Fabrication and characterization of YSZ/Al 2 O 3 nano-composite coatings on Inconel by electrophoretic deposition
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Nano-structured yttria stabilized zirconia (YSZ)/Al2O3 nano-composite coatings were prepared by electrophoretic deposition (EPD) in acetyl-acetone/ethanol solvents under the constant voltage of 40 V. High sintering temperature may damage the metal parts and also lead to high production costs. To overcome the disadvantages of high sintering temperatures, reaction bonding of Al was taken as the approach. It was found that a powder mixture of Al and YSZ can lower the sintering temperature. YSZ/Al green composites were deposited on the MCrAlY layer applied on Inconel alloy cathode. Iodine was added to the solutions as the stabilizing agent. According to differential thermal analysis (DTA) results, embedded Al particles oxidation started at 660 °C. Sintering process for YSZ/Al2O3 nano-composite coating occurred at 1150 °C for 4 h. A lower pinholes coating with the highest density due to the constraint of the substrate was obtained.
I. INTRODUCTION
Employing metallic components in aggressive conditions or in high temperature applications is restricted due to their corrosion and degradation. Ceramic coatings are often used to improve the surface behavior of metals in high temperature applications such as thermal barrier coatings (TBCs).1,2 TBCs are being used in propulsion turbines and power plants not only for reducing heat transfer but also for protecting metallic components from hot corrosion and oxidation.2–4 Yttria stabilized zirconia (YSZ) and alumina (Al2O3) coatings are currently considered as TBCs. They can be produced by air plasma spray (APS), electron beam physical vapor deposition (EBPVD) and electrophoretic deposition (EPD). In comparison with APS and EBPVD, EPD has some advantages where capability of producing controllable thicknesses, especially on complex shaped substrates, controllable microstructure, and simplicity of the process, lower equipment cost, and high deposition rate are only some of the examples.5–7 A major problem in joining a TBC to a metallic substrate is the very different thermal expansion coefficients which leads to high residual stresses.8 Inconel super alloys are usually with MCrAlY as bond-coat (where M is Ni or Ni, Co). Bond-coat has two main functions in TBC systems. The first function is to increase the adherence between the top-coat (e.g., YSZ) and the substrate. The second function, which cannot be Contributing Editor: Nahum Travitzky a) Address all correspondence to this author. e-mail: [email protected], [email protected] DOI: 10.1557/jmr.2017.258
performed by a top-coat due to its porous structure, is to protect the underlying material from chemical attacks such as oxidation. Additionally, this intermediate layer reduces thermal expansion coefficient mismatch and decreases residual stresses.9–12 According to recent studies, YSZ/Al2O3 composites have considerable mechanical strength.13,14 It is also claimed that YSZ/Al2O3 composites can effectively prevent crack growth and increase toughness.15,16 Actually, YSZ/Al2O3 composite coatings are of attraction due to the co
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