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 (10 11 XAEm). At a very high humidity level (97% RH) the electrical resistivity values for the S-HVOF coatings were in the range 10 7 -10 11 XAEm, up to five orders of magnitude higher than those recorded for the HVOF coating (orders of magnitude of 10 6 XAEm). The better electrical resistance stability of the suspension-sprayed Al 2 O 3 coatings can be explained by their specific microstructure and retention of a higher content of a-Al 2 O 3 . The dielectric strength E d of suspension-sprayed coatings was found to be 19.5-26.8 kVAEmm 21 for coating thicknesses ranging from 60 to 200 lm. These values were slightly lower than those obtained for conventional HVOF coatings (up to 32 kVAEmm 21 ). 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 kVAEmm 21) when the coating thickness varied in the range 100-200 lm.

Suspension High Velocity Oxy-Fuel (SHVOF)-Sprayed Alumina Coatings: Microstructure, Nanoindentation and Wear

“…A 73 wt% alpha alumina level is higher than previously obtained by Toma et al [10]. However, based on Rietveld refinement of the XRD pattern, the majority of the coating based on weight % is amorphous for both coatings types, albeit slightly lower for 72 kW, (58 wt% as opposed to 74 wt% for the 101 kW coating).…”

“…Toma et al [10] produced alumina coatings using SHVOF, in which the coating was comprised of a majority (60 wt%) alpha alumina phase with remainder gamma, in contrast to a majority gamma phase coating with a minority of alpha (30 wt%) produced using standard HVOF. It however must be noted that processing conditions were not the same for both tests and hence it is difficult to make a direct comparison.…”

Effects of Ambient Conditions on the Dielectric Properties of Thermally Sprayed Ceramic Coating

“…In addition, the measurement methods and conditions in previous studies are often not fully documented, and thus the evaluation and comparison of the properties of different coatings is difficult. Earlier studies of electrical properties of thermally sprayed coatings are mostly concentrated on the HVOF (high velocity oxygen fuel) and plasma sprayed alumina coatings [1,2,3,4]. Relative permittivity and dielectric loss behavior for plasma sprayed alumina (Al 2 O 3 ) coating has been studied in [1] but the results are not fully comparable to this study because of the different spraying technique.…”

“…In [1,2], the DC resistivity and DC dielectric strength of plasma sprayed alumina coatings were studied at room temperature conditions. The results of the papers [3,4] indicate that electrical properties of HVOF sprayed alumina coatings require to be examined more detailed and it has been already seen that especially the microstructure of a coating affects significantly the electrical properties. It has also been indicated that the environmental conditions affect also the electrical properties of thermally sprayed coatings.…”

“…In similar conditions the DC dielectric strength was ~22 V/µm, when the thickness of the HVOF coatings was approximately 200 µm [3]. In [4], the DC dielectric breakdown strength was studied for suspension HVOF and HVOF sprayed alumina coatings and the results were 19.5 -26.8 V/µm (thickness of a sample 60 -200 µm) and 22.5 V/µm (thickness of a sample 200 µm), respectively. The measurement conditions in [3,4] are not fully documented and because of that the results of those papers cannot be fully compared to results of this paper.…”