Hydrogen content variation for enhancing the lubricated tribological performance of DLC coatings with ester

There are many types of DLC (diamond-like carbon) coatings, which mainly differ from each other according to their hydrogen content, sp² and sp³-bonded carbon atoms and alloying elements (such as Ti, Cr, W or Zr). The lubricated tribological performance of these coatings depends on the lubricant. Controversially, the same lubricant delivers different tribological performances with differently produced DLC coatings. It has been observed that the presence of hydrogen has a remarkable effect on the tribological performance of DLC coatings in inert and vacuum environments. In this paper, the hydrogen content of two different types of DLC coatings, a-C:H:Me (metal containing hydrogenated amorphous carbon) and a-C:H, was varied, in order to obtain an optimized tribological behavior with a synthetic ester (TMP ester). The tribological performance of the coatings with TMP ester is examined in a pin-on-disk tribometer. It could be shown that increasing the hydrogen content in a DLC coating improves their tribological performance with TMP ester. Besides, a-C:H type of coatings is found to be more suitable for TMP ester regarding low friction coefficients.     

Mechanical properties and friction−wear characteristics of VN/Ag multilayer coatings with heterogeneous and transition interfaces

The heterogeneous multilayer interface of VN/Ag coatings and transition multilayer interface of VN/Ag coatings were prepared on Inconel 781 and Si(100), and the microstructures, mechanical and tribological properties were investigated from 25 to 700 °C. The results showed that the surface roughness and average grain size of VN/Ag coatings with transition multilayer interface are obviously larger than those of VN/Ag coatings with heterogeneous multilayer interface. The coatings with transition multilayer interface have higher adhesion force and hardness than the coatings with heterogeneous multilayer interface, and both coatings can effectively restrict the initiation and propagation of microcracks. Both coatings have excellent self-adaptive lubricating properties with a decrease of friction coefficient as the temperature increases, but their wear rates reveal a drastic increase. The phase composition of the worn area of both coatings was investigated, which indicates that a smooth Ag, Magnéli phase (V₂O₅) and bimetallic oxides (Ag₃VO₄ and AgVO₃) can be responsible to the excellent lubricity of both coatings. To sum up, the coatings with transition multilayer interface have excellent adaptive lubricating properties and can properly control the diffusion rate and release rate of the lubricating phase, indicating that they have great potential in solving the problem of friction and wear of mechanical parts.     

Impact of pressure on carbon films by PECVD toward high deposition rates and high stability as metallic bipolar plate for PEMFCs

Stainless-steel bipolar plates (BPPs) are widely used in place of graphite bipolar plates in proton exchange membrane fuel cells (PEMFCs). Amorphous hydrogenated carbon (a-C:H) coatings are widely used to improve the conductivity and corrosion resistance of metal bipolar plates. However, a-C:H coatings prepared by the sputtering method cannot be applied to quantity production on account of its low deposition rate. Our paper focuses on a-C:H coatings applied at metallic BPPs for PEMFCs produced by direct-current plasma-enhanced chemical vapor deposition (DC-PECVD) with high deposition rates. The effects of adjusting deposition pressures on the structure and properties of coatings have been investigated. The results show that the a-C:H coating deposited at 8 Pa deposition pressure have high stability, with a high deposition rate of 37.5 nm/min. As the deposition pressure increased, sp²-hybridized carbon atoms increased, the larger microcrystalline carbon clusters are found, and the structure will undergo different structural transformations in a-C:H coatings. Overall, the a-C:H coatings deposited at 8 Pa are attractive to be applied in metallic bipolar plate with have high deposition rates, dense coating structure, and proper carbon structure.     

Si含量对CrSiN涂层结构和性能的影响

目的在室温条件下,采用物理气相沉积(PVD)磁控溅射沉积方法,通过控制Si靶功率制备具有不同Si含量的CrSiN涂层,以探究Si元素对涂层结构和性能的影响。方法通过X射线衍射、能谱仪测试、纳米压痕测试、维氏硬度压痕测试和摩擦磨损实验,分别评价CrSiN涂层的结构、硬度、韧性和耐磨性能,并通过扫描电子显微镜对压痕形貌进行分析。结果所有CrSiN涂层均呈(111),(200)取向的Na Cl结构。随着Si含量增加,XRD峰呈宽化趋势,晶粒细化效果明显。随Si元素的加入,CrSiN涂层硬度、模量和韧性均呈先增加后降低的趋势。相比Cr N涂层,Si的原子数分数为3.2%时,CrSiN涂层的硬度由21.4 GPa增至35.7 GPa,模量由337.7 GPa增至383.9 GPa,塑性指数由0.5增至0.55,实现了强韧一体化。加入Si元素,CrSiN涂层的耐磨损性能得到改善,且Si的原子数分数为3.2%,磨损率最低,为1.0×10~(-17)m~3/(N·m),提高了约一个数量级。结论 Si元素的加入可以有效改善CrSiN涂层的结构,提高CrSiN涂层的硬度、韧性和磨损性能,但需加入适量的Si,才可实现性能最优化。     

Enhanced thermal conductivity and retained electrical insulation for polyimide composites with SiC nanowires grown on graphene hybrid fillers

Polyimide (PI) composites containing one-dimensional SiC nanowires grown on two-dimensional graphene sheets (1D–2D SiC_NWs-GSs) hybrid fillers were successfully prepared. The PI/SiC_NWs-GSs composites synchronously exhibited high thermal conductivity and retained electrical insulation. Moreover, the heat conducting properties of PI/SiC_NWs-GSs films present well reproducibility within the temperature range from 25 to 175 °C. The maximum value of thermal conductivity of PI composite is 0.577 W/mK with 7 wt% fillers loading, increased by 138% in comparison with that of the neat PI. The 1D SiC nanowires grown on the GSs surface prevent the GSs contacting with each other in the PI matrix to retain electrical insulation of PI composites. In addition, the storage modulus and Young’s modulus of PI composites are remarkably improved in comparison with that of the neat PI.     

Impulse Excitation Technique IET as a non-destructive method for determining changes during the gelcasting process

Gelcasting is a good method for obtaining ceramic components with a pre-defined shape. From the point of view of the course of the ceramic casting process, a very important issue is the so-called idle time or gelation time whose measurement is usually based on determining changes in ceramic slurry viscosity by means of rheometers. However, searches are being continued to find methods which would have the least impact on the interactions between ceramic powder particles and the forming polymeric structure and would allow more time range to observe changes in gelcasting process compared to classic measurements of viscosity.In the article, based on measurements on Al₂O₃, ZrO₂ and SiC-based gelcasting systems with various water-soluble acrylic monomers, it has been demonstrated that such requirements may be fulfilled by IET (Impulse Excitation Technique).     

Insights into high temperature oxidation of Al2O3-forming Ti3AlC2

Insights into high temperature oxidation of Al₂O₃-forming Ti₃AlC₂ are made to understand the sub-parabolic oxidation rate and the absence of Al-depletion zone beneath the protective Al₂O₃ scale. The scale results from selective oxidation of Al that migrated from Ti₃AlC₂ according to Ti₃AlC₂ + 3/2y O₂ → Ti₃Al_1−yC₂ + 1/2y Al₂O₃, leaving Al vacancy in the substrate. Inward diffusion of oxygen via grain boundaries of the Al₂O₃ scale predominates for the scale growth. The Al₂O₃ grains grow with time, yielding sub-parabolic oxidation kinetics. The Al diffuses in a fast manner, accounting for the absence of Al-depletion zone.