Scratch Velocity Research Articles

Mechanical performance of metallic glasses during nanoscratch tests

Nanoscratch experiments on four bulk metallic glasses using a pyramidal diamond tip (Berkovich) indenter have been carried out under different loads and scratching velocities. There exists a general correlation between the hardness and wear resistance, i.e., a lower hardness produces a lower wear resistance. Scratch velocity appears to have little effect on the scratch depths of the four alloys studied. However, faster scratch velocity leads to finer shear bands. Compared with ribbon sample with the same chemical composition, Ti-based bulk glassy sample that experienced slower cooling rate during solidification, and thus contained less free volume, exhibits better wear resistance.

Instrumented indentation and scratch testing evaluation of tribological properties of tin-based bearing materials

This paper describes a study of some fundamental tribological behavior of two different tin-based bearing materials using scratch and Martens hardness techniques. The scratch test results obtained from the two different tin-based bearing materials were correlated with a function of scratch hardness and coefficient of friction. It is easily observed that the friction coefficients of WM5 are lower than those of WM2 under all scratch test conditions. Beside this, it is clearly distinguished that both materials give higher friction coefficients at higher normal loads and scratch velocities. Also it is found that scratch hardness values of the WM5 materials are higher than WM2 generally. Moreover, optical microscopy was used to determine the scratch hardness of two different tin-based bearing materials by examining the scratch widths. Scanning electron microscopic observations of two different tin-based bearing materials were performed in order to identify the scratch-deformation mechanisms and trace morphology. Beside scratch tests, Martens hardness tests were carried out to evaluate the indentation of materials by considering both the force and displacement during plastic and elastic deformation. By monitoring the complete cycle of increasing and removal of the test force, hardness values were determined.

Study of scratching Mg-based BMG using nanoindenter with Berkovich probe

Nano-scratch tests using nanoindenter with a Berkovich probe are applied to examine the scratch resistance of Mg-based bulk metallic glass (BMG). To realize nano-scratch behavior, a theoretical model of abrasive wear-resistant action on Berkovich is developed to determine the wear behavior. The experimental results of friction force are carried with the application of extensive normal forces. First, a low normal force (5 mN) is applied on Berkovich at different scratch velocities to observe its effect on Mg-based BMGs. The experimental result shows that the dominating wear phenomenon is the rubbing and/or ploughing mode when a slightly normal load is applied. Second, varied normal forces (5–400 mN) are applied on Berkovich at a constant scratch velocity. The experimental result shows that wear mode changes to cutting mode when the applied normal force rises. The indented depth increases when the applied normal force increases. Residual chips appear during the scratch test when the indented depth is about 1300 μm under the normal force of 200 mN. In this study, theoretical models of the forces such as tangential, friction and normal forces are analyzed.

Numerical study of scratch velocity effect on recovery of viscoelastic–viscoplastic solids

The scratch test is a classical way to investigate the abrasive resistance of coatings and substrates. Because of the complex phenomena involved, the use of refined finite element analysis is often required to analyze the influence of specific parameters. In this paper, the influence of the tip velocity on the scratch recovery of polymer-like time-dependent solids is qualitatively investigated. More precisely the response of three constitutive models is analyzed: an elastic–viscoplastic model, a linear viscoelastic model and finally a viscoelastic–viscoplastic model. This last model is an original assembly based on the connection in series of the elastic–viscoplastic model and the linear viscoelastic model. For that, a new method allowing the connection in series of two different rheological models in a FE code is presented. To analyze the numerical results, the concept of representative stress and representative strain rate of a scratch test is introduced.

Finite element modeling of the scratch response of a coated time-dependent solid

Scratch tests is one of the most efficient tests to investigate the mechanical resistance of coated and uncoated surfaces. Nevertheless, the complexity of material and interface makes difficult the comprehension of this test. For that purpose, efficient computational modeling is required. In this paper, we present a remeshing procedure specially developed for the computational modeling of scratch tests of coated materials. This procedure allows to perform scratch tests with high ratio penetration depth over layer thickness. Then, it is used to investigate the influence of the scratching velocity on the scratch behavior of a polymer substrate coated with a hard elastic coating. The substrate is considered as an elastic–viscoplastic solids and the coating follows a linear elastic behavior. First macroscopic results such as material deformation, scratch hardness and apparent friction coefficient are presented. Then the stress distribution in the film and at the coating/substrate interface are analyzed regarding the cohesive or interfacial failure of the system. A simple interfacial failure criterion is also proposed.

Scratch resistance of a polycarbonate + organoclay nanohybrid

A polycarbonate-based nanohybrid has been created containing 1 wt% of Bentone 2010, an organically modified montmorillonite. A micro-section on the nanohybrid obtained using focused ion beam (FIB) and field emission scanning electron microscopy (FESEM) was employed to observe the orientation of the nanoclay inside a polycarbonate (PC) matrix in the cross-section FIB-milled face. A micro-scratch tester was used to measure the scratch resistance in terms of residual (healing) depth Rh under progressive load and in sliding wear. Effects of the number of scratches, normal load and scratch velocity have been evaluated as a function of nanoclay orientation. In sliding wear (multiple scratching along the same groove), our nanohybrid reaches residual depth values that remain constant after a certain number of scratches, a manifes- tation of strain hardening. The number of scratches to induce strain hardening decreases as the normal applied load increases. SEM was used to characterize deformation and wear mechanisms that operate on contacts and the results related to the wear data.

Molecular dynamics study of scratching velocity dependency in AFM-based nanometric scratching process

Three-dimensional molecular dynamics simulations are performed to investigate the AFM-based nanometric scratching process of monocrystalline copper. The effects of scratching velocities (1, 10, and 100 m/s) on the chip pattern, scratching resistance, dislocation movement, and workpiece deformation are studied. The results show that the scratching resistance increases with the increase in scratching velocity. The higher scratching velocity results in larger chip volume and closer chip shape with more amorphous structure. The dislocations move well-regulated for lower velocities (1 and 10 m/s) than that for larger one (100 m/s). The area of workpiece material deformation region beneath the tool edge decreases with the increase in scratching velocity.

Molecular dynamics simulations of nanoscratching of 3C SiC

We have carried out molecular dynamics simulations of nanoindentation followed by scratching at constant depth on the Si-terminated (0 0 1) surface of 3C SiC. The dependence of the friction coefficient, scratch hardness, and wear on scratching depth, velocity, direction, and indenter size and shape are investigated. In general, both the scratch hardness and friction coefficient increase with indentation depth but decrease with increasing scratching speed. We also find that the scratch hardness and friction coefficient are anisotropic, i.e. they are larger when scratching in the [1 1 0] direction than in the [1 0 0] direction. The friction coefficient also depends on the rake angle of the tool and is larger for a pyramidal indenter, due to the presence of a large negative rake angle in this case. While the primary mode of wear observed for scratching is ploughing in good agreement with experimental studies, there is significantly more pile-up and chip formation in the [1 1 0] direction. We have also used the shortest path ring distribution, pair-correlation function, bond-angle distribution and bond number analysis to investigate the nature of the plastic deformation due to nanoscratching. We find that scratching leads to a partial amorphization of the material along the scratching trajectory. The size of the amorphization region increases with scratching velocity, thus explaining the decrease in scratch hardness and friction coefficient for higher scratching velocities.

Nanoscale near-surface deformation in polymer nanocomposites

The objective of the study is to understand the nanoscale near-surface deformation response of two polymer nanocomposite systems with significant differences in ductility during nanoscratching with a Berkovich indenter using a load of 1mN and a scratch velocity of 1μms−1. An accompanying objective is to investigate the commonality in surface deformation behavior between nano- and microscale deformation to reinforce the underlying fundamental principles governing surface deformation. An understanding of surface deformation response is accomplished through determination of physical and mechanical properties, structural characterization and electron microscopy analysis of surface deformation tracks and residual plastically deformed structures. The study suggests for the first time that the understanding derived from microscale surface deformation studies can be extended to nanoscale surface deformation. The microscale response in a polypropylene-based system is characterized by periodic multiple ripple-type deformation tracks that form via a mechanism identical to the periodic single-ripple-type tracks during nanoscale deformation. Similarly, in a less ductile polyethylene-based system, the periodic parabolic tracks and ironing mode of deformation during microscale deformation tend to be significantly reduced in intensity, with ironing being the primary deformation mechanism at the nanoscale. The surface deformation topography suggests that both micro- and nanoscale response is material specific. Additionally, the study suggests that reinforcement of polymers with nanoclay is a viable route to significantly decrease the susceptibility of polymeric materials to micro- and nanoscale deformation and can be discussed in terms of physical and mechanical properties of materials notably percentage crystallinity and elastic recovery.

Nanoindentation and nanomachining characteristics of gold and platinum thin films

Molecular dynamics (MD) simulation and an experimental method were carried out to study the effects of applied load, hold time, and temperature on nanoscratching and nanoindentation of gold and platinum thin films. The simulated results showed that the wear depth of gold decreased as the scratching velocity was increased and the temperature was decreased. The results also indicated that when the simulated nanoindentation of gold film hold time was increased, the plastic indentation depth and the plastic energy both increased. In addition, the experimental results showed that the groove depth for gold films was larger than that of platinum films under the same machining load. The wear depth and the surface roughness of platinum films were larger than those of gold films under the same lateral machining feed. Furthermore, the simulated plastic energy of gold films was compared during nanoindentation test.

Effects of humidity on nanometer scale wear of a carbon film

Influence of environmental humidity on scanning-scratched wear tests of a carbon film deposited by electron cyclotron resonance plasma sputtering are evaluated in both air and nitrogen with an atomic force microscope and a diamond tip. Wear depth is larger in higher humidity and also larger in air than in nitrogen. Effect of humidity on scratch load dependence and repeated scanning-scratch tests are larger in nitrogen than in air. Effect of humidity on wear is clearer in the repeated scanning-scratch test than in the scratch load dependence test especially in air, and also clearer at larger scratch load. In scratch velocity dependence test, there is a tendency that wear depth decreases with increase in logarithm of scratch velocity both in air and nitrogen. Wear models to explain the experimental results are discussed.

Scratch velocity and wear resistance

AbstractUsing a micro-scratch tester we have determined the influence of scratch velocity on the sliding wear resistance of several thermoplastics: polystyrene (PS), styrene-acrylonitrile, polyamide 6, polyethersulfone and polysulfone. Variable velocities (from 1 to 15 mm/min) were applied under two different load configurations: progressively increasing loads from 0.03 to 30 N and multiple scratching under constant loads from 2.5 to 10 N. Penetration and residual depth were measured along a 5 mm length. In the case of PS the scanning electron microscopy images of the progressive load scratch reveal a transition from shear yielding to crazing at low velocity. In multiple scratching tests the residual depths obtained at 1 mm/min are substantially higher than those at 15 mm/min for all thermoplastics tested. This result is explained by contact heating at higher speeds resulting in increased chain relaxation capability and thus more viscoelastic recovery. Scanning probe microscopy was used in order to observe surface damages. Except for PS, all other materials at all velocities exhibit asymptotic behaviour of the residual depth as a function of the number of tests.

Deformation mechanisms of silicon during nanoscratching

AbstractThe deformation mechanisms of silicon {001} surfaces during nanoscratching were found to depend strongly on the loading conditions. Nanoscratches with increasing load were performed at 2 µm/s (low velocity) and 100 µm/s (high velocity). The load‐penetration‐distance curves acquired during the scratching process at low velocity suggests that two deformation regimes can be defined, an elasto‐plastic regime at low loads and a fully plastic regime at high loads. High resolution scanning electron microscopy of the damaged location shows that the residual scratch morphologies are strongly influenced by the scratch velocity and the applied load. Micro‐Raman spectroscopy shows that after pressure release, the deformed volume inside the nanoscratch is mainly composed of amorphous silicon and Si‐XII at low scratch speeds and of amorphous silicon at high speeds. Transmission electron microscopy shows that Si nanocrystals are embedded in an amorphous matrix at low speeds, whereas at high speeds the transformed zone is completely amorphous. Furthermore, the extend of the transformed zone is almost independent of the scratching speed and is delimited by a dislocation rich area that extends about as deep as the contact radius into the surface. To explain the observed phase and defect distribution a contact mechanics based decompression model that takes into account the load, the velocity, the materials properties and the contact radius in scratching is proposed. It shows that the decompression rate is higher at low penetration depth, which is consistent with the observation of amorphous silicon in this case. The stress field under the tip is computed using an elastic contact mechanics model based on Hertz’s theory. The model explains the observed shape of the transformed zone and suggests that during load increase, phase transformation takes place prior to dislocation nucleation. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigations on scratch behaviour of various polyamides

Scratch resistance of engineering polymers is one of the important performance requirements for diverse industrial applications and in particular, for the automotive industry. In general polymers are soft and can be easily scratched. This leads to the accumulation of residual stress concentration in the subsurface of the polymer, which in turn results in failure. Scratch resistance of polymers depends mainly on the nature of the polymer itself and the operating parameters such as applied load, shape, size and the nature of the indenter, its attack angle, scratch velocity, state of the interfacial lubrication, etc. Several papers are available dealing with the influence of the operating parameters on the scratch behaviour of polymers. However, little is reported on the class of polymers having identical functional group. Polyamides (PAs), very important engineering polymers, form such a class of polymers containing amide groups. Hence, it was worthwhile to carry out studies on the scratch behaviour of various PAs with different methylene-to-amide (CH 2/CONH) ratio and to analyze the data in view of their variation in chemical structure. For the present study, 10 PAs of three different types: condensation (PA 46, PA 66, PA 612 and PA 66/610), addition (PA 6, PA 11 and PA 12) and main chain modified (polyetheramide and aromatic PA) were selected. The scratch studies were carried out on a CSEM Revetest Scratch Tester under a constant load condition, within the range of 10–40 N using a diamond indenter. Scratch hardness ( H S) (which is a measure of the scratch resistance) depended highly on the applied load. It was also observed that the amorphous PA with the rigid aromatic group, and PA 66 exhibited the highest H S values in all of the selected loading conditions. The PA with the highly flexible polyether groups displayed the lowest H S values under the selected loading range. In general, all of the condensation PAs showed higher H S values as compared to the addition PAs. In the case of condensation PAs, H S decreased with increasing methylene content. Among the addition PAs, A 6 exhibited the highest H S values followed by A 12V, A 12N, and A 11. Scratched surfaces were observed via SEM to investigate the changes in morphology during scratching.

Grey theory applied to evaluate the tribological performances of the a-C:H(N) coating films prepared by differing the nitrogen content and the film thickness

The purpose of this paper is to study the tribology performances of the aC:H(N) films by using a nanotester under different scratch loads and velocities. From the measurements of the friction coefficient and wear volume, the tribological performances including wear resistance and friction coefficients were evaluated for the hydrogenated amorphous carbon films prepared by differing film thickness and nitrogen volume friction in the gas mixture of (C2H2+N2). Taguchi experimental design and the grey relational analysis were used to investigate the influence of specimen parameters (film’s thickness, nitrogen content in the film), and operating conditions in tribological tests (scratch load and scratch velocity) on the friction coefficients and the wear volume arising in the specimens with different coating films. It is found that the wear volume of thin film is increased by increasing either the nitrogen volume fraction or film thickness. Moreover, the optimal combination of the testing parameters was also determined in the use of the present model.

Mechanical analysis of the damage of a thin polymeric coating during scratching: role of the ratio of the coating thickness to the roughness of a scratching tip

It is generally accepted that the critical load generating the first damage in a scratch test is representative of the behaviour of a coating. As the properties of polymers are time and temperature dependent, a single value of the critical load cannot describe the overall mechanical behaviour. A new scratch apparatus has been designed which allows scratching over a wide range of velocities and temperatures and record real time photographs of the in situ contact area. It was observed that cracking appears within the contact area. In the case of thin solid films, the ratio of the contact radius to the radius of the grooving tip proved to be a pertinent parameter to predict the damage and did not depend on the scratching velocity or temperature. The ratio of the thickness of the coating to the roughness of the tip is another critical parameter: the coating prevents the roughness of the diamond tip from creating micro-scratches at the surface of the macro-groove. Therefore, since the absence of micro-scratches is a condition for relaxation of the macro-groove, the thickness of the coat must be greater than the roughness of the tip.

Viscoelastic effects on the scratch resistance of polymers: relationship between mechanical properties and scratch properties at various temperatures

Scratch durability of polymer surfaces and coatings is becoming critical for the increasing use of these materials in new applications, replacing other materials with harder surfaces. Scratch resistance of polymers has been the subject of numerous studies, which have led to specific definitions for plastic deformation characterization and fracture resistance during scratch testing. Viscoelastic and viscoplastic behavior during a scratch process have been related to dynamic mechanical properties that can be measured via dynamic nano-indentation testing. Yet, the understanding of the origin of the fracture process of a polymer during scratch remains approximate. Parameters like tip shape and size, scratch velocity and loading rate, applied strain and strain rates, have been considered critical parameters for the fracture process, but no correlation has been clearly established. The goal of this work is to define and analyze scratch parameters that relate to mechanical properties. The evolution of scratch resistance parameters as a function of temperature and strain rate, compared to the evolution of dynamic mechanical properties obtained from indentation and uniaxial tensile tests over a range of temperature for poly(methyl methacrylate) (PMMA) helped in identifying a correlation between the tensile stress–strain behavior and scratch fracture toughness. This correlation brings a new understanding of the origin of the fracture mechanisms during a scratch process. In particular, it is demonstrated that the characteristic strain applied by the indenter is a most relevant parameter to describe the fracture resistance during a scratch process, independently of the indenter geometry.

Humidity dependence of microwear characteristics of amorphous carbon films on silicon substrates

Influences of environmental humidity on various scanning-scratched wear characteristics, such as long-term stability of wear resistance and load dependence, scratch number dependence and scratch velocity dependence of wear depth, are evaluated by using an atomic force microscope and diamond tips. Amorphous carbon (a-C) films are deposited on silicon substrates by the electron cyclotron resonance plasma (ECR) sputtering method and the RF sputtering method combined with CVD using argon gas containing methane (CH4) as a sputtering gas. In carbon samples with a higher hydrogen content, clear influences of the humidity on various wear characteristics are observed and their wear resistances decrease with increase of the humidity. In the ECR sputtered carbon film with low hydrogen content, wear resistance is stable during long-term exposure to an environment of high temperature and high humidity. In this film, the influences of humidity on the wear resistance and adhesion forces between the films and the substrates are not observed, since it is highly wear resistant and the wear depths are shallow in each test. Thus, amorphous carbon films with low hydrogen content are suitable as wear resistant protective overcoats from the point of view of the wear resistance, in particular the influence of the humidity on the wear resistance including its long-term stability.

Scratching of polymeric coatings: experimental study and mechanical analysis

Coating with polymers is a common way of improving the scratch resistance of polymeric glasses. It is generally accepted that the critical load generating the fnst damage in a scratch test is representative of the behaviour of the coating. As the properties of polymers are time and temperature dependent, a single value of the critical load nevertheless cannot describe the overall mechanical response. A new scratch apparatus has been built that allows scratchng velocities in the range 1 to 104 pmls at temperatures varying from -70 to +120°C. The unique feature of ths experimental set-up is a built-in imaging system to record realtime pictures of the in-situ contact during scratching. Damage to the coating can thus be observed during the process. In the case of optical coatings, if damage occurs then cracking appears before flalung, irrespective of the quality of the adhesion between substrate and coating. The flalung mechanism has been found to depend on the quality of the layer adhesion. However, the damage mechanism is not related to the adhesion in a simple way. It has also been observed that cracking of the coating appears within the contact area and King and Sullivan have predicted that this behaviour will be obtained when the ratio of the elastic modulus of the coating to that of the substrate is 2. Scratching of polymeric coatings was studied over wide ranges of tip velocity and temperature to find a set of critical parameters controlling the onset of damage. The ratio of the contact radius to the radius of the tip proved to be a pertinent parameter, which indicates that the mean strain in the contact area at the onset of damage does not depend on the scratch velocity or temperature.

Micro- and nano-scale deformation processes during scratch damage in high density polyethylene

Atomic force microscopy and scanning electron microscopy techniques have been used to examine the surface deformation experienced by high density polyethylene during the scratch test. The scratch deformation process involves stretching of fibrils and microfibrils resulting in the formation of surface openings. At the molecular level the chains of molecules unfold and align in the direction of the moving indenter. In the scratch test, the scratch velocity may suggest that low strain rates are valid, but the local strain rates can be many orders of magnitiude higher as exemplified by atomic force microscopy. A number of modes of deformation are encountered during scratching. They include deformation bands, crazing, tearing, microcracking, regular cracking, and grooving. Crazing-tearing is the predominant mode of scratch deformation. It is envisaged that the sequence of tearing along the craze involves formation of deformation bands, development of craze, followed by tearing. Atomic force and scanning electron microscopy of scratch surface damage indicated that the nature and modes of scratch deformation are qualitatively similar to the case of uniaxial tensile deformation, implying similarities in the deformation behaviour between scratch and tensile deformation.