Excellent High-temperature Phase Stability Research Articles

Investigations on the mechanical and thermal properties of multicomponent dual-phase rare-earth zirconate ceramics

In this work, a series of multicomponent rare-earth zirconate (RE2Zr2O7, RE=La, Nd, Sm, Eu, Gd or Yb) ceramics were synthesized using solid-state reaction method. The phase stability and key factors affecting the thermal conductivity and mechanical performances were investigated. These materials possess pyrochlore-fluorite dual-phase structure and show excellent high-temperature phase stability. (La1/2Yb1/2)2Zr2O7 displays a lower amorphous-like thermal conductivity (1.39–1.67 Wm−1K−1, 25–1000 °C), while other specimens present a 1/T-like dependence at low temperatures. (La1/5Nd1/5Sm1/5Gd1/5Yb1/5)2Zr2O7 exhibit higher hardness (11.37 ± 0.35 GPa) and fracture toughness (2.10 ± 0.11 MPa·m1/2) in comparison with others. Size disorder shows stronger correlations with the thermal conductivity of multicomponent dual-phase RE2Zr2O7 ceramics, while the influence of configurational entropy is more significant for mechanical properties. A synergistic increase in configurational entropy and size disorder is an effective strategy to enhance the comprehensive performance of multicomponent dual-phase rare-earth zirconates.

Effects of Y doping on phase structure, mechanical properties and sintering behavior of (Yb1-xYx)2SiO5 environmental barrier coating materials

Yb2SiO5 is one of the most widely studied environmental barrier coating materials due to its excellent high-temperature phase stability and resistance to water oxygen corrosion. In this paper, a series of (Yb1-xYx)2SiO5 (x = 0, 0.1, 0.2, 0.3, 0.4) ceramics were fabricated by high-temperature solid state sintering at 1550 °C. The effects of Y doping concentration on phase composition, mechanical properties, thermal conductivity and high-temperature sintering behavior of Yb2SiO5 ceramics were investigated in detail. The results show that all the (Yb1-xYx)2SiO5 ceramic specimens are composed of single X2-Yb2SiO5 phase with relative high compactness. The introduction of Y gives rise to the reduction of the thermal conductivity of Yb2SiO5 and the value of cell parameters and volume of (Yb1-xYx)2SiO5 is proportional to the Y3+ doping concentration. Among the five (Yb1-xYx)2SiO5 ceramics, (Yb0.9Y0.1)2SiO5 possesses the lowest brittleness index and its average fracture toughness is 2.85 MPa m1/2, which is about 10 % higher than pure Yb2SiO5. In addition, (Yb0.9Y0.1)2SiO5 has good high temperature phase stability from room temperature to 1450 °C and the grain size of (Yb0.9Y0.1)2SiO5 is basically unchanged after sintering at 1400 °C for 100 h. We believe that (Yb0.9Y0.1)2SiO5 is one of the most promising candidate materials for environmental barrier coatings.

Mechanical and thermal properties evaluation of Eu2O3-Gd2Zr2O7 ceramics served as potential neutron absorber rod

Rare earth compounds (RE2O3), typically Eu2O3 and Gd2O3, are considered as absorbing materials due to their long chain neutron absorbing ability and high efficiency of neutron absorption. In this paper, a specifically designed xEu2O3-(1-x)Gd2Zr2O7 (x = 0.6, 0.7, 0.8, 0.85) composite ceramics with excellent thermal properties were synthesized through direct thermal decomposition reactions followed a high-temperature sintering method under vacuum conditions. X-ray diffraction (XRD) and Raman spectroscopy analysis indicated that the obtained ceramics consisted of monoclinic Eu2O3 and fluorite Gd2Zr2O7 phases. The hardness of xEu2O3-(1-x)Gd2Zr2O7 ceramics (x = 0.8, 0.85) have no significantly changes in the test temperature range (25–350 °C). The thermal expansion coefficients (TECs) (7.3–10 ×10-6 K-1) are lower than Dy2TiO5 (9.3–10.4 ×10-6 K-1), and the thermal expansion rate changes linearly with the increase of temperature, which indicates that the xEu2O3-(1-x)Gd2Zr2O7 ceramics exhibit excellent high temperature phase stability. Furthermore, the range of thermal conductivities is from 0.96 W·m-1·K-1 to 2.02 W·m-1·K-1 (25–800 °C), which is higher than Dy2TiO5 (0.27–0.55 W/m·K, 220–650 °C). Compared with the state-of-art Dy2TiO5 ceramics, xEu2O3-(1-x)Gd2Zr2O7 have superior thermal properties and are therefore expected to be the next generational of neutron absorbent materials used in ash rod.

Phase evolution, thermophysical and mechanical properties of high-entropy (Ce0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 ceramic for advanced thermal barrier coatings

Single-phase high-entropy rare-earth zirconate (Ce0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 with large thermal expansion coefficient (11.69 ×10−6 K−1, 25–1500 °C), low thermal conductivity (1.07 W m−1 K−1, 1500 °C), and high fracture toughness (1.84 ± 0.03 MPa m1/2, 25 °C) was successfully synthesized using reverse coprecipitation and high-temperature calcination. The underlying mechanism governing the observed results was analyzed by considering the ionic bond strength, crystal lattice energy, and bond length. X-ray diffraction and Raman spectroscopy results demonstrated that the pyrochlore structured sample has excellent high-temperature phase stability. Moreover, after being heated at 1600 °C for 10–30 h, the average grain size of this sample only increases from 0.95 µm to 1.47 µm, indicating a sluggish grain growth rate. The outstanding combination of thermophysical and mechanical properties of the (Ce0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 emphasizes its enormous potential for next-generation thermal barrier coating applications.

Structure, thermal and mechanical properties of mid-entropy thermal barrier ceramic (Y0.3Gd0.3Yb0.4)4Hf3O12 prepared by ultrafast high-temperature sintering

Thermal barrier coatings (TBCs) have an important role to play in the development of aero engines. In this paper, mid-entropy rare-earth hafnate (Y0.3Gd0.3Yb0.4)4Hf3O12(YGYbH) ceramic material was designed based on rRE3+/rHf4+ and was successfully fabricated in only 7 min using an ultrafast high-temperature sintering (UHS) process. The YGYbH ceramic bulk exhibits a single homogeneous fluorite structure and excellent high temperature phase stability, which can remain phase constant for 80 h at 1500 °C due to its unique cation radius ratio and high-entropy effect. YGYbH possesses sluggish grain growth characteristics and great anti-sintering properties, and when annealing at 1500 °C for 30 h, its grain size only grows from the initial 1.19 μm–1.61 μm, which is conducive to stabilizing the mechanical properties of YGYbH at higher service temperature. YGYbH has a low thermal conductivity of 0.89 W/(m·k) at 1100 °C, which is much lower than that of 6%–8%Y2O3 partially stabilized ZrO2(YSZ) (∼2.5 W/(m·k)), while having a high coefficient of thermal expansion (CTE), from 8.8 × 10−6/K at room temperature to 11.0 × 10−6/K at 1500 °C which is nearly to that of the YSZ. Furthermore, YGYbH has a high hardness (11.94 GPa) and a low elastic modulus (147.4 GPa), but a slightly lower fracture toughness (1.26 MPa m½) than that of 8YSZ, exhibiting great overall mechanical properties. Based on the results of this work, YGYbH with excellent overall performance exhibits great potential as the next-generation TBC material.

Highly anti-sintering and toughened pyrochlore (Dy0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 high-entropy ceramic for advanced thermal barrier coatings

Single-phase high-entropy rare-earth zirconate (Dy0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 with excellent high-temperature phase stability at 1600 °C was designed and synthesized. After being heated at 1600 °C for 1 ∼ 50 h, the average grain size of the sample increases only from 0.74 μm to 2.22 μm, indicating a high anti-sintering ability. The fracture toughness is 2.07 ± 0.03 MPa m1/2 (25 °C), which is 50% higher than lanthanum zirconate. The local lattice distortions induced by the atomic interactions and atomic-size mismatch are considered to be responsible for the strong anti-sintering ability and improved fracture toughness. In addition, the thermal expansion coefficient and thermal conductivity of (Dy0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 are 10.59 × 10-6 K-1 (25 ∼ 1500 °C) and 1.14 ± 0.09 W·m-1·K-1 (1500 °C), respectively. The outstanding combination of anti-sintering ability, mechanical behavior, and thermophysical properties of (Dy0.2Nd0.2Sm0.2Eu0.2Yb0.2)2Zr2O7 highlights its great potential for next-generation thermal barrier coating applications.

Rare-earth high-entropy aluminate-toughened-zirconate dual-phase composite ceramics for advanced thermal barrier coatings

Superb toughening is achieved by incorporating a secondary ferroelastic phase in high-entropy rare-earth zirconate 5RE2Zr2O7 (HZ). Here, we report an enhancement of 64% in fracture toughness through the addition of 30mol% high-entropy rare-earth aluminate 5REAlO3 (HA) to the HZ matrix (30HA). The aforementioned rare-earth elements RE are La, Sm, Eu, Gd, and Yb. The present dual-phase composite ceramic 30HA has a large fracture toughness of 2.77 ± 0.14 MPa m1/2, along with excellent high-temperature phase stability, resulting in good usage for potential thermal barrier coating applications. Particularly, the fracture toughness of the dual-phase composite ceramics at first increases to a maximum and then drops suddenly, as the mole fraction of HA increases from 0 to 50%. A clear definition of fitting parameters and their physical significance is provided for a better interpretation of the experimental data. The present toughening mechanism sheds light on microstructure engineering in high-entropy ceramics for excellent mechanical properties.

High-entropy (La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2(Zr0.75Ce0.25)2O7 thermal barrier coating material with significantly enhanced fracture toughness

Poor 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.

Ultrafast high-temperature sintering of high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Hf2O7 ceramics with fluorite structure

High-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Hf2O7 (5LaH) hafnate was designed and successfully prepared by the emerging ultrafast high-temperature sintering (UHS) in less than six minutes. The prepared 5LaH has an ultra-high hardness (1089 HV0.3) and an extremely low thermal conductivity of 0.93 W/(m·k), which are mainly attributed to the severe lattice distortion inside the high-entropy material. Furthermore, the grain size only grows at a rate of 0.09 μm/h at 1500 °C, which shows hugely sluggish grain growth characteristics. After annealing at 1300 °C for 30 h, the 5LaH maintains the initial fluorite structure, which exhibits excellent high-temperature phase stability owing to the higher configurational entropy that causes a decrease in the Gibbs free energy of the high-entropy material system. Therefore, 5LaH has excellent potential as the next-generation thermal barrier coating material, and the UHS has excellent prospects in developing and rapidly synthesizing new ceramic materials.

Investigation of Thermal Shock Behavior of Multilayer Thermal Barrier Coatings with Superior Erosion Resistance Prepared by Atmospheric Plasma Spraying

Gadolinium zirconate (GZ) has become a promising thermal barrier coating (TBC) candidate material for high-temperature applications because of its excellent high-temperature phase stability and low thermal conductivity compared to yttria-stabilized zirconia (YSZ). The double-ceramic-layered (DCL) coating comprised of GZ and YSZ was confirmed to possess better durability. However, the particle-erosion resistance of GZ is poor due to its low fracture toughness. In this study, a novel erosion-resistant layer, an Al2O3-GdAlO3 (AGAP) amorphous layer, was deposited as the top layer to resist erosion. Three triple-ceramic-layer (TCL) coatings comprised of an Al2O3-GAP layer as the top layer, a GZ layer, a GZ/YSZ composite layer, and a rare-earth-doped gadolinium zirconate (GSZC) layer as the intermediate layer, and a YSZ layer as the base layer. For comparison, an AGAP-YSZ DCL coating without a middle layer was prepared as well. Under the erosion speed of 200 m/s, only a small amount of spallation occurred on the surface of the Al2O3-GAP layer, indicating a superior particle-erosion resistance. In the thermal shock test, the Al2O3-GAP layer experienced glass transition and the glass transition temperature was close to 1500 °C. The hardness of the Al2O3-GAP coating after glass transition increased ~170% compared to the as-sprayed Al2O3-GAP coating. Moreover, The DCL TBC and TCL TBCs exhibited different failure mechanisms, which illustrated the necessity of the middle layer. The finite element model (FEM) simulation also shows that the introduction of the GZ layer can obviously reduce the thermal stress at the TC/BC interface. In terms of coating with a modified GZ layer, the AGAP-GZ/YSZ-YSZ coating and AGAP-GSZC-YSZ coating showed a similar failure model to the AGAP-GZ-YSZ coating, and the AGAP-GSZC-YSZ coating exhibited better thermal shock resistance.

Ultrafast high-temperature sintering of (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 high-entropy ceramics with defective fluorite structure

An entropy-stabilized rare earth hafnate (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 (5RH) with defective fluorite structure was successfully prepared by the emerging ultrafast high-temperature sintering (UHS) in less than six minutes. The 5RH ceramic possessed a higher thermal expansion coefficient (11.23 ×10−6/K, 1500 °C) and extremely low thermal conductivity (0.94 W/(m·k), 1300 ℃) owing to the larger lattice distortion of high-entropy materials. After high-temperature annealing at 1500 ℃, the 5RH showed extremely sluggish grain growth characteristics and excellent high-temperature phase stability, mainly attributed to the non-equilibrium sintering characteristic of the UHS and the sluggish diffusion effect of high-entropy materials. Therefore, (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 has excellent potential as a next-generation thermal barrier coating material to replace traditional Y2O3 stabilized ZrO2. Finally, using the UHS to prepare high-entropy ceramics provides a new technique for fast-sintering and developing next-generation thermal barrier coating materials.

Thermal Shock Behavior and Particle Erosion Resistance of Toughened GZ Coatings Prepared by Atmospheric Plasma Spraying

Gadolinium zirconate with excellent high-temperature phase stability and sintering resistance has become a very promising candidate material for a new generation of thermal barrier coatings (TBCs). However, the low fracture toughness of gadolinium zirconate greatly limits its application. In this study, gadolinium zirconate (GZ) and two kinds of toughened gadolinium zirconate (GZ/YSZ prepared by mixed powder of Gd2Zr2O7 and YSZ and GSZC prepared by (Gd0.925Sc0.075)2(Zr0.7Ce0.3)2O7 powder) double-layered TBCs were prepared by atmospheric plasma spraying (APS). The fracture toughness of the GZ/YSZ coating and GSZC coating were 9 times and 3.5 times that of GZ coating, respectively. The results of thermal shock test showed that the three TBCs exhibit different failure mechanisms. During the thermal shock test, cracking occurred at the interfaces between the YSZ layer and the BC or GZ/YSZ layer, while GSZC TBC failed due to premature cracking inside the GSZC layer. The particle erosion rate of the GZ, GZ/YSZ, and GZSC coatings were 1.81, 0.48, and 1.01 mg/g, respectively, indicating that the erosion resistance of coatings is related to their fracture toughness. Furthermore, the superior erosion resistance of the GZ/YSZ and GSZC coatings can be attributed to the conversion of crack propagation path during the erosion test.

New class of high-entropy defect fluorite oxides RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE = Y, Ho, Er, or Yb) as promising thermal barrier coatings

High-entropy ceramics exhibit great application potential as thermal barrier coating (TBC) materials. Herein, a series of novel high-entropy ceramics with RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE2HE2O7, RE = Y, Ho, Er, or Yb) compositions were fabricated via a solid-state reaction. X-ray diffraction (XRD) and energy dispersive spectrometry (EDS) mapping analyses confirmed that RE2HE2O7 formed a single defect fluorite structure with uniform elemental distribution. The thermophysical properties of the RE2HE2O7 ceramics were investigated systematically. The results show that RE2HE2O7 ceramics have excellent high-temperature phase stability, high thermal expansion coefficients (10.3–11.7 × 10−6 K-1, 1200 ℃), and low thermal conductivities (1.10-1.37 W m-1 K-1, 25 ℃). In addition, RE2HE2O7 ceramics have a high Vickers hardness (13.7–15.0 GPa) and relatively low fracture toughness (1.14-1.27 MPa m0.5). The outstanding properties of the RE2HE2O7 ceramics indicate that they could be candidates for the next generation of TBC materials.

Phase composition, microstructure and thermophysical properties of the Srx(Zr0.9Y0.05Yb0.05)O1.95+x ceramics

Srx(Zr0.9Y0.05Yb0.05)O1.95+x (x=1.0, 0.9, 0.8, 0.7) ceramics were prepared by solid state reaction sintering. The sintered Sr1.0(Zr0.9Y0.05Yb0.05)O2.95 is a single-phase solid solution while the sintered Srx(Zr0.9Y0.05Yb0.05)O1.95+x (x=0.9−0.7) are composites, and a significant grain growth inhibition is observed in the sintered Srx(Zr0.9Y0.05Yb0.05)O1.95+x (x=1.0, 0.9). Rare-earth elements distribution in the bulk materials indicates that Yb and Y preferentially substitute Zr-sites in SrZrO3, and the highest solubility of RE2O3 in pure SrZrO3 is ∼0.8 mol%. The sintered Srx(Zr0.9Y0.05Yb0.05)O1.95+x have high thermal expansion coefficients up to ∼11.0×10−6 K-1 (1200°C). Sr0.8(Zr0.9Y0.05Yb0.05)O2.75 has the lowest thermal conductivity of 1.38 W·m-1·K-1 at 800°C. Srx(Zr0.9Y0.05Yb0.05)O1.95+x (x=1.0, 0.9, 0.8) show no phase transition from 600 to 1400°C, whereas Srx(Zr0.9Y0.05Yb0.05)O1.95+x (x=0.9, 0.8) have excellent high-temperature phase stability over the whole investigated temperature range. Therefore, Srx(Zr0.9Y0.05Yb0.05)O1.95+x (x=1.0, 0.9, 0.8) are considered as promising TBCs materials that might be operated at higher temperatures compared to YSZ.

Potential thermal barrier coating materials: RE2FeTaO7 (RE = Y, Eu, Gd, Dy) compounds

In this paper, the A2B2O7-type compounds RE2FeTaO7 synthesized via a solid-state reaction were researched for high-temperature applications, and their thermophysical properties were documented for the first time. The relationships between thermophysical properties and structures of RE2FeTaO7 were established, the results show that the RE2FeTaO7 compounds have lower thermal conductivity (1.5–2.8 W m−1 K−1) than 8YSZ (2.4–3.0 W m−1 K−1) and La2Zr2O7 (2.0–2.8 W m−1 K−1) at temperature of 100–900 °C, implying their prominent thermal insulation performance. The thermal expansion coefficients (TECs, ∼9.2 × 10−6 K−1, 1200 °C) of RE2FeTaO7 are equivalent to La2Zr2O7 (∼9.0 × 10−6 K−1, 1200 °C). Furthermore, the excellent high-temperature phase stability, high hardness (∼10.2 GPa) and fracture toughness (∼1.7 MPa m1/2) jointly indicate that RE2FeTaO7 compounds are potential thermal barrier coating materials.

High temperature mechanical and thermal properties of CaxBa1-xZrO3 solid solutions

The excellent high temperature phase stability enables the perovskite BaZrO3 to be a promising candidate for thermal barrier coatings (TBCs). Further improvement of its thermo-physical properties is necessary to fulfill the increasing requirements for TBCs. In this work, CaxBa1-xZrO3 (x = 0.05, 0.10, 0.15, 0.20, 0.25) solid solutions are fabricated and the influence of the Ca2+ content on their mechanical/thermal properties is studied. They have moderate mechanical properties and their residual Young's moduli at 1473 K are around 80% of the values at room temperature. Compared with BaZrO3, CaxBa1-xZrO3 solid solutions have decreased thermal conductivities which are comparable to those of YSZ and other potential TBCs materials. Among these solid solutions, Ca0.25Ba0.75ZrO3 exhibits the lowest thermal conductivity that is in the range from 2.06 W⋅m-1⋅K-1 to 2.52 W⋅m-1⋅K-1. Meanwhile, CaxBa1-xZrO3 solid solutions have higher thermal expansion coefficients than that of BaZrO3. These good high temperature mechanical and thermal properties reveal their promising applications as high temperature structural ceramics including TBCs.

Phase structures and thermophysical properties of ZrO2-doped SmTaO4 ceramics

The [Formula: see text] ceramics have excellent high-temperature phase stability and mechanical properties and show great potential for use as next-generation thermal barrier coating (TBC) materials. However, the thermophysical properties of [Formula: see text], especially [Formula: see text]-doped, were not clearly established. The [Formula: see text] ceramics doped with different contents of [Formula: see text] (0%, 2%, 4%, 6%, and 8%) by high-temperature solid-state reaction were studied in this paper. The phase structures and microstructures of the samples were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and EDS. The thermophysical properties of the samples including specific heat, thermal diffusivity, and thermal conductivity were measured systematically. The results show that [Formula: see text]-doped [Formula: see text] ceramics have a single monoclinic phase, and that [Formula: see text] ion doping does not change the crystal structure. The bandgap of 2% [Formula: see text]-doped [Formula: see text] ceramics was narrow (4.48 eV), indicating that heat is conducted by phonons in ceramics. The [Formula: see text] doped with 2% of [Formula: see text] had lower thermal conductivity [Formula: see text] at [Formula: see text] than [Formula: see text] [Formula: see text] at [Formula: see text]. This indicates that 2% [Formula: see text]-doped [Formula: see text] ceramics have the potential to be employed as TBCs in next-generation gas turbines.

Synthesis, crystal structure and thermophysical properties of (La1-XEuX)3TaO7 ceramics

Rare earth tantalates (La1-XEuX)3TaO7 ceramics are synthesized via solid-state reaction in this work. The phase structure is characterized via X-ray diffraction and Raman spectrum, when the microstructure and element distribution are detected by scanning electron microscopy and energy dispersive spectroscopy. As for various properties, (La1-XEuX)3TaO7 ceramics own excellent absorption for the UV light and the band gap ranges from 4.0 to 4.7 eV. The highest thermal expansion coefficients reach 10.8 × 10−6 K−1 (1200 ℃) and the entire series of (La1-XEuX)3TaO7 ceramics exhibit excellent high-temperature phase stability. The minimum thermal conductivity reaches 1.04 W m−1 K−1 (500 ℃) ascribed to various phonon scattering mechanisms. The thermal radiation resistance relates to the crystal structure, which relies on the mean RE3+ ionic radius. The results reveal that (La1-XEuX)3TaO7 ceramics are candidate thermal insulation materials.

The glass-like thermal conductivity in ZrO2-Dy3TaO7 ceramic for promising thermal barrier coating application

Using the solid-state reaction method, the (ZrO2)x-(Dy3TaO7)1−x (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) ceramics are synthesized in this work. The identification of the crystal structures indicates that the (ZrO2)x-(Dy3TaO7)1−x ceramics belong to the orthorhombic system, and the space group is C2221 in spite of the value of x increasing to 0.1. The thermal conductivities of the (ZrO2)x-(Dy3TaO7)1−x ceramics range from 1.3 W/(m K) to 1.8 W/(m K), and this value is much lower than that of 7-8 YSZ (yttria-stabilized zirconia). Besides, the (ZrO2)x-(Dy3TaO7)1−x ceramics possess the glass-like thermal conductivity caused by intrinsic oxygen vacancies existing in the lattice of Dy3TaO7. Moreover, the results of thermal expansion rates demonstrate that the (ZrO2)x-(Dy3TaO7)1−x ceramics possess excellent high temperature phase stability, and the thermal expansion coefficients [(9.7–11) × 10−6 K−1] are comparable to that of 7-8 YSZ.

Effects of aging treatment on microstructure and tribological properties of nickel-based high-temperature self-lubrication wear resistant composite coatings by laser cladding

The NiCr/Cr3C2–WS2 high-temperature self-lubrication wear resistant composite coatings were fabricated on substrate of a hot-rolled AISI304 austenitic stainless steel by laser cladding. The high-temperature phase stability of the composite coatings was evaluated by aging at 600 °C for 10 h, 30 h, 50 h, and the microstructures of the as-laser clad and aged coatings were examined by means of XRD, SEM, EDS, respectively. The sliding wear resistance of the as-laser clad and aged coatings was evaluated at 600 °C. The results show that NiCr/Cr3C2–WS2 composite coating has excellent high-temperature phase stability, the γ-(Fe,Ni)/Cr7C3 eutectic phases, Cr7C3 and (Cr,W)C hard phases, CrS/WS2 mixed solid lubricant phases all existed in the as-laser clad and aged coatings. The volume fraction of eutectic phases decreased gradually with the increasing of aged time due to their dissolution. The microhardness of the aged coating decreased slightly after aging the coating 50 h at 600 °C due to the dissolution of the eutectic phases and notable breaking or granulation of the Cr7C3 hard phase, but the tribological properties were not significantly affected by aging treatment.