Surface Engineering

In order to improve the anti-corrosive and anti-oxidative properties of electroless Ni-P coating (ENPC), a vanadium conversion coating (VCC) was fabricated on the surface of the ENPC. SEM images and EDX line scanning spectra indicate that as-prepared VCC is supposed to be a thin film with a thickness of a few nanometres. XPS analyses demonstrate that the VCC is mainly consisted of V₂O₅ or V₂O₅·nH₂O. The acid exposure test data indicate that the VCC-treated ENPC, compared with the pristine one, demonstrates a highly enhanced anti-oxidative property. Furthermore, the result of neutral salt spray test manifests that the VCC-treated ENPC has also a highly enhanced anti-corrosive property, which is further confirmed by the result of electrochemical corrosion test.

In this present work, magnetic iron–nickel (Fe/Ni) alloy coatings were successfully prepared on the surface of carbon fibres (Fe/Ni–CFs) through electrodeposition, and their microstructure, phase composition and properties were also investigated. Microstructure observations indicate that Fe/Ni alloy coatings on CFs are well-distributed and smooth. The composition of Fe/Ni alloy is FeNi₃ with face-centred cubic structure. The conductivity of Fe/Ni–CFs increases dramatically with the extension of plating time and the line resistance value remains stable when the plating time reaches 15 min. Meanwhile, Fe/Ni–CFs prepared shows typical soft magnetic performance (25.5 emu g⁻¹ of saturation magnetisation) and good thermal stability. Besides, composites of Fe/Ni–CFs and epoxy resin fabricated through hot-compression moulding process exhibited good homogeneity.

Decarburisation is found as an undesirable consequence of the heat treatment cycle in hot work tool steels. In this work, an increase in the decarburisation resistance of tool steel through the electroless technique is subjected. For this purpose, the surface of the H13 tool steel was deposited by the electrodes Ni–P, Ni–B and Ni–P–B coatings. Since the carbon loss leads to a decrease in the hardness, the micro-hardness method is applied to find the influence of the mentioned coatings on the decarburisation resistance. The measurements show that all the electroless-plated steels possess higher resistance to the loss of carbon compared to the bare surface (as the reference). The results also confirmed that the incorporation of boron in the matrix of the Ni-plated layer, including Ni–B and especially Ni–P–B, prevents metal from decarburisation more effectively. The Ni–P–B coating could reduce the decarburisation depth from 0.15 to 0.035 mm.

The influence of the elemental composition of CuZnSn coatings deposited by electrodeposition on the formation of the Cu₂ZnSnSe₄ crystal structure following a selenisation process was investigated using X-ray diffraction, scanning electron microscopy and Raman spectroscopy. This study showed that the optimum alloy ratio to achieve the Cu₂ZnSnSe₄ crystal without impurity phases was in the range of 0.37–0.50 for Cu/(Zn+Sn) and 2.5–4 for Zn/Sn.

One of the important techniques for developing coatings with proper electrochemical and mechanical properties is electrodeposition method. The goal of the current work is to optimise contents of SiC particles in the bath to achieve Ni–B/SiC composite coatings with high corrosion resistance and wear properties. Pulse plating method was used to prepare Ni–B/SiC composite deposits of Watts bath including trimethylamine borane (TMAB) and SiC nanopowder addition. The effects of SiC concentration in the bath, which is the principal factor, on the main properties of the coatings were studied. Field Emission Scanning Electron Microscopy (FE-SEM), Energy-dispersive Spectroscopy (EDS) and X-ray diffraction (XRD) were used to analyse the characterisations of the coatings. In order to evaluate the corrosion behaviour of the coatings, the open circuit potential (OCP), electrochemical impedance (EIS) and potentiodynamic polarisation (Tafel) tests were applied in 3.5% NaCl solution. Moreover, the pin-on-disc procedure was designed to assess the wear behaviour of coatings. Increasing the SiC nanoparticles up to 12 g l⁻¹ in the bath leads to produce a coating including 11 wt-% SiC particles which have a positive effect on the coating microstructure. Corrosion resistance improves by raising 4–12 g l⁻¹ SiC incorporation in the coatings; so that corrosion resistance of the Ni–B/SiC12 g l⁻¹ coating reaches to nearly 62 kΩ. The presence of SiC phase in the coating can result in decrease electrochemical active regions; therefore, this phenomenon can promote corrosion resistance of the composite coatings. Ni–B/SiC 12 g l⁻¹ composite coating illustrates the best wear resistance in comparison with the others after which it shows the lowest weight losses (0.98 mg cm⁻²) and the friction coefficient of 0.57 due to the formation of the packed structures with less porosity and high content of SiC particles in the deposit.

Electroless Ni–Mo–P composite coatings with or without ultrasonic field were successfully deposited. The Sodium molybdate concentration was changeable to receive various Mo and P contents. Compared with the absence of ultrasound field, the coatings obtained smoother surface and higher Mo and P contents. The microhardness was determined by Vickers diamond indentation. The results showed that the hardness improved with increasing molybdenum and phosphorus content. The sample revealed the optimum hardness (886 HV) as it possesses 8 wt-% of Mo and 14 wt-% of P. Furthermore, the coatings exhibited higher hardness under ultrasound field. The corrosion resistance was analysed by potentiodynamic polarisation. The results showed that the corrosion resistance was enhanced in the increase of molybdenum. The sample with the highest Mo content (10 wt-%) showed the best corrosion resistance (5.81 μA cm⁻²). In addition, the coatings with ultrasound field have an influence on improving the corrosion resistance of the coating.

In this study, the functionally graded Ni–P coatings with low phosphorus top most layers were deposited on the copper substrates by the electroless plating process. The surface morphology and cross-section observations were made by using scanning electron microscopy and optical microscopy. The results showed that electroless deposition of the Ni–P coatings with gradual change of phosphorus content led to a crack-free compact nodular morphology. Also, by using the graded Ni–P coatings, the wear and corrosion resistance were improved. The multilayer coating with the sequence of Ni-high P/Ni-medium P/Ni-low P layers from the substrate to the top surface exhibited the best wear and corrosion resistance.

In this study, the effect of immersion time of the substrate in the chemical deposition bath on the physical properties of cupric oxide films is estimated by varying the immersion time as 15, 30, 45, 60 and 75 mins, respectively. The XRD studies revealed that the thin films showed a monoclinic structure with preferential orientation along (111) plane. Using X-ray profile analysis such as crystallite size, microstrain, dislocation density and stacking fault is estimated. Morphological studies revealed that the surface of films has uniformity and the particle size maximum for 45 min immersion time. In optical studies band gap, refractive index, extinction coefficient and optical conductivity have been estimated. The variations in immersion time had a considerable effect on the electrical parameters of films are confirmed. The studies revealed that the immersion time on CBD does have a significant role in deciding the property of the CuO thin films grown.

The present study investigated the preparation and characterisation of alternative black anodic films on aluminium alloy 1050 using three different inorganic colouring processes. The colouring process was performed via the standard chemical reaction and two alternative mechanisms, i.e. electrochemical reaction or a combination of electrochemical and chemical reactions. Black anodic films with high thermo-optical properties (α_s≥ 0.90 and ε_n≥ 0.90) were successfully obtained using all three processes. Differences in chemical content and location of the Co-based pigments in the coatings were highlighted and explained according to the colouring reaction mechanisms.

In this paper, the effect of zinc dipping pretreatment on corrosion resistance of electroless nickel plating on magnesium alloy was studied. The results showed that the corrosion resistance of nickel coating was influenced greatly by the complexing agent and main salt in the zinc dipping solution, and the formula and technological conditions of zinc dipping were optimized by the orthogonal test. The optimum formulation and process conditions were as follows: C₄H₆O₅ 40 g L⁻¹, ZnSO₄·7H₂O 40 g L⁻¹, NaF 4 g L⁻¹, pH 9, temperature 90°C, time 10 min. The corrosion resistance, morphology and phase composition of the nickel coating prepared under the optimum conditions were analysed by SEM, XRD and electrochemical workstation. The results showed that the self-corrosion potential of the nickel coating is 0.87 V higher than the magnesium alloy substrate, the self-corrosion current reduces by two orders of magnitude and the magnesium alloy substrate is completely covered. The corrosion resistance of magnesium alloy was effectively enhanced.

Cyanide-free electroless plating was used to deposit nickel/gold coating on the patterned circuits with fine-pitch. The relationship among the controlled process parameters, the skip plating, over-plating and pit forming were investigated. Results show that due to the uniform discharge sputtering, the copper layer is failed to be deposited around BVHs and lines of Al₂O₃ samples. The skip plating is mainly caused by the self-oxide and the leakage sputtering with copper. Excess Pd activation may result in over-plating during plating nickel. With enhancing the uniformity of the substrate materials and raising the annealing temperature, the number of pits decreases significantly. The thickness of the Au coating can reach up to 2 µm with the smooth coating surface and without the spillover. The electroless plating Au progress can be used to fine-pitch selective area plating.

Electrodeposited Ni–ZrO₂ nanocomposite coatings containing ZrO₂ nanoparticles with a mass fraction of 1%, 2% and 4% in wt-% separately used for the copperplate of crystallizer in the continuous casting mould were investigated. Their microstructure and hardness were characterized by OM, SEM, XRD and Vickers hardness tests. The tribological properties of Ni–ZrO₂ coatings rubbing with 35CrMo steel pins were evaluated by a pin-on-disc tribometer at 280°C. The results show that the ZrO₂ nanoparticles with the mass fraction of 2% in the coating have the fine size, high number density and uniform distribution. They could refine the grain of coating and have the dispersion strengthening effect; thus, they could increase the hardness of coating. The wear mechanism of Ni–1%ZrO₂ coating is the abrasive wear, adhesive wear and plastic deformation. The wear mechanism of Ni–2%ZrO₂ coating is the abrasive wear. The wear mechanism of Ni–4%ZrO₂ coating is the abrasive wear and spallation.