Advanced Materials Research Vols. 1120-1121

Abstract: The performance of bare steel’s rust stabilization was studied using cycle immersion corrosion test and X-ray diffraction. The results showed that after specimens were corroded in 3% NaCl solution, rust bubble of stabilized specimen was smaller than rust bubble of unstabilized specimen, corrosion rate of stabilized specimen was lower than corrosion rate of unstabilized specimen, and rust layers of stabilized specimen and unstabilized specimen were mainly composed of γ-FeOOH and Fe₃O₄.

Abstract: An extensive study was made of the boundary area microstructure between the matrix and dense inner sublayers of the ceramic coating formed on AZ91 magnesium alloy substrate by micro-arc oxidation(MAO).The coating was prepared in an alkaline electrolytic solution composed of 5-20g/l of NaH₂PO₄, 1-5g/l of NaOH, 5-8g/l of KF; 0.5-2g/l of Na₃C₆H₅O₇, and 0.5-2g/l of EDTA , employing a constant-current controlled AC power supply with a current density of 10-30A/dm²; and the microstructure of the coatings was characterized using transmission electron microscopy(TEM) and scanning electron microscopy(SEM). It was found that (1) the main constituent of the coating adjacent to the boundary was microcrystalline and nanocrystalline; (2) the microhardness of the coating adjacent to the boundary was improved than that of the matrix. It was due to the formation of the intermixing of Mg and MgO and their grains were refined, these leaded to fine-crystal strengthen.

Abstract: In this study, without changing the whole manufacturing process of T22 coiled tubes, high-temperature oxide films are formed through atmosphere adjustment by taking advantages of the cooling process after stress elimination heat treatment. The corrosion resistance of T22 heat-exchange tubes is improved, which are used in the steam generators (SG) of high-temperature gas cooled reactors (HTR). The surface microscopic morphology of the oxide films is observed using a scanning electron microscope, and the structure of these films is characterized using an X-ray diffractometer. The stability of the high-temperature oxide film forming process is investigated using TG-DTG, as well as the thermal expansion coefficient of the films. The results prove that: (1) The oxide film generated at 550 °C is uniform and dense; (2) The oxide films formed at various holding temperatures are mainly consisted by Fe₂O₃ and Fe₃O₄. When the holding temperature is 550 °C, the content of Fe₃O₄ is the highest as 70.1%; (3) The films are stable when placed in inert atmosphere (N2) below 900 °C, and there is not any change in the composition and structure of the films even after reacting with steam at 550 °C for 24 h; (4) The expansion coefficient of high-temperature oxide films is very close to that of the matrix of the heat exchange tube, and the difference between these two thermal expansion coefficients is 5.3×10^-9 mm/°C.

Abstract: In this work, we investigated the influence of passivation coating of aluminum oxide on cycle life of lithium-ion batteries. Al2O3 was synthesized by atomic layer deposition directly on the porous electrodes based on LiCoO2. More than 800 charge-discharge cycles were done. No increase of internal resistance due to Al2O3 coating was observed. According to the results, electrodes coated by aluminum oxide have better cycle life.

Abstract: Nano-particle reinforced composite coatings were prepared on 2A12 aluminum alloy by micro-arc oxidation and the effect of nano particles on wear performance of coating was studied. By means of SEM, CETR micro-nano-indenter friction and wear tester and white-light interferometer, the surface morphology and the friction and wear property were tested. Results showed that compared with the coating without nano-particle addition, the size and amount of pores on coatings were decreased substantially after additon with 20nm nano-SiO₂ particles, 80nm nano-SiO₂ particles and 80nm nano-TiO₂ particles, and the wear resistance of the micro-arc oxidation coating was improved.

Abstract: Tantalum carbide (TaC) ceramic coating was produced on grey cast iron by the combined process of casting and heat treatment. The microstructure, micro-hardness, and fracture toughness of the TaC ceramic coating were investigated by scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and indentation technique. The results show that TaC ceramic coating can be divided into two apparent regions, and the density of products gradually decrease from the surface to the matrix. The main phases of the ceramic coating are only TaC, α-Fe, and Fe₃C. TEM examination revealed a nanostructure ceramic, in which 200~300 nm TaC grains are cemented by iron binder. The nanoindentation hardness of TaC ceramic coating vary in the range of 14~21 GPa. A close examination of the indentation crack paths of TaC samples revealed that the TaC ceramic coating have extensive crack deflection with a great amount of intergranular fracture.

Abstract: The niobium carbide (NbC) coating on gray cast iron has been produced by in situ which combined infiltration casting and heat treatment. The microstructural observations of the coating have been obtained by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). And the growth mechanism of NbC grain was studied. The results show that the mechanism is orientation connection which means two particles of the same lattice orientation will be directly connected together and oriented attachment growth. Fine NbC grain can improve the nanohardness value of the coating with 23 GPa, meanwhile, it increase the elastic modulus with the value of 493.7 GPa.

Abstract: In the present study, a thick, uniform and crack-free sol-gel coating embedded with Al₂O₃-CeO₂ nanoparticles was successfully synthesized and deposited on aluminum alloy AA6061 by spin coating method. The coating morphology was characterized by using a scanning electron microscopy coupled with electron diffraction x-ray spectrometer (SEM-EDX), an atomic force microscopy (AFM) and water contact angle measurements. FT-IR spectra were obtained using a Fourier transformation infrared spectrometer. The corrosion resistance of this coating in 3.5 wt.% NaCl solution was evaluated with electrochemical methods including potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). The stability of the corrosion resistance of this coating was evaluated by immersion in 3.5 wt.% NaCl solution and by exposure to the UV radiation condition. In addition, the adhesion resistance of the coating was also assessed. SEM and AFM results showed that Al₂O₃-CeO₂ nanoparticles dispersed uniformly in the room temperature vulcanized (RTV) silicon rubber matrix and formed a thick and crack-free coating. Both polarization and impedance results reveal that CeO₂-Al₂O₃ nanoparticles embedded silicon rubber coating can improve the corrosion resistance of the AA6061 alloy by more than three orders of magnitude. Meanwhile, the corrosion resistance of this coating was found to be stable under immersion in 3.5 wt.% NaCl solution and UV exposure conditions. However, excessive content of CeO₂ nanoparticles in the coating made the coating morphology porous and decreased the thickness of the coating, which resulted in the decrease in the corrosion resistance of the coating.

Abstract: In this work, an experimental study of wear evaluation in combination with a finite element analysis (FEA) was carried out for austempered ductile iron (ADI) used in gears. Two different ADI materials were used to produce gears which were tested in a FZG back-to-back test rig. The experimental results were compared to those of carburized AISI 8620 steel and induction hardened AISI 4140 steel gears. The wear resistance for pitting and spalling on the gears surfaces were measured using image analysis. Comparing the two types of ADI, the one with smaller nodules showed a higher pitting resistance. In contact fatigue tests with severe load, the carburized AISI 8620 steel proved to be superior to ADI. However, ADI with smaller nodule size presented wear resistance similar to that of induction hardened AISI 4140 steel. The FEA was conducted using the commercial code ANSYS 11.0 and aimed to provide a better understanding of the microstructural effect on the stress state of subsurface regions. From the numerical results in ADIs, it was concluded that the nodule size affects the gears life independently of the mechanical properties of the matrix. The size and number of nodules affects both the nucleation and the propagation stages of cracks. ADIs with higher amount of nodules have a superior wear resistance by pitting. Also, compared to the Hertz contact theory (valid for isotropic materials), the presence of graphite nodules induced the maximum shear stress point moves toward the surface.

Abstract: The corrosion behavior of pipeline steel containing 1%Cr is studied by using immersion experiment. The corrosion rust is characteried with macroscopic/microscopic surface morphology, corrosion kinetics and corrosion phases. The results demonstrate that the main corrosion products are lepidocrocite and goethite, Cr-rich compound consists of the inner layer. The corrosion process could be divided into three stages. At stage 1, the corrosion rate decreases fast, and the distributed corrosion products are formed. At stage 2, the granular corrosion products appear on coupons surface gradually, and a plain corrosion rate is obtained. At stage 3, a compact and dense corrosion layer attaches to coupons surface, and corrosion rate decreases mildly.