Silicon coatings have been developed for environmental barrier coatings by thermal spraying. Until now, these coatings have been produced almost exclusively by Atmospheric Plasma Spraying (APS). High Velocity Oxy-Fuel (HVOF) spraying is commonly used to produce dense metallic and carbide-based coatings due to high particle velocities. However, there have been no scientific reports on HVOF-sprayed silicon coatings in the literature. This study was conducted to investigate the feasibility of fabricating silicon coatings by HVOF using a DJ2600 spray system. Both the spray powders and the parameters were varied. The coatings were investigated on their surfaces and cross-sections using scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). The hardness and indentation modulus of the silicon coatings were also determined. The results show that the particle size distribution and the stand-off distance are important influencing factors. Dense coatings could be produced by HVOF spraying, confirming the feasibility.

Thermally sprayed WC/CoCr coatings are established in the valve industry for wear protection. However, conventional coatings have to be cost-intensively postprocessed. Therefore, the aim of this study is to develop near net shaped (nns) WC/CoCr-coatings with a high wear resistance in order to avoid the expensive grinding postprocess. For the development of the nns coatings a parameter study is used to investigate the influence of the stand-off distance and hydrogen volume flow rate in the HVAF process. The parameter study indicates the influence of the hydrogen volume flow and the stand-off distance on the porosity, microhardness and wear resistance of the coatings. The developed coatings exhibit a low porosity and high wear resistance. Through the correlation of the coating properties with the process parameters, promising parameter ranges for a further development of HVAF-sprayed nns coatings of WC/CoCr can be identified. With these results, first benchmarks for HVAF process parameters, hydrogen flow rate and stand-off distance, could be delivered, advancing the overall goal of reducing manufacturing costs of valves.

Wear leads to high material and energy losses in various industries. The manufacturing of novel nano-carbide WC/Co powder feedstock materials promises a further increase in the performance of thermally sprayed wear protection coatings. A novel experimental powder and a commercial ultra-fine carbide WC/CoCr reference are thermally sprayed onto a 1.0038 substrate by High Velocity Air Fuel (HVAF) spraying. The specimens are metallographically prepared and analyzed by means of light microscopy (LM) and scanning electron microscopy (SEM). Vickers Hardness testing is conducted by microindentation and the porosities are determined by optical image analysis. X-ray diffractometry (XRD) analysis are used to investigate the phase retention. Fine nanocrystalline WC-structures are preserved in the dense coatings. A significant effect of powder type on the porosity of the coating was found. No systematic relationships could be identified between the coating structure and the parameter settings. It was possible to influence decarburization via both the powder type and the selected parameters. The resulting experimental coatings exhibit high hardness values in the range of the commercial ultrafine carbide WC reference. The novel nano-structured coating can contribute to reduced wear and therefore improve the efficient utilization of critical raw materials like tungsten.

Surface quality lifetime and wear resistance of protective coatings can be improved by decreasing carbide grain size from submicron to nanoscale. In this study, experimental WC-CoCr powders were manufactured via novel powder manufacturing approach using water-soluble raw materials. Produced powders were sprayed with the High-Velocity Air-Fuel (HVAF) spray process to control the particle temperature and to avoid in-flight decomposition of the nanocarbides. As a result, dense and wear resistant coatings with nanosized carbides were produced. Reference coatings were sprayed using commercial sub-micron WC-CoCr powder to compare the properties of the experimental coatings to the current state-of-the-art. Phase composition and microstructural characterization of the coatings were carried out with X-ray diffraction and electron microscopy, respectively. Mechanical properties were studied by using microhardness tester, as well as rubber wheel abrasion and cavitation erosion wear tests. The wear surfaces were characterized after the abrasion and cavitation erosion tests to understand the effect of nano-carbides on degradation mechanisms. Coatings with the nanosized carbides in the structure showed excellent mechanical properties in wear testing, and even outperformed reference coatings in cavitation erosion test. Based on the obtained results, these novel nano-carbide coatings are promising alternatives for demanding applications in which better surface quality lifetime is vital.

Various alumina-based materials are applied to achieve different electrical insulation properties based on the variation of the material specific relative permittivity. Thermally sprayed mullite (Al 2 O 3 · SiO 2 ) can form an amorphous phase due to the high cooling rates of the process. The formation of amorphous phases causes a change in the capacitive behaviour of the coatings. The tendency to form amorphous areas can be influenced by the composition of the feedstock material or coating parameters. On the one hand, mullite coatings based on two different Al 2 O 3 to SiO 2 ratios are investigated. On the other hand, a parameter variation is used to achieve various particle temperatures during the process. The coatings are investigated via X-ray diffraction and DSC for phase formation, electron microscopy for coating structure and impedance spectroscopy for measuring the AC-resistance. The conducted variation of the feedstock material as well as the parameters causes differences in the XRD and DSC measurements correlating with a difference in the amounts of amorphous phases. For the capacitive behaviour, coatings applied with hydrogen as process gas showed decreased AC-resistance values. The chemical composition of the feedstock material indicates that the AC-resistance decreases with increasing amounts of SiO 2 . In summary, mullite has promising insulation properties which can be modified by the feedstock material composition as well as the coating parameters. For future application, mullite is a promising candidate for increasing the electrical insulation properties in conditions under high electrical and mechanical demands.

Thermally sprayed ceramic coatings can be used for wear protection as well as thermal and electrical insulation. When exposed to environments with high humidity, the water absorption of the ceramic coating has a tremendous impact on the electrical insulation. In thermally sprayed ceramic coatings, water can easily be absorbed by the porous microstructure of the coating. A general result of the water absorption is the reduction of the dc resistivity. However, in the high frequency regime of ac loads, contrary results were observed for sealed Al 2 O 3 coatings on steel substrates. Specimens exposed to high air humidity have shown an increased ac resistance compared to dry specimens if frequencies above 1 MHz are considered. To analyse this phenomenon, a novel measurement technique was developed to investigate the influence of the water absorption of detached ceramic coatings on the ac resistivity at high frequencies. Moreover, the water absorption of the ceramic is measured gravimetrically. To ensure the results are also applicable to ceramic coatings on substrates, the morphology of the coating was analysed using electron microscopy and compared to reference specimens deposited on steel substrates from [1].

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.