Hot corrosion behavior of Al 2 O 3 laser clad plasma sprayed YSZ thermal barrier coatings Zohre Soleimanipour a , Saeid Baghshahi a,n , Reza Shoja-razavi b , Mehdi Salehi c a Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran b Department of Materials Engineering, Malek-Ashtar University of Technology, Shahinshahr, Isfahan, Iran c Department of Materials Engineering, Isfahan University of Technology, Isfahan, 84154 Iran article info Article history: Received 30 June 2016 Received in revised form 15 August 2016 Accepted 15 August 2016 Available online 16 August 2016 Keywords: Laser clad Thermal barrier coating Hot corrosion Air plasma spray YSZ abstract In the present study, the laser cladding of Al 2 O 3 on the top surface of air plasma sprayed (APSed) yttria stabilized zirconia (YSZ) coatings was carried out to improve the hot corrosion resistance of the thermal barrier coatings (TBCs) in the presence of molten salts. The coatings with and without laser cladding were subjected to a hot corrosion test at 1000 °C for 30 h in which a mixture of 55 wt% V 2 O 5 and 45 wt% Na 2 SO 4 was used as the corrosive salt. SEM micrographs and EDS analysis confirmed the formation of YVO 4 rod-shaped crystals dispersed on the surface of the APSed YSZ coatings after hot corrosion test, while these crystals were hardly detected in the laser clad coatings. The SEM micrograph of the cross section of the APSed YSZ coatings revealed cracks and a thermally grown oxide (TGO) layer in the bond coat/top coat interface, which led to the complete delamination of the coatings. Supporting the SEM micrographs, XRD patterns indicated the transformation of metastable tetragonal zirconia (t ′ -ZrO 2 ) to monoclinic zirconia (m-ZrO 2 ) after hot corrosion test. This structural transformation was due to the reaction of the molten salts with Y 2 O 3 (zirconia stabilizer) which destabilized the t ′ -ZrO 2 . To compare the hot corrosion resistance of the APSed YSZ and the laser clad coating, the volume percentage of the undesirable m-ZrO 2 was then calculated after the hot corrosion test. This calculation revealed a higher amount of m-ZrO 2 in YSZ (about 70 vol%) compared to that of the laser clad coating (about 13 vol%). & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. 1. Introduction Thermal-barrier coatings (TBCs) are advanced refractory oxide ceramic coatings applied to the surfaces of metallic components of gas-turbine engines, enabling them to operate at higher tem- peratures [1–4]. As an insulator, TBCs reduce oxidation and ther- mal fatigue. They also increase the lifespan and the operating temperature and consequently improve the efficiency of gas tur- bines. Due to the mentioned points, TBCs have been the focus of significant research and development efforts during the last dec- ade and there were a great attempt to develop new and advanced TBCs [5–8]. TBCs typically consist of two layers: a metallic bond coat and a ceramic top coat [1,3,8]. The former is mostly made of NiCrAlY or NiCoCrAlY alloys, which are more oxidation resistant than the substrate and play a key role in the improvement of adhesion of the metallic substrate to the ceramic top coat [9,10] Among all materials used as ceramic topcoat, yttria-stabilized zirconia (YSZ) has attracted more attention due to its unique properties such as low thermal conductivity, high melting temperature and high coefficient of thermal expansion [7,11]. A thermally grown oxide (TGO) layer also forms during the service at elevated temperatures between the bond coat and the top coat and acts as an oxygen diffusion barrier [6,12]. Despite the advantages, YSZ provides poor hot corrosion re- sistance, especially in an atmosphere consisting of vanadium, so- dium and sulfur, as in the case of impure fuels used in industrial gas turbines [13–16]. During combustion in the gas turbine at elevated temperatures, sodium sulfate (Na 2 SO 4 ) forms as a result of a chemical reaction between sulfur (from impure fuel) and so- dium chloride. Vanadium also reacts with oxygen and produces V 2 O 5 . Then Na 2 SO 4 and V 2 O 5 condense on the YSZ layer, react with yttria and lead to the depletion of Y 2 O 3 from YSZ. This depletion destabilizes the metastable tetragonal zirconia (t ′ -ZrO 2 ) and transforms it to monoclinic zirconia (m-ZrO 2 ). The t ′ →m-ZrO 2 transformation is introduced as the main factor of delamination and degradation of TBCs [13,15]. To improve the hot corrosion resistance of TBCs, many attempts have been made and different methods have been used by Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ceramint Ceramics International http://dx.doi.org/10.1016/j.ceramint.2016.08.090 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. n Corresponding author. E-mail address: baghshahi@eng.ikiu.ac.ir (S. Baghshahi). Ceramics International 42 (2016) 17698–17705