Optical properties of Al2O3/Al cermetsobtained by plasma spraying: role ofcomposition and microstructure

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.