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The residual stress state in cold spray coatings, which is typically compressive in nature, can be crucial for the coating integrity at high levels or for thick coatings. As an alternative, the analysis using the curvature measurement was applied in this study. The stress measurement during cold spray deposition was made by using an in situ coating property sensor (ICP sensor), which is relatively widely used in thermal spray and enables a fast comparison of several process parameters. The results were then compared to post-deposition curvature measurements. It could be revealed that the usage of a very slow robot traverse speed can lead to tensile residual stresses in cold-sprayed coatings. This finding was explained by the high local temperature during deposition and the formation of tensile stresses during cooling of the deposited material to the average substrate temperature. The increase in the powder feed rate can increase this effect. Preheating did not influence the final stress state. An analytical model is presented which can at least semi-quantitatively explain the observed findings. As an outcome of the research work, it is now possible to adjust the residual stress state in cold spray coatings from tensile to compressive and vice versa with the opportunity of a zero stress state.

A set of advanced single crystalline γ′ strengthened Co-base superalloys with at least nine alloying elements (Co, Ni, Al, W, Ti, Ta, Cr, Si, Hf, Re) has been developed and investigated. The objective was to generate multinary Co-base superalloys with significantly improved properties compared to the original Co-Al-W-based alloys. All alloys show the typical γ/γ′ two-phase microstructure. A γ′ solvus temperature up to 1174 °C and γ′ volume fractions between 40 and 60 pct at 1050 °C could be achieved, which is significantly higher compared to most other Co-Al-W-based superalloys. However, higher contents of Ti, Ta, and the addition of Re decrease the long-term stability. Atom probe tomography revealed that Re does not partition to the γ phase as strongly as in Ni-base superalloys. Compression creep properties were investigated at 1050 °C and 125 MPa in 〈001〉 direction. The creep resistance is close to that of first generation Ni-base superalloys. The creep mechanisms of the Re-containing alloy was further investigated and it was found that the deformation is located preferentially in the γ channels although some precipitates are sheared during early stages of creep. The addition of Re did not improve the mechanical properties and is therefore not considered as a crucial element in the design of future Co-base superalloys for high temperature applications. Thermodynamic calculations describe well how the alloying elements influence the transformation temperatures although there is still an offset in the actual values. Furthermore, a full set of elastic constants of one of the multinary alloys is presented, showing increased elastic stiffness leading to a higher Young’s modulus for the investigated alloy, compared to conventional Ni-base superalloys. The oxidation resistance is significantly improved compared to the ternary Co-Al-W compound. A complete thermal barrier coating system was applied successfully.

The plasma spray-physical vapor deposition (PS-PVD) process is a promising method to manufacture thermal barrier coatings (TBCs). It fills the gap between traditional thermal spray processes and electron beam physical vapor deposition (EB-PVD). The durability of PS-PVD manufactured columnar TBCs is strongly influenced by the compatibility of the metallic bondcoat (BC) and the ceramic TBC. Earlier investigations have shown that a smooth BC surface is beneficial for the durability during thermal cycling. Further improvements of the bonding between BC and TBC could be achieved by optimizing the formation of the thermally grown oxide (TGO) layer. In the present study, the parameters of pre-heating and deposition of the first coating layer were investigated in order to adjust the growth of the TGO. Finally, the durability of the PS-PVD coatings was improved while the main advantage of PS-PVD, i.e., much higher deposition rate in comparison to EB-PVD, could be maintained. For such coatings, improved thermal cycling lifetimes more than two times higher than conventionally sprayed TBCs, were measured in burner rigs at ~1250 °C/1050 °C surface/substrate exposure temperatures.

This study is focused on the variation of the microstructures of different CoNiCrAlY bond coats sprayed by the high-velocity oxy-fuel (HVOF) process for thermal barrier coating (TBC) applications. Three different size fractions of the CoNiCrAlY bond coat powder have been considered for this investigation: AMDRY 9951 (5-37 μm), AMDRY 9954 (11-62 μm), and AMDRY 995C (45-75 μm). The influence of HVOF process parameters and process conditions have been studied in detail to achieve quality bond coats in terms of low porosity level, low oxygen content, and high surface roughness. The results have been promising and have shown that dense bond coats with low porosity can be achieved by HVOF spraying through the appropriate selection of powder size and process parameters. Importantly, HVOF bond coats appear to be competitive to VPS bond coats in terms of its oxygen content and high surface roughness.

In the cold gas spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at temperatures well below their melting point. The high particle impact velocities and corresponding peening effects can lead to high compressive residual stresses in cold spray coatings. This can be advantageous with regard to mechanical properties as fatigue life and hence, cold spray is an ideal process for repair applications. In this study, INCONEL 718 particles were cold sprayed by using nitrogen as propellant gas. The deposited coatings with different thicknesses were characterized using electron microscopy techniques to study grain refinement and precipitates in the coating. In addition, depth-resolved residual stress measurements have been performed by the incremental hole drilling method. The residual stress depth profiles in the coatings indicate compressive residual stresses of several hundred MPa which are hardly influenced by the coating thickness. In addition, large compressive stress levels are found in surface-near regions of the substrate due to the grit blasting process. Furthermore, a post-heat treatment analysis was performed to investigate its influence on residual stresses and bonding strength. These findings are used to develop a consistent explanation of the dependence of strength values on thickness.

Plasma spraying at very low pressure (50-200 Pa) is significantly different from atmospheric plasma conditions (APS). By applying powder feedstock, it is possible to fragment the particles into very small clusters or even to evaporate the material. As a consequence, the deposition mechanisms and the resulting coating microstructures could be quite different compared to conventional APS liquid splat deposition. Thin and dense ceramic coatings as well as columnar-structured strain-tolerant coatings with low thermal conductivity can be achieved offering new possibilities for application in energy systems. To exploit the potential of such a gas phase deposition from plasma spray-based processes, the deposition mechanisms and their dependency on process conditions must be better understood. Thus, plasma conditions were investigated by optical emission spectroscopy. Coating experiments were performed, partially at extreme conditions. Based on the observed microstructures, a phenomenological model is developed to identify basic growth mechanisms.

The residual stresses induced by the various process conditions in engineering components can have a significant impact on their structural integrity and performance. It is essential to ensure reliable control of the mechanical properties of structural components during the repair process, as this directly affects their performance and longevity. Cold gas spray, a solid-state deposition technique, involves the high-velocity impact of fine powder particles onto a substrate, resulting in the formation of a dense, metallurgically bonded coating. The aim of this study is to investigate the suitability of cold gas spraying parameters for the repair of large cavities in components made of Inconel 718. Two sets of parameters, approaching the limits of the spraying facility, have been utilized and analyzed using particle diagnostics. Experimental methodologies involve the characterization of residual stress profiles using techniques such as in situ curvature measurement and the incremental hole drilling method after the cold gas spray repair. Additionally, the microstructure and topography of the as-sprayed repair coatings are demonstrated. The results demonstrate the ability of cold gas spray to successfully fill deep repair cavities and adjust the residual stress state of such repair coatings by varying the processing parameters. Lower residual compressive stresses in the layer were achieved by utilizing gas parameters, wherein the particles impact the substrate at an elevated temperature and at a comparatively reduced velocity. Both conditions exhibited coatings with consistent microstructure, good adhesion and uniform topography without major defects. This research demonstrates the potential of cold gas spray as a viable and efficient repair method for large repair geometries, offering a promising avenue for enhancing the reliability and lifespan of critical engineering structures.

Turbine blades in aircraft engines and land-based gas turbines are exposed to harsh environmental conditions that make them susceptible to degradation mechanisms, such as creep, oxidation, and fatigue damage. Therefore, research into effective repair methods is of high importance, especially for single crystal components, as they are cost-intensive to produce. The HVAF process is suitable for repair applications as it can produce dense layers with low oxygen content, which are essential for the repaired area. Additionally, the fine grain structure is advantageous for the subsequent directional recrystallization planned in future studies, aiming to transform the repair layer into a columnar or even single-crystal structure. This study focuses on applying CMSX-4 powder to single crystal substrates of similar composition using High Velocity Air Fuel Spraying (HVAF). Specifically, the effects of powder particle size, nozzle configuration, and various other process parameters, such as spray distance and carrier gas flow, on the characteristics of HVAF-deposited CMSX-4 were investigated. The microstructure was examined by scanning electron microscopy. Particle velocity and temperature measurements were performed to enhance comprehension of the process.

Typical plasma characteristics of the plasma spray-physical vapour deposition (PS-PVD) process were investigated by optical emission spectroscopy. Electron temperatures were determined by Boltzmann plots while temperatures of the heavy species as well as electron densities were obtained by broadening analysis of spectral lines. The results show how the plasma properties and thermodynamic equilibrium conditions are affected by the admixture of hydrogen and the ambient chamber pressure. Some experimental examples of PS-PVD coatings demonstrate the impact on feedstock treatment and deposited microstructures.

The quality of low-pressure plasma-sprayed tungsten (W) coatings for application in fusion reactors was investigated under various spray process parameter settings for the following substrate materials: carbon fiber composite, Eurofer (a ferritic/martensitic steel with reduced activation), and tungsten. The deposited coatings with a thickness of approximately 130 µm were evaluated in terms of porosity, deposition efficiency, defects, and surface roughness. Selected parameter sets were applied to produce scaled-up coatings up to 500 µm in thickness on Eurofer and tungsten substrates. Regions of very low porosity of approx. 0.3% and varying grain appearance were found. However, the subsequent grain size evaluation in terms of the aspect ratio of the fitted ellipse and the maximum Feret diameter did not reveal any significant differences in grain structure through the coating regions. The residual stress measurements performed by XRD using the sin 2 (Ψ) method validated the thermal stresses within the coatings resulting from the thermal mismatch between the coating and the substrate during cooling. The results indicate that the process settings and spraying process were effective in reducing residual stresses and producing coatings suitable for future fusion-relevant applications.