Near-Interface Crack Initiation in Thermal Barrier Coatings
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Mat. Res. Soc. Symp. Proc. Vol. 586 © 2000 Materials Research Society
Ni-based superalloy (Nimonic 263) using an air PS method. The thickness of the TBC and BC were 280 and 40 gm, respectively. Scanning Auger microprobe analysis indicated that a wavy alumina layer with the thickness of 20 gm near the TBC/BC interface formed due to inservice environmental attack [7]. Protruded TBC Specimen Testing In order to produce a stress concentration effect near the TBC/BC interface, TBC was protruded from the substrate by machining both the TBC ends of a thin plate extracted from the transition duct using a grinding machine. The protruded TBC part included the alumina and BC. By optical microscopy observation on the side surfaces of protruded specimens, it was confirmed that the ground edge of protruded TBC with the root radius (p) = 0.2 mm was located near the TBC/alumina/BC interface region. In a protruded TBC specimen without reinforcement, it was shown [7] that near-interface cracks did not form due to the extension of pre-existing transverse cracks. Thus, reinforcement made of a titanium alloy sheet (0.6 mm thick) was attached on the top of the protruded TBC using an adhesive (3M EC2214) with the thickness of 0. 1mm. The dimensions of protruded specimens and four-point bending spans are shown in Fig. 1. Mechanical tests were carried out at room tem perature using a screw-driven Instron testing machine with the cross-head speeds of 6.4 x 10- m/s. The load vs. deflection curve was monitored and stored via a computer. The morphology of TBC cracking in loadinterrupted protruded TBC specimens was examined by optical microscopy. Glue,. I
Reinforcement] Substrate
0.7 11. 1l.2
Figure 1. Geometry of reinforced protruded TBC four-point bending specimens, and the supporting and loading spans. EXPERIMENTAL RESULTS Figure 2 indicates several load vs. deflection curves obtained from tests on reinforced protruded TBC specimens. A loading rate was found to change at a critical load (PJ)before the general yielding in the substrate. In order to clarify if Pc represents the TBC crack initiation near the interface, a protruded specimen was load-interrupted just before and after Pc (BO1 specimen in Fig 2). It was shown [8] that TBC cracking took place in a protruded specimen interrupted just above Pc but not below Pc. As shown in Fig. 3, a TBC crack initiated from the root radius of the reinforced protruded TBC and extended near the TBC/alumina interface in a similar mode to in-service spalling of plasma sprayed TBC. FEM STRESS ANAYSIS In an effort to analyze the TBC delamination behavior in the protruded TBC test, FEM analysis was performed using ANSYS code. The protruded TBC specimen tested was considered as a layered structure consisting of the reinforcement, adhesive, TBC, BC and substrate with different elastic properties, as shown in Fig. 4. The elastic modulus of TBC attached with alumina (40 GPa) was directly measured using a four-point bending test on a specimen that had been extracted chemically from the BC/substrate. The modu
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