The rising industrial requirements of high-power density electrical machines (EMs) require new generations of electrically insulating (EI) coatings with high temperature (HT), T>500°C, and high voltage, V>1 kV ac/dc, ratings. Aluminium nitride (AlN) is regarded as a promising functional ceramic material thanks to its useful physical properties in the bulk form: high thermal conductivity and high electrical resistivity. However, presently a coherent picture of the overall dielectric breakdown mechanism and heat transport in thin films of AlN and their thickness dependences are yet to emerge.

Here, we used conductive atomic force microscopy to provide insights into the process of defect generation and dielectric degradation in polycrystalline hexagonal (h) phase AlN films deposited by reactive dc magnetron sputtering as adherent coatings on metal (nickel, copper) substrates. A systematic statistical approach was used to study the breakdown strength through the Weibull distribution and to provide insights into the process of defect generation and dielectric degradation. Our studies reveal record-high dielectric breakdown field strength of up to 1.6 kV/μm for thin layers (~30 nm) of h-AlN at room temperature (RT) and show that it decreases as thickness increases, saturating at 1 kV/µm for layers >90 nm. The Weibull slope parameter, β, tends to increase with increasing film thickness, indicative of a lower dispersion of breakdown electric field. The rationale for this observation is that h-AlN follows the standard percolation model with random defect generation in the volume of the film resulting in β being proportional to the thickness of the dielectric. The breakdown voltage statistics follows a tight monomodal Weibull distribution, indicating that the material is highly suitable, from a reliability perspective, for application as EI coating.

We also employed nanosecond transient thermo-reflectance (TTR) to probe thermal properties of the h-AlN thin films and our studies revealed very high thermal conductivity, κ=290±5 W/m·K at RT. This exceeds the highest values reported to date for any thin-film material of equivalent thickness. Importantly, it remains high even at elevated T, namely ~160 W/m·K at T=300°C. For comparison, the value for DuPont™ Kapton® MT+ polyimide film, which is currently one of the best commercially available options, is only up to 0.75 W/m·K.

The full potential of this EI can be revealed when combined with a conventional high-T inorganic filler material allowing for a T ceiling of an industrial motor to be raised up to 500°C. Our simulations in “MotorCAD” software showed that employing inorganic EI materials allows for five times more heat to be dissipated by the same EM design, which would significantly increase the power density by up to 50%, when considering the associated electrical effects. High dielectric strength, high thermal conductivity and good adhesion to metals of sputtered h-AlN thin films make this purely inorganic material a promising candidate for thermally stable EI coatings (e.g. insulation of magnet wires and lamination of stator/rotor cores) which meet the demanding requirements of the next generation of high-T EMs used in the aerospace and automotive industries.