Improving the hot corrosion resistance of plasma sprayed ceria–yttria stabilized zirconia thermal barrier coatings by laser surface treatment (original) (raw)
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Ceramics International, 2012
In the present work, the hot corrosion behavior of two types of multilayer plasma sprayed TBC were investigated and compared with functionally graded and conventional TBCs. These kinds of multilayer coatings consisted of nano/μ alumina as a top coat on YSZ layer, a metallic bond coat and a functionally graded intermediate layer deposited between YSZ and bond coat layers. All the layers were sprayed on the Ni-base super alloy substrate. The hot corrosion resistance of the plasma sprayed coatings was examined at 1050°C for 40 h, using a fused mixture of 45 wt% Na 2 SO 4 + 55wt%V 2 O 5. Before and after hot corrosion, the microstructure and phase analysis of the coating were studied using scanning electron microscope and X-ray diffractometer. The results showed that, the Al 2 O 3 top layer acted as a barrier against the infiltration of the molten salt into the YSZ layer during exposure to the molten salt mixture at the high temperature and the multilayer coatings of zirconia/alumina with the nanostructured alumina as a top coat showed higher hot corrosion resistance. Also, the failure mechanisms of the functionally graded coating and duplex TBC were investigated. The spallation occurred between the graded layer and the bond coat/top coat in functionally graded TBC and duplex TBC, respectively.
Hot corrosion behavior of Al2O3 laser clad plasma sprayed YSZ thermal barrier coatings
Ceramics International, 2016
In the present study, the laser cladding of Al 2 O 3 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% V 2 O 5 and 45 wt% Na 2 SO 4 was used as the corrosive salt. SEM micrographs and EDS analysis confirmed the formation of YVO 4 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 ′-ZrO 2) to monoclinic zirconia (m-ZrO 2) after hot corrosion test. This structural transformation was due to the reaction of the molten salts with Y 2 O 3 (zirconia stabilizer) which destabilized the t ′-ZrO 2. To compare the hot corrosion resistance of the APSed YSZ and the laser clad coating, the volume percentage of the undesirable m-ZrO 2 was then calculated after the hot corrosion test. This calculation revealed a higher amount of m-ZrO 2 in YSZ (about 70 vol%) compared to that of the laser clad coating (about 13 vol%).
Ceramics International, 2014
The main goal of this paper was to evaluate the effects of laser glazing on the microstructure and thermal shock resistance of nanostructured thermal barrier coatings (TBCs). To this end, nanostructured yttria stabilized zirconia (YSZ) top coat and NiCrAlY bond coat were deposited on Inconel 738LC substrate by air plasma spraying (APS). The Nd:YAG pulsed laser was used for laser treatment of top coat surface. The thermal shock behavior of plasma-sprayed and laser-glazed coatings was investigated by quenching the samples in cold water from 1000 0 C. The microstructure and phase composition of the coatings were characterized by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Energy dispersive spectroscopy (EDS) was used to analyze the interface diffusion behavior of the bond coat elements. The results of SEM revealed that the laser glazing process reduced the surface roughness, eliminated the porosity of the surface and produced network cracks perpendicular to the surface. XRD results also indicated that both as-sprayed and laser glazed coatings consisted of nontransformable (T') phase. Thermal shock test results showed that the lifetimes of the plasma-sprayed TBCs were almost doubled by laser glazing. Continuous network of segmented cracks perpendicular to the surface produced by laser glazing improved the 2 strain accommodation and recognized it as the main enhancement mechanism for TBC life extension.
Microscopic observation of laser glazed yttria-stabilized zirconia coatings
Applied Surface Science, 2010
Thermal barrier coatings (TBCs) are frequently used as insulation system for hot components in gasturbine, combustors and power plant industries. The corrosive gases which come from combustion of low grade fuels can penetrate into the TBCs and reach the metallic components and bond coat and cause hot corrosion and erosion damage. Glazing the top coat by laser beam is advanced approach to seal TBCs surface. The laser beam has the advantage of forming a dense thin layer composed of micrograins. Plasmasprayed yttria-stabilized zirconia (YSZ) coating was glazed with Nd-YAG laser at different operating conditions. The surface morphologies, before and after laser treatment, were investigated by scanning electron microscopy. Laser beam assisted the densification of the surface by remelting a thin layer of the exposed surface. The laser glazing converted the rough surface of TBCs into smooth micron-size grains with size of 2-9 m and narrow grain boundaries. The glazed surfaces showed higher Vickers hardness compared to as-sprayed coatings. The results revealed that the hardness increases as the grain size decreases.
Ceramics International, 2010
Thick plasma sprayed thermal barrier coatings are suitable for thermal and hot corrosion protection of metal components in land-based turbine and diesel engines. In this work, ceria-yttria co-stabilized zirconia coatings were deposited by atmospheric plasma spraying in a mixture of nontransformable tetragonal t 0 and cubic c zirconia phases. Free-standing coatings were isothermally annealed at 1315 8C for different times and their crystal structure was studied by XRD. No phase decomposition occurred. Columnar grains grew in the molten splats with increasing annealing time according to a preferential direction and, after 50 h of heat treatment, they were partially replaced by equiaxed grains. Both in-plane and out-ofplane thermal expansion coefficients (CTEs) were measured from coating expansion during heating. The CTE was slightly sensitive to thermal exposure in out-of-plane direction, whereas it kept almost constant in plane direction. The specific heat capacity Cp of annealed coatings, measured by differential scanning calorimetry (DSC), decreased in comparison with as-sprayed coating, due to high-temperature sintering.
Surface and Coatings Technology, 2006
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.
Key Engineering Materials, 2014
This paper presents laser surface modification process of plasma sprayed yttria stabilized zirconia (YSZ) thermal barrier coating (TBC) for enhanced hardness properties and low surface roughness. A 300W JK300HPS Nd: YAG laser was used to process YSZ TBC sample surface. The parameters selected for examination were laser power, pulse repetition frequency (PRF) and residence time. Micrographs of the TBC system were captured using EVO 15 Scanning Electron Microscope (SEM). Surface roughness was measured using 2-dimensional stylus profilometer. X-ray diffraction analysis (XRD) was conducted to measure phase crystallinity of the laser-modified coating surface. X-ray diffraction patterns were recorded in the 2θ range of 10 to 80° using Bruker D8 Advance system with 0.7Å wavelength from a copper source (~1.5Å). The laser modified surface exhibited higher crystallinity compared to the as-sprayed samples. The presence of tetragonal phase was detected in the as-sprayed and laser processed sa...
Optics and Lasers in Engineering, 2012
In this study, substrates of Inconel 738 LC superalloy coupons were first sprayed with a NiCoCrAlY bondcoat and then with a ceria and yttria stabilized zirconia (CYSZ) topcoat by air plasma spraying (APS). After that, the plasma sprayed CYSZ thermal barrier coatings (TBCs) were treated using a pulsed Nd:YAG laser. The effects of laser glazing on the microstructure and thermal shock resistance of the coatings were evaluated. Thermal shock test was administered by holding specimens at 950 1C for 5 min and then water quenching. More than 20% of the spalled region of the surface of the topcoat was adopted as the criterion for the failure of samples. The microstructures of both the as processed and the tested TBCs were investigated using scanning electron microscope (SEM). The phases of the coatings were analyzed with X-ray diffractometry (XRD). XRD analysis revealed that both as sprayed and laser glazed topcoats consisted of nonequilibrium tetragonal (T 0) phase. The results showed that the life times of the as sprayed TBCs were enhanced around fourfold by the formation of a continuous network of segmented cracks perpendicular to the surface and the increase in strain accommodation.
Laser surface treatment of plasma-sprayed yttria-stabilized zirconia coatings
A continuous wave COj laser with a defocused beam has been employed to seal the surface layer of plasma-sprayed of 8wt. % yttria-stabilised zirconia coatings on AISI 1045 steel substrate. The effect of lasef remelting on microstructure and corrosion resistance of plasma-sprayed layer was investigated. The microstructure of the sealed layer presented a cellular structure which grows perpendicular to the surface. The micrographs have shown small cracks and absence of porosity. The corrosion resistance was analysed by the EIS and polarisation tests by using a 3 % NaCl solution. Laser sealing improved the corrosion resistance of the coating and increased the microhardness.
Materials and Corrosion, 2004
The microstructural change, crack initiation and spallation of a vacuum plasma sprayed (VPS) thermal barrier coating on an INCONEL-738 superalloy substrate were investigated after successive 300 h thermal cycles at 1050 8C. The coating was characterised using Raman spectroscopy, scanning electron microscopy (SEM) energy dispersive X-ray analysis (EDX) and Auger electron spectroscopy (AES). Localised micro-cracks at the yttrium (III) oxide stabilised zirconium (IV) oxide (YSZ) ceramic coating/thermally grown oxide (TGO) interface were observed after 8 cycles. Spallation of the YSZ coating occurred after approximately 21 cycles. Significant amounts of the elements titanium, tantalum and chromium were found within the TGO together with the formation of nickel, cobalt and chromium-rich oxides at this TGO/YSZ interface.