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.

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.

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.

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.

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.

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.

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.