Sprayed TiO2 Coatings Research Articles

A Comparative Study of the Antiviral Properties of Thermally Sprayed Coatings against Human Coronavirus HCoV-229E

For decades, novel viral strains of respiratory tract infections have caused human pandemics and initiated widespread illnesses. The recent coronavirus disease 2019 (COVID-19) outbreak caused by the SARS-CoV-2 virus has raised an urgent need to develop novel antiviral coatings as one of the potential solutions to mitigate the transmission of viral pathogens. Titanium dioxide is considered an excellent candidate for viral disinfection under light irradiation, with the potential to be activated under visible light for indoor applications. This research assessed the antiviral performance of thermally sprayed TiO2 coatings under UVA and ambient light. We also report the antiviral performance of TiO2 composites with other oxides, such as Cu2O and Al2O3, produced by suspension plasma spray, atmospheric plasma spray, and suspension high-velocity oxygen fuel techniques. To evaluate the antiviral performance of the above coatings in a containment level-2 laboratory, a human common cold coronavirus, HCoV-229E, was initially used as a relevant surrogate for SARS-CoV-2. Coatings were also analyzed using SEM and XRD and were classified based on their surface roughness, porosity, and phase composition. Collectively, the thermally sprayed coatings showed comparable or slightly better antiviral activity compared to copper. The most significant level of activity observed was approximately 20% to 50% higher than that of a pure copper plate.

Anti-icing performance and durability of suspension plasma sprayed TiO2 coatings

Superhydrophobic coatings are a potential solution for mitigating the in-flight icing problem for aircraft. However, to develop a superhydrophobic coating which can be practically used for aircraft and that possesses sufficient durability is an ongoing challenge. In this work, superhydrophobic coatings are developed using suspension plasma spraying (SPS) as a flexible, versatile and scalable coating technique. The anti-icing and deicing performances of these SPS coatings are studied in icing wind tunnel experiments. Furthermore, the durability of these SPS coatings is tested in dry particle and cloud-sized water droplet erosion and icing/deicing cyclic tests. The capability of SPS superhydrophobic coatings to reduce ice accretion is comparable to that of commercial superhydrophobic coatings but perform better in deicing tests using heating. Additionally, compared to commercial superhydrophobic coatings, the SPS coatings demonstrate significantly better performance in dry particle and icing/deicing cyclic tests while showing comparable durability in cloud-sized water droplet erosion tests. It is also shown that in case of high intensity water erosion, when the superhydrophobicity of SPS coatings deteriorates, it can be restored using a simple and quick retreatment process due to the robustness of the hierarchical micro-textured TiO2 base coatings.

Improvement of corrosion resistance of aisi 410 martensitic steel using plasma coating

Historically, coating has been developed to give a shield against erosion, wear and corrosion to secure a material from its harsh working environment. The objective of this work is to study the corrosion behavior of the Ni/Cr pre coated plasma sprayed TiO2 coatings on AISI 410 steel using salt spray technique as per ASTM B117 standard. Investigation of corrosion behavior was carried out at different exposure time (50,100,150,200hrs) in 5% NaCl solution. Coatings were characterized by Scanning Electron Microscope (SEM) micrographs, Optical Microscope (OM) and EDX analysis. Vickers micro hardness test and porosity of the coated surface was also determined. Experimental results confirm that, TiO2 coatings have high corrosion resistance which can be attributed to high micro hardness and less porosity (2-3%) as compared to un-coated steel substrates.

Characterization of High-Velocity Single Particle Impacts on Plasma-Sprayed Ceramic Coatings

High-velocity impact wear can have a significant effect on the lifetime of thermally sprayed coatings in multiple applications, e.g., in the process and paper industries. Plasma-sprayed oxide coatings, such as Cr2O3- and TiO2-based coatings, are often used in these industries in wear and corrosion applications. An experimental impact study was performed on thermally sprayed ceramic coatings using the High-Velocity Particle Impactor (HVPI) at oblique angles to investigate the damage, failure, and deformation of the coated structures. The impact site was characterized by profilometry, optical microscopy, and scanning electron microscopy (SEM). Furthermore, the connection between the microstructural details and impact behavior was studied in order to reveal the damage and failure characteristics at a more comprehensive level. Differences in the fracture behavior were found between the thermally sprayed Cr2O3 and TiO2 coatings, and a concept of critical impact energy is presented here. The superior cohesion of the TiO2 coating inhibited interlamellar cracking while the Cr2O3 coating suffered greater damage at high impact energies. The HVPI experiment has proven to be able to produce valuable information about the deformation behavior of coatings under high strain rates and could be utilized further in the development of wear-resistant coatings.

Mechanical properties of plasma sprayed nanostructured TiO2 coatings on mild steel

This paper discusses the effects of plasma spray parameters on the mechanical properties of nanostructured TiO2 coatings deposited on mild steel substrates. The design of experiment method was applied to investigate the significant effects of each property and to optimize the operational spray parameters. Plasma power, powder feed rate, and stand-off distance were selected as independent variables. Agglomerated and sintered nano-TiO2 powder was deposited on A-36 commercial mild steel. The microstructural and mechanical properties of the coatings such as porosity, microhardness, surface roughness, and wear rate were evaluated. Both plasma power and powder feed rate were found to be the main factors affecting all four responses. It was also noted that the stand-off distance was a significant factor mainly in influencing the surface roughness of the coatings. All in all, the optimized properties can be achieved by applying a plasma power of 30KW (high level), a powder feed rate of 22g/min (high level), and a stand-off distance of 80mm (low level).

Formation of Pore Structure and Its Influence on the Mass Transport Property of Vacuum Cold Sprayed TiO2 Coatings Using Strengthened Nanostructured Powder

The pore structure in nano-porous TiO2 coatings influences the ion diffusion property and the photovoltaic performance of dye-sensitized solar cells. In this paper, TiO2 coatings were deposited by vacuum cold spray (VCS) using a strengthened nanostructured powder. The pore structure, ion diffusion, and dye infiltration properties were examined to understand the coating deposition mechanism. Results showed that the pores in the VCS TiO2 coatings presented a bimodal size distribution with two peaks at ~15 and ~50 nm. Based on the impact behavior of spray powder particles, a deposition model was proposed to explain the formation mechanism of the pores in the VCS coating using strengthened nanostructured powder. It was found that, compared to the conventional unimodal-sized nano-pores in TiO2 coatings, the bimodal-sized nano-pores contributed to a higher ion diffusion coefficient of the coatings and thereby a higher photovoltage of the solar cells.

Nanoindentation study of the mechanical and damage behaviour of suspension plasma sprayed TiO2 coatings

TiO2 coatings can be used as self-cleaning surfaces owing to their photocatalytic and hydrophilic properties. Suspension plasma spray (SPS) has proven to be a feasible and cheap technique for producing self-cleaning surfaces with acceptable photo-activity. This paper presents a nanoindentation study of the mechanical properties (hardness, Young's modulus and scratch resistance) of photoactive layers of suspension plasma sprayed TiO2 coatings applied on to glass substrates. Microstructure observation showed that the rutile grains were surrounded by fine anatase crystals. Under the same spraying conditions, the resulting anatase/rutile concentrations varied depending on the cooling rate (the substrate being either cooled with water or in air). The results showed that higher concentrations of anatase, which is softer than rutile, reduced the scratch damage and increased the friction coefficient.

Microstructure and photocatalytic activity of suspension plasma sprayed TiO2 coatings on steel and glass substrates

In this study, TiO2 coatings were deposited by suspension plasma spraying (SPS) from a commercial TiO2 nanoparticle suspension on two different substrates: a standard stainless steel and a Pyrex glass. Coatings were sprayed on both substrates with an F4-MB monocathode torch; a Triplex Pro tricathode torch was also used to spray coatings just on the stainless steel substrates. Spraying distance and cooling were varied.The anatase content in the coatings, determined by XRD, ranged from 32 to 72wt.%. A significant amount of anatase to rutile transformation was found to occur during cooling. Examination of the microstructure revealed that the coating microstructure was bimodal, involving a non-molten region consisting mainly of anatase nanoparticle agglomerates and a molten region. The glass substrate coatings displayed a segregated phase distribution, particularly when the surface to be coated was cooled. Photocatalytic activity was determined by a methylene blue test.The experimental data fitted well to a first-order kinetic. All the coatings exhibited high photocatalytic activity in comparison with that of a commercial sol–gel coating. However, unlike much of the previous research, photocatalytic activity did not correlate with the anatase content determined by XRD.

Influence of high velocity oxy-fuel parameters on properties of nanostructured TiO2 coatings

A liquid fuel high velocity oxy-fuel (HVOF) thermal spray process has been used to deposit TiO2 nanostructured coatings utilizing a commercially available nanopowder as the feedstock. The coatings were characterized by means of X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. Photocatalytic activity was evaluated as a rate constant of decomposition reaction of methylene blue (MB) determined from the changes of relative concentration of MB with UV irradiation time. The results indicate that the sprayed TiO2 coatings were composed of both TiO2 phases viz. anatase and rutile, with different phase contents and crystallite sizes. A high anatase content of 80% by volume was achieved at 0·00015, fuel-to-oxygen ratio with nanostructure coating by grain size smaller than feedstock powder. Photocatalytic activity evaluation results indicated that all the TiO2 coatings are effective to degradation MB under UV radiation and their activities differ in different spray conditions. It is found that fuel flow rate strongly influenced on phase transformation of anatase to rutile and by optimizing the rate which can promote structural transformation and grain coarsening in coating and improving photocatalytic activity.

Role of process type and process conditions on phase content and physical properties of thermal sprayed TiO2 coatings

Thermal spray represents an advantageous technique for depositing large-area titanium dioxide coatings that are of interest for both traditional wear-resistant coatings as well as functional applications such as photo-induced decontamination surfaces. Numerous past studies have examined the phase evolution and properties of TiO2 coatings using different thermal spray processes or parameters. In this paper, an integrated study of thermal sprayed TiO2 was conducted with different thermal spray devices and process parameters for a single feedstock powder comprising the metastable anatase phase. The aforementioned variables are correlated with in-flight particle state (particle temperature and velocity), phase evolution, and coating physical properties. The results are represented through the framework of process maps which connect process parameters with material properties. Based on the phase characterization, an initial exploration of the metastable phase evolution during thermal spray deposition of TiO2 is proposed. Furthermore, the sprayed TiO2 coatings show varying degrees of electrical conductivity associated with process-induced stoichiometric changes (vacancy generation) in the TiO2. The effects of these stoichiometric changes as well as extrinsic microstructural attributes (pores, cracks, interfaces), contribute to the complex electrical response of the coatings. This integrated study provides insights into the process–microstructure–property relationship with the ultimate goal of tailoring the functionality of spray deposited oxide thick films.

Anisotropic electrical conduction from heterogeneous oxidation states in plasma sprayed TiO2 coatings

Microstructural and electrical characterizations of air plasma sprayed TiO2 coatings were carried out to investigate the details of deoxidation during the spray process and the changes following air annealing. The coatings were found to behave as an n-type semiconductor indicating the presence of oxygen vacancies. Direct-current resistivity measurements in plane (ρIP) and through thickness (ρTT) of the coatings as a function of annealing time and temperature showed remarkably large anisotropies (=ρTT∕ρIP) of up to 105. Impedance spectroscopy of the specimens coupled with microstructural analysis revealed that the origin of this anisotropy lies in the heterogeneous deoxidation and reoxidation behavior of the coatings. Due to rapid quenching, the high temperature deoxidation state is preserved in the splat boundaries making them more conductive than the bulk of the splat in the as-sprayed coating. Upon annealing in air, the splat boundaries get selectively oxidized due to faster surface diffusion of oxygen and become more insulating. This behavior, together with the layered morphology of plasma sprayed coatings, results in anisotropy.

Photocatalytic performance of plasma sprayed Pt-modified TiO2 coatings under visible light irradiation

Abstract Plasma sprayed Pt/TiO 2 coatings were prepared by Atmospheric Plasma Spraying process. As-sprayed coatings were characterized by TEM, XRD and XPS. The photocatalytic performance was evaluated through the photo mineralization of methylene blue. All the Pt modified titanium dioxide coatings show significant absorption in the visible light range, while the pure titania coating reflects almost all the visible light. The photocatalytic efficiencies of as-sprayed pure TiO 2 coating and Pt/TiO 2 coatings are almost same under the irradiation of visible light. However, the efficiencies of all Pt/TiO 2 coating are greatly improved comparing with that of pure TiO 2 coating by applying 15 V external bias under the irradiation of visible light.

Tribological Property of Plasma Sprayed TiO<sub>2</sub> Coating

The sliding wear property of plasma sprayed TiO2 coatings with porosity of 4.2% and 5.4% mating against silicon nitride ball were comparatively investigated with a reciprocating tribometer under dry conditions. The results indicate that the wear resistance of the TiO2 coating with lower porosity is superior, which is attributed to its homogeneous microstructure and improved microhardness. The morphologies of the wear tracks of the two TiO2 coatings, as well as wear debris reveal the pore is the main location wear occur. The coefficients of friction of the two TiO2 coatings decrease with the increase of applied load, while the wear rates increase, which is attributed to the difference of the dependence of microhardness on load between the coating and the Si3N4 ball. No apparent dependence of wear resistance on sliding speed was found on these two coatings.

Characterization and Photocatalytic Properties of Plasma Sprayed TiO<sub>2</sub> Coatings Deposited from Nanoparticles

The phase and microstructure of four kinds of plasma sprayed TiO2 coatings (P25, ST, NK, KT) were characterized by XRD (X-ray Diffraction), FE-SEM (Field Emission Scanning Electron Microscope) and TEM ( Transmission Electron Microscopy). Their photocatalytic properties were discussed in relation to phase composition and microstructure. The FE-SEM observation results reveal that there are quite a number of nanoparticles on the surfaces of all four kinds of plasma sprayed TiO2 coatings, which provide photocatalytic reactive sites. Based on the observation results of FE-SEM and TEM, the microstructure schematic of plasma sprayed TiO2 coatings using nanoparticles as feedstock is suggested. The best photocatalytic property of P25 coating among the four plasma sprayed TiO2 coatings is attributed to the combination of its fine “cauliflower” structure and the highest anatase content.

Bioactivity of Plasma-Sprayed TiO<sub>2</sub> Coating in Simulated Body Fluid by Hydrofluoric Acid Treatment

TiO2 coatings on titanium alloy substrates were prepared by atmospheric plasma spraying using commercial nano-powders. Then, as-sprayed coatings were treated using 10% hydrofluoric acid (HF) at room temperature for 30 seconds. As-sprayed and HF-treated titania coatings were soaked in simulated body fluid to investigate the formation of apatite on their surface. Field-emission scanning electron microscopy was used to observe the surface morphologies, and the phase composition of the as-sprayed coating and apatite were analyzed by X-ray diffraction and energy-dispersive X-ray spectrometry. As-sprayed titania coating is composed of rutile, anatase, and a small quantity of Ti3O5. It exhibited excellent adhesion between the TiO2 coatings and titanium alloy substrates, and the bonding strength was about 38 MPa. After in vitro experiment, a new substance containing calcium and phosphate was formed on the surface of HF-treated TiO2 coatings after being soaked in SBF, while the new substance was not formed on the surface of as-sprayed TiO2 coatings. The results indicated that the bioactivity can be induced to the surface of plasma sprayed TiO2 coatings by hydrofluoric acid treatment.

Influence of Spraying Power and Distance on Porosity and Microhardness of Plasma Sprayed TiO<sub>2</sub> Coatings

The influence of the two deposition parameters on microstructure, and microhardness of plasma sprayed TiO2 coatings were investigated. It was found that a higher spraying power and a shorter distance resulted in a lower porosity and a higher microhardness for the coatings. The anisotropy on the microstructure and microhardness of TiO2 coatings was also found. The denser microstructure is attributed to the higher degree of melting and higher velocity of the TiO2 powders during spraying. The improvement of microhardness is associated with the lower porosity. The difference of porosity and microhardness between surface and cross section resulted from their different microstructures.

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The effects of plasma sprayed Al2O3, TiO2, and Al2O3-TiO2 coatings on surface properties and far infrared radiation were investigated by observation of surface roughness, measurement of relative reflection density, conductivity of the coatings, and the spectral energy distribution at wavelengths of 1.4∼14μm at 227°C. Increasing the TiO2 content of plasma-sprayed Al2O3-TiO2 coatings smoothed the surface, darkened the color of the coating, and resulted in semi conductivity and high far infrared radiation. Radiation energy increased with darkening of coating color.