表面微织构涂层-基体系统重载弹流润滑性能分析

目的研究表面涂层与织构化协同作用时摩擦副的重载弹流润滑性能,为重载传动的摩擦学设计提供参考。方法基于广义Reynolds方程、线弹性方程以及载荷平衡方程,建立表面微织构涂层-基体系统的弹流润滑模型,并无量纲化,然后运用Full-system有限元法编程求解,探讨涂层的弹性模量以及三角形织构深度、宽度、密度对系统弹流响应的影响。结果载荷一定时,薄膜涂层(2μm)的弹性模量变化(50～500 GPa)对油膜压力整体分布影响较小,但二次压力峰在硬质涂层上更为显著。在涂层与基体存在弹性模量差时,其上由微织构引起的集中应力是无涂层的2～3倍。最小油膜厚度随着涂层弹性模量的增大而增大。随着织构深度的增大(0～5μm),油膜压力和厚度波动更加明显,最小油膜厚度随之减小,系统最大等效应力也显著增大。当织构宽度增大(10～20μm)时,油膜压力和厚度波动减弱,最小油膜厚度先减小后增大。如果织构密度增大(0.5～2),油膜压力波动更为剧烈,油膜厚度波动变化不大,但其波动周期变化明显,最小油膜厚度先减小后增大。膜基界面最大剪应力出现在二次压力峰附近,织构化表面油膜压力波动越大,膜基界面剪应力波动也越大。结论存在一个最优的织构深度、宽度和密度,使得镀膜齿轮的承载能力最佳。合理的涂层选配和微织构设计,可以有效地提高齿轮的摩擦学性能,提前预防膜基系统失效。     

含颗粒缺陷涂层材料线接触力学性能数值分析

目的建立线载荷作用下含颗粒缺陷涂层材料的数值分析模型,探究颗粒缺陷对涂层材料接触力学性能的影响,以期为工程实际中机械传动部件表面的涂层工艺优化设计提供理论指导。方法利用半解析法,构建考虑颗粒缺陷的含涂层非均质材料线接触模型,所得计算结果与有限元法结果吻合良好。基于该模型,研究了摩擦系数、颗粒缺陷的位置及分布对含颗粒缺陷涂层材料最大von Mises应力及其深度位置的影响。结果随着摩擦系数的增大,最大von Mises应力值逐渐增大,其深度位置从涂层内部阶跃至近表面。随着颗粒缺陷中心x坐标从左到右变化,最大von Mises应力值先增大后减小,其深度位置位于涂层与基体界面附近或颗粒缺陷近表面端的上端。随着颗粒缺陷与涂层表面距离的增大,最大von Mises应力值先增大,后减小,并逐渐趋于稳定,其深度位置先逐渐远离涂层表面,随后稳定在涂层内部或涂层与基体界面区域。分布颗粒缺陷对最大von Mises应力的影响较为复杂,其深度则位于涂层与基体的界面或颗粒缺陷与基体的界面附近。结论摩擦系数与颗粒缺陷的位置及分布对涂层材料线接触力学性能均能产生较大影响。     

纳米级粗糙表面接触行为的分子动力学模拟

为了研究纳米级粗糙表面接触的微观机理,以金刚石为例,采用Tersoff势,用分子动力学方法研究了具有分形粗糙表面的刚性球形探头与弹性平面的接触过程.模拟结果显示,探头的表面形貌是影响趋近阶段粘附力的重要因素,分形维数越大,粘附力越大;探头的表面形貌对载荷-位移曲线也有影响,载荷较小时,探头表面粗糙形貌显著影响载荷的增长规律,载荷较大时,表面形貌对载荷-位移曲线变化的影响不再显著,载荷与位移线性增长;探头表面的纳米级粗糙峰,显著地影响了探头与基体之间的接触行为.但这种改变,并不会影响接触过程中真实接触面积与法向载荷间的线性关系.     

复合金属基/陶瓷涂层的应力分析

为了分析弯矩作用下金属基/陶瓷涂层应力分布情况,利用有限元分析软件建立计算模型,并计算不同涂层厚度和不同涂层弹性模量比条件下的拉应力和剪应力分布.计算结果表明:弯曲中心部位涂层的失效主要是由于拉应力所造成的,且拉应力值随涂层弹性模量的增加而增大,随涂层厚度的增加而减小;两端部位涂层的失效主要是由于剪应力所造成的,且剪应力值随着涂层弹性模量和涂层厚度的增加而增大;中间过渡涂层的存在可有力地减缓涂层到基体的应力变化梯度.     

基体表面形貌对Ni/Fe热喷涂及结合强度影响的原子模拟

通过分子动力学方法模拟了基体表面形貌对热喷涂的影响。研究了柱状粗糙表面和光滑表面对团簇展平、基体缺陷演化、应力分布、涂层与基体结合强度的影响。结果表明,基体表面形貌对热喷涂结合强度影响显著,粗糙表面不仅增加了团簇与基体的实际接触面积,提高了附着力,而且在界面结合处形成锚固效应,从而提高了界面结合强度。同时,基体表面形貌改变了界面区域的应力分布,柱状粗糙表面可以减小应力集中效应,降低临界应力,减轻对基体的损伤。此外,粗糙表面会阻碍团簇的滑移,减小了展平比。     

考虑表面形貌的接触应力分析

目的 研究规则和实际表面形貌对二维平面线接触模型的影响.方法 基于线接触几何特性,考虑表面受载后的弹性变形特性,结合规则和实际粗糙表面的形貌特征,并考虑磨合对表面的影响,采用移动平均滤波方法对实际粗糙表面进行光滑处理,利用共轭梯度法,求解表面接触压力和摩擦力,计算二维平面内接触近场应力分布,同时采用修正的离散卷积快速傅里叶变换方法提高计算效率.对比验证二维接触模型的准确性,并对比规则和实际粗糙形貌表面受法向载荷和切向载荷时平面应力分布云图中的应力分布和大小.结果 圆面与圆形微凸体接触时,圆形凸体半径越小,最大Mises应力值越大,半径增大,最大Mises应力减小.多个圆形微凸体及正弦微凸体对应力分布影响类似.当考虑切向力时,会对应力分布云图中的应力分布形状和大小产生巨大影响.在相同平面表面形貌时,摩擦系数越大,Mises应力越大,并且应力沿摩擦力方向发生偏移.结论 数值模型能准确计算出粗糙平面的应力.当圆面与规则表面轮廓平面接触时,与光滑表面接触时相比,Mises应力分布呈现很大不同,微凸体越小,应力越集中;摩擦力会使接触压力和近场Mises应力产生偏移,摩擦系数越大,偏移越明显.当考虑实际粗糙表面时,在粗糙界面尖端接触区域产生应力集中.经过磨合表面采用平均滤波光滑处理后,粗糙界面尖端接触区域应力集中将大幅减小.     

界面粗糙度对双层热障涂层残余应力影响的数值模拟

运用ANSYS有限元分析软件模拟了双层热障涂层（ZrO2/NiCrAIY）在制备过程中的温度场，在此基础上，进行热-应力耦合计算分析试样的残余应力，重点计算了不同基体表面粗糙度条件下的残余应力分布。结果表明：试样的温度分布呈不均匀变化，与实际喷涂过程相近；双层热障涂层的最大残余应力集中在粗糙界面的凸起与凹坑部分；涂层内的等效残余应力随表面粗糙度的增大整体呈增大趋势，在Rα=30μm附近时，等效残余应力较小，同时压应力较小。     

涂层界面性能测试的类悬臂梁模型

提出了涂层界面性能测试的类悬臂梁模型,并以有限元方法分析了涂层/基体界面处的应力及加载点位置、支撑点位置、基体厚度对界面应力状态的影响.结果表明:加载点位置和支撑点位置对界面应力状态影响显著,基体厚度变化对界面应力状态的影响没有明显的规律性.因而,有可能通过改变加载点位置和支撑点位置设计涂层,基体界面的断裂模式.同时对涂层的约束模式,即涂层表面完全约束和部分约束对涂层应力状态的影响进行了分析.分析表明将涂层完全约束有利于抑制涂层断裂.     

扭转载荷作用下熔接涂层与基体结合界面处应力特性仿真研究

目的通过有限元仿真获得熔接涂层与基体结合界面处的应力分布特性,得到熔接涂层与基体的力学性能匹配性。方法应用ABAQUS有限元软件,将模型沿轴向和径向剖切,讨论不同扭矩载荷下镍基合金涂层和基体轴结合界面的应力分布规律,分析涂层厚度对结合界面处应力的影响。结果从模型剖切后采样结点的应力变化曲线看,基体和涂层的内部应力变化均匀,在基体和涂层结合处应力存在突变,涂层上的应力大于基体上的应力。涂层厚度从0.5 mm增大到1.0 mm时最大应力减小量约为18 MPa,从1.0 mm增大到2.5 mm时最大应力的减小量约为1 MPa,涂层厚度大于1.0 mm后最大应力减小量变化不明显。结合面两侧涂层和基体应力差值的最大值Δσ_(max)随着涂层厚度的增大有所减小,在涂层厚度小于1.0 mm时,变化较为明显。结合面底面处的应力突变随涂层厚度的增加而略有减小。结论在涂层和基体结合界面处存在应力突变,最大应力出现在涂层外表面的中心位置,增大涂层厚度可以减小应力突变,在涂层厚度较小时效果明显。     

Q235钢基体表面微晶玻璃功能梯度涂层接触应力的数值模拟

接触应力状态是涂层材料表层破坏、整体分层剥落的重要影响因素。运用有限元软件ANSYS对Q235钢基体表面TiN/LZAS微晶玻璃梯度涂层在法向静载下的接触应力进行了分析,建立了该复合涂层的有限元分析模型,通过比较有限元数值解和Hertz理论解,验证了有限元模型的可靠性,探讨了不同层数和层厚对涂层体系接触应力分布的影响。结果表明:涂层的径向接触应力和Mises应力的最大值均位于接触区域中心处,最大剪切应力位于接触中心附近;涂层层数和厚度对涂层表面径向应力、界面剪切应力和整体Mises应力均有明显影响。模拟分析的结果可以为该复合材料的设计和制备提供一定的理论依据。     

球头压痕确定薄膜模量的等效膜厚法

针对名义成分为Ti-46Al-3.8(V,Cr,Ni) (at％)的挤压TiA1合金,研究了热处理工艺对高温时β相含量、形貌的影响,利用高温时β相对α相晶粒的钉扎作用获得了层片团尺寸均匀、细小的全层片组织,测试了具有该组织合金的拉伸性能.结果表明,在1320～1370℃范围内,随保温温度的升高,β相含量逐渐增加,使得合金的晶粒尺寸逐渐减小.实验合金经1340℃/5 min/AC热处理后可以获得层片团(尺寸平均为65 μm),B2相弥散分布的全层片组织.该组织的室温及高温拉伸性能均较好.少量B2相对合金的室温塑性无不利影响,对合金的高温强度可能有一定贡献.     

纳米压痕技术及其在薄膜/涂层体系中的应用

综述了纳米压痕技术的发展历程及其在薄膜领域的应用。介绍了当前实验室条件下主要采用的电磁驱动式纳米压痕仪的构造和工作过程。为了保证测试结果的准确性,要在合适的温度、湿度下进行压入实验,借助保载来消除一些可以避免的误差。阐述了压头的分类和选择原则,玻氏压头相比于维氏压头具有更小的中心线与棱面夹角,避免了尖端横刃对于压入结果准确性的影响,因此最常用的压头为玻氏压头;表征断裂韧性最合适的压头为立方角压头;表征微机电系统的弯曲采用楔形压头。总结了通过最大载荷和压入面积得到涂层力学参量的分析流程。归纳了将纳米压痕法应用于表征薄膜涂层的硬度和弹性模量、室温下蠕变性能、断裂韧性、残余应力、塑性性能等力学量的研究,如表征硬度和弹性模量的Oliver-Pharr法的应用,识别蠕变柔量的Lee-Radok模型的应用,分析断裂韧性的Lawn-Evans-Marshall模型的应用。在涂层制备过程中,制备参数的改变可以使得涂层具有不同的力学性能,涂层厚度远小于表面尺寸,硬度和弹性模量仍然存在各向异性,非晶态结构涂层具有更高的硬度和弹性模量。采用碳纳米管强化可以提高涂层的断裂韧性,涂层内存在适量的残余应力数值和合...     

双粗糙表面磨削过程微凸体曲率半径的影响分析

目的通过建立双粗糙表面磨削模型,获得微凸体曲率半径对材料磨损的影响大小。方法选取磨具上微凸体与工件上不同变化曲率的微凸体分别建立滑动磨削模型I和模型II,考虑了磨削过程中材料的弹性/塑性变形及其断裂失效,运用有限元方法分析探讨滑动过程相嵌微凸体的应变变化以及磨屑脱离情况。结果磨削滑动过程中,在同等接触干涉量δ=1.30μm条件下,接触角较小的微凸体接触对(θ_1≈19.4°)其上微凸体发生磨损断裂,而接触角较大的微凸体接触对(θ_2≈25.5°)其下微凸体发生磨损断裂。磨损微凸体最大的等效塑性应变量发生在次表层的1.5~2.0μm处。结论双粗糙表面磨削过程中,在其他影响因素相同的情况下,曲率半径较小的微凸体更易形成磨屑。磨损微凸体最大的等效塑性应变量发生在次表层的某一深度处,随着塑性变形的增大,应力三轴度减小,导致材料表层下微观裂纹的萌生形成磨屑。     

Nanoindentation on the surface of thermally sprayed coatings

The ability to quantify surface mechanical properties is valuable for assessing the quality of thermal spray coatings. This is especially important for prostheses where loading is placed directly on the surface. Hydroxyapatite was classified to small (20-40 μm), medium (40-60 μm) and large (60-80 μm) particle sizes and thermal sprayed to produce a coating from spread solidified hydroxyapatite droplets. It was revealed for the first time, that nanoindentation can be successfully used to determine the hardness and elastic modulus on the surface of well spread solidified droplets at the hydroxyapatite coating surface. Comparison with indentation results from polished cross-section exhibited comparable values and statistical variations. The hardness was 5.8 ± 0.6, 5.4 ± 0.5 and 5.0 ± 0.6 GPa on coatings produced from small, medium and large sized powder. Similarly, the elastic modulus decreased from 121 ± 7, 118 ± 7 to 114 ± 7 GPa, respectively. Use of several indentation loads gave comparable results with sintered hydroxyapatite suggesting good inter-splat bonding within the coating. MicroRaman spectroscopy and X-ray diffraction confirmed a larger degree of dehydroxylation for the smaller particles also revealing a lower elastic modulus. This shows the influence of particle size and possibly dehydroxylation of hydroxyapatite on the mechanical properties of the coating surface.     

Material and geometrical design for high reliability bilayer

Ceramic/metal, ceramic/ceramic, and ceramic/polymer bilayers are prepared to find the best design for a reliable material. The transparent bilayer enablesin situ observation of crack initiation. The elastic modulus of the substrate material in the bilayer is controlled from 2.35 GPa to 230 GPa by using different compositions of glass, metal, and polymer. The thickness of the ceramic coating layer is controlled from 120 μm to 5.6 mm. The surface of the coating material is abraded to control the strength and toughness. Classical cone cracks and transverse radial cracks are observed during Hertzian indentation. The crack initiations depend on not only material design parameters such as strength, toughness, and elastic modulus but also geometrical parameters such as coating thickness. Conditions for avoiding cracking are considered, in terms of the material and geometrical design parameters.     

Micro mechanical properties of n-Al2O3/Ni composite coating by nanoindentation

A new type of nano test system was introduced, the test principle and the indentation data analysis method were described. It was used to test the micro mechanical properties, such as hardness, elastic modulus and indentation creep property of n-Al2O3/Ni composite coating on steel prepared by brush plating, and the variety of mechanical properties with coating thickness was researched. The results show that the mechanical properties are basically identical within the whole coating, the hardness and modulus decrease in the defect fields, especially within the dendritic crystals, whereas the mechanical properties are not influenced greatly at the interspaces among dendritic crystals. The average hardness and elastic modulus of n-Al2O3/Ni coating are 6.34 GPa and 154 GPa respectively, and the hardness is 2.4 times higher than that of steel and the indentation creep curve of n-Al2O3/Ni coating is similar to that of the uniaxial compression creep, and the creep rate of steady-state is about 0. 104 nm/s. These results will supply useful data for process improvement, new type material development and application expansion.     

不同层间距对板料渐进成形影响的数值模拟与试验研究

采用有限元法对板料成形过程进行数值模拟,分析了圆锥台件渐进成形过程中不同层间距对等效应变、应力和厚度变化趋势的影响。试验表明,层间距越小,最终等效应变量越大,成形极限越高,较小的层间距有利于减少局部应力集中,数值模拟结果与试验结果基本吻合。     

Non-linear elastic stresses in a thin hard coating/an elastic substrate system subjected to a surface pressure distribution

A hard ceramic coating on elastic steel substrates has been increasingly used in the tribological applications in order to reduce wear rates and friction. In case a surface pressure is applied to the coating surface of a coating–substrate system, the stress and deformation states inside the coating and at the edges of the coating–substrate interface play important role in service life of the coating. In this study, the geometrically non-linear stress analysis of a thin hard coating–elastic substrate system subjected to a surface pressure was carried out using the incremental finite element method (IFEM) based on the small strain–large displacement theory (SSLD). The pressure distribution was considered for plane-strain and axisymmetric cases. The comparison of the stress distributions of an uncoated substrate determined analytically with the results of both the SSLD analysis and the small strain–small displacement (SSSD) analysis showed a good agreement. In addition, the stress analysis of a thin coating/substrate system was carried out based on the SSLD and SSSD theories. The stress distributions along the coating surface, coating–substrate interface and across the coating and substrate were investigated in detail. Both theories showed that the normal and shear stresses became critical in the coating and on the coating–substrate interface regions corresponding to the centre and ends of the surface pressure distribution. These stresses are a probable reason of the coating–substrate detachment encountered in practice. However, the SSSD theory can not predict accurately lower stress and strain variations arising at the edges of the coating and the coating–substrate interface since the SSSD theory neglects the effects of the large displacements whereas the SSLD theory found that the normal and shear stresses and strains at these critical locations had also non-linear variations as the applied load is increased. Increasing the coating modulus resulted in higher stresses in the coating and at the edges of the coating–substrate interface. In case of a thin coating, these critical stresses occurred in the substrate whereas they spread completely inside coating regions neighboring the coating–substrate interface for relatively thicker coatings. The SSLD analysis also predicted lower and non-linear normal, shear stress and strain variations at the critical locations in the coating and on the coating–substrate interface for different modulus ratios and coating thicknesses. Since the peak stresses arising along the coating surface and the coating–substrate interface were dependent on both the coating thickness and modulus it was not possible to determine an optimum coating thickness and modulus ratio reducing all stress components causing the coating failure. However, optimum coating thickness and modulus can be searched in a large solution space using the optimization techniques.     

Corrosion and Nano-mechanical Behaviors of Magnetron Sputtered Al-Mo Gradient Coated Steel

A gradient three-layer Al-Mo coating was deposited on steel using magnetron sputtering method. The corrosion and nano-mechanical properties of the coating were examined by electrochemical impedance spectroscopy and nano-indentation tests and compared with the conventional electroplated cadmium and IVD aluminum coatings. Electrochemical impedance spectroscopy was performed by immersing the coated specimens in 3.5% NaCl solution, and the impedance behavior was recorded as a function of immersion time. The mechanical properties (hardness and elastic modulus) were obtained from each indentation as a function of the penetration depth across the coating cross section. The adhesion resistance of the coatings was evaluated by scratch tests on the coated surface using nano-indentation method. The results show that the gradient Al-Mo coating exhibits better corrosion resistance than the other coatings in view of the better microstructure. The impedance results were modeled using appropriate electrical equivalent circuits for all the coated systems. The uniform, smooth and dense Al-Mo coating obtained by magnetron sputtering exhibits good adhesion with the steel substrate as per scratch test method. The poor corrosion resistance of the later coatings was shown to be due to the defects/cracks as well as the lesser adhesion of the coatings with steel. The hardness and elastic modulus of the Al-Mo coating are found to be high when compared to the other coatings.     

Interesting aspects of indentation and scratch methods for characterization of thermally-sprayed coatings

In order to improve the knowledge on the use and significance of instrumented indentation and scratch testing on thermally-sprayed materials, a wide range of tests was performed on thermally-sprayed ceramic, cermet and metal coatings. A scale-dependent behavior of hardness was observed as a function of indentation depth for all coatings: at low penetration depths, the hardness value depends on the intralamellar material properties, whereas at larger depths it reflects the long-range cohesive strength of the coating. In all cases, hardness becomes independent of the indentation depth above a threshold value of ~ 2000 nm. The elastic modulus is also scale-dependent, but it never stabilizes to a depth-independent value, probably on account of crack opening/closing mechanisms. Scratch test on the cross-section has been deeply investigated and identified as a comparative method to quantify the cohesion of the coatings.