In the present work, the effects of oxidising and standard flame spray techniques on microstructure and wear performance of Low Velocity Oxygen Fuel NiWCrBSi coatings produced onto mild steel substrate were studied. Results showed that coating microstructure was dependent on the stoichiometry of the reacting gas mixture. Coatings produced in oxidising flame conditions showed higher amounts of semi-molten particles and high quantity of WC phases. Those coatings showed a good behaviour in dry sliding wear test, due to their higher hardness and the formation of compact and adherent oxide tribofilms. The wear behaviour of non-molten particles against friction forces is discussed, with the aim of establishing relationships between the tribological response of the material and its process-induced microstructure defects.

This work presents microstructure and mechanical properties of NiWCrBSi coatings produced onto mild steel substrate by oxyacetylene flame spraying, using two different stoichiometries. Coatings were analysed by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray diffraction. Microhardness tests were carried out using a micro-hardness tester operating with Vickers indenter and the load being 3N. Results showed that both coating microstructures were of lamellar/stratified type, containing several inhomogeneities such as unmelted particles and pores. Coatings consisted mainly of a solid solution (Ni, Cr and Fe), containing two different phases of hexagonal structure, namely WC and Ni₃₁Si₁₂. The presence of WC hard particles contributed to the high coating hardness. Oxidizing flame conditions showed a potential interest for tribological applications, because of a reduced porosity and an increased coating hardness.

The application of hard and resistant coatings is a possibility to increase the lifetime of parts exposed to heavy loads and harsh environments. This work is focused on the investigation of mechanical properties of flame sprayed NiWCrBSi coatings, addressing in particular coating microstructure, hardness, friction behaviour and wear mechanisms. Microstructure and phase composition of coatings were characterized by optical microscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray diffraction analysis. Microhardness of coated and uncoated samples was measured and their tribological properties were evaluated by ball-on-disk tests under dry conditions at room temperature. The morphology of the worn scar at different sliding conditions was investigated using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and the corresponding wear profile by 3D profilometry. The coatings presented mainly a heterogeneous lamellar structure, containing inhomogeneities, such as unmelted particles, micro-cracks, oxides and various types of pores. The coatings underwent severe wear at low sliding speeds, where the predominating wear mechanism was identified as massive exfoliation. However, when increasing the sliding speed and the sliding distance, a transition in the predominant wear mechanism was observed, passing from the combination of oxidative/soft abrasive mechanism to fatigue delamination. These results indicate the importance of controlling coating microstructure through process parameters to optimize its tribological behaviour.

Different techniques are usually employed to evaluate the mechanical properties of arc-sprayed coatings. In many situations, comparing properties is complex, and values extracted from tensile tests are required for structural projects. X6CrNi18-8 stainless steel and molybdenum were sprayed onto a mild steel substrate using the electric arc thermal spray technique to discuss this issue. After a detailed microstructure characterization, tensile tests were performed on both coatings to determine the yield strength and total elongation. Easy techniques were also applied: Vickers hardness and Charpy impact test. Tensile tests have shown that applying coatings increased the steel substrate’s total elongation. Molybdenum coating presented a higher impact resistance than the X6CrNi18-8 one, resulting in no correlation between elongation and Charpy values. On the other hand, correlations between hardness and yield strength were identified, opening a discussion on the effects of the microstructure and type of test used.

Coating efficiency and quality can be significantly improved by carefully optimizing the coating parameters. Particularly in the flame spray method, the oxygen/fuel ratio, which is classified as oxidizing flame stoichiometry (excess oxygen) and reduces flame stoichiometry (excess acetylene), and spray distance are the most critical factors, as they correlate significantly with coating porosity and corrosion performance. Hence, understanding the effects of these parameters is essential to further minimize the porosity, improving the corrosion performance of thermally sprayed coatings. In this work, a NiWCrBSi alloy coating was deposited via the oxyacetylene flame spray/Flexicord-wire (FS/FC) method. The effect of the flame oxygen/fuel ratio and spray distance on the microstructure properties and corrosion behavior of the coatings was investigated. Afterwards, the microstructure, phases’ compositions, spray distance, and corrosion performance were studied. The equivalent circuit model was proposed, and the corrosion mechanism was discussed. The obtained results highlight that the oxygen-to-fuel ratio is a promising solution for the further application of flame spray/Flexicord-wire (FS/FC) cermet coatings in hostile environments. Depending on the flame’s oxygen/fuel ratio, careful selection of the flame stoichiometry provides low porosity and high corrosion performance.