High velocity suspension flame spraying of AlN/Al2O3 composite coatings

In applications such as electronics and optoelectronics, where high heat exchange performance is required, the thermal conductivity of aluminium oxide is not always sufficient. To enhance the thermal conductivity, direct spraying of AlN using a supersonic suspension flame spray (HVSFS) process was investigated. Aqueous suspensions containing AlN powders, or mixtures of AlN and Al2O3 were used as a feedstock. Different combinations of axial and radial injections into the HVOF torch have been investigated and results are compared. The coating morphology and phase composition were analysed using optical microscope, SEM, WDS and X-Ray diffraction. Mechanical properties like microhardness and surface roughness have been measured. The aim is to create coatings with a high AlN content and thus higher thermal conductivity. Having these properties, this type of coating could be of excellent use in numerous electronic applications. The results show the highest AlN content in the coatings with axially injected pure AlN suspension. Moreover, the thermal conductivity values clearly correlate with the AlN content in the coating. [Display omitted] •AlN/Al2O3 coating directly from the aluminium nitride suspension is possible.•Three Injection types were used to apply coatings with S-HVOF.•Aluminium nitride content can be increased by axial- compared to a radial injection.•AlN particles are incorporated in the coating when Al2O3 forms a layer.•Thermal conductivity could be increased by up to 27 % when spraying pure AlN axially.