Laser pyrolysis of an organosilazane-based glass/ZrO2 composite coating system

• Composite coatings of silazane/glass/passive filler were obtained for the first time by pyrolysis using laser irradiation.
• The applied set of processing parameters for the laser pyrolysis resulted in dense, semi-crystalline and crack-free coatings.
• Laser irradiation induced the transformation of the monoclinic ZrO2 filler into its high-temperature tetragonal phase.
• Components of the glass fillers stabilized the tetragonal phase of ZrO2.

A process for the laser pyrolysis of a ceramic composite coating system composed of an organosilazane (Durazane™ 1800) with ZrO2 and glass particles as fillers was developed. Firstly, the mild steel substrates were dip-coated with a perhydropolysilazane (PHPS) bond coat, onto which the composite coating slurry was applied by spraying. After drying, pyrolysis using a Nd:YAG laser led to the formation of a dense semi-crystalline ceramic coating system with a thickness up to 20 μm in a short time. The resulting coatings possess a significantly different morphology compared to the same coating system pyrolyzed in a furnace, due to different forming mechanisms. Laser irradiation led to the unexpected formation of oxygen vacancies in the crystalline lattice of ZrO2, which increased the absorption of the laser radiation, enabling the transformation into a ceramic coating. Simultaneously, reactions between the glasses and the monoclinic ZrO2 fillers were activated, resulting in the formation of dendritic tetragonal-stabilized ZrO2 crystals. The thermal stability of the coating components was analyzed by thermogravimetric analysis (TGA) and the coatings were investigated by attenuated total reflectance infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD).

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