Polymer-derived coatings with La2Zr2O7 and glass fillers: Preparation and characterisation

Abstract

This study investigated the impact of La₂Zr₂O₇ (LZ) and different types of glass on the performance of polymer-derived ceramic (PDC) coatings on AISI 441 stainless steel substrates. Four double-layer PDC-based glass-ceramic coatings containing LZ and different glass fillers were prepared by dip coating. The LZ powder was synthesised by solid-state reaction (SSR): powder morphology, crystal structure, and thermal stability were analysed. X-ray diffraction (XRD) detected a LZ pyrochlore phase after annealing at 1300 and 1400 °C with a trace of t-ZrO₂. Four different glass compositions, namely BaO-Al₂O₃-SiO₂ (BAS), BaO-Al₂O₃-La₂O₃-B₂O₃-SiO₂ (BALBS), CaO-B₂O₃-SiO₂ (CBS), and BaO-ZnO-MgO-B₂O₃-SiO₂ (BZMBS), were also synthesised as fillers for PDC coatings. The glass transition and crystallisation temperatures of the glasses were determined using differential scanning calorimetry (DSC). The coating systems, consisting of a Durazane 2250 bond coat and a top coat (Durazane 1800 + LZ filler + different glass sealants), were prepared. After pyrolysis of the coatings at 900 °C, some of the glasses partially crystallised. Scanning electron microscopy (SEM) revealed that the layers containing BAS, BALBS and CBS glass were dense, with good adhesion to the substrate, and with occasional presence of larger pores and cracks. Delamination of the upper layer was observed in the coating with the BZMBS glass filler.