Influence of laser‐glazing on hot corrosion resistance of yttria‐stabilized zirconia TBC in molten salt mixture of V₂O₅ and Na₂SO₄

Abstract

Plasma-sprayed 8YSZ (zirconia stabilized with 8 wt% yttria)/NiCoCrAlYTa thermal barrier coatings (TBCs) were laser-glazed using a continuous-wave CO 2 laser. Open pores within the coating surface were eliminated and an external densified layer was generated by laser-glazing. The hot corrosion resistances of the plasma-sprayed and laser-glazed coatings were investigated. The two specimens were exposed for the same period of 100 h at 900 8C to a salt mixture of vanadium pentoxide (V 2 O 5 ) and sodium sulfate (Na 2 SO 4 ). Serious crack and spallation occurred in the as-sprayed coating, while the as-glazed coating exhibited good hot corrosion behavior and consequently achieved a prolonged lifetime. The results showed that the as-sprayed 8YSZ coating achieved remarkably improved hot corrosion resistance by laser-glazing. Changes in the coatings were studied by scanning electron microscopy (SEM) to observe the microstructure and X-ray diffraction (XRD) technique to analyze the phase composition. XRD results showed that the reaction between yttria (Y 2 O 3 ) and V 2 O 5 produced yttrium vanadate (YVO 4 ), leaching Y 2 O 3 from YSZ and causing the progressive destabilization transformation from the tetragonal (t) to monoclinic (m) phase. The external dense layer produced by laser-glazing restrained the penetration of the molten salt, to a certain extent, into the coating, which led to a relatively low m-ZrO 2 content in the coating after the hot corrosion test. Additionally, the segmented cracks in the coating surface induced by laser-glazing were helpful to the improvement of strain tolerance of the coating. The two factors were important contributions to the significant enhancement of hot corrosion resistance of the as-glazed YSZ coating.

Hot corrosion behavior of La2Zr2O7 with the addition of Y2O3 thermal barrier coatings in contacts with vanadate–sulfate salts

“…At about 1020 K, ZrV 2 O 7 melts incongruently to m-ZrO 2 and a liquid mixture of m-ZrO 2 and V 2 O 5 [12,17,18]. Above 1020 K, i.e., 1173 K in this study, molten V 2 O 5 from the incongruent melting of ZrV 2 O 7 reacts with Na 2 O to form NaVO 3 , which further reacts with LZ3Y to form LaVO 4 , YVO 4 , ZrV 2 O 7 and Na 2 O, as shown in Eq.…”

“…In this case, another failure mode, hot corrosion, becomes predominant and crucial to the lifetime of TBCs [11,12]. The porous columns and inter-columnar gaps intrinsic in EB-PVD ceramic top coat provide penetration paths for molten salts such as vanadates and sulfates, to attack the EB-PVD TBC systems.…”

Study of the copper anode passivation by electrochemical noise analysis using spectral and wavelet transforms

“…The relative energy distribution concentrated on D crystals of low timescales could be related to metastable pitting [29,36] while the energy of large time scale crystals can be related to general corrosion [32,37], anodic dissolution [27] or diffusion, respectively [24,29]. The distribution of relative energy on medium time scale crystals could be indicating of pitting propagation and growth [27,29].…”

High-temperature corrosion behavior of zirconia ceramic in molten Na2SO4+NaVO3 salt mixture

“…They are usually operated in corrosive environments or burn low-quality fuels containing impurities such as vanadium, sulfur and sodium. During operation of turbine engines, these impurities can form sulfate and vanadate salts, further condense on the surface of TBCs at temperatures of 600-1100 1C [7][8][9][10]. The molten salt deposits mainly consist of Na 2 SO 4 and NaVO 3 .…”