KDP晶体单点金刚石切削加工表层力学性能的研究

压痕和刻划实验是测量材料弹性、塑性和断裂行为的最简单方法,磷酸二氢钾(简称KDP)作为优质的非线性光学材料,常用作光学频率转换器件和电光开关元件,一般采用单点金刚石削的方法加工此类零件.为了深入了解KDP晶体单点金刚石切削加工表层力学性能的变化规律,本文对KDP晶体单点金刚石切削加工表层力学性能指标如硬度H、断裂韧性Kc和残余应力σr方面的有关问题进行深入研究,并通过对KDP晶体(001)面不同晶向上的硬度检测,系统分析了KDP晶体加工表面晶向对材料硬度、断裂韧性和残余应力的影响,研究结果表明:KDP晶体的同一晶面的不同晶向硬度和断裂韧性具有强烈的各向异性.     

单晶体材料内应力研究进展

随着科学技术的发展,晶体的力学性质,如弹性、脆性、硬度和解理性等引起了人们的重视。晶体材料应力分布取决于很多复杂因素(加热、退火、提拉、切割、搬运以及生长过程中的各种力学因素)。从应力表征、硬度及断裂韧性的测试方法入手,回顾并总结了晶体材料力学参数的表征手段,阐述了晶体开裂的分布规律及原因。其中光测方法(主要包括光弹法、X射线衍射法等)因其对晶体材料无任何机械损伤、检验灵敏度高而应用广泛:光弹性是光学晶体材料的重要特性,利用光弹仪测定光程差,根据平面光弹性的应力-光学定律确定主应力差;X射线衍射方法测定样品中宏观应力具有无污染、测量精度高等特点。压痕实验和划痕实验是表征晶体硬度的主要手段,结合化学腐蚀和光学观测方法可以有效探讨晶体开裂的微观机理。     

涂层材料的断裂分析(英文)

和基体厚度相比,涂层很薄,因此细观力学模型可把基体作为半无限弹性体。由于涂层和基体材料的膨胀系数及弹性系数不匹配,涂层材料中残余热应力的解析解为 E_c/(1-γ_c)·(α_s-α_c)△T。用有限元法校核,该应力和解析解吻合得好。通过对涂层产生裂纹驱动力和断裂韧性的讨论,提出了抗裂涂层厚度公式。     

涂层材料的断裂分析

和基体厚度相比，涂层很薄，因此细观力学模型可把基体作为半无限弹性体，由于涂层和基体材料的膨胀系数及弹性系数不匹配，涂层材料中残余热应力的解析解为Ｅｃ／（１－ｙｃ）·（αｓ－αｃ）△Ｔ。用有限元法校核，该应力和解析解吻合得好，通过对涂层产生裂纹驱动力和断裂韧性的讨论，提出了抗裂涂层厚度公式。     

基于纳米试验技术的薄膜/基体体系显微力学性能表征

采用纳米压痕能够获得薄膜硬度、模量、膜层状态、粘接性能、断裂韧性等力学性能.纳米划痕可以表征薄膜/基体的表面状况、结合力、耐磨损等性能.纳米冲击可以研究薄膜/基体的脆韧性、失效机理等力学行为.综述了纳米压痕、纳米划痕及纳米冲击在薄膜/基体体系力学性能表征中的应用原理.     

纳米压痕法测试微纳米涂层的力学性能

采用G200型微/纳米压痕仪,测试了不同微纳米级涂层材料的力学性能。结果表明:使用纳米压痕法能检测和表征微纳米涂层材料的硬度和弹性模量及其随涂层厚度的变化趋势,从而了解涂层的力学性能以及与基体的结合质量情况;纳米压痕法测试涂层力学性能时存在明显的表面效应,一般建议压入深度不小于20μm,以保证表面粗糙度引起的压入深度的不确定度小于5%。     

球压法在线评价脆性材料的强度特性和残余应力

针对接触应力和压痕临界载荷的应力分布,通过应力梯度效应讨论分析了产生球压裂纹中的几个传统强度理论无法解释的问题.利用均强度准则提出了一种脆性材料局部强度和表面残余应力的压痕测试方法,并建立了一种简便的强度保证试验技术,对脆性构件的非破坏性能评价有实用意义.     

等离子喷涂纳米结构与传统结构热障涂层的残余应力对比研究

热障涂层的残余应力是影响其服役寿研究不多.在45钢基体上,用超音速火焰喷涂NiCocrAlY打底层,再用大气等离子喷涂ZrO2-8%(质量分数)Y2O3(8YSZ)工作层,制备了纳米结构与传统结构2种类型的热障涂层(TBC).采用SEM、XRD对这2种涂层的粉末及涂层进行了组织结构分析,用纳米压痕仪测得了2种涂层的弹性模量.用X射线衍射应力测试仪测得了2种涂层的表层残余应力.结果表明:喷涂工艺参数相同条件下,对于打底层及工作层的厚度均相同的2种涂层,纳米结构热障涂层的表层残余应力比传统结构热障涂层约低24.7%;相同打底层的纳米结构热障涂层表层残余应力随着陶瓷层厚度增加而增加,陶瓷层厚度在240 um以下时,表层为残余压应力;厚度超过300 um时,表层为残余拉应力.最后提出了相应的物理力学模型.     

残余应力对固体氧化物燃料电池弹塑性性能的影响

半电池结构NiO-YSZ/YSZ由于弹性模量不同和热膨胀系数不匹配,导致烧结过程中产生残余应力. 残余应力对于燃料电池的性能和使用带来一定的影响, 本文把残余应力引入到计算薄膜性能的逆向分析模型中, 建立了考虑残余应力影响的薄膜的纳米压痕分析模型. 利用纳米压痕方法测试了离双层材料界面不同距离处的载荷-位移曲线和相应的材料性能, 用本文建立的模型计算了不同点的性能的变化, 发现离界面越远, 电解质YSZ薄膜的硬度越大. 将热力分析得到的残余应力场作为压痕模拟的初始应力场, 计算三棱锥压头下压痕载荷-位移曲线, 结果显示考虑残余应力时的载荷-位移曲线更接近实验曲线, 并给出了残余应力下压痕形貌图, 发现有残余应力时的压痕形貌更深更大.     

连续纤维增强陶瓷基复合材料微观力学研究进展

微观力学参数是构建连续纤维增强陶瓷基复合材料(CFRCMCs)组分、微观结构和宏观力学性能的桥梁, 但受限于CFRCMCs的脆性和微观力学参数测试水平, 微观力学研究工作进展缓慢。随着基于纳米压痕的微观力学测试技术和基于聚焦离子束微观测试样品制备技术的飞速发展, 近年来CFRCMCs的微观力学研究工作取得显著进步。本文结合国防科技大学刘海韬课题组的研究工作, 重点对CFRCMCs组分的原位模量、断裂韧性以及界面结合强度的测试方法和典型应用进行了讨论, 最后举例说明了基于微观力学参数的CFRCMCs宏观力学行为的预判方法。     

Fracture toughness and evaluation of coating strength with an initial residual stress field

The effect of residual elastic stresses on the geometry of cracks which arise with contact and spontaneous failure of brittle coatings made of high-strength compounds is studied. Conditions are established for the correctness of fracture toughness K_Ic tests with indentation of a standard Vickers pyramid as applied to surface layers with an inhomogeneous structure and an initial residual stress field. Taking account of the anisotropy of fracture toughness established by experiment a reliable approach is suggested for evaluating the brittle strength of coatings in the presence of residual stresses.Translated from Problemy Prochnosti, No. 1, pp. 51–61, January, 1994.     

Effect of soft substrate on the indentation damage in silicon carbide deposited on graphite

Indentation-induced damage is investigated in silicon carbide (SiC) deposited on graphite substrate. The SiC films have been grown by LPCVD (Low Pressure Chemical Vapor Deposition) method using MTS (CH₃SiCl₃) as a source gas and H₂ as a diluent gas to provide highly dense deposited layer and strong interfacial bonding. The elastic-plastic mismatch is very high to induce distinctive damages in the coating and the substrate layer. The specimens with various coating thicknesses are prepared by changing CVD condition or mechanical polishing. Indentation damages with different sizes are introduced by controlling indentation load in Nanoindentation, Vickers indentation and Hertzian indentation test. Basic mechanical properties such as hardness, toughness, elastic modulus are evaluated against coating thickness. Mechanical properties are sensitive to the indentation load and coating thickness. The results indicate that coating thickness has a vital importance on the design of hard coating/soft substrate system because the soft substrate affects on the mechanical properties.     

Evaluation by Vickers indentation of fracture toughness of a phosphate biodegradable glass

Indentation tests are commonly used for the evaluation of fracture toughness of brittle materials, particularly glasses and ceramics, because this technique requires only a small polished area on the specimen surface from which a large number of data points can be generated rapidly. However, a wide variety of equations for the calculation of fracture toughness of ceramic materials by means of Vickers indentation are available. Such equations are obtained phenomenologically and their parameters adjusted in such a way that the K_IC values obtained are in good agreement with those obtained by conventional methods. This is the reason why it is necessary to check which type of equation reproduces more accurately the results obtained by means of conventional methods for the material which is going to be investigated. In the present work seven different fracture toughness equations widely used in glass and ceramic studies are considered and the results are compared with those obtained by conventional methods, such as single-edge notch beam (SENB) specimens tested in three-point bending. The role played by the applied indentation load is considered. © 1999 Kluwer Academic Publishers     

Evaluation of the fracture toughness of Nb-40Al-8Cr-1W-1Y-0.05B intermetallic material by indentation techniques

The fracture toughness of an Nb-40Al-8Cr-1W-1Y-0.05B intermetallic material was evaluated by indentation techniques at room temperature. Two widely used indentation methods, crack size measurement and indent strength, yielded excellent agreement with a conventional fracture toughness technique using straight-through precracked specimens, despite the occasional formation of poorly configured cracks. However, the modified indentation technique, using dummy indent flaws, resulted in a low fracture toughness compared to that evaluated by the other methods. The material did not exhibit rising R-curve behaviour, as evaluated from the indentation strength data. These results indicate that indentation fracture principles are applicable to this brittle intermetallic material without modification of the residual contact stress term originally calibrated for ceramic materials.     

用于测量陶瓷断裂韧性的Vickers和Knoop压痕裂纹技术

采用 Vickers 压痕裂纹测量法或 Knoop 压痕～强度法等压痕技术进行了陶瓷的断裂韧性测量。反应烧结 Si_3N_4,热压 Si_3N_4和 SiC 等结构陶瓷用于研究显微结构非均质性和热残余应力对断裂韧性值的影响。人工水晶用于研究晶体学取向和断裂韧性之间的关系。研究发现,Knoop 压痕～强度法比 Vickers 压痕裂纹测量法更适合于非等轴晶系的晶体断裂韧性测量。其原因是 Knoop 压痕的主裂纹和缺口试样的裂纹都沿着同样的晶面扩展,而 Vickers 径向裂纹对却须沿着相互正交的晶面扩展,仅其中的一个晶面与缺口试样的裂纹扩展面一致。在 Vickers 压痕裂纹技术的基础上,用以计算 K_(1c)值不含弹性模量 E 项的 Evans 式(21),似乎能显示出显微结构对 K_(1c)值的影响;含有弹性模量 E 项的 Evans 式(22),似乎能反映出残余热应力对 K_(?)值的影响。所有上述的陶瓷材料,除了反应烧结 Si_3N_4,其压痕和裂纹关系 a~2～C~(3/2)的线性回归相关系数都相当,甚至>0.99。这表明了 Vickers 压痕技术在测定陶瓷的断裂韧性方面具有一定的实用性。     

A new method of fracture toughness determination in brittle ceramics by open-crack shape analysis

Improvement of the fracture toughness of high-quality ceramics remains one of the most important goals in materials development. An associated problem is the accurate measurement of fracture toughness in such brittle or semi-brittle ceramics, particularly in small samples encountered in material development. Previously used methods relying on measurement of the size of fracture mirrors, the indentation load and crack length in Vickers hardness-induced cracking, and a variant of similar techniques, have all been less than satisfactory in discriminating quantitative differences among materials. A hitherto unused technique of inferring the fracture toughness in samples from measurements of open-crack flank displacements, which we have developed, avoids most of the theoretical and experimental difficulties of other methods. While it is somewhat intensive in terms of evaluation and requires high resolution of open cracks, the technique is fundamentally the soundest of all techniques and is capable of furnishing discriminating results. We present results of its application to the measurement of some model materials such as soda–lime glass, single-crystal silicon, alumina, and a reaction-bonded silicon nitride whose porosity would ordinarily present difficulties with other methods. This revised version was published online in November 2006 with corrections to the Cover Date.     

维氏压痕对常压固相烧结碳化硅陶瓷材料力学性能的影响

为了研究维氏压痕裂纹对常压固相烧结碳化硅陶瓷(SSiC)材料力学性能的影响, 通过扫描电镜观察了0.1~100 N的压痕载荷下产生的表面裂纹及裂纹剖面的状况, 并测试了相应载荷下的力学性质, 探讨了压痕法测量SSiC材料硬度、韧性等力学性质的适当压力载荷. 结果表明, SSiC材料压痕裂纹起始的临界压力载荷介于0.1~0.2 N; 当压痕载荷小于0.5 N时, 裂纹尺寸小于5 μm, SSiC材料的平均弯曲强度受影响程度较小. 此外, 当压痕载荷为10 N以上时, 压痕法测得的维氏硬度值趋近定值, 且所得到的裂纹是半圆形裂纹, 因此, 10 N为采用压痕法准确测量SSiC材料硬度及韧性的最低压痕载荷值.     

Vickers indentation fracture toughness test Part 1 Review of literature and formulation of standardised indentation toughness equations

Abstract

There is considerable interest in determining the fracture toughness of brittle materials by measuring the extent of cracking associated with a Vickers indentation because of the ease of specimen preparation and the simplicity of the test. However, confusion has been engendered by the multitude of models and equations in the literature relating the degree of cracking to the fracture toughness. In Part 1 of this work, nineteen of these equations are reviewed and then modified in a standard manner for both experimental convenience and direct comparison.

MST/1050a     

Contact-damage-resistant ceramic/single-wall carbon nanotubes and ceramic/graphite composites

There has been growing interest in incorporating single-wall carbon nanotubes (SWNTs) as toughening agents in brittle ceramics. Here we have prepared dense Al(2)O(3)/SWNT composites using the spark-plasma sintering (SPS) method. Vickers (sharp) and Hertzian (blunt) indentation tests reveal that these composites are highly contact-damage resistant, as shown by the lack of crack formation. However, direct toughness measurements, using the single-edge V-notch beam method, show that these composites are as brittle as dense Al(2)O(3) (having a toughness of 3.22 MPa m(0.5)). This type of unusual mechanical behaviour was also observed in SPS-processed, dense Al(2)O(3)/graphite composites. We argue that the highly shear-deformable SWNTs or graphite heterogeneities in the composites help redistribute the stress field under indentation, imparting the composites with contact-damage resistance. These composites may find use in engineering and biomedical applications where contact loading is important.     

Study of the residual stress field around Vickers indentations in a 3Y-TZP

Indentation cracks are often used as initial flaws in ceramics for different mechanical tests because of their unique advantages. The residual stresses around the indent due to the elastic/plastic contact must generally be relaxed, which can be conducted by heat treatment. The stress field around Vickers indentation and conditions of annealing have been analysed on a Y-TZP material. Different heat treatments from room temperature up to 1200 °C have been conducted. The residual stress field has been characterized by different methods. First, stable crack propagation in the residual stress field has been conducted by applying a bending stress to the indented specimens. The apparent toughness has also been measured by conducting fast fracture on indented specimens. Finally the residual stress present around the indentation has been measured by making micro-indentations and by measuring the crack lengths. The results show that at intermediate temperatures up to 600 °C an apparent stress relaxation occurs due to the tetragonal to monoclinic phase transformation which induced a superimposed compressive stress. The higher temperature of 1200 °C effectively leads to a real stress relaxation without healing the crack.