Various feedstocks are available for the thermal spraying of metallic glasses, which are often alloyed with high amounts of cost-intensive elements. In previous steps, a novel, economic Fe-based metallic glass alloy with a high Si content has been developed using melt spinning. The aim of this work is to investigate the application of the alloy using the high-velocity arc spraying (HVAS) process. On this basis, four cored wires are manufactured with the aim of maximizing in situ intermixing and amorphous phase formation during the spraying process. The cross sections of the resulting coatings are analyzed by light microscopy, scanning electron microscopy and Vickers hardness testing. Phase analysis on the coatings is conducted with regards to the formation of amorphous phases using x-ray diffractometry (XRD) and differential scanning calorimetry. The XRD patterns indicate a mixture of (nano-) crystalline ferrite and amorphous phases. In particular, the coating manufactured with wire No. 1, a Fe-B-Si-C-Nb composition, exhibited good intermixing and a highly amorphous structure. This work demonstrates that glassy metallic coatings can be produced by means of HVAS using Fe-based cored wires comprising of conventional filler materials. A successful intermixing, in situ alloying and the subsequent formation of amorphous phases is achieved.
Alumina is often used for electrical insulation. However, different material systems promise to increase the insulation due to their material characteristics. Because of the process properties including high cooling rates, thermally sprayed coatings generally differ from sintered material, which also effect the electrical properties. Within this study, different thermally sprayed coatings are analyzed via impedance spectroscopy to evaluate the capacitive and the electrical insulation behavior. Besides comparing the frequency-dependent resistance, equivalent circuit diagrams were used to calculate the relative permittivity of the coatings. X-ray diffractograms reveal the phase stability of the coatings during thermal spraying. X-ray diffraction was additionally conducted to classify the systems and the respective effects. In particular, the investigated mullite-based coatings exhibit slightly increased impedance values compared to conventionally used alumina-based coating systems.
Silicon coatings are usually produced by atmospheric plasma spraying (APS) and used as bond coats in environmental barrier coatings. The deposition efficiency (DE) of silicon powders is generally at a low level in APS processes. The reasons for the low DE values of silicon powders have not been sufficiently investigated in the literature. The aim of this study was to investigate in detail the influence of process parameters on the coating structure and deposition efficiency of a silicon powder processed with APS. A silicon powder with a size distribution of f = –53 + 15 µm was sprayed using a three-cathode plasma generator to produce coatings. The parameters such as plasma gas type, plasma gas flow rate and current intensity were varied widely. Accordingly, the power of the plasma generator increased from P = 19.4 to 51.3 kW, which allowed different melting and evaporation degrees of the powder. Particle velocities and temperatures were measured using a particle diagnostic method. The coatings were investigated in terms of their surfaces and structures using electron scanning microscopy (SEM). The porosities of the coatings were measured using an image analysis system. The deposition efficiency of the processed powder was determined. The results show that the used parameters led to high particle velocities in a range of about vp = 270–360 m/s. High particle temperatures of Tp = 2,650–3,390 °C were determined. The coating porosity varied from Φ = 2% to Φ = 15%. The porosity value of Φ = 2% is significantly lower than the values reported in the literature. The deposition efficiency of the powder changed from DE = 1.5% to DE = 28%. The value of DE = 28% is about 40% higher than the values reported in the literature. The strong grit-blasting effect was the main reason for the lowest DE value of DE = 1.5%. The strong evaporation effect was the main reason for the second lowest DE value of DE = 11.1%. Numerous melted particles and semi-melted particles splashed upon impact with the substrate, resulting in silicon melt loss. In addition, solid cores of semi-molten particles could bounce off, which also resulted in silicon loss. Splashing and bouncing were the main factors affecting DE for the parameter sets with DE values ranging from 18.7% to 28%.
The advantages of UV-curing polymers are well known and used in various coating and adhesive applications. Curing times of a few seconds and long application windows allowing an increased throughput in series production. The use of UV-curing polymers in sealers is beneficial, but so far insufficient due to only surface curing. With a newly developed dual-cure mechanism in sealers, it is now possible to combine deep penetration curing and surface curing. The hybrid sealers combine radical polymerization with subsequent polyaddition or polycondensation. The development of sealers for thermal sprayed (TS) coatings involves an extensive requirement profile. This includes properties such as corrosion protection, penetration depth and processing times. High penetration depths of the sealant into the coating system are important to ensure a protection over the full lifetime of the TS coatings. The depth of penetration of the developed sealers into various TS coatings was determined by measuring the gas permeability in a specially developed test procedure. The corrosion protection effect in combination with TS coatings was determined by measuring the cell voltage. In summary, two UV dual-cure sealers have been developed to seal TS coatings with deep penetration and corrosion protection.
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