Reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal

In order to propel the application of the developed CuNi-Xwt%Ti active filler metal in AlN brazing and get the universal reactive wetting mechanism between liquid metal and solid ceramic, the reactive wetting behavior and mechanism of AlN ceramic by CuNi-Xwt%Ti active filler metal were investigated. The results indicate that, with the increasing Ti content, surface tension for liquid CuNi-Xwt%Ti filler metal increases at low-temperature interval, but very similar at high-temperature interval, which influence the wetting behavior on AlN ceramic obviously. CuNi/AlN is the typical non-reactive wetting system, the wetting process including rapid wetting stage and stable stage. The wettability is depended on surface tension of the liquid CuNi filler metal completely. However, the wetting process of CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system is composed by three stages, which are rapid wetting stage decided by surface tension, slow wetting stage caused by interfacial reaction and stable stage. For CuNi-8wt.%Ti/AlN and CuNi-16 wt%Ti/AlN reactive wetting system, although the surface tension of liquid filler metal is the only factor to influence the instant wetting angle θ₀ at rapid wetting stage, the reduced free energy caused by interfacial reaction at slow wetting stage plays the decisive role in influencing the final wettability.     

Synthesis and characterization of Ce/SiOC nanocomposites through the polymer derived ceramic method and evaluation of their catalytic activity

Cerium oxide and silicon oxycarbide (Ce/SiOC) porous nanocomposites have been synthesized through the polymer derived ceramic route. In the synthesis of the preceramic precursors, the addition of urea facilitates the deposition of Cerium atoms on the surface of SiO₂ nanoparticles since it prevents the SiO₂ from agglomeration. Both Ce and urea affects the structural and textural parameters of the obtained ceramics. Less crosslinked structures are formed when the urea concentration increases and it also provokes a reduction of the carbon crystallite size. Cerium, on the other hand, induces an increase of the carbon size as well as the number of SiOC units. Pore anisotropy and smoothness of the surface are also dependent on the composition of the material. As expected, the better thermocatalytic behavior against CO₂ decomposition is found at the largest Ce amounts but also, smooth surfaces and low pore anisotropies favor the accessibility of the gases to the thermocatalytic centers.     

Sn-containing Si3N4-based composites for adaptive excellent friction and wear in a wide temperature range

Ceramic design based on reducing friction and wear-related failures in moving mechanical systems has gained tremendous attention due to increased demands for durability, reliability and energy conservation. However, only few materials can meet these requirements at high temperatures. Here, we designed and prepared a Sn-containing Si₃N₄-based composite, which displayed excellent tribological properties at high temperatures. The results showed that the friction coefficient and wear rate of the composites were reduced to 0.27 and 4.88 × 10^?6 mm³ N^?1 m^?1 in air at 800 °C. The wear mechanism of the sliding pairs at different temperatures was revealed via detailed analyses of the worn surfaces. In addition, the tribo-driven graphitization was detected on the wear surfaces and in the wear debris, and the carbon phase was identified by SEM, TEM, and Raman spectrum.     

Stereolithographical fabrication of dense Si3N4 ceramics by slurry optimization and pressure sintering

Photocurable gray-colored Si₃N₄ ceramic slurry with high solid loading, suitable viscosity and high curing depth is critical to fabricate dense ceramic parts with complex shape and high surface precision by stereolithography technology. In the present study, Si₃N₄ ceramic slurry with suitable viscosity, high solid loading (45 vol %) and curing depth of 50 μm was prepared successfully when surface modifier KH560 (1 wt%) and dispersant Darvan (1 wt%) were used. The slurry exhibits the shear thinning behavior. Based on the Beer-Lambert formula, D_p (the attenuation length) and E_c (the critical energy dose) of Si₃N₄ ceramic slurry with solid loading of 45 vol % were derived as 0.032 mm and 0.177 mJ/mm², respectively. Si₃N₄ ceramic green parts with complex shape and high surface precision were successfully fabricated by stereolithography technology. After optimizing the debinding and sintering process for green parts, dense Si₃N₄ ceramics with 3.28 g/cm³ sintering density were fabricated. The microhardness and fracture toughness of as-sintered Si₃N₄ ceramics are ~14.63 GPa and ~5.82 MPa m^1/2, respectively, which are comparable to those of the samples by traditional dry-pressed and pressureless sintering technology. These results show that ceramic stereolithography technology could be promising to fabricate high performance ceramics, especially for gray-colored monolithic Si₃N₄ ceramics.     

Characterization of oxidation film on SiC ceramic substrate based on indentation method

An indentation method is proposed to characterize the properties of oxidation film on a SiC ceramic substrate. In this method, a series of indentation tests on the oxidation film with different maximum indentation depths were performed. The relationship between the inverse of contact depth and the inverse of reduced modulus was fitted by an exponential function. The moduli of the oxidation film and substrate as well as the thickness of the oxidation film were estimated by analyzing the fitting parameters. In order to validate the method, indentation tests were conducted on SiC substrate to determine the reference modulus of the substrate. Microstructure observation was conducted to measure the reference thickness of the oxidation film. The estimated values agreed well with the reference values. Finite element analysis was also employed to simulate the indentation tests on the oxidation film. The simulation results agreed well with the experimental results.     

Microstructure and shear strength of ZrB2SiC/Ti6Al4V joint by TiCuZrNi with Cu foam

In this paper, brazing behaviors between ZrB₂SiC and Ti6Al4V by Cu foam interlayer were studied. The microstructure, formation mechanism, mechanical property and fracture surface of the joints were systematically studied. The results showed that the phases in the joints were α+β-Ti, TiCu, Ti₂Cu, Cu(s, s), TiC, TiB₂ and Ti₃SiC₂. An optimum shear strength reached up to 435??MPa?at a brazing temperature of 910?°C and holding time of 20?min. Such a shear strength was 90?MPa higher than the one without the Cu foam. The obtained high shear strength of joint was discussed from microstructure and residual stress. With the increase of brazing time, Cu(s,s) gradually disappeared and the content of Ti₂Cu intermetallic compound increased, which was harmful for the joint. Furthermore, the residual stress of joint with Cu foam was calculated to be 324?MPa, lower than the one without Cu foam interlayer.     

金属掺杂对高强化柴油机活塞销类金刚石薄膜涂层摩擦学性能的影响

柴油机关键部件活塞销受到周期性的交变机械载荷和热载荷作用,易因粘着导致磨损。为提高柴油机活塞销的摩擦磨损性能,采用磁控溅射方法在活塞销表面沉积了Cr、W、Cr与Ti 3种金属掺杂的类金刚石薄膜(DLC)涂层,运用扫描电镜、X射线衍射仪、能谱仪分析DLC涂层表面微观形貌和结构,并进行摩擦磨损性能和膜-基结合强度测试。结果表明：3种涂层表面微观形貌类似,致密、无明显缺陷和孔洞,但都附着少量的颗粒物,相较而言掺杂Cr的DLC涂层表面更加平滑;掺杂Cr的DLC涂层,Cr以单质和CrC化合物形式存在,具有更高的膜-基结合强度、更优的摩擦学性能;掺杂Cr与Ti的DLC涂层性能与掺杂Cr的DLC涂层类似,Cr、Ti以单质和CrC、TiC化合物的形式存在;掺杂W的DLC涂层,由于生成了WC化合物,涂层脆性高,韧性较差。     

基于方向可操作度的机器人灵巧手抓持优化研究

以三指灵巧手的速度可操作性椭球和力可操作性椭球研究为基础,定义了机构的速度方向可操作度和力方向可操作度。在给定灵巧手操作位姿的条件下,以方向可操作度为目标函数,给出了灵巧手最佳传速方向和传力方向的优化方法。最后,以三指灵巧手为算例进行演示,得出了灵巧手在给定位姿下的最佳传速方向和传力方向,为灵巧手的抓持规划提供了值得借鉴的方法。     

The novel preparation method of thermochromic VO2 films with a low phase transition temperature by thermal oxidation of V–Mo cosputtered alloy films

Vanadium dioxide (VO₂) has been studied extensively for its unique insulator-metal transition characteristics and potential applications in thermochromic smart windows, switching devices, and infrared detectors. However, how to balance the metal-insulator transition temperature, luminous transmittance (T_lum) and solar modulation ability (ΔT_sol) of VO₂ thin films remains a challenge. In this work, high-quality thermochromic VO₂ thin films were prepared by a two-step method of magnetron sputtering and thermal oxidation annealing. Metallic and alloyed V–Mo layers were first deposited by direct-current reactive magnetron sputtering, and then a thermal oxidation annealing process was used to obtain pure and Mo-doped VO₂ thin films. The Mo content in the films was regulated by changing the sputtering power of the vanadium target, and the effect of Mo doping on the crystallinity, microstructure, phase transition temperature and optical properties of VO₂ thin films was studied. The shift of the VO₂(011) peak to a lower 2θ angle in the XRD patterns showed that Mo was successfully diffused into vanadium dioxide films. The phase transition temperatures were decreased continuously from 57.4 to 32.7 °C by decreasing the sputtering power of vanadium. The thinner Mo-doped VO₂ thin films showed higher luminous transmittance and lower transition temperature. Our results were shown to be an innovative preparation method to fabricate thermochromic VO₂ films with a low phase transition temperature, balanced luminous transmittance and solar modulation ability by thermal oxidation of V–Mo cosputtered alloy films.