Sehreish Abrar,Zhuang Ma,Ling Liu,Faisal Nazeer,Abdul Malik
The materials which have low thermal conductivity , a good match of coefficient of thermal expansion with SiC-based ceramics matrix composites (CMCs) and excellent resistance against calcium-magnesium-alumina-silicate (CMAS) are suitable candidates for thermal/environmental barrier coating. To fulfil the above requirements we developed a new multicomponent rare earth (Y 0.2 Dy 0.2 Er 0.2 Yb 0.2 Ho 0.2 ) 2 Si 2 O 7 or (5RE 0.2 ) 2 Si 2 O 7 high entropy ceramics pyrosilicate by a single step solid solution method. The x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis revealed that the as-fabricated bulk composite (5RE 0.2 ) 2 Si 2 O 7 has a single phase β-type structure match well with single rare earth ytterbium disilicate (Yb 2 Si 2 O 7 ) and the different rare earth oxides mixed well with each other. The excellent coefficient of thermal expansion (4 × 10 −6 -5 × 10 −6 °C −1 ) and ultra-low thermal conductivity (0.7–1.6 W/m C) was observed as compared to other single and multicomponent high entropy ceramics composites. The (5RE 0.2 ) 2 Si 2 O 7 ceramics pyrosilicate show an excellent CMAS resistance at various temperatures (1300, 1400, and 1500) °C for 4 h compared with single principle RE disilicates. The above all mentioned results identify that this material is a suitable candidate for coating on the SiC-based ceramics composite for thermal/environmental barrier coating.
The development of YSZ TBCs has been limited by the CMAS corrosion and thermophysical characteristics. The Ytterbium Tantalate (YbTaO 4 ) ceramic was selected in this study to evaluate its potential to replace traditional YSZ. The thermal and mechanical properties of YbTaO 4 show great advantages over YSZ, especially the hardness (191.3 ± 7.6 Hv 0.3 ). The CMAS hot corrosion results indicate that the maximum thickness of the reaction layer at 1350 °C after 24 h is ~16.8 μm, which is much thinner than that of YSZ. Moreover, the porosity of the reaction layer does not increase significantly compared with the YSZ TBCs. The main product after the reaction is the Ca 2 Ta 2 O 7 phase, and there is no anorthite phase after the reaction, mainly because the formation of the Ca 2 Ta 2 O 7 phase consumes a lot of Ca atoms. In addition, the equilibrium contact angle of YbTaO 4 is higher than that of YSZ, which indicates that the YbTaO 4 possesses worse wettability . In actual conditions, low wettability is conducive to the YbTaO 4 TBCs get rid of the liquid CMAS, which can effectively reduce the corrosion effect of CMAS. Furthermore, the corrosion depth and area of YbTaO 4 causing by residual CMAS are also smaller due to the poor wettability. Based on the above results, YbTaO 4 exhibits excellent potential to replace YSZ in preparing TBCs for the next generation aero engines.
A new method is proposed to predict the formation of high-entropy single-phase oxide with a pyrochlore structure synthesis from non-equivalent principal elements. Based on this concepte, the La 2 B 2 O 7 (B = Zn 2+ , Mg 2+ /Ni 2+ , Ce 4+ /Zr 4+ /Hf 4+ /Sn 4+ /Ti 4+ , W 6+ , Mo 6+ ) were designed, in which high-entropy La 2 (ZnMgZrWMo) 2 O 7 , La 2 (ZnMgHfWMo) 2 O 7 and La 2 (ZnMgSnWMo) 2 O 7 pyrochlore ceramics were successfully prepared by solid-phase sintering. The microstructure of the entropy-stabilized phase was demonstrated using X-ray diffraction and elemental mapping, and the thermal properties including the thermal expansion coefficient and thermal conductivity were analyzed. The results show that La 2 (ZnMgZrWMo) 2 O 7 has a higher thermal expansion coefficient (13.9 × 10 –6 K –1 ) and lower thermal conductivity (0.8 W/m·K) than its corresponding single-components compound. In addition, La 2 B 2 O 7 (B = Zn 2+ , Mg 2+ , Zr 4+ /Hf 4+ /Sn 4+ , W 6+ , Mo 6+ ) pyrochlore high-entropy ceramics exhibit good calcium – magnesium – aluminosilicate (CMAS) resistance, the apatite protective layer generated after the reaction of La with CMAS effectively prevents further corrosion. This study suggests a novel method for predicting the formation of high-entropy single-phase oxides of pyrochlore structures and provides a new design concept for anti-CMAS corrosion materials.
Sehreish Abrar,Zhuang Ma,Ling Liu,Faisal Nazeer,Abdul Malik
In this work, novel equiatomic high entropy (Dy 1/4 Ho 1/4 Tm 1/4 Yb 1/4 ) 2 Si 2 O 7 or (4RE 1/4 ) 2 Si 2 O 7 ceramic pyrosilicate was fabricated through a single solid solution method to use as environmental barrier coating. The SEM analysis of high entropy powders shows the homogenous mixing and XRD proves the formation of single β-phase after milling and sintering. The coefficient of thermal expansion was reported as (2.3–4.8 × 10 −6 K −1 ) from 400 K −1 to 1723 K −1 . The ultra-low thermal diffusivity (0.4 mm 2 s −1 ) and thermal conductivity (0.8 W/m°C) were reported at 1500 °C for this novel HE ceramic disilicate. The as fabricated (4RE 1/4 ) 2 Si 2 O 7 pyrosilicate shows an excellent CMAS resistant for even up to 48 h and negligible amount of Ca is able to penetrate in the substrate. Rare earth disilicate species with intermediate radii such as Tm 3+ helps in maintaining phase stability along with passive element Yb 3+ of smaller radii which also protect the interface from severe CMAS attack. However, the rare earth species with larger radii such as Dy 3+ and Ho 3+ actively take part in apatite formation leading to reduced corrosion activity of CMAS melt by changing its composition. This result confirms the application of (4RE 1/4 ) 2 Si 2 O 7 as a potential candidate to be used as protecting coating material in harsh combustion environments.
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