Improving the Thermal Shock Resistance of Thermal Barrier Coatings Through Formation of an In Situ YSZ/Al 2 O 3 Composit
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JMEPEG DOI: 10.1007/s11665-017-2591-0
Improving the Thermal Shock Resistance of Thermal Barrier Coatings Through Formation of an In Situ YSZ/ Al2O3 Composite via Laser Cladding Zohre Soleimanipour, Saeid Baghshahi, and Reza Shoja-razavi (Submitted July 28, 2016; in revised form February 16, 2017) In the present study, laser cladding of alumina on the top surface of YSZ thermal barrier coatings (TBC) was conducted via Nd:YAG pulsed laser. The thermal shock behavior of the TBC before and after laser cladding was modified by heating at 1000 °C for 15 min and quenching in cold water. Phase analysis, microstructural evaluation and elemental analysis were performed using x-ray diffractometry, scanning electron microscopy (SEM), and energy-dispersive spectroscopy. The results of thermal shock tests indicated that the failure in the conventional YSZ (not laser clad) and the laser clad coatings happened after 200 and 270 cycles, respectively. The SEM images of the samples showed that delamination and spallation occurred in both coatings as the main mechanism of failure. Formation of TGO was also observed in the fractured cross section of the samples, which is also a main reason for degradation. Thermal shock resistance in the laser clad coatings improved about 35% after cladding. The improvement is due to the presence of continuous network cracks perpendicular to the surface in the clad layer and also the thermal stability and high melting point of alumina in Al2O3/ZrO2 composite. Keywords
air plasma spray, laser clad, thermal barrier coating, thermal shock test, YSZ
1. Introduction The durability and energy efficiency of any fuel-burning engine such as gas turbines is directly proportional to its operating temperature and oxidation resistance. In order to improve these two factors in gas turbine engines, thermal barrier coatings (TBCs) have been developed to apply on the hot sections component such as nozzle gas vanes, blades and combustors (Ref 1-5). TBCs are advanced materials systems, mostly consisting of two layers: a metallic bond coat and a heat insulating ceramic top coat. The bond coat is an oxidation resistant metal, usually made of NiCrAlY or NiCoCrAlY alloys, which increases the bond strength between the substrate and the ceramic top coat. The most widely used ceramic top coat in TBCs is yttriastabilized zirconia (YSZ) which exhibits excellent performance in applications such as gas turbines, and satisfies basic requirements, such as low thermal conductivity, excellent phase stability at high temperature and high thermal expansion coefficient (Ref 6-8). The metallic bond coat is responsible for generating a third coating layer, named as thermally grown oxide (TGO), which occurs when the coating is subjected to high temperatures. The formation of TGO is due to the oxidation of the bond coat at Zohre Soleimanipour and Saeid Baghshahi, Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran; and Reza Shoja-razavi, Department of Materials Engineering, Malek-Ashtar Unive
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