Optical properties of Al2O3/Al cermets obtained by plasma spraying: role of composition and microstructure

Abstract

Al2O3/Al cermets (ceramic/metal) have been made by plasma spraying with different metal concentration in order to study their optical properties. A metal has a high reflectivity in the visible and the IR region, but the optical behaviour of an oxide ceramic is much more complicated, mainly if synthesised by plasma spraying. Indeed, a crystallised and dense ceramic, as alumina for example, is widely transparent in the visible region up to 5 μm, whereas in the same range of wavelength a ceramic synthesised by thermal spray will be broadly reflective. Due to its specific microstructure, plasma sprayed coatings include intrinsic open and closed porosity (5 to 20%), rough surfaces and a lamellar microstructure. Plasma operating parameters have been selected during a preliminary study of pure alumina: the nature of the plasma gas, the spray distance, and the powder feed rate. Same parameters were used to realise cermet coatings, with a double injection system. Optical properties of as-sprayed and polished coatings are discussed. The reflectance rises with the aluminium concentration, but in the transparent region of alumina (1 <λ <5 μm), we notice a remarkable behaviour for low aluminium concentration where the reflectance of a cermet is lower than pure alumina.