Examination of microhardness indentation-induced subsurface damage in alumina platelet reinforced borosilicate glass using confocal scanning laser microscopy

Borosilicate glass and borosilicate glass matrix composites reinforced with 10, 15 and 30 vol.% alumina platelets have been indented and the subsurface lateral cracking examined by confocal scanning laser microscopy (CSLM). The suitability of the CSLM technique as a quick and nondestructive method of obtaining three-dimensional information of subsurface damage in dispersion reinforced brittle matrix composite materials was demonstrated. The addition of alumina platelets to the glass matrix has resulted in a reduction in the extent and depth of subsurface damage due to indentation, and hence may make the material more resistant to erosive wear. This damage development may be a consequence of the presence of residual compressive stresses in the matrix and the strong platelet/matrix interfacial bonding.     

Flaking failure in rolling contact fatigue caused by indentations on mating surface (III): Mechanism of crack growth in the direction opposite to the load-movement

In rolling contact fatigue, we have discovered that a flaking failure accompanied by cracks extending bi-directionally relative to the load-movement occurs on a defect-free surface due to the influence of indentations on a mating surface. We have also demonstrated, using stress analysis, the initiation of incipient cracks in the subsurface region resulting from indentations on the mating surface. In the present study, we focus on the stage of crack extension in the direction opposite to the load-movement. Firstly, mode II fatigue crack growth properties are investigated by conducting mode II fatigue testing, which has been previously presented. Subsequently, stress intensity factors at a subsurface crack are calculated by using a finite element method for the case of an indentation on the mating surface moving through the contact area. In this calculation, the configuration of the modeled crack is very similar to cracks actually observed. Based on the results of both the stress analysis and the cracking seen in actual tests, the mechanism for the development of opposite-directional cracking is proposed to be a mode II extension of an incipient subsurface crack.     

Impact wear of MgO single crystals II. Impact and wear damage

The impact wear damage of MgO single crystals was investigated under an impact load of 60 kgf. Two types of impact damage, a zero wear process and a measurable wear process, are distinguished. The impact scar shows three types of cracks: subsurface cracks parallel to the surface, and tangential and radial cracks on the surface. In the early stages of the wear process the impact scar is surrounded by surface and subsurface cracks and grows as a unit square cell; wear occurs at the scar bottom and on the scar wall. Consequently the profile of the wear scar changes to a hemispherical or parabolic shape. The zero wear limit under each impact load is determined from the static contact stress and the number of impacts.     

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.     

Subsurface Damage in Scratch Testing of Potassium Dihydrogen Phosphate Crystal

Potassium dihydrogen phosphate (KDP) is an important electro-optic crystal, often used for frequency conversion and Pockels cells in large aperture laser systems. To investigate the influence of anisotropy to the depth of subsurface damage and the profiles of cracks in subsurface of KDP crystal, an experimental study was made to obtain the form of subsurface damage produced by scratches on KDP crystal in [100], [120] and [110] crystal directions on (001) crystal plane. The results indicated that there were great differences between depth and crack shape in different directions. For many slip planes in KDP, the plastic deformation and cracks generated under pressure in the subsurfacewerecomplex. Fluctuations of subsurface damage depth at transition point were attributed to the deformation of the surface which consumed more energy when the surface deformation changed from the mixed region of brittle and plastic to the complete brittle region along the scratch. Also, the process of subsurface damage from shallow to deep, from dislocation to big crack in KDP crystal with the increase of radial force and etch pit on different crystal plane were obtained. Because crystallographic orientation and processing orientation was different, etching pits on (100) crystal plane were quadrilateral while on (110) plane and (120) plane were trapezoidal and triangular, respectively.     

Using vibration-assisted grinding to reduce subsurface damage

This paper discusses a technique for reducing subsurface damage and/or increasing material removal rate (MRR) in ceramics grinding. An indentation damage model shows that intermittent unloading can produce a lateral crack before the median crack fully develops. It shows that upon reloading, the lateral crack shields against further median crack penetration. Furthermore, intermittent unloading produces shielding even for oblique indentation events and intermediate locations of lateral cracks. Single-grit scratch tests provide experimental validation of the beneficial effects of intermittent unloading. A magnetostrictive actuator modulated the workpiece to create the intermittent unloading. Simulation studies predict that force per grit can be doubled under modulated conditions without an increase in normal damage in the finished workpiece. Our experimental observations show a 62% increase in MRR with minimal associated increase in depth of penetration of the median crack. Alternatively, the experimental results also show that modulations could be used to reduce the depth of median crack penetration by 24% at the same MRR. In addition, we observed that damage depth increases with an increase in the ratio of cutting speed to modulation frequency. Finally, both depth of cut modulation and cutting direction modulation were effective in reducing subsurface damage.     

EFFECT OF GRINDING ON STRENGTH OF TETRAGONAL ZIRCONIA AND ZIRCONIA-TOUGHENED ALUMINA

Abstract

The effect of grinding-induced damage, on the strength of a yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and a zirconia-tough-ened alumina (ZTA) was investigated. The four-point flexure test was used to measure fracture strength as a function of grinding conditions. Flexure bar specimens were prepared by surface grinding transverse to the tensile stress direction in flexure testing. Two series of grinding experiments were performed. In the first series, four wheels with diamond particle sizes ranging from 25 to 180 μ,m were used to grind specimens at a fixed depth of cut. The strength of the Y-TZP decreased slightly with increasing diamond particle size. For ZTA the strength loss was increased as diamond particle size was increased. In the second series, one grinding wheel with a diamond particle size of 180 μm was used to grind specimens at a depth of cut per pass ranging from 2.5 to 100 μm. The Y-TZP showed a slight decrease in strength when the depth of cut was increased. However, for ZTA, the strength first decreased, then increased with increasing depth of cut. The grinding-induced subsurface damage, observed by a bonded-interface sectioning technique, consisted of cracks for ZTA, but no subsurface cracks could be found for Y-TZP. The effect of machining damage on the strength of ZTA was explained using the observed subsurface cracks and residual stresses that were measured using an indentation technique     

Microstructural studies of the temperature-dependence of deformation structures around hardness indentations in ceramics

Various light microscopy and SEM techniques have been used to study the temperature dependence of deformation structures around hardness indentations in highly brittle ceramics (including single crystals of silicon and silicon carbide and polycrystalline forms cf SiC, Si₃N₄ and B₄C). Nomarski differential interferometry has enabled slip steps to be resolved around high-temperature indentations, allowing identification of the dominant slip systems and measurement of the extent of surface plasticity as a function of temperature. The occurrence of indentation fracture as a function of both temperature and specimen microstructure was studied by SEM methods (including stereo imaging), Nomarski differential interferometry and polarized reflection light microscopy. Generally, radial and lateral crack sizes increased with increasing temperature, due to increasing residual stresses around indentations caused by increasing indentation plasticity. Examples of microstructural control of crack paths are given for various single crystal and polycrystalline materials.     

非对称楔形压头印压玻璃裂纹产生与扩展规律的研究

研究了非对称楔形压头印压玻璃时裂纹的产生及扩展规律。根据非对称楔印压玻璃时典型的裂纹照片 ,可明显看出存在加载裂纹、横向裂纹和表面下裂纹 ,并且都偏离印压表面法线方向。试验表明 :加载裂纹的偏角αc 与楔角对称线偏角αa 总是符号相同 ;对于形状相似的压头 ,楔角对称线偏角越大 ,裂纹偏角也越大 ;对双斜压头而言 ,如果楔角不等 ,但楔角对称线偏角相等 ,裂纹偏角仍大致相等。     

The influence of complex surface geometry on contact damage in curved brittle coatings

The effects of complex geometry on contact damage in bi-layer systems composed of curved brittle coating layers on compliant polymeric substrate is investigated. Previous studies of this problem utilise relatively simple flat or singly curved (domed) model structures. It is not known the extent to which conclusions driven from such observations may extended to more complex (practical) geometry. Glass plates of 1000 μm thick are used as representative of the brittle coating layer, and epoxy filler under layer as representative of under layer support. A series of doubly curved specimens (having curvatures of 4 and 8 mm) are produced to allow investigation of the influence of complex curvature on the evolution of damage. The specimens are tested by indentation with spheres of 4 mm radius loaded along the convex axis of symmetry. For comparison, some specimens loaded parallel to the axis of symmetry but off-centre. The study explores the influence of supporting geometries on the conditions to initiate and propagate subsurface “radial” cracks, which are believed to be responsible for catastrophic failure of brittle-coating-based structures in certain applications, such as dental crowns. It is demonstrated that critical loads for initiation of radial cracks and the subsequent crack propagation are insensitive to complex geometry, so that simple monotonic indentation “axis and/or off-axis loading” with minimum geometrical complication “flat, simply curved” remains an appropriate route to study the evolution of radial cracks in practical brittle coating structures.     

磨削力对HIPSN陶瓷磨削亚表面裂纹的影响

采用金刚石砂轮是磨削热等静压氮化硅(HIPSN)陶瓷最常用的加工方法,但是被磨零件亚表面常常伴随裂纹、崩碎等加工损伤,因此研究裂纹扩展一直是工程陶瓷的热点问题。对磨削加工后的HIPSN陶瓷亚表面裂纹进行探究,分析其在磨削加工过程中产生裂纹的原因以及去除机理,研究结果表明在磨削过程中对裂纹进行适当的控制,可以提高陶瓷零件的可靠性。设置单因素实验,对不同磨削参数下HIPSN陶瓷的磨削力进行测量,通过扫描电镜(SEM)对亚表面裂纹和表面形貌进行观察,分析磨削力对亚表面裂纹的影响。实验结果表明:磨削力随着砂轮线速度的增大而减小,随着工件进给速度和磨削深度的增大而增大;当磨削力变大时,陶瓷亚表面裂纹扩展程度增加,表面形貌变差。在粗磨加工HIPSN陶瓷时,可以通过减小工件进给速度和磨削深度,提高砂轮线速度的方法来降低裂纹的扩展程度,能够有效降低后续工艺的加工时间和难度,提高表面质量。     

反复印压作用下光学玻璃的微裂纹交互作用研究

对超声振动辅助磨削加工中BK7光学玻璃材料表面及亚表面的微裂纹扩展过程中的交互作用进行研究,使用维氏金刚石压头进行了BK7光学玻璃二次印压实验来模拟超声振动作用影响下单颗磨粒对光学玻璃的反复印压作用,同时采用界面粘结法获得了不同印压载荷及印压距离下产生的压痕及微裂纹形态特征及分布情况。实验结果表明：在相同载荷加载情况下,第二次印压产生的亚表面中位裂纹扩展最大深度受到侧向裂纹影响减小了30μm,同时侧向裂纹闭合后在光学玻璃材料表面及内部产生破碎。基于压痕断裂力学理论,分析了准静态载荷作用下光学玻璃内部应力场的分布及应力场驱动下微裂纹的扩展机制,对超声振动效应影响下微裂纹扩展的交互作用进行研究。结果表明：磨削过程中使用轴向超声振动辅助,能够有效地降低光学玻璃材料亚表面裂纹的深度,改善亚表面及表面加工质量,同时促进了工件材料的去除。     

Flaking failure in rolling contact fatigue caused by indentations on mating surface (II): Formation process of flaking failure accompanied by cracks extending bi-directionally relative to the load-movement

In rolling contact fatigue, a surface crack normally extends in the direction of the load-movement (LM). However, we have found that flaking failure with bi-directional crack extension occurs on a defect-free roller surface when indentations are present on the opposed roller surface. In the present study, a stress-state analysis using the boundary element method was conducted to evaluate the effects on a defect-free surface of an indentation on the opposite roller surface moving through the contact area. It is shown theoretically and experimentally that the initiation of a small subsurface crack parallel to the contact surface takes place where the maximum stress concentration occurs induced by the indentations on the opposite roller surface. The small subsurface crack grows into a small flaking failure accompanied by cracks extending in the direction opposite to the path of the LM. Subsequently, the small flaking failure expands in the LM direction, resulting in larger flaking failure with bi-directional cracking.     

Mechanics of Hertzian cracking

We review the progress made at Oxford in recent years in the quantification of surface damage of ceramics by application of the Hertzian indentation test. This includes using micro-indenters to search out the surface density of flaws, as well as assessing the size distribution. Many analytical tools have been developed for processing the data obtained, including refined methods of analysing the cone crack using distributed strain nuclei techniques which accurately model the behaviour of cracks in steep stress gradients, and in particular the use of eigenstrain methods to quantify the growth of an initial surface thumbnail crack. The influence of elastic mismatch on the state of stress induced by the indenter is also discussed, and it is shown that the use of an indenter more compliant than the substrate leads to a more extensive annular zone of tension, thereby encouraging the formation of a ring crack. The remaining problem is an accurate analysis of the full cone crack itself, which has already been analysed using a special finite element formulation, and which is currently being re-examined by the use of axi-symmetric ring nuclei: this promises to provide a more complete and precise solution.     

Subsurface crack mechanisms under indentation loading

A linear elastic fracture mechanics analysis of plane-strain indentation of a homogeneous half-space with a subsurface horizontal crack was performed using the finite element method. Stress intensity factor results obtained for an infinite plate with a central crack subjected to far-field tension and a half-space with a frictionless subsurface horizontal crack under a moving surface point load are shown to be in good agreement with corresponding analytical results. Crack mechanism maps illustrating the occurrence of separation, forward and backward slip, stick, and separation at the crack interface are presented for different indentation load positions and crack face friction coefficients. Results for the stresses in the vicinity of the crack tips and the mode I and mode II stress intensity factors are given for different indentation positions, crack face friction coefficients, and both concentrated and distributed surface normal tractions. Although indentation produces a predominantly shear and compressive stress field, mode I loading conditions are shown to occur for certain indentation positions. However, the magnitude of the mode I stress intensity factor is significantly smaller than that of mode II, suggesting that in-plane shear mode crack growth is most likely to occur in the absence of microstructural defects. The significance of crack face friction and sharpness of the indenter on the subsurface shear mode crack propagation rate is interpreted in terms of the mode II stress intensity factor range and material behavior.     

Surface and Subsurface Cracks in Rolling Contact Fatigue of Hardened Components

The competitive aspect of surface and subsurface fatigue crack propagation in hardened components subjected to rolling contact fatigue is highlighted, the former being greatly affected by the working conditions (in particular the presence of tangential stresses and lubricant), the latter depending mainly on the inclusions content and on the hardness profile. In order to determine which one of these kinds of damage is favoured, initial data consisting of contact load, rolling and sliding speed, theological properties of the lubricant, material hardness and inclusions content are necessary.

The concurrent role of asperities and Hertzian stress field in determining surface crack propagation is explained with the approach of the “quiescent zone.” calculating the stress intensity factors range in a contact cycle and considering the pumping effect of the fluid possibly present on the contact surface.

Inherent defects (especially oxides) are thought to be responsible for subsurface cracks origin and the Murakami formula for short cracks is extrapolated to describe their growth threshold, which also depends on the hardness and therefore on the depth in surface hardened components.

A crack propagation index is then defined as a ratio of applied to threshold stress intensity factor, both for surface and subsurface cracks. Evaluating this index for a general operating condition, it is possible to determine which damage mechanism is favoured, taking into account the decisive effect of the hardness profile.     

Radon track imaging in CR-39 plastic detectors using confocal scanning laser microscopy

Confocal scanning laser microscopy (CSLM) has been used to provide the first images of radon track populations in two external CR-39 plastic detectors. Measurements of variables including track area distribution and estimates of the angle of track inclination (dip) derived from surface CSLM sections are presented. CSLM depth slices, combined with three-dimensional (3D) visualization techniques, provide a new, non-destructive way of examining the 2D and 3D geometry of the etched tracks within solid-state nuclear track detectors that may prove useful in complementing existing optical microscopy methods.     

[表面强化及损伤机理]硌伤形貌对车轮材料滚动接触疲劳特性的影响

采用重力摆锤在车轮试样表面冲击出不同形貌的硌伤坑,基于改进后的MMS-2A 型微机控制摩擦磨损试验平台,研究了未硌伤车轮以及两种硌伤形貌车轮的滚动接触疲劳特性。结合ABAQUS仿真分析的车轮硌伤后残余应力分布情况,探讨了硌伤坑附近裂纹的萌生及扩展机制。结果表明：与未硌伤车轮相比,车轮硌伤处次表层材料沿踏面垂直方向上出现的残余拉应力有助于促进裂纹的萌生和扩展。相同冲击能下,球形硌伤坑附近裂纹长度和角度均大于道砟形硌伤坑附近裂纹长度和角度。球形硌伤坑边缘次表层裂纹的裂纹面较宽,并沿着45°的扩展角度向内部扩展。     

润滑条件下中低碳空冷贝氏体钢的滚动滑动接触疲劳行为

研究了三种高强度或超高强度高韧性空冷贝氏体钢在油润滑条件下的接触疲劳行为。发现引起麻点剥落的疲劳裂纹不仅可在接触表面 ,也可以在距表面一定深度下的亚表面萌生 ,亚表面萌生裂纹的深度比理论计算最大剪切应力所在深度小近一个数量级 ,裂纹萌生由塑性变形和剪应力共同作用产生 ,一端向接触表面扩展 ,到达表面后润滑油被挤压进入裂纹中产生支点效果 ,另一端向最大剪应力深度扩展 ,最后在外力和润滑油支点共同作用下 ,由裂纹包围的金属屑被折断 ,形成疲劳剥落坑。钢的初始硬度或强度在低接触应力下对接触疲劳寿命影响不明显 ,在高接触应力下影响显著 ,初始硬度越高 ,接触疲劳寿命越长。     

Theoretical model of brittle material removal fraction related to surface roughness and subsurface damage depth of optical glass during precision grinding

Brittle material removal fraction (BRF) is defined as the area fraction of brittle material removed on machined surface. In the present study, a novel theoretical model of BRF was proposed based on indentation profile caused by intersecting of lateral cracks. The proposed model is related to surface roughness and the subsurface damage (SSD) depth of optical glass during precision grinding. To investigate the indentation profile, indentation tests of K9 optical glass were conducted using single random-shape diamond grains. The experimental results indicate that the indentation profile is an exponent function. To verify the proposed BRF model, BRF, surface roughness and SSD depth of K9 optical glasses were investigated by a series of grinding experiments with different cutting depths. The experimental results show that BRF is dependent on surface roughness and SSD depth. The relationship between BRF, surface roughness and SSD depth is in good accordance with the proposed theoretical model. The proposed BRF model is a reasonable approach for estimating surface roughness and SSD depth during precision grinding of optical glass.