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

Highlights: • 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).