Key Engineering Materials Vols. 373-374

Abstract: Substrates have major effects on coating properties. In this paper, pure magnesium, AZ91HP and Mg-Li were separately anodized in a neutral anodizing solution containing hydrofluoric acid, phosphate acid and borate acid. The coating properties such as surface morphology, thickness and compositions were separately observed and detected. The behavior of voltage with time indicated that reaching the same final voltage under the same electric parameters, anodizing time for AZ91HP was the shortest and Mg-Li the longest. The uniformity of anodic coatings formed on AZ91HP was the best and Mg-Li the worst. Analysis by EDX, the phosphorus content of anodic coatings formed on Mg-Li was the highest and that on AZ91HP the lowest. Alloying elements of aluminum and lithium slightly decreased and considerably increased the coating thickness, respectively. The effects of lithium and aluminum on coating properties can be explained by different moving speeds of Mg2+, Al3+ and Li+ as well as Pilling-Bedweorth ratios (PBRs) of MgO/Mg, Al2O3/Al and Li2O/Li.

Abstract: Anodic alumina oxide (AAO) templates with multi-branched structure were fabricated in a phosphoric acid. Atomic force microscope (AFM) was utilized to investigate the unstable growth of porous oxide film in initial stage. The morphologies of cross-section of AAO templates at different current density were also studied with FESEM. The results show that the acquired AAO templates are of multi-branched structure with large size. The diameter of pores on outer surface ranges from 150 to 220 nm, and the width of branches from 100-120 nm. This structure can be due to the unstable growth of AAO film in initial stage. Besides, the structure of AAO template is sensitive to the current density and the order degree decreases with the increase of current density: at 1.0A/dm2, most of tubes are not straight and some divided into two branches; at 2.0A/dm2, all tubes grow disorderly and form net-like structure. In this electrolyte, different types of structure of AAO templates can be acquired by adjusting current density, which may be used for the synthesis of new types of nanowires with improved mechanical properties.

Abstract: The electroless nickel-phosphor coatings containing molybdenum disulfide nanoparticles were prepared and analyzed in this paper. The effects of incorporation of MoS2 into the Ni-P coating on the morphology of the coating surface and corrosion properties were also studied. Corrosion tests were conducted inside a salt spray box with NaCl solution (5.0 wt%). The corrosional surfaces were studied and analyzed through optical microscope, X-ray spectrometer (XRD) and scanning electron microscopy (SEM). The investigation on the relationship between heat-treatment and the corrosion resistance of the coatings showed that the corrosion resistance of the composite coating became worse because of the occurrence of transformation from non-crystalline to crystalline, and then increased the metastable intergradation of the composite coating. Meanwhile, the experimental results also showed that corrosion resistance of the coating containing MoS2 was higher than that of steel substrate. The corrosion mechanism of the composite coatings was mainly ascribed to the formation of micro-cell around the nanosized MoS2 particles, and the active ion like Cl- destroyed the surface film and induced the corrosion towards the inside part of coating.

Abstract: The wear-resistance, corrosion resistance, hardness can be greatly increased by using low temperature QPQ complex salt-bath treatment. And it is a new strengthening method without distortion in the treating process. The surface of 4Cr14Ni14W2Mo steel was treated using this method. The microstructure and depth of the treated surface for the steel were analyzed using SEM. The sliding wear resistance was tested on the M-2000 tester and the micro-hardness was tested using 401MVA microscopy hardness tester. The corrosion resistance was tested in the 5%NaCl water by using spraying method. The experimental results indicate that a certain depth of white layer and diffusion layer of the steel can be obtained by using low temperature QPQ complex salt-bath treatment. The nitriding compound layer with high hardness, superior wear resistance and stable microstructure, can also be obtained on the surface of the parts. The highest hardness in the surface is HV0.11012. The surface hardness is 2.8 times higher than that of inner part. The depth of white layer is from 10 to 12μm. The experimental results and applied results show that the low temperature QPQ complex salt-bath treatment has many advantages, such as fast nitriding speed, uniform heating, short process time, low treating temperature, small distortion, high production rate, low cost, stable nitriding quality no pollution and so on.

Abstract: Amorphous diamond films were deposited on both (100) silicon and 316L stainless steel substrates at atmospheric pressure and low temperature (60°C) with a direct current pulsed power supply using methanol/additive solution as electrolyte. The morphology and microstructure of the films were analyzed by scanning electron microscopy (SEM) and Raman spectroscopy. The friction and wear behavior of the films were examined on the CERT test system. The SEM observations showed that the films consisted of fine and compact ball-like grains with about 300nm in diameter. Raman spectroscopy analysis confirmed the diamond structure by the presence of strong peaks at 1333cm-1 for both substrates. The friction coefficient of the films deposited on both substrates against WC ball was about 0.10 at ambient conditions. The active additive could play an important role not only in improving adhesive strength between the films and substrates, but also in increasing relative content of diamond phase in the films.

Abstract: With excellent wear resistance and corrosion resistance of films, simple procedures, high production efficiency and being environmentally friendly, micro-arc oxidation has become a novel technique to produce ceramic coatings on the surfaces of light metals such as aluminum and its alloys. In order to decrease the electric power consumption during the process, we put forward a improved technique of micro-arc oxidation, and introduce its technical properties and treatment effects on the surfaces of aluminum. The experimental results show that the properties, such as phase structure, thickness, roughness and hardness of the ceramic coating prepared by improved micro-arc oxidation are similar to those prepared by traditional micro-arc oxidation. The micro-arc oxidation with higher deposition efficiency will be an important research direction in the future.

Abstract: The electrodeposition of aluminum on 316L stainless steel from a molten salts based on chloride has been studied. The surface morphology of the aluminum layer has been examined through scanning electron microscope (SEM) and the structure of the aluminum layer has been analyzed by X-ray diffraction (XRD). The thickness of the deposited aluminum layer has been measured by the method of cross-section scan. It has been suggested that a white, smooth, non-porous and a high purity aluminum layer can be obtained on 316L stainless steel from the ternary chloride molten salts (AlCl3 – NaCl - KCl). And the structure of the aluminum layer was single-phase.

Abstract: Cr-Co-P alloy coating was prepared from trivalent chromium bath and the appearance and performances of the coating were characterized. The Cr-Co-P alloy electroplating bath was prepared through orderly adding cobalt chloride, sodium hypophosphite monohydrate, urea, sodium format, ammonium citrate tribasic, boric acid, and ammonium brome into distilled water. Optimum plating crafts were determined as follows: pH value 1.5~3.0, temperature 25~45°C, plating time 1~15 minutes, and current density 5~25A·cm-2. Reticulate iridium dioxide coating electrode or highly pure graphite electrode were adopted as anode. And electro deposition experiments were carried out with air disturbance. The surfaces of deposited coatings are silvery white, bright and smooth. The Cr-Co-P alloy coatings were characterized by scanning electronic microscope (SEM). The results proved that P is favour to the improvement of deposit corrosion resistance. In addition, Cobalt atoms are in favor of enhancing throwing power and cover power of plating baths. Via adjusting plating bath, the electro-deposition rate could reach at 1.0~1.3μm·min-1, and the contents of P and Co could be controlled in 15~25% and 10~65%.

Abstract: Dispersants are selected and optimized by determining the height of emulsion in the colorimetric cylinder after standing in order to ensure the homogeneity of the powder and the stability of electroless plating solution. Experimental results show that non-ionic surfactants such as PEG6000,PEG10000,OP-10 have stronger emulsifying ability to α-Al2O3 powder in ordinary condition and IW has more stability at 80~90°C. The optimum conditions are suggested as PEG6000 8g, IW 0.95g and ethanol 3mL per 50mL solution. Moreover, this composite electrolessplating solution is stable for seven days under 90°C. Therefore it has more dispersing ability. The SEM images show that the powder in the coating is homogeneous. The electroless plating coating consists of α-Al2O3 and amorphous Ni-P that will change into crystal Ni3P and Ni-base solid solution after heat treatment at 400°C for one hour, the α-Al2O3 powders are dispersed evenly in the nickel matrix.

Abstract: This paper investigated the effect of nano-SiC particle on microstructure and properties of brush electroplated nano-Al2O3/Ni composite coating. Three kinds of coatings, such as pure nickel coating, nano-Al2O3/Ni composite coating and nano-SiC-Al2O3/Ni composite coating, were fabricated on 1045 steel substrate. Before deposited, Zeta potential of nano-particles in the solution was examined. The results show Zeta potential of the nanoparticles in the nano-Al2O3/Ni electrolyte is negative, but that of the nanoparticles in nano-SiC-Al2O3/Ni electrolyte is positive when the pH value of electrolyte is about 7.5. Energy dispersive spectroscopy (EDS) results show Al element content in the nano-Al2O3/Ni coating is 5.65%, while the content of Al and Si elements in nano-SiC-Al2O3/Ni coating is 5.63% and 4.86% respectively. It confirms that nano-SiC particle co-deposited in the composite coating while nano-Al2O3 particle content keeps constant. The microhardness of nano-SiC-Al2O3/Ni coating exhibits the highest, while the pure nickel coating is the lowest. And surface morphology of nano-SiC-Al2O3/Ni coating is the most smooth and compact among the three coatings. The wear test results reveal that wear resistance of nano-SiC-Al2O3/Ni coating is 1.7 times of that of nano-Al2O3/Ni coating, and 2.3 times of that of pure nickel coating. The above results show the nano-SiC particle services a strengthening effect on the combination performance of nano-SiC-Al2O3/Ni coating.