添加CeO_2对MoS_2基复合涂层摩擦学性能的影响

针对MoS_2基复合涂层耐磨性差和承载能力低的问题,以不同含量(质量分数)的CeO_2作为添加剂,采用喷涂法在GCr15钢表面制备MoS_2基复合涂层。利用摩擦磨损试验机和划痕仪分别研究涂层摩擦磨损性能和结合强度,并借助金相显微镜对涂层磨损形貌进行表征。结果表明:添加适量CeO_2可以改善涂层的摩擦磨损性能,其最佳含量为2%,此时摩擦因数和磨损量均最小,分别为0.232和0.011 3 mm~3;同时结合强度从22 N提高到28.29 N。涂层磨损量随载荷的增大而增大;而载荷小于8 N时,涂层的摩擦因数随载荷的增大而减小,当载荷大于8 N时,摩擦因数又有回升趋势。添加稀土后涂层的承载能力有明显提高。未添加稀土时,涂层产生严重剥离,并发生磨粒磨损;添加2%CeO_2后,涂层发生轻微磨粒磨损,耐磨性得到显著提高。     

Fabrication and properties of the superhydrophobic ceria-based composite coating on magnesium alloy

A superhydrophobic ceria-based composite coating is developed to improve anticorrosion properties of AZ61 magnesium alloy, fabricating via chemical conversion method followed by hydrothermal treatment. The cerium conversion coating has a block structure with microcracks. After the hydrothermal treatment, a dense CeO₂ layer, porous CeO₂ nanorods, and stearic absorbing layers are grown stepwise on the conversion coating. And the composite coating is hydrophobic or even superhydrophobic and has almost no microcracks. As the hydrothermal reaction time increases, the water contact angle of the composite coating first increases and then decreases, and it reaches the maximum value of 152° after hydrothermal treatment for 4 h. Both the dense CeO₂ layer and the superhydrophobic stearic absorbing layer can effectively prevent the electrolyte from contacting the substrate; the corrosion current density of the superhydrophobic composite coating is lower than that of the hydrophilic composite coating and the cerium conversion coating, and has the best corrosion resistance.     

Ta2O5-Y2O3 掺杂剂对氧化锆相结构、断裂韧性和热物理性能的影响

合成制备了一系列的Ta₂O₅-Y₂O₃掺杂的ZrO₂（TYSZ）。研究了掺杂剂含量对处理相结构、相稳定性、断裂韧性、热膨胀系数和导热系数的影响,同时探讨了应力条件下TYSZ的响应机制。结果表明,掺杂Ta₂O₅和Y₂O₃引起的晶格变形有利于t相稳定。但随着掺杂含量的增加,由于应力诱导的t-m相变被抑制和高温下稳定剂析出倾向增加,高温相稳定性和断裂韧性下降。热膨胀系数和热导率的变化则是由晶体结构的混乱程度和晶格膨胀导致的。16TYSZ具有良好的相稳定性、应力敏感性和热物理性能,是先进热障涂层材料的潜在候选材料。     

Elaboration and microstructural characterization of calcareous/ceria based composite on zinc substrate

Cerium conversion coatings were prepared by cathodic electrodeposition from a low concentrated aqueous solution of Ce(NO₃)₃ with KNO₃ addition to insure the conductivity of the electrolyte. The cerium oxide film was characterized by Scanning Electronic Microscopy (SEM). Although the deposit is uniform with current density of 2 mA/cm², it shows cauliflower morphology with a crack network, giving rise to bad mechanical and electrochemical behaviour. Elaboration of a calcareous deposit inside crack network of the cerium coating by cathodic polarization from artificial seawater is investigated at different applied potential, in order to increase the quality of the cerium coating formed. For cathodic potentials lower than–1.5 V/SCE, zinc corrosion products (gordaite) were observed inside the cerium oxide film instead of calcareous deposit although the current density decrease during the deposition suggesting a partially blocked surface. A pure calcareous compound was observed at–1.5 and–1.6 V/SCE. SEM and EDX cartography of the cross section revealed that open cracks in the cerium oxide structure are filled by calcium. Calcium was also detected inside the CeO₂ film not only all around the cracks but also in all the porosity of the CeO₂ film. It has precipitated as CaCO₃ (aragonite form) as revealed by micro-Raman spectroscopy and XRD.     

Designing for the cerium-based conversion coating with excellent corrosion resistance on Mg–4Y–2Al magnesium alloy

The cerium salt chemical conversion baths containing KMnO₄ are applied to prepare protective coatings on the WA42 alloy surface, and the effect of the concentration of KMnO₄ on the microstructure and corrosion properties of the coatings is investigated by scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical tests. The results indicate that with the addition of KMnO₄ to the conversion bath, the microstructure of the coating is more uniform and denser, and the coating with the KMnO₄ concentration of 4 g/L (4M coating) has the most uniform microstructure with the least microcracks. The 4M coating exhibits a two-layered structure, and it is mainly composed of MgO, Mg(OH)₂, CeO₂, Ce₂O₃, Ce(OH)₃, MnO, and MnO₂. In addition, as the KMnO₄ concentration increases from 0 to 6 g/L, the I_corr of the coatings in 3.5% NaCl solution decreases first and then increases, and the 4M coating shows the best corrosion resistance, which should attribute to the uniform and dense microstructure.     

Preparation and Thermophysical Properties of La2Zr2O7 Coatings by Thermal Spraying of an Amorphous Precursor

Free-standing La₂Zr₂O₇ coatings were obtained by plasma spraying, using an amorphous La-O-Zr precursor as the feedstock. The La-O-Zr precursor powder was prepared by coprecipitation. During thermal spraying, the formation of coatings can be regarded as a joint process of melting-solidification, thermal decomposition, and crystallization. The time required for crystal growth was significantly shortened during spraying. Consequently, the average grain size of coatings was approximately 200 nm, with a narrow distribution (100-500 nm). Coatings prepared by this method show better thermophysical properties than those prepared with crystalline La₂Zr₂O₇ powder as the feedstock. The thermal conductivity of the as-sprayed coating was approximately 0.36-0.47 W/m K and the average coefficient of thermal expansion (CTE) is 11.1 × 10^?6/K.     

金刚石粒径及含量对超音速激光沉积金刚石/Cu复合涂层微观结构及性能的影响

目的研究不同金刚石粒径及含量对超音速激光沉积金刚石/Cu复合涂层微观结构及性能的影响。方法利用超音速激光沉积技术制备金刚石/Cu复合涂层。采用扫描电镜和摩擦磨损测试对涂层的显微组织结构和磨损性能进行了分析,用激光闪烁法测量复合涂层的热导率。结果金刚石均匀分布在复合涂层中,原始粉末中金刚石体积分数从30%增加到50%时,复合涂层中金刚石颗粒的面积占比仅从14.01%升至16.79%,远低于金刚石颗粒在原始粉末中的含量。400目金刚石/Cu复合涂层的平均热导率为296 W/(m·K),摩擦系数为0.551;800目金刚石/Cu复合涂层的平均热导率为238 W/(m·K),摩擦系数为0.545。结论原始粉末中金刚石配比的增加并未对复合涂层中金刚石含量的提升有显著作用。金刚石/Cu复合涂层的热导率随着增强相颗粒含量的增加而降低,随着增强相颗粒粒径的增大而提高。不同粒径金刚石颗粒的添加能显著降低Cu涂层的摩擦系数,且小粒径金刚石颗粒的添加使复合涂层的摩擦系数更低和更稳定,从而使其具有更小的磨损量和磨痕宽度,表现出较优的耐磨损性能。     

Improved Thermal Performance of Diamond-Copper Composites with Boron Carbide Coating

B₄C-coated diamond (diamond@B₄C) particles are used to improve the interfacial bonding and thermal properties of diamond/Cu composites. Scanning electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy were applied to characterize the formed B₄C coating on diamond particles. It is found that the B₄C coating strongly improves the interfacial bonding between the Cu matrix and diamond particles. The resulting diamond@B₄C/Cu composites show high thermal conductivity of 665 W/mK and low coefficient of thermal expansion of 7.5 × 10^?6/K at 60% diamond volume fraction, which are significantly superior to those of the composites with uncoated diamond particles. The experimental thermal conductivity is also theoretically analyzed to account for the thermal resistance at the diamond@B₄C-Cu interface boundary.     

A new semiconductor Al2Fe3Si3 with complex crystal structure

Al₂Fe₃Si₃, a new semiconductor with complex triclinic structure was synthesized by arc melting and spark plasma sintering, followed by heat treatment. The nominal compositions of samples have been changed to compensate Al evaporation during synthesis process, and single Al₂Fe₃Si₃ phase has been obtained with the nominal composition of Al: Fe: Si = 26: 37: 37 (6 at.% Al excess against stoichiometry). In this study, we measured the sound velocity, thermal expansion coefficient, Vickers hardness, fracture toughness, electrical conductivity, Seebeck coefficient, and thermal conductivity of the new semiconductor Al₂Fe₃Si₃. The Al₂Fe₃Si₃ sample displayed positive Seebeck coefficient from 300 to 850 K, with a maximum Seebeck coefficient of 110 μV/K at 430 K. The Debye temperature of Al₂Fe₃Si₃ was 640 K, which was similar to or higher than those of other Al, Fe, Si based thermoelectric materials, but the lattice thermal conductivity was lower, 4–5 W/mK, due to the complex crystal structure of Al₂Fe₃Si₃. The maximum ZT value was 0.06 at 580 K.     

Influence of CeO2 superficial coating on the high temperature corrosion behavior of 2.25 Cr-1 Mo Steel in SO2 + O2 atmospheres

The influence of superficial CeO₂ coatings on isothermal and cyclic high temperature corrosion behavior of 2.25 Cr-1 Mo steel in oxidation and sulfidation (SO₂ + O₂) atmosphere have been reported in this paper. The CeO₂ coating of three different depositions rate (0.15 mg/cm², 0.30 mg/cm² and 0.50 mg/cm²) were prepared for experimental investigations. The isothermal corrosion study of uncoated and CeO₂ coated specimens was carried out in SO₂ + O₂ environment at 973K for 150 hours. The results clearly indicate that CeO₂ coated specimen improves the high temperature corrosion resistance than uncoated specimens. The post corroded CeO₂ coated specimen shows reduction of scale growth and improved scale adhesion to the substrate. The cyclic corrosion study was also carried out in both coated and uncoated specimen in same environment at 973K for 100 h. The CeO₂ coated specimens on thermal cycling also shows significant improvement of the corrosion resistance without scale cracking and spallations. In contrary, the uncoated specimen in both isothermal as well as cyclic environment shows higher corrosion rate along with scale cracking and spallations. The results of the reaction kinetics have been substantiated by post corrosion analysis of alloy/scales combinations by SEM and XRD. The details mechanism of the corrosion in presence of CeO₂ coating was further discussed in the paper.     

Gd2 O3 -Yb2 O3 -Y2 O3 -ZrO2 热障涂层材料的热物理性能

目的通过多元稀土氧化物掺杂改性YSZ,提高传统热障涂层的性能。方法使用化学共沉淀法制备不同掺杂量的Gd2O3-Yb2O3-Y2O3-Zr O2(GYYZO)材料,并分别使用冷等静压-烧结和等离子喷涂工艺制备块材和涂层。通过测试块材的热导率和热膨胀系数,分析评价材料的热物理性能。对高温退火处理后的涂层进行X射线衍射分析,评价不同成分涂层的高温相稳定性。结果氧化锆基材料的热导率和热膨胀系数随总掺杂量升高而降低。氧化锆中稀土氧化物总掺杂量为5.5%~9.84%(摩尔分数)时,在1000℃下的热导率为1.25~1.56 W/(m·K),相对8YSZ材料下降了22%~37.5%;在200~1300℃的热膨胀系数为(10~11.1)×10-6/K,与传统8YSZ材料相当。在1400℃长时间退火处理后,低掺杂量GYYZO涂层中的单斜相含量明显低于8YSZ涂层。结论多元稀土氧化物掺杂改性氧化锆材料具有良好的高温相稳定性、低热导率和适当的热膨胀系数,可以作为高性能热障涂层的备选材料。     

Microstructure and Thermomechanical Properties of Atmospheric Plasma-Sprayed Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating

In this study, a Yb₂O₃ coating was fabricated by the atmospheric plasma spray technique. The phase composition, microstructure, and thermal stability of the coating were examined. The thermal conductivity and thermal expansion behavior were also investigated. Some of the mechanical properties (elastic modulus, hardness, fracture toughness, and flexural strength) were characterized. The results reveal that the Yb₂O₃ coating is predominantly composed of the cubic Yb₂O₃ phase, and it has a dense lamellar microstructure containing defects. No mass change and exothermic phenomena are observed in the thermogravimetry and differential thermal analysis curves. The high-temperature x-ray diffraction results indicate that no phase transformation occurs from room temperature to 1500 °C, revealing the good phase stability of the Yb₂O₃ coating. The coefficient of thermal expansion of the Yb₂O₃ coating is (7.50-8.67)?×?10^?6 K^?1 in the range of 200-1400 °C. The thermal conductivity is about 1.5 W m^?1 K^?1 at 1200 °C. The Yb₂O₃ coating has excellent mechanical properties and good damage tolerant. The unique combination of these properties implies that the Yb₂O₃ coating might be a promising candidate for T/EBCs applications.     

Thermal conductivity of Ni3V–Ni3Al pseudo-binary alloys

The effect of changes in the composition and microstructure of the Ni₃V–Ni₃Al pseudo-binary alloys on their thermal conductivity has been investigated. For Ni₃V and Ni₃Al-based single-phase alloys, the thermal conductivity shows a maximum value at the stoichiometric compositions, and it decreases as the V (or Al) content of the Ni₃Al (or Ni₃V) alloy increases, following the Nordheim rule. For Ni₃V–Ni₃Al two-phase alloys, the thermal conductivity of the constituent Ni₃Al phase exhibits a smaller value than that of the Ni₃V phase. Eventually, the thermal conductivity of the two-phase alloys decreases as the Al content increases because of the increase in the volume fraction of the Ni₃Al phase with low conductivity. As the temperature increases from 293 K to 1073 K, the conductivity increases for all of the alloys but not for stoichiometric Ni₃V. However, the dependence of the thermal conductivity on the alloy composition between 293 K and 1073 K is similar. Hence, it is confirmed that the thermal conductivity of the Ni₃V–Ni₃Al pseudo-binary alloys is controlled by the composition and volume fraction of the constituent phase.     

Novel thermal barrier coatings based on La2(Zr0.7Ce0.3)2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition

Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) and yttria stabilized zirconia (YSZ) were deposited by electron beam-physical vapor deposition (EB-PVD). The thermal cycling test at 1373 K in an air furnace indicates the DCL coating has a much longer lifetime than the single layer LZ7C3 coating, and even longer than that of the single layer YSZ coating. The superior sintering-resistance of LZ7C3 coating, the similar thermal expansion behaviors of YSZ interlayer with LZ7C3 coating and thermally grown oxide (TGO) layer, and the unique growth modes of columns within DCL coating are all very helpful to the prolongation of thermal cycling life of DCL coating. The failure of DCL coating is mainly a result of the reduction-oxidation of cerium oxide, the crack initiation, propagation and extension, the abnormal oxidation of bond coat, the degradation of t′-phase in YSZ coating and the outward diffusion of Cr alloying element into LZ7C3 coating.     

CeO2的晶格动力学性质和热输运性质的第一性原理计算

采用基于密度泛函理论的有限位移法和玻尔兹曼方程,计算了CeO2的晶格动力学性质、热力学性质和热输运性质,计算结果和实验结果基本符合。通过分析CeO2所有声子模式的振动频率、Gruneisen系数和散射率,揭示了光学声子对增强晶格振动的非简谐性和声子散射率所起的重要作用。此外,还计算了不同自由程的声子模式对热导率的贡献,发现CeO2的晶格热导率主要由声子自由程在1~10 nm之间的声子所贡献。     

Variation of the lattice parameter and thermal expansion coefficient of (U,Dy)O2 as a function of DyO1.5 content

Thermal expansions of (U,Dy)O₂ solid solutions were investigated between room temperature and 1673 K by using a thermo-mechanical analyzer. The lattice parameter of (U,Dy)O₂ pellets is lower than that of UO₂ and it decreases as Dy content increases. The linear thermal expansion and average thermal expansion coefficients of (U,Dy)O₂ are higher than that of UO₂. For the temperature range from room temperature to 1673 K, the average thermal expansion coefficient values for UO₂ and (U_0.8Dy_0.2)O₂ are 10.97 × 10⁻⁶ and 11.37 × 10⁻⁶ K⁻¹, respectively.     

Microstructure and oxidation of multi-layer MoSi2-CrSi2-Si coatings for SiC coated carbon/carbon composites

Multi-layer MoSi₂-CrSi₂-Si anti-oxidation coatings with different compositional ratios were prepared on the surface of SiC coated carbon/carbon (C/C) composites by a two-step pack cementation method. The microstructure and anti-oxidation performance of the coating were studied. The results show that the multi-layered coatings could protect the C/C composites from oxidation in air at 1773 K for 1000 h or 1873 K for 750 h, respectively. The anti-oxidation performance of the multi-layer MoSi₂-CrSi₂-Si coating is mainly attributed to their dense and microcrack-free structure, appropriate thermal expansion coefficient and the well dispersed MoSi₂ and CrSi₂ in the coating.     

Thermal stability of nanostructured Cr3C2-NiCr coatings

The thermal stability behavior of nanostructured Cr₃C₂-NiCr coatings was investigated. The nanostructured Cr₃C₂-NiCr coatings, synthesized using mechanical milling and high-velocity oxygen fuel (HVOF) thermal spraying, were thermally exposed in air at 473, 673, 873, and 1073 K for 8 h. The results show that microhardness of the conventional coating increased slightly with increasing temperature, while that of the nanostructured coating drastically increased from 1020 to 1240 HV₃₀₀ for the same temperature increases. Heat treatment led to increases in scratch resistance and decreases in the coefficient of friction for the nanostructured Cr₃C₂-NiCr coatings. A high density of Cr₂O₃ oxide particles with average size of 8.3 nm was found in the nanostructured coatings exposed to high temperatures, which is thought to be responsible for the observed increase in microhardness and scratch resistance and the decrease in the coefficient of friction of the nanostructured coatings.     

Thermodynamic properties of CexTh1−xO2 solid solution from first-principles calculations

A systematic study based on first-principles calculations along with a quasi-harmonic approximation has been conducted to calculate the thermodynamic properties of the Ce_xTh_1?xO₂ solid solution. The predicted density, thermal expansion coefficients, heat capacity and thermal conductivity for the Ce_xTh_1?xO₂ solid solution all agree well with the available experimental data. The thermal expansion coefficient for ThO₂ increases with CeO₂ substitution, and complete substitution shows the highest expansion coefficient. On the other hand, the mixed Ce_xTh_1?xO₂ (0 < x < 1) solid solution generally exhibits lower heat capacity and thermal conductivity than the ThO₂ and CeO₂ end members. Our calculations indicate a strong effect of Ce concentration on the thermodynamic properties of the Ce_xTh_1?xO₂ solid solution.     

Preparation and oxidation behavior of a novel CeO2-modified chromizing coating

By using CeO₂ particles instead of part of Al₂O₃ particles as filler, the CeO₂ was successfully entrapped into the outer layer of the chromizing coatings on the as-deposited nanocrystalline (NC) and microcrystalline (MC) Ni films using a conventional pack-cementation method at 800 °C. For comparison, chromizing was also performed under the same condition on MC Ni film using Al₂O₃ as filler without CeO₂ particles. SEM/EDX and TEM results indicate that the refinement of Ni grain and CeO₂ entrapped into the chromizing coatings refine the grain of the chromizing coating. Oxidation at 900 °C indicates that compared with the CeO₂-free chromizing coating, the CeO₂-dispersed chromizing coating exhibits an increased oxidation resistance. For the CeO₂-dispersed chromizing coating, the refinement of Ni grain size significantly decreases the transient-oxidation scaling rate of the chromizing coatings. Together with this, the CeO₂-dispersed chromizing coating formed on NC Ni exhibits a better oxidation resistance.