硬质陶瓷涂层增韧及其评估研究进展

硬质陶瓷涂层一般硬度高,但韧性差,在受到较大外加冲击荷载作用时容易产生裂纹并引发失效,从而限制了硬质涂层在工程领域的应用。硬质涂层的增韧研究是当前硬质涂层研究的热点之一。常见硬质涂层增韧方法中,有些是以降低硬度为代价来提高涂层韧性,比如第二相增韧;有些是通过优化涂层结构设计,减小缺陷尺寸,在不损失涂层高硬度的前提下,提高韧性,获得兼具高硬度和高韧性的涂层;有些是通过相变(晶体结构转变)提高涂层的韧性。通过优化涂层结构设计,改变涂层晶粒尺寸、晶界尺寸与复杂程度等,使涂层致密化,从而提高涂层硬度与韧性,从而获得增韧效果,成为研究者越来越关注的焦点。单一增韧方法的局限性,可以通过多种增韧方法协同作用获得突破,协同增韧已成为硬质涂层增韧发展的趋势。目前硬质涂层的韧性评估还没有一个公认的方法,从简单实用出发,常用的有微悬臂梁弯曲法、划痕法和压痕法。

元素掺杂类金刚石碳膜降内应力研究综述

我国汽车行业与国际同行的差距主要在于发动机关键零部件的摩擦磨损。类金刚石(DLC)碳膜技术是解决发动机关键零部件摩擦磨损问题的有效途径,但传统的DLC碳膜在制备过程中会产生较高的内应力(可达10GPa),从而导致薄膜失效。首先分析了DLC碳膜在汽车零部件领域的应用背景,探讨了薄膜内应力的来源,重点综述了异质元素掺杂降低类金刚石碳膜内应力的研究现状。常用掺杂元素按化学性质可分为碳化物形成元素与非碳化物形成元素,不同于其他分类方法,笔者按掺杂元素的组元多少进行划分,从单元掺杂、二元或多元掺杂入手,系统分析了各种掺杂方式、掺杂元素的优缺点。在此基础上,指出元素掺杂降低类金刚石碳膜内应力的研究从最初的单一元素掺杂,逐步发展到了二元或多元元素掺杂;从碳化物形成元素掺杂、非碳化物形成元素掺杂,逐步发展到碳化物形成元素与非碳化物形成元素的多元共掺杂;所研究的类金刚石碳膜的显微结构也从最初的非晶态结构向多元、多相结构发展,以期获得良好的综合机械性能。

抗固体粒子冲蚀物理气相沉积涂层的研究进展

压气机是航空发动机核心部件之一,其叶片常用钛合金制成。但钛合金本身质地较软,抗击固体粒子冲蚀能力较弱,空气中固体粒子在高速气流作用下将对压气机前级叶片造成严重冲蚀,从结构及航空动力学上降低发动机性能。同时,也使得发动机的寿命大幅下降。为提高压气机叶片的抗固体粒子冲蚀能力,通常在叶片上沉积具有一定硬度和韧性的抗冲蚀涂层。本文综述了国内外近年来抗固体粒子冲蚀物理气相沉积涂层的研究进展。指出多组元、多层膜结构涂层在拥有较高硬度的同时,可以获得较高韧性,具有良好的综合机械性能,是抗冲蚀涂层的理想选择。

Novel dye-sensitized solar cell architecture using TiO2-coated Ag nanowires array as photoanode

With the aim of reducing series resistance and increasing dye loading,novel dye-sensitized solar cell architecture was designed with TiO2 nanoparticle-coated Ag nanowires array as the photoanode.Ag nanowire array was prepared by anodic aluminum oxide (AAO) templateassisted electrochemical deposition route.Then,Ag nanowires were coated by TiO2 nanoparticles in hydrothermal process.The structures of the photoanode were characterized by field emission scanning electron microscopy (FESEM).Ag nanowires are covered by a layer of very fine nanoparticles with a diameter of less than 5 nm.X-ray diffraction (XRD) and selected-area electron diffraction (SAED) show that Ag nanowires have a strong preferred orientation in (220) direction and the TiO2 coating layer is a polycrystalline structure.With this photoanode,3.2 % conversion efficiency is achieved for the cell sensitized with N3 dye.

Mechanical size effect of eutectic high entropy alloy:Effect of lamellar orientation

The effect of lamellar orientation on the deformation behavior of eutectic high entropy alloy at the micrometer scale,and the roles of two rarely explored laminate orientations(i.e.,the lamellar orientation at~0°and 45°angles with the loading direction)in regulating size-dependent plasticity were investigated using in-situ micropillar compression tests.The alloy,CoCrFe NiTa_(0.395),consists of alternating layers of Laves and FCC phases.It was found that the yield stress of the 0°pillars scaled inversely with the pillar diameters,in which the underlying deformation mode was observed to transform from pillar kinking or buckling to shear banding as the diameter decreased.In the case of the 450 pillars with diameters ranging from 0.4 to 3μm,there exists a’weakest’diameter of~1μm,at which both constraint effect and dislocation starvation are ineffective.Irrespective of the lamellar orientations,the strain hardening rate decreased with decreasing pillar diameter due to the diminishing dislocation accumulation that originated from the softening nature of large shear bands in the 0°pillars,and the enhanced probability of dislocation annihilation at the increased free surfaces in the 45°pillars.The findings expand and deepen the understanding of the mechanical size effect in small-scale crystalline materials and,in so doing,provide a critical dimension for the development of high-performing materials used for nanoor microelectromechanical systems.

BN/SiBCN light-leakage-proof coatings of silica optical fiber for long term sensors at high temperatures

SiBCN and BN/SiBCN light-leakage-proof coatings were prepared on silica optical fibers for sensing applications at high temperatures.Scanning electron microscopy was used to characterize the surface morphology and microstructure of the coated fiber.Mechanical and optical properties of the coated fiber were characterized by Raman,optical and tensile tests.Compared with the original fiber,the tensile strength of Si BCN and BN/Si BCN coated fiber show an increase of about 60%and 90%at room temperature,respectively.In addition,the tensile strength of BN/Si BCN coated silica optical fiber was increased by about 42%at 700°C.It has been found that the improvement of the strength of BN/Si BCN coated silica optical fiber is related to the healing of defects and residual compressive stress in fibers.From the light transmittance performance of the coated fiber,it was found that BN/Si BCN double coating could be an ideal total reflection solution to protect silica optical fiber from light leakage during high temperature sensing operation.

Size-dependent deformation behavior of dual-phase,nanostructured CrCoNi medium-entropy alloy

The mechanical size effect of nanostructured,dual-phase CrCoNi medium-entropy alloy(MEA)was investigated by combining in-situ micro-compression testing with post-mortem electron microscopy analysis.The alloy possesses a superior yield strength up to~4 GPa,primarily due to its hierarchical microstructure including column nanograins,preferred orientation,a high density of planar defects and the presence of the hexagonal close packed(HCP)phase.While the yield strength of the alloy has shown sizeindependency,the deformation behaviour was strongly dependent on the sample size.Specifically,with decreasing the pillar diameters,the dominant deformation mode changed from highly localized and catastrophic shear banding to apparently homogeneous deformation with appreciable plasticity.This transition is believed to be governed by the sizedependent critical stress required for a shear band traversing the pillar and mediated by the competition between shearinduced softening and subsequent hardening mechanisms.In addition,an unexpected phase transformation from HCP to face-centered cubic(FCC)was observed in the highly localized deformation zones,leading to strain softening that contributed to accommodating plasticity.These findings provide insights into the criticality of sample dimensions in influencing mechanical behaviors of nanostructured metallic materials used for nanoelectromechanical systems.

New design for highly durable infrared-reflective coatings

The fundamental challenge in designing durable infrared-reflective coatings is achieving the ideal combination of both high reflectivity and durability.Satisfying these competing demands is traditionally achieved by deposition of durable layers on highly reflective metals.We overturn the traditional logic of‘first reflectivity and then durability’and propose an alternative of‘first durability and then reflectivity’:First,a transition-metal compound is selected as a durable base;then its reflectivity is improved by incorporating silver/gold to form an alloy or by overcoating a multilayer stack.Two validation experiments prove that the new strategy works extremely well:the coatings thus obtained have infrared reflectivities close to that of aluminum,and their hardness and acid and salt corrosion resistances are 27–50,400–1500 and 7500–25000 times that of aluminum.The traditional mirror coating(e.g.,Al/SiO2 films)is more suitable for moderate environments,while our mirror coating that was obtained by the new strategy(e.g.,an Ag-doped hafnium nitride film)is more suitable for harsh environments,such as ones with dust,windblown sand,moisture,acid rain or salt fog.This work opens up new opportunities for highly durable infrared-reflective coatings and rejuvenates the study of transition metal compounds in a completely new area of optics.