Ytterbium silicate coatings were deposited on SiCf/SiC ceramics matrix composite (CMC) substrates by plasma spray-physical vapor deposition (PS-PVD), and the microstructures and phase constituents of the coatings were studied. The results show that the Yb₂SiO₅ coating prepared with high power and low pressure (65 kW/2 mbar) had quasi-columnar structure, mainly deposited from the vapor phase, whereas the coating prepared with low power and high pressure (40 kW/10 mbar) had a typical layered structure, mainly deposited from the liquid phase. The deposition efficiency of parameter A (~2 μm/min) was also significantly lower than that of parameter B (~20 μm/min). After annealing at 1300 °C for 20 h, the coating prepared by 65 kW/2 mbar was mainly composed of ytterbium disilicate phase (77.2 wt %). The coating also contained some silicon-rich phase. The coating prepared by 40 kW/10 mbar basically consisted of ytterbium monosilicate (63.6 wt %). In addition, a small amount of silicon-rich phase and ytterbium-rich phase were also present in the coating. Accompanied with calculation results by the FactSage software, the cause of deviations in phase compositions was analyzed.

Y₂O₃-stabilized ZrO₂ (YSZ) has been the material of choice for thermal barrier coatings (TBCs) in the past decades, yet its phase decomposition limits its application above 1200 °C. In this study, Gd₂O₃ and Yb₂O₃ co-doped YSZ powders were produced, in which some amounts of monoclinic (m) phase were introduced into the cubic (c) phase matrix. XRD results showed that the fabricated powders obtained by a solid phase synthesis were composed of m and c phases, and hat the m phase content decreased in a sequence of 4Gd-2Yb-4Y, 2Gd-2Yb-6Y and 2Gd-4Yb-4Y powders. This indicated that Yb³⁺ is an excellent stabilizer in the ZrO₂-based lattice, which could largely suppress the formation of the m phase. The m phase content in the powders was almost kept unchanged with heating at 1300 °C, which could provide a toughening effect to the ceramic. All the powders exhibited no obvious sintering at 1300 °C for 150 h. As compared to YSZ, the three fabricated ceramics had lower thermal conductivities, and they increased in a sequence of 4Gd-2Yb-4Y, 2Gd-4Yb-4Y and 2Gd-2Yb-6Y.

Various Ytterbium-Silicate-Mullite composites were successfully fabricated by adding Yb₂SiO₅ into mullite ceramics and then using pressureless sintering at 1550 °C. The influence of Yb₂SiO₅ addition on the microstructure, mechanical properties, and thermal conductivity of ytterbium-silicate-mullite composites was investigated. Results show that the composites mainly consisted of a mullite matrix and some in situ formed Yb₂Si₂O₇ and Al₂O₃ phases. By the addition of Yb₂SiO₅, the Vickers hardness of composites reached ~9.0 at an additive concentration of 5 mol %. Fracture toughness increased to ~2.7 MPa·m^1/2 at the additive concentration of 15 mol %, owing to the trans-granular fracture and crack deflection of the pinning effect of the Al₂O₃ phase in the composites. With the increase of the Al₂O₃ phase in the composite, the thermal conductivity for the 15YbAM reached around 4.0 W/(m·K) at 1200 °C. Although the thermal conductivity increased, it is still acceptable for such composites to be used as environmental barrier coatings.

By virtue of plasma spray-physical vapor deposition (PS-PVD) process, coatings in possession of columnar structures can be obtained by suitable processing parameters coupled with specially designed powder feedstock. In this paper, the influence of powder characteristics on the La₂Ce₂O₇ (LC) coating microstructures was investigated by using three kinds of feedstock powders with same PS-PVD processing parameters. It was found that small agglomerated feedstock, weak binding strength, and small primary particle sizes can enhance the feedstock evaporation rate, thus obtaining well columnar structured coatings. X-ray diffraction (XRD) patterns revealed that except for a very small amount of La₂O₃ the main phase of all the coatings is LC. The La/Ce atomic ratios reduced in the coatings compared to the feedstocks, especially the coatings with better columnar structure. Super-lattice patterns were observed by transmission electron microscope (TEM), which means that the LC phase is supposed to be pyrochlore structure. Furthermore, the mechanical properties evaluated by nano-indentation tests indicated that both the hardness and Young’s modulus of each coating show negative correlations with the porosity inside the columns.