Sprayed Alumina Coatings Research Articles

Promoting microalgal biofilm formation by crushed oyster shell-hydroxyapatite layer on micropatterned aluminum coating for heavy metal ions removal

Microalgal biomass has shown inspiring potential for the heavy metal removal from wastewater, and forming microalgal biofilm is one of the sustainable methods for the microalgal biomass production. Here we report the formation of microalgal biofilm by accelerated colonization of typical algae Chlorella on thermal sprayed aluminum (Al) coatings with biologically modified surfaces. Micro-patterning surface treatment of the Al coatings promotes the attachment of Chlorella from 6.31 % to 17.51 %. Further enhanced algae attachment is achieved through liquid flame spraying a bioactive crushed oyster shell-hydroxyapatite (CaCO3-HA) composite top layer on the micropatterned coating, reaching 46.03–49.62 % of Chlorella attachment ratio after soaking in Chlorella suspension for 5 days. The rapidly formed microalgal biofilm shows an adsorption ratio of 95.43 % and 85.23 % for low concentration Zn2+ and Cu2+ in artificial seawater respectively within 3 days. Quick interaction has been realized between heavy metal ions and the negatively-charged extracellular polymeric substances (EPS) matrix existing in the biofilm. Fourier transform infrared spectroscopy (FTIR) results indicate that both carboxyl and phosphoryl groups of biofilms are crucial in the adsorption of Cu2+ and the adsorption of Zn2+ is due to the hydroxyl and phosphate groups. Meanwhile, the biofilm could act as a barrier to protect Chlorella against the attack of the heavy metal ions with relatively low concentrations in aqueous solution. The route of quick cultivating microalgal biofilm on marine structures through constructing biological layer on their surfaces would give insight into developing new techniques for removing low concentration heavy metal ions from water for environmental bioremediation.

Role of powder morphology on α-phase content in plasma sprayed alumina coatings

Despite various studies addressing the retention of α phase in thermal sprayed alumina coatings, a noticeable research gap persists regarding the impact of feedstock morphology on phase retention and properties. In this work, we explored this simple way to enhance the retention of the α phase and studied its impact on mechanical and dielectric properties. To achieve this, we deposited alumina powders with two different morphologies (such as angular and spherical) with similar particle size using plasma spraying. After deposition, we quantified the retention of α phase present in the coatings using the Rietveld refinement method. From this analysis, we noticed that spherical morphology (SM) powder exhibited the highest percentage of α phase retention (82.81 %) as compared to angular morphology (AM) powder (48.8 %). In turn, SM-coating demonstrated superior mechanical properties, with improvements in hardness, elastic modulus and fracture toughness of 21.9 % 13.7 % and 25 % respectively, compared to AM-coating. This improved mechanical properties in SM-coating is attributed to high α phase retention, synergistic effect of both fully melted (FM) and partial melted (PM) regions of coating microstructure which hindered the crack propagation. Furthermore, the SM-coating exhibited superior dielectric properties, with a dielectric strength 9.9 % higher than the AM-coating. This dielectric performance is ascribed to high retained α phase and generation of discontinuous crack due to presence of PM regions in the SM-coating. These findings suggest the impact of feedstock powder morphology on coating performance and offer avenues for optimizing coating processes in diverse technological fields.

Effect of Sulphate-Reducing Bacteria Activity on the Performance of Thermally Sprayed Aluminium and Polyurethane Coatings

In the present work, we studied whether the exposure of synthetic seawater with anaerobic sulphate-reducing bacteria (SRB) on some steel samples generates a bacterial biofilm in their surfaces. Bare steel belonging to a mooring chain as well as two coating systems applied on the steel surface were studied: polyurethane (PU) and thermally sprayed aluminium (TSA) with and without an epoxy-based sealant. After 30 days of immersion in SRB-inoculated synthetic seawater, a bacterial count was attained, and the samples were observed using scanning electron microscopy (SEM) and locally analysed using X-ray scattered energy spectroscopy (EDS). A biofilm developed on every tested surface (continuous or in the form of pustules), with evidence of metabolic activity of the SRB. Finally, a mechanism of degradation for TSA in the presence of SRB is proposed for environments with a high concentration of bacteria.

Investigation of Corrosion Mechanisms and Protective Performance of Thermal Sprayed Aluminum Coatings on Steel Structures

Investigation of Corrosion Mechanisms and Protective Performance of Thermal Sprayed Aluminum Coatings on Steel Structures

Investigation of corrosion mechanisms and protective performance of thermal sprayed aluminium coatings on steel structures

Investigation of corrosion mechanisms and protective performance of thermal sprayed aluminium coatings on steel structures

Evolution of microstructure and mechanical properties of cold spray additive manufactured aluminum deposit on copper substrate

The microstructure and mechanical properties of cold sprayed aluminum coatings on copper substrates were studied in this paper. The impact and deposition mechanisms of single and multiple particles during cold spraying were investigated using finite element modeling. The results of single particle model show that the maximum equivalent plastic strain is located at the contact surface between particle and substrate, decreasing from the jet site to the center of particles, and the equivalent plastic strain at the top of particles is almost zero. The results of multi-particle model show that the regions with large equivalent plastic strain include the contact surface between particle and substrate and the region between particles. The experimental results show that particle deposition has a significant impact on compaction during the spraying process. When the spraying distance is set to a fixed value of 30 mm, and the gas pressure is 2.2 MPa, 2.6 MPa, 3.0 MPa and 3.4 MPa, the coating porosity is always stable at about 1 %. The bonding strength of coating increases with the increase of gas pressure. When the gas pressure increases from 2.2 MPa to 3.4 MPa, the coating bonding strength has increased by 142.0 %, and the maximum value of bonding strength between the coating and the substrate is 16.9 MPa. When the gas pressure is 3.4 MPa with the gas preheating temperature of 300 °C, and the spraying distance is 20 mm, 30 mm and 40 mm, the coating porosity is 1.74 %, 0.95 % and 1.21 %, respectively. The maximum value of bonding strength between the coating and the substrate is 21.3 MPa under the spraying distance of 30 mm.

Splat Shape and Pore Size Distribution in Plasma Sprayed Alumina Coating at Various In-flight Particle Conditions

Splat Shape and Pore Size Distribution in Plasma Sprayed Alumina Coating at Various In-flight Particle Conditions

Evolution of alumina phase structure in thermal plasma processing

Alumina (Al2O3) remains one the most important engineering ceramic for industrial applications. In addition to the α phase, transition alumina phases have interesting characteristics. Controlling the obtained phase structure from alumina melt requires processes with extreme cooling rates and therefore limits the tailoring capabilities. This study investigates how the cooling rate of pure alumina affects its microstructural properties and phase structure in plasma-based processing. The paper reports phase changes in micron sized granulated alumina particles in high-temperature plasma spheroidization and compares the results to plasma sprayed alumina coatings. Both plasma processes involve melting of the material followed by subsequent rapid cooling. Direct comparison on the alumina phase transitions is obtained for the two methodically distinct processing routes, creating unique microstructures due to difference in their cooling rates.

Study on corrosion resistance of aluminium coating deposited on magnesium‐zinc‐yttrium‐calcium alloy by cold spray

AbstractPure aluminium coatings were prepared on magnesium‐zinc‐yttrium‐calcium alloy substrate via cold spraying technology with different scanning speeds and working gas temperature. The correlation between the corrosion resistance of the coatings and the different spraying process was studied. While the working gas temperature is 600 °C and the scanning speed is 1 mm/s, aluminium coating has less porosity and the coating was well combined with the substrate. Higher temperature of working gas increases the plastic deformation of particles, which lead to a dense aluminium coating. The relationship of corrosion resistance on working gas temperature and scanning speed of aluminium coatings has also been investigated by immersion corrosion test and electrochemical impedance spectroscopic techniques. The results show that the cold sprayed aluminium coatings revealed a lower porosity and higher corrosion resistance with the decreasing scanning speed and the increasing temperature of working gas. The porosity and corrosion current densities were 0.938 vol.% and 2.427 ⋅ 10−6 A/cm2. The experimental results show that the aluminium coating prepared by cold spraying has a good protective effect on magnesium alloy.

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.

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.

Growth Kinetics of Calcareous Deposits on Thermally Sprayed Aluminum Coatings in Natural Seawater

The formation of calcareous deposits on thermally sprayed aluminum (TSA)-coated surfaces immersed in natural seawater and connected to a cathodic protection system is a recurring issue. There is at present limited knowledge on the calcareous deposits’ formation kinetics on TSA in natural seawater following prolonged exposure at different temperatures. In this study, bare 25Cr super-duplex stainless steel (SDSS) and TSA-coated SDSS tube coupons (with and without a silicone-based sealer) were exposed to natural seawater under potentiostatic cathodic protection (CP) and open-circuit potential (OCP) conditions at different temperatures (i.e., 20°C, 35°C, 60°C, and 80°C) for 1.5 y. The coupons were extracted at various intervals to construct growth kinetics curves under various test conditions. The deposits were subsequently examined using scanning electron microscopy, energy dispersive spectroscopy, and x-ray diffraction. For all of the specimens, the deposits on the exposed surfaces increased with exposure time. Fewer deposits were seen under CP conditions than OCP at low temperatures (20°C and 35°C), whereas more deposits were observed with CP than those at OCP at high temperatures (60°C and 80°C). Last, a recommended protection potential of −0.90 VSSC (VAg/AgCl) and cathodic current density values of −8 mA/m2 and −15 mA/m2 were obtained based on the findings of this study.

Evaluation of Thermal Cycling Performance of Plasma Sprayed Alumina Coating on Inconel 600 with Different Bond Coats

Evaluation of Thermal Cycling Performance of Plasma Sprayed Alumina Coating on Inconel 600 with Different Bond Coats

Incorporation of graphene nano platelets in suspension plasma sprayed alumina coatings for improved tribological properties

Graphene possesses high fracture toughness and excellent lubrication properties, which can be exploited to enhance tribological performance of coating systems utilized to combat wear. In this work, suspension plasma spray (SPS) process was employed to deposit a composite, graphene nano-platelets (GNP) incorporated alumina coating. For comparison, monolithic alumina was also deposited utilizing identical spray conditions. The as-deposited coatings were characterized in detail for their microstructure, porosity content, hardness, fracture toughness and phase composition. Raman analysis of the as-deposited composite coating confirmed retention of GNP. The composite coating also showed good microstructural integrity, comparable porosity, higher fracture toughness and similar alumina phase composition as the monolithic alumina coating. The as-deposited coatings were subjected to dry sliding wear tests. The GNP incorporated composite coating showed lower CoF and lower specific wear rate than the pure alumina coating. Additionally, the counter surface also showed a lower wear rate in case of the composite coating. Post-wear analysis performed by SEM/EDS showed differences in the coating wear track and in the ball wear track of monolithic and composite coatings. Furthermore, Raman analysis in the wear track of composite coating confirmed the presence of GNP. The micro-indentation and wear test results indicate that the presence of GNP in the composite coating aided in improving fracture toughness, lowering CoF and specific wear rate compared to the monolithic coating. Results from this work demonstrated retention of GNP in an SPS processed coating, which can be further exploited to design superior wear-resistant coatings.

Role of in-situ splat sintering on elastic and damping behavior of cold sprayed aluminum coatings

The dynamic elastic behavior of cold sprayed aluminum 6061 coatings is analyzed to understand the inherent role of in-situ splat sintering from ambient to 400 °C for air and helium sprayed coatings. Higher surface energy and lower flattening ratio of air sprayed coating in as-sprayed condition result in 28% densification during splat sintering compared to 15% in heat-treated helium sprayed ones. Elastic modulus and damping behavior can be divided into three distinct temperature regimes based on sintering and progressive inter-splat structure. A theoretical analysis is established, which shows that normalized elastic modulus is an exponential function of the inter-splat porosity. This study establishes the correlation between inter-splat thermal phenomena and coating's dynamic mechanical properties at elevated temperatures.

Formation and Properties of Superhydrophobic Al Coatings on Steel.

Thermal sprayed aluminum coatings are widely scalable to corrosion protection of the offshore steel structure. However, the corrosion rate of the Al coating increases considerably due to the severe marine environment. It has remained a challenge to improve the corrosion resistance and protective ability of Al coatings. The superhydrophobic surface provides a potential way to improve the corrosion resistance of metal materials. Hence, the development of superhydrophobic Al coatings with superior corrosion resistance is of great interest. In this work, the feasibility of the preparation of superhydrophobic Al coatings on a steel substrate was explored. First, Al coatings were prepared onto the steel substrate by the arc-spraying process, followed by ultrasonic etching with 0.1 M NaOH solution, and afterward passivated using 1% fluorosilanes. The effects of the etching time on morphology, contact angle, and corrosion resistance of the Al coatings were evaluated. The schematic model of the fluorosilane passivation process on the Al coating surface was provided. The micro/nanoscale surface structure of the low-surface-energy fluorosilanes promotes the wetting angle of 153.4° and a rolling angle to 6.6°, denoting the superhydrophobic properties. The superhydrophobic Al coating surface displays excellent self-cleaning performance due to its weak adhesion to water droplets. The corrosion current density of the superhydrophobic Al coating (1.36 × 10–8 A cm–2) is 2 orders of magnitude lower than that of the as-sprayed Al coating (1.18 × 10–6 A cm–2). Similarly, the charge-transfer resistance is found to be 12 times larger for the superhydrophobic Al coating and the corresponding corrosion inhibition efficiency reaches 98.9%. The superhydrophobic Al coating displays superior corrosion resistance and promising applications in a marine corrosion environment.

Role of In-Situ Dynamic Splat Sintering on Elastic and Damping Behavior of Cold Sprayed Aluminum Coatings

The dynamic elastic behavior of cold sprayed aluminum 6061 coatings is analyzed to understand the inherent role of in-situ splat sintering from ambient to 400 oC for air and helium sprayed coatings. Higher surface energy and lower flattening ratio of air sprayed coating in as-sprayed condition result in 28% densification during splat sintering compared to 15% in heat-treated helium sprayed ones. Elastic modulus and damping behavior can be divided into three distinct temperature regimes based on dynamic sintering and progressive intersplat structure. A theoretical analysis is established, which shows that normalized elastic modulus is an exponential function of the inter-splat porosity. This study establishes the correlation between inter-splat thermal phenomena and coating’s dynamic mechanical properties at elevated temperatures.

Elaboration and characterisation of plasma sprayed alumina coatings on nickel with nickel oxide interlayer

A process implying the pre-oxidation of nickel substrates was proposed alternatively to the usual Atmospheric Plasma Spraying (APS) of alumina on sandblasted substrates. This process comprised two steps, the substrate pre-oxidation (instead of the sandblasting of the substrate) followed by APS. The pre-oxidation formed a thin layer of the NiO oxide, 0.7–3.5 μm thick, which entirely covered the nickel substrate. During APS, the deposit did not react with NiO due to the very high cooling rate of the alumina splats at the surface of the substrates. The characterisation of the interfacial zone showed the crystallographic continuity of the different lattices, from the nickel substrate (f.c.c.) to the NiO interlayer (f.c.c.) and to the monoclinic γ-Al2O3 coating. The NiO interlayer played the role of a buffer zone that adapted the crystal lattices of the three phases. The adhesion of the coatings was determined according to the ASTM C633 test modified by reducing the contact area substrate/coating, in order to quantify the adhesion of strongly-linked coatings. The coating adhesion reached 105 MPa for the pre-oxidized samples when NiO was 1.8 μm thick, instead of 48.5 MPa for the sandblasted samples and only 8 MPa for the simply polished nickel.

Statistical Evaluation of Mechanical Properties of Thermally Sprayed Alumina Coatings by Nanoindentation Method

Statistical Evaluation of Mechanical Properties of Thermally Sprayed Alumina Coatings by Nanoindentation Method

Response surface modeling and parameter optimization of detonation spraying with enhanced coating performance

Detonation spraying is a kind of thermal spray process, widely applied to produce dense, hard coatings on engineering surfaces with the least porosity and higher adhesive strength. This coating process is characterized by the significant parameters chosen, which are optimized to understand the relationship between the process parameters. For the experimental study, detonation sprayed alumina coatings on EN19 steel substrates were used. Applying Response Surface Methodology, the investigation data on the influence of the key process parameters of fuel ratio, standoff distance and nitrogen pressure on micro hardness, coating thickness and deposition rate were modeled for optimization. The optimized parameters were Fuel/ O2 Ratio of 1:2.5, Standoff distance of 191 mm and N2 Pressure of 75 kPa.