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.

In this study, APS and HVOF processes have been used to prepare alumina (Al 2 O 3 ) and magnesium spinel (MgAl 2 O 4 ) coatings designed for insulating applications. The electrical characteristics, i.e., dielectric strength and electrical resistance (electrical resistivity) were investigated using different methods: dielectric breakdown test, direct current (DC) measurements, and electrochemical impedance spectroscopy (EIS). The electrical resistance was measured at room temperature at different relative humidity (RH) levels (from 6% RH to 95% RH) as well as at 200°C. The coating microstructure, phase composition, and water vapor sorption were studied. Differences in the electrical insulating properties due to the different coating system characteristics are discussed. Of the coatings and conditions investigated in this study, the HVOF spinel coatings showed superior dielectric breakdown strength and electrical resistance stability at high humidity levels.

Die Einzeloxide im System Al2O3–Cr2O3–TiO2 und Zusammensetzungen der binären Teilsysteme sind neben dem Zirkonoxid die wichtigsten oxidkeramischen Werkstoffe für thermisch gespritzte Schichten. In diesem Beitrag werden wichtige Ergebnisse eigener Entwicklungsarbeiten der letzten Jahre zusammengefasst. Diese betreffen den Vergleich der Mikrostrukturen, Phasenzusammensetzungen und Eigenschaften von Schichten, die durch atmosphärisches Plasmaspritzen (APS) und Hochgeschwindigkeitsflammspritzen (HVOF) hergestellt wurden, aber auch vor allem die Möglichkeiten, die sich durch die Verwendung von Suspensionen als Spritzzusatz eröffnen. Besondere Aufmerksamkeit finden die Vorteile, die sich durch die Nutzung der binären Zusammensetzungen im System ergeben. Die tribologischen, elektrischen und Korrosionseigenschaften der Schichten werden diskutiert.

Boundary layers on surfaces will change from laminar to turbulent flow after a critical length. Due to the differing heat transfer coefficients of laminar and turbulent flow, the point of transition can be detected by heating the surface and measuring surface temperature by thermographic imaging. Locating the transition point is crucial for the aerodynamic optimization of components. In this study, fiber reinforced polymer composites (FRPCs) were chosen as the test substrate. Experiments were conducted using the flame spray process and NiCrAlY coatings. Multilayered coatings consisting of an aluminum bond coat, a layer of alumina as electrical insulation, and a heating layer of titania were fabricated by atmospheric plasma spraying. Free-flight tests were conducted with a functionalized winglet in order to assess the ability of thermally-sprayed heating elements to detect the location of transition of the flow regime. The results showed that the thermally-sprayed elements heat surfaces uniformly, with sufficient radiation losses for thermographic imaging. It was also shown that the change in temperature at the point of transition was readily observable using thermography.