Evaluation of surface cracking in micron and sub-micron scale scratch tests for optical glass BK7

In order to obtain the fundamental information on the deformation and fracture behavior of brittle materials during precision and ultra-precision grinding, micron and sub-micron scale scratch tests were conducted on optical glass BK7 using Vickers indenters. Three types of surface cracking were observed around the scratch grooves. They are lateral cracking, radial cracking and cracking in front of the moving indenter. It is found that lateral cracking is the main damage type due to its large damage size and low crack initiation load. The effect of surface cracking on the relationship between the normal load and the square of scratch depth was studied. The plastic zone size as well as the sliding blister field strength was expressed as a function of the contact zone size of the indenter. A prediction model for the size of damage zone induced by lateral cracking was established and was compared with experimental results.     

Characterisation of DLC coating adherence by scratch testing

In order to characterize the adherence of DLC coatings (Diamond Like Carbon), scratch testing was performed on a unit equipped with sensors for normal and tangential forces, and an acoustic detector to detect the nucleation and the propagation of cracks. The system is also equipped with a microscope permitting observation of each event on the scratch according to the friction tangential force signal or the acoustic signal. The local microscopic observation allows identification of the damage with respect to the normal load. The test was performed with a Rockwell C indenter at the relative displacement speed, v=10 mm/min under a progressive normal load from 5 to 55 N.Coating failure appears in various modes, particularly the following: propagation of the cracks along the longitudinal edges of the scratch; propagation in front of the indenter; rupture along the maximum principal stress lines; and, detachment in the subsurface by shearing of the coating. The microscopic analysis of the evolution of the scratch under a progressive normal load permits identification of the various traces and the damage mechanisms of the coating.In this study, experimental results are shown for the scratch tests on bulk glass and DLC coating. Various modes of crack initiation, damage and rupture of these materials according to the critical normal load are presented. The analysis of the contact stress field distribution in bulk glass enables identification of the crack initiation and its propagation in the coating.     

Scratch Behavior of ZrO<Subscript>2</Subscript> Thin Film Prepared by Atomic Layer Deposition Method on Silicon Wafer

To evaluate friction behaviour and adhesion strength of 130 nm-thick zirconia (ZrO₂) film produceded by atomic layer deposition (ALD) on Si substrate, scratch tests were performed at two different scales; micro- and macro-scales. Surface morphology, roughness, crack propagations and interations between the surface and sliding indenter were also investigated. Scratch test was also conducted with Si substrate as a reference. The test results showed that friction coefficient (COF) was influenced by the scale of scratch test regardless of the tested materials. In microscale test, the 130 nm-thick zirconia film failed at the critical load of 96 mN and direct relationship between the generation of micro-cracks and friction coefficient was observed. Based on the Hertzian contact theory and experimental results, the macroscratch width was much greater than that in microscratch test although the maximum contact pressure were comparable in both cases. Further discussion was made with regard to the influence of the contact pressure on COF, crack generation and film removal. Various types of failure mode were identified through analyzing the mechanical response of scratch tracks both at micro- and macroscale tests. This study suggested that 130 nm-thick ALD-ZrO₂ film showed better tribological and adhesion properties at microscale contact than macroscale contact.     

Tribological analysis of fracture conditions in thin surface coatings by 3D FEM modelling and stress simulations

A tribological analysis of deformations and stresses generated and their influence on crack generation and surface fracture in a coated surface loaded by a sliding sphere in dry conditions is presented. A three-dimensional finite element method (3D FEM) model has been developed for calculating the first principal stress distribution in the scratch tester contact of a diamond spherical tip with 200 μm radius sliding with increasing load on a 2 μm thick titanium nitride coated steel surface. The model is comprehensive in that sense that it considers elastic, plastic and fracture behaviour of the surfaces. The hard coating will be stretched and accumulates high tensile stresses. At the same time, it is carrying part of the load and thus reducing the compressional stresses in the substrate under the sliding tip. The first crack is initiated at the top of the coating from bending and pulling actions and it grows down through the coating. The fracture toughness of the coating is calculated by identifying from a scratch test experiment the location of the first cracks and the crack density and using this as input data.     

Effect of indenter geometry and relationship between abrasive wear and hardness in early stage of repetitive sliding

This paper deals with the abrasive wear of 5xxx aluminium alloys, which is a major problem in industrial applications. First, the influence of work hardening on abrasive wear resistance is investigated. A work hardened and annealed aluminium alloy is subjected to repeated scratch test using a sphere/plane configuration. It is shown that the work hardening increases the abrasive wear resistance in early stage of repetitive sliding. In the second part, the influence of the geometry indenter on abrasive wear is investigated. Repeated scratch tests with conical indenter were carried out on the same aluminium alloys. Scratch test using a conical indenter was shown that work hardening treatment (H24) on the 5xxx aluminium alloy does not improve wear resistance. Moreover, the conical indenter induces greater wear than the spherical indenter in repetitive sliding with few number cycles (10 cycles). An accurate interpretation of these phenomena is then proposed.     

On the possibility of evaluating the resistance of materials to wear by ploughing using a scratch method

Although the plastic strain that a material can withstand in sliding contact is an important mechanical characteristic determining its resistance to ploughing wear, this material property could not be measured so far. In this study, a scratch test to measure the critical plastic strain is explored. A hard spherical indenter scratches the tested surface under progressive loading to induce an increasing plastic strain on the surface. When the plastic strain exceeds the limit of plastic deformation on the surface, micro-fracture takes place at the ridges of the scratch groove. The critical load to initiate this micro-fracture is detected in the scratch test and then the critical plastic strain is calculated. A measure of the resistance of a material to wear by ploughing, this critical plastic strain to micro-fracture has been used both for evaluating the cohesion of coatings and for screening wear-resistant materials.     

Stick–Slip and Temperature Effect in the Scratching of Materials

The stick–slip process and temperature effect in scratch testing of materials have been studied. For a poly(methyl methacrylate) (PMMA) polymer scratched by a conical diamond indenter, both the amplitude and period of the stick–slip at room temperature increase with the normal load and decrease with the driving speed. An increase in temperature leads to an increase in stick–slip amplitude and period, as well as in the average horizontal force. For bismuth metal scratched by a conical tungsten indenter, the surface temperature in the contact area reveals fluctuations related to the stick–slip motion.     

Effects of Aryl Phosphate Ester Lubricant Additives on Crack Growth in Soda-Lime Glass

The growth of cone cracks in soda-lime glass during static loading with a spherical indenter shows that crack growth is accelerated at low energy release rates (low crack velocity) by tricresyl phosphate. Other aryl phosphate ester lubricant additive mixtures that contain predominantly larger molecules, as well as a synthetic hydrocarbon oil formulated with one of the mixtures, do not show enhancement of crack growth. Although reactivity in a test with static load can predict reactivity in a cyclically loaded test, lack of reactivity in a static test may not predict lack of reactivity in a cyclic test. Comparative tests with neat hydrocarbon oil and formulated oil under the same cyclically loaded conditions are needed to definitively determine whether the additives that are unreactive in these static tests influence crack growth in a bearing.     

Rheology of Thin Organic Films

This paper deals with the effects of contact pressure and temperature on the shear strength of thin organic films. The experimental method involves depositing the material as thin films (ca. 3 nm to 500 nm) on smooth glass surfaces. The film is sheared by sliding over it indenters of fired glass. By varying the indenter radius from 4 μm to 2.5 mm and the load from 10 mg to 20 g, the contact pressure may be varied from 10⁷ Pa (1.4 × 10³ P.s.i.) to 8 × 10⁹ Pa (12 × 10⁵ p.s.i.). The temperature dependence of the shear strength is also studied. Two types of organic materials have been investigated. These range from simple low molecular weight compounds such as stearates, to more complex high molecular weight polymers, for example P.M.M.A.

The shear strength of these films has been compared with the bulk shear properties and consideration has been given to the molecular processes occurring during shear.     

超高速陶瓷CBN砂轮纳米陶瓷结合剂性能实验研究

以超高速陶瓷CBN砂轮的结合剂低温高强性能要求为目标,在以化学纯原料为主的R2O+RO-B2O3-Al2O3-SiO2玻璃体系基础上,引入纳米改性剂来对陶瓷结合剂基体进行改良强化。4种成分结合剂性能的测试结果表明,纳米陶瓷结合剂的抗折强度、耐火度、浸润性与普通陶瓷结合剂相比有着显著优势。4号纳米陶瓷结合剂的抗折强度达到了83.75MPa,耐火度降至795℃,其膨胀系数也与CBN磨料更为接近,是一种更适合用于制备超高速陶瓷CBN砂轮的结合剂。     

Recurrent Morphology in the Scratching of Polymers

When a smooth sample surface is scratched by an indenter at constant normal load and driving speed, a macroscopically smooth groove is expected to be made on the surface. However, microscopically the scratch deformation is not stable, damage occurs intermittently along the scratching path so that recurrent morphology appears on the scratched surface. Examples of such morphology are described and deformation mechanisms are discussed together with the characteristics of horizontal force during the scratching of three polymers.     

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。     

陶瓷磨削表面残余应力对零件强度的影响研究

探讨了陶瓷磨削表面残余应力对断裂强度的影响。采用理论分析与实验检测相结合的方法 ,对残余应力行为包括残余应力数值、性质、作用深度和变化梯度及其对强度的影响进行了研究 ,研究结果表明 :残余应力对材料断裂强度的影响是通过磨削裂纹实现的 ;表面残余应力的数值、性质、作用深度和变化梯度的综合作用决定了断裂强度的性能 ;实质断裂强度性能受到了外加载荷与磨削残余应力的组合应力的制约。     

MD simulation of indentation and scratching of single crystal aluminum

Molecular Dynamics (MD) simulations of indentation and scratching have been conducted on single crystal aluminum in various crystal orientations and directions of scratching to investigate the anisotropy in hardness and friction. Depending on the crystal orientation, the atoms near the surface are found to be disturbed to different degrees due to repulsive forces between them as the indenter approaches the workmaterial. The hardness is found to increase significantly as the indentation depth is reduced to atomic dimensions. The calculated values of hardness are found to be an order of magnitude higher (and close to theoretical strength) than the corresponding engineering values which can be expected considering the size effect possible at indentation depths of a few nanometers or less. It thus appears that at very low depths of indentation (or nanoindentation), the plastic deformation underneath the indenter is governed by the theoretical yield strength of the material. The anisotropy in hardness and friction coefficient of single crystal aluminum with different crystal orientations and scratch directions is found to be in the range of 29%, which is close to the value of its anisotropy in the elastic range (21.9%) (stiffest in 111 and least stiff in 100) [R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 4th edn., Wiley, 1996, p. 14]. A similar observation was made in a recent investigation on the nanometric cutting of single crystal aluminum [R. Komanduri, N. Chandrasekaran, L.M. Raff, M.D. Simulation of Nanometric Cutting of Single Crystal Aluminum-Effect of Crystal Orientation and Direction of Cutting, 1998, accepted for publication in Wear]. Among the orientations investigated, hardness is maximum in (001)[100] and minimum in (01 )[221]. Friction coefficient values are found to be higher (0.6–0.9) with the maximum along (001)[ 10] and minimum along (110)[ 10]. The [ 10] scratch direction represents the close packed direction for aluminum. The minimum and the maximum scratch hardness are observed with (111)[ 10] and (111)[ 11] crystal orientations. Although, similarities are found between nanoindentation and scratching, and nanometric cutting, the rake angle effect is found to be dominated by the large negative rake angle presented by the indenter in the former case.     

纳米SiO2颗粒和玻璃微珠共混改性超高分子量聚乙烯复合材料的摩擦磨损性能

采用模压成型工艺制备了纳米SiO2颗粒和玻璃微珠共混改性的超高分子量聚乙烯复合材料;研究了相对滑动速度、载荷以及玻璃微珠含量对复合材料摩擦磨损性能的影响,并对磨损形貌和磨损机理进行了分析。结果表明:添加纳米SiO2颗粒和玻璃微珠可以提高复合材料的硬度、压缩弹性模量和摩擦磨损性能;相对滑动速度对复合材料摩擦因数和磨损率有很大的影响;载荷对复合材料的摩擦因数影响不明显,但磨损率随载荷的增加而增大;纳米SiO2颗粒和玻璃微珠混合改性后复合材料的磨损机理主要是粘着磨损和疲劳磨损。     

Effect of load and speed on the wear behaviour of woven glass fabrics and aramid fibre-reinforced composites

In the present study, wear behaviour of woven 300 and 500 glass fabrics and aramid fibre-reinforced composite materials are experimentally investigated for 500 and 710 rpm speeds and at two different loads of 500 and 1000 g using a block-on-shaft wear tester. The wear in the experiments was determined as weight loss. The weight losses were measured after different sliding distance conditions. The worn surfaces were also examined by scanning electron microscope (SEM). As a result of this study, it is shown that the applied load on the specimens has more effect on the wear than the speed. Also the weight loss in the woven 500 glass fabric reinforced is more than that in the woven 300 glass fabric-reinforced composite. The weight loss of aramid fibre-reinforced composite is quite low compared with woven glass fabric-reinforced composites.     

Dry sliding wear behaviour of PTFE filled with glass and bronze particles

Abstract

The wear behaviour of polytetrafluroethylene (PTFE) filled with 25% glass and 40% bronze particles was studied on a pin on disc test rig. Solid lubricant composite materials were prepared by compression moulding technique. The wear parameters considered for the study were applied load, sliding speed and sliding distance. The experimental results indicate that the weight loss increases with increasing load, sliding speed and sliding distance, as expected. Sliding distance has more effect on weight loss followed by applied load. The 40% bronze+PTFE composite exhibits better wear resistance compared to other types. The dominant interactive wear mechanisms during sliding of PTFE and its composites are discussed in this paper.     

3D crack network analysis during a scratch test of a polymer: A combined experimental and multigrid X-FEM based numerical approach

The scratch behavior of a thermoset solid polymer exhibiting brittle behavior in tension is investigated. The surfaces are scratched and an imaging system is used to record real time photographs. The 3D crack pattern is analyzed using fluorescence confocal laser scanning microscopy. A finite element simulation is performed to determine the indenter/specimen contact conditions. They are used as input data for the 3D crack network analysis based on combined 3D localized multigrid and X-FEM/level set techniques. The computed stress distributions within the 3D cracked specimen are of great interest to understand the crack network formation observed during the experiments.     

Sub-surface crack formation in ultrasonic vibration-assisted grinding of BK7 optical glass

Due to the inherent properties of high brittleness and low fracture toughness of optical glass materials, sub-surface cracks would be inevitably induced into the ultrasonic vibration assisted grinding process. Knowledge of the formation mechanisms of sub-surface cracks plays a key role in implementing high efficiency and precision machining of this kind of materials. In this work, the ultrasonic vibration assisted grinding experiments of BK7 optical glass were carried out. Processed by cross-sectional polishing and assisted by HF acid etching, quite a few sub-surface cracks with different shapes were observed beneath the machined surfaces. Categorized by the shapes and formation mechanisms, four kinds of sub-surface cracks (i.e., straight median sub-surface crack, arc median sub-surface crack, lateral sub-surface crack, and bifurcated sub-surface crack) and their corresponding formation mechanisms were both clarified. Experimental and analytical results suggest that the arc sub-surface cracks were formed by the relative shear stresses parallel to the plane of median crack frontiers, which are generated by nonsymmetric contacting between abrasive grains and glass specimen in ultrasonic vibration grinding. The bifurcation cracks were formed by the distortion in frontier fields caused by the impact effect provided by ultrasonic vibration as the abrasive grains vibrate down to the glass material. Effects of grinding and ultrasonic vibration parameters on the maximum depth of sub-surface cracks were also investigated in this work. The value of the maximum depth of sub-surface cracks showed a great dependence on the processing parameters.     

Tribological behaviour of steel‐alumina sliding pairs under boundary lubrication conditions

The tribological behaviour of oil‐lubricated steel‐alumina sliding pairs was investigated using a ball‐on‐disc tribometer at room temperature. Commercial bearing balls of 10 mm diameter were mated to 99.7% Al₂O₃ discs, and additive‐free mineral oil was fed into the contact area. The sliding speed and the applied normal load were varied, and the initial surface roughness of the Al₂O₃ disc was altered using different polishing and grinding procedures. The results showed that the surface roughness of the ceramic discs dominated the tribological behaviour under the given experimental conditions. The sliding speed as well as the normal load showed less effect on the friction behaviour, but the amount of wear depended strongly on the normal load. From the results it was concluded that improvement of the surface roughness and optimised surface machining of the ceramic material can be essential for improving the tribological performance for boundary‐lubricated steel‐ceramic sliding pairs.