Al A5052-H14 Table 1 2 6mm WC-12Co (high velocity oxygen fuel, HVOF) WC-12Co 25 70 120μm 3 DLC WC-12Co #2000 15μm DLC/WC-12Co A5052 Thickness Effect of Interlayer on Fatigue Behavior and Fatigue Fracture A5052 Mechanisms in Aluminum Alloy with DLC/Thermally Sprayed WC-12Co Hybrid Coatings by Toshifumi KAKIUCHI , Yoshihiko UEMATSU , Makoto TAKEKAWA Takema TERATANI , Yoshifumi KOBAYASHI and Yoshio HARADA WC-12Co was thermally sprayed by a high velocity oxygen fuel (HVOF) method on A5052 aluminum alloy as an interlayer with the thicknesses of 25, 70 and 120μm and subsequently diamond-like carbon (DLC) film was deposited with the thickness of 15μm to fabricate DLC/WC-12Co hybrid coating. Rotary bending fatigue tests were conducted using the specimens with DLC/WC-12Co hybrid coating, DLC single coating, WC-12Co single coating and without coating, and the thickness effect of interlayer on the fatigue behavior was investigated. The fatigue strengths of the specimens with DLC single coating and WC-12Co single coating were higher than those of the substrate specimens. The fatigue strengths in the specimens with WC-12Co single coating increased with increasing the thickness of WC-12Co layer. The specimens with hybrid coating exhibited higher fatigue strengths than the specimens with WC-12Co single coating when the thickness of WC-12Co layer was the same. However the thicker WC-12Co interlayer led to the lower improvement of fatigue strength by hybrid coating. In the specimens with WC-12Co layer, the boundary between WC-12Co layer and substrate was rough and uneven due to the thermal spray process and a fatigue crack initiated at the concave site of substrate on the boundary. In the specimens with hybrid coating, cracking in DLC film and WC-12Co interlayer occurred simultaneously and suddenly near the end of fatigue life. A fatigue crack in the substrate had grown up to a large size and the specimen fractured soon after a crack appeared on the surface.
A MCrAlY alloy bond coat is widely used in thermal barrier coating (TBC) systems to protect substrates from high_ tempera ture oxidizing environments. However, failure of the ceramic top coat can occur due to a thermally grown oxide (TGO) layer that grows at the interface between the bond coat and the top coat. Local stresses are produced by multiple oxides that grow separate ly at the interface. It seems that the multiple oxides may originate in the rapid oxidation of unmelted MCrAlY particles attached to the bond coat during spraying. Thus, it has become an important issue to control the growth behavior of TGOs.In the present study, the effect of chromate treatment was investigated. Prior to top coat deposition, a thin film of Cr 2 O 3 was formed on the bond coat surface. High_ temperature oxidation tests were carried out using samples coated both with and without chromate pretreatment, and the oxidation rates were determined by inspection of cross sections. Similar oxidation tests were car ried out using MCrAlY powder material assumed to be unmelted particles. Chromate_ treated bond coat showed outstanding oxi dation resistance in comparison with bond coat without chromate pretreatment. In the case of pretreated MCrAlY powder, only the surfaces of particles were oxidized, even under conditions in which most particles without pretreatment were fully oxidized. This outcome is thought to result from the preferential formation of an oxidation_ resistant Al 2 O 3 layer in the early stage of oxida tion. Calculations that take into account the oxidation of particles in the top coat indicate the generation of sufficient internal ten sile stress to cause local fracture of the top coat. Because of its simplicity and applicability to complicated shapes, the chromate treatment is quite useful as a countermeasure against bond_ coat oxidation and local fracture of the top coat near the interface.
The characteristics and adhesive strength of a high Cr Fe alloy plasma spray coating applied to the inner surface of cylindrical aluminum containers of 17 NAS battery cell used to leveling electrical power for 0.2 to 10 years were studied. The operating temperature of the batteries was 593 K±15 K, and the number of charge/discharge cycles per year was 300 on average. A Sulfide layers consisting mainly of Cr 2 S 3 and Cr 3 S 4 was formed on the surface of the high Cr Fe alloy plasma spray coating, and the growth rate of sulfide layer was approximately 2 mm per year. Chromium sulfides also formed within the spray coating. Molten sulfur and Na 2 S x which are positive electrode active materials, penetrated the coating through the interfaces of the sprayed particles. The deepest sulfides almost reached the wall surface of the aluminum container. The adhesive strength of high Cr Fe alloy plasma spray coatings operating history of 3, 5, and 10 years, was in the range of 42 44 MPa, showing that the adhesive strength from the time of coating was maintained. After tensile testing, peeling only occurred at the middle region of the sprayed layer, indicating that the reason for the peeling of the coating was formation of chromium sulfide formed at the interfaces of the sprayed particles.
Fig. 1 Construction of sodium sulfur cell. In order to improve the adhesion of 75 mass Cr Fe alloy plasma spray coatings, which provide sulfide corrosion resistance for the inner surface of NAS battery high temperature type Al cylinders, the Al substrate temperature and the flattening behavior of the plasma spray particles were studied. Also, experiments were conducted to study how changes in the preheating temperature of the Al cylinder affected the spray coating's adhesion strength. The splat morphology of the spray particles changed significantly in relation to the preheating temperature of the Al substrate, from splash shaped to disk shaped as the preheating temperature increased. It was especially evident that the splat morphology of the particles changed sharply at over 433K, forming disk shaped splats with excellent adhesion to the substrate. A good correlation was found between the flatness of the spray particles and the adhesion strength of the coating formed by changing the preheating temperature of the Al cylinder. When the preheating temperature was set at over 433K, coating's with good adhesion strength were formed. It was therefore shown that by studying the flattening behavior of the plasma spray particles, an effective method could be found for improving the adhesion strength of the plasma spray coating. (Received April 24, 2007; Accepted June 13, 2007) Keywords: sodium sulfur battery, aluminum cylindrical container, preheating temperature, high chromium iron alloy coating, spray particle, particle flattening behavior, coating adhesion strength
In order to improve the high temperature durability of the plasma sprayed thermal barrier coating (TBC) systems, high temperature oxidation test was conducted under the isothermal condition at 1100°C for 100 h for several kinds of TBC systems in connection with their coating characteristics such as the coating microstructures and interfacial natures depending on the coating process conditions. Specimens with different coating characteristics were prepared by controlling the coating parameters such as the top coat spraying conditions and the reheat treatment (RHT) conditions after spraying. Especially, the RHT was applied to the TBC specimen with different temperature, heating rate, holding time at maximum temperature and so on in inert argon (Ar) gas atmosphere.High temperature oxidation behavior was found to differ depending strongly on the RHT conditions. The RHT at lower temperature could not inhibit the heterogeneous development of thermally grown oxides (TGO) layer and the wart like oxides at the top coat/bond coat interface. On the contrary, the RHT with either the slow rate heating or the long time holding at the maximum temperature was found to be effective for an improvement of the oxidation resistance, since such a RHT process is able to develop dominantly the continuous a Al 2 O 3 layer at the interface so as to reduce sufficiently the growth rate of TGO to maintain the suitable environmental barrier function during high temperature oxidation.The influence of the RHT conditions on the high temperature oxidation property was discussed in connection with both the morphology of bond coat powders and the surface morphology of bond coat sprayed.
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