Yb2SiO5 and Yb2Si2O7 raw material powders were prepared to create the EBCs. Compared with Yb2Si2O7, Yb2SiO5 exhibits improved resistance to water vapor corrosion and CMAS attack as well as improved phase stability. However, the CTE of Yb2SiO5 (7.2 × 10–6 K−1) is larger than that of the bond-coat layer (3.5–4.5 × 10–6 K−1). Since Yb2Si2O7 has a similar CTE to that of the bond coat (3.6–4.5 × 10–6 K−1), it was used as an intermediate coat-ing layer. For the coating layers, a 100-μm top coat was deposited with 40 μm of Yb2SiO5 and 60 μm of Yb2Si2O7, and a bond coat was deposited with 30 μm of Si–Hf. The SEM images of the Yb2SiO5 and Yb2Si2O7 powders calcined at 1400 °C are shown in Fig. 1. The measured sizes of the Yb2SiO5 calcined powder were 1.2 μm at D10, 2.7 μm at D50, and 5.5 μm at D90, whereas those of Yb2Si2O7 were 0.9 μm at D10, 2.2 μm at D50, and 4.5 μm at D90. The calcined Yb2SiO5 and Yb2Si2O7 powders were ball-milled for 24 h to obtain a dense coating layer. After ball-milling, the sizes of the Yb2SiO5 powder were 0.4 μm at D10, 1.0 μm at D50, and 2.2 μm at D90, and those of Yb2Si2O7 were 0.5 μm at D10, 1.1 μm at D50, and 2.2 μm at D90, as measured using a laser scattering particle size distribution analyzer (LA-950V2, HORIBA, Japan). The observations showed that all powders were reduced to submicron particle sizes through ball milling. The XRD results of the calcined Yb2SiO5–Yb2Si2O7, SPS-coated specimen, and the coated specimen with a completed sec-ondary densification coating, are shown in Fig.2
Fig. 1 SEM image of a calcined Yb2SiO5 powder and b calcined Yb2Si2O7 powder
Fig. 2 XRD analysis of a calcined Yb2SiO5 powder at 1400 ℃ during 20 h, b calcined Yb2Si2O7 powder at 1500 ℃ during 12 h, c specimen surface after SPS coatings, and d specimen surface after secondary densification coatings
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