Effect of CMAS Composition on Hot Corrosion Behavior of Gadolinium Zirconate Thermal Barrier Coating Materials

Abstract

The resistance of synthesized pyrochlore-type Gd 2 Zr 2 O 7 bulk specimens to four calcium-magnesium aluminosilicate (CMAS) compositions at different temperatures was investigated. The reaction products were identified by X-ray diffraction and penetration depths were examined using scanning electron microscopy. A dense reaction layer is comprised mainly of Ca 2 Gd 8 (SiO 4 ) 6 O 2 and a cubic fluorite phase formed during the CMAS attack, and some unreacted CMAS was found in a transition layer below the reaction layer. The overall infiltration depth changed slightly with temperature, however, the thickness of the reaction layer and the morphology of the transition layer varied distinctly with temperature. The sintered sample underwent the most severe degradation by the CaO-lean CMAS, whereas the effect of CaSO 4 and CaCO 3 was not significant. Moreover, the Gd content of the ZrO 2 -based cubic fluorite phase depends on the temperature and the molar ratio of Ca:Si in the CMAS. The efficiency of gas turbine engines increases with increasing operating temperature. Thermal barrier coatings (TBCs) act as thermal insulators for metallic fans and vanes, which allows for a higher operating temperature and thus increases the thrust-to-weight ratio of the turbine.1 Yttria-stabilized zirconia (YSZ) with the intermediate t'-tetragonal phase has been widely used in aero engines for decades owing to its excellent mechanical and chemical properties during thermal cycling.2 However, YSZ cannot meet the requirements of modern turbines, because issues like delamination and spallation due to the phase transformation accompanied with volume changes and poor chemical resistance to molten salts at elevated temperature are detrimental to the performance and shorten the lifetimes of jet engines. [3][4][5][6][7] Conversely, the rare earth zirconates, especially gadolinium zirconate (Gd 2 Zr 2 O 7 ) with the pyrochlore structure, which exhibit relatively low thermal expansion coefficient, high-temperature phase stability, and low thermal conductivity, are of great potential. 8,9 In general, TBC materials can be deposited using atmospheric plasma spraying (APS) on the stationary components or electron beam physical vapor deposition (EB-PVD) on the rotating parts of the turbine. The corresponding specific lamellar or columnar structure of APS or EB-PVD deposited coatings with porosities of 10-25% benefits mitigation of stresses and decreases thermal conductivity. 10,11 However, the intersplat pores or intercolumn gaps are the main paths for the infiltration of molten salts during service. The molten salts, which come from sand, dust, volcanic ash and even impurities in the fuel, are mainly composed of calcium-magnesium-alumino-silicate (CMAS).4 Since Gd 2 Zr 2 O 7 is a promising alternative TBC material, the influence of CMAS on its performance has been of interest. [12][13][14][15][16] Molten CMAS wets the surface, penetrates the pores, and reacts with the pyrochlore material resulting in the establishment of a compact layer composed of a ...

Flow Kinetics of Molten Silicates through Thermal Barrier Coating: A Numerical Study

“…This model was verified on a large database of chemical compositions of molten silicates that were experimentally obtained. In literature, the Giordano viscosity model has been widely used to estimate the temperature dependent viscosity of CMAS for a wide range of chemical compositions [42][43][44][45]. For the present work, the estimated GRD viscosity for the CMAS1 shows a lower viscosity compared to the experimental obtained viscosity data [31].…”

Recent Development in Advance Ceramic Materials and Understanding the Mechanisms of Thermal Barrier Coatings Degradation

“…The possible next-generation thermal barrier covering substance gadolinium zirconate must further increase its (CMAS) resilience. Several techniques, including doping, makeup tuning, surface alteration, and stacked coatings, can be used to increase the CMAS resilience of gadolinium zirconate (GZO), a possible next-generation thermal barrier covering material [18].…”

Protocol for selecting exemplary silicate deposit compositions for evaluating thermal and environmental barrier coatings

“…Concentrations expressed in mole percent of single‐cation based oxide formulae. Sources are, from left to right: Earth's Crust (1,3‐6), 57 (); 56 Volcanic Ash (,3), 63 (2,7,8,10,17,20,22), 54 (4,15,23,26,29), 14 (5,12‐14,16,19,21,24,25), 64 (6,9,27,28), 61 (11), 65 (18); 62 Sands (), 40 (2‐11,13‐15,18‐20,24), 68 (12), 70 (16,17), 71 (21), 69 (22,26), 14 (23), 72 (25); 73 Dusts (1,7,11,13), 14 (2‐6), 70 (8‐10,12,14), 67 (15); 74 Fly Ash (), 78 (), 77 (3,4,6), 75 (); 19 Engine Deposits (1‐5,10,14), 14 (6), 40 (7), 50 (8), 51 (9), 5 (11), 53 (12), 47 (13), 44 (15), 45 (16), 46 (17), 52 (18), 48 (19); 49 Test Dusts (1‐3), 62 (); 112 CMFAS (,3), 25 (), 16 (), 26 (), 88 (6), 113 (7), 86 (8), 21 (9), 87 (10), 89 (11,14), 47 (12,13), 23 (15); 48 C(M/F)AS (1,9,11), 85 (2,12,13), 21 (3,7), 82 (), 84 (), 114 (6), 115 (8), 8 (10); 81 CAS(1‐3,5,6), 21 (),…”

“…Model compositions that have been used for research are also included in Figure 1. They range from simple ternaries of the three most common oxides (CAS), centered around the lowest eutectic; 17,21,26 to quaternary compositions with added Mg or Fe (CMAS and CFAS), designed as simplified versions of EDs 8,81,82 or sands; 21,[83][84][85] and quinaries with all five major oxides (CMFAS), selected to represent specific deposits, sands and volcanic ashes. 15,16,21,23,25,26,47,48,[86][87][88][89][90] Compositions in this section are not included in the statistical analysis but are provided for comparison with the larger compilation of EDs and naturally occurring siliceous debris (NSDs).…”