Environmental barrier coatings (EBCs) are required to protect SiC based composites in high temperature, steam containing combustion environments found in the latest generation of high efficiency gas turbine aeroengines. Ytterbium disilicate has shown promise as an environmental barrier coating, showing excellent phase stability at high temperature and a coefficient of thermal expansion close to that of SiC; however, its performance is dependent on the conditions under which the coating was deposited. In this work, a parametric study was undertaken to demonstrate how processing parameters using a widely used Praxair SG-100 atmospheric plasma spraying torch affect the phase composition, microstructure and mechanical properties of ytterbium disilicate environmental barrier coatings. Ytterbium disilicate coatings were deposited using 5 sets of spray parameters, varying arc current and secondary gas flow. The phases present in these coatings were quantified using X-ray diffraction with Rietveld refinement, and the level of porosity was measured. Using this data, the relationship between processing parameters and phase composition and microstructure was examined. Abradable coatings are used throughout gas turbine engines to increase efficiency in the compression and combustion phases of the turbine. Abradable coatings are soft enough to be worn away by turbine blade tips (without damaging the tip itself), allowing for tighter clearances to be used, limiting leakages and increasing efficiency. Using the optimum process parameter window determined in this work, a low density abradable Yb 2 Si 2 O 7 layer will be deposited in future research.

Silicon coatings have been developed for environmental barrier coatings by thermal spraying. Until now, these coatings have been produced almost exclusively by Atmospheric Plasma Spraying (APS). High Velocity Oxy-Fuel (HVOF) spraying is commonly used to produce dense metallic and carbide-based coatings due to high particle velocities. However, there have been no scientific reports on HVOF-sprayed silicon coatings in the literature. This study was conducted to investigate the feasibility of fabricating silicon coatings by HVOF using a DJ2600 spray system. Both the spray powders and the parameters were varied. The coatings were investigated on their surfaces and cross-sections using scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). The hardness and indentation modulus of the silicon coatings were also determined. The results show that the particle size distribution and the stand-off distance are important influencing factors. Dense coatings could be produced by HVOF spraying, confirming the feasibility.

Raising demands in industry lead to the request of different and better paper quality. New paper types like e.g., liquid food packaging (lfp) will replace former solutions like Tetrapak, magazine papers will be produced much cheaper on different machine designs. Since certain paper properties like roughness, printability, … still remain or step up in quality level. One machine component influencing the paper properties strongly is the calender with its extremely hard, coated roll surfaces. The hard metal coating on the calender rolls can determine the later paper quality significantly. The current work describes investigations on certain WC based HVOF coatings, which later will be in contact with the paper under high temperatures, extremely high load, and steam environment. Coating design from carbide particle size, metal matrix and other factors like apparent density and sintering grade were studied under real life influences. The various coating versions were compared in a slurry test using the same fines as added in the paper production process for increasing brightness and tear strength. Interesting results are discussed on technical, but also economical basis.

Despite their light weight, 2.3 times lighter than Al, polymers are limited to application with low thermal, wear, and abrasion demands. The enhancement of the functional surfaces of the polymers using thermal spraying techniques is a challenging task due to the thermal degradation of polymers, the low wettability, and the disparate atomic properties. The twin-wire arc spraying (TWAS) process comprises two contradictory features. Almost all spraying particles are in a molten state on the one hand, and on the other hand, the spray plume has the lowest heat output among the different thermal spraying techniques. Therefore, it is a promising spraying technique for the required surface improvement. The surface of the 3D-printed parts was metalized using two successive layers. The first layer is a TWAS coating made of low-melting ZnAl 4 to avoid thermal degradation and provide a bond coat. The topcoat is also applied using a TWAS process and was made out of Ni-WC-Co as cored wires. The top hard coating has improved the wear resistance of the polymers by 14.6 times. The erosion of the coated and uncoated specimens was determined using a low-pressure cold gas spray gun. Ni-WC-Co coating led to more than five times higher erosion resistance.

In the field of additive manufacturing, the demand for Extreme High-Speed Laser Material Deposition (EHLA) is increasing due to its unique process characteristics, economic efficiency as well as its great resource efficiency. The process is currently mostly used for surface functionalization through coating, by means of corrosion and wear protection. Thereby, almost all materials can be processed and nearly all material combinations can be created. The layers produced are dense and metallurgical bonded, and furthermore the surface roughness produced is low, so that only 20-100 μm has to be removed to produce a finished surface. However, it can also be used for the generation of 3D geometries. The greatest cost factor in the production is the coating material. With increasing requirements, for example in wear protection, cost-intensive special alloys or materials must be used. An opportunity to increase the areas of application in the field of wear resistance as well as increasing material efficiency is offered by combining EHLA with the innovative post-processing methods of hammering, solid as well as smooth rolling. Using these processes, the surface roughness can be reduced to a value of Rz 1-3 μm on the one hand and the surface hardness can be increased on the other hand. The hammering and solid rolling processes differ in their depth of impact. In the case of hammering, the impact depth can be a few millimeters and in the case of solid rolling only a few tenths of a millimeter. So far, the influence of hammering or solid rolling of additive manufactured volumes or surfaces has not been investigated. In the context of this study, the influence of hammering and solid rolling on a volume produced with EHLA is investigated. For this purpose, an EHLA produced volume of IN718 is built up and the influence of hammering as well as solid rolling on the surface roughness and hardness is analyzed.

Carbides are interesting materials for many wear resistant and high temperature applications, however, the production of coatings with these materials represents a significant challenge as they tend to oxidise or decompose into gaseous phases when they are exposed to extreme thermal spray conditions. An innovative method merging suspension and solution precursors was developed to allow the production of carbide composite coatings. Suspensions of carbides and borides were modified with the addition of oxide precursors to obtain composite coatings by high-velocity oxy-fuel (HVOF) thermal spray. The transformation of these oxides precursors and their subsequent melting during spraying contribute to protect the carbides from oxidising conditions, avoid their degradation during the spray process and support the development of dense coatings, as it was demonstrated by dispersive X-ray spectroscopy and X-ray diffraction analysis. The relationships between processing and microstructure were studied in terms of porosity phase distribution and mechanical properties, proving that this novel approach could be applied to obtain coatings of materials that are prone to decompose during thermal spraying.

Hot section components of stationary gas turbines such as turbine blades are coated with thermal barrier coatings (TBCs) to increase the high thermal strain tolerance thereby the improvement of the performance for the gas turbines. TBCs represent high-performance ceramics and are mostly composed of yttria-stabilized zirconia (YSZ) in order to fulfil the function of thermal insulation. The microstructure of conventional TBCs should be porous to decrease heat conduction. Besides porous TBCs, the recently developed vertically segmented thermal barrier coatings (s-TBCs) feature outstanding thermal durability. In this work, process parameter development for atmospheric plasma spraying (APS) of s-TBCs is presented. Within the experiments, relevant process parameters such as powder feed rate, surface speed and pathway strategy have been optimized. The aim of this work is to achieve a combination of low internal residual stress and high adhesive tensile strength for s-TBCs. For the formation of vertical cracks, the heat input into the powder feedstock material and the substrate must be controlled precisely.

Thermochemical processes are an appropriate way to improve the surface hardening of the material against wear. Thermal spraying is a group of deposition processes that can deposit different classes of materials. The use of thermomechanical process after metallic coatings deposition can result in a unique combination of bulk and surface properties. There are some studies that indicate the defects and stresses caused in the crystal lattice as one of the factors that most influence nitrogen diffusion during the nitriding process. The HVAF (High Velocity Air-Fuel) process can generate different fault conditions and stress-strain in the crystal lattice. The aim of this work is study the effect of the plasma nitriding or, as it is known, Glow Discharge (GD), on FeMnCrSiNi coating deposited with HVAF process. Initially, it was observed the formation of expanded austenite and CrN on the HVAF coating, followed by important increase on the hardness of the coating.

The exceptional properties of Ti-6Al-4V of high strength, lightweight, corrosion resistance and machinability make it one of the most widely used alloys in in the aerospace industry. Significant efforts are underway to establish powder bed additive manufacturing (AM) technologies for Ti-6Al-4V. There are also increasing attempts to use thermal and cold spray to build near net shape parts with buildup rates orders of magnitude higher than powder bed. Thermal spraying, such as HVOF, can oxidize and degrade the alloy due to the high processing temperature. Lowering the flame temperature through inert gas addition in full-size HVOF systems is a possible approach to retain solid state deposition of the feedstock particles, thereby limiting oxidation and detrimental α-case formation, while providing sufficient heat input for particle softening and plastic deformation at impact. Novel miniaturized HVOF systems, with spray jets of only a few millimetre in width, may further offer the possibility to improve the spatial resolution of the buildup for near net shape forming. The process parameter range for solid state deposition of Ti-6A-4V, using the liquid fuelled TAFA Model 825 JPid and the novel hydrogen fuelled Spraywerx ID-NOVA MK-6 with the addition of nitrogen will be discussed. Build-ups at over 80% deposition efficiency generally yield as-sprayed porosities below 3% and hardness above 200 HV100gf. Attainable microstructures and oxygen content as a function of spray parameters are delineated. Recrystallization and beta annealing of selected samples lowered the residual porosity and created equiaxed α and intergranular ß-phases. Ultimate tensile strengths of up to 1100 MPa were attained, however, at limited elongation.

Thermal spray technology keeps attracting several industries in both the manufacturing and repair sectors, thanks to its practicability and its reasonable processing time. Moreover, different kinds of materials can be successfully deposited to form coatings with potential excellent thermo-electro-mechanical properties. The resultant coating microstructure is completely different from the wrought powder material before the deposition process. In the case of metallic materials, the thermomechanical characteristics are quite dependent on the deposition conditions monitored from the spraying setup. One can mention gas temperature, impact velocity and angle, material combination, surface state, particles size, etc. Hence, one major factor which influences the final coating microstructural state is kinetic energy. In fact, in such processes where high velocity deposition is observed, intense grain refinement and sharp increase of the dislocation density are an outcome that is tightly related to high temperature and severe plastic deformation. Prediction of the mechanical properties of the produced coating is usually carried out using phenomenological models that describe very well the relationship between stress and strain under different conditions of temperature and strain rates. Most of these models fail, however, to describe the effect of the deformation mechanisms observed at ultra-high strain rates such as the viscous drag regime of dislocations or further the weak shock load regime, scenarios commonly observed in such processes. In the present paper, we present an enhanced physics-based model to describe the stress strengthening of metals upon impact and associated microstructure changes. We show that the model can accurately represent the desired effect of the dislocation drag. Modelling of the impact of a single copper particle onto a copper substrate is carried out to show the capability of the model to predict grain refinement and dislocation network modification.

Thermally sprayed WC-based hardmetal coatings offer high hardness, good sliding wear and abrasion performance and find large applications in mechanical engineering, valve construction, or offshore applications. WC-Co coatings are mainly produced by high-velocity oxy-fuel spraying (HVOF) from conventional spray feedstock powders. In our previous work, the potential of the suspension-HVOF spraying (S-HVOF) to produce dense-structured WC-12Co coatings has been shown. Significant work was devoted to the development of appropriate aqueous hardmetal suspensions starting from commercially available fine WC and Co raw powders feedstock. This contribution proposes a step forward in the development of the S-HVOF WC-12Co coatings and evaluation of their microstructural and tribological properties. Suspension spraying trials were carried out using gas-fuelled HVOF TopGun system. For comparison purposes, liquid-fuelled HVOF K2 was employed to spray WC-12Co coatings starting from commercial available spray powder. Microstructural characterization, X-Ray diffraction and microhardness of the coatings were evaluated. Oscillating sliding wear tests were conducted against sintered Al 2 O 3 and WC-6Co balls. The sliding wear performances of the WC-Co sprayed coatings were discussed in term of the microstructure, phase composition and coating-ball test couples.

In this work, a novel HVOAF process fueled with ethanol was employed to prepare NiCoCrAlYTa coatings on AISI 304 stainless steel substrate. To be able to add compressed air into the torch, it was designed to add a second-stage combustion chamber. Thereafter, investigations were carried out to determine the influence of different compressed air flow rates on the evolution of the microstructure and properties of the resulting NiCoCrAlYTa coatings. The phase composition, microstructure, porosity, microhardness, bond strength and wear resistance of the as-sprayed coatings have been studied in detail. The results reveal that the compressed air flow rate has a substantial effect on the coating's microstructure. The addition of compressed air also contributes to reduce the degree of oxidation of the coating, which could be attributable to a decrease in the temperature of the flying particles and an increase in their velocity. Although the use of compressed air diminishes the coating's bonding strength, it still has some elevated strength. Furthermore, the injection of compressed air improves the coating's sliding wear resistance dramatically. SEM and EDS were used to investigate the sliding wear mechanism of the coating. Detailed correlation between the compressed air flow rates and the coating properties are elaborated to identify the coatings exhibiting optimum performances.

Hybrid aerosol deposition (HAD) is a new coating method to deposit homogeneous nano-structured ceramic coatings. An accurate evaluation of the fabricated coating properties is required. In this study, α-Al 2 O 3 fine powder was sprayed by HAD. The obtained coatings were dense and uniform with a nanocrystalline structure. An X-ray diffraction measurement revealed that the fabricated HAD Al 2 O 3 coatings mainly consisted of α-Al 2 O 3 phase. The hardness and Young's modulus of the HAD Al 2 O 3 coatings were evaluated by a micro-Vickers method and a nanoindentation method using the Weibull distribution. The hardness of HAD Al 2 O 3 coatings measured by micro-Vickers was ~1400 HV (~15 GPa). The variation of mechanical properties of HAD coatings measured by the nanoindentation method was extremely small compared to those of plasma-sprayed coatings, which also indicates that HAD coatings contain less pores and cracks than plasma-sprayed coatings.

Thermally sprayed WC/CoCr coatings are established in the valve industry for wear protection. However, conventional coatings have to be cost-intensively postprocessed. Therefore, the aim of this study is to develop near net shaped (nns) WC/CoCr-coatings with a high wear resistance in order to avoid the expensive grinding postprocess. For the development of the nns coatings a parameter study is used to investigate the influence of the stand-off distance and hydrogen volume flow rate in the HVAF process. The parameter study indicates the influence of the hydrogen volume flow and the stand-off distance on the porosity, microhardness and wear resistance of the coatings. The developed coatings exhibit a low porosity and high wear resistance. Through the correlation of the coating properties with the process parameters, promising parameter ranges for a further development of HVAF-sprayed nns coatings of WC/CoCr can be identified. With these results, first benchmarks for HVAF process parameters, hydrogen flow rate and stand-off distance, could be delivered, advancing the overall goal of reducing manufacturing costs of valves.

In the last 15 years, the cold spray process has demonstrated a great efficiency for the deposition of metallic powders. In this case, the consolidation of coatings is achieved thanks to the high kinetic energy of unmelted particles exhibiting a ductile behaviour. Dealing with ceramics, cold spray is also of great interest because one can expect properties not reachable with classical thermal spray technologies thanks to lower involved temperatures. However, cold spray of ceramics still remains challenging because of the ceramics intrinsic brittleness. Here, in the specific case of hydroxyapatite and to overcome this brittleness issue, we investigate the role of an intermediate PEEK layer between the substrate and the deposit. We highlight how this sublayer previously deposited by FS or air APS spraying can help improving the consolidation of the coating and its growth.

The aim of this work is to study the effects of the titanium plasma spray (TPS) coating process on the fatigue resistance of a titanium-6Al-4V substrate. The combination of TPS processes and Ti alloy substrate is widely applied on components intended for cementless total hip replacement (THR). In order to understand the coating process mechanism behind the implants’ fatigue resistance decrease, one air-developed coating (Ti-APS) and one controlled atmosphere developed coating (Ti-CAPS) were considered. The effects of the most representative parameters of the plasma spray process on the fatigue resistance were analysed: the sandblasting process, the plasma and the coating powder. Fatigue resistance studies were performed by means of rotating bending fatigue testing. After fatigue failure specimens underwent morphological analyses both on the primary crack surface and on the cross-sectional area complemented by of the metallographic analyses of the coating. The titanium substrate fatigue resistance decreased after being blasted with direct relationship with the grain size. Ti-CAPS process showed a relatively limited further influence on the fatigue resistance reduction with respect to only sandblasted samples. By contrary a remarkable fatigue limit decreased was seen for Ti-APS coated samples against Ti-CAPS and simply sandblasted samples. The experiment pointed out the critical importance of cracks oxidation as a fatigue failure driving factor.

The adaptation of medium-entropy alloys (MEAs) by minor alloying constituents allows a targeted modification of the property profile of this material class for surface protection applications. In the present work, the potential of BSiC additions in the MEA system CrFeNi as base for adapted feedstock materials for thermal spraying is investigated. The alloy development was carried out in an electric arc furnace. Compared with the initial alloy, a significant increase in the wear resistance of the castings was demonstrated for the adapted alloy composition. Subsequently, powder was produced and characterized by inert gas atomization, followed by processing via high velocity oxy-fuel (HVOF) spraying. The tribological behavior was evaluated comparatively for all manufacturing variants considered. A good agreement in the property profile was determined, confirming the basic alloy development approach based on metallurgical processes. The evaluation of the process-structure property relationships confirms the great potential of adapted alloy systems for complex alloys in the field of surface engineering.

High entropy alloys (HEAs) are classified as a new class of advanced metallic materials that have received significant attention in recent years due to their stable microstructures and promising properties. In this study, three mechanically alloyed equiatomic HEA coatings – AlCoCrFeMo, AlCoCrFeMoW, and AlCoCrFeMoV – were fabricated on stainless steel substrates using flame spray manufacturing technique. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Vicker’s microhardness were utilized to characterize the fabricated HEA coatings. Furthermore, Joule heating experiments using a modified version of a two-probe test was used to measure the electrical resistivity of the HEA coatings. To prevent short-circuiting of the metallic coatings, a thin layer of alumina was deposited as a dielectric material prior to the deposition of HEA coatings. The microstructure of the HEA coatings showed the presence of multiple oxide regions along with solid-solution phases. The porosity levels were approximately 2 to 3% for all the HEA coatings. The HEA coatings had a thickness of approximately 130 to 140 μm, whereas the alumina layer was 120 to 160 μm thick. The electrical resistivity values were higher for all the HEA coatings compared to flame-sprayed Ni-20Cr and NiCrAlY coatings and AlCoCrFeNi HEA thin film, which may be attributed to the characteristics of HEAs, such as severe lattice distortion and solute segregations. The combined interaction of high hardness and increased electrical resistivity suggests that the flame-sprayed HEA coatings can be used as multifunctional wear-resistant materials for energy generation applications.

Iron-based coatings are often considered as replacement of hard chromium and WC-Co, as they pose lower health and environmental impact. In many cases the combination of mechanical and chemical properties of ferrous based alloys may be satisfactory and their relatively low cost make these coatings an interesting candidate for many applications. This study is inspired by opportunities to harden the ferrous base materials by strain hardening, solid solution strengthening, dispersion strengthening, and precipitation hardening. Already commercially available Fe-based coating materials with precipitates of mixed carbides and borides in the metastable austenitic matrix achieve a high hardness. In this study the cavitation erosion and abrasion resistance of various Fe-based coatings produced by HVAF and HVOF processes were investigated. Two experimental precipitation containing materials were prepared, and the sprayed coatings were tested for abrasive and cavitation erosion wear. In addition to precipitations, the importance of proportion of ferrite and retained austenite phases were studied by affecting the microstructure by heat treatments as the ability of different phases to affect hardening and ductility may become crucial in generating desired material properties. The properties of experimental and some commercial Fe-based alloys are compared with WC-Co and Cr 3 C 2 -NiCr coatings by property mapping.

Wear leads to high material and energy losses in various industries. The manufacturing of novel nano-carbide WC/Co powder feedstock materials promises a further increase in the performance of thermally sprayed wear protection coatings. A novel experimental powder and a commercial ultra-fine carbide WC/CoCr reference are thermally sprayed onto a 1.0038 substrate by High Velocity Air Fuel (HVAF) spraying. The specimens are metallographically prepared and analyzed by means of light microscopy (LM) and scanning electron microscopy (SEM). Vickers Hardness testing is conducted by microindentation and the porosities are determined by optical image analysis. X-ray diffractometry (XRD) analysis are used to investigate the phase retention. Fine nanocrystalline WC-structures are preserved in the dense coatings. A significant effect of powder type on the porosity of the coating was found. No systematic relationships could be identified between the coating structure and the parameter settings. It was possible to influence decarburization via both the powder type and the selected parameters. The resulting experimental coatings exhibit high hardness values in the range of the commercial ultrafine carbide WC reference. The novel nano-structured coating can contribute to reduced wear and therefore improve the efficient utilization of critical raw materials like tungsten.