Dielectric and microwave absorbing properties of low power plasma sprayed Al2O3/Nb composite coatings

Al2O3/Nb composite coatings were sprayed on graphite substrates by low power atmospheric plasma spraying (LPAPS) with an internally fed powder torch. The composite particles were agglomerated with different mass fractions by spray-drying technology. The microstructure and dielectric properties of coatings were investigated. The microstructure of composite coatings shows a uniform dispersion of metal particles in the composite coatings. Both the real (ɛ′) and imaginary (ɛ″) parts of the complex permittivity increase with increasing Nb content over the frequency range of 8.2–12.4 GHz, which is ascribed to space charge polarization and conductance loss, respectively. By calculating the microwave-absorption as a single-layer absorber, reflection loss values exceeding −10 dB can be obtained in the frequency range of 10.0–11.8 GHz with 10 wt% Nb content coating when the coating thickness is 1.5 mm.