Surface Engineering

The paper presents the influence of the nickelising parameters on the structure and tribological properties of composite coatings AHC+Ni, including nickel nanowires, which have been obtained by nickelising of anodic hard coatings on aluminium alloys. The influence of nickelising parameters on the utility properties of the coatings is presented in the diagrams. The absorbability and roughness R_a of coatings have been chosen as the utility properties important from tribological point of view. Coatings with high absorbability and low roughness feature good tribological properties, for example, during cold start of piston machines. Nickel precipitates and nanowires enable sliding of cast iron against aluminium oxide. On the basis of the elaborated diagrams, the potential user can select nickelising parameters to produce coatings featuring the required properties.

In the present study, a nickel sulphate bath containing SiC particles between 0·5 and 5·0 μm has been used as the plating electrolyte to obtain Ni–SiC composites on copper surfaces with high hardness and wear resistance for using in antiwear applications such as dies, tools and working parts of automobiles. The influence of SiC concentration in the electrolyte on morphology, microhardness, friction coefficient and wear missing volume of coatings has been studied. The coatings were analysed with scanning electron microscopy as well as X-ray diffraction. The findings indicate that increasing the concentration of SiC particles in the Watts bath results in improving the microhardness of coatings and also decreasing the friction coefficient and wear missing volume. Increasing the presence of SiC particles not only decreases the grain size of Ni crystals but also changes the morphology and preference orientation of Ni crystals.

This paper reports an experimental study of corrosion characteristics of electroless Ni–P–TiO₂ nanocomposite coatings. Coating process parameters are optimised for maximum corrosion resistance based on L₉ Taguchi orthogonal design with four process parameters, namely, concentration of nickel source solution, concentration of reducing agent, concentration of TiO₂ powder and bath temperature. Corrosion behaviour of the electroless Ni–P–TiO₂ nanocomposite coatings was evaluated in 3·5 wt-%NaCl aqueous solution using polarisation technique. Scanning electron microscopy and energy dispersive X-ray spectroscopy analysis were used for studying surface morphology and chemical composition of the electroless Ni–P–TiO₂ nanocomposite coatings. The results showed that incorporation of TiO₂ in coating causes an increase in corrosion resistance and improves surface morphology. Finally, optimum conditions were achieved as follows: concentration of nickel source solution, 50 g L⁻¹; concentration of reducing agent, 10 g L⁻¹; concentration of TiO₂ powder, 10 g L⁻¹; and bath temperature, 85°C.

The amorphous nickel–phosphorus coating and its carbon nanotube (CNT) composite coating were prepared by brush electroplating technology. After 1 h heat treatment at 400°C, the coatings translated into the crystalline state, which consists of α-Ni matrix and Ni₃P precipitated phase. The results of energy dispersive X-ray, X-ray diffraction analysis and differential thermal analysis showed that, due to CNTs, the phosphorus content in the coating decreased from 8·42 to 7·34 wt-%. Carbon nanotubes also had an effect on crystallisation kinetics of the amorphous coating. The crystallisation activation energy in Ni–P/2 g CNT composite coating was 25 kJ mol⁻¹ higher than that in Ni–P coating. The time and temperature of the 50% crystallised were 5 min longer and 29°C higher respectively than that in Ni–P coating. Further study showed that CNTs played a role in obstruction in the crystallisation because CNTs block nucleation and growth of crystallisation for Ni₃P phase.

Ni–ZrO₂ nanocomposite coatings were obtained by electroplating using Watts nickel baths with different concentrations of ZrO₂ particles in suspension. The substrate was horizontally immersed in the bath to enhance the ZrO₂ content in the Ni matrix. X-ray diffraction analysis showed that the presence of ZrO₂ leaded to the intensity of crystal face (311) sharply increasing, and, simultaneously, the crystal orientation of (220) was greatly decreased. Observation using scanning electron microscopy indicated that the incorporation of ZrO₂ nanoparticles also caused changes in the morphology and size of Ni grains. Although agglomeration of ZrO₂ nanoparticles occurs in the deposits, the nanocomposites exhibited a great improvement in wear resistance compared to those of unreinforced Ni coating.

CoWP alloy films were prepared by a typical citrate system electrodeposition on a copper substrate. The paper investigated effects of different temperatures on codeposition mechanism, magnetic property, component and structure of CoWP alloy films during the electrodeposition process. With the increase in temperatures, deposition rate and thickness rose gradually during the electrodeposition process. It was found that higher temperature improved contents of phosphorus and tungsten but reduced amounts of cobalt in the films. Almost all the deposited films were crystalline and formed tetrahedral structures Co₃W with preferred crystallographic orientation (200) and (201). Films of dissimilar surface morphology could be observed under different temperatures. Lower saturation magnetisation and higher coercivity of CoWP films could be obtained at higher temperatures.

In the present study, friction stir processing (FSP) was successfully carried out on Ti–6Al–4V alloy and nanocalcium phosphate (CaP) particles were incorporated during process. Subsequently, nanohydroxyapatite (HAp) film was effectively synthesised on Ti–CaP nanocomposite interlayer by sol–gel method. The Ti–CaP nanocomposite interlayer and nano-HAp film were characterised by XRD, OM, SEM and EDS. The substrate topography was studied by AFM analysis. The CaP particle size in the nanocomposite interlayer was in the range of 40–100 nm and nano-HAp film obtained was 10–12 μm thickness. It was found that the Ca/P atomic ratio is 1·67 in the nano-HAp film which changes to 1·21 in the Ti–CaP nanocomposite interlayer. Finally, the potentiodynamic polarisation of as received and coated samples in simulated body fluid solution was studied.

Surface activation of polyethylene (PE) samples has been carried out using low pressure plasmas (at two different operating frequencies, namely, 40 kHz and 13·56 MHz) and dielectric barrier discharge (DBD) (50 Hz frequency and at atmospheric pressure), and the results are compared. The surface of the PE samples has been exposed to these different plasmas for various time durations ranging from 1 to 30 min. The treated samples have also been studied for their aging behaviour by exposing them to the ambient atmosphere for up to 7 days. The plasma induced morphological changes were studied using an scanning electron microscope and an atomic force microscope, while the formation of various functional groups was identified using Fourier transform infrared analysis. The surface energy values were observed to increase from 27·5 dyne cm⁻¹ (of untreated PE) to 73·8 and 52·2 dyne cm⁻¹ after low pressure 40 kHz and 13·56 MHz air plasma treatment respectively and to 35·6 dyne cm⁻¹ after the DBD air plasma treatment. The low pressure plasma treatment at an operating frequency of 40 kHz has produced the best results in the surface activation of PE. During the aging process, the formation of C = C bonds was observed on the surface of PE.

Copper slag is a waste product obtained during matte smelting and refining of copper. The present work explores the coating potential of copper slag by plasma spraying. This work shows that copper slag is eminently coatable. When premixed with alumina powder, the coating exhibits higher interfacial adhesion as compared to pure copper slag coatings. Maximum adhesion strengths of about 23 and 27 MPa are recorded in for the coatings of copper slag with 15 wt.% of alumina on aluminium and mild steel substrates respectively. The input power to the plasma torch is found to affect the coating deposition efficiency and morphology of the coatings.

In order to improve corrosion resistance of pure titanium in strong acid solution, the tantalum modified layer was formed on the pure titanium surface by double glow plasma surface tantalumising. The modification layer was comprised of deposition layer and diffusion layer, which metallurgically adhered to the substrate. Tantalum element decreased with the case depth. The corrosion resistance of the tantalum modified layer in HCl solution was investigated by weight loss method and electrochemical measurements, and the corrosion morphology (SEM) was studied by SEM and X-ray diffraction. The annual corrosion rate of the modified layer was only 12 and 21% of pure titanium in 10 and 20 wt-% HCl solution respectively, and the corrosion rate was still small in 30% HCl solution. In concentrated acid for 120 h, it was found that there was significant pitting corrosion on the surface of pure titanium, while no pitting corrosion could be perceived in the modified layer. The main reason for the good corrosion resistance was a compact, stable tantalum oxidation film formed at the surface of the modified layer.

Zinc films were prepared by a typical chloride zinc system electrodeposition on iron substrate. Effects of different temperatures on deposition rate, structures and surface morphology of zinc films were investigated in the paper. Trivalent chromium passivation technology was also studied in the paper to improve corrosion resistance of zinc films. Higher temperature could dramatically improve bath conductivity, which contributed directly to the rise in mass transfer and deposition rate. The deposited films were crystalline and showed characteristic peaks of zinc. However, zinc films with bad crystallinity would be obtained at higher temperatures due to thermal decomposition of additives. With the increase in temperature, zinc films with rough surface could be observed. Zinc passivation layer with optimal corrosion resistance could be obtained at passivation time of 30 s, temperature of 303 K and pH 3.

An alternating current field (ACF) was employed in pack boriding at 800°C to produce a thick Fe₂B coating on an AISI 1045 steel. The effect of ACF on the growth of the boride coating was studied, and the coating's structure and phases were characterised. The results showed that an ACF could greatly enhance pack boriding and lead to the formation of a single Fe₂B phase coating, which is much thicker than a coating by corresponding conventional pack boriding (CPB). The ACF's enhancing effect increased almost proportionally with increasing ACF currents from 1 to 9 A. The growth rate of the coating versus boriding time showed the same parabolic character as CPB. The mechanism of the ACF's effect on the formation of the single phase Fe₂B coating was associated with enhanced chemical reactions in the boriding media and enhanced diffusion both in the media and in the treated sample.

In an effort to produce more cost effective diamond to metal interfaces, mono- and polycrystalline diamond powders were coated with nickel alloys using electroless chemical deposition techniques. Ni–P and Ni–B coatings were deposited using an acidic and an alkaline solution respectively. These deposition procedures were preceded by a thermal surface functionalisation step and a sensitisation step using a Pd–Sn colloidal solution. Imaging by SEM and chemical analyses show uniform coverage of the coating, independent of diamond's crystalline planes, and having thicknesses consistently <200 nm.