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

In this work, thermally sprayed sustainable coatings with spray additives recycled from dry alkaline batteries and solid-oxide fuel cells are developed to allow the growth of drought-resistant plants like moss, microclover and chamomile. It is assumed that these plants anchor to the coating with their rhizoids and hence can be grown without the presence of soil. Preliminary tests of a thermally sprayed Yttrium Stabilized Zirconia (YSZ) ceramic coating on sheet metal confirms the growth of chamomile plant.

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.

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 ingestion of siliceous particulate debris into the gas turbine engines during operation caused the deposition of so-called CMAS (calcium-magnesium-alumino-silicate) on the hotter thermal barrier coating (TBC) surfaces. The penetration of these particles into the TBC at temperatures above 1200°C caused the loss of strain tolerance and premature failure of the TBCs. To mimic real-world conditions, a commercially available CMAS precursor dust powder was sprayed onto 8YSZ coatings using an atmospheric plasma spraying process. The substrate temperature was maintained at an average of 1100°C and 525°C during spraying. The effect of the spraying parameters on the deposition, microstructure, and composition of the CMAS coatings was investigated. In addition, to understand the CMAS build-up on the high-temperature surfaces, the CMAS splat formation behavior was also analyzed on the polished samples at temperatures ~1100°C. SEM/EDS analyzes were performed to identify and quantify the elements of the CMAS deposits. It was found that the surface temperature, deposition time, and different nozzles could play a significant role in having different phases of CMAS deposits.

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.

One of the promising thermal barrier coatings (TBC) options for use above 1250 °C has been La 2 Ce 2 O 7 (LC). This work explored the role of dual layered ceramic coatings in the top layer of the TBC system that has been prepared using atmospheric plasma spraying (APS). Above the NiCrAlY bond coat, 8 mol.% yttria stabilized zirconia (8YSZ) coating has been deposited with optimized APS parameters. Over the top layer (8YSZ), another layer that comprises composite with LC and 8 wt.% of 8YSZ (spray dried) has been deposited. Investigations into the hot-corrosion behavior of 8YSZ-LC based TBC subjected to Na 2 SO 4 +V 2 O 5 salt at 950 °C for 4 hours. A porous layer made mostly of LaVO 4 , CeO 2 , CeO 1.66 and YVO 4 was developed on the LC+8wt.% YSZ layer after being subjected to a hot corrosion test in Na 2 SO 4 +V 2 O 5 salt. Dissociation of LC and 8YSZ leads to the formation of new phases, such as CeO 1.66 , CeO 2 , LaVO 4 and YVO 4 as the corrosion by-products in the extreme environment. The findings indicated that delamination has occurred due to the phase transformation, cavities and cracks in the 8YSZ-LC based TBCs. The molten salt's hot corrosion mechanisms of the 8YSZ-LC based TBC are discussed in detail. Further, the potential use of 8YSZ-LC based dual coatings and scope for the future work have been derived from the current study.

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.

Coating adhesion by thermal spraying method requires sufficient surface roughness on particle scale particles impacting the surface, particularly in the case of plasma spraying with particle melting state. Grit blasting process is mainly used to create the fine asperities required for spread particles to adhere. To further increase adhesion, the use of laser texturing for metallic substrates is benefit and is already well documented in literature. In the case of ceramic substrates such as alumina, grit blasting with corundum particles is no longer effective in creating a roughness of a few micrometers. Laser texturing therefore appears to be a potential candidate for generating adhesion in coatings. In this work, adhesion mechanisms of three different coatings produced by Atmospheric Plasma Spraying (APS) on a textured alumina substrate were investigated. The influence of substrate surface texturing by two different laser methods, a pulsed nanosecond laser and a continuous laser, was studied. YSZ was chosen as a potential Thermal Barrier Coating (TBC) and Al 2 O 3 and Y 2 O 3 were selected as bondcoats to observe the variation of adhesion mechanisms on ceramic substrates. Textured patterns and coating microstructures were observed by numerical and electron microscopy. Different adhesion mechanisms occurred depending on coating material. Either the geometrical parameters of the pattern and the surface roughness developed by a nanosecond laser and a continuous laser respectively, can promote mechanical anchoring and thus, a real adhesion.

Hybrid plasma spraying can be utilized to deposit novel coating microstructures by combining the simultaneous injection of a dry coarse powder and a liquid feedstock into the plasma jet. Using this approach, the coating microstructure contains both coarse powder-made splats and a dispersion of fine liquid-made splats. Furthermore, the so-called external feeding hybrid method allows the incorporation of fine particles of materials susceptible to decomposition at high temperatures thanks to the by-passing of the hot plasma jet and deposition of the temperature-sensitive material directly onto the coated surface from a suspension. In this study, microstructures of ceramic coatings with embedded self-fluxing sulfides were studied and the wear resistance of the system was evaluated using the dry sliding pin-on-disc method.

Sensors to measure gas velocities in high temperature flows need to be robust, low-profile so that they do not obstruct the flow, and easy to apply on metal surfaces. Thermal spray offers a method of making low-cost sensors that can be applied on large areas. Plasma spray was used to deposit an electrically insulating layer of alumina on a 316 stainless steel block. A 17 mm diameter heater coil was deposited on top of the alumina layer by spraying Nichrome from a twin wire arc spray system through a 3D printed polymer mask. A thermocouple junction was built next to the heater by inserting an insulated Constantan wire through a vertical hole drilled in the steel block and spraying steel on the top of the hole to close it and form an electrical connection between the wire and the surrounding substrate. The junction of the wire and the steel formed a thermocouple whose output voltage was calibrated. A flow loop was built to calibrate the sensor by passing air over it at velocities of up to 5 m/s. A series of 2 min long voltage pulses were applied to the heater, increasing its temperature by approximately 5°-10°C each time, before letting it cool. A calibration curve was developed of the air velocity as a function of the time constant for cooling of the sensor.

In plasma spraying, H2 or N2 is commonly added to the primary Ar plasma which may increase the specific enthalpy, thermal conductivity and thus improve the process efficiency. The objective of this study is to provide a process characterization of a three-cathode plasma torch with various binary gas compositions. Several process diagnostics are used to characterize the impact of binary plasma gas mixtures in plasma spraying. High-speed video analysis is utilized to capture the jet fluctuations of the studied process parameters. In addition, current and voltage measurements are performed to further complement the plasma diagnostics. The impact of the binary plasma gas mixtures is determined using particle diagnostic system DPV-2000 by measuring the particle in-flight properties of Al 2 O 3 feedstock. Furthermore, the deposition efficiency (DE) of the investigated process parameters is determined. The results show that at the identical volumetric flow rate and current, the addition of H2 yields the highest particle temperatures, followed by Ar/N2 mixtures and pure Ar plasma. In reverse order, pure Ar plasma results in the highest particle velocities. In addition, the increased DE of plasma spraying with binary gas mixtures for Al 2 O 3 coatings offers the potential to increase the deposition rate of other ceramic materials. This study provides a comprehensive correlation between plasma and particle diagnostics and the deposition efficiency of binary plasma gas mixtures.

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.

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.

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.

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.

In this study, Al 2 O 3 -based coatings with varying TiO 2 contents (0, 3, 13, and 40%) were fabricated using atmospheric plasma spraying technique. To compare the superiority of the samples, their thermal properties (thermal conductivity and thermal shock resistance) were characterized. As observed, Al 2 O 3 - 40%TiO 2 (A-40T) coating exhibited relatively superior thermal insulation and thermal shock resistance at 600°C. According to the microstructure and phase analysis, this finding can be attributed to the special phase, Al 2 TiO 5 , and the pre-existing microcracks inside the coating. Thus, A-40T manifested excellent characteristics for thermal insulation application compared with pure Al 2 O 3 and low-TiO 2 content coatings.

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.

Hybrid aerosol deposition (HAD) has been proposed recently as a new coating regime to deposit homogeneous ceramic coatings via the utilization of mesoplasma and solid particle deposition. This study will discuss the implementation of HAD for the deposition of alumina (Al 2 O 3 ) coatings on 304 stainless steel and aluminum substrates, and evaluation of the hardness and Young’s modulus using a nanoindentation method to clarify the through-thickness properties. Dense and uniform coatings with a nanocrystalline structure were fabricated on both substrate materials. The fabricated HAD coatings consisted of α-Al 2 O 3 phase. The hardness and Young’s modulus distributions along the through-thickness direction showed a significant difference across the coating-substrate interface and tended to show a slight decrease by 10-15% as the measured position went close the surface. Increasing the hardness and Young’s modulus on the substrate side near the interface is presumably related to the peeing effect of the substrate as well as the increase of interface roughness during the room temperature impact consolidation (RTIC) and deformation of the hard ceramic particles on the substrate. The decrease in the coating’s mechanical properties along the through-thickness direction is considered to be related to the particle deformation tendency during the coating build-up. At the beginning stage of the deposition, initial particles are impacting on a metallic substrate which is ductile enough to facile plastic deformation and the deposited layer can have an enough hammering effect by the subsequent impacting particles. The hardness and Young’s modulus in this location are 15.6 GPa and 246 GPa, respectively, and the highest through the thickness in case of the stainless steel substrate. However, the later particles are impacting on a hard ceramic surface (initially formed HAD Al 2 O 3 layers), which hardly undergo plastic deformation or led to less particle deformation. In addition, through-thickness measurements revealed that the deposited coatings on the stainless steel substrate showed higher hardness than deposited coatings on aluminum substrates. Thus, the stainless steel enhances the degree of deformation of the deposited particles, and the resulted smaller crystallite size and strain lead to increased hardness and modulus.