Key Engineering Materials Vols. 373-374

Abstract: Two types of Ni-based coatings were deposited by high velocity oxy-fuel (HVOF) spraying process on ASTM 1020 low carbon steel substrate, and AISI 321 stainless steel was applied to the contrast test. The influences of erosion rate on the erosion-corrosion synergism and erosion-wear mechanism of the two coatings were investigated systematically in a liquid/solid two-phase flow (5% H2SO4 +15% silica sand). The volume loss rates stemming from the erosion-corrosion synergism of materials increased from 42~66% to 78~85% of the total volume loss rates when the erosion rate of the flow increased from 5 m/s to 10 m/s. It was indicated that the erosion-corrosion synergism was primary for the material removing during the erosion-corrosion process. Cutting, wear and plowing were the main features for Type-A coatings, while cutting, plowing and plastic deformation were the main characters for Type-B coatings and the two steels.

Abstract: The exhaust pipe of a heavy duty vehicle, due to its harsh operating environment, is subject to corrosion caused by high temperature oxidization, chlorination and sulfuration, therefore, corrosion occurs very easily, which affects normal operation of the engine. The exhaust pipe of the engine can be protected by means of electro-thermal explosion spray-coating technique. The corrosion-resistant ability of coating in high temperature environment is surveyed respectively. The micro organizational structure of coating is studied in combination with SEM, EDS and other tests to search for corrosion-resistant principle of the coating in an attempt to improve the corrosion-resistance of the exhaust pipe of the engine.

Abstract: Three kinds of coating materials, namely Al2O3, Al2O3(87%)-TiO2(13%), and Cr2O3(92%) -TiO2(3%)-SiO2(5%), were plasma sprayed on specimens of 08Cr2AlMo steel. The corrosion performance of the specimens was determined by potentiodynamic polarization test, slow strain rate tensile (SSRT), and scanning electron microscope (SEM). The test results indicate that the plasma sprayed coatings do not show passivation in saturated H2S solution, but they have higher corrosion resistance than the bare 08Cr2AlMo steel. Furthermore, these coatings reduce SCC susceptibility of the steel in saturated H2S solution. The specimens treated by plasma spray have longer fracture time and larger area reduction. The SCC resistance depends largely on coating porosity, coating type and the bond strength between the coating and the substrate. Among the three coatings, the Cr2O3(92%)-TiO2(3%)- SiO2(5%) coating exhibits the lowest SCC susceptibility.

Abstract: Ni-PTFE/SiO2 composite coatings were prepared by jet electrodeposition. The influences of the particles contents and jet velocity on the properties of the composite coatings were studied. The morphology and microstructure of the coatings were observed on an S-2700 scanning electron microscopy. HX-1 microhardness tester was used to determine the hardness of the coatings. The deposition rate were determined by means of the coatings thickness. The adhesion strength was measured using a scratch tester. Wear resistance tests were performed using a pin-on-disc tribometer with the sample placed horizontally on a turning table. The corrosion behavior of the coatings immersed in 10% H2SO4 solution was analyzed using anodic polarization curve measurement. It was shown that the microhardness of the coatings increased with the increase of SiO2 content, while the addition of PTFE in the coatings resulted in better corrosion-resistant property. Enhancing the jet velocity made the coating compacter. Ni-PTFE-SiO2 composite coatings demonstrated superior wear resistance to Ni-PTFE or Ni-SiO2 coatings under the experimental conditions.

Abstract: Now the oxidation-resistant glass-ceramic coatings have been widely used in reducing surface oxidation of Ti alloys during heat process. In present investigation, a lead-free glass-ceramic coating with wider protection temperature range was developed to protect Ti-6Al-4V alloy from oxidation. The phase compositions of the present coating and the oxygen distributions in the surface layers of Ti-6Al-4V specimens were investigated by means of XRD, as well as the metallographs of cross-section were observed and the depths of oxidized layers were determined by microhardness analysis. The results show that the present glass-ceramic coating can provide the oxidation-resistant effect over the temperature range of 500-1000oC in ambient air, with an environmentally friendly lead-free coating composition.

Abstract: A new novel cored wire for producing FeCrAl/WC intermetallic coating by arc spraying was developed. The erosion wastage at high temperature of the coating was researched with different impingement angles and temperatures. The results indicated that impingement angle and temperature greatly affected erosion wastage of the coating. At the same time, when impingement angle was 90°, different temperature had a little effect on the erosion wastage. And when temperature was 650°C, different impingement angle had a little effect on the erosion wastage too. As compared with water wall tube material (G20 steel) of power station boiler, the erosion resistance at high temperature of the coating was higher. The erosion resistance rate of the coating was 1.94~2.89 times of that of G20 steel when temperature was 650°C and particle was polygonal silica. The results indicated that erosive condition of high temperature flue gas and fly ash mixture in boiler tube of pulverized coal fired power station could be solved with the FeCrAl/WC coating.

Abstract: As magnesium is an essential element for the human body, magnesium alloys are expected to be promising implant materials despite their lower corrosion resistance. The most potential advantages of magnesium lie on both excellent biocompatibility and good mechanical properties. A TiO2 coating had been successfully deposited on the surface of AM60 samples by using a thermal spray technique and was then sealed with sodium silicate. The corrosion behavior was investigated by using the potentiodynamic electrochemical technique. The experimental results demonstrated that the TiO2 coating had a porous ceramic structure. The thickness and micro hardness of the layer reached 40 μm and 886 HV, respectively. After additional sealing with sodium silicate, TiO2 coating on magnesium alloy improved corrosion resistance in Hank’s solution.

Abstract: The paper presents a simple chemical treatment to provide various non-metallic substrates such as polypropylene or polyethylene with in vitro bioactivity or ability to deposit apatite in SBF. Anatase/rutile dual layers were deposited at low temperature on the substrates when they were soaked in TiOSO4/H2O2 solution and aged in hot water: The dense bottom layer predominantly consisted of rutile, while the upper layer consisted of loosely packed aggregation of anatase particles. The titania deposition was the results of compromise among two conflicting processes: 1) hydrolysis of TiOSO4 to yield either soluble titania-H2O2 complexes or titania, and 2) dissolution of the titania layer under the presence of H2O2. The dissolution-deposition equilibrium was found associated with pH of the sulfate solution. Thus, proper pH value in the treating solution was the key factor to control the formation of dense titania layers. The resulted titania layers were easily covered with fine apatite particles when soaked in SBF supersaturated with its component ions.

Abstract: Nd, Ce and Pr in ER-magnets are very active and easy react with oxygen and other oxidizing substance [1]. So this paper deals with the formation of some organic films on the Nd8Fe80B6Co6 magnetic powders. Then the belt of the alloy was made in a melt-spun furnace and then heat-treated at certain temperature. After that the magnetic powders were prepared by special crashing techniques. Epoxy resin and titanate were chosen as coat materials. The test showed that the epoxy-titanate coating material has the lowest weight increase in the exposure of oxidation atmosphere. Effects of the coat materials on the properties of bonded alloy were also investigated. The magnetic properties of the alloy with epoxy-titanate coating material were similar to one without coat.

Abstract: 1 mol% Er3+- and 0–10 mol% Y3+-codoped Al2O3 powders is prepared in a non-aqueous sol–gel method. Two crystalline types of doped Al2O3,  and θ, are obtained for the 0–10 mol% Y3+-codoped Al2O3 powders sintered at the sintering temperature of 1000 °C by x-ray diffraction analysis. The green and red up-conversion emissions centered at about 550 and 670 nm increase with Y3+ codoping concentration, and the maximal intensity of both the green and red up-conversion emissions is obtained as about 100 and 10 times higher than that of the 1 mol% Er3+-doped Al2O3 powders, respectively. The enhancement in the green and red up-conversion emissions is ascribed to the improved dispersion of Er3+ in Er3+–Y3+-codoped Al2O3 powders.