Gu Mingyuan

The microstructure and mechanical properties of TaN/TiN and TaWN/TiN superlattice films

Abstract The microstructure and the microhardness of the TaN/TiN and TaWN/TiN superlattice films have been studied with X-ray diffraction, transmission electron microscopy and microhardness tester. The results showed that both TaN/TiN and TaWN/TiN superlattice films have a cubic crystal structure with an epitaxially grown mode of polycrystallinity. Lattice constants of superlattice films are between those of the constituent materials. The superhardness effect was found in TaN/TiN and TaWN/TiN superlattice films and the maximum hardness value was 40.0 GPa at a modulation period of 9.0 nm for TaN/TiN, and 50.0 GPa at a modulation period of 5.6 nm for TaWN/TiN. It is proposed that the lattice mismatch affects the microhardness value and the peak position of maximum hardness. The inhibition of dislocation motion by alternating stress fields of interfacial coherent strains is believed responsible for hardness anomalies.

Alternating stress field and superhardness effect in TiN/NbN superlattice films

In order to study the superhardness effect of superlattice films, a series of TiN/NbN superlattice films with various modulation periods were synthesized by reactive sputter deposition. X-ray diffraction analysis, transmission electron microscopy, and microhardness analysis were employed to characterize the modulation structure, interface structure and microhardness of these superlattice films. The results show that TiN/NbN films possess good periodic modulation structure and the modulation interfaces are straight and clear. The superlattice films have a face-centered-cubic polycrystalline structure resulting from epitaxial growth. They assume unusual microhardness which can reach a peak value of HK 39.0 GPa at a modulation period of 8.3 nm. It is considered by analysis that the superhardness effect of TiN/NbN superlattice films results from the strengthening effect of an alternating stress field, which is caused by the epitaxial growth of two kinds of materials with different lattice constants.

Magnetron sputtered NbN thin films and mechanical properties

Abstract In this study, NbN thin films were deposited by reactive magnetron sputtering at different N 2 partial pressures and at room temperature. X-Ray diffraction analysis, transmission electron microscopy and atomic force microscopy were employed to characterize their phases, microstructure and surface morphology. Their microhardness and elastic modulus were evaluated using a microhardness tester and the effects of N 2 partial pressure on the phase formation, microstructure and mechanical properties of NbN thin films were investigated. The results show that there are clear effects of N 2 partial pressure on the deposition rate, phases, hardness and elastic modulus of magnetron sputtered NbN films. At a low N 2 partial pressure, the deposition rate is higher, while hcp β-Nb 2 N and fcc δ-NbN coexist in NbN films. With the increase of N 2 pressure, the deposition rate decreases and the films are single-phase fcc δNbN; accordingly, the hardness and modulus reach peak values of 36.6 GPa and 457 GPa, respectively. A further increase of the N 2 partial pressure will cause hcp e-NbN to appear in NbN films and then the hardness and modulus of films decrease.

Growth, microstructure, and microhardness of W/Mo nanostructured multilayers

Artificially modulated W/Mo multilayers on polished stainless-steel substrates with modulation wavelength Λ ranging from 4.0 to 60.0 nm and total film thickness of 2.0 μm were prepared by magnetron sputtering. X-ray diffraction (XRD) and cross-sectional transmission electron microscopy showed that though the polycrystalline films exhibited coherent interfaces, the interfaces have a wave-like appearance due to the different orientations of individual crystals. The interplanar spacings of the W and Mo layers determined by the XRD method in W/Mo multilayers varied with the modulation wavelength. The mechanical properties of these films were investigated by a low-load microhardness indentation technique. The maximum hardness enhancement is about 51% higher than the value calculated from the role of mixtures at wavelength Λ=10.0 nm. The Koehler’s modulus difference model and Cahn’s coherent stress model have been used to estimate the hardness enhancement of W/Mo multilayers. From the comparison of theoretical ...

Microstructure and mechanical properties of polycrystalline NbN/TaN superlattice films

The polycrystalline NbN/TaN superlattice films have been grown on the substrates of 18-8 stainless steel by reactive magnetron sputtering. The microstructure and microhardness of the superlattice films have been studied with X-ray diffraction (XRD), high resolution transmission electron microscopy (HREM) and microhardness tester. The results showed that the NbN layers are of face cubic and the TaN layers are hexagonal crystal structure in the NbN/TaN superlattice films. The lattice plane (111) of NbN are coherent with the (110) of TaN and the lattice mismatch is 3.18%. The NbN/TaN superlattice film demonstrated superhardness effects. The maximum Knoop hardness value reached 5100 kgf/mm2 with a modulation period from 2.3 nm to 17.0 nm. It was proved that even if NbN layers did not take the same crystal structure as TaN layers, hardness anomalous phenomenon still can be produced as long as the coherent strains exist.

Interface evolution and its relationship with fracture strength in C/Al, C/Al-Ti and C/Al-Cu composite materials

Alloying the matrix is a convenient and effective way to improve the properties of aluminium matrix composite materials. However, the extent to which the additional elements, such as titanium and copper, would function is not very clear. The aim of this work was to compare the roles played by titanium and copper in carbon fibre-reinforced aluminium matrix composite wires. The kinetics of the interfacial reaction in modelling specimens were studied quantitatively using the depth profile of a scanning auger microprobe. The tensile strength of the composite wires was tested for the specimens in as-fabricated and annealed states, respectively. SEM observations indicated that addition of copper to the aluminium matrix deteriorates the wettability between the matrix and the reinforcements. TEM observations revealed that no significant interfacial reaction took place during fabrication processing, while Al4C3 and Ti3AlC formed near the interfaces during annealing. It can be concluded that addition of titanium re...

Effect of cold-rolling on hardness of SiCw/Al composite

A SiCw/Al composite was fabricated through a squeeze cast route and cold rolled to about 30%, 50% and 70% reduction in thickness, respectively. The length of whiskers in the composite before and after rolling was examined using SEM. Some of the rolled composites were annealed by recrystallizing to remove the work hardening of the matrix alloy. The hardness of the rolled and annealed SiCw/Al composites was examined and then associated with the change of the whisker length and the work hardening of the matrix alloy. It was found that the hardness was a function of the degree of cold rolling. For the cold rolled composites, with the increase in the degree of cold rolling, the hardness increases at furst, and decreases when the degree of cold rolling exceeds 50%. For the annealed ones, however, the hardness decreases monotonously with the increase in rolling degree. The different changes in hardness between the rolled and annealed composites could be attributed to a result of the competition between the work hardening of the matrix resulting from the cold rolling and the work softening arising from the change of whisker length.