A New Framework Based on a Discrete Element Method to Model the Fracture Behavior for Brittle Polycrystalline Materials

This work aims to develop and implement a linear elastic grain-level micromechanical model based on the discrete element method using bonded contacts and an improved fracture criteria to capture both intergranular and transgranular microcrack initiation and evolution in polycrystalline ceramics materials. Gaining a better understanding of the underlying mechanics and micromechanics of the fracture process of brittle polycrystalline materials will aid in high performance material design. Continuum mechanics approaches cannot accurately simulate the crack propagation during fracture due to the discontinuous nature of the problem. In this work we distinguish between predominately intergranular failure (along the grain boundaries) versus predominately transgranular failure (across the grains) based on grain orientation and microstructural parameters to describe the contact interfaces and present the first approach at fracturing discrete elements. Specifically, the influence of grain boundary strength and stiffness on the fracture behavior of an idealized ceramic material is studied under three different loading conditions: uniaxial compression, brazilian, and four-point bending. Digital representations of the sample microstructures for the test cases are composed of hexagonal, prismatic, honeycomb-packed grains represented by rigid, discrete elements. The principle of virtual work is used to develop a microscale fracture criteria for brittle polycrystalline materials for tensile, shear, torsional and rolling modes of intergranular motion. The interactions between discrete elements within each grain are governed by traction displacement relationships

Calculation of Boron Nitride Sputter Yields Under Low Energy Xenon Ion Bombardment

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77064/1/AIAA-2007-5313-409.pd

An atomistic investigation on the nanometric cutting mechanism of hard, brittle materials

The demand for ultra precision machined devices and components is growing at a rapid pace in various areas such as the aerospace, energy, optical, electronics and bio-medical industries. Because of their outstanding engineering properties such as high refractive index, wide energy bandgap and low mass density, there is a continuing requirement for developments in manufacturing methods for hard, brittle materials. Accordingly, an assessment of the nanometric cutting of the optical materials silicon and silicon carbide (SiC), which are ostensibly hard and brittle, has been undertaken. Using an approach of parallel molecular dynamics simulations with a three-body potential energy function combined with experimental characterization, this thesis provides a quantitative understanding of the ductile-regime machining of silicon and SiC (polytypes: 3C, 4H and 6H SiC), and the mechanism by which a diamond tool wears during the process. The distinctive MD algorithm developed in this work provides a comprehensive analysis of thermal effects, high pressure phase transformation, tool wear (both chemical and abrasive), influence of crystal anisotropy, cutting forces and machining stresses (hydrostatic and von Mises), hitherto not done so far. The calculated stress state in the cutting zone during nanometric cutting of single crystal silicon indicated Herzfeld–Mott transition (metallization) due to high pressure phase transformation (HPPT) of silicon under the influence of deviatoric stress conditions. Consequently, the transformation of pristine silicon to β-silicon (Si-II) was found to be the likely reason for the observed ductility of bulk silicon during its nanoscale cutting. Tribochemical formation of silicon carbide through a solid state single phase reaction between the diamond tool and silicon workpiece in tandem with sp3-sp2 disorder of carbon atoms from the diamond tool up to a cutting temperature of 959 K has been suggested as the most likely mechanism through which a diamond cutting tool wears while cutting silicon. The recently developed dislocation extraction algorithm (DXA) was employed to detect the nucleation of dislocations in the MD simulations of varying cutting orientation and cutting direction. Interestingly, despite of being a compound of silicon and carbon, silicon carbide (SiC) exhibited characteristics more like diamond, e.g. both SiC iii workpiece and diamond cutting tool were found to undergo sp3-sp2 transition during the nanometric cutting of single crystal SiC. Also, cleavage was found to be the dominant mechanism of material removal on the (111) crystal orientation. Based on the overall analysis, it was found that 3C-SiC offers ease of deformation on either (111) , (110) or (100) setups. The simulated orthogonal components of thrust force in 3C-SiC showed a variation of up to 45% while the resultant cutting forces showed a variation of 37% suggesting that 3C-SiC is anisotropic in its ease of deformation. The simulation results for three major polytypes of SiC and for silicon indicated that 4H-SiC would produce the best sub-surface integrity followed by 3C-SiC, silicon and 6H-SiC. While, silicon and SiC were found to undergo HPPT which governs the ductility in these hard, brittle materials, corresponding evidence of HPPT during the SPDT of polycrystalline reaction bonded SiC (RB-SiC) was not observed. It was found that, since the grain orientation changes from one crystal to another in polycrystalline SiC, the cutting tool experiences work material with different crystallographic orientations and directions of cutting. Thus, some of the grain boundaries cause the individual grains to slide along the easy cleavage direction. Consequently, the cutting chips in RB-SiC are not deformed by plastic mechanisms alone, but rather a combination of phase transformation at the grain boundaries and cleavage of the grains both proceed in tandem. Also, the specific-cutting energy required to machine polycrystalline SiC was found to be lower than that required to machine single crystal SiC. Correspondingly, a relatively inferior machined surface finish is expected with a polycrystalline SiC. Based on the simulation model developed, a novel method has been proposed for the quantitative assessment of tool wear from the MD simulations. This model can be utilized for the comparison of tool wear for various simulation studies concerning graphitization of diamond tools. Finally, based on the theoretical simulation results, a novel method of machining is proposed to suppress tool wear and to obtain a better quality of the machined surface during machining of difficult-to-machine materials

Modelling Polycrystalline Materials: An Overview of Three-Dimensional Grain-Scale Mechanical Models

International audienc

Atomistic Simulation and Virtual Diffraction Characterization of Alumina Interfaces: Evaluating Structure and Stability for Predictive Physical Vapor Deposition Models

The objectives of this work are to investigate the structure and energetic stability of different alumina (Al2O3) phases using atomistic simulation and virtual diffraction characterization. To meet these objectives, this research performs molecular statics and molecular dynamics simulations employing the reactive force-field (ReaxFF) potential to model bulk, interface, and surface structures in the θ-, γ-, κ-, and α-Al2O3 system. Simulations throughout this study are characterized using a new virtual diffraction algorithm, developed and implemented for this work, that creates both selected area electron diffraction (SAED) and x-ray diffraction (XRD) line profiles without assuming prior knowledge of the crystal system. First, the transferability of the ReaxFF potential is evaluated by modelling different alumina bulk systems. ReaxFF is shown to correctly predict the energetic stability of α-Al2O3 among the crystalline alumina phases, but incorrectly predicts an even lower energy amorphous phase. Virtual XRD patterns uniquely identify each phase and validate the minimum energy bulk structures through experimental comparison. Second, stable and metastable alumina surfaces are studied at 0, 300, 500, and 700 K. ReaxFF predicts minimum energy surface structures and energies in good agreement with prior studies at 0 K; however, select surface models at 500 and 700 K undergo significant reconstructions caused by the unnatural bias for a lower-energy amorphous phase. Virtual SAED analysis performed on alumina surfaces allow advanced characterization and direct experimental validation of select models. Third, ReaxFF is used to model homophase and heterophase alumina interfaces at 0 K. Predicted minimum energy structures of α-Al2O3 interfaces show good agreement with prior works, which provides the foundation for the first atomistic study of metastable alumina grain boundaries and heterophase alumina interfaces. Virtual SAED patterns characterize select alumina interfaces and help guide the construction of low-energy heterophase alumina interfaces by providing insight into crystallographic compatibilities. Combined, the energetic data extracted from bulk, surface, and interface simulations as well as insights gained through virtual diffraction will aid the development of mesoscale predictive models of polycrystalline alumina formation during physical vapor deposition

Applying molecular dynamics simulations to the study of micro cold spray of yttria

Micro cold spray (MCS) is a novel manufacturing process with proven success depositing dense, thick films of technical ceramics without the need for elevated processing temperatures or chemical precursors. The process transforms a loose submicrometer powder into an adherent film with mechanical properties approaching that of the bulk material through a phenomenon known as room temperature impact consolidation. While this phenomenon has been demonstrated for a wide variety of ceramic materials and film substrates, yttria stands out as a material that is both well-characterized in experiments and of significant interest for industrial applications due to its excellent refractory properties. This dissertation seeks to investigate the fundamental mechanisms that enable room temperature impact consolidation by using molecular dynamics (MD) simulations of yttria particle impacts and characterizing the deformation behavior of the system. The behavior of yttria nanoparticles impacting yttria substrates is first explored by systematically varying the impact velocity (300-1200 m/s), particle diameter (5-8 nm), and orientations of both the particle and substrate. Once the key variables have been identified, further studies were performed using much larger particles (12-50 nm), which unlike the smaller particles, exhibited shear localization. This is a key phenomenon responsible for particle sticking during cold spray of micron-scale particles used in the cold spray process. Thereafter, a plane strain geometry was developed to further extend the particle sizes accessible to MD simulations (25-200 nm), which showed that for particles with similar sizes to those used in experiments, the deformation behavior is dominated by shear localization and fragmentation. Finally, a novel technique for equilibrating ionic MD simulations that requires ∼1250 times less computational resources than the previous methodology is presented.Mechanical Engineerin

Vision 2040: A Roadmap for Integrated, Multiscale Modeling and Simulation of Materials and Systems

Over the last few decades, advances in high-performance computing, new materials characterization methods, and, more recently, an emphasis on integrated computational materials engineering (ICME) and additive manufacturing have been a catalyst for multiscale modeling and simulation-based design of materials and structures in the aerospace industry. While these advances have driven significant progress in the development of aerospace components and systems, that progress has been limited by persistent technology and infrastructure challenges that must be overcome to realize the full potential of integrated materials and systems design and simulation modeling throughout the supply chain. As a result, NASA's Transformational Tools and Technology (TTT) Project sponsored a study (performed by a diverse team led by Pratt & Whitney) to define the potential 25-year future state required for integrated multiscale modeling of materials and systems (e.g., load-bearing structures) to accelerate the pace and reduce the expense of innovation in future aerospace and aeronautical systems. This report describes the findings of this 2040 Vision study (e.g., the 2040 vision state; the required interdependent core technical work areas, Key Element (KE); identified gaps and actions to close those gaps; and major recommendations) which constitutes a community consensus document as it is a result of over 450 professionals input obtain via: 1) four society workshops (AIAA, NAFEMS, and two TMS), 2) community-wide survey, and 3) the establishment of 9 expert panels (one per KE) consisting on average of 10 non-team members from academia, government and industry to review, update content, and prioritize gaps and actions. The study envisions the development of a cyber-physical-social ecosystem comprised of experimentally verified and validated computational models, tools, and techniques, along with the associated digital tapestry, that impacts the entire supply chain to enable cost-effective, rapid, and revolutionary design of fit-for-purpose materials, components, and systems. Although the vision focused on aeronautics and space applications, it is believed that other engineering communities (e.g., automotive, biomedical, etc.) can benefit as well from the proposed framework with only minor modifications. Finally, it is TTT's hope and desire that this vision provides the strategic guidance to both public and private research and development decision makers to make the proposed 2040 vision state a reality and thereby provide a significant advancement in the United States global competitiveness

Polycrystalline Perovskite Ferroelectrics: Microstructural Origins of the Macroscale Electromechanical Response

Ferroelectrics are a class of electromechanically coupled materials which possess an electric dipole polarization that can be permanently reoriented by applied electric and mechanical stress fields. Their reorientable polarization results in complex, nano- to micrometer scale domain structures whose evolution under electric and mechanical stress fields alters the material's overall time-dependent electrical and viscoelastic properties. To understand domain structure evolution, in-situ microscopy of domain switching processes in ferroelectric thin films, single crystals and nanoparticles have been well-studied in the past. However, domain evolution in bulk polycrystals is less well understood as their local stress and electric field environment differs from thin specimens. This work seeks to understand ferroelectric domain evolution in bulk ferroelectric perovskite polycrystals using a combination of a recently-developed electromechanical characterization technique, Broadband Electromechanical Spectroscopy (BES), and theoretical-computational predictions. A constitutive material model for polycrystalline ferroelectrics is first developed and applied to simulate barium titanate single crystals and polycrystals. Simulated polarization, strain and energy dissipation hysteresis curves show good qualitative agreement to experimental data and demonstrate that macroscale properties can be efficiently predicted from microscale physics to some extent. The microstructural origins of fatigue behavior in bulk polycrystalline lead zirconate titanate (PZT) are investigated using a combination of macroscale electrical and viscoelastic property characterization via BES, and scanning electron microscopy (SEM) imaging of microstructure. The evolution of electrical and viscoelastic properties during bipolar electrical fatigue show differences in the effects of electrical vs. mechanical fatigue processes, and the latter is verified through SEM imaging and measurement of microcracks. Finally, the same electromechanical BES characterizations are performed on specimens of bulk polycrystalline barium titanate (BT). Results reveal stark qualitative differences in electrical and viscoelastic responses from PZT despite both materials being perovskite ferroelectrics. A growth vs. nucleation hypothesis is proposed to explain the observed results, guided by preliminary imaging of domain microstructure. In summary, the BES is a powerful tool to elucidate domain switching processes within bulk ferroelectric specimens, while a computational method which bridges the micro- and macroscale further adds to the diagnostic toolbox of understanding bulk ferroelectric domain switching mechanisms. This opens the pathway to designing future applications which make use of the unique electrical and viscoelastic properties of ferroelectric switching.</p

An investigation of mechanics in nanomachining of Gallium Arsenide

The first two decades of the 21st Century have seen a wide exploitation of Gallium Arsenide (GaAs) in photoemitter device, microwave devices, hall element, solar cell, wireless communication as well as quantum computation device due to its superior material properties, such as higher temperature resistance, higher electronic mobility and energy gap that outperforms silicon. Ultra-precision multiplex two dimensional (2D) or three dimensional (3D) free-form nanostructures are often required on GaAs-based devices, such as radio frequency power amplifiers and switches used in the 5G smart mobile wireless communication. However, GaAs is extremely difficult to machine as its elastic modulus, Knoop hardness and fracture toughness are lower than other semiconductor materials such as silicon and germanium. This PhD thesis investigated the mechanics of nanomachining of GaAs through molecular dynamics (MD) simulation combined with single point diamond turning (SPDT) and atomic force microscope (AFM) based experimental characterization in order to realise ductile-regime nanomachining of GaAs, which is the most important motivation behind this thesis. The investigation of mechanics of nanomachining of GaAs included studies on cutting temperature, cutting forces, origin ductile plasticity, atomic scale friction, formation mechanism of sub-surface damage, wear mechanism of diamond cutting tool. Machinability of GaAs at elevated temperature was also studied in order to develop thermally-assisted nanomachining process in the future to facilitate plastic material deformation and removal. This thesis contributed to address the knowledge gaps such as what is the incipient plasticity, how does the sub-surface damage form and how does the diamond cutting tool wear during nanomachining of GaAs. Firstly, this thesis investigated the cutting zone temperature, cutting forces and origin of plasticity of GaAs material, including single crystal GaAs and polycrystalline GaAs during SPDT process. The experimental and MD simulation study showed GaAs has a strong anisotropic machinability. The simulation results indicated that the deformation of polycrystalline GaAs is accompanied by dislocation nucleation in the grain boundaries (GBs) leading to the initiation of plastic deformation. Furthermore, the 1/2 is the main type of dislocation responsible for ductile plasticity in polycrystalline GaAs. A phenomenon of fluctuation from wave crests to wave troughs in the cutting forces was only observed during cutting of polycrystalline GaAs, not for single-crystal GaAs. Secondly, this thesis studied the atomic scale friction during AFM-based nanomachining process. a strong size effect was observed when the scratch depths are below 2 nm in MD simulations and 15 nm from the AFM experiments respectively. A strong quantitative corroboration was obtained between the MD simulations and the AFM experiments in the specific scratch energy and more qualitative corroboration with the pile up and the kinetic coefficient of friction. This conclusion suggested that the specific scratch energy is insensitive to the tool geometry and the speed of scratch used in this investigation but the pile up and kinetic coefficient of friction are dependent on the geometry of the tool tip. Thirdly, this thesis investigated formation mechanism of sub-surface damage and wear mechanism of diamond cutting tool during nanomachining of GaAs. Transmission Electron Microscope (TEM) measurement of sub-surface of machined nanogrooves on GaAs and MD simulation of dislocation movement indicated the dual slip mechanisms i.e. shuffle-set slip mechanism and glide-set slip mechanism, and the creation of dislocation loops, multi dislocation nodes, and dislocation junctions governed the formation mechanism of sub-surface damage of GaAs during nanomachining process. Elastic-plastic deformation at the apex of the diamond tip was observed in MD simulations. Meanwhile, a transition of the diamond tip from its initial cubic diamond lattice structure sp3 hybridization to graphite lattice structure sp2 hybridization was revealed. Graphitization was, therefore, found to be the dominant wear mechanism of the diamond tip during nanometric cutting of single crystal GaAs. Finally, in MD simulations study of cutting performance at elevated temperature, hotter conditions resulted in the reduction of cutting forces by 25% however, the kinetic coefficient of friction went up by about 8%. While material removal rate was found to increase with the increase of the substrate temperature, it was accompanied by an increase of the sub-surface damage in the substrate. Moreover, a phenomenon of chip densification was found to occur during hot cutting which referred to the fact that the amorphous cutting chips obtained from cutting at low temperature will have lower density than the chips obtained from cutting at higher temperatures.The first two decades of the 21st Century have seen a wide exploitation of Gallium Arsenide (GaAs) in photoemitter device, microwave devices, hall element, solar cell, wireless communication as well as quantum computation device due to its superior material properties, such as higher temperature resistance, higher electronic mobility and energy gap that outperforms silicon. Ultra-precision multiplex two dimensional (2D) or three dimensional (3D) free-form nanostructures are often required on GaAs-based devices, such as radio frequency power amplifiers and switches used in the 5G smart mobile wireless communication. However, GaAs is extremely difficult to machine as its elastic modulus, Knoop hardness and fracture toughness are lower than other semiconductor materials such as silicon and germanium. This PhD thesis investigated the mechanics of nanomachining of GaAs through molecular dynamics (MD) simulation combined with single point diamond turning (SPDT) and atomic force microscope (AFM) based experimental characterization in order to realise ductile-regime nanomachining of GaAs, which is the most important motivation behind this thesis. The investigation of mechanics of nanomachining of GaAs included studies on cutting temperature, cutting forces, origin ductile plasticity, atomic scale friction, formation mechanism of sub-surface damage, wear mechanism of diamond cutting tool. Machinability of GaAs at elevated temperature was also studied in order to develop thermally-assisted nanomachining process in the future to facilitate plastic material deformation and removal. This thesis contributed to address the knowledge gaps such as what is the incipient plasticity, how does the sub-surface damage form and how does the diamond cutting tool wear during nanomachining of GaAs. Firstly, this thesis investigated the cutting zone temperature, cutting forces and origin of plasticity of GaAs material, including single crystal GaAs and polycrystalline GaAs during SPDT process. The experimental and MD simulation study showed GaAs has a strong anisotropic machinability. The simulation results indicated that the deformation of polycrystalline GaAs is accompanied by dislocation nucleation in the grain boundaries (GBs) leading to the initiation of plastic deformation. Furthermore, the 1/2 is the main type of dislocation responsible for ductile plasticity in polycrystalline GaAs. A phenomenon of fluctuation from wave crests to wave troughs in the cutting forces was only observed during cutting of polycrystalline GaAs, not for single-crystal GaAs. Secondly, this thesis studied the atomic scale friction during AFM-based nanomachining process. a strong size effect was observed when the scratch depths are below 2 nm in MD simulations and 15 nm from the AFM experiments respectively. A strong quantitative corroboration was obtained between the MD simulations and the AFM experiments in the specific scratch energy and more qualitative corroboration with the pile up and the kinetic coefficient of friction. This conclusion suggested that the specific scratch energy is insensitive to the tool geometry and the speed of scratch used in this investigation but the pile up and kinetic coefficient of friction are dependent on the geometry of the tool tip. Thirdly, this thesis investigated formation mechanism of sub-surface damage and wear mechanism of diamond cutting tool during nanomachining of GaAs. Transmission Electron Microscope (TEM) measurement of sub-surface of machined nanogrooves on GaAs and MD simulation of dislocation movement indicated the dual slip mechanisms i.e. shuffle-set slip mechanism and glide-set slip mechanism, and the creation of dislocation loops, multi dislocation nodes, and dislocation junctions governed the formation mechanism of sub-surface damage of GaAs during nanomachining process. Elastic-plastic deformation at the apex of the diamond tip was observed in MD simulations. Meanwhile, a transition of the diamond tip from its initial cubic diamond lattice structure sp3 hybridization to graphite lattice structure sp2 hybridization was revealed. Graphitization was, therefore, found to be the dominant wear mechanism of the diamond tip during nanometric cutting of single crystal GaAs. Finally, in MD simulations study of cutting performance at elevated temperature, hotter conditions resulted in the reduction of cutting forces by 25% however, the kinetic coefficient of friction went up by about 8%. While material removal rate was found to increase with the increase of the substrate temperature, it was accompanied by an increase of the sub-surface damage in the substrate. Moreover, a phenomenon of chip densification was found to occur during hot cutting which referred to the fact that the amorphous cutting chips obtained from cutting at low temperature will have lower density than the chips obtained from cutting at higher temperatures