Papers by Author: Dong Yu Liu

Abstract: Intermetallic compound MoSi2 is useful for protective coatings against high-temperature oxidation. In this study, MoSi2-based coatings were prepared using electrothermal explosion ultra-high speed spraying methods. The spraying distance from electric exploding chamber to substrate are 5mm, 10mm, 15mm and 20mm, respectively. The microstructures, phase structures and hardness of the coatings were investigated by scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Vickers hardness tester, respectively. The result indicates that MoSi2 primary phase is synthesized. When spraying distance is 10mm, the coatings have density microstructure, submicron grains and high microhardness. The bonding of coating-substrate is metallurgical. The oxidation resistance of Mo-Si-Al coating is higher than that of Mo-Si coating.

Abstract: In this work, TiC-Ni coating was synthesized on copper substrate by electro-thermal explosion ultra-high speed spraying method at the discharge voltage of 26kV. Microstructure, phase structure, elemental distribution and microhardness of the coating were studied by means of scanning electron microscope in back-scatter-electron and secondary electron, energy-dispersive analysis X-ray spectroscopy, X-ray diffraction and Vickers hardness tester. The TiC-Ni coating, which exhibits no pores or cracks, consists of an irregular spheroidal TiC phase embedded in a nearly continuous Cu0.81Ni0.19 binder. TiC particles are uniformly distributed and the size of the TiC particle of the coating is less than 1.0 m because of the solute trapping effect. The average hardness of the TiC-Ni coating is approximately 1200 HV0.3.

Abstract: A new method, electro-thermal explosion ultra-high speed spraying, was utilized to produce the Mo-Si or Mo-Si-B coatings. Experimental results showed that submicron-grain pure MoSi2 phase coating and boron alloyed MoSi2-MoB2 multiphase coatings were in-situ synthesized on GH3039 nickel-based superalloy substrate. Erosion-resistance test of GH3039 nickel-based superalloy and MoSi2-based coatings were performed under high temperature(800°C) with GW/CS – MS erosion wear tester. Quartz sand was employed as erosive material. Experimental results showed that the erosion-resistance of the MoSi2-based coatings is better than that of the GH3039 nickel-based superalloy.

Abstract: The air cooling rate of the Low Carbon Mn-Si-Cr steel bar with different diameter after austenitizing at 910oC and 960oC was simulated by Formaster-F Phase transforming instrument and Gleeble-1500 thermal /mechanical simulating machine. Microstructure of the specimen was observed by OLYMPUS PME3 optical microscope and FEI QUANTA200F scanning electron microscope. The hardness and impact toughness of the steel was tested by HBRV-187.5 hardness tester and JCSJ300-I instrumented Charpy impact tester. The experimental result showed that with the amount of CFB in CFB+M mixed microstructure increasing the combination of strength and toughness of the steel was improved. The higher the austenitizing temperature of the steel, the wider the air-cooling rate range obtaining CFB+M mixed microstructure. However, the steel produces mixed grain after austenitizing at 960 oC. For obtaining fine prior austenite grain size, Ti and Nb alloying element need to be added.

Abstract: CaCu3(Ti1-xZrx)O12 ceramics (x = 0.02, 0.04, 0.06, 0.08) were prepared by the conventional solid-state reaction method. Phase compositions, microstructure and dielectric properties were examined by XRD, SEM and impedance spectroscopy, respectively. The experimental results indicated that there is no obvious effect on microstructure characteristic by substituting Zr4+ for Ti4+. It was found that the values of the dielectric constant for all of the samples dropped into the order of 102, which is significantly lower than the order of 105 that of normally reported in the literature. It was suggested that the presence of impurity phases played an important role for the degeneracy of dielectric properties.

Abstract: CCTO powders were prepared by using molten salt method in the NaCl-KCl system. The effects of temperature and holding time on phase compositions, particle morphology and size have been investigated by X-ray diffraction, scanning electron microscope and laser particle size analyzer. Using CaCO3, CuO and TiO2 as starting materials, CCTO compound could be synthesized at any temperature from 800oC to 1000oC in the NaCl-KCl system. The average particle size increased obviously with the increasing of temperature above 850 oC. Holding time had great effect on grain size and morphology.