The application of cold spray (CS) for additive manufacturing (CSAM) of structural components using metallic materials has recently attracted much attention. However, there are limited reports on developing thick deposits or components with high entropy alloys (HEAs) via CSAM and investigating the microstructural evolution and mechanical properties after deposition and subsequent annealing heat-treatment. This work investigated the microstructure and mechanical properties of asdeposited and heat-treated thick CoCrFeNiMn HEA deposit fabricated via CSAM. The microstructure of the HEA deposit and after heat-treatment were characterised using scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), and x-ray diffraction (XRD). The microstructural analysis reveals heterogeneous grain size distribution with ultrafine grains at the particle-particle interfacial regions and coarse grains at the particle interiors in the as-deposited sample. The as-deposited sample, characterised by moderate porosity, was consolidated following the heat treatment at different temperatures. Additionally, increasing the temperature increases grain sizes resulting from static recovery and recrystallisation, with annealing twin formed at higher temperatures. Most notably, phase decomposition of the deposit microstructure occurs at 600 ºC, with Cr-rich phase particles formed at regions of high dislocations and grain boundaries. Nano-and micro-hardness and tensile testing of micro-flat dogbones samples were performed on the as-deposited and heattreated samples. The effect of heat-treatment on the microstructure and mechanical properties of the cold-sprayed HEA deposit were analysed and discussed.

A novel composite MCrAlY coating with a fine dispersion of sub-micron Al 2 O 3 particles were sprayed by high velocity oxygen fuel (HVOF) thermal spraying for high temperature oxidation protections. The presence of Al 2 O 3 could reinforce the metallic MCrAlY coatings, in a similar way to ODS alloys (oxide dispersion strengthening). It could also enhance the oxidation resistance owing to the dispersion of Al 2 O 3 on the surface as nucleation sites, which promotes the early formation of a coherent α-Al 2 O 3 scale. This is essential for an effective protection against oxidation attack, especially for these applications at relatively lower temperatures (<900 °C) with slower growing Al 2 O 3 . In this study, a suspension route was employed to achieve a uniform dispersion of 0-10 wt.% Al 2 O 3 particles with commercial MCrAlY powders. A liquid fueled HVOF spray gun (MetJet IV) was used to deposit the composite MCrAlY-Al 2 O 3 coatings onto 304 stainless steels substrates. The composite coatings were examined thoroughly by field emission gun scanning electron microscopy (FEGSEM) with energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analysis. Isothermal oxidation at 900 °C was carried out to study the effect of Al 2 O 3 on the growth of oxide on the coatings surface. The composite coatings exhibited superior oxidation behavior against the conventional metallic coatings with the formation of nearly exclusively Al 2 O 3 on the coatings surface and inhibited NiO growth.

A known family of rare-earth oxide (REO) ceramics have recently been found to exhibit intrinsic hydrophobicity, even after exposure to high temperatures and abrasive wear. In this study, thin CeO 2 coatings were developed for hydrophobic applications using suspension high velocity oxy-fuel (SHVOF) thermal spray. It is an efficient method to produce large superhydrophobic surfaces with a unique hierarchically textured structure on a variety of substrates. The use of suspension also enables the process of fine-grained powders to form nanostructured coatings with significant improvement of mechanical and chemical properties for numerous applications. An aqueous suspension with a solid concentration of 30 wt.% sub-micron CeO 2 particles (<200 nm) was used as suspension feedstock. The as-sprayed CeO 2 coating on a stainless steel significantly improved the substrate’s surface hydrophobicity from a low contact angle of 57° to nearly 150°. The surface chemistry of SHVOF thermal sprayed CeO 2 coatings was also investigated by X-ray photoelectron spectroscopy (XPS). It was confirmed that the near-super-hydrophobicity was mainly attributed to its unique hierarchically structured surface.

Higher operating temperatures coupled with biomass-derived fuels can lead to aggressive corrosion damage to the superheater/reheater tubes in power plants. In this study, a HVOF sprayed NiCr coating was deposited onto a 9 % Cr substrate, which were exposed in simulated coal-biomass combustion gases with a screening deposit containing Na 2 SO 4 , K 2 SO 4 and Fe 2 O 3 at 700-750°C for 1000 h. The tests were carried out using the “deposit-recoat” test method and pre and post-exposure dimensional metrology was used to quantify the coating damage in terms of metal loss distributions. The exposed samples were also examined in a SEM/ EDX. The coatings developed a protective Cr 2 O 3 layer at the coatings/ deposit interface and a Cr depleted zone was observed underneath the oxide layer. NiCr coating provided suitable corrosion protection with a median metal loss of ~35μm in 1000h.