Comparison of the Microstructural Characteristics and Electrical Properties of Thermally Sprayed Al2O3 Coatings from Aqueous Suspensions and Feedstock Powders

Abstract

In this work the microstructural characteristics and electrical insulating properties of thermally sprayed alumina coatings produced by suspension-HVOF (S-HVOF) and conventional HVOF spray processes are presented. The electrical resistance at different relative air humidity (RH) levels (from 6 to 97% RH) and values of dielectric strength were investigated by direct current electrical resistance measurements, electrochemical impedance spectroscopy, and dielectric breakdown tests. Relationships between electrical properties and coating characteristics are discussed. At low humidity levels (up to 40% RH) the electrical resistivities of S-HVOF and HVOF coatings were on the same order of magnitude (1011 Ω·m). At a very high humidity level (97% RH) the electrical resistivity values for the S-HVOF coatings were in the range 107-1011 Ω·m, up to five orders of magnitude higher than those recorded for the HVOF coating (orders of magnitude of 106 Ω·m). The better electrical resistance stability of the suspension-sprayed Al2O3 coatings can be explained by their specific microstructure and retention of a higher content of α-Al2O3. The dielectric strength E d of suspension-sprayed coatings was found to be 19.5-26.8 kV·mm−1 for coating thicknesses ranging from 60 to 200 μm. These values were slightly lower than those obtained for conventional HVOF coatings (up to 32 kV·mm−1). However, it seemed that the dielectric strength of conventionally sprayed coatings was more sensitive to the coating thickness (when compared with the values of E d determined for S-HVOF coatings) and varied to a greater extent (up to 10 kV·mm−1) when the coating thickness varied in the range 100-200 μm.