基于AFM的压痕硬度自动检测算法的研究

由纳米硬度计可测得载荷-压深曲线,并由原子力显微镜可以扫描得到压痕图.在对单晶硅压痕硬度的检测试验中,用Matlab软件对获得的数据进行处理,编制了利用压痕边长法、直接面积法、Oliver-Pharr法和压痕功法等计算压痕硬度方法的程序,可以自动计算压痕的面积和体积并计算得到单晶硅的硬度值.     

Plastic deformation of copper single crystals by wedge indentations

Plastic deformations due to wedge indentation of copper single crystals with three different orientations are quantitatively assessed by analyses of deformed grid patterns on the specimens. Deformed regions, which are subdivided into six regions in terms of slip traces, are found to extend for 10–13 times the contact site size between the wedge and the specimen. The distortions near the contact site show that the deformations are generally characterized by typical features of strains and material rotation such as negative ?_x (normal strain in the indentation direction) in the deep region, positive ?_x in the shallow region, negative γ_xy(engineering shear strain) in the intermediate region and rotation contracting the material in the indentation direction. Although such distortions are affected by the feasibility of cross slip and the orientation dependence of the elasticity, they indicate that the overall material flow is similar to that in polycrystalline metals.     

磨削速度对碳化硅陶瓷磨削损伤影响机制研究

碳化硅陶瓷高速磨削过程中,磨粒对工件材料强力冲击,应变率剧增、复杂显微结构对应力波传送响应转变,材料力学行为发生变化,目前高速磨削对材料去除机制影响的物理本质认识还不清楚。为此,开展磨削速度对SiC陶瓷磨削裂纹损伤影响机制研究。通过单颗磨粒磨削SiC陶瓷试验,分析了磨削速度对SiC陶瓷磨削表面形貌、磨削亚表面裂纹损伤深度、磨削力和磨削比能的影响规律。试验结果表明,当SiC陶瓷材料以脆性方式去除时,磨削速度对裂纹损伤影响最为显著,随着磨削速度从20 m/s增加到160 m/s,磨削亚表面裂纹损伤深度从12.1μm快速降低到6μm。采用Voronoi法建立了金刚石磨削多晶SiC陶瓷有限元仿真模型,当磨粒切厚为0.3μm,磨削亚表面损伤以微裂纹为主;当磨粒切厚为1μm时,随着磨削速度增加,磨削亚表面裂纹损伤深度从14.7μm降低到4.6μm,磨削亚表面宏观沿晶裂纹逐渐变为微观裂纹。基于位错理论和冲击动力学理论,揭示了高速磨削过程中位错密度的增加和晶界反射应力波对应力场削弱作用是高速磨削SiC陶瓷裂纹损伤“趋肤效应”产生的机理。     

Evolution of residual stress and crack morphologies during 3D FIB tomographic analysis of alumina

Three‐dimensional focused ion beam (FIB) tomography is increasingly being used for 3D characterization of microstructures in the 50 nm–20 μm range. FIB tomography is a destructive, invasive process, and microstructural changes may potentially occur during the analysis process. Here residual stress and crack morphologies in single‐crystal sapphire samples have been concurrently analyzed using Cr³⁺ fluorescence spectroscopy and FIB tomography. Specifically, maps of surface residual stress have been obtained from optically polished single‐crystal alumina [surface orientation (1 ī 0 2)], from FIB milled surface trenches, from Vickers micro‐indentation sites (loads 50 g–300 g), and from Vickers micro‐indentation sites during FIB serial sectioning. The residual stress maps clearly show that FIB sputtering generates residual stress changes. For the case of the Vickers micro‐indentations, FIB sputtering causes significant changes in residual stress during the FIB tomographic serial sectioning. 3D reconstruction of the crack distribution around micro‐indentation sites shows that the cracks observed are influenced by the location of the FIB milled surface trenches due to localized stress changes.     

核主泵备用机械密封材料的摩擦性能研究

采用Falex-1506摩擦磨损试验机,研究了水润滑、室温条件下,载荷和速度对核主泵用机械密封材料:无压烧结碳化硅(WNV2)和碳化硅加碳(CHV1)、反应烧结碳化硅(R)和碳化硅加碳(R2)、石墨(MSMG)在不同配副条件下摩擦学特性的影响规律。使用扫描电镜(SEM),对磨损表面进行了观察和分析。研究结果表明,碳化硅和石墨材料自身的孔隙,在高载荷下容纳了更多的润滑流体,因此,不同配副条件下的摩擦系数均随载荷的增加而减小。另外,滑动速度引起的温度改变通过影响表面层性质影响摩擦力,而碳化硅和石墨在很宽的温度范围内机械性质保持不变,所以摩擦系数随速度的增加基本不变。     

Grooving wear of single-crystal tungsten carbide

The anisotropic nature of tungsten carbide (WC) single crystals has been evaluated in single-tip scratch testing and in multiple-tip abrasion. The single-tip grooves were made with a Vickers diamond indenter and the abrasion tests were performed with diamond and silica grits. All tests were performed on both the prism and basal planes of the WC crystals. A polycrystalline binderless carbide (Bl) was also evaluated. Optical surface profilometry was used to estimate the amounts of displaced, removed and ridge-formatted material in the scratch tests and the wear volumes in the abrasion tests. The scratches and wear scars were studied with scanning electron-, atomic force- and light optical microscopy (SEM, AFM, LOM). In situ studies of the scratch process were also performed. Wear debris were analysed with transmission electron microscopy (TEM). The results show that there are differences in both the amount of wear and the wear mechanisms between different crystallographic directions of WC. Depending on the direction of the slip planes in relation to the groove direction, the wear mechanisms change from ductile (grooves parallel to the slip planes) to brittle (grooves perpendicular to the slip planes). It is also shown that WC tends to wear by a formation of angular rod-shaped wear debris with the slip planes as the preferred surface planes.     

Effect of counterpart on the tribological behavior and tribo-induced phase transformation of Si

The tribological behaviors and phase transformation of single crystal silicon against Si₃N₄, Ruby and steel were investigated in this study. It was found that the strong chemical action between silicon and Fe was the key factor to the tribological behavior of silicon as slid against steel. SEM and Raman spectroscopy indicated that phase transformation of single crystal silicon occurred during the running-in period at low sliding velocity as slid against Si₃N₄ and Ruby, and gave birth to single or a mixture phase of Si-III, Si-XII and amorphous silicon. The high hardness of counterpart and the absence of chemical action between silicon and counterpart facilitated the phase transformation of single crystal silicon.     

Polycrystalline silicon carbide as a substrate material for reducing adhesion in MEMS

The in-use adhesion characteristics of polycrystalline cubic silicon carbide (poly-SiC) films when used as a substrate material in MEMS applications are investigated using micromachined polycrystalline Si (poly-Si) cantilever beam arrays. The detachment lengths greater than 1500 μm are obtained, corresponding to an apparent work of adhesion of less than 0.006 mJ/m². This is to be compared to the detachment lengths of less than 200 μm when poly-Si substrate is used, corresponding to the apparent work of adhesion of greater than 20 mJ/m². To help understand the mechanism leading to the significant reduction in in-use adhesion, the poly-SiC surfaces are characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle measurement. Based on the data, it is suggested that the topography as well as the slower oxidation rate of poly-SiC films may be responsible for the observed adhesion reduction.     

CVD diamond coated silicon nitride self-mated systems: tribological behaviour under high loads

Friction and wear behaviour of self-mated chemical vapour deposited (CVD) diamond films coating silicon nitride ceramics (Si₃N₄) were investigated in ambient atmosphere. The tribological tests were conducted in a reciprocal motion ball-on-flat type tribometer under applied normal loads up to 80 N (∼10 GPa). Several characterisation techniques—including scanning electron microscopy (SEM), atomic force microscopy (AFM) and micro-Raman studies—were used in order to assess the quality, stress state and wear resistance of the coatings. In addition, a novel method is presented to estimate the wear coefficient of the diamond coated flat specimens from AFM and optical microscopy (OM) observations of the wear tracks.     

The characterization of the damage introduced during Micro-erosion of MgO single crystals

The plastic damage introduced in MgO single crystals by bombardment with A1₂O₃ microspheres (0.3, 10, 20 and 27 μm diam.) has been studied using optical, transmission and scanning electron microscopy. Most of the damage is in the form of dislocation dipoles (two closely spaced dislocations of opposite sign) which lie on {110} slip planes. The dislocation density increases with the increase in the energy of the impinging particles. Using stereo-microscopy the damage has been shown to be uniformly distributed in thin foils. Studies done on indentations produced by a sharp pin show a very high density of single dislocations.     

硬度影响压痕弹塑性行为的有限元分析

以具有不同硬度40Cr为研究对象,根据集中载荷下的接触模型和赫兹理论,计算了压痕接触半径和压痕附近弹性区域的表面局部接触应力,并采用有限元法,分析硬度物理量对压痕弹塑性行为、局部接触应力、卸载后保留在内部的残余应力的影响,探讨压痕参数、压痕接触应力、残余应力与硬度之间的关系以及载荷增加时它们的发展.结果表明,相同载荷下塑性隆起量、压痕接触半径、压痕量和塑性区范围随着硬度值的提高而减小,弹性回弹量、最大接触应力和残余应力随硬度提高而增加;压痕周围处接触应力和残余应力、其分布范围和塑性区域随载荷的增加而增加.     

Comparison of the fretting wear of 100Cr6/100Cr6, Si3N4/Si3N4 and Si3N4/100Cr6 contacts in lubricated and dry conditions

The fretting wear behaviour of bearing steel against bearing steel, silicon nitride against silicon nitride, and silicon nitride against bearing steel, was investigated under lubricated and dry conditions. Amplitudes in the intermediate 5 to 50 μm range, and test durations from 10 to 360 min, were studied. Light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) were employed to determine the detailed nature of the friction and wear processes. In the silicon nitride against silicon nitride contact, brittle fracture of Si₃N₄ grains, and tribochemical reaction creating an amorphous layer on the mechanically damaged surface, were found. The main mechanism of fretting wear in the case of bearing steel against bearing steel contact was delamination. In the silicon nitride against bearing steel contact, chemical reactions predominated.     

Measurements of plastic strain around indentations caused by the impact of round and angular particles, and the origin of erosion

The aim of this paper is to examine plastic strain distributions around indentations and to consider the mechanisms of erosion damage caused by solid particle impact. A WC ball and an angular SiC particle of 3 mm in diameter were used to compare the effect of particle shape on plastic strain. Measurements of principal shearing strain distributions around the indentations were performed on surfaces of aluminum, iron and cast iron at impact angles of 20°, 30°, 40°, 60° and 90° at impact velocities from 50 to 200 m s⁻¹. It was found that the impact angle dependence was roughly consistent with the maximum principal shearing strain and erosion damage data, which have been published in previous papers and obtained during additional works in this study. The surface topography of the impact craters suggested that depth, contact area and volume of indentation are affected by the particle density and the hardness of both particle and target material. Measurements of volume ratio of lips to craters proved that material removal did not necessarily occur at a single impact of the WC ball, but occurred at the impact of the angular SiC particle at low impact angles. It is concluded that the origin of erosion is probably attributed to the conjoint actions of high plastic strains followed by subsequent removal and the cutting process caused by particle impact.     

Wear transition diagram for silicon carbide

To obtain information on the tribological behaviour of silicon carbide at elevated temperatures, unlubricated ball-on-flat wear tests were conducted on sintered silicon carbide in self-mated sliding in air. The contact load was varied from 3.2 to 98.0 N, and a temperature range of 23°C to 1000°C was used. Scanning electron microscopy, Fourier transform infrared spectroscopy and energy-dispersive spectroscopy were used to elucidate the wear mechanisms. The results of the tests and observations were employed to construct a wear transition diagram, which provides a summary of tribological information including friction coefficient, wear coefficient and wear mechanisms as a function of temperature and load. The wear transition diagram for the sintered silicon carbide studied is divided into four regions plus one transition zone. At room temperature, under high loads and high environmental humidity, the tribological behaviour is controlled by tribochemical reactions between the silicon carbide surface and water vapour in the environment. Under low loads and at temperatures below 250°C, wear occurs by ploughing and polishing. At temperatures about 250°C and under low loads, tribooxidation and formation of cylindrical wear particles control the tribological behaviour. Wear occurs by microfracture when the load is increased above a critical value; and both the friction coefficient and the wear coefficient increase.     

Estimations of work hardening exponents of engineering metals using residual indentation profiles of nano-indentation

This study was designed to predict work hardening exponent n of materials from AFM (atomic force microscope) observations of residual indentation impression in sharp indentations. FE simulations of nano-indentation were performed to 140 combinations to each parameter (elastic modulus E, yield stress σ _y , work hardening exponent n, and Poisson’s ratio gv) expressing elastic-plastic behaviors of universal engineering metals. Using the results from FE simulations and dimensional analysis, dimensionless functions were established to correlate residual indentation profiles with the work hardening exponent. This function was examined with nano-indentation, tensile test, and AFM observations after indentation for two materials (Al6061-T6 and copper).     

The effect of a weak W/SiC interface on the strength of Sigma silicon carbide monofilament

Fractography studies have shown that the strength-determining flaws in silicon carbide monofilaments are generally at the core/silicon carbide interface or in the vicinity of the outside, carbon-based coating. In tungsten-cored monofilaments like DERA Sigma, the W/SiC flaws primarily determine the strength. Fracture is accompanied by brittle failure of the tungsten. The crack propagates simultaneously outwards through the silicon carbide, inwards through the tungsten and also around the W/SiC interface before being deflected into the tungsten or out through the silicon carbide. Experiments depositing boundary layers between the tungsten and silicon carbide have resulted in significantly different fracture behaviour. The tungsten fails in a ductile manner and the strength-determining flaws are located predominantly at the outside surface of the silicon carbide. This behaviour is discussed in terms of models proposed by E. Martin and W. Curtin. It is thought that the work will ultimately lead to a significantly stronger, tungsten-based monofilament.     

Sample preparation and microscopical investigation techniques for metal and ceramics containing graphite and graphene‐like layered particles

Scanning electron microscopy (SEM) techniques are widely used in microstructural investigations of materials since it can provide surface morphology, topography, and chemical information. However, it is important to use correct imaging and sample preparation techniques to reveal the microstructures of materials composed of components with different polishing characteristics such as grey cast iron, graphene platelets (GPLs)‐added SiAlON composite, SiC and B₄C ceramics containing graphite or graphene‐like layered particles. In this study, all microstructural details of gray cast iron were successfully revealed by using argon ion beam milling as an alternative to the standard sample preparation method for cast irons, that is, mechanical polishing followed by chemical etching. The in‐lens secondary electron (I‐L‐SE) image was clearly displayed on the surface details of the graphites that could not be revealed by backscattered electron (BSE) and Everhart–Thornley secondary electron (E‐T SE) images. Mechanical polishing leads to pull‐out of GPLs from SiAlON surface, whereas argon ion beam milling preserved the GPLs and resulted in smooth surface. Grain and grain boundaries of polycrystalline SiC and B₄C were easily revealed by using I‐L SE image in the SEM after only mechanical polishing without any etching process. While the BSE and E‐T SE images did not clearly show the residual graphites in the microstructure, their distribution in the B₄C matrix was fully revealed in the I‐L SE image.     

Tribological behavior and phase transformation of single-crystal silicon in air

The tribological behaviors and phase transformations of single-crystal silicon against Si₃N₄ with different sliding velocities and durations under low contact stress at room temperature were investigated. SEM and Raman analysis indicate that phase transformation was involved and characterized by plastically deformed area which consists of Si-III, Si-XII, and a-Si, especially in the early stage at the low sliding velocity. The plastically deformed layer was wiped off at high-velocity or long sliding duration. TEM analysis indicated that the wear debris consists of amorphous silicon.