TGO Perfection

Phase constituents of TGO, especially during early stages of oxidation, have been identified as critical factors influencing the adhesion at TGO/coating interface. [8'17-20] Specifically, the formation of the transient 9-Al2O3 and its conversion to the stable a -Al2O3 in the protective oxide scale has been reported to have a profound effect on the structural integrity of TGO/coating interface during thermal cycles. An image[19] of a -Al2O3 islands nucleated within a 9-Al2O3 TGO formed by oxidation of NiAl for 1 hour at 1000° C is presented in Figure 2. It has been proposed by Clarke and coworkers[18'19] that the transformation from 9-to-a Al2O3 is responsible for additional residual stress from the volumetric constraint in the TGO scale and nucleation of sub-critical cracks, eventually leading to the spallation of a -Al2O3 TGO scale. Thus, the formation of a "perfecf oxide that consist only of stable a-Al2O3 prior to deposition of ceramic layer and thermal cyclic oxidation can lead to improved oxidation resistance, durability and reliability of both stand-alone metallic coatings and TBC bond coats.

Figure 2. Photoluminescence image of a-Al2O3 islands nucleated within a 9-Al2O3 TGO formed by oxidation of NiAl for 1 hour at 1000° C.[19]

Extensive microstructural examination of TBCs by Gell and co-workers[11'12] has identified bond coat surface features/defects as damage initiation sites. Figure 3 shows these surface features/defects to be: "ridges" associated with platinum aluminide (Pt-Al) coatings and "entrapped" oxides associated with shot-peening of MCrAlY coatings. Also presented in Figure 3 are the oxide cavities and accelerated growth of oxide scale resulting from the cyclic plasticity and oxidation of these features/defects during thermal cycling. It has been demonstrated that the removal of the ridges by fine polishing can improve the TBC lifetime by 3X.[35]

Figure 3. Surface features/defects: (a) "ridges" and (b) oxide-filled cavities in platinum aluminide (Pt-Al) EB-PVD TBCs; (c) "entrapped" oxides and (d) oxide-filled cavities associated with MCrAlY EB-PVD TBCs. These features/defects are present due to the processing of the coatings before thermal cycling and evolve into oxide-filled cavities during thermal cycling.[11'12]

Figure 3. Surface features/defects: (a) "ridges" and (b) oxide-filled cavities in platinum aluminide (Pt-Al) EB-PVD TBCs; (c) "entrapped" oxides and (d) oxide-filled cavities associated with MCrAlY EB-PVD TBCs. These features/defects are present due to the processing of the coatings before thermal cycling and evolve into oxide-filled cavities during thermal cycling.[11'12]

In addition, surface roughness has a significant effect on the level of in-plane and out-of-plane tensile stress in the TGO[13'14] as illustrated in Figure 4. In-plane tensile stresses crack the TGO, allowing molecular oxygen to reach the bond coat surface, and oxidation is accelerated. Out-of-plane tensile stress eventually leads to TGO and TBC spallation. Thus, a reliable industrial processing technique that ensures optimum surface roughness and consistent removal of the undesirable surface features/defects would provide improved durability and reliability for both stand-alone and TBC bond coats.

Under the current UCONN AGTSR contract, AGTSR 99-01-SR073, residual stress in the TGO is being measured using a laser fluorescence technique as a function of thermal cycles for various commercial TBC systems. This technique, pioneered by Qarke,[11'12'15"19'36] measures the residual stress in the TGO by examining the shifts in wave-number of Cr3+ photoluminescence in a-Al2O3 TGO scale. The shift in photoluminescence can be translated into a biaxial residual stress in TGO through piezo-spectroscopic coefficients. This technique has been applied successfully for both laboratory scale TBC specimens, and thermal barrier coated engine components.[37]

During this study, researchers at UCONN and UC-SB (subcontractor) have found that the laser fluorescence can readily provide information regarding (1) the formation and transformation of transient phases in TGO, specifically regarding the 6-to- a Al2O3 transformation[17-19] (2) changes in the TGO stress due to surface roughness/defects and (3) presence of microscopic spallation of TGO from metallic coatings. Figure 5 shows a typical spectrum collected from a TGO that contain both 6-Al2O3 and a-Al2O3. Preliminary results also reveal that the surface preparation of bond coat can significantly influence the residual stress of TGO. In addition, the laser fluorescence technique can conveniently detect the presence of microcracks that are associated with spallation (i.e., relief of residual stress) of TGO scale as shown in Figure 6.

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