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silicon nitride [13]. It was also found that the application of the silicon bond coat alone to the substrate (with or without a top layer) results in about 50% debit in room temperature strength of the substrate indicating that a coating architecture with silicon as bond coat applied on to Si3N4 is undesirable[13]. The silicon coatings are typically very well bonded to the substrate and the CTE mismatch cracks (as observed) are expected to interact with the pre-existing cracks in the ceramic resulting in a significant reduction in fracture strength of the ceramic. (a) (b) Through thickness crack Non-compliant coating Silicon nitride (a) Figure 2: (a) Effect of BSAS-based EBC coating on strength of Si3N4. (b) Schematic mechanism of strength reduction. (c) Microstructure of BSAS EBC on Si3N4 that shows the flaws in the coating. Further efforts were made to understand the mechanism for the strength debit of the substrate using numerical modeling. When a thin layer of silicon is deposited on a silicon nitride substrate and the system is cooled from a stress-free temperature of 1000°C (1832°F), the in-plane tensile residual stress in silicon given by the following equation: [σ = E∆α∆T/(1-υ)] (1) where E is the modulus of silicon, ∆αis the difference in CTE of silicon and silicon nitride, ∆T is the temperature difference and υ is the Poisson’s ratio, which is of the order of 26 ksi (=179 MPa). Although the exact value of fracture strength of silicon is unknown, it is governed by processing flaws and cracking is always observed in the silicon layer (see figure 2c). It is therefore believed that the residual stress in Si exceeds its strength. Further it is found that the magnitude of tensile stress in silicon is fairly independent of thickness of silicon layer, and thickness and moduli of intermediate and top layers, as long as thickness of coating layers is at least an order of magnitude lower than the substrate thickness (which is typically the case in most applications), as shown in Figure 3. A variety of coating architectures were considered with an objective to drive down the residual stress in the silicon layer. Some of the architectures are (c) Flame spray pattern modeled (b) Figure1:(a)TypicalSiN gasturbinecomponents. 34 (b) Study of the feasibility of thermal spraying an integral vane ring. 146 Copyright © 2007 by ASME 1000 1000 800 0 0 AS800 AS800 20C-30 MPa/s 1204 C-30 MPa/s 20C-30 MPa/s 1204 C-30 MPa/s 800 600 600 400400 200200 w/ EBC w/ EBC Flexural Strength (MPa) Flexural Strength (MPa)PDF Image | ADVANCED MICROTURBINE SYSTEMS Final Report for Tasks 1 Through 4 and Task 6
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