and Zn–Al pseudo-alloy coatings were prepared by arc spraying through parameter optimization. Furthermore, the electrochemical properties of arc-sprayed Al, ZnAl alloys and Zn–Al pseudo-alloy coatings corroded in 3.5 wt% NaCl solution for different days were studied. The electrochemical impedance spectra (EIS) investigation indicated that the pitting corrosion occurred on the surface of Al coating, and even the substrate material also appeared to be corroded. In comparison, the Zn–Al pseudo-alloy coating has both pure Zn and Al coatings in a corrosive environment with the cathodic effect of sacrificial anode protection and the shielding effect of a passivated protective film. As a result, it has an excellent corrosion resistance. Meanwhile, the corrosion mechanism of arc-sprayed Al, ZnAl alloys and Zn–Al pseudo-alloy coatings was further characterized by the combination of SEM surface morphology and XRD pattern after the corrosion. Furthermore, the wear resistance of the above-mentioned coatings was examined, and the investigations showed that the arc-sprayed Zn–Al pseudo-alloy coatings also have an excellent wear resistance.

Microstructure and Properties of Al-Based Ceramic Coating Deposited on Magnesium Alloy Surface by Cold Spraying

In this study, pure Al and Al-Al2O3 composite coatings with a low porosity and high density were prepared on magnesium alloys by cold spraying. The surface morphology, component, hardness, interfacial bonding, wear and corrosion properties were investigated. Additionally, the relationship between the interface structure and the protective coatings’ quality was preliminarily established. Pure Al powder was used to create a coating with a homogeneous and dense microstructure. The hard-phase Al2O3 in the composite coatings was non-oxidized and would reduce the porosity of the coatings to improve their density and interfacial bonding by up to 55.82 MPa. The bonding mode of the pure Al coatings is primarily mechanical, whereas the bonding mode of the Al-Al2O3 mixed coatings is mechanical and metallurgical. The tough Al2O3 particles combined to form a layer of work-hardening reinforcement that resisted wear and effectively prevented it from spreading. The three Al-based coatings had excellent corrosion properties, as evidenced by their corrosion current being several orders of magnitude lower than that of the magnesium alloy substrates. The thick coating was significantly more corrosion-resistant than the thin coating and provided greater protection to the substrate. This study offers theoretical and technological assistance for the surface protection of magnesium alloy equipment in demanding conditions.

Role of 5 wt.% Mg Alloying in Al on Corrosion Characteristics of Al-Mg Coating Deposited by Plasma Arc Thermal Spray Process.

The corrosion of steel structures in coastal areas is a major issue. Therefore, in the present study, the protection against the corrosion of structural steel is carried out by depositing 100 μm thick Al and Al-5 Mg coatings using a plasma arc thermal spray process, immersing them in 3.5 wt.% NaCl solution for 41 days (d). To deposit such metals, one of the best known processes, arc thermal spray, is frequently used, but this process has severe defects and porosity. Thus, to minimize the porosity and defects of arc thermal spray, a plasma arc thermal spray process is developed. In this process, we used normal gas to create plasma instead of argon (Ar) and nitrogen (N2) with hydrogen (H) and helium (He). Al-5 Mg alloy coating exhibited uniform and dense morphology, where it reduced more than four times the porosity compared to Al, where Mg fills the voids of the coating, resulting in greater bond adhesion and hydrophobicity. The open circuit potential (OCP) of both coatings exhibited electropositive values due to the formation of native oxide in Al, while in the case of Al-5 Mg, the coating is dense and uniform. However, after 1 d of immersion, both coatings showed activation in OCP, owing to the dissolution of splat particles from the corner where the sharp edges are present in the Al coating, while Mg preferentially dissolved in the Al-5 Mg coating and made galvanic cells. Mg is galvanically more active than Al in the Al-5 Mg coating. Due to the capacity of the corrosion products to cover the pores and defects, both coatings stabilized the OCP after 13 d of immersion. The total impedance of the Al-5 Mg coating is gradually increased and is higher than the Al, which can be attributed to the uniform and dense coating morphology where Mg dissolves and agglomerates to form globular corrosion products and deposit over the surface, thereby causing barrier protection. The defect bearing corrosion products on Al coating led to the cause having a higher corrosion rate than the Al-5 Mg coating. A total of 5 wt.% mg in the Al coating improved the corrosion rate by a rate of 1.6 times compared to the pure Al in the 3.5 wt.% NaCl solution after 41 d of immersion.

Effects of metal film coatings on surface properties of laser-textured stainless steel

Stainless steel substrates were textured with a checkboard pattern by a NIR laser with different scan pitches and laser powers. The as-textured surfaces were found to be hydrophilic. However, after holding in air for 7–15 days, the surfaces exhibited strong superhydrophobicity with contact angles close to 150° due to a reduction in the oxygen content. Thin-film Ag, Cu, Zn and Al coatings were deposited on the textured surfaces. The as-sputtered Ag and Cu films improved the hydrophobicity of the as-textured surface. However, the as-sputtered Zn and Al coatings increased the hydrophilicity. For all of the coatings, the hydrophobicity improved after aging in air for 7– 15 days. Furthermore, all of the coatings enhanced the corrosion resistance of the textured surface. Overall, the results indicate that laser texturing followed by thin-film metal deposition provides a feasible means of tuning both the wettability and the corrosion resistance of stainless steel surfaces.

Tuning the secondary reaction zone between PtAl coating and nickel based single crystal superalloy during high-temperature oxidation through regulating aluminizing time

Microstructure degradation caused by interdiffusion between coating and superalloy is an important factor affecting the performance of thermal barrier coatings. Tuning the interdiffusion microstructure of PtAl Coatings/Superalloys interface is the key to preparing high performance thermal barrier coatings. In this paper, two β-(Ni, Pt)Al coatings with different compositions and thicknesses were first deposited on the same nickel-based single-crystal superalloy by regulating the aluminizing time. After the isothermal oxidation at 1100 °C, a typical two-layer structure forms in the two different coating/superalloy specimens but with different interdiffusion microstructures. It was found that a noticeable secondary reaction zone (SRZ) with topologically close-packed (TCP) precipitates develop in the specimen with 6 h aluminizing after oxidation for 36 h and its thickness continuously increase. By contrast, no SRZ forms in the specimen with 4.5 h aluminizing after oxidation for even 100 h, but only the microstructure transformation of the interdiffusion zone (IDZ) occurred. That is mainly due to the existence of an incomplete transformed sublayer in the as-prepared specimen with a short aluminizing time, which may affect the subsequent interdiffusion of Al, Ni, and refractory elements. It is highly expected that regulating the aluminizing time is an effective strategy to tune the interdiffusion structure between coating and substrate, which should be helpful for designing the coating/substrate system with better performance.

Supercurrent reversal in ferromagnetic hybrid nanowire Josephson junctions

We report supercurrent transport measurements in hybrid Josephson junctions comprised of semiconducting InAs nanowires with epitaxial ferromagnetic insulator EuS and superconducting Al coatings. The wires display a hysteretic superconducting window close to the coercivity, away from zero external magnetic field. Using a multi-interferometer setup, we measure the current-phase relation of multiple magnetic junctions and find an abrupt switch between $\pi$ and 0 phases within the superconducting window. We attribute the 0-$\pi$ transition to the discrete flipping of the EuS domains and provide a qualitative theory showing that a sizable exchange field can polarize the junction and lead to the supercurrent reversal. Both $0$ and $\pi$ phases can be realized at zero external field by demagnetizing the wire.

Preparation, Wear Resistance, and Corrosion Performance of Arc-Sprayed Zn, Al, and Zn-Al Coatings on Carbon Steel Substrates

Preparation, Wear Resistance, and Corrosion Performance of Arc-Sprayed Zn, Al, and Zn-Al Coatings on Carbon Steel Substrates

Effect of Heat Treatment on the Mechanical and Tribological Properties of Dual-Reinforced Cold-Sprayed Al Coatings

The aluminum cold spray feedstock powder was single- and dual-reinforced with no greater than 2 vol% boron nitride nanoplatelets (BNNP) and/or nanometric boron carbide (nB4C). These powders were cold sprayed onto Al-6061 substrates and then heat-treated in an argon environment. In addition, micro- and nano-indentation hardness and wear testing were performed on the heat-treated samples. Further microscopy and optical profilometry were used to characterize the microstructure and wear track volumes. Minimal changes to the splat structure were observed after heat treatment. However, when compared to the pure Al coating, microhardness improved with reinforcement after treatment at 500 °C, while nanohardness improved only in the dual-reinforced coatings, again after treatment at 500 °C. The elastic modulus generally decreased for the reinforced coatings after treatment; however, indentation test results were mixed. The wear testing done on samples heat treated at 500 °C for one hour showed increases in the specific wear rate for single-reinforced coatings but decreases in the dual-reinforced coatings. These results indicate that both dual-reinforcement and heat treatment are required for improvements in the mechanical and tribological properties of Al nanocomposites.

Zn–Al pack co-cementation on AISI 1020 steel: structural, mechanical and tribological appraisal

ABSTRACT This study examined the effect of different Al concentrations on the structural transformation of pack cementation Zn–Al coatings on AISI 1020 steel. The resultant Zn–Al coatings were investigated using scanning electron microscopy, energy dispersive spectroscopy and X–ray diffraction to observe the coating morphology, thickness, structure, and compositional migration. A laminated coating assembly was prepared, in which an initial intermediate Zn–rich layer was followed by an Fe–Zn inner layer and Fe–Al top layer. Elemental analysis revealed a correlation between the formation of voids and the diffusion of Al, Fe, and Zn. The coated specimen demonstrated desirable mechanical strength and improved abrasive resistance, whereas the tribological capability of the Zn–Al coating could be tailored by varying the Al concentration.

Research on the technology of flame spraying aluminum coating for composite material

Aluminum coating was prepared on composite material surface by using oxygen-acetylene flame spraying technology. The H01-103H sealants were used to seal the thermal sprayed aluminum coatings. The characteristics of the aluminum coatings with and without H01-103H sealant, including surface morphologies, phase composition, and bonding strength, were investigated by means of SEM, EDS, and mechanical testing machine. Thermal shock experiment and low temperature cooling test were also used to examine the thermal shock resistance and frost resistance of the Al coatings. Electrical conductivity and electromagnetic shielding properties were then tested according to four-probe measurement principle and GJB 5239-2004, respectively. The results showed that the bond strength was 7.36 MPa and the surface porosity was about 5.21% for Al coating without H01-103H sealant, in addition, the H01-103H sealants could fill into the pores, reducing the porosity and increasing the bond strength of the coating. Furthermore, the main component of the coating was Al, the oxygen content was only 1.43%. Flame sprayed aluminum coating, with surface electrical resistance 0.89 mΩ and 5–18 GHz electromagnetic shielding effectiveness above 35 dB, was an ideal candidate for electromagnetic shielding utilization. The Al coating remained intact, compact, no cracks, peeling, layering under 50–200°C and −30°C, which showed good thermal shock resistance and frost resistance.

Galvanic synthesis of ZnO and ZnO(Al) coatings from dimethylsulfoxide electrolytic baths

Galvanic synthesis of ZnO and ZnO(Al) coatings from dimethylsulfoxide electrolytic baths

Theoretical and experimental research of hydrogen storage properties of Mg and Mg-Al hydrides

In this work, we studied the effect of aluminum atoms addition on the properties of hydrogen accumulation and desorption in magnesium hydride. Using ab initio calculations and experimental methods, the role of alloying magnesium with aluminum was determined. The hydrogen content in the Mg and Mg–10 at%Al coatings was 1.8 wt% and 4.5 wt%, respectively. For the Mg–10 at%Al coating, the peak temperature of hydrogen desorption was about 425 °C, 17 °C lower than that for the Mg coating. The desorption rate for the Mg coating and Mg–10 at%Al coating was 3 × 10−14 mol(H2)/cm2 s and 31 × 10−14 mol(H2)/cm2 s, respectively. Compared with the Mg coating, the higher hydrogen content in the Mg–10 at%Al coating with the faster hydrogen desorption is related to the energetically more favorable Al-doped Mg system than the pure Mg system with H at the distance from Al at low hydrogen concentrations. The lower hydrogen desorption temperature of the Mg–10 at%Al coating is confirmed to be the consequence of the weakened MgH bond by the doped Al atom due to the higher electronegativity of the Al atom than that of the Mg atom. In addition, the aluminum binding energy decreases more significantly with the hydrogen concentration increase for hydrogen near Al atom than for hydrogen at a distance.

Reflection-enhanced laser-induced fluorescence spectroscopy to improve the analytical sensitivity in liquids

The current work demonstrates a novel approach to enhancing the analytical sensitivity of the laser-induced fluorescence (LIF) technique in liquids. An increase in the fluorescence spectral band intensity of about sixfold compared to the conventional LIF has been achieved. Such betterment has been accomplished by having the fluorophore liquid in a cuvette having a reflecting mirror-like side facing the exciting incident laser beam. The silvered or aluminized reflecting side of the cuvette was tested. The pilot test of the proposed cuvette was performed using an excitation laser light of 405 nm wavelength and five mW average power on chlorophyll-a (Chl-a) samples of different concentrations. As a result, a pronounced improvement in the Chl-a fluorescence spectral band intensity is achieved. Such a novel approach, the reflection-enhanced laser-induced fluorescence (RELIF), has been used to analyze six Egyptian brands of extra virgin olive oil (EVOO). Compared to the LIF measurement results on the same EVOO, the RELIF was superior in terms of the fluorescence-spectrum intensity enhancement factor. Both Ag and Al coatings of the cuvette revealed similar results. Statistical analysis of the measured fluorescence spectra via the partial least square regression (PLSR) method for LIF and RELIF revealed a higher coefficient of determination R2 for both RELIF measurements (with silver and aluminum coating) than for LIF. The proposed novel RELIF approach can be utilized for other fluorophore liquids at higher analytical sensitivity than conventional LIF. On the other hand, the RELIF technique is straightforward, cost-effective, and does not complicate the traditional LIF setup.

Mechanical and Tribological Study on Aluminum Coatings with High-Pressure and Low-Pressure Cold-Spray Processes

Cold spray is a promising approach to repair all damages and defects in aluminum (Al) constituent elements. The study aims to investigate the mechanical and tribological properties of Al coatings deposited using high-pressure cold-spray (HPCS) and low-pressure cold-spray (LPCS) techniques. Al powder was sprayed on a cold-rolled plate of aluminum 1100, which was used as the substrate. The results showed that the micro-hardness of the LPCS Al coating reached up to 196.6 HV before the wear test compared to that of HPCS (174.3 HV). Moreover, more low friction coefficients obtained by LPCS (0.798) than HPCS (0.807) indicated good tribological properties with a high amount of oxide composition. Meanwhile, the wear studies reveal that the specific wear rate of the Al coating of LPCS (0.008) was lower than the HPCS (0.009) as the load increased from 3 N to 5 N, thus providing excellent wear resistance. Therefore, the results exhibited greater mechanical and tribological characteristics for Al coatings produced by the LPCS process than by the HPCS process.

Fatigue Strength of Structural Steel-Welded Connections with Arc-Sprayed Aluminum Coatings and Corrosion Behavior of the Corresponding Coatings in Sea Water

The influence of thermally sprayed aluminum coatings (Al99%; arc spraying) on the fatigue strength of gas metal arc welded (GMAW) non-alloyed structural steel specimens with respect to foundations for offshore wind turbines was investigated. Additionally, the corrosion protection effect of such coatings for water conditions similar to the Baltic Sea was determined. Wöhler tests were carried out on test specimens with different weld details in the as-welded condition as well as in the thermal spray coat under the consideration of different kinds of surface preparation (blast cleaning with corundum and grit). Substrate and coating were characterized by scanning electron microscopy and the influence on the residual stress states was determined. Corrosion rate monitoring via LPR measurements was carried out as well as the monitoring of the galvanic current between coated and uncoated steel to characterize the coatings’ sacrificial capability for minor defects. Fatigue strength was significantly increased through thermal spraying, especially for test specimens with welded transverse stiffeners (Δσc,var = 127 MPa after coating compared to Δσc,var = 89 MPa as welded). With a characteristic value of the stress range of Δσc,var = 153 MPa, the welded butt joint specimens already exhibited a high fatigue strength in the as-welded condition. The corrosion studies demonstrated that thermally sprayed Al99% coatings have a high resistance to corrosion in seawater environments and are suitable as planar sacrificial anodes sufficiently polarizing bare steel below 0.8 V. The combination of fatigue strength improvement and corrosion protection makes the thermally sprayed Al coatings promising for design and operation of e.g., offshore structures.

Wear Behaviour of Multilayer Al-PVD-Coated Polymer Gears.

A comprehensive experimental investigation of the wear behaviour of coated spur polymer gears made of POM is performed in this study. Three different thicknesses of aluminium (Al) coatings were investigated and deposited by the Physical Vapour Deposition (PVD) process. The Al coating was deposited in three steps: By plasma activation, metallisation of the aluminium by the magnetron sputtering process, and plasma polymerisation. The wear of the gears was tested on an in-house developed testing rig for different torques (16, 20, and 24 Nm) and a rotational speed of 1000 rpm. The duration of the experiments was set to 13 h, when the tooth thickness and, consequently, the wear of the tooth flank were recorded. The experimental results showed that the influence of metallisation with aluminium surface coatings on the wear behaviour of the analysed polymer gear is not significantly important. The results also showed that the gears with a thicker aluminium coating showed greater wear than gears with a thinner coating or even without a coating. This is probably due to the fact that the aluminium particles that started to deviate during gear operation represented the abrasive material, which led to the faster wear of the contacting surfaces of the meshing gear flanks.

Microstructures of Al coatings prepared through magnetron co-sputtering with microcrystalline Ti doping

Sintered NdFeB magnets are being utilised in numerous applications because of the excellent magnetic properties. However, due to its main components are Fe, B and rare earth metals. There is a potential difference between the neodymium rich phase, boron rich phase and the main phase, which was very easy to form a primary battery and cause electrochemical corrosion on the surface of the material. After the materials are easy to be corroded or the components and structures change, the performance of the materials will become weak or even lost, which will affect the service performance and life of the devices or equipment. According to the requirements of surface protection technology for NdFeB magnets, the preparation of protective coating by magnetron sputtering was an efficient means to enhance the corrosion resistance of magnets. Pure Al coating was deficient in microstructure, hardness and corrosion resistance, and the preparation of composite coating by doping co-sputtering had become a development trend. In the current work, AlTi coatings with various Ti contents were prepared from an Al target and a Ti target using direct and radio frequency hybrid magnetron co-sputtering. The surface morphology, microstructure, and elemental compositions of the AlTi coatings were explored through XRD (X-ray diffraction), SEM (scanning electron microscopy), and EDS (energy dispersive spectroscopy). The anticorrosion performances of the AlTi coatings were evaluated by performing electrochemical tests. According to SEM and EDS, the AlTi composite coating was denser and more homogeneous than that of the Al single layer. Furthermore, the results demonstrated that microcrystalline Ti doping reduced the grain size of Al coatings. Therefore, the corrosion current density of the sintered NdFeB magnets plated with Al(120w)/Ti(10w) was approximately more than a hundred times inferior to that of the pure Al layer, and the hardness of AlTi composite coatings also improved.

Structure and oxidation behavior of CoCrFeNiX (where X is Al, Cu, or Mn) coatings obtained by electron beam cladding in air atmosphere

High-entropy alloys (HEAs) are currently considered as promising materials for a wide range of applications, including high-temperature use. Since HEAs contain a large number of expensive elements, producing HEA-based coatings seems to be a reasonable approach from an economic point of view. In this study, non-vacuum electron beam cladding was used for the first time to produce CoCrFeNiX layers (where X = Al, Cu or Mn) on a surface of low-carbon steel. As-received coatings had a columnar grain structure with a 〈100〉 fiber texture. The CoCrFeNiCu cladding layer consisted of two fcc phases: Cu-enriched phase was observed in the interdendritic regions, and the Cu-depleted phase was found in dendrites. The CoCrFeNiMn coating had a single-phase fcc structure. CoCrFeNiAl alloy consisted of bcc and B2 phases. The hardness of the Cu and Mn-containing coatings was 190 and 186 HV, respectively. The hardness of the CoCrFeNiAl coating was 570 HV, which is 3.4 times higher compared to the material of the substrate. High-temperature oxidation tests carried out at 850 °C for 45 h revealed that oxide layers formed on the tested samples had a gradient structure that consisted of several different sublayers. The total mass gain due to high temperature oxidation was 2.29 mg/cm2, 2.04 mg/cm2, and 0.60 mg/cm2 for the CoCrFeNiCu, CoCrFeNiMn, and CoCrFeNiAl samples, respectively. The mass gain of the base material over the same period of time was 41.01 mg/cm2. The CoCrFeNiAl coating had the best oxidation resistance due to the formation of a dense Al2O3 scale. We show that non-vacuum electron beam cladding can be used to produce an HEA-based coating with a different chemical composition. The thickness of coatings in a single pass cladding can be as high as 1.8 mm. CoCrFeNiAl is a suitable coating for low-carbon steel in applications where high hardness and oxidation resistance are required.

Microstructural parameters estimation for sputtered Al coating via SEM, EDS, X-ray diffraction line broadening and EBSD

Aluminum (Al) coatings were deposited on Zr-4 alloy substrate by magnetron sputtering. The microstructure parameters of the as-deposited coating were characterized by means of scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electron backscattered scattering detection (EBSD). The results showed that: via the SEM and EDS, it can be found that Al atoms and Zr atoms undergo rapid interdiffusion at the interface, forming an Al-Zr diffusion layer with a thickness of about 1.5 μm; via X-ray diffraction line broadening, the estimated crystal size was 81.53 nm and the as-deposited Al coating presences lattice tensile strain; via EBSD, the fraction of HAGBs is overwhelmingly larger than that of LAGBs and the mean grain size is 0.55 μm, also, the Al coating exhibits the (100)<001> texture.