Plasticity‐Controlled Friction and Wear in Nanocrystalline SiC

Wear resistance of ceramics can be improved by suppressing fracture, which can be accomplished either by decreasing the grain size or by reducing the size of the deformation zone. We have combined these two strategies with the goal of understanding the atomistic mechanisms underlying the plasticity‐controlled friction and wear in nanocrystalline (nc) silicon carbide (SiC). We have performed molecular dynamics simulations of nanoscale wear on nc‐SiC with 5 nm grain diameter with a nanoscale cutting tool. We find that grain‐boundary (GB) sliding is the primary deformation mechanism during wear and that it is accommodated by heterogeneous nucleation of partial dislocations, formation of voids at the triple junctions, and grain pull‐out. We estimate the stresses required for heterogeneous nucleation of partial dislocations at triple junctions and shear strength of GBs. Pile up in nc‐SiC consists of grains that were pulled out during deformation. We compare the wear response of nc‐SiC to single‐crystal (sc) SiC and show that scratch hardness of nc‐SiC is lower than that of sc‐SiC. Our results demonstrate that the higher scratch hardness in sc‐SiC originates from nucleation and motion of dislocations, whereas nc‐SiC is more pliable due to additional mechanism of deformation via GB sliding.     

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.     

Towards understanding the scratchability in functional glasses

The chemical stability and transparency of functional glasses make them a suitable candidate for interactive screen displays, optical grade lenses, etc. Such applications demand glasses with increased scratch resistance to maintain their original optical and mechanical properties. For decades, indentation hardness has been used as a parameter to judge the scratch resistance of glasses. Recent technological advancements in research have shed light on the fundamental difference between the indentation hardness and scratch hardness values. Further, it is also essential to understand the fundamental behavior of glasses under various environments such as abrasive and corrosive conditions. To this extent, this review aims to provide a comprehensive overview of the experimental approaches to quantify and understand the scratch resistance of glasses. Specifically, we introduce the basic differences between indentation and scratch experiments, and various scratch quantification models available in the literature for glasses. We also discuss, in detail, the scratch experiments that have been performed on glasses and the role of various environmental conditions. Overall, through this review article, we outline the understanding on scratch behavior of glasses thus far, thereby, shedding light on the open questions which can enable the design of improved scratch-resistant glasses.     

Fracture mechanics of sharp scratch strength of polycrystalline alumina

The strength of a polycrystalline alumina containing controlled scratches introduced by translated sharp contacts is investigated and described by a multiscale fracture mechanics model. Inert strength measurements of samples containing quasi‐static and translated Vickers indentation contacts showed that scratches degraded the strength at normal contact loads an order of magnitude less than those for quasi‐static indentation. The fracture mechanics model developed to describe strength degradation by scratches over the full range of contact loads included toughening effects by crack‐wake bridging at the microscale and lateral crack‐based residual stress relaxation effects at the mesoscale. A critical element of the model is the nonlinear scaling of the residual stress field of a scratch with the normal contact load acting during scratch formation. The similarities and differences in the scratch model in comparison with prior indentation‐strength fracture mechanics models are highlighted by parallel development of both. Central to the scratch model is the use of easily controlled normal contact load as the scratch‐strength measurement variable. Scratch length and orientation are shown to have significant effects on strength. The distributions of scratch widths controlling the intrinsic strengths of as‐received samples are determined and agreement with the observed scratch dimensions is demonstrated.     

On surface deformation of melt‐intercalated polyethylene–clay nanocomposites during scratching

Electron microscopy has been used to examine the mechanically‐induced surface damage introduced during scratching of polyethylene(PE)–clay nanocomposites. The determining role of clay in reducing the susceptibility to surface deformation is predicted from the characteristics of surface morphology and the scratch deformation parameters. The reinforcement of PE with nanoclay reduces the susceptibility to scratch damage and stress whitening. Microcracks and surface deformation features namely wrinkles/ridges are the primary source of light scattering resulting in stress whitening. The scratch deformation behavior is discussed in terms of tensile modulus, percentage crystallinity, elastic recovery, and scratch hardness. Scratch hardness is a relevant parameter that can be appropriately used to determine resistance to scratch deformation. POLYM. ENG. SCI. 46:1625–1634, 2006. © 2006 Society of Plastics Engineers     

Submicrometer characterization of surfaces of epoxy‐based organic–inorganic nanocomposite coatings. A comparison of AFM study with currently used testing techniques

Surface properties (morphology, hardness) of transparent colorless epoxy‐based organic–inorganic nanocomposite coatings were investigated by atomic force microscopy, optical and scanning electron microscopy, nanoindentation, and the Persoz pendulum test. Friction and wear coefficients were obtained from tribological experiments. The influence of mechanical properties and the size, shape, and concentration of additives (colloidal silica particles and montmorillonite sheets) on the measured surface characteristics are discussed. It was found that the highest surface hardness (assigned by nanoindentation, pendulum test or expressed as the scratch resistance) exhibited materials with the glass‐transition temperature close to 20°C. Microcopy techniques revealed that surface morphology is influenced by both types of admixtures: on the nanometer scale by colloidal silica particles and on micrometer scale by montmorillonite platelets. Already 1 wt % of montmorillonite increased friction coefficients and wear resistance without distinctive changes of tensile properties. However, the addition of ? 20 wt. % of silica nanoparticles was necessary for the increase of wear and scratch resistances. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:5763–5774, 2006     

Scratch resistance of mineral‐filled polypropylene materials

Pigmented mineral‐filled polypropylene (PP‐PMF) is marketed as a potential alternative to acrylonitrile‐butyldiene‐styrene (ABS) and Polycarbonate/ABS for automotive interior components. PP‐PMF is more easily damaged by surface scratch/mar, thus limiting its acceptance for such applications. This study focuses on investigating the scratch/mar mechanisms of PP‐PMF having different mineral fillers and additives. A new method is introduced to characterize the scratch visibility by image analysis. A correlation is found between scratch visibility and scratch hardness measured by interferometer. It is found that Wollastonite filler imparts higher scratch/mar resistance. Addition of an interface modifier and a lubricant can further improve scratch/mar resistance. It is also found that Talc‐filled PP‐PMF has poor scratch/mar resistance, irrespective of the addition of an interface modifier or a lubricant.     

Constitutive response of calcium‐silicate‐hydrate layers under combined loading

Calcium‐silicate‐hydrate (C‐S‐H) is the main hydration product for ordinary Portland cement (OPC) materials that exhibits a layered structure containing interfaces that controls the system response to shear deformation at the nanometer scale. In this work, we used molecular statics simulations to study the mechanical behavior of an atomistic model of C‐S‐H under combined loading conditions that are typical of structural applications of these materials. Combined loading is implemented by first compressing or stretching the atomistic structure to impose an external hydrostatic pressure, and then loading the system through both heterogeneous and homogeneous shear deformation. By utilizing two different shear methodologies, we were able to isolate the interface behavior from the bulk response. Our results show several qualitative similarities with that of macroscale cementitious materials including pressure sensitivity of the maximum shear strength and strength asymmetry in compression and tension. This indicates that the well‐known cohesive‐frictional behavior of cementitious materials is fundamental to interfaces between C‐S‐H grains at the nanoscale. Comparing differences in our results with nanoindentation experiments motivate future investigations of the effect of C‐S‐H particle size and morphology on strength scaling properties at the mesoscale. These mesoscale model interactions should include the normal‐stress or pressure dependency that we observe.     

氮化碳超硬防划伤镀膜玻璃的制备与研究

采用工业磁控溅射设备在玻璃基板上依次沉积Al_2O_3/SiO_2/CNx多层复合纳米薄膜,并研究测试了其透过率、耐划性及耐酸碱性。结果表明,通过引入线性阳极层离子源,控制并形成新的磁控溅射镀膜工艺,膜层莫氏硬度达到8级,具有优良的防刮效果;膜层存在△T0.6%的微弱增透效果;经过酸/碱处理,透过率衰减|ΔT|0.1%。     

Scratch deformation characteristics of micrometric wollastonite‐reinforced ethylene‐propylene copolymer composites

The scratch deformation behavior of neat and wollastonite‐containing ethylene‐propylene copolymer composites has been studied by electron microscopy and atomic force microscopy techniques. The study indicates that the severity of plastic deformation during scratch testing in reinforced ethylene‐propylene copolymers is a function of debonding/detachment of wollastonite mineral particles from the ethylene‐propylene matrix. The resistance to scratch deformation was evaluated in terms of scratch hardness, scratch depth, average scratch roughness, and change in gray level before and subsequent to scratching. The data suggests that resistance to scratch deformation follows the sequence: coated + coupled wollastonite‐containing EP copolymer > fine wollastonite‐containing EP copolymer > coated wollastonite‐containing EP copolymer > coarse wollastonite‐containing EP copolymer > neat EP copolymer. EP copolymer containing coated wollastonite and coupling agent is characterized by highest scratch hardness and minimum scratch depth and scratch roughness. The visibility of scratch, quantified in terms of gray level, suggests that coated + coupled wollastonite‐containing copolymer exhibits significantly reduced susceptibility to stress whitening, and is characterized by a lower gray level in the scratch‐deformed regions. In the present case of wollastonite‐containing copolymer composites, the resistance to scratch deformation follows a trend similar to that of gray level or scratch visibility. Polym. Eng. Sci. 44:1738–1748, 2004. © 2004 Society of Plastics Engineers.     

Nature of Interatomic Bonding in Controlling the Mechanical Properties of Calcium Silicate Hydrates

Calcium silicate hydrate (C–S–H) is the most important phase of hydrated cement gel which is the key material in construction industry. It is well accepted that hardened cement paste consists of either poorly crystalline or completely disordered phases. Although a myriad of speculative atomistic models of disordered C–S–H have been proposed, the fundamental basis of structure–property relationships remain elusive. This study focuses upon the correlations between mechanical properties and electronic structure based on well‐defined quantum mechanical parameters. We use 20 CSH minerals with known structure to gain fundamental understanding of structure–property relationship. The results indicate Si–O bond order density, which represents the cumulative bond strength of SiO bonds, has no direct correlation with bulk mechanical properties which is counterintuitive and against conventional wisdom. The variations are determined more precisely by the overall atomic and electronic structure dictated by bond order density of the Ca–O and hydrogen bonds (HB). Most importantly, there is a multifaceted balance between different types of interatomic bonds including the HBs in controlling mechanical properties. HBs categorized in relation to next nearest neighbor (NNN) enable us to identify specific types of HBs that are prevalent in CSH. In certain crystals such as suolunite, the HB network is organized in such a unique way that enhances its mechanical properties. The approach and findings presented in this paper points to a broad roadmap for the developing next‐generation cements.     

Mechanical properties of block poly(propylene carbonate‐cyclohexyl carbonate) investigated by nanoindentation and DMA methodologies

To extend the practical application of poly(propylene carbonate) (PPC), the chemical methods were used to improve its mechanical properties. In this connection, random copolymer poly(propylene‐cyclohexyl carbonate) (PPCHC) and di‐block copolymers poly(propylene carbonate‐cyclohexyl carbonate) (PPC‐PCHC) were synthesized. Dynamic mechanical analysis (DMA), nanoindentation and nanoscratch test were applied to evaluate their mechanical properties. The storage's modulus, Young's modulus (E) and hardness (H) obtained from DMA and nanoindentation tests showed that the introduction of the third monomer cyclohexene oxide (CHO) can greatly improve the mechanical properties of PPC, and that the block copolymer PPC‐PCHC hand better mechanical properties than the random copolymer PPCHC. The annealing treated PPC‐PCHCs exhibited deteriorated mechanical properties as compared with untreated PPC‐PCHC. From the results of scratch tests, the plastic deformation of PPC‐PCHC was smaller than those of PPC and PPCHC. Meanwhile, the plastic deformations of the heat‐treated PPC‐PCHCs were smaller than the untreated PPC‐PCHC because of the possible rearrangement of the molecular chains of PPC‐PCHC. The scratch hardness (H_s) of the block copolymer PPC‐PCHC is larger than random polymer PPCHC and PPC, but lower than the values of heat‐treated samples indicating that the surfaces' hardness of block polymers increase after heat treatment. These different measurement methodologies provide a more precise assessment and understanding for the synthesized block polymers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

An integrated approach towards the study of scratch damage of polymer

To seek a better understanding of the scratch damage of polymers, an integrated analysis approach is proposed in this article. This integrated approach essentially involves (a) the use of a new scratch test device for testing, (b) employing microscopy techniques and image an analysis tool, VIEEW^®, for studying material damage and scratch visibility, and finally (c) performing finite element (FE) modeling to examine the mechanical response of the polymeric substrate involved during the scratch process. Applying this approach to five model material systems and employing linearly increasing load tests, the findings of the fundamental material science study of the scratch damage of these materials are presented. From the three-dimensional FE analysis, the numerical results generated were able to reasonably predict the scratch damage and provide corresponding mechanistic interpretation. The essential link between material science and mechanics outlines the uniqueness of this approach for studying the scratch damage of polymers.     

A new model for simulating particle transport in a low‐viscosity fluid for fluid‐driven fracturing

Fluid‐driven fracture (i.e., Hydraulic fracturing) is an important way to stimulate the well productivity in the development of unconventional reservoirs. A low‐viscosity fluid called slickwater is widely used in the unconventional fracturing. It is a big challenge to simulate the particle (or proppant) transport in the low‐viscosity fluid in a field‐scale fracture. A new model to simulate particle transport in the low‐viscosity fluid in a field‐scale fracture is developed. First, a new parameter is defined, called the rate of proppant bed wash‐out, by which we incorporated the mechanism of proppant bed wash‐out into Eulerian‐Eulerian proppant transport model. Second, we proposed a novel way to consider the effect of proppant settling on the proppant concentration in the upper layer (or suspending layer) to accurately simulate the proppant transport. Additionally, a dimension reduction strategy was used to make the model quickly solved. Our simulation results were compared with published experimental data and they were consistent. After validation, the effect of fluid viscosity, injection rate, fracture height, and proppant concentration on the proppant distribution in a fracture is investigated. This study provides a new model to simulate particle transport. Meanwhile, it gives critical insights into understanding particle transport in the field‐scale fracture. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3542–3552, 2018     

Scratch behavior of epoxy nanocomposites containing α‐zirconium phosphate and core‐shell rubber particles

The effects of nanoscale core‐shell rubber (CSR) particles and α‐zirconium phosphate (ZrP) nanoplatelet fillers on the scratch behavior of epoxy have been examined using a newly established ASTM scratch testing method. The critical load for onset of microcrack formation is utilized to determine scratch resistance of the epoxy nanocomposites. Optical microscopy and scanning electron microscopy were performed to determine failure and fracture patterns caused by the scratch. The findings of this study suggest that the introduction of nanoparticles or nanoplatelets does not necessarily enhance the scratch resistance of epoxy. This implies that increases in ductility and fracture toughness alone, i.e., the epoxy/CSR case, and enhancements in modulus and tensile strength alone, i.e., the epoxy/ZrP case, will not necessarily improve scratch resistance of epoxy matrix. A combination of material property attributes is needed to prepare scratch resistant polymers. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Quasi‐static and dynamic nanoindentation and scratch behavior of multifunctional titania/poly(methyl methacrylate) composite

Nanoindentation (NI) and nanoscratch testing was used to determine the dynamic viscoelastic properties of titania reinforced poly(methyl methacrylate) nanocomposites. It was observed that the dynamic NI have a significant effect on the measured indentation modulus and nano‐hardness of the polymer‐based composite. Agreement was found between quasi‐static and dynamic NI result of the nanocomposites. The sinus‐nanoindentation had a limited effect on the measured viscoelastic properties of the composite. However, tribological properties and scratch hardness confirmed that the titania nanofillers act as the friction coefficient modifier in polymer matrix. POLYM. COMPOS., 35:1372–1376, 2014. © 2013 Society of Plastics Engineers     

Indentation and scratch behavior of functionalized MWCNT–PMMA composites at the micro/nanoscale

Polymethylmethacrylate (PMMA) and functionalized multiwalled carbon nanotube (F‐MWCNT) based composite films were prepared using solution casting method. Nanoindentation and scratch measurements were carried out to study the influence of F‐MWCNT as the reinforcement on the mechanical properties of the composite at the sub‐micron scale. The composites were prepared with varying weight percentages of F‐MWCNT in the PMMA matrix. The composites containing an adequate amount (0.25 wt%) of F‐MWCNT was found to demonstrate the maximum nanomechanical properties, viz. hardness, elastic modulus, recovery index. Scratch resistance measured in terms of coefficient of friction, also showed maximum value for the PMMA composite reinforced with 0.25 wt% of F‐MWCNT. POLYM. COMPOS., 35:948–955, 2014. © 2013 Society of Plastics Engineers     

Mechanical,Tribological, and Thermal Properties of Hot‐Pressed ZrB2‐B4C Composite

The effect of addition of submicrometer‐sized B₄C (5,10 and 15 wt%) on microstructure, phase composition, hardness, fracture toughness, scratch resistance, wear resistance, and thermal behavior of hot‐pressed ZrB₂‐B₄C composites is reported. ZrB₂‐B₄C (10 wt%) composite has V_H1 of 20.81 GPa and fracture toughness of 3.93 at 1 kgf, scratch resistance coefficient of 0.40, wear resistance coefficient of 0.01, and ware rate of 0.49 × 10^?3 mm³/Nm at 10N. Crack deflection by homogeneously dispersed submicrometer‐sized B₄C in ZrB₂ matrix can improve the mechanical and tribological properties. Thermal conductivity of ZrB₂‐B₄C composites varied from 70.13 to 45.30 W/m K between 100°C and 1000°C which is encouraging for making ultra‐high temperature ceramics (UHTC) component.     

Application of multivariate latent variable modeling to pilot-scale spray drying monitoring and fault detection: Monitoring with fundamental knowledge

This work describes the use of multivariate latent variable modeling (LVM) to enhance fundamental understanding of the operational space, the scale differences and the common-cause variability present in the operation of a pharmaceutical spray-dryer. LVM provided a real-time process monitoring and fault detection tool for continuous quality assurance. A latent variable model was built and tested using commercially available software in a pilot-scale facility at Bend Research Pharmaceutical Process Development Inc. (BRPPD) in Bend, OR. The key learning from the exercise at the pilot-scale helped identify and understand the normal variability of the commercial scale equipment.A key advantage of the LVM approach is that the variability that drives the process is easily understood in a fundamental way by interpreting the model parameters in light of fundamental engineering knowledge (e.g., transport phenomena, thermodynamics). The understanding of the common-cause variability enables the better understanding of the differences across scales for this unit. In monitoring the process, the faults are not only detected in a statistical way, but also understood in a fundamental way by using the model to track down the driving forces that were involved in detecting such fault (e.g., an abnormal behavior of the gas momentum across the unit).