A key technology to minimize CO 2 -emissions is the production of hydrogen from water electrolysis. The proton exchange membrane water electrolysis (PEMWE) consists of a stacked system out of bipolar plates (BPP), porous transport layers (PTL) and a membrane electrode assembly (MEA). Research activities are ongoing to minimize material input, reduce costs and increase the performance. For example, the BPP on the anodic side of the stack is currently manufactured of bulk titanium and its substitution by a Ti-coated steel substrate is economically interesting. The main requirements for the BPP-coating are a high coating density, a low electrical resistance and a long lifetime in a harsh electrochemical environment. Coating application on substrates of s ≤ 0.5 mm thickness is conducted with three thermal spraying technologies: Cold Gas Spraying (CGS), High Velocity Air-Fuel (HVAF) spraying and High Velocity Oxy-Fuel (HVOF). Substrate preparation is examined as well. Coating development is conducted with regards to coating thickness, density and oxidation. The examination of coatings includes roughness analysis, structural and chemical analysis. The results allow an evaluation of the suitability of thermally sprayed Ti-coatings by the structural properties for the PEMWE application. Among the three tested processes, CGS is the most suitable for this type of application. The three chosen thermal spraying processes are examined for coating application on metal sheets in context of PEMWE for the first time.

The polymer cold spray (CS) process has been demonstrated as a promising coating and repair technique for fiber-reinforced polymer composites (FRPs). However, a noticeable variation in coating thickness (herein referred to as checkerboard pattern) often occurs in the initial (bond) layer of low-pressure CS deposition. The checkerboard pattern occurs due to essentially periodic variations in matrix thickness above the subsurface fiber weave pattern. When the bond layer exhibits the so-called checkerboard pattern, the CS deposition for subsequent layers may be negatively affected in terms of deposition efficiency, porosity, adhesion, surface roughness, and surface thickness consistency. The present work compares results of both numerical simulations and experimental studies performed to reveal the governing mechanisms for and elimination of checker-boarding. Numerical single particle impact simulations are conducted to observe various thermomechanical domains for CS impact on the FRP surface in different regions of the composite material. Complementary experimental CS studies of exemplar powders onto FRPs with various surface interlayer thicknesses are also presented. Experimental analyzes of deposits include microstructural observations to compare against the simulations while also providing practical strategies for the elimination of checkerboarding effects.

Design, manufacturing, and utilization of efficient heating systems for pipelines and closed-pressure equipment are necessary for cold regions to compensate for heat loss and prevent damages that are caused by freezing of the enclosed liquid. Given large-scale financial losses that stem from failure and bursting of the pipes, the development of novel, efficient, and affordable heaters, which can lead to improved efficiency, cost savings, and environmental benefits across various industries and applications, is of crucial importance. Heating systems have already been produced via different high-temperature thermal spraying techniques to achieve higher efficiency compared to conventional heating cables. In this study, tin, as the heating element, was deposited by using the cold spray process onto alumina coating that was fabricated by flame spraying (FS) to provide electrical insulation. Techno-economic assessment of fabrication and utilization of the coating-based heaters was conducted. It was found that cold-sprayed heater coatings exhibit improved performance compared to other thermally sprayed heater coatings and conventional heater cables. Further, their fabrication and utilization were more economically feasible. The results suggest that the new generations of coating-based heating systems may be competitive with conventional heat tracers that are widely used in industry.

In cold gas spraying, successful bonding occurs when particle impact velocities exceed the critical velocity. The critical velocity formula depends on material properties and temperature upon impact, relying mainly on tabulated data of bulk material. However, rapid solidification of powder particles during gas atomization can result in strengths up to twice that of bulk materials, causing an underestimation of the critical velocity. Thus, a re-adjustment of the semi-empirical calibration constants could supply a more accurate prediction of the requested spray conditions for bonding. Using copper and aluminum as examples, experimentally determined particle strengths for various particle sizes were 43% and 81% higher than those of the corresponding soft bulk materials. Cold gas spraying was performed over a wide range of parameter sets, achieving deposition efficiencies ranging from 2% to 98%. Deposition efficiencies were plotted as functions of particle impact velocities and temperatures, as calculated by a fluid dynamic approach. By using deposition efficiencies of 50%, the critical velocities of the different powders and the corresponding semi-empirical constants were determined. Based on particle strengths, the results reveal slight material-dependent differences in the mechanical pre-factor. This allows for a more precise description of individual influences by particle strengths on critical velocities and thus coating properties. Nevertheless, the general description of the critical velocity based on bulk data with generalized empirical constants still proves to be a good approximation for predicting required parameter sets or interpreting achieved coating properties.

This work focuses on the processing and deposit by suspension plasma spraying (SPS) of ZrO 2 -based ceramic materials for Thermal Barrier Coatings (TBC's) applications. The system of interest is ZrO 2 -16mol%Y 2 O 3 -16mol%Ta 2 O 5 (16YTZ). This ceramic has been reported to keep a non-transformable tetragonal phase (t'-phase), suitable to overcome the thermodynamic limits of the mostly used conventional 7-8wt.% yttria stabilized zirconia (YSZ). The research consists into evaluate the t'-phase stability and performance of the 16YTZ SPS coating. Synthesis of 16YTZ and, the evolution of the resulting microstructure in the dense ceramic and in the coating are a central part of the study. Sintering behavior in dense ceramics prepared from both precursor derived and milled powders is evaluated. Microstructural characterization by XRD, SEM and RAMAN spectroscopy of the as-deposited ceramic coating is presented and discussed.

Tantalum and silver are recognized for their outstanding biocompatibility and antibacterial ability, respectively. However, owing to their distinct chemical and physical properties, synthesizing alloys and composites by using Ta and Ag presents a considerable challenge. In this study, Ta-Ag composites, exhibiting good antibacterial ability, were successfully produced by using a solid-state cold spray technique. Notably, intriguing correlations were observed between Ag microstructure and antibacterial ability. To unravel this correlation, a comprehensive experimental and simulation analyze were conducted. It is found that the volume ratio of Ta to Ag in the feedstock powder result in different deformation histories for Ag during the cold spray process. This, in turn, leads to the formation of distinctive Ag microstructures within Ta-Ag composites. The varied Ag microstructures results in different Ag dissolution ability and the formation of an insoluble AgCl layer exhibiting varying morphologies, when Ag exposed in a high chorine ion environment, like in human body fluids. This consequently influences the concentration of Ag ion and ultimately determines antibacterial ability. The study demonstrates that Ag release rate and the related antibacterial properties could be alternatively controlled by changing Ag contains or by creating different deposition process by adjusting CS parameter.

Robot-guided cold spraying is currently developing as a technique with great potential for the repair of metallic components, particularly for depositing heat- and oxidation-sensitive materials. In this regard, the use of automation and robotics enables flexible control of the repair process. To ensure an optimal repair process, it is essential to consider the various requirements of robot-guided cold spraying already during the simulative planning phase. However, conventional robotic repair trajectories often do not fully consider the geometric constraints of material deposition, efficient material use, and the underlying limitations of robot kinematics. This work proposes the application of trajectory optimization by mathematical optimization for repair by robot-guided cold spraying. In this context, the optimal repair strategy must handle the constant material flow by the spray jet, which inevitably couples local material deposition with the robot motion. For this purpose, decision variables, objective function, constraints and a material deposition model are formulated to control the amount of deposited material accordingly. The goal is to generate an optimized trajectory that incorporates the requirements of cold spraying and robot kinematics to guarantee high-quality repair and efficient material use. This includes minimizing excess material and minimizing the jerk of the robot motion. The results demonstrate successful application of the trajectory optimization for component repair by cold spraying.

In this study, the in-situ technique was used to observe crack formation and growth in multilayer suspension plasma spray (SPS) thermal barrier coatings (TBCs). Utilizing synchronized three-point bending (3PB) and scanning electron microscopy (SEM), coupled with digital image correlation (DIC), we provide real-time insights into strain field dynamics around cracking zones. Bending-driven failure was induced in both single and composite-layer SPS coatings to investigate the crack behavior in these columnar-structured multilayer TBCs. The real-time observations showed that columnar gaps can facilitate crack initiation and propagation from the coatings' free surface. The composite-layer SPS coating exhibits lower susceptibility to vertical cracking than the single-layer SPS coating, possibly due to the presence of a gadolinium zirconate (GZ) dense layer at the coating's free surface that enhances the bonding strength within the coating's columnar structure. The splat structure of the bond coat (BC) layer contributes to the crack path deflection, thereby potentially improving the SPS coating' fracture toughness by dissipating the energy required for crack propagation. Moreover, it was revealed that grit particles at the BC/substrate interface seem to promote crack branching near the interface, localized coating delamination, and serve as nucleation sites for crack development. Hence, optimizing the grit-blasting process of the substrate before BC layer deposition is crucial for minimizing the possibility of crack formation under operational conditions, contributing to enhanced durability and prolonged lifespan. This study underscores the critical role of in-situ observation in unravelling the complex failure mechanisms of multi-layered coatings, paving the way for the design of advanced coatings with enhanced structural complexity and improved performance for more extreme environments.

Erosion-corrosion is a severe problem observed in the coal fired thermal power plant boilers which lead to premature failure of boiler tubes. Thermal spray coatings have been applied successfully to check the erosion-corrosion of boiler tubes. In the present research work NiCrTiCRe coating powders were successfully deposited on T22 boiler steel by two different coating processes i.e. high velocity oxy-fuel (HVOF) and cold spray process. The performance of the coatings in actual power plant boiler were investigated and compared. The uncoated and coated T22 boiler steels were subjected the superheater zone of the coal fired boiler for a total of 15 consequent cycles. The thickness loss data and weight change analysis were used to establish kinetics of the erosion-corrosion. X-ray diffraction, surface field emission scanning electron microscope/energy dispersive spectroscopy (FE-SEM/EDS) techniques were used in the present work for the analysis. The results of thickness loss data indicated that the cold sprayed coating performed better in thermal power plant boiler environment.

This contribution gives an overview concerning basic principles of cold spraying (CS) and current trends in respective applications. As powder spray technique dealing with solid impacts, cold spraying results in coatings of high purity and unique properties, not attainable by other spray methods. Particularly within the last two decades, cold spraying developed from laboratory scale to a reliable industrial process. The presentation summarizes current models and key parameters in order to achieve and to improve bonding and coating qualities, and gives examples for applications in electronics, mechanical part repair and additive manufacturing.

The US Navy has adopted High-Pressure Cold Spray (HPCS) as a repair technique for corroded and worn components in their fleet of aircrafts, ships, and submarines. HPCS repairs are not only used for depositing corrosion and wear resistant coatings but is being successfully used for dimensional restoration in metal parts and components. By utilizing HPCS, the Navy ensures the longevity and reliability of critical components, even in harsh environments. Whether safeguarding against corrosion or restoring worn parts, HPCS is playing an increasingly critical role in maintaining operational readiness for the US Navy and other DOD agencies, as well as the commercial industry.

For the application of thermally sprayed titanium coatings, the high oxygen affinity and tendency to nitride formation in the presence of nitrogen represents a major challenge. Consequently, thermally sprayed titanium coatings are usually applied by cold gas spraying, vacuum plasma spraying and shrouded spraying processes. Nevertheless, the formation of oxides cannot be completely avoided with these methods. The pre-sent study demonstrates an alternative coating strategy for the application of oxide and nitride free thermally sprayed titanium coatings. Thereby, the previous limitations are overcome by transferring the coating process into a silane-doped argon gas environment to achieve an extremely low oxygen and nitrogen partial pressure. Thus, the created titanium coatings are oxide and nitride free and have an extremely low porosity. Moreover, by transferring of the corundum blasting process to this environment, the native oxide layer on the substrate surface can be removed and its reformation is suppressed. This results in full material bonding conditions with extremely high adhesive tensile strengths.

Suspension plasma spraying (SPS) is increasingly studied to produce finely structured coatings with dense and columnar microstructures for promising thermal barrier coatings especially in aerospace application. However, this process involves many parameters and complex phenomena with large spans of time and space scales in many physical mechanisms, like droplet break-up, liquid droplet evaporation, and various physical phenomena occurring within the suspension droplet, making it difficult to master. Especially, understanding the interactions of liquid drop submitted to plasma with the submicronic suspended particles is essential for material process optimization and control. For SPS understanding, a meaningful modelling of suspension treatment requires a prior analysis of these physical mechanisms and their characteristic times. This study details the different phenomena, their significance and characteristic timescales as well as the selection of the main governing forces acting between the different continuous and discrete phases (plasma, liquid, submicronic particles). We explore associated mechanisms: droplet breakup, carrier liquid evaporation, convective mixing and submicronic particle diffusion within the droplets. These mechanisms involve mass and heat transfer, that should condition particle agglomeration morphology before melting.

Due to their promising photocatalytic properties under visible light irradiation, thermally sprayed ZnO-TiO 2 coatings are of interest as substitute for TiO 2 for various industrial applications, like hydrogen production via water splitting or the reduction of organic pollutants in water. Suspension spraying is an effective method to produce coatings in the binary ZnO-TiO 2 system to form Zn 2 TiO 4 in-situ during the spraying process. Aqueous suspensions containing fine dispersed ZnO and TiO 2 particles are mixed at tailored composition and sprayed using the SHVOF and SPS spraying processes. Coatings with homogeneous distribution of elements and different surface structures and phase compositions are obtained. The phase composition is analyzed via XRD. UV-Vis spectroscopy measurements and photocatalytic tests of Rhodamin B degradation are performed. The potential to use appropriate binary suspension feedstock to produce ZnO-TiO 2 -Zn 2 TiO 4 coatings with different microstructures and photocatalytic properties is presented.

Bond coats are used to protect the superalloy from oxidation and to serve as a bond between the ceramic thermal barrier coating (TBC) layer and the superalloy. During high temperature exposures, a thermally grown oxide (TGO) layer forms between the bond coat and the topcoat due to oxygen diffusion, leading to coating failure in the components. This study aimed to investigate the microstructure evolution of three TBCs with different cold-sprayed bond coat alloys after undergoing isothermal heat treatments. The TBCs were heat treated at 1100 °C for durations of 12, 25, and 50 hours to observe the effects of temperature on the microstructure and phase distribution. The microstructure of heat-treated bond coat alloys was examined using scanning electron microscopy and x-ray diffraction. The findings are discussed in relation to the characteristics of the coating alloy and the application process.

Surface coatings play a pivotal role in enhancing mechanical and functional properties of various materials. High Entropy Alloy (HEA) annealed coatings have garnered significant interest due to their potential to improve wear resistance and overall durability. This research presents a comprehensive study focused on the characterization of HEA annealed coatings. It focuses on evaluating their roughness and wear performance. In this research, a systematic approach is adopted to assess the effects of annealing on coating surface properties. The investigation begins with the deposition of the Al 0.1-0.5 CoCrCuFeNi and MnCoCrCuFeNi coatings using a well-established cold spray (CS) technique, followed by a controlled annealing process. The coating surface roughness is analyzed using profilometry and microscopy techniques. This offers insights into the changes induced by annealing. The wear performance of the annealed coatings is evaluated through tribological tests.

Cold spray (CS) is a progressive method for the deposition of metals and alloys whose principles involve considerable plastic deformation of the produced material at extreme strain rates. Positron annihilation spectroscopy (PAS) is an analytical technique capable of studying deformation on the atomic scale level, even in extremely deformed materials. In our study, the PAS method was used to characterize the deformation character at the lattice level and quantify the open-volume defects in four cold sprayed metals: Al, Cu, Ni, and Ti. As counterparts, bulk samples of these materials with ultrafine-grained structures were also produced by high-pressure torsion (HPT), a process exceeding cold spray in the total deformation, but having several orders of magnitude smaller strain rates, and by a traditional cold rolling process. The results show that the CS and HPT processes lead to the formation of similar lattice defects (dislocations and vacancy clusters), and both exhibit significantly higher dislocation densities than conventionally cold-rolled materials. Further, the vacancy clusters present in CS and HPT materials were not present in the rolled counterparts due to the lower vacancy production rate.

By approaching the glass transition temperature it was possible to realize well quality metallic coatings on two different glasses using Cold Spray deposition, CS. A roughness is introduced on the glass surface and is proportional to the energy deposited. Using the thermal pressure memory effect of glass, Raman spectroscopy mapping allowed determining that the CS introduced a strong heterogeneity of the glass substrate characterized by zones with different cooling rate and residual stress. Using a simplified Eshelby's inclusion approach, it is demonstrated that the residual stress can be in first approximation explained by the introduction of local density fluctuation induced with high cooling rate of micrometric regions related with the impact of the deposited metallic particles.

Cold spray additive manufacturing (CSAM) is an emerging process that has garnered significant attention from researchers due to its unique advantages. These include higher deposition rates, no need for a protective atmosphere, and the ability to connect or combine dissimilar materials. While CSAM allows for near-net-shape fabrication of workpieces, the accuracy and properties of the final products often fall short of user requirements. Furthermore, there is an urgent need to develop a generalized manufacturing strategy for workpieces with complex geometries. It appears that integrating various processes throughout the entire manufacturing workflow, from design to delivery, could address these challenges. However, few researchers have explored this area. To fill this gap, this study presents an integrated modular CSAM system designed for efficient and flexible workpiece fabrication. The system comprises two main components: software for modeling and simulation, and hardware for precise fabrication, each containing multiple modules. These modules do not operate independently but are coupled through direct or indirect decentralized and event-driven physical links. The system described in this paper offers a generalized strategy for precision manufacturing of workpieces using CSAM, potentially advancing the field and addressing current limitations in accuracy and versatility.

Thermal spray (TS) technology has attracted the attention of numerous industrial sectors due to its apparent simplicity and versatility. It has been used across the world for over 80 years in the conservation and creation of art. Despite the creativity involved in the creation of an art piece, the TS artistic endeavors are limited and insufficiently explored. Unique material combinations, usually not observed in conventional engineering applications, can be achieved with TS technology. Although the material amalgamation possibilities are infinite, their combined deposited characteristics, interfacial compatibility and color palette require further study. In this work, the fields of photography, image processing and TS are combined to produce a large art-piece using the cold gas dynamic spray (CGDS) process. Aluminum, zinc, nickel, alumina, steel and titanium alloy powders are sprayed to replicate in three-dimensions a photograph of a crinoid from the Silurian period found on the Anticosti Island, located in the Gulf of St. Lawrence in Canada. The numerous steps required to produce the artistic 3D piece, namely numerical segmentation of the photograph, conversion to a computer-assisted design (CAD), manufacturing of steel masks and CGDS deposition of the selected powders to reach the sought color palette are described. Powder deposition efficiency, material compatibility and microstructural characteristics are analyzed. and the resulting art piece is presented.