试样倾斜和正压力对球形压头微米划痕测试紫铜的影响

采用球形压头对紫铜进行微米划痕试验,研究了不同恒定正压力下试样倾斜对划痕测试紫铜的影响. 结果表明:试验测得的名义摩擦系数与试样倾斜角度线性相关,且斜率不受正压力的影响. 建立了球形压头与试样倾斜状态的位置关系模型,发现试样倾斜对名义摩擦系数中的黏着组分无影响,而犁耕组分随倾斜角度线性变化. 试样倾斜时的摩擦系数可通过球-面接触力学模型进行校正,获得试样无倾斜时的摩擦系数. 摩擦系数与正压力之间呈非线性关系,其中犁耕组分线性增大,黏着组分非线性增大并趋于稳定. 通过划痕形貌测量的残余划痕宽度几乎不受试样倾斜的影响,但与正压力的0.5次方存在线性关系. 分析了划痕硬度随正压力的变化,发现由残余划痕宽度计算得到的划痕硬度几乎为恒定值,不受正压力影响,而由接触投影面积计算得到的划痕硬度随正压力增大而增大,随后趋于稳定.      

利用球形压头研究聚碳酸酯的微米划痕性能

采用100Cr6球形压头系统研究了法向载荷、划痕速度和划痕次数对聚碳酸酯划痕性能的影响. 结果表明:在单次和第15次划痕中,法向载荷增加时,摩擦系数和压入深度增加;划痕速度增加时,摩擦系数先增加后减小,压入深度减小;法向载荷增加或划痕速度减小时,残余深度增加,弹性恢复率减小. 对于多程单向滑动磨损,随着划痕次数增加,划痕宽度线性增加,划痕硬度线性减小;摩擦系数、压入深度和残余深度均呈增加趋势,但增长的速率逐渐降低,一定次数后达到稳定;压入深度和残余深度达到稳定时的划痕次数随法向载荷的增加而减小. 第15次划痕中,随着法向载荷增加,划痕宽度增加,残余划痕硬度和几何划痕硬度变化趋势相反;划痕宽度随划痕速度的增加而减小,划痕硬度随划痕速度的增加而增加,最后均趋于稳定.      

用自动划痕仪测定硬质镀层的结合强度

作者利用自制的自动划痕试验仪对通过等离子增强化学气相沉积法(PECVD)制备的TiN和TiC镀层进行了划痕试验,考察了底材硬度、镀层厚度、镀层硬度、加载速率、划痕速度及压头(亦称划针)、与镀层间的摩擦状况等因素对表征镀层结合强度的临界载荷的影响。结果表明,临界载荷随底材硬度及镀层硬度的增加而增大,镀层厚度存在一个最佳值,在此值下的临界载荷最大,当加载速率高于100N/min时,临界载荷基本不再受其影响,临界载荷与划痕速度无关,镀层上覆盖固体润滑剂可使临界载荷增大,而覆盖纯油对临界载荷没有影响。     

凸结构的形成-低载下单晶硅表面的划痕损伤研究

利用纳米划痕仪及曲率半径为3μm的球形金刚石针尖,在单晶硅(100)表面进行了不同载荷下的划痕试验.结果表明:随载荷的增加,单晶硅表面的划痕损伤先后经历了从凸起形成、凸起与凹槽并存到材料去除的变化过程.当载荷为0.5～3.0mN时,单晶硅上的划痕损伤表现为凸结构的形成,且凸起的高度和体积随载荷的增加而增大;当载荷为3～50mN时,凸起和凹槽同时出现,但损伤区域体积未见减少,损伤仍以凸结构形成为主导;当载荷大于50mN时,凹槽深度和磨损体积明显增大,划痕损伤表现为典型的材料去除.进一步的分析显示,单晶硅的划痕损伤特征与其接触区的应力状态密切相关,低载下的摩擦剪切是凸结构产生的主要原因.     

用于ICF的三种典型光学玻璃的AFM纳米划痕行为研究

本文作者通过原子力显微镜,以球形金刚石针尖作为对摩副,在大气环境下对磷酸盐激光玻璃、K9光学玻璃、熔融石英玻璃三种用于ICF系统的典型光学玻璃的纳米划痕行为进行了定量研究.结果表明:随着载荷的增加,三种玻璃的摩擦系数均表现为先保持恒定再剧烈上升的变化趋势.这是由于随着载荷的增加,摩擦机理由界面摩擦主导逐步转变为犁沟摩擦和界面摩擦共同主导所致.在相同的法向载荷作用下,磷酸盐玻璃的摩擦系数最高,K9光学玻璃次之,熔融石英玻璃的摩擦系数最小,这与三种玻璃的机械性能以及它们的表面亲水性密切相关.在相同的载荷下,磷酸盐玻璃和K9玻璃的划痕损伤表现为材料凹陷和堆积并存,而熔融石英玻璃的划痕损伤仅表现为划痕区域明显的凹陷变形.在所有载荷下,熔融石英玻璃的划痕残余深度均略高于磷酸盐玻璃;K9玻璃在低载时的划痕深度在三种玻璃中最大,中载时居中,而高载时最小.不同载荷下玻璃表层的机械性能、塑性流动方式以及其致密化程度都将对最终的划痕残余深度产生影响.     

线接触状态下激光加工微造型表面摩擦特性研究

为了研究表面形貌对线接触弹流状态下摩擦副摩擦特性的影响,采用激光微造型技术,通过控制微造型的形状、深度、间距和面积占有率等参数制造了两组表面高度算术平均值Sa分别相同的试件.使用Talysurf CCI Lite非接触式三维光学轮廓仪对表面进行测量,采用ISO25178参数及连通性系数Us对测量表面进行表征,最后在JPM-1型双盘摩擦磨损试验机上对试件进行富油摩擦试验,得到不同转速、不同载荷工况下的摩擦系数.结果表明:线接触状态下,微造型形状、方向及深度均会对表面的摩擦特性产生影响,顺向箭头微造型表面表现出了最优的摩擦特性;试件摩擦系数随着转速的增大呈线性增长趋势,随着载荷的增大而减小,减小的幅度随载荷的增大逐渐变得缓慢;表面形貌三维表征参数Ssk、Sku、Vvv、Vvc、Us与摩擦特性均有一定的关联性.     

纳米压痕试验中压头尺寸效应的准连续介质法分析

采用准连续介质法模拟了单晶铝纳米压痕试验过程,分析了不同宽度的刚性矩形压头所引起的初始塑性变形特点,获得了载荷-压深、应变能-位移和硬度-压深曲线.从位错理论的角度分析了压头尺寸对纳米压痕测试结果的影响.研究发现:随着压头宽度的不断增大,压头下方位错形核所需要的载荷和压深程度增大,需要的应变能增加,应变能的变化速率递增,纳米硬度值减小,呈现出明显的尺寸效应.同时表明在一定的压入深度下,硬度与压头尺寸之间存在着一定的比例关系,不同尺寸压头获得的硬度值可以相互换算,但当矩形刚性压头宽度大于或等于120时这种尺寸效应消失.研究结果为纳米压痕实验过程中压头尺寸的选择提供了参考依据.     

基于机器学习的金属材料划痕响应预测

确定材料的划痕响应对评价其抗划擦及摩擦磨损性能有着重要的价值.采用有限元仿真与经典多输出多层感知器(MLP)神经网络的方法，建立了划痕输入参量(材料的屈服应力、应变硬化指数和界面摩擦系数以及划痕过程中施加的法向加载力)与划痕响应(表观深度、划痕宽度以及划痕切向力)之间的关系.由有限元结果与机器学习预测结果的对比可知：采用960组金属材料划痕仿真数据集训练的MLP神经网络预测结果与有限元仿真结果吻合较好.采用304不锈钢、黄铜和18CrNiMo7-6合金钢的划痕试验对MLP神经网络进行了试验验证.结果表明：MLP神经网络预测的划痕响应与试验中获得的结果较为接近.本文中结果可为评价材料抗划擦性能提供了1种可行的方法.     

利用圆锥压头微米划痕测试材料断裂韧性

利用Rockwell C金刚石压头对铜、聚碳酸酯、熔融石英和钠钙硅玻璃4种材料进行了微米划痕测试，这是国内首次利用划痕方法测量材料的断裂韧性. 研究表明:在划痕深度较浅时，必须要考虑压头顶端的球体区域，考虑和未考虑压头顶端球体区域时的压头形函数差别较大；考虑压头顶端的球体区域后，采用线弹性断裂力学(LEFM)模型评估断裂韧性的结果和能量尺寸效应(SEL)模型的计算结果接近，与单边切口梁(SENB)法、山形切口梁(CNB)法或压痕断裂(IF)法评估的断裂韧性相吻合，而未考虑压头顶端球体区域时计算断裂韧性的结果偏大；采用SEL模型评估断裂韧性时，按照投影接触面积等效的压头半顶角评估的断裂韧性相对按照周长等效的压头半顶角评估的断裂韧性误差更小，与LEFM模型的评估结果更接近.      

聚甲基丙烯酸甲酯材料(PMMA)纳米压/划痕变形热恢复的实验研究

聚甲基丙烯酸甲酯(PMMA)是常见的光伏电池封装材料.本文采用锥形和球形两种压头,利用纳米压痕仪开展了PMMA的纳米压痕和划痕实验.基于表面形貌扫描得到的PMMA材料在不同温度热处理后压痕与划痕变形随时间恢复的演化规律,分析了时间、温度和压头形状等对PMMA材料压/划痕变形恢复过程的影响机理.结果表明,压/划痕变形恢复...     

Novel Experimental Method to Determine the Cutting Effectiveness of Grinding Grits

This work presents a novel experimental apparatus to determine the cutting effectiveness of grinding grits. The apparatus consists of a custom high-speed scratch tester, a force measurement system, and an offline 3D optical profilometer. Preliminary results based on a spherical tool are presented to demonstrate the usefulness of the system. Experiments were performed at depths of cut ranging from 0.3 μm to 7.5 μm at cutting speeds of 5 m/s to 30 m/s in 5 m/s increments. High resolution scans of the scratch profiles provided insight into the change in the cutting mechanics as the depth of cut and cutting speed were increased. In general, lower cutting speeds produced higher pile-up heights while higher cutting speeds produced lower pile-up heights. The force measurements indicated that the normal forces increased with cutting speed due to strain rate hardening of the workpiece material while the tangential forces decreased with cutting speed due to a reduction in the coefficient of friction and a change in the cutting mechanics. The force ratio data and the specific energy data both demonstrated high slopes at low depths of cut due to asperity contact between the tool and the workpiece. The modular nature of the developed system allows different grit geometries to be investigated.     

Influence of Surface Scratches on the Flexure Strength of Soda-Lime Silicate and Borosilicate Glass

Borosilicate (BS) and soda-lime silicate (SLS) glasses are being considered for use as the strike-face in transparent armor systems. This effort examined the effect of surface scratches generated with 1, 5, 10, and 30 N scratching loads on the equibiaxial flexure strength of both glasses including versions of each that had been thermally-tempered. At lower scratch loads there is more lateral cracking associated with the scratches in the BS while the scratches in the SLS predominantly exhibit plastic deformation. At the highest scratch load the damage is dominated by chips and lateral cracking within and along the scratch length. The tempered versions of the SLS appear to have less lateral cracking than the tempered version of the BS glass. In all instances the flexure strength decreases significantly when a 1 N scratch is introduced on the tin surface but when the scratch load increases any additional strength loss is minimal.     

The frictional sliding response of elasto-plastic materials in contact with a conical indenter

Over the past decade, many computational studies have explored the mechanics of normal indentation. Quantitative relationships have been well established between the load–displacement hysteresis response and material properties. By contrast, very few studies have investigated broad quantitative aspects of the effects of material properties, especially plastic deformation characteristics, on the frictional sliding response of metals and alloys. The response to instrumented, depth-sensing frictional sliding, hereafter referred to as a scratch test, could potentially be used for material characterization. In addition, it could reproduce a basic tribological event, such as asperity contact and deformation, at different length scales for the multi-scale modeling of wear processes. For these reasons, a comprehensive study was undertaken to investigate the effect of elasto-plastic properties, such as flow strength and strain hardening, on the response to steady-state frictional sliding. Dimensional analysis was used to define scaling variables and universal functions. The dependence of these functions on material properties was assessed through a detailed parametric study using the finite element method. The strain hardening exponent was found to have a greater influence on the scratch hardness and the pile-up height during frictional sliding than observed in frictionless normal indentation. When normalized by the penetration depth, the pile-up height can be up to three times larger in frictional sliding than in normal indentation. Furthermore, in contrast to normal indentation, sink-in is not observed during frictional sliding over the wide range of material properties examined. Finally, friction between indenter and indented material was introduced in the finite element model, and quantitative relationships were also established for the limited effects of plastic strain hardening and yield strength on the overall friction coefficient. Aspects of the predictions of computational simulations were compared with experiments on carefully selected metallic systems in which the plastic properties were systematically controlled. The level of accuracy of the predicted frictional response is also assessed by recourse to the finite element method and by comparison with experiment.     

Mechanics of indentation of plastically graded materials—I: Analysis

The introduction of controlled gradients in plastic properties is known to influence the resistance to damage and cracking at contact surfaces in many tribological applications. In order to assess potentially beneficial effects of plastic property gradients in tribological applications, it is essential first to develop a comprehensive and quantitative understanding of the effects of yield strength and strain hardening exponent on contact deformation under the most fundamental contact condition: normal indentation. To date, however, systematic and quantitative studies of plasticity gradient effects on indentation response have not been completed. A comprehensive parametric study of the mechanics of normal indentation of plastically graded materials was therefore undertaken in this work by recourse to finite element method (FEM) computations. On the basis of a large number of computational simulations, a general methodology for assessing instrumented indentation response of plastically graded materials is formulated so that quantitative interpretations of depth-sensing indentation experiments could be performed. The specific case of linear variation in yield strength with depth below the indented surface is explored in detail. Universal dimensionless functions are extracted from FEM simulations so as to predict the indentation load versus depth of penetration curves for a wide variety of plastically graded engineering metals and alloys for interpretation of, and comparisons with, experimental results. Furthermore, the effect of plasticity gradient on the residual indentation pile-up profile is systematically studied. The computations reveal that pile-up of the graded alloy around the indenter, for indentation with increasing yield strength beneath the surface, is noticeably higher than that for the two homogeneous reference materials that constitute the bounding conditions for the graded material. Pile-up is also found to be an increasing function of yield strength gradient and a decreasing function of frictional coefficient. The stress and plastic strain distributions under the indenter tip with and without plasticity gradient are also examined to rationalize the predicted trends. In Part II of this paper, we compare the predictions of depth-sensing indentation and pile-up response with experiments on a specially made, graded model Ni-W alloy with controlled gradients in nanocrystalline grain size.     

Mechanics and mechanism of cut resistance of protective materials

A sliding sharp edge penetrating material is one of the most dangerous cases of cutting because it requires the smallest applied load. A better understanding of the cutting mechanism is a fundamental step to develop new and more performing protective materials. This study aims at analyzing cutting mechanics and mechanism in the presence of friction. The International Standard ISO 13997 cut test method consists in measuring the distance that a straight blade slides horizontally to cut through a material under a constant applied normal force and was used to investigate cutting phenomena.In practice, cut resistance of a material is contributed by the intrinsic strength of material and the frictional distribution. Two types of friction distributions are involved in cutting: a macroscopic friction induced by the gripping of the material and by the applied normal load on the two sides of the blade; and the other the sliding friction associated with cut through of the material that occurs along the face of the blade tip. For most materials, frictional forces due to lateral gripping could be several times greater than the friction due to the applied normal force. Thus, the cutting energy required for breaking molecular chains is much smaller than the energy dissipated for friction. The elastic modulus, the structure of the material as well as the sliding velocity have significant influence on the friction. Therefore all these properties can affect the cutting resistance results.     

角蛋白组织摩擦学性能及其损伤自修复的初步研究

采用纳米压痕/划痕仪和维氏硬度仪,初步研究了2种不同角蛋白材料-指甲和鸡爪的摩擦学性能及其损伤自修复特性.结果表明:指甲和鸡爪微观结构的不同导致其摩擦学性能和自修复能力有较大差异.由于具有更好的纤维取向性,指甲表面的划痕摩擦系数略低于鸡爪.指甲横截面上垂直于纤维方向的划痕摩擦力比平行于纤维方向的大,划痕宽度小;鸡爪内外层结构的不同使得其外层的摩擦力较大,划痕宽度较小.浸泡在水中,指甲和鸡爪上的压痕变形和低载下的划痕变形能够分别在5和30 min内完全恢复.     

Mechanics of mixed-mode ductile material removal with a conical tool and the size dependence of the specific energy

The mechanics of material removal during a single-grit rotating scratch has been investigated both analytically and experimentally. The models for cutting, plowing and mixed modes of material removal are analyzed based on the pressure and the frictional resistance. The mixed-mode model takes into account the contribution of built-up edge (BUE) ahead of the tool. To validate the model, single-grit rotating scratch experiments were conducted with a conical diamond tool on pure titanium. It was noticed that the adhesion between the tool and the deformed material, and the hardening properties of material play active roles in the scratching process and provide a driving force to the formation of the BUE. The overall frictional coefficient was found to oscillate strongly on both ends of the scratch but increases steadily over the central span of the scratch length. It is shown that the mixed-mode model captures the salient features of material removal and the size dependence of specific energy during the formation of a rotating scratch. The size dependence of specific energy may be attributed to the size effect of the yield pressure in titanium.