In this study, the high-temperature molten salt corrosion resistance of bare steels and steels with protective coatings, fabricated by thermal diffusion processes (boronizing, aluminizing and chromizing), were investigated and compared. Surface engineering through thermal diffusion can be used to fabricate protective coatings against corrosion, while alleviating issues around possible cracking and spallation that is typical for conventional thermal-sprayed coatings. In this regard, samples of low carbon steel and 316 stainless steel substrates were boronized, chromized, and aluminized through a proprietary thermal diffusion process, while some of the samples were further coated with additional thin oxide and non-oxide layers to create new surface architectures. In order to simulate the actual corrosion conditions in recovery boilers (e.g., from black liquor combustion), the surfaces of the samples sprayed with a modeling salt solution, were exposed to low-temperature (220 • C) and high-temperature (600 • C) environments. According to microstructural and X-ray diffraction (XRD) studies and results of hardness determination, the coatings with multilayered architectures, with and without additional oxide layers, showed successful resistance to corrosive attack over bare steels. In particular, the samples with boronized and chromized coatings successfully withstood low-temperature corrosive attack, and the samples with aluminized coatings successfully resisted both low-and high-temperature molten salt corrosive attacks. The results of this study conducted for the first time for the thermal diffusion coatings suggest that these coatings with the obtained architectures may be suitable for surface engineering of large-sized steel components and tubing required for recovery boilers and other production units for pulp and paper processing and power generation.2 of 36 materials and process features, low-melting temperature ash or smelt is produced. The produced smelt mainly consists of different salts, e.g., sodium and potassium chlorides, sulfates, and carbonates. At elevated temperatures, mostly greater than 500 • C, the aforementioned salts in certain combination likely become molten. In the presence of oxygen, the molten salts can be highly corrosive [2,3]. Combustion of the black liquor fuel also results in the formation of hot corrosive gases, such as SO 2 , CO 2 , Cl 2 , and some others [5].Generally, high-temperature black liquor corrosion occurs according to two main mechanisms, namely (i) high-temperature active oxidation and (ii) corrosion due to the formation of sulfidic and chlorine gasses and residual deposits of molten salts and their interaction with the steel surface [6]. In the former mechanism, which generally occurs in high temperatures (above 450 • C), the anions of the molten salt, such as chlorides (Cl − ), sulfates (SO 4 2− ) and sulfides (S 2− ), continuously diffuse into the oxide-metal interface and actively sustain the oxidation. On the other hand, in the latter mechanism, the presence of the an...
decades because of their utility in bistable and multistable switches for potential applications in accelerometers, [1] microrelays, [2] logic gate, [3] actuators, [4] radio frequency switches, [5] nonvolatile memory devices, [6] neuromorphic systems, [7] etc. A resonating device is bistable, if its resonance frequency changes from an initial value to a final stable value (only one ON state) under an external trigger (e.g., photo, thermal, electric field, etc.) and returns to the original state (OFF state) upon removal of the triggering pulse. Unlike a bistable system, a tristable (or multistable) system can toggle between all the available stable states under external triggering. [8,9] A multistable resonating device can have multiple equilibrium frequency, phase, or amplitude states depending on the changes in its physical properties (stiffness, coefficient of thermal expansion, dielectric constant, etc.) induced by external stimuli. In silicon MEMS resonators, the changes in the resonance frequency usually show bistable states under heating since its mechanical modulus varies uniformly with temperature. However, by integrating a phase change material (PCM) with conventional silicon MEMS, it is possible to engineer a device with multiple stable states, with negative and positive changes Vanadium dioxide (VO 2 ), a promising phase change material, exhibits insulator to metal transition at 68 °C, manifests a drastic change in multiple physical properties, such as electrical resistance, mechanical modulus, lattice parameters, etc. From technological perspective, the transition temperature can be reduced by precise strain engineering. Here a noncontact, all-optical, and highly energy efficient platform is demonstrated to study macroscopic dynamics related to the localized structural rearrangements at room temperature. A thin layer (≈25 nm) of polycrystalline VO 2 deposited on a platinum coated silicon nitride microstring resonator shows a fast controlled mechanical resonance frequency response upon variations in optical power and wavelength. It is shown multiple stable frequencies of the resonator, designated as different equilibrium states, can be activated at different optical powers (≈200 µW) and wavelength, i.e., 450, 520, and 635 nm. The observed multiple resonance states of the microstring are explained because of the generation of stress due to the interplay between thermal expansion and the temperature-induced phase change of VO 2 . It is believed this change in frequency states under the controlled external optical excitation can have potential applications in ultrafast optical switching, intelligent temperature sensors, and neuromorphic devices operated at room temperature.
Aircraft gas turbine blades operate in aggressive, generally oxidizing, atmospheres. A solution to mitigate the degradation and improve the performance of such components is the deposition of thermal barrier coatings (TBCs). Specifically for bond coats in aerospace applications, High Velocity Air Fuel (HVAF) is very efficient for coating deposition. However, internal diameter (ID) HVAF has received little attention in the literature and could be a promising alternative to limit oxidation during spraying when compared to conventional methods. The main objective of this study is to analyze how the ID-HVAF process influences the microstructure of NiCoCrAlY coatings. To that end, an i7 ID-HVAF torch is used to deposit NiCoCrAlY splats on a steel substrate with different stand-off distances. The deposited splats showed the presence of craters, and both partially melted and deformed particles at the surface. The particle velocity data was recorded, and the splat deformation and amount of particles deposited was shown to be directly corelated to the stand-off distance. The material composition analyzed and quantified by Energy Dispersive Spectroscopy (EDS) did not reveal any traces of in-flight of particle oxidation, but further investigation is required. This study provided a preliminary understanding towards the importance of stand-off distance on the splat deformation and in-flight oxidation.
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