ICMCTF2011 Session AP: Symposium A Poster Session

To prevent Co diffusion from cemented carbides at high working temperatures, we fabricated CrTaN coatings by reactive direct current magnetron co-sputtering onto 6 wt.% cobalt cemented carbide substrates, to form diffusion barrier layers. The nitrogen flow ratio, N₂/(Ar+N₂), during the sputtering process set at 0.4. The deposition rates of CrTaN coatings, 19 to 25 nm/min, depended on the sputter yield of targets. The CrTaN coatings crystallized into a columnar structure, without heating the substrates during the sputtering process. The CrTaN coatings were annealed at 500, 600, and 700^oC for 4 hours in air. We evaluated the performance of the diffusion barrier using both Auger electron spectroscopy depth-profiles and X-ray diffraction techniques. We also investigated oxidation resistance of the CrTaN coatings annealed in air, and under a 50 ppm O₂-N₂ atmosphere, to evaluate the fabricated layers effectiveness as a protective coating for glass molding dies.

Thermal properties and failure mechanisms of thermal barrier coatings (TBCs) are closely related to their microstructure, which is usually determined by the feedstock powder as well as the spraying condition. Recently, TriplexPro^TM-200 system has been launched to offer an advanced TBC performance resulting from higher particle velocity, lower particle oxidation, and higher coating density, compared with the commercial air plasma spray (APS) system using 9MB gun. Therefore, in this study, TBC samples were prepared by TriplexPro^TM-200 system using different commercialized powders and the microstructure of TBC was controlled by the reheating the surface of TBC without powder feeding in same equipment. The relatively porous TBC was prepared with METCO 204 C-NS and the relatively dense TBC with METCO 204 NS. The microstructure of the top coat in TBCs was just controlled, and the bond coat with about 300 mm thickness in the both top coats was prepared with AMDRY 962. The cyclic thermal exposure tests were performed at the surface temperature of 1100℃ with the temperature difference of 150℃ between the surface and bottom of sample, with a dwell time of 1 h till 24000 EOH (1143 cycles) or delamination, in a specially designed apparatus. The distance parameter is more effective in creating the vertical cracks than the gun speed, with less and shorter vertical cracks in the relative porous TBC. The hardness values on the sectional plane are gradually changed in the both vertical-cracked TBCs, indicating that those of the relatively porous TBC are changed from 6.5 (interface of the top and bond coats) to 4.2 GPa (surface of the top coat) and those of the relatively dense TBC being changed from 7.8 to 4.5 GP. After the thermal exposure tests, the vertical cracks reach down to the interface of the top coat and the TGO layer in the relatively dense TBC, and the hardness and toughness values are decreased and increased, respectively, in the both TBCs. The both vertical-cracked TBCs show a sound condition without any evidence of delamination, while the relatively dense TBC without the vertical cracks is delaminated within 8000 EOH (381 cycles) or shows an evidence of delamination at the interface of the top coat and the TGO layer after 8000 EOH. Consequently, in the both cases, the TBCs with the vertical cracks are more efficient in thermal durability than those without the vertical cracks, especially in the relatively dense TBC. The relationship between the microstructural evolution and thermomechanical characteristics of the TBCs with and without the vertical cracks is discussed.

During the two last decades, plasma Spray and EBPVD TBCs have been intensively used in turbines for aircraft propulsion and power generation. However, interfacial damages due to thermal exposure and oxidation remain not totally understood. Thus, the knowledge on lifetime prediction of TBCs before spallation is still limited. In this work, the laser shock spallation has been applied to generate interfacial defects as those occurring in TBCs during thermal cycling. This method is derived from the LASAT (Laser Shock Adhesion Test) protocole to determine adhesion of ceramic coating. Depending on the laser energy and dimensions of the LASATested coatings (layers thicknesses, laser diameter mainly), it has been evidenced that the circular crack size located at the zirconia-bond coat interface can be easily varied for further non destructive assessments by photo-stimulated luminescence. Thus, different damage areas at bond coat-zirconia interface have been realized without the removal of TBC ceramic top-coat for coated samples with different aging periods of one-hour cycles at 1100°C (0 and 100 cycles). The photo-stimulated luminescence was used as a powerful Non-Destructive Assessment of the damages induced by laser shock through the measure of the released stress in the TGO and the corresponding variation in the photo-excitation of Cr3+ ions within the thermally grown oxide (TGO). The damaged areas were detected by this method and the dimensions were also measured accurately using scanned images obtained by photoluminescence piezospectroscopy. The crack damages were also investigated by visual inspection and SEM cross-sectioned observations for further comparison with actual dimensions of the interfacial defects that were deliberately induced by LASAT.

Presented investigations will concern characteristics of the gradient TBC double-layer type Gd₂Zr₂O₇ - ZrO₂Y₂O₃ (YSZ). The layers were placed on a nickel superalloys type AMS5599 with an bond-coat type NiCrAlY, obtained by the VPS (vacuum plasma spraying) method. Ceramic layers were applied in a result of plasma spraying by the APS method with powders of a general formula Gd₂Zr₂O₇ and ZrO₂Y₂O₃ obtained by a spray drying method. Four types of samples were prepared. They varied according to volumetric fraction of ceramic powders situated in loaders during APS process (Gd₂Zr₂O₇ and ZrO₂Y₂O₃ powders volumetric fractions, respectively: 25% - 75%; 50% - 50%; 75% - 25%; 100% -100%).

The last sample with 100% - 100% volumetric fraction - bilayer coating was obteined in different way than others: ZrO₂Y₂O₃ layer was applied firstly and than Gd₂Zr₂O₇ layer was sprayed on it.

A range of investigations, presented in the paper will comprise:

· evaluation of microstructure of a ceramic layer from a point of view of thickness, quality of a layer and quantity characteristics of cracks and pores architecture;

· evaluation of thermal diffusivity of a ceramic layer within a range of temperature 25-1100°C;

· determination of thermal conductivity of the TBC louble-layer type Gd₂Zr₂O₇ - ZrO₂Y₂O₃.

Results, presented in the paper, are effects of long-running investigations, carried out by the Department of Materials Science in the Silesian University of Technology, and these investigations concerned heat- resisting coatings and layers type TBC.

The scope of this study covers characterization of the microstructure and chemical composition of the TGO zone in the TBC layers isothermally annealed at 1100°C in air or argon. Conventional layers based on 8YSZ powders and a new type of layer based on a zirconate powder Gd₂Zr₂O₇ with a pyrochlore structure were studied. Annealing proceeded over 2 or 10 h in argon, and over 10 h in air. The samples were microstructurally analyzed using scanning microscopy and an assessment of the chemical composition of the micro-areas was conducted. The thickness and structure of the oxide zone was determined for the different annealing atmospheres and annealing times. TBC coatings were obtained by the APS method, in which the standard parameters for the spraying process were applied. Analysis showed that the TGO zone thickness of the new type of layer was approx. 50% smaller in comparison with the thickness of the conventional TBC (YSZ). In both cases, the main ingredient of this zone was aluminum oxide. The thickness of the argon-annealed oxide layer was larger than the thickness of the air-annealed layer, and complex oxides formed with nickel and chromium on the outer parts of the TGO in the second case.

One suggested strategy for improving the performance of thermal barrier coating (TBC) systems used to protect hot section components in gas turbines is the addition of low levels of dopants to the Ni-base superalloy substrate. For the more aggressive environment expected for coal-derived, synthesis gas-fired turbines, this strategy may be effective in retaining TBC durability. To quantify the benefit of these dopants, coupons of three commercial alloys with different Y and La contents were coated with a NiCoCrAlYHfSi bond coating by high velocity oxygen flame spraying. Coupons were oxidized in cyclic exposures at 1050°, 1100° and 1150°C; and the oxidation rate and alumina scale adhesion were compared in an effort to better understand the benefit of superalloy dopants. In addition, some of these substrates will be spray coated with yttria-stabilized zirconia to determine the effect of dopants on coating lifetime.

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This research was sponsored by the U.S. Department of Energy, Office of Coal and Power R&D, Office of Fossil Energy, (R. Dennis program manager).