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.

The need for effective electrical insulation coupled with good thermal conductivity in power electronics has led to an exploration of suitable solutions for components like Insulated-Gate Bipolar Transistors (IGBTs). Considering its material properties, AlN emerges as a promising candidate for this application due to its high thermal conductivity, good electrical insulation and ample dielectric strength. However, aluminium nitride (AlN) has a low deposition efficiency when applied by atmospheric plasma spraying (APS). In contrast to AlN, alumina has a very good deposition efficiency during thermal spraying. Feedstock development was conducted to enhance the coating deposition for AlN. Therefore, a parameter study was carried out with AlN feedstock material to form a protective alumina shell around the AlN particles. Subsequently, the heat-treated powder was applied on an aluminium substrate by APS. X-ray diffraction (XRD) analysis displayed that, the heat-treated feedstock material contained AlN and α-Al 2 O 3 phases. It was observed from scanning electron microscopy (SEM) analysis that the AlN particles formed an oxide shell which led to an enhanced deposition efficiency with a high amount of AlN in the coating. The coatings were also investigated by XRD and SEM to prove the presence of AlN and alumina. For the first time, oxide shelled AlN was successfully applied by thermal spraying with sufficient coating deposition and enhanced AlN-content in the coating.

Aerosol deposition (AD) is a novel method for producing dense nanocrystalline ceramic films at room temperature. Previous studies primarily used flat substrates with varying hardness and roughness. However, the development of micro-device applications is increasing the demand for deposition on structured/patterned surfaces. To investigate the impact of substrate patterns on coating microstructure and growth mechanisms in AD, alumina coatings were deposited on patterned Si substrates. Si wafers with patterns of micropillars were employed. The coatings were characterized using laser scanning microscopy, scanning electron microscopy, and x-ray diffraction. The microstructure and density of coatings in the valleys were influenced by the size of and the spacing between the patterns. The results revealed that coatings initially formed in the valleys before covering the entire pattern. Fragments of the initial powder particles became trapped between the patterns, adhering to the groove bottoms and pillar sides. Subsequent particle impacts and densification processes transformed these fragments, ultimately filling the gaps between the walls. With further deposition, a uniform coating surface was achieved.

Hybrid plasma spraying combines deposition of coatings from coarse powders and liquids (suspensions or solutions) so that the benefits of both routes may be combined. In this study, failure evolution of early-stage thermal barrier coatings (TBCs) with hybrid YSZ-YSZ and YSZ-Al 2 O 3 top-coats deposited by hybrid water/argon-stabilized plasma torch was evaluated. In-situ bending experiment was carried out in SEM to assess potential influence of the secondary miniature phase addition on the coating failure during mechanical loading. Adapted high-resolution open-source strain-mapping code GCPU_Optical_flow was used to track evolution of the local coating failure. For the tested coatings, addition of miniature phase did not weaken the hybrid coating microstructure as the crack propagation was practically insensitive to the presence of the secondary phase and dissimilar splat boundaries. Main micromechanisms of the top-coat failure were thus splats cracking, loss of cohesion (splat debonding), and mutual splat sliding.

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.

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.

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.

Light alloys are being investigated as an alternative to ferrous-based engineering components. The manufacturing of such components requires a surface modification step necessary to eliminate the top surface's poor wear and corrosion response for improved functionality. Thermally sprayed cermet coatings offer improved surface resistance to wear and/or corrosion. This work presents a customized composition of WC-CoCr feedstock cut in fine and coarse powder size distribution (PSD) to fabricate different coatings on aluminium alloy and steel substrates using two high velocity spray techniques. The WC-CoCr coatings sprayed using the high velocity air-fuel (HVAF) technique at varied parameters consist of six different coatings (four thick, ~ 200 μm and two thin ones, 60-80 μm) to investigate the relationship between processing conditions, microstructure, and performance. Using scanning electron microscopy (SEM) and electro-dispersive X-ray spectroscopy (EDX) offered a comprehensive characterization of the respective coatings. Micro indentation, dry sliding wear, dry sand abrasion, and cavitation erosion tests conducted on the samples show the performance of the coatings based on the processing techniques and spray conditions. However, despite the similarities in the microstructural makeup of the coatings and the measured micro indentation hardness of the coatings (1000-1300 HV0.1), their respective specific wear rate (SWR) varied based on spray processing techniques and the substrate on which the coatings were deposited. Three of the HVAF coatings showed ~ 60 % more wear on the aluminium alloy substrate compared to the same coating deposited on a steel substrate. However, irrespective of the substrate used the HVAF coatings showed better wear resistance than the HVOF coating. The dry sand abrasion wear results of the two thick HVAF coatings show them superior to the HVOF coating in the three-body wear experiment conducted. The cavitation erosion resistance of the coatings varied based on the processing conditions and the driving mechanisms but the best two were the AF-2 and AF-6 samples.

Recently, laser deposition technologies have made significant advancements in their ability to manufacture high temperature metals and ceramics. One of these technologies, known as Direct Energy Deposition (DED), has the potential to deposit a wide range of materials from polymers to refractory materials, ceramics and functionally graded materials. This study evaluates major microstructural characteristics of WC-Co additively manufactured by DED technology. This material is commonly used for deposition of protective coatings due to its high hardness and excellent wear resistance. To this end, hardness and wear resistance of the DED processed samples were also investigated in this study. WC-Co coatings are generally deposited using various thermal spray technologies. However, it is speculated that DED deposited WC-Co could provide superior properties such as higher hardness and wear resistance. A DED manufactured WC-Co sample was examined by Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and X-ray Diffraction (XRD). Those studies could provide information about important microstructural features, chemical compositions and phase distribution. All the tests were also repeated on High-Velocity Oxygen Fuel (HVOF) deposited WC-Co with the same composition. Both DED and HVOF produced WC-Co coatings experience decomposition of the carbides into compound phases; however, the DED deposited sample displays unique dendritic and eutectic structures that improve the hardness and wear properties compared to the homogenous HVOF coating. In addition, DED produced samples show higher hardness and relatively better wear resistance compared to the HVOF deposited ones. The obtained results could establish a relationship between microstructural characteristics with hardness and wear properties of both samples.

Thermally sprayed wear resistant coatings have proven their effectiveness in many applications. Their benefit is unquestionable in the case of mutual sliding contact or abrasive stress caused by hard particles. However, for the case of dynamic impact loading, either single or cyclic, the lifetime of different types of coatings is rarely described, probably due to the complex influence of many parameters. The paper deals with the evaluation of resistance to dynamic impact loading of two types of HVOF-sprayed Cr3C2-rich binary hardmetal coatings (Cr3C2-42%WC-16%Ni and Cr3C2-37%WC-18%NiCoCr) with respect to the variation of their deposition parameters and compares them to a well established Cr3C2-25%NiCr coating. For each coating, a Wohler-like curve was constructed based on a failure criterion of sudden increase in impact crater volume. Besides, coatings deposition rate, residual stress, microstructure and hardness were evaluated. Differences in the coatings dynamic impact wear resistance was found, related to their residual stress. The failure mechanism and crack propagation mode are analyzed using SEM of impact surface and cross-sections. Deformation and related stress changes in coated systems during dynamic impact loading are described using FEA analyzes.

To achieve higher engine combustion efficiency while reducing emissions, it is necessary to address the challenges posed by elevated operating temperatures. High Entropy Alloys (HEAs) have emerged as promising materials for this purpose, offering exceptional properties at high temperatures, including synergistic effects and excellent resistance to oxidation and corrosion. In this study, a FeCoNiCrAl HEA was investigated as a bond coat material due to its excellent balance of strength and ductility, coupled with outstanding oxidation resistance. It was deposited using HVAF M3 and i7 guns equipped with different nozzles/powder injectors and pressures. Notably, this research marks the first study of the i7 gun globally for the HEA bond coat, coupled with the optimization of HVAF parameters for both i7 and M3 guns. Characterization of both powder and as-sprayed samples was carried out using X-ray Diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), and Field Emission Scanning Electron Microscopy (FESEM) techniques. The results revealed the formation of a dense and homogeneous microstructure. Additionally, isothermal oxidation tests were conducted to analyze the behavior of the thermally grown oxide. After 50 hours at 1000 °C, a dense, uniform, and thin alumina TGO layer was observed to have formed. These tests revealed that FeCoNiCrAl HEA exhibits significant potential to enhance oxidation resistance at high temperatures.

The development of materials and alloys for coatings has been increasingly important for reducing costs in different manufacturing processes. The Inconel alloy is widely used due to its chemical inertness and high resistance to high temperatures, but it does not present adequate resistance to erosive wear. In this context, the resistance to wear from cavitation erosion and slurry erosion was evaluated of samples with depositions obtained by laser cladding (Laser directed energy deposition - L-DED) of Inconel 718 and Inconel 718+10%NiNb. The cavitation erosion wear tests were carried out following the ASTM G32 standard (2016), and the ASTM G73-10 standard (2017) was used to evaluate the resistance to slurry erosion wear. The scanning electron microscopy technique (SEM-EDS), and X-ray diffraction (XRD) were used to characterize the cross-section and the surface after wear. The wear mechanism was checked and identified. Microhardness profiles of the cladding cross-section were carried out. The mass loss and wear rate due to cavitation and slurry jet erosion of Inconel 718 and Inconel 718+ 10% NiNb coatings were determined. It was proven that the addition of 10% NiNb in the formation of the cladding caused a 45% increase in average microhardness in the cross-section of the Inconel 718 cladding. The addition of 10% NiNb to the Inconel 718 cladding caused a decrease in mass loss due to slurry erosion from 38.9 mg to 21.9 mg (33%) when the erodent impact angle was 60°.

The multi-layered thermal barrier coatings (TBC) are commonly used in the systems exposed to extensive heat, such as jet engines or gas turbines. The testing of coatings' performance is usually carried out using electric or gas furnace. Concentrated solar power (CSP) could provide cost-effective and environmentally friendly alternative using natural energy source. Moreover, it can also simulate materials exposure in real applications, e.g., in solar power-plants. In this study, possibility of using concentrated solar power to test the performance of hybrid YSZ-based TBCs prepared by hybrid water/argon-stabilized plasma (WSP-H) technology was studied for the first time. In service, TBC top-coat layer may be exposed also to so-called CMAS air-borne particles occurring in the atmosphere which may melt at elevated temperatures and penetrate the coating microstructure, inducing crystallographic and volumetric changes therein. Therefore, testing with the presence of CMAS particles was also included in this study to observe its influence on the coating microstructure under solar irradiation. Changes of the coating microstructures were studied using SEM analysis and X-ray diffraction.

A previous study on the pull-off testing of metallized carbon fiber reinforced polymers (CFRPs) via cold spray showed that better adhesion strengths could be obtained when features such as carbon fibers or surfacing elements were present, by providing potential mechanical interlocking features. In this work, the effect of the fiber orientation on the deposition and bonding of the metallic coating to the thermoplastic composite substrate is explored. Pure Sn powder was cold sprayed onto two thermoplastic Polyether-Ether- Ketone (PEEK) CFRP substrates, containing carbon fibers with different orientations: one had fibers in the plane of the substrate (uni-directional tape), while the other had fibers mostly perpendicular to the substrate (ZRT film). Characterization of the coatings was performed via scanning electron microscopy (SEM) and confocal microscopy, and some aspects of mechanical testing (namely wear and scratch testing) were carried out to assess the effect of the substrate on the properties of the coatings.

Zinc oxide (ZnO) is known for its rich diversity of microstructures and has been attracting attention for its unique combination of mechanical and physical properties. It has been a material of interest in different areas such as optoelectronics, sensors and the general ceramic industry. It also has been a material of interest in biomedicine due to its antimicrobial characteristics and biocompatibility properties. A simple processing route to produce ZnO micro/nanostructures is the thermal oxidation of zinc, which results in a wide range of ZnO nanostructures depending on the oxidation conditions. The main objective of this study was to investigate the influence of a severe plastically deformed zinc microstructure on the formation of ZnO nanostructures produced by oxidation, with a special attention to the zinc oxide growth mechanism and nanostructures characteristics. For this purpose, the cold spray process was used to produce Zn coatings using different feedstock powders that required different process parameters in order to obtain Zn coatings with severely deformed particles. A non-catalytic thermal oxidation method was then used to successfully produce ZnO nanostructures at the surface of the heavily deformed cold sprayed Zn coatings. The as-grown ZnO nanostructures were investigated in detail using scanning electron microscopy and X-ray photoelectron spectroscopy. These investigations revealed that the chemical fingerprint of the oxides grown in the cold sprayed samples was different from that of conventional ZnO. It was also observed that in the oxidized cold sprayed Zn coatings, the formation of ZnO nanowires was hindered due to the formation of blisters generated during the high temperature exposure, revealing nonoptimized process parameters.

Hybrid plasma spraying has been proved to provide novel coating microstructures as a result of the simultaneous injection of a dry coarse powder and a liquid feedstock into the plasma jet. Such microstructure contains both large splats originating from the conventional dry powder and finely dispersed miniature splats deposited from the liquid. This approach enables preparation of coatings from virtually all materials which are conventionally processed using plasma spraying. However, incorporation of materials susceptible to decomposition at high temperatures is still challenging even using this concept due to the high thermal energy provided to all feedstocks to be deposited. Hereby, we propose an innovative approach of incorporation of thermally-sensitive materials into a coating sprayed using a high-enthalpy plasma torch. As a case study, Al 2 O 3 was sprayed from dry coarse powder and MoS 2 was sprayed from the suspension which was deposited directly onto the substrates, i.e., by-passing the hot plasma jet. The retention of the added material in the coating was evaluated using scanning electron microscopy and X-ray diffraction.

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.

A micro-plasma system was investigated for its capability in additive manufacturing (AM). Micro-plasma AM system has the advantage of lower cost and higher deposition rate over the laser-based AM systems, and generates leaner and cleaner weld deposit than other arc-based AM systems. However, the microplasma system is complex and involves a large number of process variables. In this study, the effects of two arc and wire feed modes on dimensional consistency and hardness were firstly examined. Subsequently, one set of the specimens was further subjected to oxidation tests and the results were compared to that from conventional wrought Inconel 718. It was found that all four processes could produce crack free samples without measurable distortion. Some surface discoloration was observed, ranging from light straw to a purple tint. After heat treatment, the hardness of the samples varies from 403 to 440 HV, with the transverse surface showing slightly lower hardness values. The oxidation tests at 900 °C yielded similar weight change for AM Inconel 718 and its counterpart wrought alloy; however, the rate constant for wrought alloy was slightly higher. Microstructural analysis with SEM and EDS revealed a dendritic structure in the AM Inconel 718 and the presence of Nb-rich compounds in the interdendritic region. The polycrystal grain structure was not delineated in AM material as that in wrought 718. With the increase of exposure time, the oxide layer continues to increase at a higher rate, along with a sublayer of Ni 3 Nb above the metal substrate. In addition, after 200 hours, the wrought alloy developed porous chromia, while AM material exhibited uneven oxide thickness. In consideration of all aspects of the evaluation carried out thus far, it is concluded that the AM material produced by micro-plasma process is equivalent to wrought material in mechanical properties and oxidation performance.

As a supersonic solid-state deposition process, Cold Spray (CS) has a unique role among other thermal spray techniques as it uses compressed and heated gas to accelerate particles to a critical velocity. CS can be an expensive process, especially when helium is used as a processing gas. In recent thermal spray developments, High-Velocity Air Fuel (HVAF) has taken a specific place in terms of providing dense and strong coatings similar to CS, but also coatings with less oxidation than High- Velocity Oxy-Fuel (HVOF). In contrast to these techniques, HVAF uses a mixture of fuel and air, instead of pure oxygen as in HVOF, to accelerate particles. Therefore, HVAF appears as a relatively cheaper and environmentally friendly alternative for the deposition of a wide variety of materials. The aim of this research is to produce fully dense copper coatings with limited oxidation using an inner diameter (ID) HVAF system and to compare the microstructure with CS copper coatings. Coating microstructures, surface roughness, and microhardness are studied using different characterization methods such as Scanning Electron Microscopy (SEM). Through this paper, the influence of both spray processes, CS and ID-HVAF, on the deposition of copper coatings is discussed. Cross-sectional studies of different coatings show a fairly dense microstructure for CS and ID-HVAF coatings. Moreover, it is discussed how the copper coating properties can change upon modifying the spray parameters.

This research presents a novel approach for producing metal matrix composite powders using a nanoparticle decoration technique. A 1wt% stable suspension of 30nm Al 2 O 3 particles was decorated onto primary AA6061using a redispersion method. The resulting AA6061-1wt% Al 2 O 3 composite powder was mixed in a rotary mixer for one hour and subsequently dried at 45°C. Scanning electron microscopy of the composite powder confirmed the successful material composition. The composite powders were then deposited onto an AA6061 substrate using a low-pressure cold spray system, with the coating quality, deposition efficiency, surface roughness, and hardness of the deposited materials analyzed. After heat treatment at 430oC, the role of the nanoparticles in hindering recrystallization was studied, with Orowan strengthening shown to be the main mechanism for preventing recrystallization and grain growth. This technique provides a promising alternative method for producing metal matrix composites and offers potential for further exploration of their properties and applications.