Improved sealing treatments for thick thermal barrier coatings

Zirconia-based 8Y2O3–ZrO2, 22MgO–ZrO2 and 25CeO2–2.5Y2O3–ZrO2 thick thermal barrier coatings (1000 μm) were studied with different sealing methods for diesel engine and gas turbine applications. The aim of the sealing procedure was to improve the hot corrosion-resistance and mechanical properties of porous, thick thermal barrier coatings (TTBC). The surface of the coatings was sealed with three different methods: (1) laser glazing; (2) an aluminium phosphate sealing treatment; and (3) detonation gun spraying of a dense top coating on the TTBC. Sealant penetration into the coating and the coating microstructure were determined by scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) and optical microscopy. Coatings were characterised by X-ray diffraction (XRD), microhardness and porosity measurements. The thickness of the densified top layers in all cases was 50–400 μm. XRD analysis showed some minor phase changes and reaction products caused by the phosphate-based sealing treatment and some crystal orientation and phase changes in laser-glazed coatings. The porosity of the outer layer of the sealed coating decreased in all cases, which led to increased microhardness values.