Papers by Author: Guan Jun Yang

Abstract: Superalloy coating was deposited by cold-spraying using a commercial NiCoCrAlTaY powder. The coating microstructure was investigated by scanning electron microscopy and X-ray diffraction to reveal the change of the b-NiAl phase in the as-received powder particle during coating deposition. The oxidation behavior of the cold-sprayed MCrAlY coating and its microstructural evolution during the isothermal treatment were examined. The results show that significant microstructural change occurred to NiCoCrAlTaY superalloy during cold spraying and the thermal exposure. The intensive plastic deformation upon high velocity impact of spray particles results in transformation of b-NiAl to the matrix phase, forming metaltable b-NiAl depletion zones (b-PDZs) which are distributed around the boundaries of deposited particles in the coating. The central part of the deposited particles with limited deformation retained the original phase constitutions of the starting powder. The b-phase with fine grains is re-precipitated uniformly in the areas in b-PDZs in the as-sprayed coating during high temperature exposure. A stable Al2O3 scale was formed on cold-sprayed NiCoCrAlTaY during oxidation possibly due to active b-PDZs on the top surface of the coating.

Abstract: Diamond/NiCrAl composite powder with a diamond content of 40vol.% is prepared by mechanical alloying process with diamond powder and NiCrAl alloy powder. Cold spraying is employed to deposit diamond/NiCrAl composite coating. To enhance the bonding between diamond particles and NiCrAl matrix, annealing treatment is applied to the as-sprayed coating. The effect of annealing treatment conditions on the microstructure and microhardness and abrasive wear performance of the coating is investigated. The results show the microstructure of the composite powder is completely retained in the coating owing to the low temperature of spray powder during cold spray process. The interface between diamond and NiCrAl matrix is enhanced by the CrxCy layers formed during annealing treatment. Moreover, the cohesion of deposited particles in the coating is improved through metal element diffusion during heat treatment. The hardness and wear performance of the coating is related to coating microstructure and the bonding at the interfaces between diamond particles and NiCrAl matrix.

Abstract: TiCl4 treatment was used to chemically agglomerate TiO2 primary nano-particles to form a nanostructured powder in size of submicrometer. Nanocrystalline TiO2 coatings were fabricated by vacuum cold spraying at room temperature using the powder and were employed to assemble dye-sensitized solar cells. TiO2 coating of 10-20μm in thickness was deposited successfully on both F-doped tin oxide (FTO) conducting glass and plastic conducting substrate. The assembled solar cell with an FTO conducting glass yielded a short-circuit current density of 9.7 mA/cm2 and an energy conversion efficiency of 3.3%. Using the plastic substrate, the cell efficiency was 1.9%. These results suggest that TiCl4-treated nanocrystalline TiO2 agglomerates can be used to deposit TiO2 coating by vacuum cold spraying at low temperature for flexible dye-sensitized solar cells.

Abstract: Nanostructured FeAl/WC intermetallic composite coatings were prepared by cold spaying of the ball-milled powders. The effect of annealing on the coating structure and microhardness was examined. It was found that the nanocrystalline structure of the milled feedstock was retained in the cold sprayed coatings. The FeAl intermetallic phases were formed from the milled Fe(Al) solid solution during the post-spraying annealing at 550oC. The microhardness of the as-sprayed coating was about 680HV0.1 and it decreased a little with increasing the annealing time at 550oC.

Abstract: YSZ molten droplets created by plasma spraying were deposited on the YSZ substrate preheated to different temperature from 75oC to 1000oC. The results showed that there is little change in crack spacing when substrate temperature is less than about 740oC, and a significant increase in crack spacing from 3.54 μm at 740oC to 10.91 μm at 1000oC was observed. A simple qualitative model was proposed on the basis of the origin of thermal stresses to explain the influence of substrate initial temperature on crack density characterized by crack spacing. The analysis indicates that the results can be ascribed to the changes in tensile stress and splat/substrate adhesion resulting from changes of substrate initial temperature.