In this study, high-emissivity NiCr oxide coatings, one with TiO 2 and one with TiB 2 compounds, were deposited on stainless steel substrates by spray granulation and atmospheric plasma spraying. The main phases in both coatings are NiTiO 3 , spinel (NiCr 2 O 4 ), and residual Cr 2 O 3 . The emissivity of the layer with TiO 2 was found to be higher in the 2.5-8 μm range than that of the layer with TiB 2 , which is attributed to higher surface roughness and larger particle size. These results indicate that the NiCr oxide coatings with TiO 2 compounds are more suitable for high-temperature IR applications.

In this study, (La 0.9 Ca 0.1 )(Cr 0.9 Mg 0.1 )O 3 ceramic powders prepared by solid-state synthesis were deposited on nickel-base superalloy substrates by atmospheric plasma spraying. Powder morphology and coating surfaces were examined by SEM, and composition and phase structure were evaluated by EDS and XRD. Coating porosity and bond strength were measured and emissivity and thermal shock tests were carried out. The results show that the powders maintained their perovskite structure during spraying and that no impurities were introduced in flight. The emissivity of the coatings was found to be 0.88 at 600 °C and 0.89 at 800 °C, which is attributed to lattice distortion stemming from differences between doping and original ions and the valence states of Mg 2+ and Cr 3+ . Coating crystal structure was stable over the thermal shock range from room temperature to 1100 °C and no spalling or fracture occurred after ten shock cycles.

In this study, ceramic-matrix composites consisting of elongated metal (CoNbZr) particles in a cordierite (MgAlSiO) matrix were produced by plasma spraying. The metal powder was injected into the plasma jet downstream of the ceramic powder to minimize metal decomposition and oxidation. The microstructure and composition of cermet coatings containing 5, 10, and 20 vol% metal were analyzed by SEM and XRD and their electromagnetic properties were evaluated via saturation magnetization, permittivity, and permeability measurements. As expected, flake-shaped metallic particles were obtained and all coatings exhibited soft ferromagnetic behavior.

This paper describes how effective medium theory and fractal analysis are used to investigate nonlinear microstructure-property relationships in HVOF-sprayed composite coatings produced from nano Fe-(β-SiC/SiO 2 ) agglomerate powders in order to optimize microwave absorption performance. The powder used in the study was prepared by spray granulation and deposited on Fe substrates. The microstructure of the powder and coatings was examined by SEM, the phase structure was determined by XRD analysis, and electrical permittivity and permeability were measured. To simplify calculations, electromagnetic absorption phases in the coating were assumed to be periodically distributed cubes. The results of the study indicate that multi-fractal diffraction in the coating microstructure facilitates the absorption of microwaves and is optimized when the mass fraction of nano βSiC in the composite is 28 wt%.

Concentrating solar power (CSP) systems represent a zero emission method for conversion of sunlight to electricity. CSP systems use an array of mirrors to concentrate sunlight on the surface of a heat exchanger and heat a working fluid. These heat exchanger surfaces must have high absorptivity and low emissivity in the solar spectrum. In addition, they must be capable of extended operation at temperatures in excess of 600°C. Initial development of solar selective coatings using the air plasma spray process will be discussed. Eight different coating materials were deposited onto 304L stainless steel substrates. Solar absorptance and emittance were measured from each coating in three conditions: as-deposited, after heat treatment at 600°C for six hours, and after polishing to a 1 µm finish. A figure of merit based upon solar power tower (SPT) operation was calculated from these data and compared to the industry standard solar selective coating for SPT receivers, Pyromark Series 2500 high temperature paint. This comparison shows that Ni-5Al, 80WC-20Co, and CeO plasma-sprayed coatings all have potential as solar selective surfaces for SPT receivers.

The physico-chemical and thermo-mechanical properties of aluminosilicate ceramics (high melting point, low thermal expansion coefficient, excellent thermal shock resistance, low density and good corrosion resistance) make this class of materials a good option for high temperature structural applications. Al 2 O 3 -SiO 2 compounds show an excellent refractory behaviour allowing a wide use as wear resistant thermal barrier coatings, in metallurgical and glass plants and in high temperature heat exchangers. Moreover the low values of thermal expansion coefficient and of complex permittivity allow to extend the use of this ceramic for microelectronic devices, radome for antennas and electromagnetic windows for microwaves and infrared. The present paper presents the results of an extensive experimental activity carried out to produce thick aluminosilicate coatings by plasma spray technique. APS deposition parameters were optimized on the basis of a surface response approach, as specified by design of experiments (DoE) methodologies. Samples were tested for phase composition, total porosity, microstructure, microhardness, deposition efficiency, fracture toughness and modulus of rupture. Finally, coatings were characterized for their particularly interesting electromagnetic properties: complex permittivity was measured at microwave frequency using a network analyzer with wave guide.

Single-wavelength pyrometers are widely used as a noncontact temperature measurement tool in material processing, petrochemical and laser-machining industries. In addition, these intensity-based IR sensors are used extensively as a diagnostic and health monitoring in the development and research of advanced high-temperature military and commercial gas turbine engines. In contrast to thermocouples, optical pyrometers have several advantages. First, they are easy to install and second they do not bring any disturbance to the measured system. However, they suffer from some problems, in particular the variation of the material emissivity and perturbations introduced by extraneous radiations. Yttria stabilised Zirconia (YSZ) thermal barriers are known to be more emissive and opaque in the 8-14 µm spectral band, therefore we can take advantage using this spectral band in temperature measurement. Spectral emissivities of an YSZ sample were measured using two commercial pyrometers. And a method for adapting commercial wide-band pyrometers (generally used for low temperature measurements) for high temperature measurements of thermal barrier coatings was tested.

Substrate temperature is nowadays recognized as a key parameter to optimise the coating quality in the thermal spraying process. Generally parts being processed are in motion and therefore non contact temperature measurement devices are appropriate. In contrast to thermocouples, optical pyrometers have several advantages. First, they are easy to install and second they do not bring any disturbance to the measured system. Meanwhile, several problems may arise with those devices which are not always considered as they should be and in particular the variation of material emissivity temperature, the effect of the reflection of the external radiation or the attenuation of the optical signal due to the variable transmissivity of the optical path. The aim of this work was to develop algorithms for correcting optical pyrometer temperature measurements during thermal spraying by taking into account emissivity variations and radiation reflexion on the components. Emissivity of some materials with respect to the specific spectral band of the pyrometer and the influence of reflected radiations were measured. Results are discussed in order to point out the influence of each parameter on the temperature value.

This paper describes the development and evaluation of multifunctional coatings for outdoor parabolic antennas. The plasma-sprayed composite coatings provide corrosion protection, reduce solar heating, and improve the absorption of EM radiation at the edges of the antenna while impeding its flow in other areas. The coatings also reduce the impact of the support structure on antenna performance. Paper includes a German-language abstract.