Papers by Keyword: Thermally Grown Oxide (TGO)

Abstract: Finite element simulation of stress distribution of thermal barrier coating system (TBCs) is presented. Two dimensional periodic unit cells are used to examine the stress development and critical sites with high potential of cracking during thermal cycling. During cooling, high tensile out-of-plane stresses in the peak of the thermally grown oxide (TGO) are formed, which lead to crack initiation in the vicinity of TGO and the interface. At the same time, high compressive stresses developed in the valley domain. The influence of crack within the top coat in the vicinity of the TGO is also investigated. The finite element analysis shows that crack seriously affects stress field development and the thermal-mechanical behavior of TBCs. Based on the finite element analysis results one can conclude that imperfections and its development should be always considered to be a crucial parameter for TBCs life.

Abstract: TBCs is a type of multilayer systems, mainly used in the thermal parts of aero engine, acting as the part of heat insulation. The using temperature of parts can be improved because of the being of TBCs. TBCs is one of the most advanced high temperature coatings, and it has many perfectly properties, including the chemical property at high temperature, anti-erode and thermal insulation [1,2]. If TBCs spall from the base, the base will expose in the high temperature, then, rapidly destroy. TBCs generally include three layers, superalloy, bonding layer, insulation layer, and the thermally grown oxide (TGO) between the bonding layer and insulation layer. Justly because of the being of TGO, the TBCs easily failed [3,4]. Although many researchers studied the oxidation of TBCs, the investigation of the relations between oxidation and oxide temperature has not been done by far. So, the work put the emphases on the discussion of APS TBCs’ oxidation property in different temperatures by experiment.

Abstract: Thermal fatigue damage evolution behavior in thermal barrier coatings (TBCs) was studied, by employing the originally designed two dimensional ring-shape TBC specimen. The TBC specimen consisted of Ni-based superalloy IN738LC substrate, bond coat, and 8 wt.% Y2O3-stabilized ZrO2 (YSZ) top coat. The top coat was fabricated by electron-beam physical vapor deposition (EB-PVD) method with 250 micron-meters in thickness. Three kinds of MCrAlY bond coat alloys were specified as an experimental variable. Through the work, special attention was paid not only to the failure life of TBC specimen, but also to the underlying failure mechanisms. Some problems have been also pointed out, on feeding back these experimental findings to engineering applications.

Abstract: The effects of bond coat nature in thermal barrier coating (TBC) systems on the delamination or fracture behavior of the TBCs with different bond coats prepared using two different processes—air plasma spray (APS) and high velocity oxyfuel (HVOF)—were investigated by cyclic thermal fatigue tests. The TBCs with the HVOF bond coat were delaminated or fractured after 3–6 cycles, whereas those with the APS bond coat were delaminated after 10 cycles or show a sound condition. These results indicate that the TBC system with the APS bond coat has better thermal durability than the system with the HVOF bond coat under long-term cyclic thermal exposure. The hardness values of the TBCs (top coats) in both systems are dependent on applied loads, irrespective of the hardness of the bond coats and the substrate. The values are not responded to the bond coat nature or the exposure time. Thermally grown oxide (TGO) layers in both cases consist of two regions with the inner TGO layer containing only Al2O3 and the outer TGO layer of mixed-oxide zone containing Ni, Co, Cr, Al in Al2O3 matrix. The outer TGO layer has a more irregular shape than the inner TGO layer, and there are many pores within the outer layer. At failure, the TGO thickness of the TBC system with the HVOF bond coat is 9–13 m, depending on the total exposed time, and that of the TBC system with the APS bond coat is about 20 m. The both TBC systems show the diffusion layer on the side of substrate in the interface between the bond coat and the substrate. The relationship between the delamination or fracture behavior and the bond coat nature has been discussed, based on the elemental analysis and microstructural evaluation.

Abstract: It is well known, that thermally grown oxide (TGO) forms at the interface between the thermal barrier top coating (TBC) and the bond coating during service. In previous work, SEM images showed that the TGO layer contained at least two layers with different oxides. One layer was Al2O3, and the other was a mixed oxide of NiO, CoO, Cr2O3, and their spinels. It was supposed that a reason for macro-crack formation near an interface is due to a decrease in bond strength or to the formation of stress concentration sites caused by the formation of pores in the mixed oxide. In order to improve the bond strength, a modified bond coating material was developed, which is MCrAlY with Ce and Si added. Four- point bending tests were carried out to measure the bond strength of conventional TBC and of the modified TBC with MCrAlYCeSi bond coating. As a result, the TBC with modified bond coating had a bond strength superior to the conventional one. It is likely that the reason for the superior bond strength is due to a notable difference in oxidation behavior.