Evaluation of laser-glazed plasma-sprayed thermal barrier coatings under high temperature exposure to molten salts

Abstract

Thermal Barrier Coating (TBC) systems are frequently used in gas turbine engines to provide thermal insulation to the hot-section metallic components and also to protect them from oxidation, hot corrosion and erosion. Surface sealing treatments, namely laser-glazing, have been showing a high potential for extending in-service lifetimes of these systems by improving chemical and thermo-mechanical resistance. In this investigation, both as-sprayed and laserglazed TBCs were exposed to hot corrosion in molten salts. The glazed coatings were obtained by scanning the surface of the plasma-sprayed coatings with either a CO 2 or a Nd:YAG laser.The hot corrosion investigation was accomplished by subjecting the specimens to an isothermal air furnace testing under V 2 O 5 and/or Na 2 SO 4 in a temperature of 1000ºC for 100 hours.Spallation has been observed in coatings in the as-sprayed condition under V 2 O 5 or -2 -V 2 O 5 +Na 2 SO 4 . Na 2 SO 4 itself had no or minimal effect on the degradation of the laser-glazed or as-sprayed condition coatings, respectively. The degradation in V 2 O 5 was accomplished by destabilization of YSZ as a consequence of depletion of yttria from the solid solution to form YVO 4 and therefore led to the disruptive transformation of the metastable tetragonal phase to the monoclinic phase. Moreover, the presence of both corrosive salts induced the formation of large high aspect ratio YVO 4 crystals that introduced additional stresses and contributed to the degradation of the coatings. The laser-glazed specimens were not efficient in avoiding the molten salt penetration along the thickness direction due to the presence of cracks on the glazed layer.However due to a reduced specific surface area of the dense glazed layer, the corrosion reaction of the molten salts with the YSZ has been lower than in coatings in the as-sprayed condition.-3 -

An Investigation on Hot Corrosion Resistance of Plasma Sprayed YSZ‐Ceria TBC in Na 2 SO 4 +V 2 O 5 at 1050°C

“…It seems that the molten NaVCb has increased atom mobility and enhancing the depletion of yttria and hence promoting the growth of YVO4 crystals [10]. According to the XRD Patterns after hot corrosion for YSZ+CeC>2 and YSZ/CeC>2 samples (Fig.…”

Recent advances in laser surface treatment of titanium alloys

“…26 All of the above contribute to a better behavior during oxidation tests in the presence of molten salt at 1000°C than that achieved with sprayed condition coatings. 27 Laser glazing transforms the metastable ␥-Al 2 O 3 present in the APS coating to ␣-Al 2 O 3 . There is also a significant reduction in residual stresses and increased hardness when compared with sprayed coatings ͑2000 HV͒.…”

Degradación de barreras térmicas por sales fundidas

“…La transformación a ZrO 2 monoclínica lleva asociado un brusco aumento de volumen, que introduce tensiones en el recubrimiento produciendo grietas y su delaminación y desprendimiento posterior (figura 2). A continuación se expresan las reacciones posibles que se producen durante el ensayo de corrosión (14)(15)(16) Aunque esta última reacción no parece haberse producido al no aparecer en las difracciones de rayos X el compuesto NaYO 2 .…”

“…El NaVO 3 fundido aumenta la movilidad atómica aumentando la reducción de contenido en itria y por lo tanto, favoreciendo la formación de YVO 4 (16). La aparición de este compuesto y la desestabilización, y consecuente transformación de fase cúbica a monoclínica de la ZrO 2 , va acompañada de un aumento de volumen de un 5% aproximadamente, perjudicial para el recubrimiento, pues introduce tensiones en la ZrO 2 .…”