In our previous work, the potential of the suspension-HVOF spraying (S-HVOF) to produce dense-structured WC-12Co coatings has been shown. This contribution proposes a comparative study of the corrosion properties of the S-HVOF WC-12Co coatings and conventional sprayed HVOF coatings. The corrosion properties were evaluated at room temperature in NaCl electrolytes with different pH values and in a pH neutral 0.5 M Na 2 SO 4 solution. By varying the pH value, the corrosion mechanism of the cemented carbide coatings should be assessed more precisely, since the two components, WC and Co, show strongly different pH dependencies. The electrochemical properties of the sprayed coatings were investigated using open circuit potential measurements, linear sweep voltammetry and potentiodynamic polarization methods. Before and after corrosion tests, microstructural evaluations of the coatings were performed. Moreover, element analyses of the eluates have been performed to determine soluble corrosion products. The S-HVOF coatings show a similarly good corrosion resistance as the conventional HVOF WC-Co coatings. Generally, the coating properties, i.e. microstructure and phase compositions, as well as the electrolyte significantly influence the corrosion performance of the sprayed coatings.

Thermal spray WC-NiCr coatings generally requires grinding processing to meet the surface finish requirements. The cost associated with grinding can potentially be reduced through the deposition of finer (– 30 + 5 μm) feedstock rather than the more conventional commercial (– 45 + 15 or – 53 + 20 μm) feedstock. Additionally, such a fine powder is likely to require lower energy spray parameters, resulting in less heat input to the substrate, which could be beneficial in application on heat sensitive substrates. However, the spray parameters need to be optimised to mitigate increased degradation of coatings due to unacceptable brittleness caused by decarburisation and oxidation, and to produce defect-free dense coatings. In the present work, a theoretical model to optimise spray parameters was developed, which suggested an oxygen-to-fuel ratio slightly more than 3.3 and a shorter barrel to avoid decarburisation in coatings. In total four parameter sets suggested by the theoretical model were selected to spray the fine-cut powder using a 100 mm long barrel. Scanning electron microscopy, X-ray diffraction, microhardness, and 3Dprofilometer were used to analyse the produced coatings. The coatings deposited using optimised parameters exhibited the best performance in terms of low porosity, inter-splat cracks, brittleness, and roughness. Coating deposited at lower kerosine and oxygen flow rates, with reduced stand-off distance, was denser, crack-free, and ductile. Hence, the fine-cut powder can be used to produce a finer as-sprayed finish, thereby demonstrating the potential in reducing grinding efforts. Additionally, successful deposition of coatings using low energy parameters, making this an attractive option for thermally sensitive substrates.

Atmospheric plasma sprayed (APS) CuNiIn coatings have been widely used for fretting wear protection in many important areas such as aircraft engines for decades. The oxides in CuNiIn coating prepared by APS hinder splat bonding formation and thus degrade the coating fretting performance. In this study, CuNiIn powders of different boron contents were designed to realize the self-oxide-cleaning effect for in-flight molten droplets and thus deposit the dense CuNiIn coating with high fretting performance. Scanning electron microscope was used to characterize the microstructure. The oxygen content in the coating was measured by the inert gas fusion technique. Fretting test was performed for three coatings under different loadings. The results show that CuNiIn2B and CuNiIn4B coatings presented the oxide content of 0.40wt% and 0.38wt%, which are lower than 1.6wt% of the CuNiIn coating. The oxygen content in the CuNiIn4B coating decreased with the increase of spray distance while the oxygen content in CuNiIn coating increased with the increase of the spray distance. Such results clearly reveal the boron in-situ deoxidizing effect of inflight molten droplets. As a result, the dense CuNiIn2B and CuNiIn4B coatings were deposited with oxide-free molten droplets. The test results showed that the fretting wear performance of B-alloyed CuNiIn coatings were increased by a factor over three comparing with conventional CuNiIn coating.

Stellite 6 is a widely applied hardfacing alloy with good corrosion and wear resistance. Several different techniques have been used to deposit this alloy, like oxyacetylene, Tungsten Inert Gas welding overlay, CO 2 laser cladding and thermal spray methods. This paper evaluates the wear and corrosion resistance of Stellite 6 coatings applied by high velocity oxygen fuel (HVOF) spraying and gas tungsten arc welding (GTAW) hot wire technique. Mechanical properties and microstructural characteristics of the coatings are assessed and compared. The influence of these properties in the wear results is further discussed.

Three-cathode plasma spraying is characterized by high process robustness in terms of particle in-flight properties due to high stability of plasma jet. However, during coating parts with complex geometries, process parameters such as stand-off distance and spray angle usually cannot be kept at optimized conditions. In this study, the process robustness during three-cathode plasma spraying of a novel FeCrMnBC alloy was investigated by varying stand-off distance and spray angle to simulate spray conditions for complex geometries. A three-cathode plasma generator TriplexProTM-210 was used to spray FeCrMnBC powder with a fraction of -45 +20 µm onto substrates of cast iron EN-GJL-250. The stand-off distance and the spray angle were varied from d=90 mm to d=110 mm and between ø=90° and ø=60°, respectively, while the other process parameters were kept constant. The results revealed that the reduction of spray angle caused an about 15 % decrease of coating thickness, about 40 % porosity increase and about 15% increase of bond strength. In contrast, the variation of stand-off distance changed only insignificantly the coating thickness, porosity and bond strength. The variation of stand-off distance and spray angle did not result in significant changes in the coating microhardness. The reduction of spray angle resulted in significant worsening of the corrosion protection ability of the coating system.

Fe-based coatings, such as novel FeCrMnBC alloys, have both economic and ecological advantages compared to other coatings like Ni-based or Co-based coatings. In recent years, high performance Fe-based wear and corrosion resistant coating systems have been developed. Some of them have even been introduced into the market. However, the suitability of the FeCrMnBC alloy as coating for cast iron under complex erosive and corrosive stresses in particle-loaded fluids for pump parts has not been investigated yet. Especially the impact of the process robustness of three-cathode plasma spraying coatings applied with variable process parameters like stand-off distance and spray angle is in the focus of interest. The objective of the present work has been the characterization of novel FeCrMnBC alloys, for the first time deposited via Thermal Spray processes. The corrosion resistances as well as the cavitation and erosion properties were separately evaluated by current density-potential measurements and supersonic cavitation in artificial sea water. Erosion corrosion behavior has been investigated in a pump test rig with 10 wt.-% corundum (Al 2 O 3 ) particles. The results show that the reduction of spray angle and the variation of stand-off distance limit the corrosion and cavitation resistance in different ways. The erosion behavior shows only small variations for the tested parameters. The results reveal that the FeCrMnBC coatings exhibit high process robustness for the chosen parameter variations and a large potential to improve the protection of cast iron even for not optimized conditions.