RESUMEN
This study is focused on the high-temperature corrosion evaluation of selected thermally sprayed coatings. NiCoCrAlYHfSi, NiCoCrAlY, NiCoCrAlTaReY, and CoCrAlYTaCSi coatings were sprayed on the base material 1.4923. This material is used as a cost-efficient construction material for components of power equipment. All evaluated coatings were sprayed using HP/HVOF (High-Pressure/High-Velocity Oxygen Fuel) technology. High-temperature corrosion testing was performed in a molten salt environment typical for coal-fired boilers. All coatings were exposed to the environment of 75% Na2SO4 and 25% NaCl at the temperature of 800 °C under cyclic conditions. Each cycle consisted of 1 h heating in a silicon carbide tube furnace followed by 20 min of cooling. The weight change measurement was performed after each cycle to establish the corrosion kinetics. Optical microscopy (OM), scanning electron microscopy (SEM), and elemental analysis (EDS) were used to analyze the corrosion mechanism. The CoCrAlYTaCSi coating showed the best corrosion resistance of all the evaluated coatings, followed by NiCoCrAlTaReY and NiCoCrAlY. All the evaluated coatings performed better in this environment than the reference P91 and H800 steels.
RESUMEN
At present, due to advanced fatigue calculation models, it is becoming more crucial to find a reliable source for design S-N curves, especially in the case of new 3D-printed materials. Such obtained steel components are becoming very popular and are often used for important parts of dynamically loaded structures. One of the commonly used printing steels is EN 1.2709 tool steel, which has good strength properties and high abrasion resistance, and can be hardened. The research shows, however, that its fatigue strength may differ depending on the printing method, and may be characterized by a wide scatter of the fatigue life. This paper presents selected S-N curves for EN 1.2709 steel after printing with the selective laser melting method. The characteristics are compared, and conclusions are presented regarding the resistance of this material to fatigue loading, especially in the tension-compression state. A combined general mean reference and design fatigue curve is presented, which incorporates our own experimental results as well as those from the literature for the tension-compression loading state. The design curve may be implemented in the finite element method by engineers and scientists in order to calculate the fatigue life.
RESUMEN
In order to extend the life of boilers by applying an anti-corrosion coating without the need to dismantle them, it is advisable to find coatings that can be applied using cheaper and portable techniques, such as Twin Wire Arc Spray technology (TWAS). In this study, we compare selected NiCr-based coatings and two uncoated steel substrates (steel 1.7715 and 1.4903). Two coatings, Cr3C2 - 25% NiCr and Hastelloy C-276 are deposited using High velocity oxygen-fuel technology (HVOF) and three coatings, NiCrTi, NiCrMo, and Inconel 625, are deposited using TWAS. In addition to the corrosion weight gain during 50 cycles of loading in an 18% Na2SO4 and 82% Fe2(SO4)3 salt environment at 690 °C evaluated using the gravimetric method, the microstructure and phase composition of the coatings were analyzed on the samples after the exposure in order to compare the properties and gain a deeper understanding of the corrosion kinetics. Coating cross-sections and free-surfaces were observed with a scanning electron microscope (SEM) with an energy-dispersive (EDX) system. The phase composition was investigated using X-ray diffraction (XRD) and Raman spectroscopy. No significant differences were observed between the TWAS and HVOF coating methods for the coatings compared. Due to the similar corrosion products found on all coatings, a very effective corrosion protective layer was formed on the surface, forming a barrier between the corrosive environment and the coating regardless of the used deposition technology. Therefore, for industrial use on the inner surface of coal-fired boilers we recommend NiCrTi, NiCrMo, or Inconel coatings prepared with the more cost-effective and portable TWAS technology.
RESUMEN
Pulsed thermography is a common technique for nondestructive testing (NDT) of materials. This study presents the apparent effusivity method for the quantitative evaluation of coating thickness in a one-sided thermal NDT procedure. The proposed algorithm is based on determining a threshold value of apparent effusivity, which can be found for particular coating-on-substrate structures. It has been found that the square root of the time at which the apparent effusivity curve reaches this threshold is proportional to the coating thickness. The efficiency of the proposed approach is demonstrated by analytical modeling and experimentation performed on thermally-sprayed coatings.
RESUMEN
High velocity oxygen-fuel (HVOF) prepared CrC-based hardmetal coatings are generally known for their superior wear, corrosion, and oxidation resistance. These properties make this coating attractive for application in industry. However, under some loading conditions and in aggressive environments, the most commonly used NiCr matrix is not sufficient. The study is focused on the evaluation of dynamic impact wear of the HVOF-sprayed Cr3C2-25%NiCr and Cr3C2-50%NiCrMoNb coatings. Both coatings were tested by an impact tester with a wide range of impact loads. The Wohler-like dependence was determined for both coatings' materials. It was shown that, due to the different microstructure and higher amount of tough matrix, the impact lifetime of the Cr3C2-50%NiCrMoNb coating was higher than the lifetime of the Cr3C2-25%NiCr coating. Differences in the behavior of the coatings were the most pronounced at high impact loads.
RESUMEN
Ceramic Al2O3-ZrO2-SiO2 coatings with near eutectic composition were plasma sprayed using hybrid water stabilized plasma torch (WSP-H). The as-sprayed coatings possessed fully amorphous microstructure which can be transformed to nanocrystalline by further heat treatment. The amorphous/crystalline content ratio and the crystallite sizes can be controlled by a specific choice of heat treatment conditions, subsequently leading to significant changes in the microstructure and mechanical properties of the coatings, such as hardness or wear resistance. In this study, two advanced methods of surface heat treatment were realized by plasma jet or by high energy laser heating. As opposed to the traditional furnace treatments, inducing homogeneous changes throughout the material, both approaches lead to a formation of gradient microstructure within the coatings; from dominantly amorphous at the substrate-coating interface vicinity to fully nanocrystalline near its surface. The processes can also be applied for large-scale applications and do not induce detrimental changes to the underlying substrate materials. The respective mechanical response was evaluated by measuring coating hardness profile and wear resistance. For some of the heat treatment conditions, an increase in the coating microhardness by factor up to 1.8 was observed, as well as improvement of wear resistance behaviour up to 6.5 times. The phase composition changes were analysed by X-ray diffraction and the microstructure was investigated by scanning electron microscopy.