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The finite element analysis of the uncoated SN282 silicon nitride vane was performed using the thermal boundary conditions and temperature dependent material properties. Figure 5 shows the temperature distribution and maximum principal stress on the vane in steady state condition. The maximum temperature in the vane (1300°C/2376°F) was at mid-span and the minimum temperature was 640°C (1185°F). The thermal results were then used to predict the stresses in the vane. The maximum principal stress at the trailing edge (TE) of the vane was 245 MPa (35.5 ksi). The analysis was performed for two extreme cases to determine the effect of residual stresses on the stresses during operation. In the first scenario, it is assumed that above certain elevated temperature (called stress-free temperature), the substrate and coating have zero thermal stress and when the system is cooled to RT, residual stresses are developed due to difference in material’s CTEs. In the second scenario, it is assumed that the residual stresses during cooling from annealing temperature are fully relieved, either by micro-cracking or local plastic deformation, and therefore at room temperature, the EBC and vane are stress-free. Figure 6(a) shows the temperature gradients in the EBC at steady state and Figure 6(b) shows the temperature distribution in the vane underneath the EBC for the first scenario. The maximum temperature in the EBC was 1323°C (2413°F) and that in the substrate was 1289°C (2352°F). Compared to the case of no EBC layer, the EBC provides a temperature difference of 13°C (24°F). The EBC surface temperature is higher than that of silicon nitride due to much lower thermal conductivity. Figure 7 shows the corresponding thermal stresses in the vane. In the second scenario described above, the stresses calculated are those generated during the heat-up from RT to operation (~1300°C/~2400°F). The stress at the trailing edge for this case is 364 MPa (52.8 ksi), ~49% higher than that predicted for without EBC case. Table 1 lists the analysis steps and maximum and minimum temperatures and stresses in the vane and EBC for both cases. The most likely scenario is that a part of the residual stress will be relieved by local cracking to between 260 to 364 MPa (37.8 to 52.8 ksi), an increase of 6-49% compared to no EBC case depending on the degree of cracking. In addition to the stresses in silicon nitride, as shown in Table 1, the tensile stresses in EBC can also be fairly high (172 MPa/~25 ksi) and these stresses can initiate cracking in the coating itself or at the coating- vane interface during the cool-down after annealing. 3.0 2.5 2.0 1.5 1.0 0.5 0 500 1000 1500 2000 2500 Temperature (F) SN282 AS800 EBC Figure 4. CTEs of Silicon Nitride (SN282 and AS800) and EBC (dense BSAS) The analysis was repeated with a 15-mil EBC on the vane. While the EBC coated component is cooled from the annealing temperature to room temperature (RT), below a certain stress-free temperature, residual stresses are developed in the EBC and vane due to CTE mismatch. The coating will be under tensile residual stresses (due to higher CTE) and the substrate will be under compression. However, during cooling from the annealing temperature, part of the residual stresses can be relieved by cracking. When the vane and EBC are heated to operating condition, the stress-state is reversed and the EBC is in compression and vane surface in tension. 135 Copyright © 2004 by ASME CTE (ppm /F)PDF Image | ADVANCED MICROTURBINE SYSTEMS Final Report for Tasks 1 Through 4 and Task 6
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