Wet Chemistry based Development of LaMgAl11O19 and its Hot Corrosion Studies in Molten Salt Eenvironment for Thermal and Environmental Barrier Applications /

Abstract

Gas turbines for higher efficiency require high-temperature and severe environment tolerant materials and coatings for environmental and thermal barrier applications. Yttria-stabilized zirconia (YSZ) has been widely used as an environmental and thermal barrier material for applications up to 1200 °C. At temperatures, above 1200 °C, its thermo-physical properties degrade because of undesirable microstructural and phase modifications. The overall impact is an increase in volume, crack formation and spallation of coatings. On the other hand, rare-earth hexaaluminates, having magnetoplumbite structure, exhibit high-temperature phase stability and possess excellent thermo-physical properties at temperatures well above 1200 °C. This research highlights the development of lanthanum magnesium hexaaluminate (LaMgAl11O19) via wet chemistry route and silicon coating using non-thermal plasma process for environmental and thermal barrier applications. The aim of this study is to the synthesize LMA via wet chemistry route, to investigate the effects on its crystal growth as a result of N, N-dimethylformamide (DMF) organic addition and to examine its hot corrosion behavior. The as synthesized xerogel (without DMF) were subjected to calcination treatment for 5 hours at 1000 °C, 1100 °C, 1200 °C, 1300 °C and 1400 °C. Structural purity and surface morphology is enunciated using X-Ray Diffraction (XRD) and Scanning electron microscopy (SEM) techniques. Effects arising from changing the calcination temperature of LMA revealed that the crystallite size was increased with rise in temperature. The in-house synthesised LMA powder was deposited on Inconel-600 (Nickle based superalloys) using slurry based dip coating method. Furthermore, the as-synthesized doped DMF xerogel powders were calcined at 1100 °C to1400 °C and subjected to XRD and SEM to analyse the structural and morphological characteristics. The average crystallite size was decreased about 14% due to the addition of DMF as compared to LMA prepared without DMF. Hot corrosion studies at 700 °C, 800 °C and 900 °C for 5 hours exhibited the degradation mechanism of LMA in V2O5 environment. The nucleation and growth of different phases were elaborated by employing the knowledge of binary phase diagram of V2O5 & Al2O3 and La2O3 & V2O5. At 700 °C AlVO4 and at 800 °C LaVO4, MgAl2O4 and AlVO4, whereas at 900 °C α-Al2O3, MgAl2O4 & LaVO4 were confirmed as the main corrosion products. Since silicon is actively being used as a bond coat on Si-based ceramic matrix composite (CMC) substrate for environmental protection applications. Therefore, during research in the USA, silicon coating process was developed by using non-thermal plasma and various deposition parameters were evaluated.