ICMCTF2014 Session B5-3: Hard and Multifunctional Nano-Structured Coatings

Nano-scaled multilayer CrN/NbN coatings were produced in an industrial-size cathodic arc physical vapor deposition (PVD) chamber, with an alternate three cathodes (Cr/Nb/Cr) configuration. Microstructural and compositional characterization of the CrN and NbN nanolayers was obtained. Sharp interfaces between the nanolayers were obtained. Four multilayer NbN/CrN coatings were produced with different periodicities (20 nm, 10 nm, 7.5 nm and 5 nm) with total thickness of 30µm in all cases. The coatings were characterized by X-Ray diffraction (XRD) and Transmission Electron Microscopy (TEM), which provided the lattice parameter in each of the constituent layers and structural description of the multilayers, respectively. Coatings were also analyzed in terms of hardness, using an instrumented micro-hardness tester. The multilayer coating system was composed of CrN and NbN with similar structures, but with a lattice mismatch, which varies the fraction of the region with lattice strain depending on periodicity. For the thicker individual layers, the adjustment of lattice parameter at the interfaces does not represent a predominant fraction of the entire structure, i.e. the separate peaks of NbN and CrN can be distinguished in the XRD analysis. In the presence of low periodicity (lower than 10nm), the lattice of each constituent may be coherently strained to each other and just one intermediate lattice (d-spacing) is detected for NbN/CrN multilayer. TEM observations showed that the interfaces were sharp with no apparent interdiffusion. The formation of columnar polycrystalline grain structure in the multilayers of several micrometers in size appear, indicating that despite of stratified structure, grain growth is not always interrupted by the different interfaces. From high to low periodicity, the multilayer NbN/CrN coatings show promising enhancements on hardness. Hence, the interfacial effects can dominate multilayer structure and properties, leading to unusually large strain and a trend for stabilization of metastable structure.

Hard protective coatings have been undergoing development for decades and further improvement to their mechanical and tribological properties is more and more challenging. Nowadays most research is aimed at improving the tribological behaviour of hard and functional coatings through the optimisation of their microstructure. The combination of different layers is another possible way of improving the tribological behaviour of protective coatings. The use of bonding layers mitigates the differences in mechanical and thermal properties between coating and substrate. As a consequence, the nature of this bonding interlayer plays an important role on the response of the coatings to the complex stress conditions occurring in a tribological scenario.

Among the possible interlayer candidates a layer with the capability of accommodating large deformation, thus protecting the substrate from plastic deformation as well as improving the adhesion of the top layer to substrate, was chosen. One of the potential classes of materials satisfying the above mentioned requirements is the Ni-Ti-based alloys which are known to exhibit superelastic properties also when sputter deposited.

In this study two different superelastic layers were fabricated by magnetron sputtering, namely a Ni-rich Ni-Ti and a (Ni, Cu)-rich Ni-Ti-Cu film, 2 µm thick, were deposited on steel substrates. In order to obtain superelastic properties, the films [#] were isothermally annealed for 1 hour at 500°C in a high vacuum environment. Subsequently the superelastic layers as well as bare steel substrates were coated with a 2 µm thick protective layer (hard and self-lubricant coatings) by magnetron sputtering.

The chemical composition of every single layer was measured by Energy-dispersive X-ray spectroscopy (EDS), while the structure was evaluated by grazing-incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). The mechanical properties of the single layers as well as those of the bilayers were measured by nanoindentation. Finally, the tribological behaviour of the bilayers and of the single layers were characterised by pin-on-disc testing.

The microstructural and mechanical properties of the different designs were correlated and discussed in relation to the measured tribological properties. A first comparison was performed in order to evaluate whether the use of a superelastic interlayer was beneficial to the tribological behaviour of a functional top layer. In addition the use of different superelastic interlayers, i.e. Ni-Ti and Ni-Ti-Cu, in the bilayers was investigated, the Ni-Ti-Cu being characterised by a different microstructure compared to the Ni-Ti composition.

In this study, a single ZnO nanowire detector is described. ZnO and Copper doped ZnO nanowires have been prepared by low-temperature chemical vapor deposition method at 575°C. The morphology and microstructure of the nanowires were characterized by field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM), respectively. Low temperature photoluminescence spectroscopy is employed to analyze optical emissions of nanowires. With X-ray diffraction (XRD), we can investigate that Cu-doped ZnO nanowires are hexagonal phase with c-axis orientation. The X-ray photoelectron spectroscopy (XPS) data indicated that doping concentration of copper was high to 3.0 at%. The sensor device was fabricated by the e-beam lithography technology.

Keywords: ZnO nanowires, Cu-doping, chemical vapor deposition, gas sensor

In this work, n-type ZnO and p-type Cu₂O nanostructures e lectrodeposited via high aspect ratio anodic alumina oxide (AAO) template assistance, using ZnSO₄ and CuSO₄ electrolyte, respectively. The ZnO and Cu₂O nanostructures of morphology and crystallinity were characterized by using X-ray diffraction (XRD), field emission scanning microscopy (FE-SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). The thickness and channel diameter of AAO template were about 150 μm and 110~160 nm, respectively. ZnO nanostructures fabricated under various electrolyte concentrations of 0.1M and 0.5M ZnSO₄ were nanotubes and nanowires, respectively. The Cu₂O contained in Cu/Cu₂O complex nanowires will increase with decreasing deposition current density. Both ZnO and Cu/Cu₂O nanostructure were uniformly corresponded to AAO’s pore size, the diameter was about 140 nm. The carrier concentrations of the films were calculated by Mott–Schottky equation from Electrochemical Impedance Spectroscopy (EIS). The films were examined by one-step and two-step systems for hydrogen evolution from water splitting under visible light illumination, hydrogen generation efficiency were calculate by photocurrent from photoelectrochemical analysis (PEC) in Na₂SO₄ solution.