Material Removal Characteristics of Single-Crystal 4H-SiC Based on Varied-Load Nanoscratch Tests

Single-crystal silicon carbide(SiC)has been widely applied in the military and civil fields because of its excellent physical and chemical properties.However,as is typical in hard-to-machine materials,the good mechanical properties result in surface defects and subsurface damage during precision or ultraprecision machining.In this study,single-and double-varied-load nanoscratch tests were systematically performed on single-crystal 4H-SiC using a nanoindenter system with a Berkovich indenter.The material removal characteristics and cracks under different planes,indenter directions,normal loading rates,and scratch intervals were analyzed using SEM,FIB,and a 3D profilometer,and the mechanisms of material removal and crack propagation were studied.The results showed that the Si-plane of the single-crystal 4H-SiC and edge forward indenter direction are most suitable for material removal and machining.The normal loading rate had little effect on the scratch depth,but a lower loading rate increased the ductile region and critical depth of transition.Additionally,the crack interaction and fluctuation of the depth-distance curves of the second scratch weakened with an increase in the scratch interval,the status of scratches and chips changed,and the comprehensive effects of the propagation and interaction of the three cracks resulted in material fractures and chip accumulation.The calculated and experimental values of the median crack depth also showed good consistency and relativity.Therefore,this study provides an important reference for the high-efficiency and precision machining of single-crystal SiC to ensure high accuracy and a long service life.

Understand anisotropy dependence of damage evolution and material removal during nanoscratch of MgF_(2) single crystals

To understand the anisotropy dependence of the damage evolution and material removal during the machining process of MgF_(2) single crystals,nanoscratch tests of MgF_(2) single crystals with different crystal planes and directions were systematically performed,and surface morphologies of the scratched grooves under different conditions were analyzed.The experimental results indicated that anisotropy considerably affected the damage evolution in the machining process of MgF_(2) single crystals.A stress field model induced by the scratch was developed by considering the anisotropy,which indicated that during the loading process,median cracks induced by the tensile stress initiated and propagated at the front of the indenter.Lateral cracks induced by tensile stress initiated and propagated on the subsurface during the unloading process.In addition,surface radial cracks induced by the tensile stress were easily generated during the unloading process.The stress change led to the deflection of the propagation direction of lateral cracks.Therefore,the lateral cracks propagated to the workpiece surface,resulting in brittle removal in the form of chunk chips.The plastic deformation parameter indicated that the more the slip systems were activated,the more easily the plastic deformation occurred.The cleavage fracture parameter indicated that the cracks propagated along the activated cleavage planes,and the brittle chunk removal was owing to the subsurface cleavage cracks propagating to the crystal surface.Under the same processing parameters,the scratch of the(001)crystal plane along the[100]crystal-orientation was found to be the most conducive to achieving plastic machining of MgF_(2) single crystals.The theoretical results agreed well with the experimental results,which will not only enhance the understanding of the anisotropy dependence of the damage evolution and removal process during the machining of MgF_(2) crystals,but also provide a theoretical foundation for achieving the high-efficiency and low-damage processing of anisotropic single crystals.

氧化镓单晶的磨削材料去除机理和损伤演化研究

为了探究氧化镓单晶在磨削过程中材料去除机理和亚表面损伤演化规律,通过变切深纳米划痕试验模拟单颗磨粒去除材料的过程来探究磨削过程中的材料去除机理,使用粒度分别为SD600、SD1500和SD5000的金刚石砂轮对氧化镓单晶进行磨削试验,分析磨削表面形貌和亚表面的损伤演化规律.使用扫描电子显微镜和透射电子显微镜作为主要表征手段,采用有限元法分析划痕过程中的应力分布.研究结果表明,氧化镓单晶在材料去除过程中沿不同晶向扩展的交错滑移带可能导致不规则的破碎坑,取向裂纹受到(-3-10)滑移面的严重影响.随着砂轮粒径的减小,磨削表面形貌表现为破碎坑和取向裂纹主导的脆性表面逐渐演化为完全塑性表面.

Nanoscratching and mechanical behaviors of high-entropy alloys with different phase constituents

High-entropy alloys(HEAs)exhibit unique microstructural features and properties in nanoscale and atomic scale because of their multi-element alloy system.The nanoscratching behaviors of three HEAs with different phase constituents,relative to the microstructure and mechanical properties of the HEAs,were investigated.Three typical phase constituents were selected:face-centered cubic(FCC)structure,body-centered cubic(BCC)structure,and a dual-phase structure containing both FCC and BCC phases.Despite the fact that the FCC alloy has the highest ductility and strain hardening capability,it exhibited inferior scratch resistance due to the over-softening of hardness.Due to the brittle failure mode,the BCC alloy hardly exhibited desirable scratch resistance despite its highest hardness.By contrast,the nanostructured dual-phase alloy exhibited the best scratch resistance because of its good combination of strength and ductility,as well as the ductile failure mode.This research suggests that the HEA with structure comprising nanoscale hard and soft phases is desirable for nanoscratch resistance,and possesses appropriate hardness for industrial applications.

Material removal mechanisms and characteristics of potassium dihydrogen phosphate crystals under nanoscratching

Potassium dihydrogen phosphate(KDP)crystals are important materials in high-energy laser systems.However,because these crystals are brittle and soft,machining-induced defects often emerge in KDP components.This study aimed to investigate the material removal mechanisms and characteristics of KDP during nanoscratching using Berkovich,spherical,and conical indenters.We found that KDP surface layers could be removed in a ductile mode at the micro/nanoscale and that dislocation motion was one of the main removal mechanisms.Removal characteristics are related to the stress fields generated by indenter geometries.The spherical indenter achieved a ductile removal mode more easily.The lateral force of nanoscratching increased with an increase in the normal force.The coefficient of friction(COF)followed the same trend as the lateral force when spherical and conical indenters were used.However,the COF was independent of the normal force when using a Berkovich indenter.We found that these COF variations could be accurately described by friction models.

Study of nanoscratching of polymers by using molecular dynamics simulations

Molecular dynamic simulations are performed to study the nanoscratching behavior of polymers.The effects of scratching depth,scratching velocity and indenter/polymer interaction strength are investigated.It is found that polymer material in the scratching zone around the indenter can be removed in a ductile manner as the local temperature in the scratching zone exceeds glass transition temperature Tg.The recovery of polymer can be more significant when the temperature approaches or exceeds Tg.The tangential force,normal force and friction coefficient increase as the scratching depth increases.A larger scratching velocity leads to more material deformation and higher pile-up.The tangential force and normal force are larger for a larger scratching velocity whereas the friction coefficient is almost independent of the scratching velocities studied.It is also found that stronger indenter/polymer interaction strength results in a larger tangential force and friction coefficient.

(0001)面氧化锌单晶微纳米尺度划痕特性实验研究

目的对(0001)面ZnO单晶微纳米尺度划痕特性进行实验研究,为ZnO单晶器件性能提升及ZnO单晶精密加工工艺优化等提供必要的科学依据。方法采用Berkovich金刚石压头棱向前和面向前2种划痕方式,在不同划痕速度下对(0001)面ZnO单晶进行了纳米划痕实验,分析了划痕速度和划痕方式对其微纳米尺度划痕特性的影响。结果当划痕速度从2μm/s增加到100μm/s时,棱向前划痕方式下的深度从352.9 nm降到了326.9 nm,面向前划痕方式下的深度从352.7 nm降到了289.9 nm;棱向前划痕方式下的切向力从4.15 mN降到了3.93 mN,面向前划痕方式下的切向力从5.12 mN降到了4.45 mN;棱向前划痕方式下的摩擦因数从0.21降到了0.19,面向前划痕方式下的摩擦因数从0.25降到了0.2;棱向前划痕方式下的残余划痕深度从162.2 nm降到了138.4 nm,面向前划痕方式下的残余划痕深度从148.3 nm降到了129.9 nm;棱向前划痕方式下的残余划痕两侧塑性堆积高度从23 nm降到了17 nm,面向前划痕方式下的残余划痕两侧塑性堆积高度从18nm降到了11nm。结论随划痕速度的增加,(0001)面Zn O单晶的划痕深度、切向力、摩擦因数、残余划痕深度及划痕两侧塑性堆积高度均在下降。在相同划痕速度下,棱向前划痕方式下的划痕深度、残余划痕深度及划痕两侧塑性堆积高度都比面向前划痕方式下的要大,而棱向前划痕方式下的切向力和摩擦因数都比面向前划痕方式下的要小。

基于纳米压痕与纳米划痕实验的单晶硅超精密切削特性研究

为研究单晶硅超精密切削特性,采用纳米压痕仪配合Berkovich金刚石压头对单晶硅<100>晶面进行纳米压痕与纳米划痕实验。纳米压痕实验分别以10、30和50 mN载荷将压头压入单晶硅表面,发现30 mN载荷下载荷-位移曲线产生微小波动,而在50 mN载荷下发生“pop-out”现象,说明材料此时有突然的应力变化并有脆性破坏发生,预测了单晶硅脆塑转变的临界载荷略小于30 mN。开展变载荷纳米划痕实验,用0~100 mN的载荷刻划单晶硅表面,根据载荷-位移曲线观察到单晶硅在变载荷刻划中分为弹塑性去除和脆性去除阶段。弹塑性去除阶段,载荷-位移曲线波动平稳,而脆性去除阶段曲线波动较大,得到单晶硅脆塑转变的临界载荷为27 mN,临界深度为392 nm。通过恒载荷纳米划痕实验,在塑性加工域内分别以5、10和20 mN的恒载荷刻划单晶硅表面,并通过扫描电子显微镜(SEM)观察恒载荷划痕后的单晶硅表面形貌,分析刻划数据发现切削力和弹性回复率随着载荷的增加而增大,摩擦系数则先增大后减小。因此单晶硅超精密切削加工应选择合理的载荷,并充分考虑弹性回复的影响。

氧化层对纳米划痕诱导金属定向沉积的影响研究

金属微纳结构在电磁及等离子体传感、能量收集、信息存储、生物技术等方面具有广泛的应用前景,而实现金属微纳结构的可控制备是保障其应用的基础。采用光刻胶掩膜实现金属(Au)定向沉积,研究了划痕表面去氧化层处理对定向沉积行为的影响,据此实现高质量金属微纳结构的制备。结果表明,光刻胶可有效起到掩膜作用,使金属能够在单晶硅表面的划痕上实现选择性定向沉积;去除划痕表面的氧化层可使沉积的金属微纳结构更为平整致密,划痕区域导电位点增多且分布均匀,更利于电化学反应发生,因而在其表面析出的单质金属更为均匀。

类金刚石薄膜的表面纳米划擦性能评价

用射频等离子体增强化学气相沉积在钛合金表面制备了类金刚石薄膜，利用纳米压入仪及其附件研究了薄膜与纳米划擦有关的力学性能．结果表明：随薄膜厚度的增加，其硬度略有增加，但增幅较小，弹性模量没有明显的相应规律；薄膜在划擦过程中，随载荷增加，先后经历薄膜变形、薄膜与基体共同变形及薄膜剥离三个阶段；在薄膜变形阶段，划擦对薄膜的损害较小；当压头进入薄膜一定深度后，划擦后薄膜与基体的变形不同步，造成薄膜沿划痕向两边形成整齐排列的小裂纹，呈鱼骨状；达到临界载荷值时，薄膜在界面处发生脆性剥落；随膜厚增加，薄膜的临界载荷增大，因其残余应力的相应增大而发生大面积脆性剥落．

纳米硬度技术的发展和应用

摘 要 近二十年来,主要用于检测材料表面微米和亚微米尺度力学性质的纳米硬度技术发展迅速.首先,概述硬度的定义、分类及其适用范围.然后,系统地总结纳米硬度技术的发展,重点介绍纳米压痕硬度的测量原理及其影响因素,连续刚度测量原理,高分辨率的载荷位移测量原理,几种常用压头的几何形状,试样表面的准备和确定,相关的测试方法,仪器校准和显微观察等问题.通过压痕实验可获得硬度、弹性模量、断裂韧性、存储模量和损耗模量、蠕变应力指数等.最后,简要介绍纳米划痕硬度测量技术的发展和应用.

基于纳米划痕实验和有限元仿真的KDP晶体断裂性能研究

通过变深度纳米划痕实验对KDP的断裂特性进行了研究,测量了在KDP晶体(001)晶面上沿不同方向进行划痕实验时首条裂纹出现的位置。随后建立了该划痕过程的有限元模型,计算得到了导致KDP晶体沿不同方向发生断裂时的拉应力,并解释了划痕实验中出现微裂纹和崩坑的原因。结果表明,在KDP晶体(001)晶面上沿0°方向加工时材料最容易发生断裂,对应的拉应力为107 MPa;而沿45°方向时材料表现出较好的可加工性能,此时导致KDP晶体发生断裂的拉应力为160 MPa。

亚微米氮化钛膜的纳米压痕和划痕测定

采用等离子电弧沉积的方法,分别在GT35和40CrNiMo钢上沉积厚约为0.5μm的氮化钛(TiN)膜。为了筛选基材,采用纳米压痕和划痕技术,评价膜基界面结合和固体润滑效果。纳米压痕结果,GT35,40CrNiMo和TiN的纳米硬度/弹性模量的典型值分别约为11.5GPa/330GPa,6.0GPa/210GPa,30GPa/450GPa。纳米划痕结果,GT35有较理想的膜基结合能力;GT35,40CrNiMo,TiN及其有机膜的摩擦系数分别约为0.25,0.45,0.15,0.10。同40CrNiMo相比,GT35是较为理想的基体材料。纳米压痕和划痕技术能提供丰富的近表面的弹塑性变形、断裂和摩擦等的信息,是评价亚微米薄膜力学性能的有效手段。

纳米压痕和划痕法测定氧化硅薄膜材料的力学特性

为了研究不同制备工艺对材料力学性能的影响 ,选择了热氧化、LPCVD和PECVD三种典型工艺 ,在硅片上制备 1μm氧化硅薄膜。通过纳米压痕和划痕检测可知 ,热氧化工艺制备的SiO2薄膜的硬度和模量最大 ,LPCVD制备的样品界面结合强度高于PECVD。纳米压痕和划痕技术为此提供了丰富的近表面弹塑性变形和断裂等的信息 。

加热温度对Ni-P/Ti/DLC多层膜力学性能的影响

目的研究不同加热温度对Ni-P/Ti/DLC多层膜力学性能的影响。方法用化学镀镍磷工艺在模具钢基体上镀Ni-P层作切削层,采用过滤阴极真空电弧(FCVA)技术分别沉积Ti过渡层和DLC保护层。利用拉曼光谱分析了多层膜表层在不同加热温度下的结构成分,采用纳米压痕、纳米划痕和扫描电镜对多层膜的硬度和弹性模量、膜层结合性能以及划痕表面形貌进行了表征。结果拉曼光谱检测结果表明,随着加热温度的升高,多层膜表层DLC膜中的AD/AG值及sp2键含量增大,且400℃时AD/AG值的变化幅度明显增大。纳米压痕实验结果表明,多层膜的硬度和弹性模量随着温度的升高呈先增后减的趋势,且在300℃时达到最大,纳米压痕过程中膜层未出现破裂现象。纳米划痕实验及SEM观测结果表明,多层膜的临界载荷A1随着加热温度的升高而增加,临界载荷A2在25~200℃区间没有明显变化,而在300~400℃区间显著增大。结论在加热温度达到400℃时,多层膜表层DLC膜的石墨化倾向显著。加热温度为300℃时,多层膜的力学性能及膜层间的结合性能较优,而400℃时膜层间的结合性能及抑制裂纹扩展能力减弱,且膜层具有较大的塑性。因此,适宜的加热温度有利于提高多层膜的力学性能和膜层间的结合性能。

用纳米压痕法研究玻璃表面复合薄膜的力学性能

采用溶胶凝胶法在玻璃基体上制备了CeO2-TiO2单层膜、TiO2-SiO2单层膜和CeO2-TiO2/TiO2-SiO2双层膜,采用纳米压痕和划痕法对薄膜的机械性能(纳米硬度、弹性模量、临界载荷、摩擦系数)进行了分析。实验结果表明,双层膜与玻璃基体的附着力以及弹性模量、硬度等指标均大于单层膜,由TiO2-SiO2组成的内层,对强化附着力起到了关键作用。

晶体铜微探针纳米刻划的分子动力学建模

分别从分子动力学基本原理、晶体铜原子结构建模和晶体缺陷分析三方面开展了晶体铜微探针纳米刻划的三维分子动力学建模研究。研究结果表明:在合理地选择分子动力学模拟参数的基础上,基于晶体结构、重位点阵和维诺图分别建立单晶铜、双晶铜和纳米晶体铜的原子结构模型,并结合先进的晶体缺陷分析技术,可为系统开展晶体铜微探针纳米刻划的分子动力学模拟提供技术保障。

纳米硬度技术在表面工程力学性能检测中的应用

结合纳米硬度技术测量各类薄膜和块体材料表层的纳米压痕硬度、弹性模量、断裂韧性、膜厚、微结构的弯曲变形,采用纳米划痕硬度技术测量各类薄膜和块体材料的粗糙度、临界附着力、摩擦系数、划痕横剖面。纳米硬度计是检测材料表层微米乃至几十纳米力学性能的先进仪器,可广泛应用于表面工程中的质量检测。

聚碳酸酯透明材料表面耐磨涂层的纳米力学性能和耐磨性研究

利用纳米压痕和划痕技术对不同纳米硅溶胶含量的两种有机硅耐磨涂层的表面与体相的纳米力学性能和耐磨性分别进行了对比实验研究。结果表明:纳米硅溶胶会增加涂层Si-O-Si网络结构的刚性,提高涂层的力学性能和耐磨性,并且涂层近表面区的硬度、模量以及耐磨性明显优于体相的性质。

纳米压痕和划痕法对含氧化铈薄膜机械性能的测定

采用溶胶-凝胶法在玻璃基体上制备了CeO2-TiO2单层膜、TiO2-SiO2单层膜和CeO2-TiO2/TiO2-SiO2双层膜,采用纳米压痕和划痕法对薄膜的机械性能(纳米硬度、弹性模量、临界载荷、摩擦系数)进行了分析。实验结果表明,双层膜与玻璃基体的附着力以及弹性模量、硬度等指标均大于单层膜,由TiO2-SiO2组成的内层,对强化附着力起到了关键作用。