On the dielectric behavior of annealed plasma sprayed forsterite and alumina coatings

Atmospheric plasma spray (APS) is an efficient technique for depositing high performance dielectric coatings at a low cost, with high deposition rate and on complex-shaped parts. Due to its availability, low price, ease to spray and good dielectric properties, alumina (Al2O3) is a very common choice of material for insulating applications. However, the coefficient of thermal expansion (CTE) of alumina does not represent the best match for the CTE of most metallic substrates hence alternative high CTE ceramic materials are of interest. We report on the microstructural and electrical properties of a dense high CTE forsterite (Mg2SiO4) coating deposited by APS. Annealed forsterite coatings' microstructure and dielectric properties are compared to that of annealed alumina coatings. The ceramics' electrical properties were evaluated by performing impedance measurements on as-sprayed coatings and coatings exposed to different annealing temperatures. We show that both materials exhibit important degradation after exposure to a threshold temperature. An equivalent electrical model is used to extract relevant coating properties such as resistivity and dielectric constant. The coating electrical degradation mechanisms are discussed.Peer reviewed: YesNRC publication: Ye