High-entropy(La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2(Zr0.75Ce0.25)2O7thermal barrier coating material with significantly enhanced fracture toughness

Donghui GUO,Feifei ZHOU,*,Bosheng XU,Yigung WANG,You WANG

aInstitute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081, China

bDepartment of Materials Science, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001,China

KEYWORDSFracture toughness;High-entropy ceramics;High-temperature phase stability;Rare-earth zirconate;Thermal barrier coatings

AbstractPoor fracture toughness leads to premature failure of La2(Zr0.75Ce0.25)2O7(LCZ) thermal barrier coatings in an elevated temperature service environment.A novel coating material,namely (La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2(Zr0.75Ce0.25)2O7(LNSGY) based on the high-entropy concept, was successfully fabricated by solid-state sintering.The microstructure of LCZ and LNSGY was investigated by X-Ray Diffraction(XRD),Raman Spectrometer(RS),Transmission Electronic Microscopy(TEM)and Scanning Electron Microscopy(SEM).The fracture toughness of the LCZ and LNSGY ceramics was evaluated.The LNSGY has excellent high-temperature phase stability,and the grain size of LNSGY ceramic is smaller than that of LCZ ceramic at an elevated temperature due to the sluggish diffusion effect.Compared with LCZ (fracture toughness is (1.4 ± 0.1)MPa?m1/2), the fracture toughness of LNSGY is significantly enhanced (fracture toughness is(2.0±0.3)MPa?m1/2).Therefore, the LNSGY can be a promising advanced thermal barrier coating material in the future.

1.Introduction

Thermal Barrier Coatings (TBCs) are applied to the hot components of gas turbine engines to protect the inner superalloy components from overheating and corrosion.TBCs can dramatically improve service life and efficiency of engines.Yttria-Stabilised Zirconia (YSZ) has become the most widely used ceramic TBC material because of its high thermal expansion coefficient and fracture toughness.However, the operating temperature of YSZ is limited to below 1200 °C due to the non-transformable tetragonal phase degradation and poor sintering resistance.1–3Rare-Earth Zirconates (REZ) have the advantages of high temperature stability,low thermal conductivity and superior CMAS (CaO-MgO-Al2O3-SiO2) resistance.4Nevertheless, the fracture toughness of REZ is lower than that of YSZ, which restrict the wide application of REZ.

Doping or defect engineering is an effective way to improve thermo-physical and mechanical properties of REZ.5,6Additionally, high entropy ceramics, also known as multiprincipal element ceramics, usually refer to the solid solution formed by five or more ceramic components.It has become a popular topic in the ceramic field in recent years due to the unique high entropy effect.The earliest research of high entropy ceramics can be traced back to 2015.Rost et al.7reported a bulk entropy-stabilised oxide ceramic for the first time,i.e.high entropy ceramics.Subsequently,more and more high entropy ceramics, including high entropy oxide ceramics with fluorite structure, perovskite structure and spinel structure,as well as nitride and silicide,gradually attracted research attention.In recent years,High Entropy Rare-Earth Zirconate(HE-REZ) exhibited superior performance due to structural stability, a sluggish diffusion effect, corrosion and oxidation resistance.8Fan et al.9investigated the microstructure of dual-phase REZ medium-and high-entropy ceramics,and proposed the principal element design criterion of pyrochlorefluorite dual-phase ceramics.Moreover, A2B2O7compounds with pyrochlore and/or defective fluorite structures have many advantages as TBCs.Chen et al.10fabricated the RE3NbO7ceramics(RE=La,Nd,Sm,Eu,Gd,Dy)by solid state reaction and reported that the thermal expansion coefficient and thermal conductivity of Dy3NbO7ceramics with fluorite structure were 11.0 × 10-6K-1(1200 °C) and 1.0 W?m-1?K-1(25 °C), respectively.Wu et al.11synthesised the novel ZrO2-Dy3TaO7ceramics and found the thermal conductivity was lower than that of YSZ.Ren et al.12prepared a single defective fluorite structure (Sm0.2Eu0.2Tb0.2Dy0.2Lu0.2)2Zr2O7, and found that the multi-component HE-REZ had lower thermal conductivity and larger thermal expansion coefficient.Zhou et al.13reported that (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7/YSZ coatings had excellent thermal insulating property and hightemperature phase stability compared with the La2Zr2O7/YSZ coatings.Zhu et al.14found that dual-phase HE-REZ(La0.2Nd0.2Y0.2Er0.2Yb0.2)2Zr2O7exhibited enhanced Vickers’hardness and thermal expansion coefficients.To date, the change of elements located at A or B sites have received extensive attention.However, studies on both A and B sites are lacking.

In our previous studies, the La2(Zr0.75Ce0.25)2O7(LCZ)coatings were fabricated, which exhibited lower thermal conductivity and higher thermal expansion coefficient compared with single rare-earth La2Zr2O7(LZ) coatings.15–17However,the main drawback of LCZ coatings is poor fracture toughness.Based on the high-entropy engineering, a new high-entropy (La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2(Zr0.75Ce0.25)2O7(LNSGY) ceramic with pyrochlore structure was designed,where the phase composition and microstructure were characterised.In addition, the fracture toughness of LNSGY was studied.The results would give guidance for the design of advanced TBCs materials.

2.Experimental procedure

2.1.Synthesis of LNSGY ceramic

To obtain LNSGY powders,a solid-state reaction method was conducted.Different raw ceramic powders, such as La2O3,Nd2O3, Sm2O3, Gd2O3, Yb2O3, ZrO2, and CeO2(99.9wt%,Beijing InnoChem Science and Technology Co., ltd.) were used.Firstly,according to the designed molecular formula,different ceramic powders were precisely weighed and then mixed with zirconia milling balls and ethanol by using a planetary mill (Fritsch, Pulverisette 6) for 24 h at 260 r/min.The weighing accuracy of the equipment is two decimal points.The mass ratios of powder and ball, and powder and ethanol were 1∶10 and 1∶1.5, respectively.Subsequently, the obtained slurries were dried at 80 °C for 12 h in a drying oven, and the dried powder was sieved by a screen (200-mesh).To further synthesise LNSGY powders, the sieved powders were calcined at 1500 °C for 10 h.Then, the calcined powders were granulated by using the 5wt% Poly Vinyl Alcohol (PVA), and drypressing was carried out at 7.5 MPa for 60 s.Following that,green bodies were fabricated by uniaxial cold pressing at 200 MPa for 2 min.Finally, the green bodies were sintered at 500 °C for 2 h to burn out PVA and then sintered at 1600 °C for 40 h in air.For comparison, dense LCZ ceramics were synthesised by using the same method.

2.2.Characterization

The phase composition of synthetised powder and ceramic samples was identified using X-Ray Diffraction(XRD,Ultima IV,Japan)with a scan rate of 5(°)/min and Raman spectrometer (HORIBA Scientific LabRAM HR Evolution, France)with a 532 nm laser source.The crystal structure and elemental distribution of powder were observed by a Transmission Electronic Microscopy(TEM,FEI Talos F200s,Netherlands).The structure and grain size were characterised using a fieldemission Scanning Electron Microscope (SEM, JSM-7500F,Japan).To further observe and analyse the grain boundaries,the surface of ceramic samples was thermally etched at 1500 °C for 40 min.

To measure fracture toughness (KIC), each sample was tested eight times by using a micro-hardness testing instrument(HV-10Z,China)at 9.8 N pressing load.The Vickers hardness can be calculated by

where F is the pressing load of indentation; d is the length of diagonal of indentation.

The thermal conductivity of LCZ and LNSGY ceramics was determined by a laser flash analyser (Netzsch HFM 446 Lambda, Germany) with argon gas.The samples were machined into dimensions of ?12.7 mm×1.5 mm discs before the test.The measurement was conducted in the temperature range of 200–1200 °C at an interval of 200 °C.The thermal conductivity can be calculated by

where Cpis the thermal capacity calculated based on Neumann-Kopp law; q is the density of ceramic obtained by the Archimedes method; λ is the thermal diffusivity measured by a laser flash analyser with argon gas.

The LCZ and LNSGY ceramics were machined into dimensions of 4 mm × 4 mm × 12 mm cuboids before the test.The thermal expansion coefficient of the LCZ and LNSGY ceramics was measured by a high-temperature dilatometer(NETZSCH DIL402C, Germany) with argon gas.The measurement was conducted at the temperature range of 25–1300 °C with a heating rate of 5 °C/min.

3.Results and discussion

Figs.1 (a) and (b) display the XRD patterns of the LCZ and LNSGY powder and sintered ceramic,respectively.The phase structure is a single-phase cubic pyrochlore structure whether it is the powder or the sintered ceramic.No impurity phases exist, which indicates that the LCZ and LNSGY materials have excellent phase stability.18In general,the phase structure of a typical A2B2O7ceramic is determined by the radius ratio rA/Bof A3+and B4+.19For instance, the extent of structural disordering in REZ,is related to the RE3+radius,which leads to different phase structures.The cubic pyrochlore structure(P) is stable when it ranges from 1.46 to 1.78, whereas the defective fluorite structure (F) is easier to form when rA/Bis below 1.46.20The rA/Bof LNSGY can be expressed as

where r(RE) is the cation radius of RE.The cation radii of La3+, Nd3+, Sm3+, Gd3+, Yb3+, Zr4+and Ce4+can be found in a previous study.21Thus, according to Eq.(3), the phase structure of LNSGY is expected to be fluorite, because rA/B= 1.422.In contrast, the XRD patterns imply that the phase structure of LNSGY was a single-phase pyrochlore.In addition, as previously reported,22–25it is worth noting that nano-sized REZ ceramics usually exhibit defective fluorite structure, whereas sub-micron- or micron-sized ceramics demonstrate pyrochlore structure, when the rA/Bvalue lies at the boundary of 1.46.According to the theory of Navrotsky,because the surface/interface energy has an effect on the Gibbs free energy of the system,the ultra nano powder always forms a high temperature phase (fluorite phase).The surface/interface energy of sub-micron or micron level is smaller than that of the ultra nano powder.Therefore,nano powder can provide abundant surface energy,which is conducive to the generation of fluorite phase during the subsequent sintering,whereas submicron or micron powder tends to generate pyrochlore phase.The phase structures of REZ ceramics may also conform to this rule.

Fig.1 XRD patterns and Raman spectra of LCZ and LNSGY.

Fig.2 HRTEM photograph, SAED pattern, and corresponding compositional mapping of LNSGY powder.

The Raman spectra can obtain the precise information of phase composition with short-range and long-range order,especially vibrations about oxygen ions, compared with the XRD patterns, thereby distinguishing between the pyrochlore and fluorite structures.26Therefore, to further verify phase structures, the Raman spectra of the synthesised LCZ and LNSGY powder and sintered ceramic are shown in Figs.1(c) and (d).The synthesised LCZ and LNSGY powder and sintered ceramic demonstrate four typical Raman bands of the pyrochlore structure, and this result further confirms the results of XRD.For the LCZ ceramic, the band at around 683 cm-1is interestingly detected as well, which might be attributed to the optical mode.27Two Raman bands at approximately 292 cm-1and 389 cm-1represent the O–La–O bending vibration and Zr(Ce)–O stretching mode, respectively.In addition, the bands at about 493 cm-1and 567 cm-1indicate the La–O stretching modes.Meanwhile,the Raman bands of LCZ sample are consistent with the results in another study.28For the LNSGY ceramic, the Raman band of LNSGY ceramic at approximately 303 cm-1imply the O–RE–O bending vibration, and the Raman band at approximately 374 cm-1indicates Zr(Ce)–O stretching mode.Moreover, the bands at about 526 cm-1and 598 cm-1are RE–O stretching modes.The band at about 598 cm-1is a unique peak for pyrochlore.

Fig.2 shows the TEM images of synthesised LNSGY ceramic powder.The High-Resolution Transmission Electron Microscopy(HRTEM)photograph demonstrates the structure with a long-range order of LNSGY powder, where the interplanar distance of 0.32 nm corresponds to the (222) plane of pyrochlore(Fig.2(a)).Fig.2(b)shows the Selected Area Electron Diffraction (SAED) pattern.The single-phase structure has diffraction spots along the[10 1]zone axis, which confirms the pyrochlore structure.This result is in accordance with that of XRD and Raman spectra.Fig.2(c) displays the compositional distribution of La, Nd, Sm, Gd, Yb, Zr, Ce and O.The result indicates the elemental homogeneity of LNSGY powder and further corroborates the formation of singlephase structure.The results of TEM analysis are consistent with that in a previous study.29

The morphologies of synthesised LCZ and LNSGY powder are exhibited in Figs.3(a) and (b), respectively.The irregular particles are composed of raw particles with a size less than sub-micron.Additionally, the average particle sizes of LCZ and LNSGY powders are 1.03 μm and 0.91 μm, respectively(Fig.4).This result is consistent with SEM observation.The density of the LCZ and LNSGY ceramic are 6.25 g/cm3and 6.67 g/cm3, respectively.The densification rates of the two samples are 98.8% and 96.2%, respectively.Figs.3(c) and(d) show the microstructure of sintered LCZ and LNSGY ceramic.The two ceramic samples are dense with little pores.Dramatically,the average grain size of LCZ is 22.85 μm,which is obviously larger than the grain size of LNSGY (approximately 3.92 μm), as shown in Fig.4.Compared with LCZ,the grain size of LNSGY is obviously lower, and this is due to the sluggish diffusion effect of HE-REZ.30

Fig.5 exhibits SEM-Energy Dispersive Spectroscopy(EDS)mapping of sintered LCZ and LNSGY ceramics.Figs.5(c)and(d)show that the elements of LCZ and LNSGY are still homogeneously distributed.Furthermore, the EDS is carried out to further discuss the element content.The results of element content are shown in Table 1.The compositions marked A and B are LCZ and LNSGY,respectively.The results of EDS analysis are consistent with the XRD pattern.

The macroscopic change of green bodies is volume shrinkage after high temperature sintering.Meanwhile, the density and strength increase.It is necessary to characterise the sintering degree by volume shrinkage ratio.Fig.6 depicts the volume shrinkage ratio of LCZ and LNSGY ceramic sintered at 1600 °C for 40 h.The volume shrinkage ratio of LCZ is(30.6 ± 1.8)%, which is slightly larger than that of LNSGY(25.8 ± 2.1)%.This finding may be due to the high entropy effect of HE-REZ,31which means the anti-sinterability of the LNSGY is better than that of LCZ.

The fracture toughness which reveals interfacial strain tolerance, determines the reliability of TBCs.17The fracture toughness can be measured by indentation method.5The typical indentation photograph of LNSGY ceramic is exhibited in Fig.7(a).The fracture toughness can be expressed as32

where a is the length from the centre point to the edge corner of the indentation;c is the total length of the radial crack plus a.Based on Eq.(4), the fracture toughness of LZ, LCZ and LNSGY ceramic is depicted in Fig.7(b)33.Fig.7(b)33shows that the fracture toughness of LZ,LCZ and LNSGY ceramics is(1.1±0.1),(1.4±0.1),(2.0±0.3)MPa?m1/2,respectively.Among them, the fracture toughness of LZ was demonstrated in a previous study.33Moreover, YSZ is a state-of-the-art TBCs material and its fracture toughness is 2.4 MPa?m1/2.10No obvious variation exists between the fracture toughness of LNSGY and YSZ ceramics.The TBCs materials have a good high temperature fracture toughness, mainly because TBCs materials with lower fracture toughness are prone to delamination during service.Compared with LCZ,the fracture toughness of LNSGY ceramic is significantly enhanced by more than 40%.

Fig.4 Grain size of LCZ and LNSGY.

To further investigate the high toughness of LNSGY, the crack propagation path is observed, as shown in Fig.8.For LNSGY ceramics, the crack bridging and deflection can be found in the crack propagation path.Their existence means cracks need to absorb more energy for propagation, thereby improving the fracture toughness.Moreover, LNSGY ceramics have higher fracture toughness, which might be due to the presence of ferroelastic domains, similar to YSZ.34Compared with LCZ,LNSGY ceramics can adjust the density of dislocations due to the entropy gain.The strain field of dislocation can be enlarged by multivalent cations of LNSGY ceramics,which can enhance the chance of crack bridging and deflection.High density dislocations increase the local plasticity of LNSGY ceramics surface and may become an additional toughening mechanism of LNSGY ceramics.35In short, the crack direction changes due to the local stress around the crack tip during crack propagation,which is the main reason for the excellent fracture toughness.

The thermal conductivities of LCZ and LNSGY ceramics are depicted in Fig.9.The thermal conductivity of LNSGY ceramics(1.63–1.84 W?m-1?K-1at 200–1200°C)is lower than that of LCZ ceramics(2.10–2.27 W?m-1?K-1at 200–1200°C).The thermal conductivity of LNSGY ceramics decreases as a result of the enhanced phonon scattering with increasing cation numbers.36–38

Generally speaking, the thermal conductivity of insulating crystalline could be explained by

where v is the velocity of phonons; l is the phonons mean free path.Hence, the thermal conductivity is proportional to the phonons mean free path.The phonons mean free path could be expressed as39

where li（w;T）is the mean free path due to phonon–phonon scattering,the subscript i means the the abbreviation of intrinsic, w is the frequency, T is the temperature; lp（w;T）is the mean free path owing to defects scattering, the subscript p means the abbreviation of point defect.Based on Eqs.(5) and(6), the phonon–phonon scattering enhances with increasing temperature.This leads to a shorter mean free path of phonons, thereby causing a lower thermal conductivity.

Fig.5 SEM-EDS mapping of LCZ and LNSGY.

Table 1 EDS analysis of A and B positions.

Fig.6 Volume shrinkage ratio of LCZ and LNSGY ceramic sintered at 1600 °C for 40 h.

Fig.10 displays the thermal expansion coefficients of LCZ and LNSGY ceramics under different temperatures,where L0and dL are initial length and change of sample length cuased by temperature change, respectively.Fig.10(a) shows that the deformation variable increases with increasing temperature, which indicates that LCZ and LNSGY ceramics have good thermal stability.However,the thermal expansion coefficient of LNSGY ceramic(10.98 × 10-6K-1at 1300 °C) is higher than that of LCZ ceramic (9.95 × 10-6K-1at 1300 °C), as shown in Fig.10(b).This may be attributed to the lower cationic bonding strength of LNSGY.40

Considering that the binding energy of Nd, Sm, Gd, Yb and O is lower than that of La and O.Therefore, the ionic bonding strength at A and B sites is considered.The ionic bonding strength could be expressed by14

Fig.7 Indentation photograph of LNSGY ceramic and fracture toughness of LZ, LCZ and LNSGY ceramics.33

Fig.8 SEM photographs of indentation of LNSGY ceramic and locally enlarged tip of radial crack.

Fig.9 Thermal diffusivities and thermal conductivities of LCZ and LNSGY ceramics.

Fig.10 dL/L0and thermal expansion coefficients of LCZ and LNSGY ceramics under different temperatures.

where IABis the cationic bonding strength of A and B;xAand xBare average electronegativity of A3+and B4+.According to the electronegativity data, the average electronegativity of La,Nd,Sm,Gd,Yb,Ce,and Zr is 1.10,1.14,1.17,1.20,1.30,1.12,and 1.33, respectively.The cationic bonding strength of LCZ and LNSGY is 0.0078 and 0.0023 by Eq.(7).Hence, the thermal expansion coefficient of LNSGY ceramic is higher than that of LCZ ceramic.In summary,LNSGY can be a potential material for TBCs applications.

4.Conclusions

The novel LNSGY high-entropy ceramics are successfully prepared by solid-state sintering.The microstructure and fracture toughness of the LCZ and LNSGY are comparatively investigated.Overall, meaningful conclusions are drawn as follows:

(1) Both LCZ and LNSGY exhibit a single-phase pyrochlore structure, and the element distribution of LNSGY is homogenous.

(2) The average grain size of LNSGY ceramic is 3.92 μm after sintering at 1600 °C for 40 h, which is extremely smaller than that of LCZ ceramic (approximately 22.85 μm).Moreover, LCZ ceramic exhibits a larger volume shrinkage ratio((30.6 ± 1.8)%) than LNSGY ceramic ((25.8 ± 2.1)%).

(3) Compared with the fracture toughness of LCZ((1.4 ± 0.1) MPa?m1/2), the fracture toughness of LNSGY ceramic ((2.0 ± 0.3) MPa?m1/2) is significantly enhanced by more than 40%, which is favourable for the service life of TBCs material.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China(No.52202057)and the National Science and Technology Major Project, China (2017-VI-0020-0093).