Scratch Depth Research Articles

Tribological properties of 304 stainless steel with rainwater corrosion

Aiming at exploring the influence of rainwater corrosion on the tribological properties of 304 stainless steel, the test samples were immersed in rainwater for corrosion with 6 h, 12 h, and 18 h respectively. The morphology, element content, and wettability of them before and after rainwater corrosion were obtained by white light interferometer, scanning electron microscope, energy spectrum, and contact angle measuring instrument. Then, the hardness and tribological properties were measured by Vickers hardness tester and MWF-500 reciprocating friction and wear tester. The experimental results show that the rainwater has an obviously corrosive effect on the 304 stainless steel surfaces. With the conducting of the corrosive process, the contents of N and S elements are increased which increase the hardness value, surface roughness, and wettability of test sample surface. In this case, the interfacial tribological properties is changed. Small friction coefficient is obtained, and the quantity and depth of surface scratch is decreased in dry friction. For lubricated case, the interfacial tribological properties is improved by rainwater, and small friction coefficient is obtained for corrosive samples. But, the quantity of scratch crack is increased due to the increasing of wear particles. The dissolution of metal impurities on the sample surface leads to the deterioration of the surface morphology, and the diffusion of N and S elements on the sample surface leads to the hardening and embrittlement of the sample surface, The main contribution of this work is in providing a reference for the 304 stainless steel corrosion protection.

Atomistic study on the nano-scratch mechanism of CoCrFeMnNi high-entropy alloy at different morphology densities

The physical nature of the scratch behavior of CoCrFeMnNi HEA and its deformation mechanism at different morphology densities are investigated by molecular dynamics simulations. The results show that the groove morphology contributes to the reduction of surface plastic deformation and exhibits a friction-reducing effect. As the morphology density decreases, the surface deformation and atom pile-up decrease, and the plastic deformation in the scratch region decreases, resulting in a further enhancement of the friction reduction effect. The increase of scratch depth intensifies the plastic deformation of the specimens, and the average scratch coefficient of friction increases with the increase in scratch depth. The dominant plastic deformation mechanism in the scratch deformation of CoCrFeMnNi HEA with different morphology densities is the slip deformation of Shockley partial dislocations. The MD simulations are verified further by qualitatively comparing them with corresponding experimental observations of CoCrFeMnNi HEA.

Characterization of graphene reinforced 3C-SiC composite as a metal-free friction material using molecular dynamics simulation

In this work, the tribological, mechanical and thermal properties of graphene (Gr)-based 3C-SiC nanocomposite as a metal-free friction material were first studied by using molecular dynamics (MD) simulations. In tribological properties, the frictional force, normal force, effective coefficient of friction and wear rate were investigated. The effects of scratching depth, temperature, scratching velocity and the number of Gr layers in the composite to the friction process were considered. Young’s modulus, ultimate tensile stress (UTS) and failure strain in the mechanical properties, and thermal conductivity in the thermal property were also included to understand the tribological performances of the composite. The interfacial interaction energy was evaluated to characterize the effect of interface on material properties. The validations were performed on some representative results. In the simulation results with the selected configurations which are listed in the subsequent content, the nanocomposite showed the lowest wear rate with an appreciable coefficient of friction at the scratching depth of 10 Å. 10 m/s was the optimal scratching speed to maintain the best wear performance. A temperature of 300 K yielded the highest coefficient of friction and the lowest wear rate. The model with three Gr layers showed a relatively high coefficient of friction and a relatively low wear rate. The model with five Gr layers stacked in 3C-SiC exhibited the best mechanical performance. The nanowire model with two Gr layers showed a higher thermal conductivity. The research findings will provide guidance in design and manufacturing of Gr/3C-SiC friction materials. The study will also help understand the effect of Gr as a potential reinforcement for metal-free friction materials.

Atomic-scale analysis of mechanical and wear characteristics of AlCoCrFeNi high entropy alloy coating on Ni substrate

High entropy alloys (HEAs) are potential materials for coating due to their high hardness and wear resistance. This study builds molecular dynamics models to explore the mechanical characteristic and wear behavior of AlCoCrFeNi HEA coating on Ni substrate during scratching. The scratching force and friction coefficient of HEA coating on Ni are larger than the pure Ni due to the HEA coating with high strength and good adhesion. The Ni substrate protection mechanism exhibits deformation absorbed by the HEA coating, so the shear strain of HEA coating is more severe than the pure Ni. The Ni substrate is protected by HEA coating, which causes less wear, as shown by the higher wear atom number of pure Ni than the HEA coating on Ni. More resistance to tool motion as increased scratching depth results in an increased friction coefficient, while the friction coefficient decreases with increasing scratching speed due to the influence of thermal softening and strain rate hardening. The accumulation of scratched atoms and energy due to insufficient relaxation time around the tool leads to considerable deformation and temperature increase. The HEA coating reduces the wear of Ni substrate when the scratching depth is smaller than the HEA coating thickness.

Friction and Wear in Nanoscratching of Single Crystals: Effect of Adhesion and Plasticity.

Friction and wear are two main tribological behaviors that are quite different for contact surfaces of distinct properties. Conventional studies generally focus on a specific material (e.g., copper or iron) such that the tribological result is not applicable to the other contact systems. In this paper, using a group of virtual materials characterized by coarse-grained potentials, we studied the effect of interfacial adhesion and material plasticity on friction and wear by scratching a rigid tip over an atomic smooth surface. Due to the combined effects of adhesion and plasticity on the nanoscratch process, the following findings are revealed: (1) For shallow contact where interfacial adhesion dominates friction, both friction coefficient and wear rate increase as the adhesion increases to a critical value. For deep contact where plasticity prevails, the variation of friction coefficient and wear rate is limited as the adhesion varies. (2) For weak and strong interfacial adhesions, the friction coefficient exhibits different dependence on the scratch depth, whereas the wear rate becomes higher as the scratch depth increases. (3) As the material hardness increases, both the friction coefficient and wear rate decrease in shallow and deep contacts.

Modeling and quantitative study of soft contact between abrasive medium and active abrasive particles in abrasive flow machining

In abrasive flow machining (AFM), material is abraded by active abrasive particles powered by solid-liquid contact between them and the abrasive medium. This contact characteristic is a key and unfortunately it is investigated scarcely.In this work, a soft-contact model was firstly proposed. Soft-contact parameters including protrusion ratio and covering inclination angle were then defined to characterize soft-contact characteristics. Finally, the effect of medium pressure and particle diameter on soft-contact parameters was studied by full factorial design. Results showed that average scratch depth of single active abrasive particle increases with increasing particle diameter and medium pressure. In addition, active abrasive particles are actually deeply wrapped to a large proportion by abrasive medium, and the descriptive protrusion ratio decreases with increasing medium pressure and particle diameter. Moreover, medium pressure offers tangential and normal force for active abrasive particle through a covering inclination angle, and the angle increases with increasing medium pressure and particle diameter. It provides guidance for further improvement of prediction accuracy of material removal in AFM process.

Microstructure and mechanical properties of NiTi nanoporous structures fabricated with dealloying process

Dealloying is an emerging material processing technology to fabricate nanoporous functional surface. Systematic characterization of microstructure and mechanical properties for dealloyed surface is a basis for tailoring dealloying parameters. This paper aims to investigate the effect of dealloying treatment on the evolution of microstructures and mechanical properties of NiTi alloy. Firstly, nanoporous layer is fabricated with electrochemical dealloying method to dissolve nickel element selectively from NiTi alloy. Secondly, the microstructures of dealloyed layer are characterized and analyzed with microscopic test methods. The release characteristics of nickel ions from the sample surface before and after dealloying treatment were evaluated using SBF immersion tests. Thirdly, the mechanical properties of NiTi nanoporous structure are evaluated with nanoindentation and micro scratch tests. The results show that both elastic modulus and nanohardness of the NiTi nanoporous layer are over 80% lower than those of the matrix material. The super-elastic property of NiTi alloy is weakened. Meanwhile, the scratching force at the same scratching depth is reduced by over 30% after dealloying process. Instead of severe plastic deformation, irregular brittle fracture occurs along the scratched groove and it demonstrates the plasticity degradation of dealloyed surface. Finally, the weakening mechanism of nanoporous structures fabricated with dealloying treatment is discussed.

Self‐healing corrosion‐resistant coatings based on fluorinated alkyl silane microcapsules

AbstractIn this study, microcapsules are prepared by in situ polymerization using the repairing agent fluorinated alkyl silane (FAS) as the core material and urea‐formaldehyde as the covering shell. Self‐healing coatings (FEP coatings) with repairing ability in water are prepared by adding FAS microcapsules in an epoxy resin matrix. The morphology, composition, and thermal properties of the microcapsules are studied. In addition, electrochemical impedance spectroscopy is used to study the barrier properties and self‐healing properties of the coating. The results show that FEP coating has good self‐healing ability, with the semiquantitative index of 105 Ω·cm2, which is greater than that of pure epoxy coating (EP coating). This is because that FAS leaked from the microcapsules reacts with water molecules to form a dense water‐insoluble film, which blocks the contact of corrosive ions with the substrate and inhibits the progress of metal corrosion. Moreover, the scratch width and depth results of the FEP coating after self‐healing also confirm the formation of the new film by FAS.

Probing the atomic-scale origins of anti-friction and wear-resisting in graphene-coated high-entropy alloys

Multilayer graphene (Gr) reinforced high-entropy alloys (HEAs) matrix composite shows promising prospects in anti-friction and wear-resisting. Nevertheless, a potential reinforcement mechanism at the atomic level urgently needs to be revealed. In the present work, through the molecular dynamics (MD) simulations, the friction and wear behavior of Gr coating reinforced HEA matrix composite on the basis of various normal loads and the number of Gr layers during scratching were analyzed. The coating effect of Gr was demonstrated to induce a substantial enhancement of anti-friction and wear resistance of HEAs matrix. Specifically, the narrowed distribution range of atomic forces combined with the counteraction between local pinning force and actuation force significantly weakened the friction. Meanwhile, the Gr coating-dependent inhibition of the pile-up effect was verified to substantially enhance the wear resistance. Especially, the increased Gr layers were validated to further reduce the friction and wear damage, attributing to the interlayer repulsion effect. Additionally, the experimentally detected reduction in scratch depth and inhibition of bulge behavior effectively supported our simulation findings. The analysis provides a theoretical insight to achieve superior surface performance and facilitates to prolong the service life of friction pairs.

Special architecture and anti-wear strategies for giant panda tooth enamel: Based on wear simulation findings.

Giant pandas are the flagship species in world conservation. Due to bamboo being the primary food source for giant pandas, dental wear is common owing to the extreme toughness of the bamboo fiber. Even though research on tooth enamel wear in humans and domestic animals is well-established, research on tooth enamel wear in giant pandas is scarce. The purpose of this study is to evaluate tooth enamel wear resistance in giant pandas to provide a basis for a better understanding of their evolutionary process. From microscopic and macroscopic perspectives, the abrasion resistance of dental enamel in giant pandas is compared with that of herbivorous cattle and carnivorous dogs in this study. This involves the use of micro-scratch and frictional wear tests. The results show that the boundary between the enamel prism and the enamel prism stroma is well-defined in panda and canine teeth, while bovine tooth enamel appears denser. Under constant load, the tribological properties of giant panda enamel are similar to those of canines and significantly different from those of bovines. Test results show that the depth of micro scratches in giant panda and canine enamel was greater than in cattle, with greater elastic recovery occurring in dogs. Scratch morphology indicates that the enamel substantive damage critical value is greater in pandas than in both dogs and cattle. The analysis suggests that giant panda enamel consists of a neatly arranged special structure that may disperse extrusion stress and absorb impact energy through a series of inelastic deformation mechanisms to cope with the wear caused by eating bamboo. In this study, the excellent wear resistance of giant panda's tooth enamel is verified by wear tests. A possible theoretical explanation of how the special structure of giant panda tooth enamel may improve its wear resistance is provided. This provides a direction for subsequent theoretical and experimental studies on giant panda tooth enamel and its biomaterials.

Determination of plastic properties of surface modification layer of metallic materials from scratch tests

A new test method was proposed to extract plastic parameters, i.e., yield stress and strain hardening index, and interfacial coefficient of friction of metallic materials via scratch tests with a pyramid indenter based on dimensional analysis and finite element analysis. Yield stress and strain hardening index of scratched materials can be calculated with scratch width, residual scratch depth and normal force obtained from the scratch tests. Interfacial coefficient of friction between the scratched materials and scratch tip can be determined by the ratio of tangential force and normal force. The accuracies of the proposed formulas were verified by comparing the results of uncarburized and carburized 18CrNiMo7-6 alloy steel obtained by conventional methods with those by scratch tests. This work provides a convenient method to determine plastic parameters and interfacial coefficient of friction of metallic materials simultaneously, which is important for the mechanical analysis, design and safety evaluation of mechanical parts.

Morphological characteristics and formation mechanism of latent scratches in chemical mechanical polishing

Latent scratches are extremely shallow scratches inevitably introduced during chemical mechanical polishing (CMP), which deteriorates the performance of functional crystals. Although latent scratches can currently be detected by some advanced microscopes, there is still a lack of in-depth understanding of them. In this paper, we describe the morphological characteristics of latent scratches using atomic force microscopy and develop a mathematical model to explain the formation mechanism of latent scratches with YAG crystals as an example. The observations showed that the distributions of the depth and width of latent scratches were always Gaussian-like. And the pressure was positively correlated with the number of latent scratches but did not significantly affect the depth and width. The proposed mathematical model describes the contact state between the particles and the crystal surface, and identifies the formation of latent scratches resulting from plastic deformation of the crystal surface, a conclusion further supported by transmission electron microscopy observations. The model also explains that the distribution of depth and width is mainly determined by the material properties and that pressure affects the number of particles in contact with the workpiece but does not significantly change the contact stress between individual particles and the workpiece. There is an upper limit to the contact stress, resulting in scratches always below 1 nm in depth. The analysis of latent scratches is of great significance to removing latent scratches and manufacturing the ultra-smooth surface in the CMP process.

Gross fracture pits at the intersection of two single scratches during grinding of silicon

Infeed grinding is used in the industry for grinding thick silicon wafers. The grinding process being a series of overlapping scratches, leaves behind the surface and sub-surface damages in the silicon. Continuous in-feed grinding of a thick wafer when used to produce thin wafer leads to breakage upon release from the fixture. This happens because of not only damage induced by a single scratch, but also due to cumulative damage induced as the grinding proceeds with several overlapping and intersecting scratches. In a single scratch, depths below a critical depth can cause material removal without gross fracture and pits; however, it is not clear if this applies to multiple intersecting scratches while grinding silicon. In this study, the grinding operation is performed for a very short period of time, gradually removing the material over only a partial region of the wafer surface while leaving the remaining portion unground. This produces multiple intersecting grinding scratches of increasing depth. Scanning electron microscopy and surface profile measurements of these scratches show that while in a single scratch, material is removed without gross fracture and pitting; when two such scratch intersects, it results in gross fracture at the site of their intersection. Such gross fractures at intersections can happen even at a very shallow depth of scratch as low as 28 nm. This gross fracture at the intersections act as local sites to initiate wafer damage and material removal. Based on the results, we attempted to redefine the critical depth of cut for grinding operation. The study paves the way to manufacture thin damage-free wafers needed for the next generation thin silicon solar cells.

Porous but Mechanically Robust All-Inorganic Antireflective Coatings Synthesized using Polymers of Intrinsic Microporosity.

Here, we introduce polymer of intrinsic microporosity 1 (PIM-1) to design single-layer and multilayered all-inorganic antireflective coatings (ARCs) with excellent mechanical properties. Using PIM-1 as a template in sequential infiltration synthesis (SIS), we can fabricate highly uniform, mechanically stable conformal coatings of AlOx with porosities of ∼50% and a refractive index of 1.41 compared to 1.76 for nonporous AlOx that is perfectly suited for substrates commonly used in high-end optical systems or touch screens (e.g., sapphire, conductive glass, bendable glass, etc.). We show that such films can be used as a single-layer ARC capable of reduction of the Fresnel reflections of sapphire to as low as 0.1% at 500 nm being deposited only on one side of the substrate. We also demonstrate that deposition of the second layer with higher porosity using block copolymers enables the design of graded-index double-layered coatings. AlOx structures with just two layers and a total thickness of less than 200 nm are capable of reduction of Fresnel reflections under normal illumination to below 0.5% in a broad spectral range with 0.1% reflection at 700 nm. Additionally, and most importantly, we show that highly porous single-layer and graded-index double-layered ARCs are characterized by high hardness and scratch resistivity. The hardness and the maximum reached load were 7.5 GPa and 13 mN with a scratch depth of about 130 nm, respectively, that is very promising for the structures consisting of two porous AlOx layers with 50% and 85% porosities, correspondingly. Such mechanical properties of coatings can also allow their application as protective layers for other optical coatings.

Subsurface damage minimization of KDP crystals

In nanocutting and nanopolishing of potassium dihydrogen phosphate (KDP) crystals, microscopic abrasive particles scratch the material surfaces and cause subsurface damage. This paper aims to establish scratching maps for subsurface damage minimization in KDP crystals by revealing the deformation and material removal mechanisms of KDP crystals based on large-scale molecular dynamics analyses. It was found that both the indenter size and scratching depth have significant influences on the surface integrity of a machined KDP component. The material can experience no-wear, adhering, ploughing and cutting deformation stages under different scratching conditions, and the variation range of the coefficient of friction reflects the stages of the deformation regimes. Scratching maps were also established as guidelines to quantitatively define the “depth/radius — removal regimes” in different lattice surfaces of KDP, which is particularly helpful for the industry in selecting surfacing conditions for high surface integrity.

Molecular dynamics study on mechanical cleavage mechanisms of GaAs and experimental verification

Laser bars are fast becoming a key device in a range of specific industrial applications. Mechanical cleavage technology is a new and efficient method for fabricating laser bars. However, there is little detailed analysis of the relationship between scratching step and cleavage step. To close this research gap, a molecular dynamics study on cleavage mechanisms of GaAs are reported investigating surface and subsurface damage, and molecular dynamics method could accurately describe the nano-scale processes in semiconductors at atomic scale. Simulation results show that the scratching depth has a significant effect on the scratch quality as compared with other parameters during the scratching process. Then, a series of cleavage experiments were carried out to verify the simulation results and further explore the influence of key scratching parameters on cleavage plane morphology. Experimental results correlate well with the simulations. Consequently, the achieved optimal combination of parameters have been found to be a scratching load of 10 g, scratching speed of 20 mm/s and scratching length of 0.6 mm, which provides direct guidance for the cleavage processing of GaAs materials. Under optimal conditions, the length and surface roughness of the undamaged area can reach 11.77 mm and 0.43 nm respectively.

Effect of grinding damage on cutting force and ductile machining during single grain scratching of monocrystalline silicon

Understanding the initiating and suppression mechanisms of existing defects are the key to optimizing the ultra-precision grinding process of hard and brittle materials. This paper discusses the effects of grinding damage on material removal mechanism from the perspectives of residual scratch depth, scratch morphology and normal scratching force, through conducting the contrast variable-depth nano-scratch experiment on polished and ground monocrystalline silicon and establishing the single grain scratching Smoothed Particle Hydrodynamics (SPH) model. The experimental and simulation results show that radial crack on ground silicon surface is almost invisible due to the existence of grinding marks, which indicates that it is difficult to judge the ductile brittle transition (DBT) point by the traditional method of observing cracks. With the increase of penetration depth, pop-in will occur in the residual scratch depth of polished and ground silicon, indicating that the material removal mode has transformed, which provides an important basis for judging the DBT point. The DBT critical normal scratching force (38.93 mN) and residual scratch depth (−0.14 μm) of ground silicon are much higher than those of polished silicon, implying ground silicon is easier to achieve ductile processing. Meanwhile, when the penetration depth is the same, compared to polished silicon, the normal scratching force of ground silicon is generally larger. Finally, a single grain scratching SPH model of pre-stressed silicon is established. And the simulated phenomenon and conclusions are consistent with the experiment, which proves that the surface compressive stress is an important factor that causes the removal characteristics of ground silicon to be different from polished silicon. This study provides a theoretical basis for understanding the ultra-precision grinding removal mechanism of hard and brittle materials.

Damage behavior and removal mechanism of different yarn orientations 2.5D SiCf/SiC composites under single-abrasive scratch test

This study aims to comprehensively investigate the effects of yarn orientation and grinding depth on the damage behavior and removal mechanism of 2.5D SiCf/SiC composites via single-abrasive scratch tests. Results show that brittle matrix damage and fiber fracture are the primary removal modes of the composites. As the scratching depth increases, the fiber fracture removal modes of warp yarns are cutting fracture, cutting and shear fracture, and shear fracture, whereas those of weft yarns are cutting fracture, shear fracture, and bending fracture. The matrix is primarily removed via cracking, damage, and peeling. The resulting debris is composed of fiber segments, fiber microsegments, interface fragments, matrix fragments, and microdebris. The scratching force and cross-sectional area gradually increase with increasing scratching depth; however, the specific scratching energy decrease. This study clarifies the damage behavior and removal mechanism of 2.5D SiCf/SiC composites and provides meaningful guidance for improving processing quality.

Surface Morphology Evaluation and Material Removal Mechanism Analysis by Single Abrasive Scratching of RB-SiC Ceramics

Reaction-Bonded Silicon Carbide (RB-SiC) ceramics possessing excellent mechanical and chemical properties, whose surface integrities have an essential effect on their performance and service life, have been widely used as substrates in the core parts of aerospace, optics and semiconductors industries. The single abrasive scratching test is considered as the effective way to provide the fundamental material removal mechanisms in the abrasive lapping and polishing of RB-SiC ceramics for the best surface finish. In this study, a novel single abrasive scratching test with an increasing scratching depth has been properly designed to represent the real abrasive lapping and polishing process and employed to experimentally investigate the surface integrity regarding different scratching speeds. Three typical and different material removal stages, including the ductile mode, ductile–brittle transition mode and brittle mode, can be clearly distinguished and it is found that in the ductile material removal stage by increasing the scratching speed would inhibit the plastic deformation and improve its surface integrity. It is also found that in the ductile–brittle transition and brittle material removal stages, to increase the scratching speed would inhibit the plastic deformation due to the fast scratching speed that limits the time of plastic deformation on the target, but it also results in the increased length of lateral cracks with the increased scratching speed which can reflect that the size of brittle chips, like brittle fractures and large grain fragmentations, increases as the scratching speed increases. It can provide the references for the optimization of the abrasive lapping and polishing of RB-SiC ceramics with high efficiency and surface quality.

Investigation of the adhesive and abrasive wear mechanisms at the atomic scale using molecular dynamic simulations

In this study, the effect of adhesion on material removal is numerically investigated at the nanoscale level. In this regard, Molecular Dynamics (MD) simulations are conducted to distinguish the contribution of adhesive and abrasive wear mechanisms during a scratching process in terms of the degree of interfacial adhesion and scratching depth. The numerical model simulates the scratching of a flat workpiece made of a single crystalline aluminum using a rigid conical indenter with a blunted spherical tip at four different indentation depths (i.e. 0, 3, 7, and 10 Å). The classical Leonard-Jones interatomic potential is used to mimic the degree of adhesion by varying the adhesion parameter between 5–70% of the aluminum bonding energy. It is shown that, at shallow scratching depths, the contribution of adhesive work to the frictional work is much larger than the ploughing work. On the contrary, the ratio of adhesive to ploughing work reduces by increasing the scratching depth.