Advanced Materials Research Vols. 97-101

Abstract: Ceramic coatings were prepared on Ti6Al4V in aluminate-based electrolyte by microarc oxidation at various voltages using potentiostatic regime. The phase composition, microstructure and morphology of the coatings were characterized by eddy thickness gauge, XRD and SEM respectively. The corrosion resistance of the coatings was investigated by polarization curve and electrochemical impedance spectroscopy (EIS) in 3.5% NaCl aqueous solution. The results show that all the coatings are mainly composed of Al2TiO5. When the voltage increases from 360V to 510V, the coating thickness grows significantly, and the pores in the coatings get large and deep simultaneously. As a result, the corrosion resistance of the coatings increases first and then decreases with the turning point of 410V, at which the coating prepared shows the smallest corrosion current density of 1.965E-9 A/cm2. The coating prepared at a higher voltage before the discharges grow large and intense (410V in this work) shows the best corrosion resistance.

Abstract: Both of coating with lead nitrate as stabilizer (common Ni-B coating) and coating with thallium sulfate as stabilizer (Ni-Tl-B coating) were prepared on steel substrates. Meanwhile, different Ni-Tl-B coatings were also prepared by changing the content of thallium salt in solution. Several kinds of process of electroless plating Ni-B were introduced. Surface morphology was observed with TM-1000 scanning electron microscope, and the alleviating fretting damage property was tested on DELTALAB-NENE high-precision hydraulic servo fretting wear tester. The results show that: The more content of thallium salt, the rougher the surface morphology of coatings is. When the content of thallium salt in plating solution is 2mg/L, the coating has the best alleviating fretting damage property with the coefficient of friction 0.12 in running-in stage.

Abstract: Three different WC grain size powder were used to prepare WC-Co coatings employing HVOF technique. The effect of WC grain size on the abrasive wear resistance and failure mechanism were analyzed. The results show that the abrasive wear resistance increases with the decrease of WC grain size. Nanostructured coating is high in surface plasticity. It can only be pressed out of prints, the opportunity of cutting is small. Micro-cutting wear dominates the main abrasive wear mechanism of nanostructured coating.

Abstract: A kind of copper multilayer coating was prepared on stainless steel substrate using ultrasonic-electrodepositing method. The microstructure, hardness, wear resistance, adhesion and corrosion resistance of it were investigated respectively. The results show that the coating exhibits a multilayer structure and the thickness of each layer is about 0.3µm. The crystallite average diameter of it is obviously smaller than that of the ordinary electrodeposited copper coating. The diffraction peaks of Cu (200) and Cu (111) for it are obviously stronger, while Cu (220) for it is much weaker. Comparative investigations on the mechanical properties show that only the hardness of it is a little lower, whereas the wear resistance and the adhesion of it to the substrate are all evidently enhanced. The average start corrosion time of it in 20％ HNO3 solution is nearly three times delayed, and the corrosion rate is obviously decreased; anode polarization curves in 3.5% NaCl solution also show that the corrosion resistance of it is greatly improved.

Abstract: A nanostructure surface layer was produced on low carbon steel and commercially pure titanium using high-speed rotating wire-wheel deformation (HRWD). The microstructural features of the surface layer were systematically characterized by cross-sectional optical microcopy observations, transmission electron microscopy, and microhadness measurement was conducted along the depth from top surface layer to matrix of the samples. The results show that nearly equiaxed nanocrystalline layer is formed on the surface of the low carbon steel and pure titanium, in which the average grain size is about 8 nm and 15 nm respectively. The microhardness of the top surface is enhanced obviously compared with that of the coarse-grained matrix.

Abstract: Molybdenum alloying or modified layer was fabricated on the H13 steel substrate by electronic spark deposition technique (EDS). The microstructure, element distribution and microhardness of the modified layer were analyzed using metalloscope, X-ray diffraction (XRD), Glow Discharge Optical Emission Spectroscopy (GDOES) and microhardness tester, respectively. The modified layer is composed of white layer, diffuse layer and heat affected zone. The interdiffusion between Mo element and other elements in H13 substrate results in Metallurgical bonding. Microhardness of the coating is 1482HK0.025 which is 5 times higher than that of H13 steel base (280HK0.025). The ball-on-disc dry sliding test indicates that the friction coefficient of H13 substrate is decreased obviously by the molybdenum modified layer. The wear mass loss of Mo modified H13 substrate is only one seventh of that of the substrate. The wear resistance of H13 is improved significantly by the Mo modified layer.

Abstract: Nickel-iron-silicon nitride nanocomposite thin films were prepared by electrodeposition technique. The deposition was performed at current density of 11.5 A dm-2. Nano-size silicon nitride was mixed in the electrolyte bath as dispersed phase. The effects of silicon nitride nanoparticulates in the nickel-iron nanocomposite thin films were investigated in relation to the amount of silicon nitride in the plating bath. X-ray diffraction (XRD) analysis showed that the deposited nickel iron film has face-centered cubic structure (FCC). However, a mixture of body-centered cubic (BCC) and face-centered cubic (FCC) phases were observed for nickel iron-silicon nitride nanocomposite films. The crystallite size of Ni-Fe nanocomposite coating decreased with increasing amount of silicon nitride in the film. From elemental mapping procedure, Si3N4 nanopaticles were uniformly distributed in the Ni-Fe film. The presence of silicon nitride increased the hardness of the film. The microhardness of the nickel-iron nanocomposite increased from 495 HV for nickel-iron film to 846 HV for nickel-iron nanocomposite film with 2 at. % Si. The coercivity of Ni-Fe nanaocomposite films increases with decreasing crystallite size.

Abstract: The wear behavior of chromizing-titanizing coating is examined in different media at room temperature and at elevated temperature, and compared with the wear behaviors of chromizing coating, titamizing coating and basis carbon steel. The structure of chromizing-titanizing coating is analyzed using electron probe, X-ray diffration and hardness. The results show that the chromizing-titanizing coating has better wear resistance than that of chromizing coating, titanizing coating and basis material. The wear resistance of chromizing-titanizing coating is mainly affected by the microstructure and composite of the coating which consists of complex compounds containing chromium carbides and TiC. The chromizing carbides exist mainly outside the coating and TiC exists mainly inside the coating. The complex compounds make the coating have both advantages of chromizing coating and titanizing coating, resulting in good wear resistance of chromizing-titanizing coating at room temperature and at elevated temperature.

Abstract: Zinc protective coatings on high carbon SWRH82B-1 steel were sherardized to markedly improve corrosion resistance of the high-strength steel bridge cable wires (SBCW). Sherardizing parameters have been optimized by optical microscopy (OM) /scanning electron microscopy (SEM), X-ray diffraction (XRD) and potentiodynamic polarization tests. The sherardizing coatings are composed of the loose outer layer (§-FeZn13 phase) and the dense inner layer (δ- FeZn7 phase) with higher hardness. Addition of Y2O3 activator slightly increases the corrosion resistance of sherardized steel wire in comparison with CeO2. A thicker coating corresponds to a higher sherardizing temperature or a longer heating duration, but an extra thick coating is unfavorable for thru-microcrack existed in the inner layer. Good quality of sherardized wires ( higher corrosion resistance and longer duration than conditional hot-dip-galvanized one) can be produced with the zinc-rich powder containing 7.5wt.% CeO2 activator and 25wt. % SiO2 filler under 400°Cfor 6h.

Abstract: A nickel aluminide coating was formed on P92 steel substrate using a two step process of electro-Ni plating followed by pack aluminising at 650 °C. The coating was tested in 100% steam to assess its resistance against steam oxidation at 650 °C using a purpose-built rig for steam oxidation tests. The data obtained were compared with those measured from air oxidation test at the same temperature. It was revealed that steam is a more severe oxidising environment than air for the coating. Oxidation kinetics and degradation mechanisms affecting the lifetime of the coating were discussed.