熔石英材料特性分析及实验研究

对熔石英材料的光学特性进行研究,利用分光光度计得到了不同波长下元件的光透过率、吸收率及其变化规律。通过熔石英材料纳米压痕实验,研究了熔石英材料的机械特性,得到了熔石英晶体的弹性模量和材料硬度。研究了熔石英材料的断裂特性,分析熔石英光学元件超精密加工亚表层损伤形成机理,对亚表层损伤裂纹的结构、组成以及裂纹扩散的形成过程进行了分析和研究。利用纳米压痕实验,模拟研磨加工时,材料表面受单颗粒磨粒静态压印和动态冲击时形貌演变的过程,得到了维氏硬度、断裂韧性及临界压力载荷。     

光学元件亚表面缺陷的损伤性检测方法

在磨削、研磨和抛光加工过程中产生的微裂纹、划痕、残余应力等亚表面缺陷会导致熔石英元件抗激光损伤能力下降,如何快速、准确地检测亚表面损伤成为光学领域亟待解决的关键问题。采用HF酸蚀刻法、角度抛光法和磁流变斜面抛光法对熔石英元件在研磨加工中产生的亚表面缺陷形貌特征及损伤深度进行了检测和对比分析,结果表明,不同检测方法得到的亚表层损伤深度的检测结果存在一定差异,HF酸蚀刻法检测得到的亚表面损伤深度要比角度抛光法和磁流变斜面抛光法检测结果大一些。且采用的磨粒粒径越大,试件表面及亚表面的脆性断裂现象越严重,亚表面缺陷层深度越大。     

动载下准脆性材料的泛形裂纹研究

为建立动态拉伸载荷作用下准脆性材料裂纹扩展路径的泛形表征,提出了一种非均匀准脆性材料动态裂纹扩展的泛形模型,计算得到的泛形裂面复杂度与已有实验数据吻合较好。结果表明:动态拉伸载荷作用下的裂纹扩展路径是泛形的,其复杂度随加载应变率的增大而减小,并与材料动态拉伸承载能力的空间随机分布无关,且随Weibull分布形状参数m的增加而减小。研究结果为分析动态拉伸载荷作用下准脆性材料的裂纹扩展机理即泛形表征提供了依据。     

酸蚀深度对熔石英三倍频激光损伤阈值的影响

 采用干涉仪和台阶仪测试蚀刻深度随时间的变化,结合材料去除速率测量,研究了HF酸蚀液对熔石英表面蚀刻的影响。测试了蚀刻后损伤阈值和表面粗糙度的变化。研究表明,熔石英表面重沉积层厚度约16 nm,亚表面缺陷层大于106 nm;重沉积层去除后损伤阈值增大,随亚表面缺陷层暴露其阈值先降低后又增加,最后趋于稳定;然而,随蚀刻时间的增加,其表面粗糙度增大。分析表明,蚀刻到200 nm能有效地提高熔石英的低损伤阈值,有利于降低初始损伤点数量和提高熔石英表面的机械强度。     

晶体相场法研究应力状态及晶体取向对微裂纹尖端扩展行为的影响

采用晶体相场模拟研究了单向拉伸作用下初始应力状态、晶体取向角度对单晶材料内部微裂纹尖端扩展行为的影响, 以(111)晶面上的预制中心裂纹为研究对象探讨了微裂纹尖端扩展行为的纳观机理, 结果表明: 微裂纹的扩展行为主要发生在<011>(111)滑移系上, 扩展行为与扩展方向与材料所处的初始应力状态及晶体取向紧密相关. 预拉伸应力状态将首先诱发微裂纹尖端生成滑移位错, 进而导致晶面解理而实现微裂纹尖端沿[011]晶向扩展, 扩展到一定程度后由于位错塞积, 应力集中, 使裂纹扩展方向沿另一滑移方向[101], 并形成锯齿形边缘; 预剪切应力状态下, 微裂纹尖端首先在[101]晶向解理扩展, 并诱发位错产生, 形成空洞聚集型长大的二次裂纹, 形成了明显的剪切带; 预偏变形状态下微裂纹尖端则直接以晶面解理形式[101]在上进行扩展, 直至断裂失效; 微裂纹尖端扩展行为随晶体取向不同而不同, 较小的取向角度会在裂纹尖端形成滑移位错, 诱发空位而形成二次裂纹, 而较大的取向角下的裂纹尖端则以直接解理扩展为主, 扩展方向与拉伸方向几近垂直.     

三倍频熔石英激光损伤的吸收前驱体

 在三倍频熔石英激光损伤研究中,吸收前驱体的存在是造成熔石英材料表面损伤阈值远低于体损伤阈值的主要原因。以吸收作为主线,建立吸收前驱体引起的熔石英表面损伤模型,并讨论吸收前驱体与熔石英表面损伤的联系。针对金属元素微粒残留理论模拟了激光辐照过程中微粒周围温度场的变化,结果表明在脉冲时间内微粒周围温度很快到达损伤判据所给出的临界温度,通过激光损伤实验,证实吸收性微粒含量与损伤密度有密切联系。针对亚表面裂纹,建立一维模型计算受激后裂纹表面附近自由电子密度分布。计算结果和实验结果均表明：金属元素微粒和亚表面裂纹是熔石英激光损伤中重要的吸收前驱体,金属元素微粒对激光能量的吸收致使周围材料温度在脉冲时间内达到损伤判据温度2 000 K;受激后裂纹表面的自由电子密度达到1021 cm-3的水平,可导致对后续激光的强烈吸收;两种吸收前驱体极大降低了熔石英材料的表面抗损伤性能。     

熔石英再沉积层结构的纳米级表征和杂质分析

利用原子力显微镜观察熔石英不同蚀刻时间的表面形貌,结合二次离子质谱分析,研究了熔石英的再沉积层结构和杂质分布。结果表明,熔石英表面深度10nm的再沉积层内存在大量微裂纹和杂质,经蚀刻展开形成nm级划痕和坑点,其分布随着深度增加呈指数衰减。根据nm级划痕密度、宽深比随蚀刻深度变化的规律,估算出再沉积层厚度,估算结果与二次离子质谱测得的杂质嵌入深度基本一致。杂质元素嵌入深度与抛光微裂纹分布特征的关联性表明,杂质很有可能藏匿在抛光微裂纹中。     

脆性材料动态断裂的介观格子模型

岩石、陶瓷、玻璃、固体炸药等脆性材料在爆炸与冲击施加的强动载荷作用下易发生迅速的裂纹扩展和灾难性的断裂破碎,造成材料、器件、装置的功能失效和事故危害。理解脆性断裂过程中介观裂纹网络演化与宏观动态响应的关联是提升脆性材料可靠性和安全性的关键,但同时也是计算建模与数值模拟研究面临的难点。为了解决爆炸与冲击加载下脆性材料中裂纹网络随机萌生、裂纹面挤压摩擦、大量裂纹交错扩展等复杂过程带来的算法困难,一种无网格/粒子方法——"格子模型"得到了持续的关注和长足的发展。本文综述了格子模型的原理和方法,介绍了运用格子模型开展脆性断裂研究的代表性成果,分析了格子模型存在的不足与改进的方向。     

激光诱导熔石英表面损伤修复中的气泡形成和控制研究

基于熔石英材料在CO_2激光作用下的温度分布和结构参数变化的计算结果,对熔石英损伤修复中的气泡形成和控制进行了研究.针对损伤尺寸介于150—250μm之间的损伤点,提出了一种能够有效控制气泡形成的长时间低温预热修复方法.基于低温下熔石英材料结构弛豫时间常数较长的特点,该方法在不引起熔石英材料结构发生显著变化的同时,能够解吸附表面和裂纹处所附着的气体和杂质,可有效降低裂纹闭合过程中气泡形成的概率.实验结果表明,长时间低温预热修复方法的成功修复概率可达到98%.     

抑制损伤发展的CO_2激光修复技术及机理研究

熔石英表面激光损伤发展问题一直制约着激光器的运行通量。采用CO2激光在线熔融修复损伤点,修复后形成一个光滑的高斯坑,去除了损伤点中的裂纹,平滑了凹凸不平的表面,并且在紫外脉冲激光作用下,修复斑再次产生损伤的阈值高于熔石英元件的损伤生长阈值。因此CO2熔融修复技术能有效地抑制损伤发展。通过分析CO2激光作用下熔石英表面的温度分布,讨论修复坑的形成过程,确定激光参数对修复效果的影响,为寻找最佳修复参数提供理论基础。同时利用原子力显微镜(AFM)、轮廓仪细致分析损伤点和修复斑的微细结构,采用有限差分时域方法计算损伤点和修复斑周围的光强分布,探索消除裂纹和平滑表面对抑制损伤生长的作用。     

Lattice wave emission from moving cracks

Antiplane shear cracks moving in square lattices have been studied in a molecular dynamics simulation. They become unstable when a critical stress intensity at the tip is reached. Within picoseconds they accelerate to steady state velocity, which can be supersonic or not, depending on the interatomic potential. If the surface energy is low, they become supersonic; if the surface energy is high, they reach only subsonic speeds before the crystal reaches the theoretical shear strength. A simple modification of the interatomic potential switches the crystal behaviour from brittle to ductile, substituting dislocations to daughter cracks. The steady state velocities are the same, whether the model crystal is brittle or ductile. The steady state velocity is a function of the applied stress, not the stress intensity.     

Structure-properties changes in ZnO-PbO-GeO<Subscript>2</Subscript> glasses

Molecular dynamics simulations are used to study brittle/ductile interfaces Ni/B2-NiAl under mechanical loading. Uniaxial tensile tests perpendicular to the interface are performed. It is shown that interfaces have influence on strain induced material failure by nucleation of defects. Crack propagation in the interface is investigated by applying load via fixed displacement boundary conditions. Determined crack velocities in the interface are found to be clearly above those in each of the materials.     

塑性加工条件下射流颗粒冲击去除效应

针对在微观上存在尖锐突起、凹坑和划痕等缺陷的光学元件,提出用低质量分数磨料水射流冲击的方式对其进行处理。从弹性接触出发,对射流中粒子与元件发生塑性接触的临界速度进行了推导,并引入了塑性转入脆性加要的临界速度,从而对射流的塑性去除阶段作了明确的界定。针对常用的两种光学材料K9和石英玻璃,结合具体参数对使其处于塑性去除阶段的射流速度进行了模拟计算,利用单颗粒冲击去除模型,在塑性去除范围内对两种材料的冲击去除进行了模拟计算。结果表明:石英玻璃进入塑性去除的临界速度高于K9玻璃,而进入脆性去除的临界速度低于K9玻璃,因而使石英玻璃处于塑性去除阶段的射流速度范围为K9玻璃相应速度范围的子区间;在塑性去除阶段,各材料的去除量皆随着冲击速度的增大而增大,但较硬的石英玻璃更不耐冲击,较K9玻璃更容易被去除。     

Generation and removal of pits during chemical mechanical polishing process for MgO single crystal substrate

Magnesium oxide (MgO) single crystal is an important substrate for high temperature superconductor, ferroelectric and photoelectric applications. The function and reliability of these devices are directly affected by the quality of polished MgO surface because any defect on the substrate, such as pit or scratch, may be propagated onto device level. In this paper, chemical mechanical polishing (CMP) experiments were conducted on MgO (1 0 0) substrate using slurry mainly comprised of 1-hydroxy ethylidene-11-diphosphonic acid (HEDP) and silica or ceria particles. Through monitoring the variations of the pits topography on substrate surface, generation and removal mechanism of the pits were investigated. The experimental results indicate that the pits were first generated by an indentation or scratch caused by particles in the slurry. If the rate of chemical etching in the defect area is higher than the material removal rate, the pits will grow. If chemical reaction in the defect area is slower than the material removal rate, the pits will become smaller and eventually disappear. Consequently, these findings may provide insight into strategies for minimizing pits during CMP process.     

The research on mechanical effect etching Si in pulsed laser micromaching under water

To explore further the influencing of mechanical effects on laser machining in the liquid, in the process of great-energy and short-pulsed laser irradiating matter in the liquid, the experiments of 248 nm laser etching n-Si under water were carried out. The removal mechanism of brittle material etched by mechanical effects, which is induced during high-energy and short-pulsed laser machining in the liquid, was discussed. In the paper, the approximate mechanics model of indentation fracture was used to analyze the mechanical effects for removing brittle materials of silicon when laser machining in the liquid. Based on this, a theoretical model of material removal rate was proposed; the experiment of laser machining under water was adopted to validate the model. The experimental results indicate that the removal rate of brittle material caused by shock forces is relatively great.     

The existence of a lateral size effect and the relationship between indentation and scratch hardness in copper

Indentation size effects (ISEs) are well known in static indentation of materials that deform by dislocation-based mechanisms. However, whilst instrumented indentation techniques have become rapidly established as a means of determining the near-surface mechanical properties of materials, scratch testing has been much less widely used. Hardness is used in wear models as a proxy for the yield stress, and the design of materials and hard coatings has often sought to exploit size-derived performance enhancements through length-scale engineering. Yet, it is not known directly whether (or not) length-scale effects also apply to scratch (and thus wear) performance at small scales, or what the functional form of this effect is. This work directly demonstrates that there is a lateral size effect (LSE) and shows that there are questions to be answered if the use of hardness as an indicator of wear performance is to remain valid. We report on constant load scratch experiments using a Berkovich indenter on single-crystal, annealed copper, using a range of applied normal forces and compare results from three scratch hardness calculation methods to indentation hardness (ISO 14577:2002) measured on the same sample at the same loads. Scratch tests were performed with the Berkovich indenter aligned either edge forward or face forward to the scratch direction. In all cases, we demonstrate that there is a very significant (approximate factor of two) effect of scratch size (an LSE) on scratch hardness. The results also show that the deformation mechanisms occurring in scratch tests are different to those occurring beneath a static indentation and that different mechanisms dominated for different stylus orientations (face-forward vs. edge-forward orientation). This is, to our knowledge, the first direct demonstration of an LSE akin to the ISE in metallic materials. The results have significant implications for using static indentation as a predictor of deformation during wear processes.     

Formation of indentation cracks and origin of indentation size effect in cadmium tartrate pentahydrate single crystals

The load dependence of the Vickers microhardness on the as-grown (0 1 0) and (0 0 1), and cleaved (0 0 1) faces of cadmium tartrate pentahydrate (CTPH) single crystals has been investigated. The experimental results showed that, with an increase in the applied load, the microhardness of the as-grown (0 1 0) and (0 0 1) faces decreases, while that of the unheated and heated (0 0 1) cleavage faces decreases first up to a load of 2.5 N and then increases. Analysis of the experimental results revealed that: (1) radial crack length, indentation size and applied indentation load are mutually related, and these dependences related with fracture mechanics are the basis of Meyer’s empirical law, (2) with increasing indentation load, changes in the mechanism of development of indentation cracks from radial cracks to lateral cracks and surface chipping of the material, followed by predominantly surface chipping of the material are responsible for indentation size effect in CTPH crystals, (3) proportional specimen resistance model and Meyer’s law not only explain the indentation size effect but also can be used to determine load-independent hardness H^∗, and (4) there is no direct relationship between microhardness and fracture toughness of different CTPH samples, while the values of load-independent hardness H^∗, and brittleness indices β and B of CTPH crystals increase linearly with the Meyer constant A. Procedures are given to determine load-independent hardness H^∗ from the transition values of load and corresponding indentation size.     

Elimination of surface scratch/texture on the surface of single crystal Si substrate in chemo-mechanical grinding (CMG) process

Si wafers are widely used as a substrate material for fabricating ICs. The quality of ICs depends on the quality of Si wafers. The chemo-mechanical grinding (CMG) with soft abrasive grinding wheels (SAGW) has been recently found to be a great potential process for machining Si wafers to generate superior surface quality at low cost. However, there have been very few studies on observing variation of topography of scratch/texture and understanding basic eliminating process of scratch/texture on the ground Si wafer. Furthermore, few reports on the variation of surface roughness and material removal rate (MRR) during CMG process and relationship between MRR and surface roughness during CMG process are presented. In this paper, a series of CMG experiments have been conducted to study the elimination process of artificial scratches created on etched Si surfaces and residual textures induced by SD1500 diamond wheel in CMG process, and to understand the topography variations of Si surfaces and some basic grinding characteristics during CMG process.     

Probing mechanical properties of thin film and ceramic materials in micro- and nano-scale using indentation techniques

In this study, we report on the mechanical properties, failure and fracture modes in two cases of engineering materials; that is transparent silicon oxide thin films onto poly(ethylene terephthalate) (PET) membranes and glass-ceramic materials. The first system was studied by the quazi-static indentation technique at the nano-scale and the second by the static indentation technique at the micro-scale. Nanocomposite laminates of silicon oxide thin films onto PET were found to sustain higher scratch induced stresses and were effective as protective coating material for PET membranes. Glass-ceramic materials with separated crystallites of different morphologies sustained a mixed crack propagation pattern in brittle fracture mode.     

Investigation of material removal mechanism of silicon wafer in the chemical mechanical polishing process using molecular dynamics simulation method

Chemical mechanical polishing (CMP) technology, being the mainstream technique of acquiring global planarization and nanometer level surface, has already become an attractive research item. In the case of CMP process, the indentation depth lies in the range of nanometer or sub-nanometer, huge hydrostatic pressure induced in the local deformation area which makes the material removal and surface generation process different from traditional manufacturing process. In order to investigate the physical essence of CMP technique, the authors carry out molecular dynamics (MD) analysis of chemical mechanical polishing of a silicon wafer. The simulation result shows that huge hydrostatic pressure is induced in the local area and leads to the silicon atom transform from the classical diamond structure (α silicon) to metal structure (β silicon). This important factor results in the ductile fracture of silicon and then in the acquisition of a super-smooth surface.