van der Waals effect on the nanoindentation response of free standing monolayer graphene

Using molecular mechanics simulations we investigate the elastic properties of monolayer graphene determined from free standing indentation and the effects of graphene size and indenter tip size are considered. In free standing indentation, the overall system potential energy includes two parts: the strain energy of graphene monolayer and the van der Waals (VDW) interaction energy between indenter tip and graphene. The VDW interaction will strongly affect the elastic properties of graphene monolayer determined from free standing indentation. Without considering the VDW interaction, the classical free standing indentation analysis is not able to accurately estimate the second-order elastic stiffness (E) and the third-order nonlinear elastic constant (c_m) (E will be underestimated and c_m will be overestimated). It is found that both E and c_m determined from free standing indentation are quite close to their counterparts determined from in-plane tension after properly considering the VDW interaction. The results can provide a useful guideline to understand the elastic properties of graphene monolayer determined from free standing indentation tests.     

Nanoindentation experiments for single-layer rectangular graphene films: a molecular dynamics study

A molecular dynamics study on nanoindentation experiments is carried out for some single-layer rectangular graphene films with four edges clamped. Typical load–displacement curves are obtained, and the effects of various factors including indenter radii, loading speeds, and aspect ratios of the graphene film on the simulation results are discussed. A formula describing the relationship between the load and indentation depth is obtained according to the molecular dynamics simulation results. Young’s modulus and the strength of the single-layer graphene film are measured as about 1.0 TPa and 200 GPa, respectively. It is found that the graphene film ruptured in the central point at a critical indentation depth. The deformation mechanisms and dislocation activities are discussed in detail during the loading-unloading-reloading process. It is observed from the simulation results that once the loading speed is larger than the critical loading speed, the maximum force exerted on the graphene film increases and the critical indentation depth decreases with the increase of the loading speed.     

Electroconductive composite of zirconia and hybrid graphene/alumina nanofibers

Novel type of hybrid nanofillers representing graphene encapsulated alumina nanofibres was selected as an additive to develop toughened electroconductive partially stabilized zirconia. The sinterability, mechanical and electrical properties of the produced nanocomposites were studied as function of the filler/graphene content. Composites containing just 0.6 vol.% of graphene corresponding to 3 vol.% of hybrid nanofibres exhibited high electroconductivity of 58 S/m without deterioration of mechanical properties. They also showed a slight toughening effect that is reflected by an increase in the indentation fracture toughness by 20% as compared to monolithic zirconia.     

Stiffness-dependent interlayer friction of graphene

The friction of graphene depends on thickness, but little is known how it is dependent on stiffness. Based on a graphene-spring model, using molecular dynamics simulations, we investigate the friction behavior of a graphene flake sliding on a supported graphene substrate. We show that the friction force increases exponentially with the decreasing stiffness. The stiffness is a dominant parameter for the friction of a soft substrate, e.g., where superlubricity may be completely impeded. We relate the friction to the substrate deformation and find that the indentation depth can be an indicator for the friction of soft substrates. These findings may provide a fundamental understanding for the stiffness dependent nanoscale friction.     

Microstructure,fracture behaviour and mechanical properties of conductive alumina based composites manufactured by SPS from graphenated Al2O3 powders

Conductive Al₂O₃/graphene composites were manufactured by SPS from Al₂O₃ powders coated with a few graphene layers. Composite powders with a total carbon content of 0.1, 0.6 and 1.0 wt. % were manufactured by chemical vapour deposition. The effect of the graphene content on the microstructure, mechanical and electrical properties of the compacts were studied. Graphene, homogenously located along the grain boundaries, dramatically hindered the Al₂O₃ grain growth. The continuous interconnected graphene network enhanced electrical properties, achieving percolation threshold as low as 0.6 wt. % of graphene. The content of 1 wt. % of graphene increased electroconductivity by 13 orders of magnitude as compared to the monolithic alumina. The indentation fracture toughness increased by 20 % in specimens with 0.6 wt. % graphene content as compared to pure alumina. The presence of 1.0 wt. % of graphene resulted in a slight decrease of elastic modulus and hardness, but strength decreased by 40 %.     

An atomistic study of the abrasive wear and failure of graphene sheets when used as a solid lubricant and a comparison to diamond-like-carbon coatings

The failure mechanisms of graphene under nanoscale sliding conditions are examined using atomistic simulations to evaluate its use as a solid lubricant and to simultaneously answer principal questions regarding wear of lamellar films comprised of atomically-thin sheets. To determine the failure behavior of graphene and the impact of adhesion on wear and failure, an asperity is slid over a substrate-supported graphene film with various adhesion strengths. For a purely-repulsive asperity, the graphene never delaminates and lower substrate-membrane adhesion appears to reduce the overall damage to the graphene layer and permits the recovery of more of the load-bearing capability of the graphene post-tearing. When tri-layer graphene is benchmarked with a 2 nm repulsive asperity against an 86% sp³ content diamond-like-carbon (DLC) coating of the same thickness (1.0 nm), the graphene supports up to 8.5 times the normal load of DLC during indentation, and up to twice the normal load of DLC during sliding even after failure of one or more layers. The preliminary results indicate that graphene has promise as a solid lubricant with thickness on the order of nanometers due to its atomically-thin configuration and high load carrying capacity.     

Microstructure,mechanical properties and toughening mechanisms of graphene reinforced Al2O3-WC-TiC composite ceramic tool material

The low fracture toughness of Al₂O₃-based ceramics limited their practical application in cutting tools. In this work, graphene was chosen to reinforce Al₂O₃-WC-TiC composite ceramic tool materials by hot pressing. Microstructure, mechanical properties and toughening mechanisms of the composite ceramic tool materials were investigated. The results indicated that the more refined and denser composite microstructures were obtained with the introduction of graphene. The optimal flexural strength, Vickers hardness, indentation fracture toughness were 646.31?±?20.78?MPa, 24.64?±?0.42?GPa, 9.42?±?0.40?MPa?m^1/2, respectively, at 0.5?vol% of graphene content, which were significantly improved compared to ceramic tool material without graphene. The main toughening mechanisms originated from weak interfaces induced by graphene, and rugged fractured surface, grain refinement, graphene pull-out, crack deflection, crack bridging, micro-crack and surface peeling were responsible for the increase of fracture toughness values.     

Graphene nanopatterns with crystallographic orientation control for nanoelectronic applications

The possibility of parallel processing of several features was investigated experimentally for the two methods allowing the crystallographically controlled nanopatterning of graphene: scanning tunneling lithography (STL) and carbothermal etching (CTE). It was found that with multitip systems both methods are suitable for parallel processing. CTE has the advantages that only in the atomic force microscope (AFM) indentation phase is needed the multitip system and it can reveal the location of grain boundaries, so that the nanodevices can be placed in a way that they do not cross grain boundaries. STL is well suited for purposefully producing twisted graphene multilayers with precisely-know misorientations of the individual layers, as also evidenced by Moiré-type patterns observed in the atomic resolution scanning tunneling microscopy (STM) images.     

Graphene reinforced alumina nano-composites

Graphene was prepared using liquid phase exfoliation and dispersed in an alumina matrix using an ultrasonication and powder processing route. Al₂O₃–graphene composites with up to 5 vol% content were densified (>99%) using SPS. The fracture toughness of the material increased by 40% with the addition of only 0.8 vol% graphene. However for higher graphene contents the improvement in fracture toughness was limited. Graphene changed the mechanism of crack propagation for the alumina matrix from inter-granular to trans-granular. The formation of an inter-connecting graphene network promoted easy fracture for concentration ⩾2 vol%. Elastic modulus remained nearly constant for up to 2 vol% and decreased significantly for 5 vol% due to the formation of the inter-connecting graphene network. Fracture toughness measured with the indentation and chevron notch methods were consistent up to 2 vol% and at 5 vol% the percolating network of graphene resulted in easy crack propagation with significant discrepancy between the results for the two methods.     

Contact-mechanical properties at pre-creep temperatures of fine-grained graphene/SiC composites prepared in situ by spark-plasma sintering

Hertzian indentation was used to investigate for the first time the elastic–plastic behavior and contact damage evolution with temperature (in the 25–850 °C range) in air of different in situ grown graphene/SiC composites. Three SiC starting powders differing in polytypes and particle sizes were liquid-phase densified by spark-plasma sintering, thus producing ceramic composites that contain about 4 vol.% of epitaxial grown graphene flakes at the grain boundaries and have interesting differences in their microstructure. The general trend in the contact mechanical behavior is qualitatively similar among the different composites, with the elastic modulus, yield stress, and critical loads for the onset of quasi-plasticity and ring/cone cracking all decreasing with increasing temperature, with the differences at the quantitative level being explained in terms of the particular microstructure of each graphene/SiC composite.     

Residual stress and fracture toughness of sintered body of ZrO2-GO composite ceramics material

Zirconia ceramics and carbon-based materials are widely adopted in medical and dental applications due to their excellent biocompatibility and aesthetics. However, fracture toughness of ceramic materials limits their application in clinical dentistry because of the existence of residual stress. In this study, zirconia/graphene oxide (ZrO₂-GO) composite ceramics were fabricated by hot-press sintering. Residual stresses developed on the surface of ZrO₂-GO composite ceramics were evaluated by X-ray residual stress analysis and indentation techniques. The variation of surface residual stress with GO content was evaluated, and found to be consistent with that of fracture toughness. The generation of residual stress was found to be directly related to fracture toughness. Residual stress calculated by theoretical formula of indentation method was consistent with that measured by X-ray diffraction in line with the content of GO. Based on above results, it is concluded that 0.1–0.15 wt% GO composite ceramics possessed better mechanical properties.     

Orientation dependence of the fracture behavior of graphene

Graphene has unique mechanical properties in that it is simultaneously very strong and stretchy, which severely hampers the prediction of its orientation-dependent fracture behavior based on conventional theories used for common brittle or ductile materials. For the first time, by exploring the entire range of available tensile orientations, this study reveals the unique anisotropic fracture response of graphene using molecular dynamics simulations. We found that, as the uniaxial tensile direction rotates from armchair (0°) to zigzag orientation (30°), both the tensile strength and strain remain almost constant up to an orientation angle of ∼12°, then they rapidly increase (exponential growth), resulting in a remarkable degradation of the tensile strength compared to brittle fracture counterpart (inverse-sinusoidal growth). This typical fracture pattern holds for 100–700 K. We propose a model that can explain its physical origin in good agreement with the simulation results. We also found that the elastic behavior of graphene is quasi-isotropic for all tensile orientations, in contrast to its anisotropic fracture behavior. Using indentation simulations of graphene, we showed that the anisotropic/isotropic features of fracture/elasticity are also well-preserved in the two-dimensional tensile systems but its fracture anisotropicity is greatly attenuated due to the inherent sixfold symmetry of graphene.     

Molecular dynamics of cleavage and flake formation during the interaction of a graphite surface with a rigid nanoasperity

Computer experiments concerning interactions between a graphite surface and the rigid pyramidal nanoasperity of a friction force microscope tip when it is brought close to and retracted from the graphitic sample are presented. Covalent atomic bonds in graphene layers are described using a Brenner potential and tip-carbon forces are derived from the Lennard-Jones potential. For interlayer interactions a registry-dependent potential with local normals is used. The behavior of the system is investigated under conditions of different magnitudes of tip-sample interaction and indentation rates. Strong forces between the nanoasperity and carbon atoms facilitate the cleavage of the graphite surface. Exfoliation, i.e. total removal of the upper graphitic layer, is observed when a highly adhesive tip is moved relative to the surface at low rates, while high rates cause the formation of a small flake attached to the tip. The results obtained may be valuable for enhancing our understanding of the superlubricity of graphite.     

Low-weight fractions of graphene and hydroxyapatite enhance mechanics in photocured methacrylate adhesives

In many applications, it is desirable for photocured adhesives to have high-mechanical strength in the cured state, but relatively low viscosity when liquid. This was achieved by adding less than 0.5 wt% hydroxyapatite and graphene to methyl methacrylate with diurethane dimethacrylate (UDMA-MMA). Nanoindentation shows hardness increasing by 30–40% and indentation modulus by >30% compared to UDMA-MMA on its own. Rheometry shows only a small increase in uncured viscosity for the liquid state. The additives affect the optical properties, mobility of free radicals, photocuring, and degree of conversion, the effects of which are seen in Fourier transform infrared and micro-Raman spectra. Thermographic images taken during curing show that the additives impact the photocuring process. In addition, changes in intermolecular bonding are seen in the vibrational spectra when the additives are present. The enhanced mechanical properties are attributed to the observed changes in photocuring and bonding.     

Influence of processing on fracture toughness of Si3N4 + graphene platelet composites

Silicon nitride + 1 wt% graphene platelet composites were prepared using various graphene platelets (GPL) and two processing routes; hot isostatic pressing (HIP) and gas pressure sintering (GPS). The influence of the processing route and graphene platelets’ addition on the fracture toughness has been investigated. The matrix of the composites prepared by GPS consists of Si₃N₄ grains with smaller diameter in comparison to the composites prepared by HIP. The indentation fracture toughness of the composites was in the range 6.1–9.9 MPa m^0.5, which is significantly higher compared to the monolithic silicon nitride 6.5 and 6.3 MPa m^0.5. The highest value of K_IC was 9.9 MPa m^0.5 in the case of composite reinforced by the smallest multilayer graphene nanosheets, prepared by HIP. The composites prepared by GPS exhibit lower fracture toughness, from 6.1 to 8.5 MPa m^0.5. The toughening mechanisms were similar in all composites in the form of crack deflection, crack branching and crack bridging.     

Microstructure and toughening mechanisms of Al2O3/(W,Ti)C/graphene composite ceramic tool material

To toughen the Al₂O₃ matrix ceramic materials, Al₂O₃/(W, Ti)C/graphene multi-phase composite ceramic materials were fabricated via hot pressing. The effects of the graphene nanoplates (GNPs) content on microstructure and mechanical properties were investigated. Results showed that the fracture toughness and flexural strength of the composite added with just 0.2?wt% GNPs were markedly improved by about 35.3% (~ 7.78?MPa?m^1/2) and 49% (~ 608.54?MPa) respectively compared with the specimens without GNPs while the hardness was kept about 24.22?GPa. However, the mechanical properties degrade with the further increase of GNPs’ content owing to the increased defects caused by agglomeration of GNPs. Synergistic toughening effects of (W, Ti)C and GNPs played an essential role in improving the fracture toughness of composites. By analyzing the microstructures of fractured surface and indentation cracks, besides GNPs pull-out, crack deflection, crack bridging, crack branching and crack arrest, new toughening mechanisms such as break of GNPs and crack guiding were also identified. Furthermore, interface stress can be controlled by means of stagger distributed strong and weak bonding interfaces correlated with the distribution of GNPs.     

铝合金车轮涂膜压痕影响因素研究

为探究铝合金车轮表面涂膜压痕产生因素,根据铝合金车轮涂膜表面压痕状态,表征了涂膜硬度、涂层固化度、涂层组分等性能,研究了涂装工艺及包装工艺对铝合金车轮涂膜表面压痕的影响。结果表明：仓储运输过程中车轮表面温度为影响压痕的主要因素,温度超过 45 ℃将大幅度提升压痕产生概率;涂料相变点硬度变化是压痕产生的根本原因,通过控制车轮包装温度、储存温度、运输温度可以杜绝压痕产生;延长包装后日光下存放时间会增加压痕产生概率。     

基于能量平衡方法的压痕尺寸效应分析

针对硬度测试中出现的压痕尺寸效应现象,采用纳米压痕技术与原子力显微技术相结合,得到压痕过程中的载荷与压深加载卸载曲线和压痕三维图,从中可以得到压痕过程中压头所做的塑性功、最大压深、塑性变形面积、塑性变形体积等,并以此为变量提出了一个基于能量平衡方法的改进模型,此模型能更好地解释压痕尺寸效应.单晶硅实验结果表明:在较大压深下,由于形成新表面所消耗的功是出现压痕尺寸效应的主要因素;随着压痕深度的减小,用于克服材料对压头的阻力而消耗的功所起的作用越来越大;而在更小的压深下,用于产生塑性变形的初始能量和测试系统误差所造成的影响会越来越大,不可忽视.     

Effects of water molecules on tribological behavior and property measurements in nano-indentation processes - a numerical analysis

Nano/micro-manufacturing under wet condition is an important consideration for various tool-based processes such as indentation, scratching, and machining. The existence of liquids adds complexity to the system, changes the tool/work interfacial condition, and affects material behaviors. For indentation, it may also affect material property measurements. However, little effort has been made to study this challenging issue at nano- or atomistic scale. In this study, we tackle this challenge by investigating nano-indentation processes submerged in water using the molecular dynamics (MD) simulation approach. Compared with dry indentation in which no water molecules are present, the existence of water molecules causes the increase of indentation force in initial penetration, but the decrease of indentation force in full penetration. It also reduces the sticking phenomenon between the work and tool atoms during indenter retraction, such that the indentation geometry can be better retained. Meanwhile, nano-indentation under wet condition exhibits the indentation size effect, while dry nano-indentation exhibits the reverse indentation size effect. The existence of water leads to higher computed hardness values at low indentation loads and a smaller value of Young''s modulus. In addition, the friction along the tool/work interface is significantly reduced under wet indentation.     

准静态压痕作用下复合材料层合板损伤阻抗与损伤容限实验研究

参照标准实验方法,开展了复合材料层合板对准静态压痕力的损伤阻抗和损伤容限实验研究,获取了接触力、压痕深度、压头位移等实验数据,并对含静压痕损伤层合板进行了剩余压缩强度试验。研究了压痕深度-接触力与剩余压缩强度-压痕深度的变化关系,并讨论了准静态压痕过程中的损伤演变过程和层合板的压缩破坏模式。结果表明:当层合板表面出现目视勉强可见压痕时,初始损伤发生,压痕深度随接触力增大而明显增大,同时剩余压缩强度随压痕深度增加而明显降低;当达到最大接触力时,层合板失去承载能力,背面可看到大量纤维断裂。对于含静压痕损伤的层合板,压缩破坏模式为贯穿损伤区域的层合板断裂。