超音速火焰喷涂铝青铜涂层微动磨损行为

目的 改善铝合金的抗微动磨损性能.方法 采用超音速火焰喷涂技术在ZL114A铝合金表面制备铝青铜涂层,在不同温度(25、200、300℃)下对有、无涂层的ZL114A铝合金样品进行微动磨损测试,通过对涂层性能和磨痕形貌进行表征分析,探索铝青铜涂层的抗磨损性能.结果 铝青铜涂层均匀致密,与铝合金基体结合良好,显微硬度为279HV0.3,结合强度为74 MPa.不同温度(25、200、300℃)下,涂覆铝青铜涂层样品的平均微动摩擦系数分别为0.898、0.886、0.744,磨损率分别为10.249×10–7、0.035×10–7、0.207×10–7 m3/(N·m),相比基体的平均微动摩擦系数和磨损率,3种温度下分别下降了34.5％、42.9％和58.9％.对磨痕的形貌和三维轮廓的分析表明,在25、200、300℃下,铝青铜涂层的磨损机制不相同,25℃下为磨粒磨损和剥层,200℃下为磨粒磨损、剥层、氧化磨损和粘着磨损,300℃下为塑性变形、氧化磨损和粘着磨损.结论 制备的铝青铜涂层改善了基体的抗微动磨损性能.     

Machinability improvement in milling of SiCf/SiC composites based on laser controllable ablation pretreatment

The poor machinability of SiC_f/SiC composites greatly limits its application and promotion. The laser-induced ablation products of SiC_f/SiC composites are powdery, loose and porous. Milling of laser ablated samples demonstrated that the force and heat were almost negligible when milling ablation products. Accordingly, a laser ablation pretreatment milling (LAPM) process of SiC_f/SiC composites was proposed. Under the LAPM process, after the laser ablation treatment with controllable depth, the cutting allowance could be achieved in only one pass, which greatly improved the machining efficiency compared with the conventional milling process. The material removal rate was greatly improved on the premise of ensuring the machining quality. Taking the milling of tensile specimens as an example, compared with conventional milling, the total processing time of the specimen was reduced by 31.29 % by LAPM process. Therefore, LAPM provides a potential feasible process scheme for greatly improving the machinability and machining efficiency of SiC_f/SiC composites.     

Bulk amorphous FC20 (Fe–C–Si) alloys with small amounts of B and their crystallized structure and mechanical properties

The addition of a small amount (0.4 mass%) of B to a commercial FC20 cast iron was found to cause the formation of an amorphous phase in melt-spun ribbon and cast cylinders with a diameter of up to 0.5 mm. The structure of a melt-spun B-free FC20 alloy consisted of α-Fe, γ-Fe and Fe₃C. The effectiveness of additional B is presumably due to the generation of attractive bonding nature among the constituent elements. The amorphous alloy ribbon exhibits a high tensile strength of 3480 MPa and good bending ductility. The annealing causes the formation of an amorphous phase containing α-Fe particles with a size of about 30 nm. The mixed phase alloy exhibits an improved tensile strength of 3800 MPa without detriment to good ductility. With further increasing temperature, the mixed amorphous and α-Fe structure changes to α-Fe+Fe₃C+graphite through the metastable structure of α-Fe+Fe₃C. The structure after annealing for 900 s at 1200 K has fine grain sizes of about 0.5 μm for α-Fe, 0.3 μm for Fe₃C and 1 μm for graphite. The graphite-containing alloy exhibits high tensile strength of 1200–2000 MPa and large elongation of 5–13%. The high tensile strength and good ductility were also obtained for the 0.5 mm cylinder annealed at 1200 K. The good mechanical properties are due to the combination of fine subdivision of crack initiation sites by the homogeneous dispersion of small graphite particles and the dispersion strengthening of Fe₃C particles against the deformation of the α-Fe phase. The synthesis of the finely mixed α-Fe+Fe₃C+graphite alloys having good mechanical properties by crystallization of the new amorphous alloy in the melt-spun ribbon and cast cylinder forms is encouraging for the future development of a new Fe-based high-strength and high-ductility material.     

Mechanical properties of textured,multilayered alumina produced using electrophoretic deposition in a strong magnetic field

The mechanical properties of ceramics materials can be tailored by designing their microstructures. It had generally been difficult to utilize a magnetic field for controlling the texture of diamagnetic ceramics because of their extremely small susceptibility; however, the possibility of controlling the texture by a magnetic field occurred with the development of superconducting magnets. We demonstrate in this study that alumina/alumina laminar composites with alternate crystalline-oriented layers are produced by electrophoretic deposition in a strong magnetic field and that the bending strength of the laminar composite depended on the direction of the multilayered microstructure.     

Preparation and characterization of Ce0.8Sm0.2O1.9(SDC)–carbonates composite electrolyte via molten salt infiltration

Ce_0.8Sm_0.2O_1.9 (SDC)–carbonates composite electrolytes for SOFCs have been prepared for the first time via molten salt infiltration approach by infiltrating LiNaCO₃ molten carbonates into porous SDC ceramic. A range of techniques including XRD, SEM, Mercury Porosimeter and A.C. impedance, have been employed to characterize SDC and composite electrolytes. It has been found that the porous SDC shows a monomodal pore size distribution with average pore size of 300 nm. SDC–carbonates composite electrolytes via molten salt infiltration show a kind of SDC/carbonates co-continuous composite structure, with one SDC ceramic skeleton of bonded grains surrounded by carbonates phase. Both phases provide a continuous pathway for ionic transport and a large interfacial surface area. SDC–carbonates composite electrolytes prepared by molten salt infiltration have a higher mechanical strength than that of mixing–pressing composite electrolytes. Moreover, they show higher conductivity and lower activation energy than that of mixing–pressing composite electrolytes according to the measurements of AC impedance in the temperature range of 350 °C–650 °C.     

Ablation behavior and mechanisms of 3D-Cf/Ta0.8Hf0.2C-SiC composite at temperatures up to 2500 °C

In this work, ablation behavior and mechanisms of 3D-C_f/Ta_0.8Hf_0.2C-SiC composite were studied via air plasma test at temperatures up to 2500 °C. At temperatures below 2000 °C, a continuous oxides layer composed of o-Ta₂O_5(H) – t-Ta₂O₅ – Hf₆Ta₂O_17(H) – SiO₂ is formed on the ablation surface, which turns to o-Ta₂O_5(H) – SiO₂ above 2200 °C. During ablation, o-Ta₂O_5(H) precipitates from the glassy SiO₂ and grows up following Oswald ripening. Although the volatilization of SiO₂ aggravates with ablation temperature increase and ablation time extension, o-Ta₂O_5(H) – SiO₂ melt can still serve as effective self-healing similar to SiO₂ glass. Accordingly, the multiphase oxides layer formed on the ablation surface provides a stable protective effect for the internal composite under all the tested ablation conditions. As a consequence, the 3D-C_f/Ta_0.8Hf_0.2C-SiC composite presents outstanding ablation-resistant performance even at 2500 °C for 300 s, with a linear recession rate of ～ 5.7 µm/s and a mass recession rate of 2.91 mg/s.     

Binder stabilization and rheology optimization for vat-photopolymerization 3D printing of silica-based ceramic mixtures

Enhancing inlet gas temperature in aero/gas turbines to reduce their carbon-footprint, has led to a strive for better performing inlet cooling mechanism of the turbine blades. The internal cooling of the blades is made by ceramic cores in their casting process, but conventional ceramic molding has long reached its maximum possible geometrical complexity, hence shedding light on 3D printing of these cores. The objective of this study is to develop low-viscous, fully stabilized, commercially viable ink for vat-photopolymerization of silica-based ceramics. This paper investigates the best dispersion type and amount for different formulated monomer mixtures, and explains the best correlation between viscosity, solid loading, binders, dispersants, peeling forces and mechanical properties, and offers an optimized mixture to avoid the common ceramic printing issue, namely crack propagation of cores during sintering. Among five dispersant agents, the SOL20, SOL24 and FA4611 exhibited better performance than other dispersion agents, and the optimum concentration level for each binder and dispersant agent was ensured through sedimentation test. Their dispersion capability and long-term stability were further investigated to designate the best dispersion agent for each binder system. Further verification was made by sedimentation study of the samples at 40 °C for 40 days and reducing the superficial area of the used powder mixture. According to the result of the rheology analysis, the best dispersions were achieved using SOL20 for the loaded binder mixtures of M1 and M4, SOL24 for M3and FA4611 for M2. The instability of M1 and M2 with their respective dispersant agent was coordinated through the thixotropic agent of TX/2, and complete stabilization and near-Newtonian behavior were achieved. However, the research showed that the addition of TX/2 to fully stabilized M4 and M2 suspensions negatively impacts the mixtures’ rheological behavior from near-Newtonian to shear-thickening. In the final stage of this study, peeling forces, sintering and three-point bending tests were conducted to determine the final formulated suspension to print ceramic core components. M4 and SOL20 combination was selected for SiO₂-ZrSiO₄ loading and dispersing, respectively. The impact of solid loading between the range of 58 and 65 vol% on the rheological behavior of the final suspension and the mechanical properties of sintered bodies were investigated to assign an optimum solid rate. The adequate strength on sintered and degree of viscosity for ceramic vat-polymerization processing was achieved at 58 vol%. Lastly, a validation study is conducted by printing a complex ceramic core model by a commercial LCD hobby printer. This validation shows the significance of this study to scale up the manufacturing of complex-shaped ceramic cores and to revolutionize the sector, by printing inexpensive and readily available irregular-shaped (non-atomized) ceramic powder, using the most cost-effective LCD printers (non-specialized expensive ceramic printers).     

Amino acid-driven hydrophobization of alumina and production of particle stabilised foams in a broader pH range

The interaction of amino acids (glycine, L-valine, L-isoleucine and L-leucine) with alumina surface was studied to induce partial hydrophobization and production of ultrastable particle-stabilised foams. The evaluation of these amino acids was carried out by mechano-quantum simulations followed by experimental tests (foamability, zeta potential, contact angle and foam lifetime measurements). The experimental results agreed with the trends pointed out by the simulations. The selected amino acids interact with alumina particles in aqueous media and in a broader pH range, leading to hydrophobization of surfaces, which was more intense for amino acids with higher molecular mass (isoleucine and leucine). As a consequence, ultrastable foams with a longer lifetime (> 100 h) were produced and the foam microstructure was preserved from ageing phenomena. Moreover, the attained foams were stable at different pH, opening up new possibilities to develop macroporous multiphasic ceramics, which can result in novel materials for thermal insulation at high temperatures.     

尖端结构对PS-PVD YSZ涂层沉积的影响

采用等离子喷涂-物理气相沉积(PS-PVD)技术在设计的尖端结构试样表面制备了YSZ涂层,研究了YSZ涂层在尖端不同区域的形貌、厚度、孔隙率和微区力学性能的差异.结果表明:尖端部位各区域涂层呈现羽柱状结构,在涂层生长过程中柱状晶之间会因生长的空间受限而存在“挤压”竞争效应,尖端边角的几何特性会削弱“挤压”竞争效应,因此其涂层厚度远高于尖端其他区域;此外,尖端尖角附近的涂层羽柱状结构呈放射状,并沿基体表面法线方向向上生长;纳米压痕表明涂层硬度由顶部至底部呈现逐渐下降趋势,这与涂层结构和残余应力相关;尖端结构的尖角区域涂层微区硬度,显著高于尖端两侧区域;结合数值模拟分析结果发现,半封闭型面会导致等离子射流紊乱度上升,由于不同方向等离子体存在“卷流”和“冲击”现象,其对应沉积的涂层厚度下降,孔隙率上升.     

等离子喷涂-物理气相沉积热障涂层的表征技术研究进展

等离子喷涂-物理气相沉积技术(PS-PVD)是一门新型的热障涂层制备技术。概述了PS-PVD的技术优势,包括高的沉积效率、制备不同结构涂层的灵活性和优良的复杂结构工件适用性等。基于此,简单介绍了不同表征手段的测试原理与过程,重点总结了近年来应用于PS-PVD涂层表征手段的研究进展。包括组织结构相的表征手段:X射线衍射、扫描电子显微镜、透射电子显微镜、电子背散射衍射、工业用计算机断层成像技术等;热防护性能的表征手段:隔热性能、静态/循环氧化、抗热震性能、热腐蚀性能测试等;力学性能的表征手段:拉伸性能实验、三点/四点弯曲实验、粒子冲蚀性实验以及纳米压痕实验等;并详细介绍了交流阻抗谱这一电化学无损检测手段。最后,对不同表征手段的准确性、便捷性、客观性等特性进行了总结,根据不同技术的优缺点提出了相应的评价与建议。展望了未来PS-PVD表征手段的三个需要发展的方向:便捷的非接触式诊断方法、高精度的涂层微区结构与性能表征和高效的仿真模拟技术应用。