Surface Engineering

Nano Ag/N–TiO₂ films with porous hadites as carriers have been successfully prepared by a modified sol–gel process. The performance of the films was characterised by X-ray diffraction, UV-vis and energy dispersive X-ray spectroscopy. The photocatalytic activities and sterilisations of the films were estimated with the waste water contained oil and the fresh cocci as polluted models. The experiments showed that the codoping of Ag and N modified observably the photocatalytic activity of TiO₂. The degradation rate of Ag/N–TiO₂ films on oil in water and bacteria was three times more than pure TiO₂ films on porous hadites. The porous hadites loaded Ag/N–TiO₂ films may be used for waste water processing, sterilisation and filtering.

This research was focused on the study of delamination phenomena in metal–polymer laminates used in containers. The selected material was an electrolytic chromium coated steel (ECCS), protected by polyethylene teraphthalate (PET). The study employed an acetic acetate medium, which has been proved to degrade the PET coating depriving it from its protective capacity. The experiments consisted of electrolyte induced delamination of the PET by anodic–cathodic polarisation. The main objective was to investigate the degradation mechanisms in order to explain how the presence of electrochemical activity in the delamination cross-section front detaches the PET coating and leads to subsequent propagation of this delamination. Finally, specific delamination models were proposed for the electrochemical conditions investigated, and the degradation was characterised by SEM. The induced delamination experiments applied to the ECCS–PET composite in an acetic acetate medium allowed to discriminate between the delamination processes proper for anodic polarisation and those for cathodic polarisation.

The results of the investigations of multilayer TiAlN/CrN coatings deposited using cathodic arc evaporation applying TiAl (50∶50 at-%) and elemental Cr cathodes are presented. The bilayer TiAlN+CrN thickness was ∼250 nm with the thickness ratio of TiAlN/CrN layers of approximately 1∶1. The atomic ratio of Ti/(Al+Ti) in the coating was 0·60. As the reference, the monolayer TiAlN and multilayer TiAlN/TiN coatings were selected. All the coatings were deposited on CrN sublayer to reduce their stress and improve their adhesion. The investigated coatings show good adhesion in the range 80–90 N. The TiAlN/CrN coating shows a lower friction coefficient and a significantly lower wear rate than those of reference coatings (more than 10 times).

In order to investigate the effect of the gas oxynitrocarburising on the properties of the medium carbon railway axle steel, the steel was gas oxynitrocarburised at five different temperatures for 2 h by applying a low temperature gas multielement penetrating system. The results show that the compound layer composed of γ′-Fe₄N and ϵ-Fe_2–3N and the oxide layer consisting of Fe₃O₄ and Fe₂O₃ are formed on the surface after treatment. The thickness of the compound layer increases with increasing temperature; however, the case depth decreases when the temperature is above the temper temperature of the origin material. The increase in treatment temperature resulted in an increase in the surface roughness. The properties of wear and corrosion resistance are improved, and the fatigue limit of the specimen treated at the optimum temperature, which is 570°C, improves largely compared to that of the untreated one and the one treated by microshot peening.

Ni–3Al and Ni–3Ti alloys were deposited on AISI 1045 steel substrate using air plasma spraying process. In this regard, mechanically alloyed feedstock powders were used. Open circuit potential E_OCP measurements and Tafel polarisation tests were carried out to study corrosion behaviour of the coatings in 3·5%NaCl solution. These tests indicated that the corrosion performance of the Ni–3Ti coating is better than that of the Ni–3Al coating. The corrosion current densities were three times lower in the Ni–3Ti coating than in Ni–3Al coating. In addition, Ni–3Ti coating showed better passive behaviour than Ni–3Al coating. After 10 h of immersion, the surface of Ni–3Al coating became pitted, and rust spots appeared on the surface. In contrast, there was no rusting and pitting on the surface of Ni–3Ti coating even after 10 h of immersion.

Boride coatings were synthesised on AISI H13 steel using pulsed plasma electrolytic boronising technique in a modified aqueous solution of borax. Hard and strongly adherent coatings with various desirable properties were obtained by applying different frequencies and duty cycles during the treatment. The analytical characteristics of the coating were assessed using X-ray diffraction and scanning electron microscopy techniques. The thickness of boride layers was increased by increasing frequencies. The corrosion performances of these coatings were investigated by a potentiodynamic polarisation test and ac impedance spectroscopy. Equivalent circuits have been proposed to represent the boride phases, oxide film and electrolyte systems examined. The general corrosion resistance of the produced coatings has been found to increase with the frequency.

With the application of scanning electrical microscopy, surface profilometer and surface mapping microscopy, the two- and three-dimensional surface morphologies of the different substrate sheets, including cold rolling, hot dipped galvanised and electrogalvanised sheets, were characterised. The overlaid effect of paint on the surface texture of substrate sheets was analysed with the aid of low pass filtering. The specular gloss and distinctness of image (DOI) of organic coating were also measured, which are finally related to the texture of substrate sheets. The results showed that the DOI of organic coating on cold rolling, hot dipped galvanised and electrogalvanised sheets were ranked in descending order, which was identical to the tendency presented by the roughness and waviness of substrate sheets. A series of parameters was postulated under the consideration of DOI control for organic coating. In this study, the DOI score was >69% when R_a<0·86 μm, R_z<5·43 μm, R_pm<2·26 μm, S_m<78·81 μm and W_ca<0·411 μm.

Cu–Ni alloys were electrodeposited by the brush plating technique from sulphate/citrate electrolyte at various pH values. The effects of pH on composition and surface morphological properties of alloys were investigated by X-ray fluorescence and atomic force microscopy. The Cu content increased in the composition of Cu–Ni alloy at electrolyte pH 2. The deposition mechanisms of Cu, Ni and Cu–Ni were investigated by cyclic voltammetry. The lower electrolyte pH leads to the higher reduction current density. The corrosion behaviours of the deposits at different pH values were investigated by Tafel analysis and electrochemical impedance spectroscopy. The corrosion studies indicate that the Cu–Ni alloy exhibits better corrosion resistance at higher pH values than at lower pH values. The surface roughness decreases with increasing solution pH and this observation was confirmed by atomic force microscopy measurements.

Owing to excellent mechanical properties, such as high strength, high elastic modulus and large elastic as well as fracture strain, carbon nanotubes (CNTs) are attracting significant interests as reinforcements in metallic coatings. In the present investigation, CNT reinforced nickel composite coatings were deposited on a stainless steel substrate using pulse electrodeposition process employing a nickel Watts bath. The presence of CNTs in the composite coating prohibited the columnar growth of the nickel grains resulting in random/weak texture and smaller thickness of the composite coatings. The Ni–CNT composite coatings exhibited significantly improved microhardness (580±15 HV) compared to pure nickel coatings (320±15 HV). The ball-on-disc wear testing data indicated that the reinforcement of CNTs significantly improved wear resistance of the composite coatings compared to pure nickel coatings.

A bias voltage apparatus is attached to the filtered cathode vacuum arc system for coating a diamond-like film containing Ti on WC substrate. Ti, which acts as the cathode target, is sputtered by plasma, and reacts with C₂H₂ gas, then deposits on the WC under working conditions of 60 A current and voltage between −50 and −275 V. Such processed diamond-like film containing Ti(Ti–C:H) is analysed by glow discharge spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and nanoindentation for determining Ti concentration, crystallographic structure, bonding mode and mechanical properties. Characteristic parameters are determined to obtain suitable properties, and the underlying reasons are also addressed.

Additives usually affect the electrochemical corrosion behaviours of metals which are immersed in the electrolyte. In this study, five additives including thiourea (TU), thiosinamine (ATU), thiosemicarbazide (TSC), sodium benzoate (Bz) and sodium citrate (SC) were added into the electrolyte respectively to investigate the electrochemical corrosion behaviours of AZ91D magnesium alloy immersed in neutral 0·05 wt-%NaCl solution. Open circuit potential (OCP), potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) measurements were employed in the electrochemical tests. The results revealed that TSC showed a good inhibiting effect, and TU as well as ATU slightly improved the corrosion resistance of AZ91D magnesium alloy in the studied solutions. A negative inhibiting effect with the addition of Bz or SC was also observed. Moreover, the electrochemical results were analysed by computational chemistry method and ultraviolet absorption spectra, and the possible inhibiting mechanism was also discussed in detail.

Molybdenum disulphide composite particles made of both hollow nanospheres and nanoplatelets were synthesised in a single process using a chemical method that was optimised by adjusting the relative proportions of Na₂S and thioacetamide (TAA) precursors. The composite nanoparticles were characterised using powder X-ray diffraction, transmission electron microscopy and thermal analysis; four-ball tribological tests were used to assess their lubricating effect when used as additives in liquid paraffin dispersions. The results showed that composite nanoparticles of appropriate shape distribution could only be synthesised with a high dosage of TAA: too high concentrations of Na₂S were shown to destroy the template chain of TAA and disturbed the formation of nanospheres. The MoS₂ composite particles exhibited a better lubricating effect as compared to composite particles produced by mechanical mixing of separately produced nanospheres and nanoplatelets, confirming that the chemical method can lead to a better synergistic lubrication between two kinds of MoS₂ nanoparticles.

Knowledge of internal stresses in thin copper film structures is essential in understanding the film properties, such as stress migration, adhesion, hardness and elasticity. Internal stresses and nanomechanical properties in thin copper films were investigated by the curvature method, nanoindentation and atomic force microscope. Variations in the abovementioned parameters were studied with decreasing deposition temperature for the films. Irrespective of deposition temperature, the stress was observed to be compressive and increased at low electrolyte temperatures. Hardness and elasticity of the films were found to be increased with reduced deposition temperature. With increasing compressive stress, the hardness of the films increased. The surface adhesion of the film deposited at 5°C was minimum, indicating increased cleanliness and chemical stability with low deposition temperatures.