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.

The method of simulating the wear performance under working conditions using a high-temperature ultra-high-speed wear testing machine was adopted to study the effect of feed rate variation on the wear behavior and scraping performance of the AlSi/hBN sealing coating and TC4 simulated blades. The macro and micro morphology of the coating and blades were analyzed by stereomicroscope and scanning electron microscope (SEM). The phase composition of the coating was analyzed by energy dispersive spectrometer (EDS) and X-ray diffraction. The results showed that, under the conditions of temperature of 450°C, line velocity of 300m/s, and feed depth of 500μm, the change in feed rate significantly affected the macro and micro morphology and wear mechanism of the AlSi/hBN sealing coating-TC4 simulated blades. At low feed rates, severe wear occurred, mainly manifested as grooving, adhesion transfer, and overheating mechanisms. At medium to high feed rates, good machinability was observed, mainly manifested as cutting and transfer of coating material to the blades.

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.

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.

Hybrid plasma spraying combines plasma spraying of dry powders and liquids (suspensions and solutions). Combination of these two approaches allows deposition of microstructures consisting of both conventional coarse and ultrafine splats. Moreover, splats with dissimilar size may have also different chemistry. Such combination is potentially interesting for many fields of thermal spraying, including thermal barrier coatings (TBCs), as novel microstructures may be economically and relatively easily obtained. The technology has recently reached a level, where coatings with interesting hybrid microstructures may be reliably deposited, so that their potential for practical applications may be evaluated. In this study, first experimental TBCs with YSZ-based hybrid topcoat were deposited by hybrid water/argon stabilized plasma (WSP-H) technology. Al 2 O 3 and YAG were selected as secondary phase deposited from suspension as both provide strong materials contrast in scanning electron microscope (SEM) so they can be used as “markers” in the coating microstructure. Samples were exposed to thermal cycling simulating in-service TBC conditions in order to test their thermal shock resistance. Changes of the coating microstructure were studied by SEM analysis and X-ray diffraction.

Graphite is useful in high-temperature applications in many engineering fields, such as heat-treating, brazing, and sintering industries. As the operation becomes severe, carbon experiences degradation leading to failure. In this study, a protective coating of W and Mo as the intermediate layer by air plasma spraying on graphite substrate was investigated to find a better intermediate layer. Their performance was explored as a bonding layer in a protective alumina-YSZ ceramic topcoat. X-ray diffraction and scanning electron microscope were used to observe the cross-section of coatings and the difference in the bonding characteristics between W and Mo, respectively. W was found inferior to Mo as a bonding performance over 1450 °C in view of carbide formation against the thermodynamic data. It seems to be related to the formation of a barrier layer as oxide during air plasma spraying.

Atmospheric plasma sprayed (APS) CuNiIn coatings have been widely used for fretting wear protection in many important areas such as aircraft engines for decades. The oxides in CuNiIn coating prepared by APS hinder splat bonding formation and thus degrade the coating fretting performance. In this study, CuNiIn powders of different boron contents were designed to realize the self-oxide-cleaning effect for in-flight molten droplets and thus deposit the dense CuNiIn coating with high fretting performance. Scanning electron microscope was used to characterize the microstructure. The oxygen content in the coating was measured by the inert gas fusion technique. Fretting test was performed for three coatings under different loadings. The results show that CuNiIn2B and CuNiIn4B coatings presented the oxide content of 0.40wt% and 0.38wt%, which are lower than 1.6wt% of the CuNiIn coating. The oxygen content in the CuNiIn4B coating decreased with the increase of spray distance while the oxygen content in CuNiIn coating increased with the increase of the spray distance. Such results clearly reveal the boron in-situ deoxidizing effect of inflight molten droplets. As a result, the dense CuNiIn2B and CuNiIn4B coatings were deposited with oxide-free molten droplets. The test results showed that the fretting wear performance of B-alloyed CuNiIn coatings were increased by a factor over three comparing with conventional CuNiIn coating.

High-pressure cold spraying has shown significant potential in manufacturing metallic composite coatings for a wide range of industrial applications, including wear and corrosion protection. Quasi-crystalline materials, in turn, are promising candidates due to their unique microstructural features. Combining these concepts, metallic composite coatings were generated using high-pressure cold spraying to produce functional and protective coatings. Several spray trials were done to detect the effect of compositions and size of quasi-crystalline feedstock materials mixed with metal powders, Al6061, and stainless steel 316L, on coating microstructure, integrity, and surface properties. A scanning electron microscope was used to examine the microstructure of the feedstock materials and composite coatings. A 3D surface optical profilometer was also used to investigate surface texture. The wettability of the coating surfaces was measured by static water contact angles using a droplet shape analyzer. Cold-sprayed quasi-crystalline composite coatings showed denser and well-integrated deposits with a random distribution of phases across the composite surface, indicating promising structural reliability and hydrophobic behavior.

In order to investigate the potentials to improve the deposition efficiency and to functionalize the polymer-based substrates, six configurations of microparticles Sn, Zn, Al, Sn+Al 2 O 3 , Al+Al 2 O 3 , Cu+Al 2 O 3 were cold sprayed on the substrate of Carbon Fiber Reinforced Polymer (CFRP) composites equipped with Cu-based sublayer or Al-based sublayer. The process conditions were kept unchanged. Microanalysis of sublayers and coatings was performed via a Scanning Electronic Microscope (SEM), the deposition mechanisms of different powders couplings on CFRP substrate were then discussed. The results indicated that although the deposition efficiencies were negative, the systems of Zn, Al and Al+Al 2 O 3 perform better among all the configurations. It was found that the addition of alumina led to a lower deposition efficiency (DE), compared to the corresponding pure coatings. For single-component Sn, Zn and Al powders, they all showed an increasing trend of DE when changing the substrate from Cu-based systems to Al-based systems. The aim of this present work is to elaborate the intrinsic causes of erosion issues and to provide a reference value for picking spraying materials and preparing functionalized CFRP substrates. According to the SEM analysis, the insufficient deformation and escape behaviours of spherical copper powders explained for the difficulty of coating formation. It was noticeable that the surfaces of Al-based systems were more uniform than those of Cu-based ones, due to their desirable deformation abilities. Besides, the significant flattened particles, material mixing and melting phenomenon were observed in Al-involved systems, which would definitely contribute to the adhesive bonding between coating and substrate.

In this study, pure spherical Ta powders made by induced plasma sphero technique were used in the laser cladding process. The powders were sent into high-energy laser zone and were melted at the surface of a steel substrate to create a Ta layer. The microstructure development in the Ta layer was investigated in a scanning electron microscope. The results showed that the layer was basically dense with some pore/crack defects. In the layer, typical dendritic crystalline structures were formed. With the help of an energy dispersive spectroscope, Fe was detected in the Ta layer. The top surface had about 5% Fe while at the bottom of the cladded layer 15% Fe was detected. So, the diffusion of Fe upwards occurred. With the participant of Fe, the microstructure of the Ta layer was changed. Thermocalc software was used to simulate the phase constitution at different Ta-Fe compositions. The results by the simulation basically agreed with the experimental observations.

In this study, pure Al coating was deposited via in-situ shot-peening-assisted cold spray method in order to study the effect of the in-situ tamping effect which was caused by the impact of large sized shot-peening particles on grains size evolution of coatings. The microstructures of the as-sprayed Al coating were observed by using Scanning Electron Microscope and Electron Backscatter Diffraction. A commercial gas atomized Al powder with a grain size range of 10-20 μm was used as the spraying powder. The cross section of the as-sprayed Al particles presented elongated rectangular morphologies, which indicated that the nearly spherical particles experienced severe plastic deformation by the impact of large sized shot-peening particles. It was found that dynamic recrystallization of dislocations-ridden regions was responsible for the grain refinement of cold sprayed coating. Aluminum grains with size of several tens to several hundred of nanometers can be apparently recognized at the whole cross section of the particle. Therefore, in-situ shot-peening-assisted cold spray method can deposit completely nanocrystalline coating using micrometer-grain powder, and thus can be employed to develop high quality coatings of commercial importance.

Thermally sprayed coatings are mechanically bonded to the substrate and present porosities and a lamellar microstructure that make them less attractive in applications requiring high coating toughness and impermeability to gas and liquids, these properties been obtained with more technically advanced overlaying processes. This paper presents the research work carried out to increase the erosion resistance of arc-sprayed coatings containing hard Fe 2 B crystals dispersed in mild and alloyed steel-based matrices. These arc-sprayed coatings were a) heat-treated in furnace up to 1000°C and b) fused with an oxy-acetylene torch. The sprayed specimens were tested in a particle erosion device at the impact angles of 25° and 90°. The evolution of microstructure was done by SEM and wear damage by Time- Domain Optical Coherence Tomography. It was shown that both the heat treatment and fusing processes considerably enhanced the erosion resistance of coatings particularly at the impact angle of 90°. This increase in erosion resistance is attributed to the disappearance of stringers between sprayed lamellae. Liquid phase sintering is the mechanism responsible for the homogenization of arc-sprayed coatings containing Fe 2 B. Grain growth observed in arc-sprayed coatings heat-treated up to 1000°C or fused with an oxyacetylene torch does not have a detrimental effect on erosion resistance.

Cu and Cu-MoS 2 coatings were fabricated by cold gas dynamic spray and the fretting wear performance of the two coatings was compared. A mixture (95 wt.% Cu + 5 wt.% MoS 2 ) was used as feedstock for the composite coating. Coatings were sprayed with identical gas flow conditions on the substrates preheated to approximately 170°C. The cross section of the coatings was analyzed by scanning electron microscope (SEM) and MoS 2 concentration was measured, as well as coating microhardness. Fretting tests were carried out under gross slip conditions in ambient environment. SEM observation on wear scars and counterspheres revealed the development of third bodies, by which the sliding was accommodated. For the Cu-MoS 2 coating, solid lubrication effects in the form of friction drops occurred in early cycles (< 5k), but eventually (> 5k) the coating's friction behavior was similar to the pure Cu coating. Third body morphology and wear of the two coatings were distinctly different, which could largely be attributed to the hardness reduction of the Cu-MoS 2 composite due to poorly bonded interfaces induced by the effect of MoS 2 during particle impact and coating formation.

Solid resin rods including ceramics nanoparticles were fed successfully into a Rokide flame gun to create dense coated layers without micro cracks and pores applying for electric, magnetic and dielectric components. In this investigation, alumina particles of 170 nm in average diameter were dispersed into acrylic liquid resin at 40 % in volume fraction. The paste material was injected into an brass mold of φ4 ~ 200 mm in inner dimension and thermally cured through heating at 120 °C for 60 min. Formed solid rods were fed coaxially into an oxyacetylene gas flame by using the Rokide spraying system. Sprayed particles were collected in a water bath for microstructure observations by a scanning electron microscope and crystal phase analyses by an X-ray diffraction spectroscopy. Fine ceramics layer formations will be discussed systematically by the feeding speed of solid rods and gas flame condition of air pressure and oxygen pressure.

The deposition mechanism of cold spray process has not been fully understood at present. It has been widely accepted that particle velocity prior to impact is one of the most important parameter for cold spray process, and bonding occurs when the impact velocities of particles exceed a critical value. For cold spray, the splat is the basic element of coatings and determines the coating properties, such as porosity, bonding strength. Therefore, the study of splats is helpful to understand the deposition mechanism of cold spray process. In this work, copper powder was utilized to prepare splats on three substrates, aluminum alloy, copper and stainless steel, under different spray conditions. Particle velocities were measured by DPV-2000 system experimentally. The morphologies of copper splats and cross sections were characterized by scanning electron microscope (SEM). In order to control the cross sections of splatted particle passing the center of particle as much as possible, the cross sections were polished by a new method with ion beam. The influences of particle velocity on cold-sprayed splat morphologies and cross-sections were discussed.

Excimer laser annealing provides a rapid and efficient means for surface alloying and modification of ceramic materials. In this study, Alumina-13% Titania coatings were sprayed with a water-stabilized plasma spray gun. The coated surface was treated by Excimer laser having a wavelength of 248 nm and pulse duration of 24 ns. The surface structure of the treated coating was examined by field emission scanning electron microscope and X-ray diffraction (XRD). A detailed analysis of the effects of various laser parameters including laser energy density (fluence), pulse repetition rate (PRR), and number of pulses on the morphology and the microstructure of the coatings are presented.

Titanium oxide coatings were suspension plasma sprayed onto different substrates. The suspension was formulated using fine rutile pigment in the mixture of water with ethanol. The zeta potential of the suspension was determined. The spray process parameters were designed using a full factorial plan using spray distance and torch scan velocity as the variables. The temperature at spray process was monitored using a pyrometer. The X-ray diffraction (XRD) analysis enabled to find out the crystalline phases in sprayed deposits and, in particular, the anatase content. Scanning electron microscope (SEM) enabled to characterize the coatings’ microstructure. The coatings included well molten lamellas and zones of loosely agglomerated and sintered grains. The scratch test of the coatings enabled to determine their mechanical properties such as critical load and scratch hardness.

In this paper, an iron/aluminum composite coating was prepared by cold spraying using iron and aluminum powder mixture and then annealed to aim at forming iron aluminides by suitable annealing treatment. The annealed coating was characterized using X-ray diffraction (XRD) to determine the coating phases and scanning electron microscope (SEM) with an EDXA energy dispersive spectroscopy (EDS) to examine the coating microstructure evolution. Results showed that the Fe 2 Al 5 intermetallic layer along some regions of the aluminum-iron boundaries forms after annealing at a temperature of 450°C, where true metal to metal contact had occurred. The content of Fe 2 Al 5 phase increased with raising annealing temperature. It was observed that some cracks were developed in Fe 2 Al 5 layer after annealing treatment at a high temperature of 600°C.

Agglomerated and sintered Cr 3 C 2 -25%NiCr powders possess excellent flow ability and appearance that have been extensively applied to resist abrase and erosion in high temperature applications such as power boiler and turbine blade. Microstructure of Cr 3 C 2 -25%NiCr coatings were observed through scanning electronic microscope (SEM), and bond strength and microhardness of coatings were measured by tensile shearing test and Vickers hardness test. It is indicated that ultrafine Cr 3 C 2 -25%NiCr coatings have some outstanding properties to traditional Cr 3 C 2 - 25%NiCr coatings by plasma sprayed.

In the oil industry, logging systems involving geological sensors are designed to operate under increasing severe service conditions of deep and horizontal boreholes. Under these conditions, metal matrix composites (MMCs) with ceramic reinforcement are applied on components to achieve wear and corrosion resistant systems. The ‘cold spray’ could be described as a cold and inert process to form coating layers through severe plastic deformation of a ductile metal. Ceramic/metal MMC coating could be achieved by co-deposition of a ceramic with a ductile material. In this work, it was it was investigated the use of MMC B 4 C-Ni coating from both mechanically milled blends or B 4 CNi CVD coated batches. Powder blends involving Ni powder with fine or coarse B 4 C powders were prepared by mechanical milling. Three CVD coated B 4 C-Ni powder batches were synthesized with 30, 40 and 50 Ni wt% respectively. Cold spray coatings were achieved with 1 pass and 5 passes to investigate the building-up mechanisms and interfaces with AISI316L. Powders and cold sprayed coatings microstructures were observed by optical and scanning electron microscopies and further quantitative image analysis were carried out to determine the content of B 4 C embedded in the Ni matrix of B 4 C-Ni cold spray coatings. The highest B 4 C vol.%, up to 45%, could be reached in the case of B 4 C-Ni coated powder. Micro-hardness values of such MMC coatings were also determined through Vickers micro-indentation. The beneficial role of the Ni surrounding layer on coating formation is discussed in relation to the unique features of the microstructures obtained by cold spray of B 4 C-Ni coated powders.