Spray coatings composed of zirconia, zirconium boride, and zirconium boroxide were reactively produced by Electrothermally-Exploded-Powder-Spray (ELTEPS) technique by using a mixed powder of zirconium and boron oxide particles. Substrates used were mild steels on which a Ni-20%Cr coating was plasma-sprayed as an undercoat for such a composite coating. Energy-controlled multiple sprays brought the hetero-structure of a top layer of Zr-O-B ceramic composite and a graded layer which composition continuously changed from the ceramics to the metal under-coat. The microstructure of the top ceramic layer was composed of Zr-O-B ceramic particles the size of which was on the order of several nanometers to a few hundred nanometers.

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