Surface Engineering

Organic–inorganic nanocomposite coatings were prepared from tetraethylorthosilicate and 3-glycidoxypropyltrimethoxysilane (GPTMS) by sol–gel method. These ormosil films were deposited by dip coating on aluminium alloy 5083. The effect of the concentration of GPTMS on the adhesion, mechanical and corrosion performance of coated samples was investigated. Adhesive strength of sol–gel coatings to the substrate was evaluated using pull-off and tape tests. Bending, impact resistance, pencil scratch and potentiodynamic polarisation tests were employed to characterise mechanical and corrosion performance of coated substrates with different GPTMS content. The chemical composition and the microstructure of the hybrid films were studied by attenuated total reflectance–Fourier transform infrared spectroscopy and field emission scanning electron microscopy respectively. The results indicate that the hybrid films had good mechanical properties, and the coating with minimum organic content had minimum corrosion current density, but the corrosion current density did not have any specific trend with an increased organic content.

A simple evaporation method was applied to deposit films of hydrophobic organic compounds, either polyethylene (PE) or 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDS), on various steel surfaces. The endurance of these films was evaluated after ultrasonic cleaning, ultraviolet (UV) light irradiation and heat treatment, and the corrosion behaviour of the PFDS coated steels was measured in NaCl solutions. The results show that in order to achieve high hydrophobicity on steel, the most suitable deposition conditions are exposure to saturated PE or PFDS vapours for a time of 1·8 ks at 393 K, in the case of PE, and 423 K, in the case of PFDS. The hydrophobicity of the PFDS coated surfaces was significantly more stable than that of the PE coated surfaces under ultrasonic cavitation, UV light irradiation and heating to 373 K. The deposition of a PFDS film on the surfaces of SUS304, SUS316 and SS400 steels improved their corrosion resistance in NaCl solution.

The ZrN films were deposited on type 304L stainless steel using dc magnetron sputtering technique at different substrate temperatures from room temperature up to 600°C. The ZrN films have been analysed for their composition, microstructure, surface morphology and their corrosion resistance in different nitric acid concentrations. The grazing incidence angle X-ray diffraction analysis showed the texture orientation of (111) or (200) plane depending on the substrate temperature. The atomic force microscopy analysis reveals the formation of ZrN agglomerates with no visible cracks or pores. However, increase in grain size (10–20 nm) and roughness (10–50 nm) with increase in the substrate temperature was observed. The potentiodynamic polarisation and electrochemical impedance spectroscopy results indicated that the corrosion resistance of different ZrN films, although high in 1 mol L⁻¹ HNO₃, suffered a marked decrease in 8 mol L⁻¹ HNO₃ concentration.

Alumina ceramic coating was prepared on Q235 carbon steel by plasma electrolytic oxidation. The discharge process was analysed according to the voltage–time curve. The microstructure of the ceramic coating was investigated by scanning electron microscopy, X-ray diffractometry and energy dispersive X-ray spectroscopy. The bonding strength, thickness, hardness, surface roughness and corrosion properties of the coatings were studied. The results indicated that coating on Q235 carbon steel mainly consisted of α-Al₂O₃ and γ-A1₂O₃. The coating showed coarse and porous surface. The surface roughness of the coating was 1·8 μm, and the average diameter of the pores was 3–4 μm. The hardness of the coating was 1854 HV. The thickness of the coating was ∼28 μm, and the bonding strength was 23 MPa. The corrosion tests indicated that the corrosion current density of alumina coated Q235 in 3·5%NaCl was 8·289×10⁻⁷ A cm⁻², which was decreased by two orders of magnitude compared with the uncoated one. The corrosion potential was increased to −0·379 V, which shifted positively by 0·303 V compared to the uncoated one.

Cr buffer layers of six different thicknesses (100, 200, 300, 400, 500 and 600 nm) were respectively deposited on silicon substrate so as to constitute a series of Cr implantation layer/Cr buffer layer/tetrahedral amorphous carbon (ta-C) films. Analysis by SEM suggests that the implantation of Cr layer favourably improves the interfacial transition between ta-C film and Si substrate. X-ray diffraction detection reveals that the difference in thickness of Cr buffer layer does not significantly influence the crystalline structure. Raman result shows that the ductility of Cr buffer layer favours to remit stress of the ta-C film. Increase in Cr buffer layer thickness results in the reduction of nanohardness value. However, its elastic modulus and adhesion exhibit an initial increase followed by a decreasing fluctuation. Result of the study shows the accomplishment of superior mechanical properties for the ta-C film with 200 nm thick Cr buffer layer.

Al–W_x alloys with the W content ranging from 0·6 to 11 at-% were sputter deposited at a low temperature on AA7075 alloy substrates. The sputtered Al–W alloys exhibit a remarkably broad range of microstructures and were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). For the alloys with a W content from 0·6 to 3·5 at-%, increasing tungsten additions lead to a decrease in the grain size from a few hundred nm to 10 nm. At 11 at-%W, a lamella-like microstructure forms, accompanied by an amorphous phase. Nanoindentation tests revealed that the increasing tungsten additions lead to an increase in the hardness. A polarisation corrosion test in a near neutral 0·1M NaCl solution revealed that increasing the tungsten additions leads to an increasing passive range and, therefore, the pitting potential.

This paper aims to investigate the parametric effect of deposition and target frequency on the mechanical properties and machining performance of the TiAlN coatings deposited in a dual cathode pulsed dc CFUBMS system. Coating composition is not directly dependent on deposition temperature or target frequency individually but increase in both the parameters has led to Al rich coating. The coating thickness is influenced by target frequency only. The fatigue fracture resistance of the TiAlN coating has been evaluated by the nanoimpact test and it has been found to be at least as good as commercial Ti_0·5Al_0·5N coating deposited on cemented carbide. In dry machining SAE 1037 steel, it has been observed that the resistance to crater wear is influenced by target frequency. Simultaneous increase in the deposition temperature and target frequency has provided improved resistance to crater wear due to their favourable influence on the coating thickness and Al percentage.

In order to overcome the difficulties of laser melt injection (LMI) on aluminium alloys, the process of tungsten inert gas (TIG) arc enhanced LMI ceramic particles was developed. The influence of process parameters, including laser power, TIG current, the direction of powder feeding, the carrier gas flow of powder and the shield gas flow of TIG, was investigated. The results have shown that to achieve appropriate surface enhancement effect, the powder should be fed from the back side of the laser beam. The depth of the arc enhanced LMI layer increases monotonously with increasing TIG current. The depth of the LMI layer increases first and then decreases with increasing laser power. The carrier gas flow and gas flow of TIG should be matched. Through optimisation of process parameters, WCp/Al layer was prepared successfully on the surface of aluminium alloy. The microstructure of the surface layer prepared by arc enhanced LMI was analysed.

Surfaces are the primary place of contact between bone and implant without an intervening soft tissue layer. Titanium based alloys are widely used for orthopaedic implants due to their high biocompatibility and high corrosion resistance. However, pure Ti is a soft material with a low shear resistance, and its poor wear resistance limits its application. TiO₂ biological coating has not yet been produced by double glow plasma surface alloying, which is a technique based on both ion nitriding and sputtering. Wear resistance and blood compatibility of materials were studied in this paper. The results showed that friction coefficient and wear rate of oxygenised layer were lower than that of the matrix. In the dynamic clotting time, uniform rutile TiO₂ (1 0 1) film after plasma oxygenising due to high electron density, low cavity density and electrostatic attraction of Ca²⁺ ions on the TiO₂ surface exhibited the best biocompatibility.

In this study, silver particles were embedded in Ni–P matrix by electroless deposition on medium carbon steel substrates to produce Ni–P–Ag composite coating. The structure of as plated and heat treated coatings were evaluated by X-ray diffraction analysis. Tribological properties of the coatings were investigated by pin on disc test method using 52100 steel pin as counter body at room and high temperatures. Three-dimensional optical profiler was employed to determine wear rate of the deposits. Surface morphology, cross-section of coatings and wear scars were studied by using scanning electron microscopy equipped with EDS analysis. The results showed that nanocrystalline Ni–P–Ag composite coating had a self-lubricating property at room and high temperatures. It was concluded that diffusion and nucleation of silver on the surface of Ni–P–Ag coating and also formation of a silver thin layer on the sliding surface led to decrease the friction coefficient of Ni–P–Ag coating.

The influence of Pd and Ni coatings on the oxidisation of hydrogen that permeated through a thin iron membrane was studied for the development of a hydrogen sensor. The minimum electroplating period and current density to produce compact Ni and Pd coatings were investigated. The ideal potential ranges for complete oxidation of the hydrogen that permeated through an iron membrane were found to be +0·20 to +0·50 V(Ni) (voltage versus the voltage on nickel wire) and +0·30 to +0·40 V(Ni) for the Pd and Ni coatings respectively; the Pd coating provides a wider window for oxidation. The optimum potential for the oxidation of hydrogen on Ni coating was found to be 0·35 V(Ni). Ni coating could achieve the same detection efficiency as a Pd coating with the appropriate applied potential.

Chromium aluminium nitride (CrAlN) coatings were prepared by a modified ion beam enhanced magnetron sputtering system in this study. Friction and wear tests were carried out by a (pin on disc) tribometer with Si₃N₄ (at 600°C) balls as a counter material without lubricant. The results show that CrAlN coating deposited at −120 V has a higher Al content and is more resistant to high temperatures than that deposited at 0 V. During the wear test at 600°C, the friction coefficient of CrAlN coating deposited at −120 V is higher than that of CrAlN coating deposited at 0 V, but the increased hardness, fracture toughness and improved microstructure in CrAlN coating deposited at −120 V can be attributed to its increased wear resistance and its wear rate is nearly unchanged. The wear resistance of the CrAlN coated H13 steel specimen has been greatly improved compared to the uncoated reference H13 steel specimen.

The excellent properties of titanium and its alloys make them a perfect choice for many applications. In this paper, the effects of thermal oxidation condition on the surface morphology and surface roughness have been studied. The results showed that the average roughness at temperatures 450 and 600°C did not change significantly. Despite the constant average roughness, skewness and kurtosis parameters have been changed considerably during the thermal oxidation. The results suggest that the valleys are preferred sites for nucleation and growth of oxides. This mechanism changes the surface morphology from a scratched surface to a bumpy one.