Surface Engineering

Biomedical implants such as prosthetic hips and heart stents and instruments used in vivo all have negative performance issues. A common solution, which can also be appropriate for other devices, is to apply a diamond-like carbon thin film in order to enhance the performance of the devices. The films were produced by pulsed laser deposition and characterised by Raman spectroscopy, atomic force microscopy, ball on flat tribometry, contact angle measurements and spectrophotometry. The stability of the films was checked by soaking in simulated body fluid (SBF). The root mean square surface roughness was found to be <1 nm, and a coefficient of friction of 0·08 was produced in a lubricated environment. Contact angles ranged from 65 to 88°. The coatings produced on fused silica had high transparency and showed no delamination after 43 weeks of immersion in SBF. These films have potential to enhance the performance of biomedical implants and instruments.

In this article, a protective Ni–Co alloy coating was prepared on AZ91D magnesium alloy which was pretreated with electroless plating. The surface morphologies, microstructure and chemical composition of the coatings were studied using environmental scanning electron microscope, X-ray diffraction and energy dispersive spectroscopy respectively. The surface morphologies and microstructure analysis showed that the Ni–Co alloy coating possessed cone shaped grain congeries and formed face centred cubic solid solution. The energy dispersive spectroscopy analysis revealed that the content of cobalt element in the alloy coating was ∼31 wt-%. Potentiodynamic polarisation curves and electrochemical impedance spectroscopy were employed to investigate the corrosion resistance of different corrosive systems in neutral 3·5 wt-%NaCl and 0·5 mol L^–1 Na₂SO₄ solutions, which were chosen to simulate the effects of marine and industrial environments respectively. The results showed that the corrosion resistance of the AZ91D magnesium alloy was obviously improved by the Ni–Co alloy coating.

Different surface preparation processes and post-treatments were applied to ‘electroless’ E-coated AZ31B magnesium alloy. The corrosion performance of the differently treated samples was assessed by means of ASTM B117 salt spray, anodic polarisation curve and electrochemical impedance spectroscopy measurements respectively. The results show that the corrosion resistance of the ‘electroless’ E-coated AZ31B can be ranked in the following order: wet polishing+curing>dry polishing+curing≈wet polishing>dry polishing>commercial benchmark treatment. It suggests that a suitable pre- or post-treatment can significantly further improve the corrosion performance of an ‘electroless’ E-coated Mg alloy. The surfaces of the differently pre- and post-treated AZ31B were examined by SEM. It appears that the different corrosion resistances can be attributed to different microstructures of the surface films on AZ31B.

A layer of nickel–phosphorus alloy was electroless plated on the surface of microarc oxidised AZ31 magnesium alloy first, and then sealing treatment was performed on the electroless nickel plated coating using bis-(triethoxysilylpropyl)tetrasulphide silane. Fourier transform infrared spectroscopy, scanning electron microscope (SEM), energy dispersed X-ray spectroscope analyses, electrochemical techniques and immersion test were used for the analyses of surface morphology and structure and corrosion resistance of electroless nickel plated samples before and after sealing treatment. The results showed that the silane film by sealing treatment improves its corrosion resistance significantly.

This paper addresses the residual stress in the microarc oxidation (MAO) coating on AZ31 magnesium alloy. The MAO coating was deposited by pulsed direct current (dc) and examined by scanning electron microscopy and X-ray diffraction for its microstructure and composition. The porosity of the coatings was measured by potentiodynamic polarisation tests and found to be in the range of 3–6%. The residual stresses in the MAO coatings prepared at 300, 500, 1000 and 3000 Hz respectively were found to be between −860 and −1272 MPa. Using a high pulse frequency at 3000 Hz can produce a less porous and less stressed MAO coating, a desirable feature in the biomedical applications of MAO coated AZ31. This finding was further verified by establishing a first principle relation between the residual stress and the porosity via Stoney equation, which enables extrapolation of the experimental data measured.

For the first time, electroless nickel plated ZrO₂ (Ni-NCZ) particles were used for the co-electrodeposition of Ni-NCZ composite coating. Optical microscope, SEM and X-ray diffraction studies showed that Ni-NCZ has a rough surface with a smaller Ni crystallite size than Ni-ZrO₂ due to the conductive properties of NCZ particles, which provide more nucleation sites for Ni clusters. The energy dispersive X-ray spectroscopy measurement results showed that Ni-NCZ has more co-electrodeposited particles than Ni-ZrO₂. This is due to the more positive zeta potential of NCZ particles compared to that of ZrO₂. The microhardness measurements demonstrated that the Ni-NCZ composite coating has a higher microhardness than Ni-ZrO₂ due to the higher amounts of co-electrodeposited particles and lower crystallite size. Electrochemical impedance spectroscopy and potentiodynamic polarisation test showed that the corrosion resistance of Ni-NCZ is higher than that of Ni-ZrO₂.

Electrochemical impedance spectroscopy of Ni and Ni–ZrO₂ composite coatings was studied. Investigation of corroded surfaces showed that the cluster boundaries in pure Ni and weak bonds between Ni matrix and ZrO₂ particles in Ni–ZrO₂ composite coating are the appropriate paths for corrosion to proceed. An equivalent circuit diagram based on blocked and partially corroded surface characterisations was proposed, and good agreement was observed between theoretical impedance spectra obtained on the basis of the equivalent circuit and spectra recorded during the measurements. Changes of microstructure and corrosion proceeding paths were recognised as the reasons for the higher corrosion resistance of Ni–ZrO₂ with respect to pure Ni.

Ni_40·8Fe_27·2B₁₈Si₁₀Nb₄ (at-%) coating was deposited on mild steel substrate using high power diode laser cladding followed by laser remelting process. The phase composition and microstructures of the as cladded coating and remelted coating were analysed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The microhardness and wear properties of the coatings were also tested and analysed. It was observed that the remelted coating has an amorphous matrix embedded with some dendrites. The remelting of the coating increased the hardness of as cladded coating by ∼25%. The remelted coating exhibited a characteristically lower friction coefficient (0·21) than the as cladded coating (0·27) and the substrate (0·31). Significant increase in wear resistance was found for the remelted coating compared with the as cladded coating and the substrate under identical conditions.

Aluminium alloy parts are often formed using cold forming. The transfer of aluminium to the tool is a major problem in these operations. TiB₂ has low reactivity with aluminium and has shown promising results in other forming tests. Here, cold forming is simulated in equipment comprising a TiB₂ coated tool cylinder and an aluminium cylinder in sliding contact. The coated surfaces are prepared to two surface finishes, and the aluminium cylinders were prelubricated or unlubricated respectively. The test is focused on friction level and number of contacts to reach a threshold friction level. The aluminium surface pretreatment was found to be the most important factor; lubrication lowers the friction significantly. The tool surface finish is also important; polishing lowers and stabilises the friction. The TiB₂ coating offered 20–30% better galling performance than the uncoated steel. However, this improvement was far from that of the best available diamond-like carbon coatings.

The low erosion resistance of titanium and its alloys has prevented their widespread application as joint implants. In addition, one essential requirement for the implants to bond with the living bone is the formation of a bone-like apatite on their surfaces in the host body. To enhance the erosion resistance of the surface, a diffused layer of TiB₂ was formed at 1000°C on the commercial pure titanium. Hydroxyapatite was then coated on the boronised titanium by means of dip coating in a sol–gel solution. In order to confirm the biocompatibility of the specimens, they were soaked in a simulated body fluid for several days. The surface morphology of the specimens after exposure was studied by scanning electron microscopy, whereas X-ray diffraction patterns clearly revealed the growth of a calcium phosphate phase on top of the surface. Results showed that both wear resistance and biocompatibility of the hydroxyapatite coated samples on boronised films were improved.

In this study, Al–Ti–Co was first used to improve the surface performance of pure Ti. The synthesis of hard composite coating on pure Ti by laser cladding of Al–Ti–Co+TiB₂/Si₃N₄ preplaced powders was investigated in detail. The SEM result indicated that a composite coating with metallurgical joint to the substrate was formed, and the coating can also have major dilution from the substrate. X-ray diffraction result indicated that this composite coating mainly consisted of Ti₃Al, TiB₂, TiB, TiN, Ti₅Si₃ and Co–Ti intermetallics. Compared with pure Ti, the improvement of the microhardness and wear resistance was observed for this composite coating.

The annealing parameters play a vital role in determining and understanding the microstructures and mechanical properties of Ta–Mo nanostructured coatings in the annealing processes. This investigation aims at finding the best annealing parameters and predicting the behaviour of the annealing parameters during the preparation of Ta–Mo coatings by cosputtering deposition. This study reveals that phase transformation and grain growth of the coatings were strongly dependent on the annealing processes. Low temperature annealing process within 600°C can increase the hardness H and elastic modulus E due to the increased grain size and decreased crystal defect. However, severe oxidation occurred in the coatings by the high temperature annealing process that exceeded 700°C. Finally, a three stage model related to the low and high temperature annealing mechanisms of Ta–Mo coating was proposed.

Amorphous gallium nitride doped with Mn thin films was deposited on sapphire (0001) substrates by laser molecular beam epitaxy. After simple processing of annealing at different temperatures for 30 min in ammonia atmosphere, good quality Ga_1−xMn_xN films were obtained. In order to investigate the influence of annealing temperature on the crystalline quality and structural and magnetic properties, the films are analysed by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and alternating gradient magnetic field meter. The results show that the annealing can improve the quality of Ga_1−xMn_xN films, and the optimum annealing temperature is 1050°C; moreover, there is no second phase forming. Through magnetic analysis, we obtained the annealed samples of room temperature ferromagnetic property and paramagnetism of the as grown film and obtained the way of changing the paramagnetism into ferromagnetism using the simple method.

Si–Mo–Cr oxidation protective coating has been prepared on the surface of C/SiC coated carbon/carbon (C/C) composites by pack cementation technique. The phase composition and microstructure of the as prepared coatings were characterised by X-ray diffraction, scanning electron microscopy (SEM) and energy dispersion spectrum (EDS) analyses. The oxidation behaviour of the coated C/C specimens was also investigated at 1500°C in air. The results show that the as prepared coating is mainly composed of SiC, MoSi₂, CrSi₂ and Si. The coating characterised by excellent oxidation resistance and thermal shock resistance can effectively protect C/C composites from oxidation for 186 h at 1500°C in air and endure the thermal cycle between 1500°C and room temperature for nine times, while the corresponding weight loss is only 0·33%. The excellent oxidation protection ability of the coating is primarily attributed to the formation of a compound glass layer, including SiO₂ and Cr₂O₃.

Failed rail can be repaired using self-shielded flux cored wire. Aluminium powder is always added to the flux cored wire to protect the weld pool. However, the Al content affects the microstructure and property of the surfacing layer significantly. Therefore, the microstructure and impact property of the surfacing layer of the rail with different Al contents were analysed. Meanwhile, the formation mechanism of the inclusions was calculated by thermodynamic method. The results show that the pore sensitivity of the surfacing layer is high when the Al content is <0·79 wt-%. The impact energy decreases with the increase in Al content. The thermodynamic calculated results show that the inclusion in the surfacing layer is AlN when the Al content is >1·89 wt-%. In contrast, they are mainly Al₂O₃ when the Al content is <1·45 wt-%. The thermodynamic calculated results are well agreed with the experimental ones.