PHASE COMPOSITION OF OXIDE FILMS FORMED ON THE SURFACE OF THE FE-CR-AL SYSTEM COATINGS

The phase composition of the oxide films on the surface of the Fe-Cr-Al system coatings is studied using glancing angle X-ray diffraction. It is shown that at 900 °С the formed oxide films consist of αAlO and (FeCr)O, to which FeAlO oxide is added during long-term exposure. An increase in temperature to 1100 °C intensifies the growth of oxide films, and an increase in the aluminum content ensures a stable growth of αAlO and FeAlO oxides. When the aluminum content in the coating is more than 10 at. % at large exposure times, metastable alumina δAlO is formed, the formation of which is associated with a decrease in the concentration of chromium in thin surface layers.

Development of Highly Durable Thermal Barrier Coating by Suppression of Thermally Grown Oxide

Durability of thermal barrier coating (TBC) systems is important because of recent rising of turbine inlet temperature (TIT) for improved efficiency of industrial gas turbine engines. However, high-temperature environment accelerates the degradation of the TBC as well as causes spalling of the top coat. Spalling of the top coat may be attributed to several factors, but evidently the growth of thermally grown oxide (TGO) should be considered as an important factor. One method for reducing the growth rate of TGO is to provide a dense α-Al2O3 layer at the boundary of the bond coat and top coat. This α-Al2O3 layer will suppress the diffusion of oxygen to the bond coat and consumption of aluminum of the bond coat is suppressed. In this study, we focused on thermal pre-oxidation of the bond coat as a means for forming an α-Al2O3 barrier layer that would be effective at reducing the growth rate of TGO, and we studied the suitable pre-oxidation conditions. In the primary stage, we analyzed the oxidation behavior of the bond coat surface during pre-oxidation heat treatment by means of in situ synchrotron X-ray diffraction (XRD) analysis. As a result, we learned that during oxidation in ambient air environment, in the initial stage of oxidation metastable alumina is produced in addition to α-Al2O3, but if the thermal treatment is conducted under some specific low oxygen partial pressure condition, unlike in the ambient air environment, only α-Al2O3 is formed with suppressing formation of metastable alumina. We also conducted transmission electron microscope (TEM) and XRD analysis of oxide scale formed after pre-oxidation heat treatment of the bond coat. As a result, we learned that if pre-oxidation is performed under specific oxygen partial pressure conditions, a monolithic α-Al2O3 layer is formed on the bond coat. We performed a durability evaluation test of TBC with the monolithic α-Al2O3 layer formed by pre-oxidation of the bond coat. An isothermal oxidation test confirmed that the growth of TGO in the TBC that had undergone pre-oxidation was suppressed more thoroughly than that in the TBC that had not undergone pre-oxidation. Cyclic thermal shock test by hydrogen burner rig was also carried out. TBC with the monolithic α-Al2O3 layer has resistance to >2000 cycle thermal shock at a load equivalent to that of actual gas turbine.

Development of Highly Durable Thermal Barrier Coating by Suppression of Thermally Grown Oxide

Durability of thermal barrier coating (TBC) systems is important because of recent rising of TIT (Turbine inlet temperature) for improved efficiency of industrial gas turbine engines. However, high-temperature environment accelerates the degradation of the TBC as well as causes spalling of the top coat. Spalling of the top coat may be attributed to several factors, but evidently the growth of thermally grown oxide (TGO) should be considered as an important factor. One method for reducing the growth rate of TGO is to provide a dense α-Al2O3 layer at the boundary of the bond coat and top coat. This α-Al2O3 layer will protect the bond coat against oxidation and prevent outward diffusion of aluminum of the bond coat which causes further oxidation. In this study, we focused on thermal pre-oxidation of the bond coat as a means for forming an α-Al2O3 barrier layer that would be effective at reducing the growth rate of TGO and we studied the suitable pre-oxidation conditions. In the primary stage we analyzed the oxidation behavior of the bond coat surface during pre-oxidation heat treatment by means of in-situ synchrotron X-ray diffraction (XRD) analysis. As a result, we learned that during oxidation in ambient air environment, in the initial stage of oxidation metastable alumina is produced in addition to α-Al2O3, but if the thermal treatment is conducted under some specific low oxygen partial pressure condition, unlike in the ambient air environment, only α-Al2O3 is formed with suppressing formation of metastable alumina. We also conducted TEM and XRD analysis of oxide scale formed after pre-oxidation heat treatment of the bond coat. As a result, we learned that if pre-oxidation is performed under specific oxygen partial pressure conditions, a monolithic α-Al2O3 layer is formed on the bond coat. We performed a durability evaluation test of TBC with the monolithic α-Al2O3 layer formed by pre-oxidation of the bond coat. An isothermal oxidation test confirmed that the growth of TGO in the TBC that had undergone pre-oxidation was suppressed more thoroughly than that in the TBC that had not undergone pre-oxidation. Cyclic thermal shock test by hydrogen burner rig was also carried out. TBC with the monolithic α-Al2O3 layer has resistance to >2000 cycle thermal shock at a load equivalent to that of actual gas turbine.

Effects of Different Raw Materials in the Synthesis of Boehmite andγ- andα-Alumina

Two alumina polymorphs, the metaestableγ-Al2O3and the stableα-Al2O3, were obtained from thermal treatment of the precursorγ-AlOOH (boehmite). This precursor was prepared by a precipitation method employing different raw materials in order to study their effect on the synthesis process and several characteristics of the materials, such as the crystallite size, the thermal behavior, and the surface area. Aluminum chloride (AlCl3·6H2O) and an aluminum waste were used as the source of aluminum. A 1 M NaOH solution and a 1 M n-butylamine solution were used as alkalizing agents, due to their strong and weak alkaline characteristics, respectively. The XRD profiles of the boehmites obtained from waste show lower crystallinity than samples obtained from aluminum chloride. The content of water, from TG studies, was higher in the samples obtained from waste, which fit well with the smaller crystallite size. The use of n-butylamine as alkalizing agent favors the formation ofγ-alumina with higher surface area (177.2 cm2 g−1, for aluminum waste, and 159.4 cm2 g−1, for aluminum pure reagent). The temperature of transformation from gamma to alpha, from DTA results, is higher for samples obtained from the waste, and accordingly the presence of impurities in the waste stabilizes the metastable alumina phase.