PSI - Issue 10

I. Georgiopoulos et al. / Procedia Structural Integrity 10 (2018) 280–287

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I. Georgiopoulos et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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increased thermal stability will facilitate higher combustion temperatures and thus improved engine efficiency, in numerous fields including power generation, aerospace, marine propulsion etc (Padture et al. (2002); Clarke et al. (2012); Xu & Guo (2011)). The pertinent market is growing fast and is expected over the next 10 years reaching nearly 228,000 aviation gas turbine engines valued in $1.232 trillion through 2030 and 5480 power generation engines worthing $105.3 billion (Bakan & Vaßen (2017)) . In general, a typical TBC consists of two deposited layers, the bond coat (BC) and the top coat (TC). The state-of-the art BC consists of (Ni, Co) CrAlY or aluminides of Pt and Ni and the TC consists of Yttria-stabilized Zirconia (YSZ). The YSZ better-off performance is leveraged by a combination of con venient properties, i.e., high melting point, 2680 °C, high thermal ex pansion coefficient (TEC) 11.5 10 -6 K -1 at 1273 K, low thermal conductivity (k) 2.12 W/mK at 1273 K and high fracture toughness 1-2 MPa m 1/2 (Fu et al. (2011); Sub ramanian et al. (2002)). However, YSZ exhibits accelerated sintering above 1200 o C while its insufficient phase sta bility after long-term exposure in these high temperatures reduces the lifetime of the TC (Gadow & Lishka (2002)). Recently lanthanum aluminates have been used due to their refractory properties (high melting point, coefficient of thermal expansion > 8.5x10 -6 K -1 and thermal conductivity < 2.2 W/mK), towards increasing the performance of TBC systems. Even though LaAlO 3 is claimed as a potential YSZ replacement in various patents, (Fu et al. (2011); Subra manian et al (2002)) the focus is more for the substituted lanthanum aluminates exhibiting the hexaaluminate structure (Gadow & Lishka (2002); Chen et al. (2011); Ovaneysyan et al. (2014); Yeganan et al. (2015)). Even in hexa aluminates, as well as in lanthanum zirconates (Cao et al. (2001); Wang & Xiao (2014) etc), LaAlO 3 gradually evolves with thermal aging (Chen et al. (2011)), due to the reaction of La with the Al from the evolved TGO. More interestingly La oxide seems to stabilize the sintering and hence the creep resistance of Al oxide (Schaper et al. (1983)). Its evaluation as material in multi-layered TC structures in conventional TBC systems as well as in nano composites has been reported recently, (Stathopoulos et al. (2016); Georgiopoulos et al. (2014); Vourdas et al. (2018)). Moreover, use of the Suspension or Solution Precursor Plasma Spray techniques (SPS & SPPS, respectively) provides the advantage of finer sized droplets deposition resulting in different microstructures, compared to con ventional Atmospheric Plasma Spraying (APS), exhibiting a high cumulative porosity mainly consisting of sub micrometer range pores and/or high segmentation crack density with excellent thermal cycling performance. TBC ’ s performance (lifetime and failure) depends on several parameters. During service, they undergo thermal cycling loading due to the cycling variation of the environmental temperature and the different thermal expansion coefficients of the different TBC layers. Stress variations induced by thermal shock loading generated during start up and shut down of the turbine engine result in coating system failure with the highest stresses and consequent failure occurring at the TC-BC interface. This work aims to develop, optimize and study the properties and the performance of innovative TBC systems involving LaAlO 3 (LA) as an overlayer. More specifically, experimental results for the evaluation of LA as overlayer in double-layered TC structures of YSZ-based TBCs using different thermal spraying techniques (SPS, SPPS and APS) for the deposition of the different layers are presented. Consequently, different multi-layered structures in terms of materials and deposition techniques have been developed, evaluated and compared. LA overlayer is expected to provide high strain tolerance and low thermal conductivity in the TBC systems. Macroscopic together with micro structural analysis of thermal cycling and thermal shock experiments in temperatures up to 1200 o C reveal interesting results for the perovskite overlayer.

2. Experimental protocol

2.1. Materials synthesis and suspension optimization

The multi-layered TBC structures investigated in this study consisted of different materials. Commercial NiCrAlY (H. C. Starck Amperit 413) was used as BC deposited using APS, commercial YSZ (H. C. Starck Amperit 827) as TC deposited with APS technique, commercial YSZ by Unitec Materials (after water based suspension optimization) as TC deposited using the SPS technique and nitrate precursor LaAlO 3 solution as overlayer deposited using the SPPS technique. Particle size distribution of raw powders and suspensions were measured using a Malvern 2000 laser particle sizer. LaAlO 3 (perovskite) powder was synthesized based on a citrate-precursor technique described in detail elsewhere (Vourdas et al. (2018)). La(NO 3 ) 3 .6H 2 O and Al(NO 3 ) 3 .9H 2 O (Sigma Aldrich) were used as La and Al sources. An