In this study, the influence of spray parameters on the electrical resistivity of thermally sprayed ceramic coatings from the system Cr 2 O 3 -TiO 2 was investigated. Fused and crushed feedstock powders with contents of 10 wt. % and 20 wt. % chromium oxide were deposited by APS and HVOF. Temperature and velocity of the particles in the spray jet as well as the coating surface temperature were analyzed during the deposition process. Impedance spectroscopy was used to investigate the electrical resistivity of the coatings and the results were correlated to coating microstructure and phase composition. It was found that phase transformations occur during the spray process. In the coatings a high temperature phase (n-phase) and rutile were observed. Though, the ratio of rutile depends on the spray methods employed for coating deposition. The electrical resistivity of coatings obtained by HVOF can be correlated to the content of chromium oxide. Furthermore, the surface temperature of the coating during deposition also shows some influence. Concerning the coatings resulting from APS, the different mixtures of the plasma gases (Ar-H 2 and Ar-N 2 ) are supposed to have the most important influence on the electric resistivity.

This study investigates the microstructure and efficiency of coating-based heating elements produced by deposition of various powders, including aluminum oxide (Al 2 O 3 ), alumina-titania (Al 2 O 3 -TiO 2 ), nickel-chromium (NiCr), and copper, using flame spraying, suspension plasma spraying, high-velocity oxyfuel (HVOF) spraying, and cold spraying techniques. The main goals are to assess the dielectric strength of flame and plasma sprayed alumina, compare the electrical resistivity of HVOF and flame sprayed NiCr, and obtain coating cross-sectional images to shed light on the challenges and potential of different heating element designs. The Al 2 O 3 layer produced by suspension plasma spraying appeared to be more reliable due to its cauliflower-like structure, corundum content, and hygroscopic properties. Resistivity was found to be higher in the flame sprayed NiCr than in the HVOF deposit mainly due to discontinuities and imperfections such as cracks, pores, and oxygen content. The micrographs taken from sample cross-sections show penetration of flame-sprayed NiCr into the flame-sprayed Al 2 O 3 and Al 2 O 3 -TiO 2 layers, which decreases the effective thickness of the dielectric. However, interlocking between NiCr and Al 2 O 3 -TiO 2 coatings can be beneficial when cohesion is a concern.

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

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.

A method for evaluating the adhesion of metallic thermally sprayed coatings by measuring the electrical resistance of the coating sprayed on a metal substrate was carried out. The thermal sprayed coatings were made of zinc alloy on carbon steel substrates. The electrical resistance levels between the substrates and coatings were evaluated. The electrical resistance increases with increasing measurement time. The larger the rate of increase of the electrical resistance, the lower the adhesive strength. There is a possibility to evaluate qualitatively the adhesion properties.

In this paper the characteristics (microstructure, phase compositions) and electrical insulating properties of thermally sprayed alumina coatings produced by suspension-HVOF (S-HVOF) process and conventional HVOF spray method are compared. The electrical resistance (electrical resistivity) and dielectric strength were investigated using DC-electrical resistance measurements, electrochemical impedance spectroscopy (EIS) and dielectric breakdown test. The electrical resistance was determined at room temperature at different relative air humidity (RH) levels, from 6% RH up to 97% RH. Differences in the electrical insulating properties due to the different coating characteristics are discussed. The suspension-sprayed Al 2 O 3 coatings showed better electrical resistance stability at high humidity levels (> 75% RH), which could be explained by a specific microstructure and retention of a higher content of α-Al 2 O 3 . Nonetheless, the values of dielectric breakdown voltage and dielectric strength recorded for suspension sprayed coatings were lower than those of HVOF coatings.

In the present study, Cu coating was deposited by cold spraying and the electrical resistivity of the coating in both directions parallel and perpendicular to the coating surface was measured to investigate the anisotropy of the coating. Annealing treatment was applied to the coating to examine its effect on the microstructure and properties of cold-sprayed Cu coating. The examination of coating microstructure evidently revealed that the coating was constituted by flattened particles and the interfaces were clearly observed between the deposited particles. The anisotropy in microstructure and electrical resistivity was present in cold-sprayed Cu coating. The electrical resistivity of the as-sprayed coating was higher than that of Cu bulk. Moreover, the electrical resistivity along the direction parallel to the coating surface was lower than that along the perpendicular direction. On the other hand, it was found that the annealing led to the enhancement of particle interface bonding and evident recrystallization of the elongated grains and remarkable grain growth as well. The annealed coating presented an equiaxed grain structures similar to annealed Cu bulk with particle interfaces almost disappeared under certain annealing condition. The coalescence of voids or oxides in the coating was clearly observed at high annealing temperature. Moreover, the annealed coating yielded an electrical resistivity and microhardness comparable to Cu bulk.

In the present work, APS and HVOF processes have been used to prepare alumina (Al 2 O 3 ) and magnesium aluminate spinel (MgAl 2 O 4 ) coatings designed for electrical insulating applications. The microstructures and the phase compositions of the sprayed coatings were evaluated by microscopic and XRD analysis. The electrical characteristics electrical resistance, electrical resistivity and dielectric breakdown strength were investigated using different methods: direct current (DC) measurements, electrochemical impedance spectroscopy (EIS) and dielectric breakdown testing. The electrical resistance was measured at room temperature at different humidity levels. Differences in the insulating properties due to the different natures of the coating materials, microstructures and the measurement methods used for electrical characterisation are discussed.

By means of In-Mold-Metal-Spraying (IMMS), wire arc sprayed metal coatings are transferred onto plastic parts during the injection molding process for the efficient production of metallized plastic parts. One potential field of application of IMMS parts are electrical applications such as electrically conductive tracks or electromagnetic shielding. In the current study, the properties of the transferred coatings, especially the electrical resistivity, are determined. Different feedstock materials are used for the application of the coatings. In the first investigation, pressurized air is used as atomizing gas for wire arc spraying. In contrary to Zn coatings, Cu coatings applied with pressurized air have a significantly higher electrical resistivity in comparison to massive copper. One possible reason for this is the oxidation of the Cu particles during the spraying process. Therefore, N 2 and a mixture of N 2 and H 2 are used as atomizing gas to reduce the oxidation of particles. Consequently, the electrical resistivity of IMMS parts can be significantly reduced. Furthermore, spraying distance, current and pressure of the atomizing gas are varied to investigate the influence of these process parameters on the coating properties.

An efficient temperature control on tool surfaces is essential in processes like injection moulding or die casting. A thermally sprayed heating coating could combine dynamic heating properties with a small assembly space as it is sprayed directly onto the cavity surface. With their intrinsically high electrical resistivity and low thermal expansion as compared with traditional alloys, High Entropy Alloys (HEA) show promising properties for the use as heating elements. Thus, the well-studied HEA Al 0.5 CoCrFeNi was used as a starting material for additional alloying with Zr and Si to force further lattice distortion in the solid solution. HEAs of differing compositions were melted and characterized. In the process, the potential of HEAs was assessed by characterizing their phase composition, thermal stability, and electrical resistivity. The characterized HEAs show a solid solution mainly consisting of fcc and bcc structure. Moreover, the composition Al 0.5 CoCrFeNiZr 0.2 Si 0.2 was determined as stable after heat treatment at 600 °C for 324 h. In addition, the electrical resistivity was raised by over 20 % relative to the starting material. As a result, a hitherto unknown HEA composition was detected to possess superior properties to traditional alloys for the application as heating coating.

Titanium oxide is established as an oxide material for thermally sprayed coating solutions and has received increasing interest over the last few years. Scientific and technological research focuses on electrical conductivity, solid lubrication and photocatalytic properties of titanium oxide (TiOx) coatings of differing stoichiometry. The aim of this study was to investigate the effect of oxygen loss by reduction with hydrogen occurring in the conditions of vacuum plasma spraying (VPS) from commercial titanium oxide feedstock on coating microstructure, hardness, phase composition, abrasion wear resistance and electrical resistivity. The X-ray diffraction pattern showed the presence of rutile, with peaks decreasing in intensity with increasing hydrogen content in the plasma-forming gas. The intensities of the peaks showed significant deviations from those of the standard. An increase in hydrogen flow rate did not influence the coating microstructure, hardness or abrasion wear resistance, but it caused the electrical resistivity to decrease. VPS coatings prepared from commercial fused and crushed powder show a resistivity in the range of 0.01-0.1 Ohm*cm, which corresponds exactly to the range published in the literature. Comparison with results for APS- and HVOFsprayed coatings reveals that VPS coatings yield the best combination of abrasion wear resistance and electrical resistivity when commercial titanium oxide spray powder is used.

In this work, completely ceramic heating elements have been developed by the combination of conductive and insulating thermally sprayed oxide coatings. These heating elements with a total thickness of less than 1 mm have been directly applied on metallic substrates. APS- and HVOF-sprayed Al 2 O 3 and spinel (MgAl 2 O 4 ) coatings were employed for insulation. A comparative analysis of the insulating properties (dielectric strength, electrical resistivity) of these coatings is presented. The HVOF-sprayed spinel coatings show better dielectric breakdown strength and higher electrical resistance stability. TiO x , TiO 2 -10%Cr 2 O 3 and TiO 2 -20%Cr 2 O 3 powders have been used to prepare the electrical conductive coatings. The thermal and oxidation stabilities at high temperature, as well the electrical properties have been investigated. Addition of Cr 2 O 3 reduced the oxidation rate of titanium oxide and increased the operational temperature of the heating coating. A ceramic heater consisting of spinel coating as insulator and TiO 2 - 20Cr 2 O 3 as conductor was sprayed on a metallic roller and the electrical stability during the long-term (300h) thermo-cycling (from RT to 300°C) was successfully tested.

Copper coatings were deposited on ferrous substrates by utilizing high-velocity oxy-fuel spraying (HVOF). Three different coating process parameters have been used in order to optimise the required electrical characteristics. Microstructure and phase formation in the coatings were analyzed by scanning electron microscope (SEM), X-ray diffraction (XRD), and oxygen analyzer (ELTRA). Electrical resistivity of coatings was measured in-plane and through-thickness using the four-contact method. Results shown that dense coatings with high purity and low level of porosity are required to achieve high electrical conductivity. The coatings exhibited an anisotropic electrical resistivity due to the nature of the thermal spray coating microstructure. Microstructural evaluation shown that individual splats morphology and splats interfaces play important roles in the electrical conductivity.

In order to meet the increased requirements for power electronics in the automotive sector, an effective utilization of difficult installation spaces is necessary. A new production concept to realize this 3D integration of electronic circuit boards directly on components is the combination of thermal spraying and cold gas spraying to create multilayer-coating systems consisting of conducting and insulating coatings. In this study two- and tree-dimensional demonstrators were developed, showing the flexible use of thermal spraying in mechatronics and power electronics. In contrast to past studies on this construction concept, the main focus of this study was on the optimization of the ceramic insulting coatings and bond strength of the metallization. The ceramic coatings showed a dielectric strength and electrical resistance, which was suitable for most applications. Additional post treatment improved the electrical resistance in humid conditions. Already 150 µm thick electrical insulation layers showed a breakdown voltage of more than 5 kV AC and a specific electrical resistance of 5.1011 Ω.m.

The influence of flame spraying parameters on coating microstructure and electrical conductivity of aluminum- 12silicon coatings deposited on polyurethane substrates was studied. In order to evaluate the effect of the spray parameters on temperature distribution and its corresponding effect on coating characteristics, an analytical model based on a Green’s function approach was employed. It was found that the addition of air to the flame decreased the temperature within the substrate. Dynamic mechanical analysis (DMA) of the PU substrate revealed that the PU softened as the temperature increased. Therefore, by increasing the pressure of the air injected into the flame spray torch from 35 kPa to 69 kPa, the particles impacted a stiffer substrate. This led to increased deformation of the particles into splats upon impact, improved interlocking, and the overall coating had lower porosity and lower electrical resistance. The results obtained indicated that coating properties are sensitive to both thermal spraying parameters and temperature distribution within the substrate when depositing on elastomeric materials. The effect of torch stand-off distance on coating properties was also evaluated. It was found that higher air pressure can cool the substrate and, therefore, allow for a decrease of the stand-off distance. As a result of shorter stand-off distances, a coating with lower porosity and electrical resistance was deposited.

In a fusion reactor (i.e. ITER), the use of a ceramic coating on structural material (SS3I6LN-IG) has been considered as electrical insulator. Al 2 O 3 including TiO 2 (Al 2 O 3 -TiO 2 ) is one of most promising materials as coating from a point of high electrical resistivity and so on. However, crack and peeling occur by difference of thermal expansion between substrate material and coating material. Therefore, 80Ni-20Cr was selected as the undercoating between SS316LN-IG substrate and Al 2 O 3 -TiO 2 coating. In this characterization, Al 2 O 3 -3%TiO 2 and Al 2 O 3 -13%TiO 2 were used as ceramic coating material. 80Ni-20Cr undercoating was fabricated by atmospheric plasma spray method. The thickness of 80Ni-20Cr was 50µm. Al 2 O 3 -3%TiO 2 and Al 2 O 3 -13%TiO 2 were fabricated by atmospheric plasma spray method. Thickness of these ceramic coatings are 200 and 500µm. From the results of out-of-pile test, it was clear that Al 2 O 3 -3%TiO 2 and Al 2 O 3 -13%TiO 2 coating had a good mechanical and electrical properties.

Electrical properties of plasma-sprayed aluminum oxide coatings were measured at temperatures up to 600 °C. High purity (>99.5 wt% pure Al 2 O 3 ) alumina powders were plasma-sprayed on stainless steel substrates over a range of power levels, using two gun configurations designed to attain different spray velocities. Key electrical properties were measured to evaluate the resultant coatings as potential insulating materials for electrostatic chucks (ESCs) being developed for semiconductor manufacturing. Electrical resistivity of all coatings was measured under vacuum upon heating and cooling over a temperature range of 20 to 600 °C. Dielectric constants were also measured under the same test conditions. X-ray diffraction was performed to examine phase formation in the coatings. Results show the importance of powder composition and careful selection and control of spray conditions for optimizing electrical behavior in plasma-sprayed aluminum oxide, and point to the need for further studies to characterize the relationship between high temperature electrical properties, measured plasma-spray variables, and specific microstructural and compositional coating features.

The plasma spray process is used to create titanium oxide coatings under the current stoichiometry of titania and titanium suboxides. This study used feedstock powder with Magnéli phases TinO 2n-1 , slightly reduced titania TiO 2-x , and rutile. A factorial design of experiments approach was used to better understand the influence of operational parameters on coating quality, in particular, the electric resistivity and the degree of oxidation of the titanium oxide during the spraying. Firstly, arc current intensity and stand-off distance were studied; the results show strong correlations between particle temperatures and the electric resistivity of the coating. Then, different plasma compositions were used in order to understand the influence of hydrogen in the formation of titanium sub-oxides. The hardness of the most significant coatings was analyzed.

In this work two different thermal spray techniques were used to deposit La 0.9 Sr 0.1 CrO 3 interconnect material: high velocity oxy-fuel (HVOF) using a modified nozzle and atmospheric plasma spray (APS). Two different APS torches were employed: A commercial torch that uses Ar/H 2 as plasma forming gases and a new torch design that uses CO 2 gas mixtures. A substitute powder with similar physical properties to La 0.9 Sr 0.1 CrO 3 was employed for the development and optimization of the process parameters to achieve the highest density before the deposition of the La 0.9 Sr 0.1 CrO 3 on zirconium oxide substrates. The microstructures observed by scanning electron microscopy (SEM) and the phase composition of the coatings obtained from X-ray diffraction analysis are correlated to the spraying characteristics of the different techniques employed. The electrical resistivity of the as-sprayed coatings is discussed in terms of microstructure features and the phase composition. Post-deposition heat treatments were studied in order to reduce the electrical resistivity.

Relationships between the electrical properties of thermally sprayed titania coatings and their microstructure have been investigated. As far as possible, a broad range of microstructures was produced by using various processes of plasma spraying with different powder size ranges and variations of the plasma operating parameters. The two spraying processes consisted of DC plasma spraying and RF plasma spraying. Physical properties of plasma-sprayed coatings are generally influenced by their microstructure. But the electrical properties of plasma-sprayed titania coatings are known to be strongly influenced by their stoichiometry. It is the reason why coatings with identical stoichiometry were compared. It was found that electrical resistivity was directly linked to the quality of the contact between the splats and their density through the titania plasma-sprayed coatings.