For design of multi-layered environmental barrier coatings (EBCs), it is essential to assure mechanical reliability against interface crack initiation and propagation induced by thermal stress owing to a misfit of the coefficients of thermal expansion between the coating layers and SiC/SiC substrate. We conducted finite element method (FEM) analyses to evaluate energy release rate (ERR) for interface cracks and performed experiment to obtain interface fracture toughness to assess mechanical reliability of an EBC with a function of thermal barrier (T/EBC; SiC/SiAlON/mullite/Yb-silicate gradient composition layer/Yb₂SiO₅ with porous segment structure) on an SiC/SiC substrate under thermal stress due to cooling in fabrication process. Our FEM analysis revealed that a thinner SiAlON layer and a thicker mullite layer are most suitable to reduce ERRs for crack initiation at the SiC/SiAlON, SiAlON/mullite and mullite/Yb₂Si₂O₇ interfaces. Interface fracture tests of the T/EBC with layer thicknesses within the proposed range exhibited fracture at the SiC/SiAlON and SiAlON/mullite interfaces. We also estimated the approximate fracture toughness for the SiC/SiAlON and SiAlON/mullite interfaces and lower limit of fracture toughness for the mullite/Yb₂Si₂O₇ interface. Comparison between ERR and fracture toughness indicates that the fabricated T/EBC possesses sufficient mechanical reliability against interface crack initiation and propagation.

Ceramic matrix composites (CMCs) based on silicon carbide (SiC) are used for high-temperature applications such as the hot section in turbines. For such applications, the mechanical properties at a high temperature are essential for lifetime prediction and reliability design of SiC-based CMC components. We developed an interatomic potential function based on the artificial neural network (ANN) model for silicon-carbon systems aiming at investigation of high-temperature mechanical properties of SiC materials. We confirmed that the developed ANN potential function reproduces typical material properties of the single crystals of SiC, Si, and C consistent with first-principles calculations. We also validated applicability of the developed ANN potential to a simulation of an amorphous SiC through the analysis of the radial distribution function. The developed ANN potential was applied to a series of creep test for an amorphous SiC model, focusing on the amorphous phase, which is expected to be formed in the SiC-based composites. As a result, we observed two types of creep behavior due to different atomistic mechanisms depending on the strain rate. The evaluated activation energies are lower than the experimental values in literature. This result indicates that an amorphous region can play an important role in the creep process in SiC composites.

Ideal shear strength under superimposed normal stress of cubic covalent crystals (C, Si, Ge, and SiC) is evaluated by ab initio density functional theory calculation. Shear directions in [112] and [110] on the (111) plane are examined. The critical shear stress along the former direction is lower than that along the latter in all the crystals unless the hydrostatic tension is extremely high. In both the [112]-shear and [110]-shear, critical shear stress is increased by compression in C but is decreased in the other crystals. The different response of the critical shear stress to normal stress is due to the strength of the bond-order term, i.e., dependence of the short-range interatomic attraction on the bond-angle.

The theoretical shear strength (τ _c ) and its temperature dependence for a series of FCC and HCP metals have been calculated. Despite the obtained differences in temperature dependences of τ _c , critical deformation (γ _c ), and shear modulus (G), the fact of a weak temperature dependence of ratio Gγ _c /τ _c relating these quantities is established for all studied materials. The deviation of Gγ _c /τ _c from the average value in the temperature ranges under consideration is no more than 12%.

To elucidate the mechanism responsible for structural instability at the atomic level, atomistic modeling simulation of tension in a Cu thin film containing a notch was performed using an embedded-atom method potential and dislocation nucleation was observed. Mechanical stability during tension was analyzed by solving the eigenvalue problem of the Hessian matrix taking into account all the degrees of freedom of the atoms in the system. Since an eigenvalue designates the curvature of the potential energy landscape in the direction of the corresponding eigenvector, which indicates a deformation mode, the system is unstable under vanishing temperature at the critical strain ($\epsilon${}c) when any eigenvalue is zero or negative. At a strain smaller than $\epsilon${}c where all the eigenvalues are positive, atomic fluctuations due to finite temperature may cause structural instability. We found that the path of activated instability (dislocation emission from the notch) could be written with a linear combination of the eigenvectors having small eigenvalues obtained under a corresponding external strain at zero temperature. The energy landscape has a much lower hill along the mixed-mode path than along any single-mode paths. In a molecular dynamics simulation under finite temperature, components of deformation modes having small eigenvalues fluctuate at low frequency, which dominate the activation of instability.