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.

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.

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.

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 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.

In thermal spraying, one of the fundamental elements to achieving good bonding strength of the applied coating is surface preparation. Traditionally grit blasting using hard particles such as corundum is used to achieve suitable roughness on the substrate. Lately, there is an effort to find a suitable alternative from ecological and economical aspects. A promising possibility is laser texturing which enables the preparation of defined structures on the surface. Within a research project, procedures are developed to texture various substrates to direct application of HVOF coatings. The main goal is to achieve speeds of texturing comparable to grit blasting – more than 500 mm 2 /s while ensuring good bonding strength of the applied coating. This study focuses on HVOF spraying of Stellite 6 and WC-CoCr Coating. Selected substrates are steel, and then materials that cannot be traditionally grit blasted – nitrided steel and alumina ceramics. The study presents the analysis of laser textures on substrates, analysis of coating substrate-coating interface, and adhesion tests by tensile test. The most suitable textures – regarding the processing speed and achieved adhesion are selected.

Thermal spraying enables a fast and effective way to additively deposit various ceramics as electric insulators, which are used in conditions where polymers are not suitable. Alumina (Al 2 O 3 ) is among the most widely employed materials in the coating industry because it exhibits good dielectric properties, high hardness, and high melting point, while still being cost-effective. Various parameters (e.g., feedstock type, plasma gas mixture, plasma power) significantly influence the resulting coating in terms of microstructure, porosity, crystallinity, and degree of unmolten and molten particles. As a consequence, these parameters need to be investigated to estimate their impact on the electrical insulating properties of thermally sprayed alumina. This study focuses on the development of a novel electric insulation coating from Al 2 O 3 feedstock powders deposited via atmospheric plasma spray (APS). The microstructure, porosity, and corresponding crystallographic phases have been analyzed with optical microscopy, XRD, and SEM images. To achieve an understanding of the parameters influencing the electrical insulation performance of the manufactured coatings, an in-depth analysis of the fundamental dielectric parameters (e.g., DC resistance, breakdown strength, dielectric loss tangent, and permittivity) is presented.

During the impact and solidification of thermal spray droplets on a substrate, the density increases when the droplet solidifies. Depending on the material, the changes in density could be significant. For example, aluminum oxide's density changes by 66%, while the changes are 12% and 19% for nickel and copper, respectively. For zirconia, this change is 24%. The effect of such densification on the dynamic of the droplet impact and the formation of porosity could be dramatic. In this study, the effect of shrinkage of a molten droplet during solidification on droplet impact is numerically investigated for several materials. Results for the impact of molten alumina, nickel, copper, and zirconia droplets on both smooth and rough surfaces are presented. The results of variable density cases are compared with those assuming constant density. The effect of thermal shrinkage is particularly vital in the interaction of two impacting droplets. The shrinkage promotes the formation of additional pores.

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.

Various alumina-based materials are applied to achieve different electrical insulation properties based on the variation of the material specific relative permittivity. Thermally sprayed mullite (Al 2 O 3 · SiO 2 ) can form an amorphous phase due to the high cooling rates of the process. The formation of amorphous phases causes a change in the capacitive behaviour of the coatings. The tendency to form amorphous areas can be influenced by the composition of the feedstock material or coating parameters. On the one hand, mullite coatings based on two different Al 2 O 3 to SiO 2 ratios are investigated. On the other hand, a parameter variation is used to achieve various particle temperatures during the process. The coatings are investigated via X-ray diffraction and DSC for phase formation, electron microscopy for coating structure and impedance spectroscopy for measuring the AC-resistance. The conducted variation of the feedstock material as well as the parameters causes differences in the XRD and DSC measurements correlating with a difference in the amounts of amorphous phases. For the capacitive behaviour, coatings applied with hydrogen as process gas showed decreased AC-resistance values. The chemical composition of the feedstock material indicates that the AC-resistance decreases with increasing amounts of SiO 2 . In summary, mullite has promising insulation properties which can be modified by the feedstock material composition as well as the coating parameters. For future application, mullite is a promising candidate for increasing the electrical insulation properties in conditions under high electrical and mechanical demands.

In a novel approach for guiding elements of sawing machines wear resistant coatings were applied on open-porous AlSi 7 Mg substrates by means of high velocity suspension flame spraying (HVSFS). The challenge is to establish a wear resistant coating but simultaneously maintain the open-porous structure that is necessary to serve as a permeable structure for liquid cooling or lubricant media under operation. In a first approach, a water-based suspension containing a mixed Al 2 O 3 -TiO 2 powder for HVSFS was used to deposit dense and well adherent mixed-oxide coatings. As the substrates exhibit an open-porous structure and a well-defined pore size distribution, transpiration cooling through the pores is possible and even necessary in order to ensure a low thermal impact on the fragile pore structure, preserving the open-porosity of the substrate. The coatings are characterized and compared by the means of light microscopy and hardness indentation.

Deposition of protective dense environmental barrier layers is a promising solution to improve the reliability and environmental durability of the next-generation turbines and other industrial applications. In this context, spraying of fine particles could enhance the formation of fine dense coating microstructures with improved properties. In AIST we are focusing on the spraying of the fine particles via different spraying technologies including suspension plasma spraying, as well as deposition of the fine solid particles directly by aerosol deposition (AD) and plasma-assisted aerosol deposition (so-called Hybrid Aerosol Deposition; HAD. The HAD is a new coating window to spray the fine ceramic particles via the implementation of a low-power rf-plasma source to assist the aerosol deposition at room temperature. This study introduced the feasibility of utilization of HAD as an outstanding technology for deposition of dense ceramic coatings on different substrate materials and 3D deposition capability. Highly dense and well-adhered Al 2 O 3 coatings without obvious observable cracks and bulk-like properties were successfully fabricated on different substrate materials of SUS 304, Aluminium, Al 2 O 3 and glass, via HAD of fine particles. The substrate material and its hardness significantly influenced the first deposition step, which determined the coating adhesion and properties. Furthermore, homogeneously uniform, dense, and crack-free coating with a strong adhesion has been fabricated successfully on cylindrical substrates with 6.3 mm diameter. During HAD spraying the plasma activated the surface of the particles without reaching to the molten state, then the activated particles impact and stuck with the substrate by room temperature impact consolidation mechanism. Therefore, the fabricated coatings had the same crystal structure as the starting feedstock powder, and the activated surface act as glue and improved the deposition efficiency and 3D capabilities. Herein, the deposition phenomena of HAD makes it as a promising candidate technology for development of environmental and sealing layers of highly dense microstructure, with the targeted crystalline phase structure, without stoichiometric composition nor phase transformation and improved deposition efficiency on multi-shape components in different fields such as environmental, thermal barrier coatings (TBCs), environmental barrier coatings (EBCs) and gas turbine applications.