Al2O3-TiO2 Coatings Research Articles

Surface Modification and Analysis of St37 Steel with Al2O3-TiO2, ZrO2, and Cr2O3 Ceramic Coatings: Structural, Mechanical, and Tribological Properties

St37 steel, widely used in many engineering applications, experiences significant losses in its mechanical and physical properties over time due to reasons such as corrosion or wear. In this study, the aim was to improve the surface properties of St37 steel using the plasma spray method. To achieve this, the surface of St37 was coated with four different oxide layers: zirconium oxide (ZrO2), chromium oxide (Cr2O3), and aluminum oxide-titanium oxide at different ratios [Al2O3-%13 TiO2 (AT13) and Al2O3-%40 TiO2 (AT40)]. Microhardness measurement, wear test, scratch test, and structural analyses (XRD and SEM) were performed on the samples. The highest coating thickness (311 µm) was observed in the chromium oxide-coated sample. All applied coatings significantly increased the hardness of St37 steel, with the chromium oxide-coated sample exhibiting the highest hardness value (1664 HV0.1). The highest adhesion strength was determined to be 59 N in the aluminum oxide-titanium oxide (AT13) coated sample, while the lowest adhesion strength was found to be 28 N in the chromium oxide-coated sample. In this study, the analysis results of untreated and differently oxide-coated St37 steel samples are presented comparatively.

High Photocatalytic Efficiency of Al2O3-TiO2 Coatings on 304 Stainless Steel for Methylene Blue and Wastewater Degradation

This work explores the novelty of achieving high photocatalytic efficiency and remarkable bactericidal activity with Al2O3-TiO2 coatings on perforated 304 stainless steel (SS) substrates, placed transversely along an airlift reactor of 0.980 L for wastewater treatment under visible light irradiation. The Al2O3-TiO2 coatings achieved methylene blue and total organic carbon (TOC) concentration reductions of 97.3 and 96.51%, respectively, in a wastewater sample with heterogeneous photocatalsis. The Al2O3-TiO2 coatings resulted in a 33.30% reduction in total and fecal coliforms and a remarkable 94.23% decrease in Salmonella spp. in the wastewater sample. XRD confirmed the TiO2 anatase–rutile phases and Al2O3 α-γ phases in the coating. The particle size distribution ranges from 100 to 500 nm, and the coating surface was homogeneous without cracks confirmed using SEM and AFM, respectively. The roughness and thickness of the coatings were 85 ± 5 nm and 250 ± 50 nm, respectively.

Chemical composition optimization of Al2O3-TiO2 composite coatings for enhanced wear and corrosion resistance

This study aimed to optimize the chemical composition of wear- and corrosion-resistant Al2O3-TiO2 composite coatings. 2-layered and 4-layered Al2O3-TiO2 coatings were fabricated on a 316L substrate with various Al2O3 and TiO2 content (20 %, 35 %, 50 %, 65 %, and 80 % in weight) at 900 °C by sol-gel technique. Wear properties were measured by dry sliding tests against an Al2O3 ball, and corrosion behavior was determined through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. All Al2O3-TiO2 composite coatings consisted of α-Al2O3 and rutile phases, regardless of the chemical composition. Increasing TiO2 content significantly increased coating thickness at the same number of layers. The 2-layered configurations resulted in lower wear rates in low-TiO2 compositions, whereas the 4-layered configurations provided higher wear resistance in high TiO2-containing coatings. The high Al2O3 concentration caused extremely high corrosion rates compared to those consisting of high TiO2. Wear and corrosion losses were decreased with increasing TiO2 content due to its nanosized structure, allowing well-adhered, continuous, and less porous coating morphology. The maximum improvement in wear and corrosion resistance was obtained in the 4-layered 65 % TiO2-containing composite coating.

Adhesion of selected thermally sprayed coatings on additive manufactured maraging steel

Additive manufacturing (AM) technology is increasingly expanding into different types of industries. Still, many potential applications are restricted due to the limited choice of materials used in AM. A suitable solution could be a combination of AM and thermal spraying of functional parts of the surface to achieve superior surface properties. This work aims to study the possibility to prepare the surface with AM technology to direct application of coating by thermal spraying without any additional surface preparation. To verify this possibility, the parameters of the 3D printing technology were adjusted to achieve suitable surface structures for the following thermal spraying. Several variants of modifications with different surface properties were selected. The samples were printed from maraging steel using the DMLS technology. The coatings chosen for testing were WC – CoCr and Stellite 6 sprayed by HVOF and further NiAl and Al2O3-TiO2 sprayed by APS. The adhesion of the coating was tested by an adhesion test performed under the ASTM C633-79 standard. The bonding of the coating on the surface was studied by metallographic analysis. The achieved results indicate that the adhesion of HVOF sprayed WC-CoCr coating on 3D printed surfaces is excellent for most surface modifications. The Stellite 6 exhibits good adhesion on several surface modifications. The APS sprayed coating shows significantly lower adhesion, the NiAl and Al2O3-TiO2 coatings can be successfully sprayed only on selected surface modifications.

Deposition and study of plasma sprayed Al2O3-TiO2 coatings on AZ31 magnesium alloy

In this study, Al2O3, TiO2 and Al2O3+ 3wt%TiO2 coatings were deposited on AZ31 Mg alloy substrate by plasma spraying. The coatings were structurally characterized by scanning electron microscopy and X-ray diffraction. Corrosion experiments were carried out in 3.5% NaCl solution. Adhesion tests were performed according to daimler benz VDI-3198 standard. A coating layer of approximately 70 microns in thickness was deposited. High plasma enthalpy caused phase transformations in alumina-based ceramics. As a result of electrochemical corrosion study, it was determined that the coatings increased the corrosion resistance of AZ31 Mg alloy. While the most corrosion resistant coating is Al2O3+ 3wt%TiO2, the weakest coating against corrosion is TiO2. The adhesion behavior of all coatings to the substrate was at an acceptable quality levels.

The importance of type of Ti-based additives on the PEO process and properties of Al2O3-TiO2 coating

Al2O3-TiO2 coatings were obtained from a silicate-based electrolyte using pulsed bipolar current by PEO process. Nano-particle titania (NP-TiO2) and potassium titanyl oxalate (PTO) were used as Ti-based additive sources in the PEO electrolytic solution, separately. The coatings were characterized using scanning electron microscope, energy dispersive spectroscopy, and an X-ray diffractometer. The mechanical properties of the coatings were investigated using nanoindentation and ball-on-disk tests. SEM results showed that the PTO developed a more compact inner layer besides the increase of coating thickness. However, NP-TiO2 created an inner layer with less thickness but with higher compactness, without any effect on the outer layer morphology. XRD and Raman spectroscopy analyses showed that the NP-TiO2 had inert incorporation into the alumina. However, TiO2 produced by PTO had reactive incorporation into alumina and made a polymeric titanium oxide structure on the coating with doped rutile and anatase phases. Nanoindentation and tribology analyses approved that the Ti incorporation through PTO provides appropriate mechanical properties owing to the more compact and thicker inner layer. The mechanism of PTO performance in the PEO process was discussed regarding its effect on coating characteristics.

Improving wear and corrosion protection of AISI 304 stainless steel by Al2O3-TiO2 hybrid coating via sol-gel process

ABSTRACT Al2O3, TiO2, and Al2O3-TiO2 coatings were deposited on AISI 304 stainless steel plates by the sol–gel dipping technique, and the effects of the coating composition on microstructure, wear, and corrosion characteristics were examined. Tribological features were determined by ball-on-disc sliding tests against an Al2O3 ball under a 10 N load, while corrosion properties were evaluated by potentiodynamic polarisation tests performed in 3.5% wt. NaCl solution. γ-Al2O3, α-Al2O3, and rutile phases were detected in the hybrid coatings. The highest coating thickness was obtained in single Al2O3 film and decreased with an increasing amount of TiO2 nanoparticles. The single Al2O3 layer had both the highest wear resistance and the lowest corrosion resistance. Although TiO2 addition caused an increase in the wear rate, it improved the corrosion resistance of the coating by filling the voids in the Al2O3 film. The Al2O3-TiO2 hybrid film with a relative ratio of 6:1 as a candidate for the wear- and corrosion-resistant coating decreased wear and corrosion losses of the 304 substrates by 83% and 97%, respectively.

Development of CNT reinforced Al2O3-TiO2 coatings for boiler tubes to improve hot corrosion resistance

This study examines the hot corrosion behaviour of plasma-coated T12 steel for 10 cycles of 100 h each in an industrial boiler. The coating contains CNT (carbon nanotubes) reinforced alumina-titania powders. The substrates were exposed to the boiler at 750 °C. A thermogravimetric examination was conducted to evaluate the kinetics of corrosion. Corroded samples were examined with scanning electron microscopy and x-ray diffraction analysis after the end of the corrosion cycle. This research study concludes that CNT-reinforced coatings provide better corrosion resistance than conventional alumina coatings in the boiler environment.

Investigation into microstructure and mechanical properties effects on sliding wear and cavitation erosion of Al2O3–TiO2 coatings sprayed by APS, SPS and S-HVOF

In this work, different Al2O3–TiO2 coatings, i.e., Al2O3, Al2O3-13 wt% TiO2 and Al2O3-40 wt% TiO2, produced by various thermal spray processes, namely: (i) atmospheric plasma spraying (APS), (ii) suspension plasma spraying (SPS), and (iii) suspension high-velocity oxy fuel spraying (S-HVOF), were investigated. For spray purposes, micrometer-sized powders and water-based suspensions of fine submicrometer-sized powders were used. The study aimed to investigate the influence of spray technology and spray feedstock characteristics, mainly chemical composition, i.e., TiO2 content, on the properties of the resulting coating.The sprayed coatings were characterised in terms of selected mechanical and wear properties, including fracture toughness, Vickers and instrumental hardness, Young's modulus, sliding wear resistance, and cavitation erosion resistance.The study showed that the sliding wear rate decreased with the increasing hardness, Young's modulus, and the content of the α-Al2O3 phase in the coating. The dominant wear mechanism was fatigue-induced brittle delamination, followed by the adhesive smearing of the wear debris. The dense microstructures characterised by a high hardness and an increased content of α-Al2O3 favored the resistance of coatings to cavitation erosion. Cavitation erosion was initiated at the microstructural discontinuities like pores or microcracks, resulting in damage by brittle failure with cracking and spallation, which ended up forming large cavitation craters.

Comparative investigation on thermally sprayed Al2O3, Al2O3–13%(TiO2) and Al2O3–40%(TiO2) composite coatings from room to 400 °C temperature

In the present study, three coatings of Al2O3, Al2O3–13%(TiO2) and Al2O3–40%(TiO2) composite coatings were deposited by the thermally flame spray process. The comparative wear behaviour of the Al2O3–TiO2 coatings has been studied under high temperature levels up to 400 °C with the help of a high temperature tribometer at a constant load of 40N. Prior to the tribo-test, the coating morphology and elemental analysis has been examined with the help of field emission scanning electron microscope (FESEM), elemental dispersion spectroscopy (EDS) and x-ray powder diffraction (XRD) techniques. The mechanical properties have been also analyzed and reported. The results reveal that specific wear rate decreases with an increase in temperature for all the deposited coating except Al2O3 coating at 400 °C. The overall friction coefficient of the coatings is decreasing with the increasing temperature. The Al2O3–40%TiO2 coating showed maximum sustainability against wear as well as a low coefficient of friction because of its low hardness and high adhesion properties. The examined worn surface validates the presence of brittle fracture and abrasive wear behaviour mechanism along with the oxidation wear, from room temperature to 400 °C.

Silicate and Hydroxide Concentration Influencing the Properties of Composite Al2O3-TiO2 PEO Coatings on AA7075 Alloy

This work evaluates the effect of sodium meta-silicate pentahydrate (SMS) and potassium hydroxide concentrations on properties of Al2O3-TiO2 coatings produced through plasma electrolytic oxidation in a solution containing 3 g L−1 potassium titanyl oxalate, (PTO), using a unipolar waveform with constant current density. The surface and cross-section characteristics of PEO coatings including morphology, elemental distribution, and phase composition were evaluated using FESEM, EDS, and XRD techniques. Voltage-time response indicated the concentration of SMS and KOH had a significant effect on the duration of each stage of the PEO process. More cracks and pores were formed at the higher concentrated solutions that resulted in the incorporation of solution components especially Si into the coating inner parts. Ti is distributed throughout the coatings, but it had a dominant distribution in the Si-rich areas. The coating prepared in the electrolyte containing no silicate consisted of non-stoichiometric γ-Al2O3 and/or amorphous Al2O3 phase. Adding silicate into the coating electrolyte resulted in the appearance of α-Al2O3 besides the dominant phase of γ-Al2O3. The corrosion behaviour of the coatings was investigated using the EIS technique. It was found that the coating prepared in the presence of 3 g L−1 SMS and 2 g L−1 KOH, possessed the highest barrier resistance (~10 MΩ cm2), owing to a more compact outer layer, thicker inner layer along with appropriate dielectric property because this layer lacks the Si element. It was discovered that the incorporation of Ti4+ and especially Si4+ in the coating makes the dielectric loss in the coating.

The Effect of Heat Treatment on the Microstructure and Phase Composition of Plasma Sprayed Al2O3 and Al2O3-TiO2 Coatings for Applications in Biomass Firing Plants

This study presents the thermal and chemical resistance of plasma-sprayed Al2O3 and Al2O3 doped with 13 wt.% of TiO2 coatings and their suitability for the fire grate of straw pellet furnaces. Coatings were deposited on steel substrates using direct current atmospheric pressure plasma spraying. The surface structure, elemental, and phase composition of formed coatings were analyzed before and after the thermal treatment, imitating natural application conditions. For the experiment, the annealing temperature was 500 °C for twenty-five cycles (80 min each). It was found that the steel substrate oxidized after five thermal cycles, and the formation of iron oxides was observed. The elemental composition of the Al2O3 and Al2O3-13 wt.% TiO2 coatings remained unchanged even after 25 cycles of heat treatment. The X-ray diffraction (XRD) results revealed that the alpha-Al2O3 to gamma-Al2O3 phase ratio in the Al2O3-TiO2 coating was reduced by only 8.7% after 25 cycles.

Flame sprayed and plasma sprayed Al2O3-TiO2 coatings

Al2O3–TiO2 coatings were received by flame sprayed of flexible cord and plasma sprayed using powder. Shown higher mechanical and tribological properties of flame sprayed coating compared plasma sprayed coating.

Friction energy map for sliding wear of Al2O3-TiO2 and Cr2O3-TiO2 coatings

The present study deals with the development of friction energy map for plasma sprayed Al2O3-TiO2 and Cr2O3-TiO2 coatings. Al2O3-TiO2 and Cr2O3-TiO2 coatings were deposited by atmospheric plasma spray technique. The structural and mechanical properties of coatings were studied using Scanning Electron Microscopy (SEM), x-ray Diffraction (XRD) and Vickers macro and micro hardness test. Sliding wear tests were performed on coated specimens over a range of sliding speed and load using pin on disc test apparatus. The analysis of worn surface and wear debris was carried by SEM, XRD and Raman Spectroscopy. Cr2O3-TiO2 coating showed higher hardness and elastic modulus and relatively lower fracture toughness than Al2O3-TiO2 coating. Al2O3-TiO2 coatings exhibited 1.8 times better wear resistance and 11%–30% lower friction coefficient, as compared to Cr2O3-TiO2 coating, depending upon the wear test conditions. The reduction in friction coefficient and wear rate at higher sliding speed was due to the formation of tribofilms on worn surface. A good correlation was observed between friction energy and contact severity; which incorporates mechanical properties of coatings. Friction energy map was used to rationalize wear rates of Al2O3-TiO2 and Cr2O3-TiO2 coatings, for the identification of various wear regimes and failure modes of the coating.

Microstructure evolution and mechanical properties of Al2O3-40%TiO2 coating by Hybrid-Low Velocity OxyFuel process

This study presents a new thermal spray coating manufacturing process to deposit high performance ceramic coatings named ‘Hybrid-LVOF’ (Low Velocity OxyFuel). By this process, both thin and thick ceramic coatings can be deposited on metallic or ceramic substrates. In this work, the microstructural and mechanical properties of H-LVOF sprayed Al2O3-TiO2 coatings were compared with plasma sprayed (PS) Al2O3-TiO2 coatings; with the additional condition that typical bond coat was not applied. The α-Al2O3 and γ-Al2O3 contents have been quantified in the as-sprayed coatings. As-sprayed Hybrid-LVOF coatings show better properties than conventional plasma-sprayed coatings including better fracture toughness. Noise and radiation levels are very low in the hybrid-LVOF process. Furthermore, a 50% reduction in overall production cost was achieved. H-LVOF could be therefore a possible alternative to a conventional plasma spray process.

A Study on the Microstructural Characterization and Phase Compositions of Thermally Sprayed Al2O3-TiO2 Coatings Obtained from Powders and Water-Based Suspensions.

In this work, the alumina (Al2O3) and alumina-titania coatings with different contents of TiO2, i.e., Al2O3 + 13 wt.% TiO2 and Al2O3 + 40 wt.% TiO2, were studied. The coatings were produced by means of powder and liquid feedstock thermal spray processes, namely atmospheric plasma spraying (APS), suspension plasma spraying (SPS) and suspension high-velocity oxygen fuel spraying (S-HVOF). The aim of the study was to investigate the influence of spray feedstocks characteristics and spray processes on the coating morphology, microstructure and phase composition. The results revealed that the microstructural features were clearly related both to the spray processes and chemical composition of feedstocks. In terms of phase composition, in Al2O3 (AT0) and Al2O3 + 13 wt.% TiO2 (AT13) coatings, the decrease in α-Al2O3, which partially transformed into γ-Al2O3, was the dominant change. The increased content of TiO2 to 40 wt.% (AT40) involved also an increase in phases related to the binary system Al2O3-TiO2 (Al2TiO5 and Al2−xTi1+xO5). The obtained results confirmed that desired α-Al2O3 or α-Al2O3, together with rutile-TiO2 phases, may be preserved more easily in alumina-titania coatings sprayed by liquid feedstocks.

Appraisal on corrosion performances of CrNi, TiAlN/CrNi and CrNi–Al2O3–TiO2 coatings on H13 hot work mold

PurposeThe purpose of this paper is to evaluate the salt spray corrosion (SSC) and electrochemical corrosion performances of CrNi, TiAlN/NiCr and CrNi–Al2O3–TiO2 coatings on H13 steel, which improved the corrosion resistance of H13 hot work mold.Design/methodology/approachCrNi, TiAlN/NiCr and CrNi–Al2O3–TiO2 coatings were fabricated on H13 hot work mold steel using a laser cladding and cathodic arc ion plating. The SSC and electrochemical performances of obtained coatings were investigated using a corrosion test chamber and electrochemical workstation, respectively. The corrosion morphologies, microstructure and phases were analyzed using an electron scanning microscope, optical microscope and X-ray diffraction, respectively, and the mechanisms of corrosion resistance were also discussed.FindingsThe CrNi coating is penetrated by corrosion media, producing the oxide of Fe3O4 on the coating surface; and the TiAlN coating is corroded to enter into the CrNi coating, forming the oxides of TiO and NiO, the mechanism is pitting corrosion, whereas the CrNi–Al2O3–TiO2 coating is not penetrated, with no oxides, showing the highest SSC resistance among the three kinds of coatings. The corrosion potential of CrNi coating, TiAlN/CrNi and CrNi–Al2O3–TiO2 coatings was –0.444, –0.481 and –0.334 V, respectively, and the corresponding polarization resistances were 3,074, 2,425 and 86,648 cm2, respectively. The electrochemical corrosion resistance of CrNi–Al2O3–TiO2 coating is the highest, which is enhanced by the additions of Al2O3 and TiO2.Originality/valueThe CrNi, TiAlN/CrNi and CrNi–Al2O3–TiO2 coatings on H13 hot work mold were firstly evaluated by the SSC and electrochemical performances.

A Critical Review on Nano structured Coatings for Alumina-Titania (Al2O3-TiO2) Deposited by Air Plasma Spraying Process (APS)

Abstract The thick coatings obtained from nanostructured powders with fine layered microstructure are usually deposited using Air Plasma Spraying process (APS). In APS, nitrogen or argon gas is used as primarily. Microstructural characteristics of coatings for nanostructure powders are typically affected by the proportion of undissolved or partly melted zone (PM regions) present in coatings microstructure. It is observed that volume portion of partly melted regions is seen to be very much smaller in coatings obtained from nanostructurerd Al2O3-TiO2 powders deposited with nitrogen as compared to argon as the essential plasma gas. Coatings acquired from agglomerated powders of nanostructured alumina-titania shown to qualify superior resistance to wear over coatings by conventional powders. The Bi-modal representation of microstructures in nanostructured coatings with homogeneous dispersion of partially melted areas in fully melted region are considered to be responsible for improved properties of nanostructured coatings. This paper attempts to critically review the recent works done on atmospheric plasma sprayed nanostructured Al2O3-TiO2 coatings deposited by either argon or nitrogen as primary gas

Impact of Al2O3-40 wt.% TiO2 feedstock powder characteristics on the sprayability, microstructure and mechanical properties of plasma sprayed coatings

Atmospheric plasma sprayed (APS) Al2O3-TiO2 coatings have found a wide range of industrial application due to their favorable properties, combined with low costs and a high availability. However, the detailed effect of the phase composition and the element distribution of the feedstock powders on the coating properties and the spraying process have only crudely been investigated so far. Here the impact of aluminum titanate (Al2TiO5) on the microstructural features and mechanical properties of Al2O3-40 wt.% TiO2 APS coatings is demonstrated by investigating the detailed phase composition and the distribution of aluminum and titanium in three fused and crushed feedstock powders and the respective coatings. Thereby, a direct influence of Al2TiO5 content on the deposition efficiency, the porosity, the elastic modulus, and the hardness of the coatings is revealed. The results emphasize the need for a more detailed specification of commercial Al2O3-TiO2 feedstock powders to ensure a high reliability of the coating properties.

Tribological behavior of SAPS sprayed Al2O3-TiO2 and NiCr-Cr3C2 coatings under severe load conditions

Plasma sprayed Al2O3-TiO2 and NiCr-Cr3C2 coatings have been widely used in various components with severe wear conditions. In this study, the microstructure and friction-wear behavior of both coatings were comparatively studied in order to provide an insight into their tribological properties under heavy load. The results suggested that although both of the coatings showed almost the similar porosity and hardness, the wear rate of NiCr-Cr3C2 coating was only 44% as much as that of Al2O3-TiO2 due to a higher toughness. The delamination, grooves and pits were observed on the worn surface of Al2O3-TiO2 or NiCr-Cr3C2 coating. The wear behavior of both coatings was associated with abrasion, accompanied by a certain extent of delamination and adhesion. For Al2O3-TiO2 coating, the high hardness Al2O3 particles that deeply embedded in the surface of softer friction pair (Ni55) caused serious adhesive tearing and scratch under normal and radial stresses.