Hot corrosion behavior of Al₂O₃ laser clad plasma sprayed YSZ thermal barrier coatings

In the present study, the laser cladding of Al₂O₃ on the top surface of air plasma sprayed (APSed) yttria stabilized zirconia (YSZ) coatings was carried out to improve the hot corrosion resistance of the thermal barrier coatings (TBCs) in the presence of molten salts. The coatings with and without laser cladding were subjected to a hot corrosion test at 1000 °C for 30 h in which a mixture of 55 wt% V₂O₅ and 45 wt% Na₂SO₄ was used as the corrosive salt. SEM micrographs and EDS analysis confirmed the formation of YVO₄ rod-shaped crystals dispersed on the surface of the APSed YSZ coatings after hot corrosion test, while these crystals were hardly detected in the laser clad coatings. The SEM micrograph of the cross section of the APSed YSZ coatings revealed cracks and a thermally grown oxide (TGO) layer in the bond coat/top coat interface, which led to the complete delamination of the coatings. Supporting the SEM micrographs, XRD patterns indicated the transformation of metastable tetragonal zirconia (t^′-ZrO₂) to monoclinic zirconia (m-ZrO₂) after hot corrosion test. This structural transformation was due to the reaction of the molten salts with Y₂O₃ (zirconia stabilizer) which destabilized the t^′-ZrO₂. To compare the hot corrosion resistance of the APSed YSZ and the laser clad coating, the volume percentage of the undesirable m-ZrO₂ was then calculated after the hot corrosion test. This calculation revealed a higher amount of m-ZrO₂ in YSZ (about 70 vol%) compared to that of the laser clad coating (about 13 vol%).