Al2O3-based Coatings Research Articles

Hot Corrosion Performance Analysis of Microwave Heat-Treated Al2O3-Based Coatings Deposited on SA 210 Grade-I Boiler Steels

Hot Corrosion Performance Analysis of Microwave Heat-Treated Al2O3-Based Coatings Deposited on SA 210 Grade-I Boiler Steels

Effect of TiO2 on mechanical and thermal properties of Al2O3-based coating via atmospheric plasma spraying

ABSTRACT In this study, Al2O3-based coatings with varying TiO2 contents (0, 3, 13, and 40%) were fabricated using atmospheric plasma spraying technique. To compare the superiority of the samples, their mechanical (Vickers hardness and adhesion strength) and thermal properties (thermal conductivity and thermal shock resistance) were characterized. As observed, Al2O3-40%TiO2 (A-40T) coating exhibited relatively inferior mechanical properties but superior thermal insulation and thermal shock resistance at 600°C. According to the microstructure and phase analysis, this finding is attributed to the special phase, Al2TiO5, and preexisting microcracks inside the coating. Thus, A-40T manifested excellent characteristics for thermal insulation application compared with pure Al2O3 and low-TiO2 content coatings.

Influence of different introduction modes of metal Ag into plasma sprayed Al2O3 coating on tribological properties

Al2O3 coating and Al2O3/Ag (10%) composite coating were prepared on the surface of GH4169 superalloy by the atmospheric plasma spraying technology. And an in-situ synthesis method was applied to introduce the Ag particles into a part of Al2O3 coatings to obtain Al2O3/Ag(synthesis) composite coating. Then, the microstructure and mechanical properties of these three Al2O3-based coatings were systematically studied in this work. In order to reveal the lubrication characteristics of Ag, their friction tests were carried out at room temperature (RT), 400 °C, 600 °C and 800 °C, respectively. The results showed that both microstructure and mechanical properties of Al2O3/Ag(synthesis) composite coating were better than that of Al2O3/Ag (10%) composite coating because many pores and cracks produced during the direct spraying. Although the friction coefficients of two kinds of composite coatings were close to that of Al2O3 coatings at RT, their wear rates were both greatly decreased due to the introduction of Ag. In addition, the lubricating performance of Ag was not enough to reduce their friction coefficients when friction temperature is lower than 600 °C. However, the friction coefficients of these composite coatings were both reduced to about 0.3 at 800 °C . At this time, the Al2O3/Ag(synthesis) composite coating also exhibited a lower wear rate because of its dense microstructure and excellent mechanical properties.

Preparation and Characterization of NiCr/NiCr-Al2O3/Al2O3 Multilayer Gradient Coatings by Gas Detonation Spraying

This paper investigates the influence of the technological parameters of detonation spraying on the phase composition of NiCr- and Al2O3-based coatings. It was determined that the phase composition of Al2O3 coatings during detonation spraying strongly depends on the barrel filling volume with the gas mixture. The acetylene–oxygen mixture, which is the most frequently used fuel in the detonation spraying of powder materials, was used as a fuel gas. To obtain a ceramic layer based on Al2O3, spraying was performed at an acetylene–oxygen O2/C2H2 mixture ratio of 1.856; the volume of filling of the detonation gun barrel with an explosive gas mixture was 63%. To obtain a NiCr-based metallic layer, spraying was performed at the O2/C2H2 ratio of 1.063; the volume of filling of the detonation gun barrel with an explosive gas mixture was 54%. Based on a study of the effect of the detonation spraying mode on the phase composition of NiCr and Al2O3 coatings, NiCr/NiCr-Al2O3/Al2O3-based multilayer coatings were obtained. Mixtures of NiCr/Al2O3 powders with different component ratios were used to obtain multilayer gradient coatings. The structural-phase composition, mechanical and tribological properties of multilayer gradient metal–ceramic coatings in which the content of the ceramic phase changes smoothly along the depth were experimentally investigated. Three-, five- and six-layer gradient coatings were obtained by alternating metallic (NiCr) and ceramic (Al2O3) layers. The phase composition of all coatings was found to correspond to the removal of information from a depth of 20–30 μm. It was determined that the five-layer gradient coating, consisting of the lower metal layer (NiCr), the upper ceramic layer (Al2O3) and the transition layer of the mechanical mixture of metal and ceramics, is characterized by significantly higher hardness (15.9 GPa), wear resistance and adhesion strength.

Assessment of plasma sprayed carbon nanotube reinforced Al2O3-based nanocomposite with micro-scratching

Abstract An efficient Al2O3-based nanocomposite coating was developed to fabricate the top thermally insulative layer of the thermal barrier coatings (TBCs). Here, Al2O3-based coatings were prepared by atmospheric plasma spraying and the scratch properties were investigated using progressive-load scratching. Scratch mechanism and the role of carbon nanotube (CNT), 3 and 8 mol% Y2O3 stabilized ZrO2 (3YSZ and 8YSZ, respectively) during progressive load scratching were analysed. The investigations revealed that the Al2O3-based nanocomposite coatings were showed a high scratch coefficient (0.165–0.219) compared to that of monolithic Al2O3 (0.128). Also, the monolithic Al2O3 coating showed a decent scratch resistance (0.35 mm3N−1 m−1) compared to 3YSZ/8YSZ and CNT reinforced nanocomposite (0.49–69 mm3N−1 m−1). It was attributed to the synergistic effect of hardness, fracture toughness and the bonding of the reinforcement with the matrix Al2O3. The work of adhesion of lamella in the YSZ and CNT reinforced nanocomposite was measured as 1.16–2.75 × 103 J, which is lower (up to 2.5 times) than the monolithic Al2O3 (2.91 × 103 J). It is attributed to the poor bonding of CNTs with the lamella contains Al2O3 and YSZ. Further, the failure of the nanocomposite coatings, such as flaking and chipping were resisted by CNT. Nevertheless, CNT addition declines the scratch toughness of the nanocomposite coatings (1.05–2.04 MPa.m0.5). It endorses no effective contribution of CNT in the scratch toughness of the coatings. Meanwhile, the mere addition of 8YSZ and 3YSZ in the Al2O3 matrix showed an increment (10.5% and 67.5%, respectively) in the scratch toughness compared to that of monolithic Al2O3 (2.37 MPa.m0.5) coating.

Laser cladding of manganese oxide doped aluminum oxide granules on titanium alloy for biomedical applications

Due to the low hardness and poor tribological behavior of titanium alloys, surface coating is a very critical technique in order to increase their commercial exploitation. In this case, Al2O3-based coatings are very promising for these alloys to improve their surface properties. To increase the absorption of the laser light in the visible range, MnO2 is proposed as a dopant for Al2O3, in order to achieve dense coatings on Ti6Al4V substrates via cladding with a JK fiber laser. To ensure a homogeneous doping, MnO2 and Al2O3 were dispersed by means of ammonium citrate, and to fully cover the substrate, spray dried MnO2 doped Al2O3 granules were prepared and applied to the substrate. The effects of the solid loading on the properties of these granules were investigated. It is found that with 1.0 wt% MnO2 addition, a dense and uniform Al2O3 layer, accompanied with an interlayer, was successfully fabricated through laser cladding with low laser power of 20 W. Owing to the better adhesion on the substrate, Al2O3 coating on sandblasted substrates feature a higher hardness of 1204.1 ± 66.9 HV0.05 and an elastic modulus of 212.0 ± 11.8 GPa, which is an increase of 165.5% and 53.3%, respectively, compared to the as-received substrate.

Robust hydrophobic surface driven by Al2O3/glass composite coatings

Fabrication of hydrophobic surface with exceptional mechanical robustness has been of considerable interest for its urgent practical application. Herein, we demonstrate a facile approach to fabricate mechanically robust, hydrophobic surface by combining Al2O3, glass frits, and hydrophobic solution. Al2O3-based composite coatings with different roughness were prepared by controlling the ratio of Al2O3 particles and glass frits. Surface properties of Al2O3-based composite coatings were carefully modified by hydrophobic fluorine-containing sol-gel solution. Hydrophobic sol-gel solution-coated Al2O3-based coatings (H-Al2O3) show strong hydrophobicity, and water contact angle of 132.38° was achieved at an optimal Al2O3/glass frits ratio of 50. To demonstrate the mechanical robustness of our H-Al2O3, the hydrophobicity was evaluated after performing sandpaper (400 mesh) abrasion and ultrasonication. The H-Al2O3 still retained great water repellency even after sandpaper abrasion for 100 cm and ultrasonication for 60 min, exhibiting excellent mechanical robustness. We believed that the presented method could provide a straightforward and effective route to fabricate mechanically robust and hydrophobic surface on various substrate materials for a great number of potential applications.

An effective low-temperature strategy for sealing plasma sprayed Al2O3-based coatings

Pores and cracks are usually inevitable in the coatings fabricated by plasma spraying. Herein, thermal chemical reaction technique was developed as an effective low-temperature strategy for addressing this issue. Dense α-Al2O3/AlPO4 coating was synthesized on the surface of the plasma sprayed Al2O3-based coating by thermal chemical reaction at 500°C. The α-Al2O3/AlPO4 composites permeated into the coating and effectively filled up the pores and cracks inside. The sealed coating showed a uniform and dense morphology. The hydrogen permeation rate of the sealed coating sharply decreased by a factor of 15, which further confirmed its significantly improved compactness. In addition, the coating exhibited good thermal shock resistance and bond strength after sealing. Considering its low-temperature heat treatment, facile fabrication process and wide applicability, thermal chemical reaction method can be potentially used as a cost-saving strategy for sealing plasma sprayed ceramic coatings.

Thermal stability of WAlN/WAlON/Al2O3-based solar selective absorber coating

The solar absorptance property of W/WAlN/WAlON/Al2O3-based coatings, deposited by DC/RF magnetron sputtering on stainless steel substrate was studied by measuring the reflectance spectra in the wavelength range of 250 - 2500 nm. The effect of thermal annealing on the optical properties of the solar selective absorber coatings was investigated. Annealing the coatings at 450°C for 150 hrs in air did not show any significant change in the spectral properties of the absorber coating indicating the excellent thermal stability of the coating. The W layer acts as infrared reflective layer and diffusion barrier on stainless steel substrate. The top Al2O3 layer serves as dense shield to protect the under layers from oxidation in air. In summary, the present study indicates the potential application of W/WAlN/WAlON/Al2O3-based selective coatings in high temperature photo thermal conversion systems.

Corrosion Resistance of APS- and HVOF-Sprayed Coatings in the Al2O3-TiO2 System

In this article, the results of corrosion investigations performed on thermally sprayed ceramic coatings with different compositions in the Al2O3-TiO2 system (Al2O3, Al2O3-3%TiO2, Al2O3-40%TiO2, and TiO x ) are presented. The coatings were deposited on corrosion-resistant steel substrates using atmospheric plasma spraying (APS) and high-velocity oxy-fuel (HVOF) spraying processes and characterized by means of optical microscopy, scanning electron microscopy (SEM), and x-ray diffraction (XRD). The corrosion properties were investigated in 1 N solutions of NaOH and H2SO4, at room temperature, 60 °C, and 85 °C, as well as in hydrothermal conditions with deionized water at 100 °C and 200 °C. The corrosion stability of the coatings depended on coating characteristics (spraying method, microstructure, and crystalline phase composition) and the corrosive environment (media, test temperature, and duration). In contrast to expectations, APS-sprayed coatings were found to be more corrosion-resistant than the HVOF-sprayed coatings. Addition of TiO2 to Al2O3 increased the corrosion stability, especially for the HVOF-sprayed coatings. In this work, TiO x coatings were found to be more corrosion-resistant than the Al2O3-based coatings.