In situ laser remelted thermal barrier coatings: Thermophysical properties (original) (raw)
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Modified thick thermal barrier coatings: Thermophysical characterization
Journal of the European Ceramic Society, 2004
Thermophysical properties of modified zirconia based thick thermal barrier coatings (8Y 2 O 3-ZrO 2 , 25CeO 2-2.5Y 2 O 3-ZrO 2 and 22MgO-ZrO 2) were characterized at temperature range of RT-1300 C. Coatings were studied in laser-glazed and phosphate sealed state, and their properties were compared to as-sprayed coatings. Laser glazing affected only slightly on thermal conductivity of the studied coatings. If the segmentation cracks, induced by laser glazing, were oriented vertically like in the case of laser-glazed 8Y 2 O 3-ZrO 2 coating, thermal conductivity was increased in some degree. But if the orientation of the segmentation cracks was deviated from the vertical direction or if the cracks were branched, thermal conductivity was decreased. This was the case with the laserglazed 25CeO 2-2.5Y 2 O 3-ZrO 2 and 22MgO-ZrO 2 coatings. Phosphate based sealing treatments were found to increase the thermal conductivity of all coatings. Aluminium phosphate sealing also lowered the high temperature stability of the 8Y 2 O 3-ZrO 2 coating down to 1000 C. In 8Y 2 O 3-ZrO 2 and 25CeO 2-2.5Y 2 O 3-ZrO 2 based coatings thermal conductivity was increased in consecutive measurement cycles, caused mainly by the sintering based phenomena in which the contact between overlapping lamellae was improved. Thermal conductivity of the 22MgO-ZrO 2 based coatings was increased significantly in the first measurement cycle because of the unstabilization of zirconia-caused by precipitation of MgO.
Advances in Materials Science and Engineering : An International Journal (MSEJ), 2016
Thermal Barrier Coatings (TBCs), routinely prepared from Ceramic based compositions (typically 8%Y2O3-ZrO2or 8YSZ) are being engineered to protect the metallic components from degradation in applications like gas turbines, jet and automotive engines. With a goal of finding improved TBC materials a wide variety of ceramics are being researched worldwide. Before physically preparing the TBCs of uncommon compositions in the laboratory, their suitability to perform can be predicted. Limited accessibility to detailed and realistic information on the influence of newer compositions (other than 8YSZ) on TBCs warrants methods to obtain this information.
Materials
In this work, functionally graded lanthanum magnesium hexaluminate (LaMgAl11O19)/yttria-stabilised zirconia (YSZ) thermal barrier coating (FG-TBC), in as-sprayed and laser-glazed conditions, were investigated for their thermal shock resistance and thermal insulation properties. Results were compared with those of a dual-layered coating of LaMgAl11O19 and YSZ (DC-TBC). Thermal shock tests at 1100 °C revealed that the as-sprayed FG-TBC had improved thermal stability, i.e., higher cycle lifetime than the as-sprayed DC-TBC due to its gradient architecture, which minimised stress concentration across its thickness. In contrast, DC-TBC spalled at the interface due to the difference in the coefficient of thermal expansion between the LaMgAl11O19 and YSZ layers. Laser glazing improved cycle lifetimes of both the types of coatings. Microstructural changes, mainly the formation of segmentation cracks in the laser-glazed surfaces, provided strain tolerance during thermal cycles. Infrared rapid...
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Increasing the turbine hot gas inlet temperature is a potential way to improve the efficiency of the land base gas turbines. Since the nickel and cobalt based superalloy materials can not face temperatures higher than 950 o C, thermal barrier coatings (TBC) with better insulation properties are needed. For this reason thicker TBCs (> 500µm) are needed to improve the thermal insulation. However, the increased thickness of TBCs may lead to a reduced coating lifetime. In order to overcome this drawback, two modifications of the thick 8Y 2 O 3-ZrO 2 TBC's structure were studied. Within these two modifications the TBC coating surface layer was sealed by using phosphate impregnation or laser glazing. These procedures are expected to improve coating hot corrosion and thermal cycling resistance, due to the denser coating surface or with the controlled vertical crack network. In this study thermal diffusivity and specific heat analysis together with microstructural characterization were carried out considering the sintering effect and possible phase transformations at elevated temperatures, up to 1250 o C. Qualitative explanation of experimental results was taken account by modelling the effect of porosity on the thermal properties of TBC. Broad variations in microstructural and thermophysical properties were observed within modified coatings.
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.
Microstructural evaluation of laser remelted gadolinium zirconate thermal barrier coatings
Surface and Coatings Technology, 2015
Thermal barrier coatings having NiCoCrAlY bond coat and Gd 2 Zr 2 O 7 top coat were produced utilizing HVOF and APS processes. Then the coatings were subjected to CO 2 continuous wave laser remelting by using different laser parameters. The effect of laser remelting process on the surface roughness, microstructure, grain size, hardness and phase transformation was investigated. The microstructural characterizations showed that a smooth, flat and dense surface having a network of segmented cracks was observed and open porosities were sealed throughout with a thin (~35 μm) remelted layer. The surface roughness value decreased from 8.3 μm to 2.9 μm for as-sprayed and laser remelted specimens respectively. Furthermore, equiaxed and columnar grains that are perpendicular to the surface were formed and the grain sizes decreased from 7.03 μm to 3.69 μm due to laser process parameters. The hardness value increased from 10.66 GPa to 12.73 GPa with decreasing grain size. XRD patterns of as-sprayed and laser remelted top coats indicated that the structure of the coating has been changed due to preferred orientation after surface modification by laser remelting.
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...
Modified thick thermal barrier coatings: microstructural characterization
Journal of The European Ceramic Society, 2004
Thick thermal barrier coatings were modified with laser glazing and phosphate based sealing treatments. Surface porosity of the sealed coatings decreased significantly in all cases. Structural analysis showed a strong preferred crystal orientation of the t 0 ZrO 2 phase in direction [002] in laser-glazed 25CeO 2 -2.5Y 2 O 3 -ZrO 2 coating. In laser-glazed 22MgO-ZrO 2 coating the major phase was rhombohedral Mg 2 Zr 5 O 12 . In phosphate sealed 8Y 2 O 3 -ZrO 2 coating the strengthening mechanism was identified as adhesive binding without chemical bonding. Coating microstructures were determined by scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy and optical microscopy. Coatings were also characterized by X-ray diffraction, microhardness and porosity. #
2 3 Surface Modification of Thermal Barrier Coatings by Single-Shot Defocused Laser Treatments
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Air plasma spray Electron beam-physical vapor deposition Thermally grown oxide Yttria-stabilized zirconia Thermal conductivity a b s t r a c t Thermal barrier coating is a crucial thermal insulation technology that enables the underlying substrate to operate near or above its melting temperature. Such coatings bolster engineers' perpetual desire to increase the power and efficiency of gas turbine engines through increasing the turbine inlet temperature. Advances in recent years have made them suitable for wider engineering and defense applications, and hence they are currently attracting considerable attention. A thermal barrier coating system is itself dynamic; its components undergo recurrent changes in their composition, microstructure and crystalline phases during its service life. Nevertheless, the performance of multi-layered and multi-material systems tailored for high temperature applications is closely linked to the deposition process. The process improvements achieved so far are the outcome of increased understanding of the relationship between the coating morphology and the operating service conditions, as well as developments in characterization techniques. This article presents a comprehensive review of various processing techniques and design methodologies for thermal barrier coatings. The emphasis of this review is on the particle technology; the interrelationship between particle preparation, modification and the resulting properties, to assist developments in advanced and novel thermal barrier coatings for engineering applications.