Abstract

In this paper we calculated the phase stability, electronic structure and mechanical properties of Nb₄AlC₃ by means of a first-principles pseudopotential total energy method. Based on thermodynamical calculations of the two possible crystal structures of Nb₄AlC₃, a type Nb₄AlC₃ is confirmed to be the preferred equilibrium phase at ambient conditions. The chemical bonding displays layered characteristics that have commonly been reported for MAX ceramics. The equation of state and compressibility of a-Nb₄AlC₃ were investigated.
The material exhibits anisotropic elasticity under hydrostatic pressure: it is more compressible along the c direction than along the a and b directions. The second-order elastic coefficients, bulk modulus, shear modulus and Young’s moduli were reported and compared with those of Nb₂AlC. Since the salt-rock-type Nb–C slab is thicker in Nb₄AlC₃ than that in Nb₂AlC, the former material shows higher elastic stiffness than the latter one; at the same time, Nb₄AlC₃ may display quasi-ductility, which has been well documented for
MAX ceramics.

Abstract

The theoretical elastic stiffness of Y3Si5N9O, the only Y-Si-O-N quaternary crystal that contains a framework of corner-sharing SiN4 and/or SiON3 tetrahedron in three dimensions, was investigated using the firstprinciples total energy calculations. The full set of second order elastic coefficients, polycrystalline bulk and shear moduli, and anisotropic elastic moduli were reported and further compared with those of Y2O3 and β-Si3N4. The equation of state and compressibility of Y3Si5N9O were investigated at pressures up to 50 GPa. The crystal structure is stable up to 50 GPa and exhibits anisotropic compressibility under hydrostatic pressure. The relatively softer YN6 and/or YN5O polyhedra are more prone to distort or deform than the SiN4 and/or SiON3 tetrahedra. Therefore, although the crystal structure of Y3Si5N9O contains a Si-N-O framework similar to that in β-Si3N4, it displays elastic stiffness between those of Y2O3 and β-Si3N4.

Abstract

Dense bulk Ta2AlC ceramic was synthesized by an in situ reaction/hot pressing method using Ta, Al, and C as initial materials. The average grain size of Ta2AlC is 15μm in length and 3μm in width. The physical and mechanical properties were investigated. Ta2AlC is a good electrical and thermal conductor. The flexural strength and fracture toughness of Ta2AlC were measured to be 360MPa and 7.7MPam^1/2, respectively. The typical layered grains contribute to the damage tolerance of this ceramic. After indentation up to 200N at the tensile surface of the beam specimens, no obvious decrease of the residual flexural strength was observed. Even at above 1200 ^oC, Ta2AlC still retains a high Young’s modulus and shows excellent thermal shock resistance, which renders it a promising high-temperature structural material.

Abstract

New layered compounds, (V_0.5Cr_0.5)₃AlC₂, (V_0.5Cr_0.5)₄AlC₃, and (V_0.5Cr_0.5)₅Al₂C₃ were synthesized by reactive hot pressing V, Cr, Al, and graphite powders. The crystal structures of these new phases were determined using a combination of X-ray diffraction and scanning transmission electron microscopy. (V_0.5Cr_0.5)₃AlC₂ is isotypical to Ti₃AlC₂; while (V_0.5Cr_0.5)₄AlC₃ has the Ti₄AlN₃ or a-Ta₄AlC₃-type crystal structure. (V_0.5Cr_0.5)₅Al₂C₃ is formed by periodically stacking of halfunit cells of (V_0.5Cr_0.5)₂AlC and (V_0.5Cr_0.5)₃AlC₂.

Abstract

Two kinds of composites (i.e., conductive and strong Cu–Ti3AlC2 composites) were prepared at 850 °C, while high-strength in situ Cu–TiCx composites were prepared by consolidation at 850 °C and then hot pressing at 1000 °C. In both kinds of composites, the reinforcements were uniformly distributed within the Cu matrix. In Cu–Ti3AlC2 composites, strengthening was achieved by the load transfer through a strong interfacial layer consisting of TiCx and Cu(Al), which was formed by the partial deintercalation of Al from Ti3AlC2. For the in situ Cu–TiCx composites, the higher modulus of TiCx as well as the highly twinned structure formed during processing contributed to the enhancement of strength. It was demonstrated that the
deintercalation of Al from Ti3AlC2 formed substoichiometric Ti3AlxC2 (with x < 1), and no detrimental effect on the electrical conductivity was observed.

Abstract

We investigated the stable (0001) surfaces of M2AlC (M = Ti, V and Cr) using the first-principles plane-wave pseudopotential total energy method. Four possible (0001) terminations were considered by breaking the M–Al and M–C bonds. The corresponding surface energies were calculated and compared. The Al- and M(C)-terminated (0001) surfaces demonstrated better stability than the C- and M(Al)- terminated surfaces by their much lower surface energies. In addition, the stability of surfaces was predicted under various chemical environments as a function of chemical potentials. We further investigated the character of surface relaxations. The electronic structures of the stable Al- and M(C)-terminated surfaces were analyzed.

Abstract

The mechanical and thermodynamic stabilities of M4AlC3 (M = V, Nb and Ta) and Ti4AlN3 polymorphs were investigated by means of the first-principles pseudopotential total energy method. All compounds satisfied the Born criteria for mechanical stability, but had different thermodynamic stabilities. Only Ta4AlC3 showed a possible polymorphic phase transformation driven by thermodynamic competition. The present theoretical results support the polymorphism of Ta4AlC3 in experimental synthesis and explain the underlying thermodynamic mechanism. Polymorphism was excluded from V4AlC3, Nb4AlC3 and Ti4AlN3.

Abstract

Magnetron sputtering deposition Cu and subsequent annealing in the temperature range of 900–1100 ?C for 30–60 min were conducted with the motivation to modify the surface hardness of Ti3SiC2. Owing to the formation of TiC following the reaction Ti3SiC2 + 3Cu→3TiC0.67 +Cu3Si, the surface hardness was enhanced from 5.08 GPa to a maximum 9.65 GPa. In addition, the surface hardness was dependent on the relative amount of TiC, which was related to Cu film thickness, heat treatment temperatures and durations of annealing. Furthermore, after annealing at 1000 ?C for 30 min the Cu-coated Ti3SiC2 has lower wear rate and lower COF at the running-in stage compared with Ti3SiC2 substrate. The reaction was triggered by the inward diffusion of Cu along the grain boundaries and defects of Ti3SiC2. At low temperature and short annealing time, i.e. 900 or 1000 ^oC for 30 min, Cu diffused inward Ti3SiC2 and accumulated at the trigonal junctions first. At higher temperature of 1100 ?C or prolonging the annealing time to 60 min, considerable amount of Cu diffused to Ti3SiC2 and filled up the grain boundaries leaving a mesh structure.

Abstract

In this work, a bulk Nb4AlC3 ceramic was prepared by an in situ reaction/hot pressing method using Nb, Al, and C as the starting materials. The reaction path, microstructure, physical, and mechanical properties of Nb4AlC3 were systematically investigated. The thermal expansion coefficient was determined as 7.2×10^-6
K^-1 in the temperature range of 200^oC–1100 ^oC. The thermal conductivity of Nb4AlC3 increased from 13.5 W. (m . K)^-1 at room temperature to 21.2 W. (m . K)^-1 at 1227 ^oC, and the electrical conductivity decreased from 3.35×10⁶ to 1.13×10×6 Ω^-1 .m^-1 in a temperature range of 5–300 K. Nb4AlC3 possessed a low hardness of 2.6 GPa, high flexural strength of 346 MPa, and high fracture toughness of 7.1 MPa .m^1/2. Most significantly, Nb4AlC3 could retain high modulus and strength up to very high temperatures. The Young’s modulus at 1580 ^oC was 241 GPa (79% of that at room temperature), and the flexural strength could retain the ambient strength value without any degradation up to the maximum measured temperature of 1400 ^oC.

Abstract

In situ synthesis of bulk Al3BC3 was achieved via a reactive hotpressing method using Al, B4C, and graphite powders at 1800 ^oC for 2 h. The reaction path for synthesizing Al3BC3 was investigated. It was found that Al3BC3 formed via the reaction of C, B4C, and Al4C3 above 1180 ^oC. Dense Al3BC3 was prepared with a little B4C and graphite remained. Microstructure observations revealed the plate-like morphology of Al3BC3 grains. Moreover, the mechanical properties of Al3BC3 were characterized (Vickers hardness of 11.1 GPa, bending strength of 185 MPa, fracture toughness of 2.3 MPa .m^1/2, and Young’s modulus of 163 GPa). Young’s modulus decreased slowly with increasing temperature, and at 1600 ^oC remained 79% of that at ambient temperature. These results show that Al3BC3 is a promising lightweight high temperature structural material.

Abstract

γ-Y2Si2O7 is a promising candidate material both for hightemperature structural applications and as an environmental/ thermal barrier coating material due to its unique properties such as high melting point, machinability, thermal stability, low linear thermal expansion coefficient (3.9×10 ^-6/K, 200– 1300 ^oC), and low thermal conductivity (＜3.0 W/m.K above 300 ^oC). The hot corrosion behavior of γ-Y2Si2O7 in thin-filmmolten Na2SO4 at 850–1000 ^oC for 20 h in flowing air was investigated using a thermogravimetric analyzer (TGA) and a mass spectrometer (MS). γ-Y2Si2O7 exhibited good resistance against Na2SO4 molten salt. The kinetic curves were well fitted by a paralinear equation: the linear part was caused by the evaporation of Na2SO4 and the parabolic part came from gas products evolved from the hot corrosion reaction. A thin silicafilm formed under the corrosion scale was the key factor for retarding the hot corrosion. The apparent activation energy for the corrosion of γ-Y2Si2O7 in Na2SO4 molten salt with flowing air was evaluated to be 255 kJ/mol.

Abstract

Si2N2O/BN composites were successfully fabricated. With increasing BN content, the elastic modulus and hardness almost linearly decrease while the flexural strength does not exhibit a dramatic degradation. This is attributed to the fact that the homogeneously dispersed nanosized BN particles inhibit the grain growth of Si2N2O. The critical thermal-shock resistance temperature of the Si2N2O/30 vol% BN composite is enhanced by 4001C than monolithic Si2N2O. The introduction of BN significantly improves the dielectric properties and machinability. The Si2N2O/BN composites show a combination of high strength, low dielectric constant, good thermal shock resistance, and machinability, indicating that they are promising structural/ functional materials.

Abstract

Transmission electron microscopy characterizations and elastic properties of two quaternary carbides, i.e. Zr2(Al(Si))4C5 and Zr3- (Al(Si))4C6 are reported. The space group and atomic-scale microstructures of both compounds were determined using a combination of selected area electron diffraction, convergent beam electron diffraction, high-resolution transmission electron microscopy and Z-contrast scanning transmission electron microscopy. In addition, the combined experimental and theoretical studies on elastic properties for Zr2(Al(Si))4C5 are presented. A full set of second-order elastic constants, bulk modulus, shear modulus, and Young’s modulus were calculated using first-principles calculations. Both experimental and theoretical works demonstrated that quaternary Zr–Al–Si–C ceramics possess close elastic properties to ZrC. Furthermore, Zr2(Al(Si))4C5 retained a high Young’s modulus up to about 1580 ^oC, which can be attributed to its comparable activation energy of lattice drag process to that of ZrC.