Scratch Path Research Articles

Effect of hydrogen concentration on the friction evolution mechanism of few-layer graphene: A molecular dynamics study in nanoscratch processes

This article explored the friction evolution mechanism of graphene under different hydrogen concentrations and scratch depths based on molecular dynamics. In a vacuum, graphene exhibits excellent frictional properties when the scratch depth is less than the layer distance of graphene. However, as the depth increases, the diamond tip contacts more CC bonds, and the phenomenon of fracture and recombination of CC bonds in the graphene layer intensifies, increasing surface friction. In a low concentration of hydrogen environment, compared with the friction in a vacuum, it increases due to the interlocking effect of the protruding hydrogen atoms on the diamond tip and the scratch path. As the concentration of the hydrogen environment increases, the friction force does not increase monotonically. In addition, in a hydrogen environment, the stability of the planar structure of graphene will be weakened, thereby affecting the wrinkling effect and wear path.

A Preliminary Geometrical Characterization of Surface Scratches on Aluminium Alloys Used in the Aerospace Industry

Surface marks, such as scratches or cosmetic marks, commonly appear during the manufacturing phase of metallic components, because of the contact between tools and sharp edges with the surface of the parts. Scratches, depending on their width, depth, and root radius, cause a decrease in the fatigue life of metallic alloys. In particular, the presence of scratches with a size comparable to the grain size favors the generation of fatigue cracks in these features. In the aerospace industry, the presence of surface marks is a common cause of rejection. The low hardness of aluminium, a material widely employed in the manufacture of aerospace structures, contributes to the generation of surface marks. In this paper, a preliminary geometrical characterisation of scratches is established. It aims to define a set of parameters to characterise exhaustively the different scratches and to generate different behavior models for each type of scratch. Parameters such as scratch length, path radius, and burr height are considered in addition to the well-known parameters such as scratch depth, root radius, and open angle.

EBSD evaluation of damage in micro-tribology experiments on WC/Co hardmetals

WC/Co hardmetals are used in many applications where resistance to abrasive wear is important. To provide further information on the mechanisms of damage that can occur in abrasion, a series of micro-tribology experiments were carried out on a number of WC/Co hardmetal samples. Samples were traversed under a diamond indenter with a sequence of controlled applied loads. The resultant damage was examined by scanning electron microscopy (SEM). Electron back scattered diffraction (EBSD) analysis of the damage was also carried out, both of the test surface, and also at some distance below the surface. To ensure that no damage was introduced to the structure of the materials, broad ion beam polishing was used to polish the original test surfaces and form the sub-surface section.It was found that plastic deformation of the WC grains and the Co binder phase occurred at all test loads. The visibility of slip increased with increasing test load, with fracture to the WC grains starting to occur at higher loads. EBSD analysis confirmed the trend for increasing plastic deformation with increasing load, but also showed that the degree of deformation had a strong relationship with orientation of the WC grains. Some transformation of the normal fcc Co phase to hcp was also detected close to the scratch path.

Direct visualization of the molecular orientation and microstructure of glassy transparent polymers after the scratch test based on optical microscopy and X-ray scattering

The scratch test was conducted for polycarbonate (PC) and poly(methyl methacrylate) (PMMA) with a load-progressive mode. Changes in the molecular aggregation structure of PC and PMMA induced by the scratch test were investigated based on polarized optical microscopic observation and small-angle X-ray scattering (SAXS) measurement. The scratching coefficient of friction (SCOF), the ratio of tangential load to normal load, of PC was much larger than that of PMMA. The width and depth of the damage on the scratch path was increased monotonically with the increase in the normal load. Hardness obtained by nanoindentation and microindentation tests were similar to those with SCOF, indicating that compression resistance mainly governs the scratch properties. The polarized optical microscopic images revealed that molecules were oriented from the edge to the center of the scratch path at a certain acute angle for both PC and PMMA. SAXS measurement revealed that microfibrils were formed along the direction of stress for PC.

Improving of tribology properties of TiAl6V4 with nanostructured Ti/TiN-multilayered coating deposited by high-vacuum magnetron sputtering

TiAl6V4 is widely used in the medical and aerospace industry due to the good corrosion resistance and mechanical properties. In this work, single-layer TiN and multilayer Ti/TiN coatings are deposited on TiAl6V4 by high-vacuum magnetron sputtering and the phase, structure, and morphology are investigated by GIXRD, XPS, FE-SEM, and AFM. The tribological properties are determined by pin-on-disk tests with a tungsten carbide pin (WC). The 1.4 µm thick coating contains TiN, TiOxNy, and TiO2 phases. The friction coefficients, hardness, and elastic modulus of the TiN and Ti/TiN coatings are 0.43 and 0.49, 19.744 and 22.462 GPa, and 192.709 and 183.565 GPa, respectively. The tiny cracks along the scratch path on the TiN coating arise from the lower toughness compared to the Ti/TiN-multilayered coating. The larger friction coefficient in the presence of the Ti interlayer may be due to network failure at the Ti and TiN boundaries which show different crystalline structures and the stress formed at both layers prohibits dislocation movement in the Ti layer. More brittle cracks are observed from the TiN mono-layered coating and larger plastic deformation occurs on the multilayered coating, indicating that the latter is more resistant to delamination. The adhesion mechanism is dominated by the Ti intermediate layer and can be exploited to improve the effectiveness of TiN coatings in applications such as biomedical implants.

Multi-scale tribological and nanomechanical behavior of cold sprayed Ti2AlC MAX phase coating

Ti2AlC based MAX phase coatings were successfully deposited on Inconel 625 substrate by a cold spraying technique. A dense coating of 70μm thickness was deposited. Ball-on-disk wear behavior of Ti2AlC coating at room temperature (25°C), and high temperature (600°C) were studied. The coefficient of friction (COF) and wear volume loss at 600°C reduced by ~21% and ~40% respectively, due to the lubricious nature of oxide layer formed at a higher temperature. Mechanical properties of the Ti2AlC coating were also studied by carrying out nanoindentation and nano-scratch tests at room temperature and 300°C and varying loads. For a low load of 7000μN at room temperature, Ti2AlC coating exhibited a higher elastic modulus of 273GPa compared to the elastic modulus of 191GPa at high temperature (300°C). The room temperature nano-scratch at 7000μN displayed brittle behavior with fracture, chipping and wear debris formation along the scratch path. However, high temperature (300°C) scratch path exhibited ductile nature with plowing, cutting and no wear debris formation. The wear volume loss was several orders of magnitude higher at 8N load scratch. The overall wear behavior in MAX phase Ti2AlC coating at multiple load scales is elucidated in terms of the interaction volume varying from a single to several splats in the cold sprayed structure.

Analysis of Grinding Surface Creation by Single-Grit Approach

This paper presents some new research findings in the investigation of single-grit grinding in terms of surface creation. The investigation demonstrated that rubbing–plowing–cutting hypothesis of grinding material removal mechanism is valid in both experiments and simulations. A finite element model (FEM) was developed to simulate the material deformation during the grit interacts with the workpiece. It was found that the cutting mechanism is the more effective in the first half of the scratch where the grit penetrates the workpiece. The plowing is a prominent mechanism in the second half of the scratch where the grit is climbing up along the scratch path and uplifting the material at the front and the sides of it. This observation is very important to provide a greater insight into the difference between up-cut and down-cut grinding material removal mechanisms. Multipasses scratch simulations were performed to demonstrate the influence of plowing on the ground surface formation. Moreover, by analyzing the effects of grinding conditions, the shape of cutting edges, and friction in grinding zone on the grinding surface formation, some useful relations between grinding performance and controllable parameters have been identified. It has demonstrated that plowing has significant influences on ground surface formation and concluded that the influence of grit shape, friction, and grinding kinetic condition should be considered together for the plowing behavior control, which could provide a good guidance for the improvement of grinding efficiency.

Repeated nanoscratch and double nanoscratch tests of Lu2O3 transparent ceramics: Material removal and deformation mechanism, and theoretical model of penetration depth

The single nanoscratch, repeated nanoscratch and double nanoscratch tests of Lu2O3 transparent ceramics are carried out on a nanoindenter. Theoretical models of the penetration depth in single scratch, repeated nanoscratch and double nanoscratch tests are established by taking the elastic recovery into account. The surface morphology, friction characteristics of repeated nanoscratch and elastic recovery of double nanoscratch are researched. Ductile surface with hardly cracks and burrs, and obvious pile-up at the leading edge and both sides of the scratch path are obtained. TEM results indicate that the ductile deformation mechanism of Lu2O3 transparent ceramics is a combination of poly-crystalline nano-crystallites in the inner grain and amorphous transformation. Many defects including dislocations, stacking faults, nano twins, torsion of atomic plane, fracture of atomic plane and wrong arrangement of atomic plane are induced in nano grains generated during the ductile removal process. The dominant way of subsurface crack propagation is intergranular fracture.

Effect of Sandblasting, Annealing and Hydrophobic Treatment on the Nano-Mechanical and Corrosion Behaviour of n-TiO<sub>2</sub>-Coated 316L Stainless Steel

In this article, the effect of hydrophobic coated and uncoated surfaces on the corrosion resistance, tribological property, and surface morphology and nanomechanical properties is presented. A hierarchical surface (micro/nano) was prepared by ultra shot penning of 316L stainless steel at 20 kHz. Chemical vapour deposition technique was used for deposition of the coating, and coating thickness of 0.5 μm was obtained. A maximum hardness of 6.7 and 6.32 GPa was exhibited by the sandblasted (SB), annealed (SBAT) and coated (SBAT) samples respectively. The beneficial effect of annealing and surface roughness was clearly indicated by the sandblasted, annealed and coated specimens. They showed the highest value of reduced modulus maximum resistance to scratch tests and a high adhesion as indicated by scratch path profiles. The lowest corrosion rates were obtained by SBAT specimens (1.32 mpy) in 3.5 wt% NaCl. After application of 3.5% fluoroalkylsilane (FAS13), the corrosion rate was reduced to 0.04 mpy with no evidence of localized pitting. The water contact angle using DSA-100 system was measured to be 120° for SBAT and 80-90° for the samples.

Effect of Carbide Particles on the Behaviour of the Oxide Layer

In this study, an FE simulation was used to simulate the contact established between a High-Speed Steel (HSS) work roll and a hot strip material in hot rolling, in which the top layer and the substrate represented the HSS roll and the tip of the indenter represented a particle from oxide scale formed on the strip steel. As the oxidation behaviour of carbides affects the homogeneity of the oxidised surface of work rolls, this has an important influence on wear and therefore it is instructive to investigate the effect that carbide particles have on the behaviour of the oxide layer. This study focused on the interaction between the carbide and the asperity by an abrasive. The evolution of strain fields during the simulation tests was recorded at room and high temperature. The 3D FE model was able to show the interaction between the indenter and the carbide particle reinforcements of these following scenarios: particles above, along and below the scratch path. The results indicated that the strain at high temperature is higher than at room temperature which leads to a higher sensitivity to failure. Increasing the strain means high probability of particle movement that produces delamination of the layer. The strains for particles on the oxide-substrate interface at both temperatures are higher than the particle in the substrate. The interaction between the tip and the particle significantly influences the strains.

Experimental study on single grit grinding of Inconel 718

This article presents an important investigation of material removal mechanism in grinding utilizing single grit scratch tests. The investigation helps people to understand the abrasive cutting behaviour when the abrasive cutting edge shape alters during single grit grinding. The results provide fundamental knowledge of the grinding material removal process, which helps to improve grinding performance and quality. Cubic boron nitride grits of 40/50 mesh size were used to perform scratch tests on the alloy Inconel 718. The concepts of material pile-up ratio and actual material removal area were introduced to measure the material removal efficiency during grinding. It is found that pile-up ratio decreases and actual material removal area increases when the depth of cut increases, albeit the material removal mechanism is highly dependent on the abrasive grit cutting edge shape. The material removal mechanism along the scratch length shows different behaviours at the entrance and exit sides of the scratching passes. When a grit was moving along its scratch path, it pushed material forward resulting in high material accumulation at the exit side of the scratches. Consequently, cutting is more prominent at the entrance side of the scratch, whereas ploughing or pile-up is extremely high at the exit side of the scratches. The research finding provides crucial information for grinding process optimization.

Scratch resistance analysis of plasma-nitrided Ti–6Al–4V alloy

A modified plasma nitriding treatment developed for Ti–6Al–4V alloy at a low temperature (600°C) and in a dilute nitriding atmosphere (3% N2) has shown to improve the alloy tribological properties and resistance to surface crack initiation and propagation. This modification formed a thin compound layer (CL) (1.9μm) on the surface of Ti–6Al–4V followed by an α-Case (1.8μm) and a deep nitrogen diffusion zone (DZ) (44.4μm). Transmission electron microscopy (TEM) and x-ray diffraction (XRD) analyses revealed that the compound layer consisted of Ti2N, TiN, and TiN0.3 nitrides. The failure mechanisms of the nitrided samples subjected to micro-scratch testing under both constant and progressive loading were investigated using scanning and transmission electron microscopy, focused ion beam milling (FIB), and optical surface profilometry. The onset of failure events was observed as the applied load reached 0.75N (mean Hertzian pressure of 4.4GPa) with the appearance of angular cracks on scratch path rims and subsurface microcracks within the CL likely at grain boundaries. Normal loads higher than 8N (9.7GPa) indicated the advent of tensile surface cracks perpendicular to the scratch direction and subsurface cracks which propagated beyond the CL. As the normal load exceeded 15.7N (12.1GPa), angular and tensile cracks joined and formed semi-circular cracks. FIB observations indicated that these cracks penetrated into the diffusion zone but were effectively stopped by ductile β particles at α grain boundaries. The plasma-nitriding also led to a significant reduction in COF without any signs of compound layer spallation and delamination during micro-scratch testing.

Rubber content effect on scratch behavior in acrylonitrile‐styrene‐acrylate copolymers

AbstractThe scratch behavior of butyl‐acrylate rubber‐modified styrene‐acrylonitrile thermoplastics is investigated following the ASTM D7027 linearly increasing normal load test methodology. The critical normal loads at the onset of the major transitions along the scratch path, such as groove formation, scratch visibility, microcrack formation, and plowing, are reported and quantitatively analyzed. It is found that the scratch resistance generally deteriorates with increasing butyl‐acrylate rubber content, and is strongly related to the tensile and compressive yield stresses of the blends. Microscopy investigation indicates that a rubber content of up to 30 wt % in a styrene‐acrylonitrile copolymer (SAN) does not alter the scratch‐induced damage mechanisms, but only reduces the critical onset loads for the observed damage transitions. The present finding suggests that addition of rubber causes reductions in modulus, tensile, and compressive yield stresses, thus leading to deterioration in scratch resistance. It appears that the improvement in ductility for SAN after the rubber toughening does not benefit scratch resistance. Implication of rubber toughening on scratch behavior of polymers is discussed. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Effect of high temperature annealing on scratch behavior of acrylonitrile styrene acrylate copolymers

The scratch behavior of butyl-acrylate rubber-modified styrene-acrylonitrile (ASA) upon high temperature annealing is investigated following the ASTM D7027/ISO 19252 linearly increasing normal load test methodology. The critical normal loads at the onset of the major deformation transitions along the scratch path, such as groove formation, scratch visibility, microcrack formation, and plowing, are reported and quantitatively analyzed. It is found that the scratch resistance improves with high temperature annealing, i.e., 30 °C above T g , as compared to annealing below or around T g . Microscopic investigation suggests that the increase in scratch resistance is related to the changes in surface morphology of the polymer. It is concluded that performing high temperature annealing enhances the scratch performance without compromising ASA bulk properties. Implication of the present study for improving scratch resistance of polymers is discussed.

Quantitative scratch visibility assessment of polymers based on Erichsen and ASTM/ISO scratch testing methodologies

A lack of effective quantitative tools for evaluating scratch visibility resistance of polymers has hindered the fundamental understanding of visible scratch-induced defects and effective approaches to improve performance. Thanks to the recent establishment of the Erichsen and ASTM/ISO scratch testing protocols, researchers are now able to give quantitative assessments of scratch visibility resistance with ease. Unfortunately, the above two methods differ in testing and evaluation approaches. The main objective of this work is to disseminate the similarities and differences between the two industrial scratch visibility determination methodologies, i.e., delta L (ΔL) for Erichsen and contrast for ASTM/ISO. A good correlation between the two methods can be established if the ΔL value is normalized against the original L value for the undamaged surface, similar to the concept of contrast. The surface deformation that gives rise to light scattering, such as scratch groove formation and the surface roughness on the scratch path, was studied for both methods using a set of three model thermoplastic olefin systems. The implication of the current research for quantitative scratch visibility assessment is discussed.

Quantitative determination of scratch-induced damage visibility on polymer surfaces

A new methodology to quantify scratch-induced damage visibility on polymer surfaces is proposed. The color image analysis using a hue–saturation–intensity (HSI) color space combination provides satisfactory results in quantifying scratch visibility and in monitoring surface damage features along the scratch path. The effectiveness of the HSI color image analysis methodology is validated via comparison of the results from a model of polypropylene blend using three linearly increasing normal load conditions. The scratch damage transitions are correlated with the HSI color transitions and show good agreement. Implication of the present finding for the evaluation of scratch visibility resistance of polymers is discussed.

Inelastic deformation under indentation and scratch loads in a ZrB 2–SiC composite

Inelastic deformation features induced in an ultra-high temperature ceramic composite, ZrB 2–SiC, due to static indentation (rate of deformation of the order of 10 −5 s −1), dynamic indentation (rate of deformation of the order of 10 3 s −1), and high-velocity scratch (500 mm/s) experiments are presented. It was found that this ceramic composite has up to 30% higher dynamic hardness compared to static hardness. Dynamic indentations resulted in extensive transgranular microcracking within the indented regions compared to static indentations. In addition, significant plastic deformation features in terms of slip-line formation were observed within statically and dynamically indented regions. The high-velocity scratch studies revealed extensive transgranular microcracking perpendicular to the scratch direction and slip-lines in and around the scratch path. Preliminary transmission electron microscopy (TEM) observations from regions of slip-lines surrounding the scratch grooves revealed dislocation activity in the composite.

Enhanced mineralized tissue adhesion to titanium over polystyrene assessed by the nano‐scratch test

The critical load determined by the scratch test is regarded to be a representative measure of coating adhesion in the field of engineering. This study aimed to evaluate the method for its usefulness for assessing the mineralized tissue-titanium interface strength. Osteoblastic cells derived from rat bone marrow were cultured on polystyrene, titanium-coated polystyrene, and titanium disks with either a machined or dual-acid etched surface. Nano-scratch testing was performed on mineralized tissue specimens at culture day 28. The scratch path was monitored by light microscopy until complete delamination of mineralized tissue from the substrate occurred, and the required force was recorded as the critical load. Energy-dispersive spectroscopic analysis was used to verify the delamination. The mean critical load values (+/- standard deviations) were as follows: polystyrene 31 mN (+/-1), titanium-coated polystyrene 67 mN (+/-4), machined titanium 76 mN (+/-4), DAE titanium 107 mN (+/-3), with statistical differences (P < 0.05; ANOVA). No elemental calcium and phosphorous were observed in the delaminated areas. The nano-scratch test applied to cultured mineralized tissue differentiated the critical load from various culture conditions: polystyrene vs. titanium; titanium with different surface topographies. Culturing mineralized tissue on titanium, especially on roughened surfaces, increased the tissue critical load. The nano-scratch test may be useful to evaluate mineralized tissue adhesion properties in titanium cultures.

Investigation of wear mechanisms through in situ observation during microscratching inside the scanning electron microscope

Scratch experiments have been used in the past to investigate various contact phenomena such as abrasion mechanisms or asperity contacts. Depending on tip geometries, loading conditions and materials investigated, different crack propagation modes (radial or lateral cracks, etc.) or deformation modes (ploughing, chipping, etc.) may dominate the scratching process. The residual scratch path can yield some information about dominant deformation and fracture modes. It is, however, often not possible to uniquely correlate cracks and other phenomena with events on the recorded load–displacement curves. We have built a miniaturized microscratch device for use inside a scanning electron microscope (SEM) that allows the observation of the surface around the tip with sub-micrometer resolution during scratching. Using a conical indenter with spherical tip we demonstrate on different materials that the device is a powerful tool to observe initiation and propagation of cracks, to observe the flow of the material near the indenter (piling-up and sinking-in) and to study chip and particle formation mechanisms during microscratching. In GaAs, particles were observed to form in front and on the rear side of the tip via interaction of chevron cracks. In the case of a Fe-based bulk metallic glass, shear bands were observed to form in front of the tip leading to serrated chip formation. Discontinuities in the tip penetration during scratching of a polymer thin film were related to the onset of crack formation behind the tip and to the propagation of semi-circular cracks in front of the tip. The observed large elastic recovery of the polymer film at the rear side of the tip has to be taken into account for accurate contact area calculations.

Nano-indentation of scratched poly(methyl methacrylate) surfaces

The paper describes an experimental study of the nano-indentation hardness profiling of scratches, of various depths, produced upon the surface of a poly(methyl methacrylate). The method has limited spatial resolution and typically the indent size, and hence the volume of deformed material sensed, was in the worst cases one-tenth of the scratch width; in exceptional cases the resolution was ten times better than this figure. Additionally, the various topographical and morphological features are evaluated using laser profilometry and optical and scanning electron microscopy. The data described indicate that the scratching process introduces damage in subsurface regions, which may be tentatively ascribed to the formation of subsurface crazing or voidage in certain regions of the scratch damage area; in particular within the base of the scratch formation. Such structures could potentially develop in the substrate tensile fields developed at the rear of the indenter. In addition, there is evidence for densification of the polymer adjacent to the primary damage zone—on the “shoulder” areas adjacent to the main scratch path. An analysis of the periodic fluctuations of the nanohardness data, in certain zones, allows a speculative description of the topology of the subsurface damage in terms of the induced voidage and its distribution. Such features would naturally lead to a “positive” deformation volume; an analysis of an orthogonal topographical trace of a scratch allows a simulation of volume with respect to the unpurturbed surface—a “positive” comparison infers an additional volume and vice versa. In the current study, such “positive” volume deformation regions were not unequivocally identified with the proposed craze zones using the topographical estimation procedures employed.