Papers by Author: Ji Qiu

Abstract: Deck steel with anti-skid coating is treated by induction heating for coating removal. The microstructure of cross-section before and after coating removal is observed by metallographic microscope and scanning electron microscope. Z-direction micro-hardness of cross-section before and after coating removal is tested by micro-hardness tester. Results show that the softening layer of about 3 mm, which the average micro-hardness of the layer is 244 HV, is formed on the surface of the sample. The micro-hardness decreases by 3.5% compared with the untreated sample. Coating removal by induction heating will not have an impact on the metallographic structure. But carbide particles will be precipitated at the grain boundary. At last, the influence of induction heating on the performance of deck steel is briefly analyzed.

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: In this article, a nano-SiO₂/micro-arc oxide composite coating layer was prepared on the surface of 7A52 Aluminum alloy by addition of SiO₂ nanoparticles into the micro-arc oxidation electrolyte to enhance the performance of the formed oxidative layer. Then, the composite oxidative layer was characterized by X-ray Photoelectron Spectroscopy (XPS) to investigate its elemental and chemical compositions, as well as their respective distributions. The results revealed that at elevated temperatures resulted from micro-arc discharge, the SiO₂ in the composite reacted with Al₂O₃ (the major content of the micro-arc oxidative layer) to form a new compound known as mullite. In addition, the contents of SiO₂ and γ-Al₂O₃ in the inner layer of the oxidative layer were lower than those in the surface layer. This can be explained by the higher temperature in the inner layer which motivated the formation of mullite and α-Al₂O₃ from SiO₂ and γ-Al₂O₃ respectively during the micro-arc oxidation process.

Abstract: The temperature change process of the single sprayed composite powder during the self-reactive spray forming process for preparing the Ti (C,N)-TiB₂ ceramic preforms was numerically simulated by means of finite element analysis. The results show that after the sprayed composite powder with grain size of 50μm has entered the flame field for 0.35ms, the surface temperature of it will reach the igniting temperature and the self-propagating high-temperature synthesis (abbr. SHS) reaction will take place. The heating rate of the particle in this period is about 2.82×10⁶°C/s. After the SHS reaction has taken place, the heating rate becomes quicker because of the double function of the flame and the reactive heat release. When the temperature of the sprayed particle is higher than that of the flame, the heat exchange process will turn into heat absorption from heat release, which leads to the great drop of the heating rate (about 1.20×10⁶°C/s). The composite powder completes the reaction in 0.88ms and reaches the highest temperature of 2920°C, which makes it become a ceramic droplet. After the reaction has finished, the droplet cools down quickly from exterior to interior, and the surface temperature of it descends to the theoretic eutectic melting point of the composite ceramics (2620°C) after 0.34ms. Then the droplet begins to solidify at some degree of supercooling and becomes ceramic particle. The numerically simulated results before, during and after the reaction match the water-quenching experiments of the sprayed particle with particle size of 50μm during the corresponding period. It indicates the heat process of the sprayed composite powder on the whole, which is composed of being heated, heat releasing, cooling and solidifying.

Abstract: The multiphase coatings with major phases Al2O3-AlxCuy were prepared by the reactive flame spray technology. The melting and reactive behavior of the Al-CuO agglomerated particles and the forming process of the coatings were studied by means of water-quenching experiments and the methods of XRD and SEM. It was shown that during the spray distance between 60 and 150 millimeters, CuO in the agglomerated particles was heated to decompose into Cu2O, Cu and O2. During the flying course, a little of Al reacted with Cu2O and produced Al2O3 and Cu. The reduced Cu mutually dissolved with Al and formed liquid Al-Cu alloy. After the spray particles bumped into the substrate, Cu2O reacted with Al richly, then lots of Al2O3 was produced and Cu was reduced at the same time. These products were wallowed up by the liquid Al-Cu alloy soon. When the temperature of the system dropped quickly, the structure transforming process began. The intermetallics of Cu9Al4 and Al2Cu3 were deposited from the liquid Al-Cu alloy via complicated eutectic reaction and eutectoid reaction.

Abstract: Good dispersing stability is required for nano-SiO2 which, however, tends easily to conglomerate, to be widely applied as an important functional material of high toughness, high resistance to corrosion and good high-temperature performance. Through ultrasonic treatment, the suspension of SiO2 nano-powder in water was prepared utilizing sodium dodecylbenzenesulfonate (SDBS) as the dispersant. The effect of SDBS content and ultrasonic treatment time on the suspension stability was investigated by way of testing the particle size of SiO2 nano-powder, the Zeta potential and transmittance of the suspension. The results show that with increasing SDBS content, the particle size of SiO2 nano- powder decreases and then increases, which is similar to the case of increasing the ultrasonic treatment time. With optimum SDBS content, the suspension of SiO2 nano-powder possesses a good dispersing stability due to the existence of electrostatic and steric effects resulting from the particular structure of SDBS. The optimum dispersing condition is SDBS of 1.6 wt% with the ultrasonic time of 18 min.

Abstract: Micro-arc oxidation (MAO) is an effective approach to improve the properties of aluminium and its alloys by forming ceramic coatings on the surface. However, the oxide layers often have a porous surface structure, which limits their mechanical properties. In order to enhance the properties of the layers produced by micro-arc oxidation, SiO2 nanoparticles reinforced Al2O3 composite coatings were produced on 7A52 aluminium alloy by adding SiO2 nanoparticles into the electrolyte. With the addition of SiO2 nanoparticles in the electrolyte, the formation rate of Al2O3 coating enhanced considerably and the current density through the sample surface became much higher than that without SiO2 at the same voltage. The coatings were investigated with X-ray fluorescence spectrometry (XRF), Scanning electron microscopy (SEM), Vickers hardness test, and reciprocating friction and wear test. Compared with the Al2O3 coatings without SiO2 nanoparticles, the n-SiO2 reinforced Al2O3 composite coatings are much denser and harder, and the wear resistance is also improved significantly. The improvement can be attributed to the enhancement of the surface structure and morphology of the n- SiO2 reinforced Al2O3 composite coatings.