Dried particle plasma spray in-flight synthesis of spinel coatings

Powder particle diameters currently used for spraying are generally between 5 and 100 μm with a preferred size range around 40–60 μm. Future trends in plasma spraying involve the use of fine or ultrafine powders and the reduction of the number of steps between raw material preparation and coating. The use of non-sintered spray dried ceramic aggregates as feedstock material for plasma spraying has accordingly been investigated. Al₂O₃ based coatings were prepared by this route of dried particle plasma spray (DPPS). The microstructure and crystallographic phases of these deposits were characterised using scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). Given the intimate mixing of the starting oxides, reactions occur during spraying leading to the formation of spinel (MgAl₂O₄ and/or ZnAl₂O₄) and zinc aluminum oxide (Zn₄Al₂₂O₃₇). The layered structure of the deposit is characteristic of conventional plasma-sprayed coatings but the features are smaller in size. Depending on the operating conditions (plasma characteristics and powder injection), two different melting modes of the particles were identified; the first leads to individual well-melted droplets that splash regularly even if generating some fingers and the second leads to aggregates that are well-melted on their outer parts and strengthened in their cores.