Adhesion and friction behaviors of a γ-Fe/diamond heterogeneous contact interface: a density functional theory study.

Diamond tools play a vital role in precision machining. However, the adhesive wear restricts their application when Fe-based workpieces are cut by diamond tools. Thus, it is significant to theoretically explain the interface binding mechanism between the diamond and Fe alloy matrix. In this study, the adhesion and friction behaviors of a γ-Fe/diamond (denoted as Fe/C) heterogeneous contact interface were investigated employing density functional theory (DFT). The results show that the transfer of the Fe atom to C atom occurs when the interaction energy for a given configuration is larger than the separation energy of the corresponding Fe surface layers. The energy barriers of the Fe/C(100), (110) and (111) sliding interfaces along the minimum energy path are 1.45, 0.48 and 0.42 J m-2, respectively, indicating that the Fe/C(111) interface is the easiest to slide. Furthermore, the friction potential barrier increases with an increase in the load (1-5 nN) according to the potential energy curves. Moreover, the friction coefficient of the Fe/C interface is larger than 0.2 and provides a theoretical minimum friction coefficient for the Fe/C sliding interface.

Well-defined double hysteresis loop in NaNbO3 antiferroelectrics.

Antiferroelectrics (AFEs) are promising candidates in energy-storage capacitors, electrocaloric solid-cooling, and displacement transducers. As an actively studied lead-free antiferroelectric (AFE) material, NaNbO3 has long suffered from its ferroelectric (FE)-like polarization-electric field (P-E) hysteresis loops with high remnant polarization and large hysteresis. Guided by theoretical calculations, a new strategy of reducing the oxygen octahedral tilting angle is proposed to stabilize the AFE P phase (Space group Pbma) of NaNbO3. To validate this, we judiciously introduced CaHfO3 with a low Goldschmidt tolerance factor and AgNbO3 with a low electronegativity difference into NaNbO3, the decreased cation displacements and [BO6] octahedral tilting angles were confirmed by Synchrotron X-ray powder diffraction and aberration-corrected scanning transmission electron microscopy. Of particular importance is that the 0.75NaNbO3-0.20AgNbO3-0.05CaHfO3 ceramic exhibits highly reversible phase transition between the AFE and FE states, showing well-defined double P-E loops and sprout-shaped strain-electric field curves with reduced hysteresis, low remnant polarization, high AFE-FE phase transition field, and zero negative strain. Our work provides a new strategy for designing NaNbO3-based AFE material with well-defined double P-E loops, which can also be extended to discover a variety of new lead-free AFEs.

Electronic, Tensile, and Sliding Characteristics of the C/Ti Interface: A First-Principles Study.

In this paper, according to the C(111) surface and Ti(112̅0) surface relative positions, three stacking interface models were constructed by the first-principles method, and they were defined as 1st-C(111)/Ti(112̅0), 2nd-C(111)/Ti(112̅0), and 4th-C(111)/Ti(112̅0), respectively. After calculation, the work of interfacial adhesion of the 1st-C(111)/Ti(112̅0), 2nd-C(111)/Ti(112̅0), and 4th-C(111)/Ti(112̅0) interface models is found to be 9.689, 10.246, and 9.714 J/m2, respectively, and their interface energies are observed to be 1.064, 0.507, and 1.039 J/m2, respectively. Moreover, the electronic characteristics of C(111)/Ti(112̅0) interfaces are dominated by polar covalent bonds, supplemented by certain metallicity. When the strain reaches 13, 15, and 12%, respectively, the maximum tensile stress values of 1st-C(111)/Ti(112̅0), 2nd-C(111)/Ti(112̅0), and 4th-C(111)/Ti(112̅0) interface models are observed to be 16.207, 19.183, and 17.393 GPa, respectively. After all C(111)/Ti(112̅0) interfaces fracture under tension, the Ti atoms of the Ti(112̅0) surface are transferred to the C(111) surface, indicating that the strength of Ti-C bonds at the interface is higher than the strength of Ti-Ti bonds inside the Ti(112̅0) surface. The maximum value of the sliding potential energy surface is 1.709 J/m2; the maximum value of the potential energy curve is 0.445 J/m2; and the ideal shear strength of the C(111)/Ti(112̅0) interface is 0.386 GPa. In summary, the interfacial adhesion property of the 2nd-C(111)/Ti(112̅0) interface is better than those of 1st-C(111)/Ti(112̅0) and 4th-C(111)/Ti(112̅0) interfaces.

Pressure-induced structure, elasticity, intrinsic hardness and ideal strength of tetragonal C4N.

A tetragonal C4N (t-C4N) structure was predicted via CALYPSO code, and the effects of pressure on its structural and mechanical properties were studied. The results show that t-C4N is different from various 2D CxNy compounds with a new type 3D crystal structure, which is similar to diamond. Bulk t-C4N is equipped with excellent elastic properties. When the pressure is increased from 0 GPa to 350 GPa, its bulk modulus B, shear modulus G and Young's modulus E are increased from 426.9 GPa to 1123.1 GPa, 371.4 GPa to 582.9 GPa and 863.7 GPa to 1490.9 GPa, respectively. The anisotropic Bmax, Gmax and Emax are increased from 582.38 GPa to 1751.41 GPa, 478.29 GPa to 1033.97 GPa and 1281.26 GPa to 2490.14 GPa, respectively. When the pressure is 0 GPa, the hardness calculated by Chen's and Tian's models are 51.15 GPa and 51.81 GPa, respectively. Its ideal tensile strength in [111] orientation is the smallest (63.46 GPa), which indicates that the (111) planes allow easy cleavage. The smallest ideal shear strength (67.98 GPa) can be obtained in the (111)[11̄0] orientation, which suggests its theoretical hardness is about 67.98 GPa. Due to its excellent mechanical properties, t-C4N can be used as an industrial superhard material.

Effects of Ti Doping on Structure and Internal Stress of Amorphous Carbon Films on the γ-Fe Substrate: Molecular Dynamics Simulation.

In this paper, C and Ti were used as the deposition atoms in the molecular dynamics model. The effects of Ti doping percentages (2, 8, 14, and 20 atom %) on the structure and internal stress of a-C films were investigated. The results showed that with the increase in the Ti percentage, the density of the intrinsic zone is gradually increased, while the growth rate is slowly decreased. The internal stress is gradually changed from compressive stress to tensile stress. Among them, when the Ti percentage is 14 atom %, the internal stress is closer to 0. The C percentage with sp3 hybridization is decreased, while those with sp2 and sp hybridizations are increased. The positions of the first and second peaks in the RDF were shifted to the left. Moreover, the distribution of bond lengths and bond angles in the intrinsic zone tend to change from diamond to graphite, which proves that Ti doping leads to the graphitization of a-C films. In addition, Ti doping affects the internal stress of a-C films by changing the C percentage with sp3 hybridization in them.

High-Temperature Sliding Friction Behavior of Amorphous Carbon Films: Molecular Dynamics Simulation.

With the development of the aerospace industry, the requirement for mechanical parts, which are serviced under extreme conditions such as high temperature, is more and more severe. Amorphous carbon (a-C) films are widely used in the aviation field as a protective coating because of their excellent antiwear and friction-reduction properties. However, a-C films are vulnerable to failure in a high-temperature environment, and a series of complex changes in the friction process make it a challenge to put forward the friction mechanism. Here, the sliding friction behaviors of amorphous carbon (a-C) films at different simulated temperatures (STs) (300-1300 K) were analyzed by molecular dynamics. The density, average coordination number, and local residual stress as well as the hybridization of sp, sp2, and sp3 of a-C films were analyzed to reveal the high-temperature sliding friction mechanism of a-C films. The results show that the friction coefficient (µ) of a-C films increased with increase in ST. Meanwhile, the friction mechanisms of a-C films are different at an ST lower than 800 K and higher than 1100 K. Compared with those before sliding, the local residual stress of all a-C films is relaxed, which causes transformation of sp3 into sp2. Moreover, when ST is lower than 800 K, the µ increased with increase in sp3%. When ST is higher than 1100 K, the stability of a-C films is broken, which results in the rapid increase in µ.