In order to avoid ice accretion on structures that are exposed to cold environments, nickel-chromium-aluminum-yttrium (NiCrAlY) and nickel-20 wt.% chromium (Ni-20Cr) coatings have been deposited on fiber-reinforced polymer composite plates by using flame spraying. Electrical current was supplied to the coatings to increase the substrate temperature by way of Joule heating. The coatings were assessed under free and forced convection conditions at -25°C and 23°C. The electrical resistance of the coating was estimated at different temperatures. At ambient temperatures below 0°C, the temperature on the coating surface remained above 0°C for both the forced and free convection conditions. A nearly homogeneous temperature distribution over the coating surface was observed. The coating materials were found to be Ohmic and their resistance was weakly dependent on temperature. The results suggest that the coating systems may also be used in anti- and de- icing systems.

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.

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.

This study investigates the effect of incorporating different reinforcing particles on the microstructure, electrical resistance, and heating efficiency of flame-sprayed nickel-based coatings. Feedstock powders were prepared by adding Al 2 O 3 , TiO 2 , and WC particles to NiCrAlY powder, and the various combinations were applied to alumina-coated carbon steel substrates. A number of Joule heating experiments were conducted by creating voltage differences across the coatings and measuring temperature changes due to induced electron flow and associated resistive heating. It was found that the electrical properties of the ceramic particles have a major effect on heat generation and that there is considerable room for improvement.

In this study, NiCr alloy coatings were deposited by arc spraying using different combinations and mixtures of pressurizing gases and other process modifications. Coating properties were examined mainly by SEM, EDS, and conductivity measurements. The results show significantly reduced oxygen contents and improved coating morphologies compared to reference coatings produced using current plasma processes. Improved microstructure is shown to have a positive effect on surface quality and specific resistivity, making it possible to texture arc-sprayed coatings just as successfully as the plasma-sprayed reference layers. Moreover, the temperature coefficients and resistivities of arc-sprayed NiCr were found to be superior to those of conventionally manufactured coatings.

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.

In recent years, laser-based post-processing of thermally sprayed coatings has gained significant attention as an alternative post-processing route; to mitigate the microstructural defects such as pores, microcracks, and splat boundaries associated with thermally sprayed coatings. Optimisation of the parameters for the laser post-processing is of paramount importance to maintain the required properties of these coatings. The current thermo-mechanical model simulates the impact of laser heat treatment on thermally sprayed Tungsten Carbide Cobalt (WC-17Co) coating and AISI 316L as substrate. A sequentially coupled transient thermal and structural analysis is performed. Transient temperature field from thermal analysis due to laser source will become input loads for the subsequent stress-strain analysis with appropriate boundary conditions. Both the coating and substrate are given temperature-dependent material properties. A gaussian heat flux distribution is used to model the laser source. The finite element analysis results underline the importance of temperature gradients and the presence of thermally induced stress-strain fields responsible for promoting coating degradation. The obtained results also revealed that heat input and dimensional characteristics play a vital role in the annealing treatment's efficacy. Three separate test cases were considered wherein the hatch spacing was varied, keeping the other parameters (scan speed, laser power, and laser spot diameter) constant. The impact of hatch spacing on the temperature and residual stress distribution across the coating was assessed by this simulation. Residual compressive stress was observed in the coating for two out of the three test cases, which further improved the durability of the coating.

A multi-layered thermal-sprayed coating system, developed as a resistive heating system, was deposited on a carbon steel pipe. The feasibility of using a 50Cr-50Ni coating as a heating element on top of a conductive substrate was studied. Alumina was deposited to serve as an electrically insulating layer between the metal coating and the substrate to restrict the flow of electrons from the metal alloy heating element to the steel substrate. Continuity, homogeneity, and adhesion of the coating were qualitatively analyzed by studying scanning electron microscope images. The performance of the heating system was determined by measuring the ice temperature and the times required to heat and melt the solid ice that was formed within the pipe. It was found that the coating system was able to generate the heat required to melt the ice in the pipe, thus avoiding the detrimental effects on the pipe of internal liquid freezing. This suggests that the proposed novel resistive heating system can be used on an industrial scale to mitigate or avoid the detrimental effects of ice accumulation in steel and other metallic pipes.

Many processes and systems require hot surfaces. These are usually heated using electrical elements located in their vicinity. However, this solution is subject to intrinsic limitations associated with heating element geometry and physical location. Thermally spraying electrical elements directly on surfaces can overcome these limitations by tailoring the geometry of the heating element to the application. Moreover, the element heat transfer is maximized by eliminating the air gap between the heater and the surface to be heated. This paper is aimed at modeling and characterizing resistive heaters sprayed on metallic substrates. Heaters were fabricated using a plasma-sprayed alumina dielectric insulator and a wire flame sprayed iron-based alloy resistive element. Samples were energized and kept at a constant temperature of 425°C for up to four months. SEM cross-section observations revealed the formation of cracks at very specific locations in the alumina layer after thermal use. Finite element modeling shows that these cracks originate from high local thermal stresses and can be predicted according to the considered geometry. The simulation model was refined using experimental parameters obtained by several techniques such as: emissivity and time-dependent temperature profile (infra-red camera), resistivity (four probe technique), thermal diffusivity (laser flash method) and mechanical properties (micro and nanoindentation). The influence of the alumina thickness and the substrate material on crack formation was evaluated.

The use of suspensions in the thermal spraying process, makes it possible to apply sufficiently thin (<30 μm), metallic coatings made of nickel-chromium alloy 2.4869 (NiCr8020). High velocity oxy-fuel suspension flame spraying (HVSFS) is used to manufacture these thin metallic coatings in order to be able to effectively use them as electric panel heaters. Area heating capacities of 25 W cm -2 are possible with them and heating rates of 15 K s -1 even outperform many ceramic heating elements. In addition, it provides a flexible way to apply the heating coatings directly to the components to be heated. The use of fine powders in the micron and sub-micron ranges allows a more precise adjustment of the coating thickness, compared to conventional thermal spraying techniques, even in the thickness range below 10 μm. Therefore, an adaption to customer needs is possible regarding the electric panel heater characteristics, like electric resistance, applied voltages and current range and heating rates.

The economic feasibility of using thermal-sprayed heat generating coatings for temperature control in steel pipes was investigated. A data-intensive model was developed to compare fabrication, installation, operation, and maintenance expenditures with those of conventional heating cables. The multi-layered coating consists of flame-sprayed Al 2 O 3 and NiCr layers and cold-sprayed copper. Scalability factors were incorporated in the model to estimate the total projected costs for fabricating the coatings as opposed to installing heat tracing. Although material costs for the coating and heat tracing were approximately the same, the cost of fabrication for the coating was higher due mainly to labor expenses. However, the coating-based system was found to be more energy efficient than heat tracing due to the good adhesion and reduced thermal contact resistance between the heating elements and pipe.

Boundary layers on surfaces will change from laminar to turbulent flow after a critical length. Due to the differing heat transfer coefficients of laminar and turbulent flow, the point of transition can be detected by heating the surface and measuring surface temperature by thermographic imaging. Locating the transition point is crucial for the aerodynamic optimization of components. In this study, fiber reinforced polymer composites (FRPCs) were chosen as the test substrate. Experiments were conducted using the flame spray process and NiCrAlY coatings. Multilayered coatings consisting of an aluminum bond coat, a layer of alumina as electrical insulation, and a heating layer of titania were fabricated by atmospheric plasma spraying. Free-flight tests were conducted with a functionalized winglet in order to assess the ability of thermally-sprayed heating elements to detect the location of transition of the flow regime. The results showed that the thermally-sprayed elements heat surfaces uniformly, with sufficient radiation losses for thermographic imaging. It was also shown that the change in temperature at the point of transition was readily observable using thermography.

Developing effective heating systems to prevent ice accretion on the surface of wind turbine blades and aircraft wings is of great significance for extreme cold environments. However, due to high velocity impingement of water droplets and solid particles on the surface of these components, an appreciable degree of surface material degradation may occur. In this study, nickel-chromium-aluminum-yttrium (NiCrAlY) was chosen as a metal matrix material for a coating-based heating system. Pure ceramic powders, namely, alumina and titania, and a cermet powder, tungsten carbide-cobalt (WC-12Co), were mechanically admixed with NiCrAlY powder and deposited to fabricate reinforced metal matrix composite (MMC) coatings. The powders were deposited on cylindrical low carbon steel bars by using flame spraying. The specimens were placed in a wind tunnel to conduct a comparative investigation of their erosive wear resistance under water droplet impact. A cold spraying unit was used for solid particle impact erosion tests. The erosive wear rates were quantified by measuring mass loss. The experimentally obtained results showed noticeably lower wear rate in NiCrAlY-WC-12Co and NiCrAlY-titania coatings compared to the other coatings. The results suggest that certain MMC coatings could be effectively employed to decrease the erosion rate of coating-based heating elements.

Coating operations over glass ceramic substrates represent a new field for thermal spray applications. Due to the unique thermal and mechanical properties of glass ceramics, especially the low or even negative CTE, coating processes must be adapted to reduce the distribution of thermal stresses in the system and to not damage the substrate. This study investigates the deposition of a complex-shaped ceramic-metallic multilayer coating system that could potentially serve as a heating element in a glass ceramic cooking plate. To ensure coating adhesion, the substrates are preheated and their surfaces are grit blasted. In order to minimize stresses associated with the deposition of metal, the movement of the spraying mechanism was automated with robot control and new masking concepts were developed to ensure the accuracy of the shape and placement of the coating. The influence of spraying parameters on coating properties and residual stress distribution is analyzed as well.

A new coating process called polymer thermal spraying (PTS) was recently developed to accommodate the deposition of heat sensitive polymeric materials over a broad range of substrates. The novel process uses an electro-resistive element to heat the main process gas, which could be air, any pure gas, or gas mixture. This paper describes the process and presents three case studies in which it is used to produce blast mitigation coatings for civil structures, super-hydrophobic coatings for corrosion protection, and flame resistant polyimide syntactic foams for thermal insulation.

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.

Traditional low-cost bulk materials are unable to fulfil the increasing requirements of actual technology and functional coatings – e.g. APS alumina coatings on aluminium substrates for tribological properties – are a suitable alternative. The development of new thermal sprayings is usually based in experimental procedures which involve long development times and high costs. Nowadays, numerical simulation allows the researcher a better understanding of thermal spray processes as well as reducing the time and cost for the optimization of processes, but it requires a deep insight into the physics of the phenomenon. The submodelling approach allows the researchers to work with a local model, a thin layer – just a few microns – on the metal substrate surface, and more realistic boundary conditions, which requires a little specialised knowledge. The current study involves the workflow to manage the modelling at piece scale, and the transfer and interpolation of boundary conditions in each scale. The coating is divided into several layers, which represent the successive splats deposited during the process, and the heat flow from the torch is modelled by radiation and convection. A code is implemented in order to generate the routine needed by the FEM software, in which the results are processed and interpolated for the subsequent submodel. Furthermore, the material plasticity is considered and several tests are performed in order to check the simulation results.

The properties of thermally sprayed coatings significantly depend on the alloy composition and the adjusted process parameters. In addition to the powder certificate it may be useful to analyse the chemical composition of the sprayed powder during the spraying process itself. The principle of composition analysis is similar to the chemical analysis in an ICP plasma but the boundary conditions are more complex because the sprayed powder should not be completely evaporated in a thermal spray process. Nevertheless all thermal spraying processes lead to a certain evaporation of the species and to excitation of atomic states. The transition into the ground state occurs under emission of characteristic lines. The intensity of these lines is influenced by the plasma temperature, the particle temperature, the temperature dependent evaporation rate of the alloying elements and the powder feed rate. In consideration of the boundary conditions and the information from a detailed analysis of the emitted spectra the lines can be used to quantify the chemical composition of the sprayed alloys online. The theory of the principle for on-line analysing the chemical composition will be deduced and the first experimental validation will be presented.

MCrAlY(M=Ni, Co, or Ni-Co)coatings with good high temperature oxidation resistance have attracted great interest. They are widely used in gas turbines as protecting layers, such as thermal barrier coatings and seal coatings. Among many methods developed for preparing MCrAlY coatings, electroplating has drawn great attention due to its perfect bond strength, precise controllability, good coating ability for complex shape and so on. In this paper, the MCrAlY coatings have been prepared by a composite plating way. During this process, the CrAlY particles are wrapped with Ni clad layer. The thickness of the composite coatings is controlled at 150- 200 μm. The plating tests results indicate that the density of the clad layers mainly depend on the electroplating time. After that, these coatings are heat treated under the vacuum condition to make elements diffuse, forming homogeneous M(Ni)CrAlY component. The high-temperature oxidation resistance tests of the prepared coatings show good antioxidant ability at 1000 °C under air condition.

Fe-B composite powder was produced by spray-drying of elemental iron and boron powders. Fe-ZrB 2 composite powder was made by ball-milling of elemental iron and ZrB 2 powders. These powders were plasma-sprayed in low-pressure argon atmosphere to produce iron matrix composite deposits with dispersed boride particles. As-sprayed deposits formed using Fe-B composite powder are composed of ferrite (α) and austenite (γ) phase that is supersaturated with boron due to the high cooling rate of molten particles on a substrate. Heat treatment of the deposit at 673 K leads to the formation of Fe 3 B. The deposit heat-treated at 1073 K is made up of α, Fe 2 B, and FeB. With increasing heat-treatment temperature up to 1073 K, the hardness of the deposit decreases. As-sprayed deposit produced using Fe-ZrB 2 composite powder is composed of α, ZrB 2 , Fe 2 B, and Fe 3 Zr. Heat treatment of the deposit at 1073 K results in the formation of FeB. Heat treatment at 1073 K lowers hardness of the deposit.