Effect of Surface Geometry on Stress Generation in Thermal Barrier Coatings During Plasma Spray Deposition
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Effect of Surface Geometry on Stress Generation in Thermal Barrier Coatings During Plasma Spray Deposition Guosheng Ye and Soumendra Basu Department of Manufacturing Engineering Boston University, Brookline, MA 02246, U.S.A. ABSTRACT A fully coupled thermo-mechanical finite element model was used to study the buildup of stresses during splat solidification, and to understand the effect of deposition conditions on crack formation during plasma spray deposition. Through the simulation, the locations and magnitudes of maximum stresses were identified, where crack formation would presumably initiate. The model showed that the stresses scaled with the temperature difference between the superheated splat and the substrate. The simulation further showed that the stresses scale with the three geometric parameters, and two independent geometric ratios were defined; z (defined as t/l) and y (defined as A/l). 2D maps of maximum S11 and S 22 under different combinations of z and y were constructed. The mappings showed that only roughness features on the scale of splat thickness were important in providing locations of maximum stress concentration. INTRODUCTION The need for cleaner and more efficient operation mandates the use of increasingly higher temperatures in advanced gas turbines. In order to protect the metallic components in the hot sections of these turbines, thermal barrier coatings (TBCs) are used. These TBCs can maintain a significant temperature drop across their thickness due to their low thermal conductivities. This allows the metallic components, typically made of high temperature superalloys, to be exposed to lower temperatures, leading to an increase in their service lifetimes. Typically, in land-based gas turbines, air plasma sprayed TBC top-coats are deposited over a vacuum plasma sprayed bond coat. The plasma spray deposition deposition process involves introducing ceramic powders, typically yttria stabilized zirconia (YSZ), into the high temperature plasma plume. The powders melt in the plume and the molten droplets are accelerated towards the substrate. On impingement, the molten droplets spread on the substrate forming rapidly quenched splats. Typically, several successive splats are deposited over a given area in one pass, and the coating thickness is built up using several passes over the substrate. The rapid quenching associated with the plasma spray process combined with the brittle nature of the ceramic top-coat typically leads to a high density of microcracks. Typically, plasma sprayed TBCs contain horizontal cracks at splat boundaries and vertical cracks that extend through the splat thickness. Under appropriate conditions, longer vertical cracks that extend through the thickness of the coatings (termed as 'segmentation cracks') can be formed [1]. These cracks increase coating compliance and extend their lifetimes by resisting TBC spallation. Horizontal cracks reduce the thermal conductivity of the coatings in the direction of heat transfer, making it a more effective thermal barrier. Howe
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