Air Plasma Sprayed Yttria-stabilized Zirconia Research Articles

The Effect of APS-HVOF Bond Coating Thickness Ratio on TBC Furnace Cycle Lifetime

Combinations of NiCoCrAlY APS and high-velocity oxygen fuel (HVOF) bond coatings were deposited on alloy 247 disk substrates with APS yttria-stabilized zirconia (YSZ) top coatings to assess the benefit of air plasma sprayed (APS) ‘flash’ bond coatings. Using 1-h cycles at 1100 ℃ in air with 10% H2O and HVOF-only and APS-only bond coatings as a baseline, it was found that APS flash coatings extended the average coating lifetime by 16% to 35% with the thicker flash coating performing best. Principal component analysis and energy dispersive spectroscopy (EDS) compositional mapping on coatings characterized after 0, 100, 300 and 500 cycles and after failure showed that the Al in the bond coatings was depleted due to both oxidation and back diffusion into the substrate. The APS-only bond coating had significant oxidation throughout the bond coating and was so depleted in Al, that Al diffused from the substrate to the coating. Residual stress maps of the thermally-grown alumina scale were obtained every 100 cycles using photo-stimulated luminescence piezospectroscopy (PLPS) revealing that the Thick Flash coating had the slowest rate of damage accumulation in the oxide scale.

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 30h in which a mixture of 55wt% V2O5 and 45wt% 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 70vol%) compared to that of the laser clad coating (about 13vol%).

The Effect of Environment on Thermal Barrier Coating Lifetime

While the water vapor content of the combustion gas in natural gas-fired land-based turbines is ∼10%, it can be 20–85% with coal-derived (syngas or H2) fuels or innovative turbine concepts for more efficient carbon capture. Additional concepts envisage working fluids with high CO2 contents to facilitate carbon capture and sequestration. To investigate the effects of changes in the gas composition on thermal barrier coating (TBC) lifetime, furnace cycling tests (1-h and 100-h cycles) were performed in air with 10, 50, and 90 vol. % water vapor and CO2-10% H2O and compared to prior results in dry air or O2. Two types of TBCs were investigated: (1) diffusion bond coatings (Pt-diffusion or Pt-modified aluminide) with commercial electron-beam physical vapor-deposited yttria-stabilized zirconia (YSZ) top coatings on second-generation superalloy N5 and N515 substrates and (2) high-velocity oxygen fuel (HVOF) sprayed MCrAlYHfSi bond coatings with air plasma-sprayed YSZ top coatings on superalloys X4, 1483, or 247 substrates. For both types of coatings exposed in 1-h cycles, the addition of water vapor resulted in a decrease in coating lifetime, except for Pt-diffusion coatings which were unaffected by the environment. In 100-h cycles, environment was less critical, perhaps because coating failure was chemical (i.e., due to interdiffusion) rather than mechanical. In both 1-h and 100-h cycles, CO2 did not appear to have any negative effect on coating lifetime.

Buckling failure in air-plasma sprayed thermal barrier coatings induced by molten silicate attack

The buckling phenomenon in air-plasma sprayed yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) induced by calcium–magnesium-alumino-silicate (CMAS) attack was investigated. The liquid CMAS infiltration due to capillary force weakened the interface and introduced a significant volume expansion (32%) in the top coat, leading to a large scale buckling of the TBC at high temperature. The dissolution and tetragonal-mononiclic phase transformation of YSZ induced by CMAS only has a negligible contribution.

The role of beam dispersion in Raman and photo-stimulated luminescence piezo-spectroscopy of yttria-stabilized zirconia in multi-layered coatings

Laser beam dispersion affects the resolution of Raman and photo-stimulated luminescence piezo-spectroscopy measurements of transparent materials. In this paper, we investigate the lateral spreading of the laser beam and the axial sampling depth of Raman spectroscopy measurements within thermal sprayed yttria-stabilized zirconia (YSZ) thin coatings. The lateral diameters of the laser beams (λ=632.8nm and 514nm) reach approximately ∼160μm after travelling through a thickness of 200μm of air plasma sprayed (APS) YSZ and ∼80μm after travelling through 120μm of electron beam physical vapour deposited YSZ. The Raman spectroscopy sampling depth was found to be between 30 and 40μm in APS YSZ. The beam dispersions within these two coatings were simulated using the ray-tracing software ZEMAX to understand the observed scattering patterns. The results are discussed with respect to the application of these two spectroscopic techniques in multi-layered thermal barrier coating systems.

Degradation of Yttria‐Stabilized Zirconia Thermal Barrier Coatings by Vanadium Pentoxide, Phosphorous Pentoxide, and Sodium Sulfate

The presence of vanadium, phosphorus, and sodium impurities in petcoke and coal/petcoke blends used in integrated gasification combined cycle (IGCC) plants warrants a clear understanding of high‐temperature material degradation for the development of fuel‐flexible gas turbines. In this study, degradation reactions of free‐standing air plasma‐sprayed (APS) yttria‐stabilized zirconia (YSZ) in contact with vanadium pentoxide (V2O5), phosphorus pentoxide (P2O5), and sodium sulfate (Na2SO4) were investigated at temperatures up to 1200°C. Phase transformations and microstructural development were examined using X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. Molten V2O5 reacted with solid YSZ to form zirconium pyrovanadate (ZrV2O7) at temperatures below 747°C. However, at temperatures above 747°C, molten V2O5 reacted with YSZ to form yttrium vanadate (YVO4). The formation of YVO4 led to the depletion of the Y2O3 stabilizer and deleterious transformation to the monoclinic ZrO2 phase. In addition, studies on YSZ degradation by Na2SO4 and a Na2SO4+V2O5 mixture (50–50 mol%) showed that Na2SO4 itself had no effect on the degradation of YSZ. However, in the presence of V2O5 at high temperatures, Na2SO4 forms vanadate compounds having a lower melting point such as sodium metavanadate (610°C), which was found to degrade YSZ by the formation of YVO4 at a relatively lower temperature of 700°C. P2O5 was found to react with APS YSZ by the formation of zirconium pyrophosphate (ZrP2O7) at all the temperatures studied. At temperatures as low as 200°C and as high as 1200°C, molten P2O5 was observed to react with solid YSZ to yield ZrP2O7, which led to the depletion of ZrO2 in YSZ (i.e., enrichment of Y2O3 in t′‐YSZ) that promoted the formation of the fluorite‐cubic ZrO2 phase.