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.

In Laser Cladding, a differentiation must be made between cladding by brazing and cladding by welding regarding process parameters and the resulting material properties. Results of investigations of bronze cladding on steel parts produced by Laser Deposition Brazing will be presented. This means that a strong metallurgical bond is realized by diffusion processes by Laser Deposition Brazing, but the steel base material is not molten. The coatings were characterized by hardness distribution measurements from the bronze cladding to the steel substrate, by measuring the size of the heat-affected zone and by porosity measurements. This combination of a steel substrate and a local bronze coating is used industrially in many tribological applications, such as plain bearings or hydraulic pumps etc. The bronze offers excellent tribological properties. In some cases, the bronze is used as a complete solid part. However, applying the bronze locally to a steel base body instead of using a complete solid bronze component, offers the advantage of the higher modulus of elasticity of the steel, which provides greater stability of shape with regard to possible elastic deformations as these coated parts are exposed to high mechanical loads, it is essential that a high coating quality is achieved by laser cladding and that the properties are extensively and purposefully characterized. The production technology, the characterization and the industrial applications of such bronze coated steel parts are presented and explained in this contribution.

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.

Gas-fuel HVOF for thermal spraying of WC-CoCr powder is widely known and well described in literature. Focus are the various influencing factors like fuel-to-oxygen ratio, standoff-distance and powder feed rate on the coating characteristics like hardness and porosity. However, the total gas flow is usually not being described in this context despite its wide influence on particle characteristics and therefore on coating properties. In this study, the characteristic influence of the total gas flow on roughness, hardness and porosity is described as well as its effect on the particle characteristics. The study performed was based on technical standard values for thermally spraying WC-Co-Cr via gas-fuel HVOF (DJ2700 hybrid) and additional trials for increased and decreased total gas flow. It was possible to determine that with higher gas flow the deposition rate increases while the roughness and porosity decrease. However, these results cannot be viewed in isolation as other factors, such as the fuel-to-oxygen ratio, are affecting the particle and coating characteristics at the same time. Therefore, the total gas flow is also considered in combination with other factors.

Titanium porous transport layers (PTL) are important components in proton exchange membrane water electrolysis (PEMWE) cells. The performance enhancement and the reduction of manufacturing cost of PTLs are of importance for market expansion of PEMWE. Vacuum plasma spraying (VPS) was used to prepare PTL or modify PTL of sintered titanium powders and the PTLs by VPS showed a high performance. Regarding the cost efficiency, it is of great interest to produce PTLs using more economical spray processes than VPS. In this study, high velocity oxy-fuel spraying (HVOF) was used to produce highly porous titanium coatings for this purpose. The spray process was developed to achieve a high porosity of up to Φ = 30 % using three titanium powders with size distributions of fA = -90 +45 μm, fB = -63 +20 μm and fc = -45 +11 μm. The coating structures were examined on the cross sections of the titanium coatings with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The porosity was determined using the image analysis system ImageJ. The deposition efficiency of the titanium powder fC = -45 +11 μm was determined. The results show that the coating structure significantly depends on the titanium powders. Highly porous titanium coatings of Φ = 24 - 40 % can be produced with the titanium powders of fB = -63 +20 μm and fc = -45 +11 μm. Titanium oxides are hardly visible on the cross-sections of the titanium coatings. A high deposition efficiency of approximately DP ≈ 70 % was measured for the titanium powder of fc = -45 +11 μm.

Cold spray is an emergent sold-state manufacturing process based on high-speed consecutive collision of small sized particles powders. Such a cold process principle led to the recent development of coatings for various surface functionalization and direct component manufacturing applications. This study investigates the mechanisms of porosity formation during the additive growth of Al1050 powders. To this end, a thermo-mechanical computational model based on the Eulerian computational approach using the Johnson-Cook constitutive law is applied on the case of an additive growth from a stacking of powder layers. The model uses in-flight velocities measured by a DPV2000 system during a cold spray test and an isentropic gas flow model. The measurements show the velocity distribution within the powder jet at the nozzle exit and consists of a Gaussian-like distribution within the typical range of 450-650m/s. The centerline zone at the nozzle exhibits the highest velocity. These data used as input data of the model allowed to apprehend some circumstances of pore formation, in terms of site occurrence, pore behavior over time, and deficient in-flight velocity.

The influence of air plasma sprayed alumina coating geometry, microstructure, interface roughness on its delamination and crack propagation resistance during low temperature thermal cycling, i.e. thermal mismatch stress, is investigated both numerically and experimentally. Previous studies on thermal cycling loading concentrate on flat, numerically designed locally curved specimens and/or mathematically modeled roughness without extension towards real coating morphology, which renders the conclusions less practically driven. Results show that arbitrarily oriented cracks originate predominantly near the coating/substrate interface and propagate along zones of high tensile and shear residual stress. The crack path deflection was attributed to the complex stress concentration structure resultant from the intricate microstructural porosity and coating general convex geometry. Microstructural features such as porosity increase the interfacial and coating tensile stress, which may lead to important delamination processes even during low temperature thermal cycling.

A variety of process parameters affect the properties of the deposited coatings in the High Velocity Oxygen Fuel (HVOF) spraying process. In fact, the quality of coatings can be improved without changing feedstock or deposition technology by the application of optimized spraying process parameters. In this study, a large set of data “Big Data” is used to create a variety of machine learning models for prediction of porosity content and hardness values of HVOF deposited coatings. A set of process parameters was selected as validation run and actual HVOF coating was deposited using those parameters. The porosity level and hardness were measured and compared to those predicted by models. The models differ based on the number of neurons utilized in each layer for the calculations. A model with six neurons could predict closest porosity level and the one with three was the best in prediction of hardness. The final model could be obtained by running data through both models. Through this study, a robust machine learning model for the optimization of HVOF process parameters will be developed that could be used for other coatings and thermal spraying techniques.

Alkaline water electrolysis is currently the most promising approach to produce hydrogen. However, a main limitation for large-scale application originates from the significant energy loss caused by the coverage of bubbles on the electrode surface. Here, pore-graded Ni electrodes with a positive and negative gradient porous structure that boosts the desorption and release of gas bubble are reported, resulting in a greatly advanced mass transference. The electrodes are obtained from a blend of Ni and Al via high-pressure cold spray. The gradient porosity is realized by varying the addition of Al and chemical etching. As-sprayed electrodes are annealed to eliminate the residual stress and strengthen the adhesion of layers, hence improving their durability. As a result, the electrode with a positive pore-graded structure exhibits a better HER/OER performance when tested with a carbon rob counter electrode. Notably, when tested with an annulus counter electrode of Nickel foam, the electrode with a negative pore-graded structure achieves minimal HER/OER overpotential, outperforming other porous electrodes. This is benefited from improved bubble removal and mass transference capability. All prepared electrodes showed an excellent stability that after 500 cycles of HER/OER test without a large potential fluctuation.

The objective of this work is to assemble an aluminum alloy to a steel to reduce the final mass of this assembly. Doing that, cold spray is considered as an efficient solution. Surfaces are previously prepared with a texturing laser to improve the adhesion of the coating on the substrate. Deposits are slightly rough (Ra < 10 μm), porosity is less than 1% and adhesion is higher than 80 MPa for textured surfaces. These high values are also due to the high filling rates in holes (100% for steel and 65% for aluminum alloy). Shear values obtained through the combination of laser texturing and cold spray for multi-material assembly are of 90 MPa (a heat treatment of 3h at 300°C applied on the joining point improves mechanical strength and increases it by three). By analogy with linear joining methods such as Laser Welding (190 MPa), the values obtained in uniaxial tension by this assembly method are significantly lower (around 50 MPa). It can be explained by the nature of the joining bead, which is made of aluminum alloy.

Cold Spraying is an emerging additive manufacturing method that uses a high-speed collision of micrometre sized powders capable of producing a solid-state bonding. Such a principle has led to the recent development of a coating for various surface functionalization and additive manufacturing applications. This paper is the result of an experimental study on the evolution of the deposit properties (ultimate strength, and porosity) generated by the additive growth during cold spraying. The deposit characterization shows the existence of ultimate strength gradient. For samples taken from the bottom to the top of the deposit, the ultimate strength decreases but there is no significant change in porosity value. The porosity evolutions do not allow to establish a generalized law of variation. The numerical analysis of the additive growth shows that the thermomechanical response of the stacking powder during the additive growth can decrease the bonding capacity, the thermomechanical heating (due to the plastic work) and the gradient of thermal kinetics.

The performance of two distinct coating materials under alumina particle impingement was tested in this study. CrMnFeCoNi and WC-Ni coatings were applied to 2205 duplex stainless steel substrates using cold spray method with nitrogen as the process gas. In between the substrate and the high entropy alloy coating, an interlayer coating of 316 stainless steel was used. The presence of WC particles in the WC-Ni composite coatings was confirmed by SEM cross sectional inspection. Following deposition, the coatings were heat treated in an air furnace. The influence of heat treatment holding time on the WC-Ni coatings was studied using chemical analysis by X-ray diffraction. Heat treatments peak temperatures for the WC/Ni- Ni and high entropy alloy coatings were 600°C and 550°C, respectively. Coatings microhardness and porosity volume fraction were measured for all the samples. The HEA coating outperformed the WC/Ni-Ni hardness but exhibited a higher level of porosity. The coatings were then subjected to erosion experiments using alumina particles with variable impact angles (30°, 60°, and 90°). To compare the different materials, an average erosion value was calculated for each target specimen. The WC/Ni-Ni as-sprayed coating was the most effective against a 60° impingement angle. The HEA coating, on the other hand, demonstrated greater resistance to impact angles of 30° and 90°. SEM was utilized to examine the eroded areas and determine the main mechanisms of erosion.

Cold spraying (CS) of high strength materials, e.g., Inconel 625 is still challenging due to the limited material deformability and thus high critical velocities. Further fine tuning and optimization of cold spray process parameters is required, to reach higher particle impact velocities as well as temperatures, while avoiding nozzle clogging. Only then, sufficiently high amounts of well-bonded particle-substrate and particle-particle interfaces can be achieved, assuring high cohesive strength and minimum amounts of porosities. In this study, Inconel 625 powder was cold sprayed on carbon steel substrates using N 2 as propellant gas under different refined spray parameter sets and powder sizes for a systematic evaluation. Coating microstructure, porosity, electrical conductivity, hardness, cohesive strength and residual stress were characterized in as-sprayed condition. Increasing the process gas temperature or pressure leads to low coating porosity of less than 1 % and higher electrical conductivity. The as-sprayed coatings show microstructures with highly deformed particles and well bonded internal boundaries. X-ray diffraction reveals that powder and deposits are present as γ- solid-solution phase without any precipitations. By work hardening and peening effects, the deposits show high microhardness and compressive residual stresses. With close to bulk material properties, the optimized deposits should fulfill criteria for industrial applications.

Ni/Co-based alloys have been widely employed as bond coats (BCs) in thermal barrier coatings (TBCs) to provide oxidation resistance through the formation of a dense thermally grown oxide (TGO) layer. TGO thickening is a major contributor to TBC failure. Conventional approaches to minimize its growth have included refinement/optimization of the BC composition, deposition techniques, and post-treatments. However, these approaches have only led to incremental improvements in TBC performance and do not directly address the effect of the thin interfacial oxide layer on the TBC lifetime. In a shift from conventional thinking, the development of an Al 4 C 3 -Ni alloy composite BC aims to overcome the challenges generated by current TGOs. Post-deposition heat treatment tailors the coating microstructure to form a continuous internal carbide network. At elevated temperatures, the Al 4 C 3 preferentially oxidizes to form an interlacing protective Al 2 O 3 “root” that provides better TGO anchoring and reduces TBC thermal mismatch with the substrate. In this paper, the coatings were manufactured through gas-shrouded plasma spraying using various parameters to optimize the degree of inflight carbide dissolution and minimize the extent of coating porosity and cracking. XRD and carbon analysis were performed on the coatings and the microstructure was observed using SEM. Differences between coatings are discussed in relation to the spraying parameters.

Thermally sprayed WC/CoCr coatings are the most established coatings in the valve industry. However, due to the high wear resistance and as-sprayed surface roughness, the surface post processing costs are very high. Near-net-shaped fine powder coatings have the possibility to reduce the costs effectively. Due to the high specific surface to volume ratio of the powders, undesired phase transformations can occur during the spraying process. To avoid such phase transformations, the novel thermal spraying process Ultra-HVOF (UHVOF) is used in this study. An extensive parameter study is carried out on the influences of the process parameters on microhardness, porosity, as-sprayed surface roughness, phase composition and wear resistance. With suitable process parameters, near-netshaped and almost pore-free coatings can be applied. Compared to a conventional HVOF sprayed WC/CoCr coating, a wear reduction by a factor of three can be achieved in a pin-on-disktest against Al 2 O 3 at a load of F = 15 N. Due to the pore-free and highly wear-resistant coatings, significantly thinner coatings can be used for the protection against corrosion and wear in valves. In addition, the required surface quality of the near-net-shape coatings can be achieved by polishing only. Thus, the novel UHVOF coatings represent a cost-effective alternative to conventionally used valve coatings.

Atmospheric plasma spraying (APS) is characterized by complex interactions between input, process and output variables. Process control relies heavily on human expert knowledge and experience. Process diagnostics can provide additional information to the operator and support cognitive processes in task execution. When using non-cascaded torch systems, significant plasma fluctuations occur, affecting the coating quality. High-frequency fluctuations can only be detected by suitable diagnostic systems and interpreted by experienced APS operators. In this study, the state of the plasma jet (area, fluctuation) is investigated depending on total plasma gas flow rate (50 vs. 65 l/min) and the H 2 content of plasma gases (17, 20 and 23 vol. %) using high-speed camera pictures. To evaluate plasma fluctuation effects, particle temperature and velocity as well as resulting coating properties (thickness and porosity) are determined for two ceramic systems. The results show that fluctuations of the plasma jet have a significant effect on the particle state and coating quality. The use of a high-speed camera to evaluate the stability of the plasma jet is an attractive method that, when properly integrated, has the potential to provide the human operator with important information to allow rapid assessment of input parameters or the condition of the plasma torch.

Hardmetal coating compositions containing both WC and Cr 3 C 2 are less intensively studied than WC-Co(Cr) and Cr 3 C 2 -NiCr. In particular, compositions with Cr 3 C 2 as the main phase are relatively new in the market. In this contribution, two commercial agglomerated and sintered feedstock powders with similar compositions (42Cr 3 C 2 -42WC-16Ni and 45Cr 3 C 2 - 37WC-18NiCoCr) were studied. Both powders differ in their porosity and the melting behavior, as was found by DSC experiments. Coatings were deposited with a liquid-fueled HVOF process (JP 5220). Optimization of the spray conditions was evaluated with five different spray parameter sets. Coating microstructures and phase compositions, as well as microhardness HV 0.3 and abrasion wear resistance were less influenced by the spray parameter sets. At the same time, significant differences in deposition efficiency between the two compositions were observed, which might be related to the differences in the melting behavior of the compositions and the powder porosity. However, coating microhardness and abrasion wear resistance were similar for each of the spray parameter sets. Coating microstructure and phase composition were studied with a focus on the interaction between Cr 3 C 2 and WC and will be discussed in detail.

The feasibility of processing various polymers by cold spray has been exemplified by depositions with low porosity and properties comparable to the bulk material. However, cold sprayed polymers are generally deposited with low deposition efficiency compared to more extensively studied metal sprays. Low efficiencies in polymer sprays are attributed to characteristic differences in material properties between metals and polymers. Notably, the thermophysical properties of polymers limit heat transfer and promote intra-particle thermal gradients that develop during cold spray processing. These properties (e.g., thermal conductivity, heat capacity, density) and low deposition efficiencies demand alterations to the cold spray process equipment outside typical metal powder spray conditions. Herein, a modified powder feed tube is used to pre-heat powder to temperatures (~84 °C) below the powder melting point, or cool it (~-55 °C) below room temperature before contacting the high velocity carrier gas in the nozzle of a CSM 108 cold spray system. Numerical simulation demonstrated that pre-heating/cooling the powder feedstock is a viable means of adjusting particle temperature upon impact with the substrate; however, this technique has generally not been deliberately utilized in the cold spray of polymers. In the present work, no significant increase in deposition efficiency (~65% for all sprays) was found by increasing the pre-heat temperature. However, pre-heated particles had a mechanical strength 28% higher than particles injected at room temperature and -55 °C. Despite this, scanning electron microscope images indicated no notable differences between the deposit microstructures. Future works are planned to study the effect of pre-heat at higher particle impact velocities and degrees of pre-heat to improve powder consolidation.

Polymer cold spray has yielded lower deposition efficiency (DE) and quality deposits compared to metal cold spray. The disparity stems from metals being studied far longer than polymers in cold spray; in addition, polymers exhibit richer thermo-mechanical behavior. An experimental study was conducted to examine the effects of polymer feedstock degree of crystallinity (D) on cold sprayed deposits of polyetherketoneketone (PEKK), a thermoplastic used in aerospace and other high-performance applications. As deposition relies on the plastic deformation of the impacting particle, polymers with high D may inhibit deposition, reducing deposit quality and efficiency. This study evaluates three PEKK grades produced using different ratios of terephthalic (T) to isophthalic (I) monomer moieties (T/I = 60/40, 70/30, 80/20). The ratios control D, with higher proportions of T monomers corresponding to higher crystallization rates and degrees of crystallinity. A parametric study was completed to evaluate functional process set points of system carrier gas temperature and powder mass flow rate. Using operational parameters common among the PEKK grades, spray cycles were completed for each material and quantitative responses to variation in crystallinity were evaluated through a suite of analyses. DE of the materials was assessed gravimetrically, deposit porosity was evaluated by scanning electron microscopy, and thermophysical changes to the feedstock during the spray cycle were determined by differential scanning calorimetry. Overall, we found that cold spray processing of powders of lower D formed less porous deposits with a higher DE than more crystalline powders sprayed at the same process conditions. PEKK grades with lower T/I ratios achieved DEs in the range of 60-75%, whereas the most T enriched grade only reached ~10% DE.