Mao Wen

Effects of substrate bias on the preferred orientation, phase transition and mechanical properties for NbN films grown by direct current reactive magnetron sputtering

NbN films are deposited using direct current reactive magnetron sputtering in discharge of a mixture of N2 and Ar gas, and the effects of substrate bias (Vb) on the preferred orientation, phase transition, and mechanical properties for NbN films are explored by x-ray diffraction, selective area electron diffraction, and nanoindentation measurements. It is found that Vb has a significant influence on the stress in NbN films, leading to a pronounced change in the preferred orientation, phase structure, and hardness. As the substrate is at voltage floating, the stress is tensile. In contrast, as negative Vb is applied, the stress becomes compressive, and increases with increasing the absolute value of negative Vb. It is observed that a phase transition from δ (face-centered cubic) to δ′ (hexagonal) for NbN films occurs as Vb is in the range of −80to−120V, which can be attributed to a decrease in the strain energy for NbN films. In order to explore the relationship between the stress and phase transition as w...

Enhanced tensile strength and thermal conductivity in copper diamond composites with B 4 C coating

Boron carbide (B4C) coating on diamond particle is synthesized by heating diamond particles in a powder mix of H3BO3 and B in Ar atmosphere. The composition, bond state and coverage fraction of boron carbide coating on diamond particles are investigated. The boron carbide coating favors to grow on diamond (100) surface rather than on diamond (111) surface. Cu matrix composites reinforced with B4C-coated diamond particles were made by powder metallurgy. The addition of B4C coating gave rise to a dense composite. The influence of B4C coating on both tensile strength and thermal conductivity of the composite were investigated. When the B4C fully covered on diamond particles, the composite exhibited a greatly increase in tensile strength (115 MPa) which was much higher than that for uncoated-diamond/Cu (60 MPa) composites. Meanwhile, a high thermal conductivity of 687 W/mK was achieved in the B4C-coated-diamond/Cu composites.

Modulation periodicity dependent structure, stress, and hardness in NbN/W2N nanostructured multilayer films

NbN/W2N nano-multilayer films with a modulation periodicity, Λ, ranging from 5.1 to 157.4 nm have been deposited on a Si(100) substrate by reactive magnetron sputtering in Ar/N2 mixtures. The Λ dependent structural and mechanical properties for the resulting NbN/W2N multilayers have been evaluated by means of low-angle x-ray reflectivity, x-ray diffraction, high-resolution transmission electron microscope, and nanoindentation measurements. The finding is that for films with Λ ≤ 10.6 nm, fcc NbN layers are coherent with cubic W2N layers, resulting in NbN layers and W2N layers that are in the compressive and tensile states, respectively. In contrast, as Λ is larger than 10.6 nm, a phase transition from W2N to W occurs in the W2N layer, which is a result of the coherent interface strain relaxation. For this case, all layers are in the compressive state, and the coherent interface disappears. The intrinsic compressive stress evolution with Λ can be interpreted in terms of interface stress. The formation of co...

Enhancement of oxidation resistance via a self-healing boron carbide coating on diamond particles

A boron carbide coating was applied to diamond particles by heating the particles in a powder mixture consisting of H3BO3, B and Mg. The composition, bond state and coverage fraction of the boron carbide coating on the diamond particles were investigated. The boron carbide coating prefers to grow on the diamond (100) surface than on the diamond (111) surface. A stoichiometric B4C coating completely covered the diamond particle after maintaining the raw mixture at 1200 °C for 2 h. The contribution of the boron carbide coating to the oxidation resistance enhancement of the diamond particles was investigated. During annealing of the coated diamond in air, the priory formed B2O3, which exhibits a self-healing property, as an oxygen barrier layer, which protected the diamond from oxidation. The formation temperature of B2O3 is dependent on the amorphous boron carbide content. The coating on the diamond provided effective protection of the diamond against oxidation by heating in air at 1000 °C for 1 h. Furthermore, the presence of the boron carbide coating also contributed to the maintenance of the static compressive strength during the annealing of diamond in air.

Correlation between hardness and pressure of CrB4

The correlation between hardness and pressure for two different structures of CrB4 is investigated by a first-principles approach. With increasing pressure, the hardness gradually decreases, in contrast to the bulk modulus, shear modulus, Youngs modulus and B/G ratio, which monotonically increase. Pressure gives rise to a hardness transition from a superhard to a hard material, which is in good agreement with the experimental data. Moreover, the pressure leads to a brittle-to-ductile transition at 200 GPa based on the analysis of the B/G ratio, which is consistent with the hardness trend. The analysis of the density of states and chemical bonding implies that pressure induces electronic compression and collapse in localized regions, and the variation in hardness originates from the bond reversal between the B–B (3) and B–B (2) covalent bonds, which are located at the applied load plane. Finally, we conclude that the hardness of CrB4 under pressure is related to the B/G ratio and bond characteristics.

Relationship between dielectric coefficient and Urbach tail width of hydrogenated amorphous germanium carbon alloy films

It is found in hydrogenated amorphous germanium carbon alloy films that the dielectric coefficient (e) plays an important role in Urbach tail width (E0), besides the degree of structural disorder that has previously proved to be an important factor contributing to E0 in amorphous semiconductor alloy films. The quantitative relationship between E0 and e has been well explored by means of hydrogen-like atom model of energy levels in defective crystalline semiconductors. It is shown that E0 is proportional to the variation in 1/e2 if the degree of structural disorder remains constant, which agrees well with the existing experimental findings.

Highly hard yet toughened bcc -W coating by doping unexpectedly low B content

Either hardness or toughness has been the core interest in scientific exploration and technological pursuit for a long time. However, it is still a big challenge to enhance the hardness and toughness at the same time, since the improvement of one side is always at the expense of the other one. Here, we have succeeded in dealing with this pair of conflict based on tungsten (W) coating by doping boron (B) via magnetron co-sputtering. The results reveal that the introduction of low concentrations of B (6.3 at. %), in the doping regime, leads to the formation of W(B) supersaturated solid solution with refined grains. Meanwhile, the doping-induced higher compressive stress, higher H/E* and denser microstructure result in a surprising combination of improved hardness (2 × larger than pure W) and superior toughness (higher crack formation threshold compared to pure W). We believe this is an innovative sight to design new generation of transition-metal-based multifunctional coatings. Besides, our results are applicable for industrial application because it can be realized by simple manufacturing approaches, e.g. magnetron sputtering technology.

Temperature-dependent evolution of interfacial zones in SiCf/C/Ti17 composites

The continuous SiC fibre reinforced Ti matrix composites are usually subjected to elevated service temperature, representing a wide range of possible applications relying on the corresponding interfacial stability. SiC fibres coated by a turbostratic C reinforced Ti17 matrix composites (SiCf/C/Ti17) were fabricated and the evolution of interfacial zones subsequently to processing of thermal exposure to distinct conditions: 450 °C/600 h, 800 °C/600 h and 1100 °C/2 h was investigated. The corresponding low-temperature long term and high-temperature short term applications were evaluated. It was discovered that the interfacial zone of the as-processed SiCf/C/Ti17 could be described as turbostratic C||amorphous C||fine-grained TiC||transition TiC||coarse-grained TiC, remaining stable subsequently to 450 °C/600 h of exposure. The same thickness of each sub-layer was observed as the as-processed sample was identified. The 800 °C/600 h thermal treatment induced apparent increment in the coarse-grained TiC sub-layer thickness, reaching twice the as-processed sample increment as well as the grains growth in the fine-grained TiC sub-layer. On the contrast, the 1100 °C/2 h thermal treatment not only induced a remarkable increment in thickness of the coarse-grained TiC sub-layer, but also actuated the grain growth of the fine-grained TiC sub-layer. Consequently, it merged the sub-layer with the transition TiC sub-layer. The diffusion behavior of C atoms activated by different temperatures could be responsible for the aforementioned temperature-dependent evolution of the interfacial zones in the SiCf/C/Ti17.

Thermal Stability of Microstructure and Mechanical Properties of NbNhard Films

Cubic δ-NbNfilm with (200) texture, hexagonalδ′-NbN films with a mixed (100)+(110) texture and (110) texture have been deposited on Si (100) substrate at-40, -160 and-200Vsubstrate bias, respectively. Vacuum heat treatments were performed to investigate the effects of annealing temperature on structural stability and hardness of δ-NbN and δ′-NbN films. The results show that for δ-NbN film and δ′-NbN films with a strong (110) texture,no phase transition occuredafter heat treatments.But for δ′-NbN films with a mixed (100)+(110) texture, phase transition from δ′-NbN to δ-NbNtook place, which can be ascribed to small lattice mismatch between δ′-NbN (100) and δ-NbN (111) and low phase transition barrier. In addition, the high substrate bias can improve the interface adhesion due to interface mixing resulting from high energy ions bombardment. Even after annealing at 900°C, the hardness for δ′-NbN deposited at-200V still remains 32GPa, which shows a potential application at the field of protect coatings.

Correlation between hardness and bond orientation of vanadium borides

The relationship between hardness and bond characteristic of vanadium borides was investigated by first-principles approach. The calculated lattice parameters of V–B system are in good agreement with previous experimental data. The convex hull indicates that the VB are most stable at ground state. The vanadium borides have higher bulk modulus, shear modulus and Youngs modulus, and lower B/G ratio. These vanadium borides are brittle. We predict that the V5B6 and VB2 are potential superhard materials. The nature of hardness is related not only to covalent bonding but also to bond orientation. The B–B and V–B covalent bonds parallel to the load plane are the origin of high levels of hardness.