Synthesis and characterization of sol-gel derived non-stoichiometric aluminum phosphate coating

The aim of this study was to synthesize a non-stoichiometric aluminum phosphate coating with high thermal stability. The aluminum phosphate precursor solution, in the presence of polyvinylpyrrolidone (PVP) as a crack-limiting agent was prepared by a sol–gel process and then applied on AISI 304 stainless steel substrate using the dip coating technique. Phase composition analysis of the coating material was performed by X-ray diffraction (XRD). The surface and cross sectional morphology of the coating after annealing at 500 °C for 15 min were observed using scanning electron microscopy (SEM). Topography and roughness of coated surface were investigated by atomic force microscopy (AFM). The amorphous-nanocrystalline structure and distribution of nanocrystals in the amorphous matrix were studied by transition electron microscopy (TEM). Thermal analysis and study of amorphous to crystalline transformation were investigated by thermal-gravimetric and differential scanning calorimetry (TG-DSC). Molecular spectroscopy of the synthesized powder was studied by Raman spectroscopy. According to SEM and AFM images a smooth, uniform, continuous and crack-free coating with 6.7 nm surface roughness was achieved. XRD analysis showed that the amorphous structure of the coating remained almost unchanged after annealing at 500 °C for 15 min. Conversely, an amorphous-nanocrystalline structure was obtained after air annealing at 1100 °C for 1 h. Also, oxide peaks number and intensity after oxidation at 1100 °C for 10 h were remarkably less in the coated sample than the bare one indicating the surface protection of aluminum phosphate coating against oxidation. The presence of graphitic and amorphous carbon (ID:IG = 0.97) in the coating structure was confirmed by Raman spectroscopy. TEM observations also confirmed the amorphous structure of the coating after annealing at 500 °C for 15 min and the partial transformation into a mixed amorphous-nanocrystalline structure after annealing at 1100 °C for 1 h. Accordingly, the thermal analysis of the coating material showed that the onset temperature of the amorphous to crystalline transformation was around 1050 °C, a value higher compared to previous reports (below 1000 °C) for the same material.