Hexavalent chromium has been widely used in the coating industry and more specifically in gas turbine hot end component protection. UK REACH (registration, evaluation, authorization, and restriction of chemicals) have given an end date of September 2024 for the use of hexavalent chromium and as such, the industry must adapt to these regulations. Indestructible Paint LTD have developed a new aluminium diffused slurry coating, CFIPAL, that does not contain hexavalent chromium like its predecessor, IP1041. Both CFIPAL and IP1041 were deposited onto Nimonic 75 alloy and underwent metallurgical and chemical analysis which included scanning electron microscopy, energy dispersive spectroscopy, electron backscatter diffraction, hardness testing, contact angle testing, surface roughness testing and finally, salt spray corrosion testing. The results indicated that CFIPAL is a suitable alternative for hexavalent chromium-containing coatings, such as IP1041.

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.

In this study, pure Al coating was deposited via in-situ shot-peening-assisted cold spray method in order to study the effect of the in-situ tamping effect which was caused by the impact of large sized shot-peening particles on grains size evolution of coatings. The microstructures of the as-sprayed Al coating were observed by using Scanning Electron Microscope and Electron Backscatter Diffraction. A commercial gas atomized Al powder with a grain size range of 10-20 μm was used as the spraying powder. The cross section of the as-sprayed Al particles presented elongated rectangular morphologies, which indicated that the nearly spherical particles experienced severe plastic deformation by the impact of large sized shot-peening particles. It was found that dynamic recrystallization of dislocations-ridden regions was responsible for the grain refinement of cold sprayed coating. Aluminum grains with size of several tens to several hundred of nanometers can be apparently recognized at the whole cross section of the particle. Therefore, in-situ shot-peening-assisted cold spray method can deposit completely nanocrystalline coating using micrometer-grain powder, and thus can be employed to develop high quality coatings of commercial importance.

Different Ti-6Al-4V feedstock powders (i.e., plasma atomized, gas atomized and hydride de-hydride) and corresponding cold spray depositions were characterized using electron back scatter diffraction and X ray diffraction. In addition, a low temperature heat treatment was applied to the coatings and characterized using the above techniques to understand the microstructural evolution after annealing. XRD did not show BCC phase in the three powders and coatings. The axial ratio for all the powders and coatings were similar to what has been reported for Ti-6Al-4V. Atomized powders and hydride dehydride (HDH) powders were characterized by α' microstructure, and equiaxed α phase microstructure, respectively. Phase maps for HDH powders revealed β phase microstructure distributed around equiaxed α grains. Microstructural evolution during cold spray deposition resulted in the atomized coatings retaining as-received powder microstructure at particle interiors and formation of nanograins near the interface. On the other hand, HDH coatings were characterized by slightly deformed as-received powder microstructure with no evidence for nanograin formation. Higher average grain size of HDH powders resulted in the higher percentage of low angle grain boundaries observed after cold spray deposition. Subsequent heat treatment resulted in recovery, some recrystallization and grain growth for all the coatings. Nonetheless, the extent to which this took place was higher for atomized cold sprayed coatings.

Epitaxial grain growth during the rapid solidification of molten TiO 2 in plasma spraying was studied. The crystallographic structure of the TiO 2 splats deposited on rutile and α-Al 2 O 3 substrates at 150, 300 and 500 °C was characterized by high resolution transmission electron microscopy and electron back scattering diffraction. The results reveal that homoepitaxial and hetero-epitaxial TiO 2 splats can be formed at the deposition temperature of 500 °C. Epitaxial growth is significantly influenced by the crystal orientation. It is easier to form an epitaxial TiO 2 splat with a <001> orientation in the direction perpendicular to the substrate surface. In order to explain the formation of epitaxial splat during plasma spraying, a competition mechanism between heterogeneous nucleation and epitaxial growth was proposed. It was indicated that the face (001) of rutile crystal exhibits the largest growth velocity, which is conducive to form an epitaxial splat for the melt with a largest undercooling degree. In addition, the effect of deposition temperature and crystalline orientation on the epitaxy was simulated. The simulation results are in agreement with the experimental observations.

The thermally grown oxide (TGO), formed on the bond coat (BC) surface of thermal barrier coatings (TBCs), has a significant impact on the TBCs life time. In order to reduce growth kinetics of TGO, vacuum pre-treatment was taken for some bond coats before pre-oxidation and isothermal oxidation in air. The other bond coats were directly subjected to pre-oxidation and isothermal oxidation. The grain size and grain boundary distribution of the TGO were analyzed by electron backscattered diffraction (EBSD). Element diffusion was taken into account to analyze growth kinetics of the TGO. Results indicate that the change of Al 2 O 3 TGO thickness with isothermal oxidation duration doesn’t fit well to conventional oxidation theory, i.e. exponential function. It is attributed to the change of the grain structure during the pre-treatment. This mechanism would benefit the development of long-life and highly reliable TBCs.

The Cold spray method of material deposition is widely used for surface enhancement, to improve properties such as corrosion and wear. A detailed microstructural analysis of cold sprayed Ni based coatings (IN625 (Inconel 625 is a Trademark of Huntington alloys corporation) using the transmission electron microscope, revealed the occurrence of three distinct types of microstructures in the as sprayed condition, adjacent to each other. These include the occurrence of large (> 1 μm) grains having a high dislocation density, along with regions comprising shear bands (20-30 nm wide) and twins with a large aspect ratio (> 1000), along with locations having a very fine grained structure (20-30 nm). The crystallite size, using a Hall-Williamson plot measures an average 50 nm. The substrate deformation indicates a bilayer dynamic recrystallization, as a means to accommodate the strain. The microscopy studies will serve to correlate the bonding mechanism of cold sprayed IN625 on 4130 steel.

This study investigates a new evaluation method that has the potential to differentiate between particle interfaces and grain boundaries in cold spray coatings. The method uses confidence index (CI) and image quality (IQ) values obtained from EBSD analysis to determine the location of grain boundaries as well as grain orientation and crystallinity of the deposit. In the case of a cold-sprayed copper deposit, the new method helps to explain the observed characteristics of the coating.