Slow sintering in garnet‐containing Y and Gd zirconate–aluminate mixtures for thermal barrier coatings

Abstract

Mixtures of rare‐earth zirconates and aluminates containing Y or Y + Gd that form a two‐phase garnet–fluorite mixture exhibit much slower sintering than pure fluorite at 1400°C. An equivalent Y‐free, Gd‐containing composition that forms a perovskite aluminate instead of garnet showed faster densification after the metastable garnet decomposes. At 1500°C, the Y‐free sample also showed the fastest initial sintering rate, whereas there was more divergence in the sintering rate for the samples containing Y + Gd. The zirconate–aluminate with equimolar Y + Gd shows the slowest densification at 1500°C and retains ∼25% porosity after 250 h. The results highlight possibilities for designing compliant thermal barrier coatings that can retain significant porosity at 1400°C or higher.

Phase stability and cation partitioning in compositionally complex rare earth aluminates and aluminate‐zirconate mixtures

“…Finally, these aluminatezirconate mixtures with RE:Zr ≥ 1 are more likely to have desirable characteristics for T/EBC applications, and facilitate comparison with results from prior studies related to that application. 38,46,47 In addition to the equimolar Gd, La, Nd, Yb, and Y (RE = GdLaNdYbY henceforth), mixed aluminate-zirconate compositions were formulated with equimolar mixtures of Gd and Y (RE = GdY) and Gd, Yb, and Y (RE = GdYbY). These combinations have average radii near the center of the garnet and monoclinic ranges, respectively, shown in Figure 1, and were selected to probe cation partitioning in these phases that are less stable in the RE = GdLaNdYbY mixture.…”

“…Well-mixed oxide powders were prepared by coprecipitation using the previously reported method. 38 Solutions were prepared from nitrates of Al, Gd, La, Nd, Yb, and Y (99.9% pure, Alfa Aesar) in ethanol and Zr oxy-nitrate (99.9%, Alfa Aesar) in water. The calibrated solutions were mixed to yield the desired cation stoichiometries and added dropwise to excess aqueous ammonium hydroxide.…”

“…This phase is stable for the smaller RE 3+ and is also known for persistent metastability for other stoichiometries. [38][39][40] Finally, REAl 11 O 18 is an alkali-earth deficient magnetoplumbite (β-Al 2 O 3 ) variant formed for the largest RE. 41,42 Available experimental and calculated thermodynamic data 37,43 suggest that perovskite and monoclinic aluminate stability increases and decreases, respectively, with increasing RE size.…”

“…In addition to providing insights about the stability limits for compositionally complex materials, the results for these systems have practical importance for the design of multiphase aluminate-zirconate materials of interest as prospective thermal and environmental barrier coatings (T/EBC) in turbine engines. 38,44,45…”

“…Ternary AlO 1.5 -REO 1.5 -ZrO 2 isothermal sections representing the expected equilibria at 1500 • C with increasing RE 3+ size from left to right; adapted from literature reports 38,[48][49][50]. For Yb, Y, and Gd, the REO is the cubic (bixbyite) structure and for Nd and La it is the A-type (P32/m).…”