Electrically conductive ceramic coatings

Although thermally sprayed coatings in the system Cr₂O₃-TiO₂ have a high potential, this system has been, till date, scarcely used. Only a few mixing ratios along the region rich in Cr₂O₃ are commercially available as thermal spray feedstock. Therefore, the main goal of a research project founded by the AiF, was the knowledge-based development of coatings in the system Cr₂O₃-TiO₂ this time along the region rich in TiO₂ with Magneli-Phases structure. The investigations were focused on the tribological and electrical properties. The so called Anderson-Phases (with Magneli-Phases structure) rich in TiO₂ feature, for ceramic materials, a rather high electrical conductivity.

Among the various processes available in thermal spraying, atmospheric plasma spray (APS) is the most suited to obtain Cr₂O₃-TiO₂ coatings. Moreover, high velocity oxy-fuel (HVOF) was also used to comparison. With both processes, coatings were obtained overall the Cr₂O₃-TiO₂ system. Therefore, commercially available powders rich in Cr₂O₃ and experimental powders rich in TiO₂ (developed at Fraunhofer IKTS) were employed. The electrical properties of the coating were then correlated with the microstructures and phase transformations.

In the thermoanalytical analyses till 1500 °C, conducted on two of the experimental powders, was shown that no loss of Cr₂O₃ occurred under these conditions that could limit the application of these powders in thermal spraying. After process optimization, it was possible to obtain, by both APS and HVOF, homogenous and dense coatings. By means of the spraying process, significant phase transformations occurred, and some phases present in the powders (like E-Phase or TiCr2O5) where no longer present in the coating.

The eskolaite structure (Cr2O3) is the only phase present in the coatings composed of powder rich in Cr₂O₃. In the APS sprayed coatings, escolaite is found as a secondary phase for compositions up to 76,5% TiO₂ – 23,5 % Cr₂O₃. As principal constituents of the coatings rich in TiO₂, high temperature Magneli-Phase (n-phase) and rutile mix crystal are detected.

Regarding the electrical resistivity of the coatings, a dependence on the Cr₂O₃ content is observed. Figure 1 demonstrates this relationship for APS and HVOF coatings. The electrical resistivity of the HVOF coatings is, when both values can be compared, slightly higher than the one measured for the APS coatings. The already expected low electrical resistivity and its dependence on the phase composition for the coatings rich in TiO₂ were confirmed. Due to their high temperature stability, electrically conductive ceramic coatings have high application potential.