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Searching for layered MAX phase-like materials with properties of both ceramics and metals is a topic in its infancy. Herein, through a combination of crystal structure, electronic structure, chemical bonding, and elastic property investigations, we report two MAX phase-like layered materials Rh2YSi and Ir2YSi. Rh2YSi and Ir2YSi have bulk modulus B of 150 and 185 GPa, respectively, which are comparable to the typical MAX phases like Ti2AlC, Ti3AlC2, and Ti3SiC2, but much lower shear modulus G (82 and 97 GPa for Rh2YSi and Ir2YSi, respectively) than MAX phases. The high stiffness is due to the presence of rigid Si2-M-Si3-M (M = Ir, Rh) units, while the low shear deformation resistance is due to the presence of metallic bonds and the weak bonds that link the rigid Si2-M-Si3-M (M = Ir, Rh) units. Based on the low shear deformation resistance and low Pugh's ratio, Rh2YSi and Ir2YSi are predicted as damage-tolerant silicides and promising water vapor-resistant interphase materials for SiCf/SiC composites if yttria or yttrium silicates are formed to protect the SiC fibers in oxygen containing environments. The possible slip systems are {0001} < [ABSTRACT FROM AUTHOR]

Weak interphases play a pivotal role by acting as mechanical fuse to deflect matrix cracks in future ultrahigh-temperature ceramic fiber reinforced ceramic matrix ( UHTCf/ UHTC) composites. However, the interphase materials are not available yet. In this work, the electronic structure, chemical bonding, and mechanical properties of NiB, which is a promising interphase material for UHTCf/ UHTC composites, were investigated. NiB has relatively low shear modulus of 116 GPa, moderate Young's modulus of 307 GPa, but high bulk modulus of 287 GPa. The Pugh's ratio G/B is only 0.404 and the micro hardness is predicted to be 8.2 GPa, indicating that NiB belongs to 'soft' and 'ductile' UHTCs. The possible slip systems are [100](001), [010](001), and [001](010) due to the presence of metallic Ni-Ni and weak Ni-B bonding. Details on the electronic structure and directional dependence of shear and Young's moduli are disclosed to highlight the mechanisms that underpin the properties. [ABSTRACT FROM AUTHOR]

Abstract: A systematical ab initio analysis of ZrP2O7 is presented in this work. Density functional theory (DFT) computations were performed for the electronic, mechanical, lattice dynamical and thermal properties of ZrP2O7. The lattice constants determined from the theoretical calculation are consistent with the experimental results. Based on the analyses on the electronic density of states, charge density and electron localization function of ZrP2O7, heterogeneous bonding nature is revealed and confirmed by the phonon density of states. We also reported the second-order elastic constants and polycrystalline mechanical properties of ZrP2O7 for the first time. According to the calculated polycrystalline moduli, the minimum thermal conductivity of ZrP2O7 is estimated to be 1.15Wm−1 K−1. Our theoretical results illustrate that ZrP2O7 is a promising candidate as thermal barrier coating and high temperature binding material. [Copyright &y& Elsevier]

The electronic structure, mechanical properties and lattice dynamics of YB 4 are investigated using first-principles calculations. The Y–B bonding is ionic-covalent, the Y–Y bonding is metallic, the B–B bonding within B 6 octahedron is weak τ-type covalent and that connecting the octahedra is strong σ-type covalent. The chemical bonding anisotropy has also been confirmed by lattice dynamic properties, and is reflected by the anisotropic elastic stiffness. The maximum Young’s modulus (427 GPa) is 1.5 times larger than its minimum (285 GPa). The lowest shear moduli are associated with the [1 1 0](1 0 0) and [1 1 0](1 1 0) systems. The linear compressibility of a (1.70 × 10 −3 GPa) is larger than that of c (1.47 × 10 −3 GPa). Based on the low c 44 , c 66 and Pugh’s ratio G/B, YB 4 is predicted as a damage to lerant ceramic. YB 4 also has a relatively low Young’s modulus (339 GPa), which ensures it good thermal shock resistance. [ABSTRACT FROM AUTHOR]