Microstructural Characterization of a Zirconia-Titania-Yttria Thermal Barrier Coating
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Most of above the mentioned zirconia alloys have been used as thermal barrier coatings. However, ZrO 2 alloyed with 6-10 wt% Y 20 3 is accepted widely as the TBC exhibiting the most desirable properties in service [12-14]. It is thought that the ZrO2-TiO2 -Y20 3 ceramic coating exhibits improved properties such as erosion, abrasion, and corrosion resistances compared with the ZrO 2-Y20 3 based coatings[15]. In a previous study[161, a comparison between these two systems was carried out in this laboratory. In the present work, the phase transformations in a plasma sprayed ZrO 2-TiO 2-Y 20 3 TBC, subjected to different thermal treatments, in the temperature range 900-1400'C, were characterized by XRD. The fracture surfaces of the as-sprayed and thermally treated ceramic coatings were investigated using SEM. As-sprayed and thermally treated (1400'C) ceramic microstructures were also studied using TEM. EXPERIMENTAL DETAILS Specimens The specimens, consisted of pieces (3x3x0.5 cm) of 304A Ni-Cr stainless steel with an termediate thermally sprayed bonding coat (17.5wt%Cr, 5.5wt%A1, 2.5wt%Co, 0.5wt% Y 20 3 and the balance being Ni) and a top plasma sprayed zirconia coat (18wt%TiO 2, lOwt% Y 20 3). 253 Mat. Res. Soc. Symp. Proc. Vol. 403 @1996 Materials Research Society
Thermal Treatments A set of these specimens were soaked at either 900, 1200 or 1400'C for 24 hours. Subsequently these were taken out of the muffle furnace and left to reach room temperature in approximately 60 minutes. Another set of specimens, heated at either 900, 1000, 1100 or 1200°C for 24 hours under an argon atmosphere, were left to cool down, over approximately 8 hours inside the muffle furnace under a slow argon gas flow. Characterization After these thermal treatments, the cubic, monoclinic and tetragonal ZrO 2 mole fractions in the ceramic coat of each specimen were determined using the integrated intensities of the 1111 and {400} XRD reflections, as described previously[16,171. The fracture surfaces of the ceramic coats in the as-sprayed and thermally treated conditions were studied by SEM using a JEOL JXA 840A microscope. Thin samples of the as-sprayed and thermally treated (1400'C) ceramic coats were prepared by ion milling and studied using a JEOL 2000FX transmission electron microscope. REiSULTS AND DISCUSSION Phase Transformations The changes in the proportion of the zirconia phases, observed in the thermally treated ceramic coat samples are presented in table I. The monoclinic (M) phase content of the ZrO 2 remained relatively constant. When the specimens were thermally treated in air, the cubic (C) phase transformed to the tetragonal (T) phase. Considering that the specimens were cooled rapidly to room temperature after being soaked at a high temperature, a diffusionless phase transformation process should have taken place in this case. A different trend was observed in the specimens thermally treated in argon and cooled slowly. The proportion of the T phase grows at expense of the C. However, after 800'C the T phase changes to the C, p
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