Qingnan Meng

Synthesis of graphene on a polycrystalline Co film by radio-frequency plasma-enhanced chemical vapour deposition

Graphene of 1–5 layers was synthesized on a polycrystalline Co film by radio-frequency plasma-enhanced chemical vapour deposition at a relatively low temperature of 800 ◦ C for only 40 s in a mixture of gases of CH4/H2/Ar (1/5/20, with a total gas flow rate of 78 sccm). The obtained graphene is of high quality as revealed by Raman spectroscopy and x-ray photoelectron spectroscopy. It exhibits a high optical transmittance of more than 70% in the wavelength range 500–1200 nm, and a sheet resistivity of 2661 � /sq. A possible formation mechanism is proposed. The significant influence of octahedral and tetrahedral lattice interstitial sites in the Co crystallites on the formation of graphene is discussed. (Some figures in this article are in colour only in the electronic version)

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.

Enhanced bending strength and thermal conductivity in diamond/Al composites with B 4 C coating

Diamond/Al composites containing B4C-coated and uncoated diamond particles were prepared by powder metallurgy. The microstructure, bending strength and thermal conductivity were characterized considering the B4C addition and diamond fraction. The influence of B4C coating and fraction of diamond on both bending strength and thermal conductivity were investigated. The bending strength increased with decreasing diamond fraction. Moreover, addition of B4C coating led to an obvious increase in bending strength. The peak value at 261.2 MPa was achieved in the composite with 30 vt.% B4C-coated diamond particles, which was about twice of that for 30 vt.% uncoated diamond/Al composite (140.1 MPa). The thermal conductivity enhanced with the increase in diamond fraction, and the highest value (352.7 W/m·K) was obtained in the composite with 50 vt.% B4C-coated diamond particles. Plating B4C on diamond gave rise to the enhancement in bending strength and thermal conductivity for diamond/Al composites, because of the improvement of the interfacial bonding between diamond and aluminum matrix.

Deposition and Characterization of Reactive Magnetron Sputtered Tungsten Carbide Films

Tungsten carbide thin films were deposited on silicon (100) substrates by DC reactive magnetron sputtering using CH4 as a carbon source. The microstructure, compressive stress, hardness and tribological behaviors showed great dependences on the rates of CH4 flow (FCH4). Increasing the FCH4 from 2 to 5 sccm, the film exhibited a phase transition from hexagonal-W2C to cubic-WC1-x. Further increasing the FCH4 larger than 10sccm, the film presented amorphous state. As the FCH4 increased, the Raman revealed that the films showed a graphitization trend, meanwhile, the surface of the films became smoother and smoother. The hardness of tungsten carbide films first increased, and then decreased after reaching the maximum 38.5GPa (FCH4=10 sccm). While the sample deposited at 15 sccm obtained the lowest wear rate (2.17×10-6 mm3/Nm) and low coefficient of friction (CoF, 0.24) and still maintained a high hardness of 32.1 GPa. The lowest wear rate could be ascribed to the highest ratio of H3/E2.

Microstructure, Mechanical and Tribological Properties of NbN/Ni Coatings

The NbN/Ni coatings were deposited by co-sputtering on Si (100) substrates. The structure, hardness and tribological properties were characterized by X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscopy (SEM), nanoindentation test and ball-on-disc tribometer. XRD revealed that the NbN/Ni coatings exhibited a NaCl-type NbN structure but no sign of any nickel phase. For the coatings with various nickel powers ranged from 20 W to 40 W, the shrinkage of the lattice parameter of NbN indicated that Ni atoms might be incorporated into the NbN lattice with a substitution of Nb atoms by the smaller Ni atoms. Further increasing of Ni powers, the degree of crystallinity of the coatings became worse. The NbN coating doped with a certan power of Ni (40 W) exhibited the best degree of crystallinity among all samples. It also displayed a maximum microhardness of 25 GPa combined with a better resistance to plastic deformation, which could attribute to the grain refinement and the solid solution strengthening. Tribilogical properties of NbN/Ni coatings were also found to be depentent on nickel powers significantly. For the pure NbN coating, the coefficient of friction (CoF) was 0.7 approximately, while it decreased to 0.54 when the power of Ni increased to 40 W. Simultaneously, the wear resistance of the NbN/Ni coatings was improved due to the spontaneous oxidation of the wear track surfaces caused by the addition of a certain amount of nickel to the niobium nitride coatings.