Surface Engineering

Nowadays, atmospheric pressure cold plasmas, particularly in dielectric barrier discharge (DBD) configuration, attract significant interest in the field of surface processing of materials. Fluorocarbon containing DBDs have also been studied, but the state of the art in this field is at its early stages, especially if compared to low pressure plasmas, which have been widely and successfully employed for the etching of inorganic and organic materials, for the deposition of fluoropolymers as well as for the treatment of synthetic and natural polymers. This contribution will provide an overview of our recent studies on fluorocarbon containing DBDs and will present results on the deposition of fluoropolymers concerning the tuning of the chemical composition of the deposits, the etching–deposition competition and the influence of feed gas contaminants (i.e. air and water vapour).

In this work, the adsorption and reactivity of Zn and ZnO on Pd(111) have been investigated using scanning tunnelling microscopy, temperature programmed desorption, density functional theory and reflection absorption infrared spectroscopy. Using this approach, it was shown that an ordered PdZn surface alloy already forms at room temperature. The electronic structure of this surface alloy resembles that of copper. Depending on the preparation method and temperature, different surface alloys and Zn layers can be formed. The ZnO growth on Pd(111) forms different ZnO structures. Both a (4×4) and a (6×6) structure are formed depending on the oxygen pressure during ZnO growth. The (4×4) structure is most likely a hydrogen terminated Zn₆O₅ structure. In an attempt to verify the hydrogen termination, this structure and the (6×6) structure, which corresponds to a graphite-like Zn₆O₆ structure, were investigated using reflection absorption infrared spectroscopy. The results show a vibrational feature that could correspond to the O–H stretch vibration of the hydrogen terminated structure.

Microwaves at 2·45 GHz have been used to ignite the self-propagating high temperature synthesis of Ni and Al powder mixtures (50∶50 at-%) to produce a duplex intermetallic coating on Ti substrates. Owing to the high β-NiAl synthesis reaction enthalpy and the energy provided by the electromagnetic field, the newly formed aluminide is in the liquid phase. Thus, it can react with the underlying Ti substrate, forming a ternary eutectic at the interface, belonging to the Ni–Al–Ti system. The peculiar interlocking microstructure of such ternary layer is responsible for an increase in toughness and hardness compared to the parent NiAl coating. Moreover, the ternary layer's oxidation resistance was tested, showing the possibility of replacing the functionality of hard and brittle NiAl, but limitedly up to 750°C. This can be ascribed to the relatively low Al content in the ternary layer, which hinders the formation of a continuous and protective Al₂O₃ scale. For this reason, the increase in Al content, or the addition of further elements such as Cr or Si, in the starting powder mixture, helps improve the oxidation resistance of the complex intermetallic coating at the interface, up to the tested temperature of 900°C.

The aim of this work is to investigate the effects of different power modes [constant current (CC), constant voltage (CV) and constant power (CP)] on the structure and corrosion resistance of the plasma electrolytic oxidation ceramic coatings containing Ca and P on AZ91D Mg alloy. The phase composition, morphology and element distribution of the coatings were studied by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The corrosion resistance of the coatings in the simulated body fluid was examined by electrochemical impedance spectroscopy and polarisation curve methods. The results showed that the coatings were of porous structure and mainly composed of MgO. The thickness, surface roughness, the size of the surface micropores and the amount of MgO were all increased with the cell voltage under the same power mode or increased in the sequence CV>CC>CP under the different power modes. The corrosion resistance of coatings was related to the structure characteristics, which were determined by different power modes. Among the different power modes, the corrosion resistance of the coatings is generally increased in the sequence CV>CP>CC, and under the same mode, the proper increase in the cell voltage is liable for improving the corrosion resistance of the coatings.

Hydrogen doped In₂O₃ (In₂O₃:H) films show high conductivity, small dispersion of refractive index and very low extinction coefficient in the visible to near infrared wavelengths. The improved properties make this transparent conducting oxide an ideal candidate for a window electrode of optoelectronic devices. This article describes the control of microstructure of In₂O₃:H, the relationship between the structure and transport properties and the Si based solar cells incorporating the In₂O₃:H window electrode.

Gold plated surfaces are widely applied in several technical and decorative fields. The two main issues regarding the discussion on the field of precious metal coatings concern the increase in the use of thinner gold layers and ‘Ni free’ substrates. In order to ensure the quality of the final products, the effects of the plated surfaces on their performance require thorough and accurate research. In this paper, the corrosion resistance of gold plated nickel, copper and bronze was investigated by electrochemical methods specifically potentiodynamic polarisation and electrochemical impedance spectroscopy in phosphate buffered saline. The cytotoxicity of the gold plated substrates was also evaluated and compared. The results showed that the substrate related to the best corrosion resistance and cytotoxicity among the tested ones was bronze, and the one with the lowest performance was nickel.

In this paper, TiC–W–Cr powders were alloyed on grey cast iron by plasma transferred arc (PTA). The alloying samples were characterised with reference to the microstructure, microhardness, fatigue life and fatigue crack growth. From the results, it is indicated that two distinguishing region, i.e. alloying zone and heat affected zone, were formed on the surface after PTA alloying. The alloying zone mainly consists of primary austenite, martensite, a eutectic of (Fe,Cr)₇C₃ carbide and austenite as well as the uniformly distributed unmelted TiC particles. Plasma transferred arc alloying TiC–W–Cr eliminated the stress concentration at the edge of graphite and produced hard carbide, resulting in frequent crack deflection. As a result, the Weibull distribution of fatigue life demonstrated that PTA alloying TiC–W–Cr exhibited longer lives compared to matrix and PTA hardening without reinforcement, but more scattered. In addition, on the basis of the careful observation of fatigue crack growth, it was shown that the fatigue crack growth rate could be retarded by PTA alloying TiC–W–Cr at a low stress intensity, but speeded up at high stress intensity due to the brittleness.

Infrared radiative properties of thermal barrier coatings have been investigated for developments of solar absorbing surface coatings. Various types of multilayer broadband optical filters were designed incorporating 7 wt-% yttria stabilised zirconia (7YSZ) and aluminium oxide (Al₂O₃). Electron beam physical vapour deposition method was employed to deposit 7YSZ thin films on quartz substrates to study their optical properties. The refractive index and thickness of 7YSZ films were estimated using spectroscopic ellipsometry technique, giving the index as 1·95 at 633 nm. Multilayer thermal barrier coating structures were fabricated by depositing alternate layers of 7YSZ and Al₂O₃ on quartz using electron beam physical vapour deposition. The spectral characteristics of multilayer 7YSZ–Al₂O₃ coatings were evaluated using a spectrometer in the visible and near infrared range. Preliminary results obtained from multilayer 7YSZ–Al₂O₃ coatings had exhibited 75% transmission over a wide band of 0·4–1·0 μm.

This paper describes an amine free sol–gel method for silica coating of Au nanoparticles and shows an X-ray image of a colloid solution of the silica coated Au (Au–SiO₂) particles. The Au nanoparticles that had a size of 16·9±1·2 nm were prepared through a conventional citrate reduction method. The silica coating was performed with a sol–gel reaction of tetraethylorthosilicate (TEOS) catalysed with NaOH in the presence of Au nanoparticles. The silica shell thickness was varied from 37 to 68 nm for TEOS concentrations of 1×10⁻³–20×10⁻³M at 4·3×10⁻⁵M of Au, 10·7M of H₂O and 1·0×10⁻³M of NaOH. The optical properties of the Au–SiO₂ particle colloid solution were related to the refractive index around the Au particles and the intensity of scattering from silica shells. The as prepared colloid solution could be concentrated up to an Au concentration of 4·3×10⁻²M with salting out. The concentrated colloid solution showed a high contrast X-ray image.

Synthesis of nanocomposite thin films of CrN/Cu on (100) Si and titanium modified stainless steel (D-9 alloy) substrates by pulsed magnetron sputtering from a composite target of Cr–Cu using sputtering gas mixture of argon and nitrogen is investigated. X-ray diffraction analysis of the films deposited at 773 K and 10 sccm of nitrogen flowrate indicated that the films are nanocrystalline and biphasic (fcc CrN and fcc Cu). The films showed a peak hardness of ∼15 GPa and a Young's modulus of ∼200 GPa for those with a copper content of 15·1 at-%, and these values were found to decrease significantly with a Cu content of ⩾18·4 at-%. The evaluation of the antibacterial activity as a function of copper content in the range of 15·1–38·5 at-% indicated excellent antibacterial properties for CrN/Cu films with the copper content of 18·4 at-%. The phase stability of CrN of the nanocomposite is also discussed in relation to pure CrN thin films.

This review surveys the recent developments in the plasma deposition of polycrystalline diamond (PCD) films from highly diluted (1%CH₄ in H₂) gas mixture and superhydrophobic fluorocarbon films from C₂F₄ gas. Specifically, the pulsed plasmas are also used and examined at different duty cycles and pulse periods. Emphasis is given to the role of pulsed plasmas with respect to continuous ones in controlling the gas surface interaction and the growth chemistry and in determining the material properties. The obtained materials have been characterised using a wide range of methods: scanning electron microscopy, atomic force microscopy, Raman spectroscopy, X-ray diffractometry, spectroscopic ellipsometry, water contact angle, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Power modulation affects the PCD film morphology but deteriorates their optical and structural properties: the refractive index and the quality factor of continuous films are better than pulsed ones. As for fluorocarbon films, the control of both the plasma phase and the film deposited is improved under pulsed conditions. The pulsed plasmas, contrary to the continuous ones, allow to tune the CF₂ growth precursor and correspondingly the composition, structure and morphology of the film, exhibiting a superwater repellent surface with contact angles of up to 170°.

The post-treatment of ion implantation on hard coatings attracts a great deal of attention because of the improvement in coating surface properties. The present research investigates the effect of nitrogen (N), carbon (C) and carbon followed with nitrogen (C+N) ion implantations on the structural and mechanical properties of the Ti–Al–Si–N coatings. Ion implantations were performed at an energy of 50 keV and different doses. The surface properties of the implanted layer were identified by a variety of analytic techniques, such as cross-sectional transmission electron microscopy, energy dispersive spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and nanoindentation measurement. Additionally, the wear performance of the samples was evaluated by a typical ball on disc tribometer in dry conditions. The results showed that the surface properties depended strongly on the implanted species and doses. In addition, a great improvement in the wear was observed on the samples with the post-treatment process of C and C+N ion implantations.

The specification of the powder mixtures used in any thermal spray process has a significant effect on the coating properties. In this research work, Ni–TiC and Ni–(Ti,W)C composite powders, synthesised by self-propagation high temperature synthesis process, were used to prepare coatings by high velocity oxygen fuel (HVOF) method. The coatings were characterised and compared with that produced from conventional Ni+TiC powder mixture. Using the composite powders resulted in homogeneous coatings with no carbide oxidation during the HVOF process, while a significant oxidation of TiC occurred when conventional powders were used. The coatings produced from composite powders also show superior wear resistance over that of conventional powders. Details of the wear mechanism when using different powders as feedstock were explained.