Electromagnetic and mechanical properties of silica-aluminosilicates plasma sprayed composite coatings

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. Al2O3-SiO2 compounds show an excellent refractory behavior 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 article presents the results of an extensive experimental activity carried out to produce thick aluminosilicate coatings by plasma-spray technique. The 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.