The piezoresistivity of flame-sprayed NiCoCrAlTaY on an electrically insulated surface of a steel substrate was investigated through cyclic extension and compression cycles between 0 and 0.4 mm for 1000 cycles and uniaxial tensile test. The sprayed NiCoCrAlTaY was in grid form with grid thickness of 3 mm and grid length of 30 mm while the electrical insulation was fabricated by flame spraying alumina on the surface of the steel. During mechanical loading, instantaneous electrical resistance measurements were conducted to evaluate the corresponding relative resistance change. Images of the loaded samples were captured for strain calculations through Digital Image Correlation (DIC) technique. After consolidation of the pores within the coating, the behavior of the flame-sprayed NiCoCrAlTaY was consistent and linear within the cyclic compression and extension limits, with strain values of approximately -1000 με and +1700 με, respectively. The coating had a consistent and steady maximum relative resistance change of approximately 5% within both limits. The tensile test revealed that the coating has two gauge factors due to the bi-linearity of the plot of relative resistance change against strain. The progression of damage within the coating layers was analyzed from its piezoresistive response and through back-scattered scanning electron microscopy images. Based on the results, the nickel alloy showed high piezoresistive sensitivity for the duration of the loading cycles, with little or no damage to the coating layers. These results suggest that the flame-sprayed nickel alloy coating has great potential as a surface damage detection sensor.

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.

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.