Sealing procedures for thick thermal barrier coatings

Zirconia-based 8Y₂O₃-ZrO₂ and 22MgO-ZrO₂ thick thermal barrier coatings (TTBC, 1000 μm), were studied with different sealing methods for diesel engine applications. The aim of the sealing procedure was to improve hot corrosion resistance and mechanical properties of porous TBC coatings. The surface of TTBCs was sealed with three different methods: (1) impregnation with phosphate-based sealant, (2) surface melting by laser glazing, and (3) spraying of dense top coating with a detonation gun. The thicknesses of the densified top layers were 50–400 μm, depending on the sealing procedure. X-ray diffraction (XRD) analysis showed some minor phase changes and reaction products caused by phosphate-based sealing treatment and some crystal orientation changes 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. The hot corrosion resistance of TTBCs against 60Na₂SO₄-40V₂O₅ deposit was determined in isothermal exposure at 650 °C for 200 h. Corrosion products and phase changes were studied with XRD after the test. A short-term engine test was performed for the reference coatings (8Y₂O₃-ZrO₂ and 22MgO-ZrO₂) and for the phosphate-sealed coatings. Engine tests, duration of 3 h, were performed at the maximum load of the engine and were intended to evaluate the thermal cycling resistance of the sealed coatings. All of the coatings passed the engine test, but some vertical cracks were detected in the phosphate-sealed coatings.