Durable aluminate toughened zirconate composite thermal barrier coating (TBC) materials for high temperature operation

Abstract

Research on advanced thermal barrier coating (TBC) materials capable of operating beyond 1200°C has primarily focused on the rare earth zirconate pyrochlores, particularly gadolinium zirconate (Gd 2 Zr 2 O 7 -GZO). The drawback of this material is a significant reduction in durability due to a low fracture toughness. This study investigates utilization of a thermodynamically compatible gadolinia alumina perovskite (GdAlO 3 -GAP) toughening phase to improve the durability of GZO. Dense pellets were fabricated to assess the material properties with minimal microstructural influence. Thermal stability, elastic modulus, hardness, indentation fracture resistance and erosion durability were evaluated for GZO, GAP, and composite pellets containing 10, 30, and 50 wt.% GAP. It was demonstrated that GAP and GZO are thermodynamically compatible through 1600°C and thus capable of operating well beyond the limits of traditional 7 wt.% yttria stabilized zirconia (YSZ). Grain sizes are maintained due to a lack of diffusion, and thus microstructural stability is enhanced. The GAP fracture toughness was shown to be over 2X that of GZO while exhibiting a lower elastic modulus and similar hardness. The 50:50 GZO-GAP composite exhibited a 63% reduction in the absolute erosion rate, demonstrating the immense toughening capabilities of this system. The implications for composite TBCs utilizing this system are discussed, along with future work. K E Y W O R D Saluminates, composites, durability, gadolinium zirconate, thermal barrier coatings

Enhanced calcium–magnesium–aluminosilicate (CMAS) resistance of GdAlO3 (GAP) for composite thermal barrier coatings

“…Last, this study builds on the results of our previous work suggesting composites of rare earth zirconates and rare earth aluminates as toughened, high temperature phase stable and thermochemically compatible TBCs. 15 The positive results for GAP-CMAS resistance establish that using this material as a toughening agent for GZO will not hamper CMAS resistance in the composite and will in fact likely improve CMAS resistance. Thus, GAP-GZO composites will provide not only toughening,…”

“…To combat the low toughness of zirconates, the authors have explored utilizing multiphase composites consisting of GZO modified with a secondary toughening phase in the form of multilayers 10,11 and composites 12–15 . It was demonstrated that minor additions of a toughening phase can have substantial improvement on the erosion durability of coatings while having minimal debit on thermal conductivity for both electron beam‐physical vapor deposition and air plasma spray‐derived coatings.…”

“…Therefore, the requirements for the toughening phase are less stringent than the matrix, which enabled exploration of materials not traditionally considered for TBC application. Chief among these was gadolinium aluminate perovskite (GAP), a material that exhibits thermochemical compatibility with GZO well beyond 1400°C and which possesses the higher fracture toughness necessary to act as a toughening agent 15 . Although this material has demonstrated attraction for improved mechanical performance, its effect on CMAS durability is unknown.…”

Apatite and garnet stability in the Al–Ca–Mg–Si–(Gd/Y/Yb)–O systems and implications for T/EBC: CMAS reactions

“…This behavior can be rationalized by the need to have a combination of large, intermediate, and small cation sizes to fill the three distinct sites. Likewise, coatings based on or containing rare earth aluminates are likely to cause a shift in reaction equilibria toward garnet formation 33 . Garnet is observed less frequently as an equilibrium reaction product at higher temperatures, and in some cases has been observed to grow on cooling rather than at temperature 26,29,34 …”

“…Likewise, coatings based on or containing rare earth aluminates are likely to cause a shift in reaction equilibria toward garnet formation. 33 Garnet is observed less frequently as an equilibrium reaction product at higher temperatures, and in some cases has been observed to grow on cooling rather than at temperature. 26,29,34 Initial insight into the relative stability of apatite and garnet is gained by examining the endmembers RE 9.33 (SiO 4 ) 6 O (the apatite terminus in the REO 1.5 -SiO 2 binary), Ca 2 RE 8 (SiO 4 ) 6 O 2 (the vacancy-free nominal apatite stoichiometry), and RE 3 Al 5 O 12 (the rare earth aluminate garnet, RE-AG).…”