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In this work, molecular dynamics (MD) and Car-Parrinello molecular dynamics (CPMD) simulation-based analyses are performed to understand the influence of grain boundary (GB) misorientation on the tensile strength of SiC bicrystals. The tensile strength is governed by the changes in electron density and bond strength of atoms in GBs. An investigation of dislocation activity during mechanical deformation shows that the extent of the propagation of dislocations across the bicrystal grains is directly proportional to the extent of GB misorientation. An analytical relation that predicts the tensile strength as a function of GB misorientation is developed. [ABSTRACT FROM PUBLISHER]

The effect of knock-on atom induced damage on the peak tensile strength of cubic silicon carbide (3C-SiC) is examined using an ab-initio simulation framework based on Car–Parrinello molecular dynamics (CPMD) method. The framework examines the effect of impact damage caused by a knock-on atom with velocities corresponding to four different kinetic energy levels (50 eV, 500 eV, 1 keV, and 2 keV) in three different SiC structure samples with different grain boundary (GB) configurations. Analyses show that peak tensile strength of the examined structures decreases by up to 37% in samples with GBs due to the impact damage caused by knock-on atom when compared with the case of single crystalline SiC under similar conditions. Analyses reveal new insights regarding the influence of bond strength change under knock-on atom induced impact damage on peak tensile strength of the examined structures. It is found that the peak tensile strength of the examined structures is a function of change in temperature, impact energy, and GB configuration. In order to extend the observed correlation of the peak tensile strength with atomic configurations to other structure types, a fractal dimension based approach is adopted to predict structure peak tensile strength as a function of knock-on atom impact energy, temperature, and GB configuration. Analyses show that the tensile strength of the examined SiC structures increases as a function of their fractal dimension increase. Fractal dimensions also change as a function of change in impact energy level and the corresponding damage in an inversely proportional manner. Based on the observed correlations, an empirical relation to predict structure peak tensile strength as a function of simulation parameters is developed. The developed relation is found to predict strength data of structures not included in the fitting with good accuracy. [ABSTRACT FROM PUBLISHER]

Recent developments in SiC based ceramic materials have shown a possibility of graphene dispersion at grain boundaries (GBs) significantly modifying bulk thermomechanical properties. Graphene and SiC have been known to have significant electronic affinity for each other. This work presents an electronic density of states-mechanical strength correlation based understanding of the effect of graphene layer defects on the tensile strength of selected 3C-SiC GBs that incorporate graphene layers. The tensile deformation simulations are performed using Car-Parrinello molecular dynamics (CPMD) method. Three different SiC GBs are examined (unit GB, ∑3 tilt SiC GB, and ∑9 tilt SiC GB). For each examined GB type, graphene layers with four different defect types are examined. Analyses establish quantitative correlation between the tensile strength of the examined GBs and graphene defect type in GBs. Of the defects examined, double vacancy defect most significantly deteriorates GB strength. GB failure strain is mainly determined by graphene cleavage fracture strain. Graphene cleavage is accompanied by monoatomic carbon chain formation, pentagon ring formation, and heptagonal ring formation. The specific mechanism closely depends upon the graphene-SiC interaction strength defined based on total electron density of states. The interaction strength variation is found to be closely correlated to the tensile strength variation in GBs. Examination of electron density variation points to specific sites of graphene cleavage fracture that correlates to earlier failure mechanism observations. Observations are used to predict strength of SiC GBs as a function of GB misorientation angle. The developed relation predicts other reported values in literature quite closely. [ABSTRACT FROM AUTHOR]

Highlights: [•] Grain boundary damage under impact in 3C–SiC examined using ab initio simulations. [•] Higher impact velocity leads to lower grain boundary peak tensile strength. [•] Grain boundary peak tensile strength is a function of their unique fractal dimension. [•] Grain boundary structure-strength relationship established. [ABSTRACT FROM AUTHOR]

Abstract: This work focuses on understanding correlations between thermal conduction and mechanical strength in a model high temperature material interface. Analyses examine single crystal ZrB2, single crystal SiC, and a 〈0001〉–〈111〉 ZrB2–SiC interface using a framework based on Car Parrinello molecular dynamics (CPMD) ab initio simulation method from 500K to 2500K. Analyses indicate that the strength reduction with increase in temperature is strongly correlated to phonon and electron thermal diffusivity change. With increase in temperature, phonon thermal diffusivity increases in the case of ZrB2 and reduces in the cases of SiC as well as the interface. Electron contribution to thermal diffusivity increases with temperature increase in the case of interface. Examination of change in thermal properties at different mechanical strain levels reveals that the mechanisms of strength and thermal property change with increase in temperature may be similar to the mechanisms responsible for property change with change in applied strain. [Copyright &y& Elsevier]

A simulation-based robust design optimization methodology to predict the most suitable microstructures of SiC–Si3N4nanocomposites for desired high-temperature toughness is presented. The focus is on finding robust nanocomposite microstructures with maximum toughness at two temperatures: 1500°C and 1600°C. Within this context a sequential approximate optimization algorithm under uncertainty is applied to six different test problems addressing different aspects of robust microstructure generation. During optimization, statistical uncertainties inherent to the computational microstructural generation are quantified and introduced in the optimization framework. The results show that the SiC volume fraction, the number of Si3N4grains, the grain size distribution of the Si3N4grains, and the grain size of the SiC particles have varied effects on the microstructure toughness at different temperatures. At 1500°C, the preferred microstructure is the one with higher Si3N4volume fraction, whereas at 1600°C, the preferred microstructure is the one with higher SiC volume fraction. [ABSTRACT FROM PUBLISHER]

Silicon carbide (SiC) is an important ceramic material usually found in polycrystalline form with grain boundary thickness ranging from a few nanometers to a few hundred nanometers and grains with multiple orientations with sizes of the order of few micrometers. The present work focuses on analyzing how the interplay between different orientations of SiC grains and different grain boundary thicknesses can be exploited for targeted improvement in the fracture resistance properties of SiC. Crack propagation simulations using the cohesive finite element method (CFEM) are pelformed on the finite element meshes developed on experimentally processed SiC morphologies. Analyses were performed at two different length scales: 300 µm x 60 µm (scale-l :Microscale) and 75 µm x 15 µm (scale-2 :Mesoscale). Lower resolution microstructure at scale-1 does not explicitly consider the presence of grain boundaries (GBs). Higher resolution microstructure at scale-2 explicitly models GBs. Results indicate that the effect of change in grain orientation is on crack path only. The fracture resistance is not significantly affected. The presence of GBs may directly aid in strengthening a microstructure's fracture resistance. However, indirectly it may weaken a microstructure by favoring the formation of microcracks. Significantly higher crack formation in grain interior while lower interfacial energy dissipation in comparison to interfaces indicates overall lower fracture strength of grain interiors in comparison to interfaces. If GBs are not accounted for, the second most influencing factor affecting fracture strength is the average grains size. Overall, it is mainly the GBs not the grain orientation distribution and grain size that significantly affects fracture strength. [ABSTRACT FROM AUTHOR]

Abstract: Experimentally obtained Silicon Carbide (SiC)–Silicon Nitride (Si3N4) nanocomposites have SiC particles with nearly circular cross-section placed in Si3N4 matrix either along grain boundaries (GBs) or in inter-granular locations. In the present investigation, 3-D molecular dynamics (MD) analyses of SiC–Si3N4 nanocomposite deformation are performed at 300 K, 900 K, and 1500 K to understand the effect of SiC particle position with respect to the Si3N4 GBs on the nanocomposite mechanical strength. A range of SiC–Si3N4 nanocomposite phase morphologies having cylindrical SiC particles with diameter of 4 nm distributed in different manners in three different types of Si3N4 phase matrices (single crystalline, bicrystalline, and nanocrystalline) are generated. Analyses reveal that the second phase particles act as significant stress raisers in the case of single crystalline Si3N4 phase matrix. However, the particle’s presence does not have a significant effect on the mechanical strength of bicrystalline or nanocrystalline Si3N4 phase matrices. In order to understand the effect of grain boundary (GB) contamination on mechanical strength of the nanocomposites, structures having GBs with two different configurations: (1) sharp and (2) diffused; are analyzed. The strength of structures with diffused GBs decreased with increase in temperature with one exception for structures where due to the particle clustering the strength improved with increase in temperature. The findings indicate that the SiC–Si3N4 interface plays an important role in strengthening the nanocomposite microstructure with increase in temperature through stronger Si–C–N bonding with increasing temperature. GBs on the other hand soften the structures by facilitating GB sliding based deformation mechanism. Overall, analyses confirm that the temperature dependent strengthening of the nanocomposite owing to SiC second phase particles is a strong function of particle placement along GBs, particle clustering, relative volume fractions of atoms in the interfaces and GBs, and GB thickness. [Copyright &y& Elsevier]

New developments in high temperature ceramic materials technology have focused on obtaining nanocomposite materials with nanoscale features for an optimal control of thermal and mechanical properties. One example is the silicon carbide (SiC)-silicon nitride (Si3N4) nanocomposites with nanosized SiC particles placed either in microsized Si3N4 grains or along Si3N4 grain boundaries (GBs). This work focuses on analyzing the influence of GBs, interfaces, and impurities on thermal and mechanical properties of a set of SiC-Si3N4 nanocomposites at three different temperatures (300 K, 900 K, and 1500 K). Nanocomposite thermal conductivity values predicted in this study are smaller in comparison to the bulk Si3N4 values (-30 W/m K). Even with the volume fraction of SiC phase being limited to maximum 40%, it is shown that the thermal conductivity values could be reduced to less than those of the bulk SiC phase (~3 W/m K) by microstructural feature arrangement. Nanocomposite phonon spectral density values show a short rage structural order indicating a high degree of diffused phonon reflection. Visual analyses of the atomistic arrangements did not reveal any loss of crystallinily in the nanocomposites at high temperatures. This indicates that structural arrangement, not the phase change, is a factor controlling thermal conduction as a function of temperature. The nanocomposite deformation mechanism is a trade-off between the stress concentration caused by SiC particles and Si3N4-Si3N4 GB sliding. The temperature increase tends to work in favor of GB sliding leading to softening of structures. However; microstruc- rural strength increases with increase in temperature when GBs are absent. GBs also contribute to reduction in thermal conductivity as well as increase in fracture strength. Replacement of sharp GBs by diffused GBs having C/N impurities, lowered thermal conductivity, and increased fracture strength. Decrease in SiC-Si3N4 interfaces by removal of SiC particles tends to favor an increase in thermal conductivity as well as fracture resistance. Overall, it is shown that for high temperature mechanical strength improvement, judicious placement of SiC particles and optimal control of GB atomic volume fraction are the main controlling factors. [ABSTRACT FROM AUTHOR]

Abstract: This investigation presents nanoindentation and microindentation creep analyses on polymer derived Si–C–O ceramic coatings at temperatures ranging from room temperature to 500°C. The properties of focus include elastic modulus, hardness, creep exponent, and creep strain rate. Analyses show that at the nanoscopic length scale the deformation mechanism is dominated by dislocation climb and diffusion. With increase in length scale to microscale the thermal activation volume increases by approximately 10 times. The increase in free volume leads to the deformation mechanism switching to volumetric densification and dislocation pile up. An important physical effect analyzed is the effect of increase in temperature on the observed deformation mechanism. At the nanoscale, with increase in temperature, both hardness and elastic moduli show an increase. At the microscale, however, hardness reduces with increase in temperature. The indentation size effect is observed at both scales. However, at the nanoscale the indentation size is linked with strain hardening. At the microscale, a strain softening behavior is observed. [ABSTRACT FROM AUTHOR]

Abstract: 3-D molecular dynamics (MD) analyses of SiC–Si3N4 nanocomposite deformation and SiCO nanocomposite deformation are performed at 300K, 900K, and 1500K. In SiC–Si3N4 nanocomposites, distribution of second phase SiC particles, volume fraction of atoms in GBs, and GB thickness play an important role in temperature dependent mechanical behavior. The deformation mechanism is a trade-off between the stress concentration caused by SiC particles and Si3N4–Si3N4 GB sliding. The temperature increase tends to work in favor of GB sliding leading to softening of structures. However, microstructural strength increases with increase in temperature when GBs are absent. In the case of SiCO nanocomposites, findings indicate that temperature change dependent amorphization of nanodomains, the nanodomain wall placement, the nanodomain wall thickness, and nanodomain size are important factors that directly affect the extent of crystallinity and the strength against mechanical deformation. [Copyright &y& Elsevier]

Silicon carbide (SiC) composites are under consideration for cladding and structural materials in various types of reactors. The effects of ion irradiation and corrosion on stress distribution due to mechanical loading on chemical vapor deposited (CVD) SiC were investigated in this paper by using nanomechanical Raman spectroscopy (NMRS). The stress distribution was analyzed as a function of the oxide formation on a corroded specimen and as a function of ion-induced irradiation damage in an irradiated specimen. A finite element method (FEM) based model was developed based on local mechanical properties measured using nanoindentation to predict the NMRS measured stress distribution. The stress distribution was also predicted theoretically by using a stress concentration factor, which is a function of sample geometry and boundary conditions. The maximum stress obtained theoretically was in good agreement with the FEM model and NMRS based measurements. FEM results captured the stress variation trends and maximum stress value in the analyzed samples. NMRS measurements predicted that corrosion had a greater influence on increasing the maximum value of stress in comparison to ion irradiation. The increase in stress attributed to corrosion in comparison to ion irradiated samples was approximately 10%–20%. [ABSTRACT FROM AUTHOR]