G.-J. Yang

Characterization of microstructure of Nano-TiO2 coating deposited by vacuum cold spraying

Control of the microstructure of TiO2 coatings through preparation methods significantly influences the coating performance. In this study, a vacuum cold-spray process, as a new coating technology, is used to deposit nanocrystalline TiO2 coatings on conducting glass and stainless steel substrates. TiO2 deposits were formed using two types of nanocrystalline TiO2 powders with mean particle diameters of 200 and 25 nm. Coating microstructures were characterized by scanning electron microscopy and x-ray diffraction analysis. Results demonstrate that a thick nanocrystalline TiO2 coating can be deposited by the vacuum cold-spray process. The coating was found to consist of particles stacked as agglomerates that build up to several hundred nanometers. The coating also presents a mesoporous microstructure that could be effective in such applications as photocatalytic degradation and dye-sensitized solar cells.

Effect of solid carbide particle size on deposition behaviour, microstructure and wear performance of HVOF cermet coatings

Abstract The deposition process of WC – Co and Cr3 C2 – NiCr cermet coatings through a high velocity oxyfuel (HVOF) spray process was examined using powders of different carbide particle sizes that ranged from 1.5 m m to over 15 m m. The effect of carbide particle size on the thickness of flattened particles was investigated. The carbide size in the coating was compared to that in the original feedstock. Results showed that most of the carbide particles remained in a solid state after passing through the flame, and the deposition of the cermet coating involves lateral flattening of a solid – liquid two phase droplet rather than a single liquid droplet, as in plasma spraying of metallic and ceramic materials under optimised conditions. It was found that the size of carbide particles in a sprayed coating depends greatly on the corresponding carbide size in the starting powder. The large solid carbide particles in a two phase droplet tend to rebound easily during HVOF spraying when the droplet impacts on a substrate surface, while small carbide particles in the droplet may follow the flattening of the liquid binder phase and be easily retained in the deposit. Using a powder consisting of small carbide particles leads to substantial retention of carbide particles in the coating and consequently an improved wear performance. A model is proposed to explain the effect of carbide particle size on the splat formation by a solid – liquid two phase droplet.

Influence of Substrate Temperature on Microcracks Formation in Plasma-Sprayed Yttria-Stabilized Zirconia Splats

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.

Microstructure Features of Cold-Sprayed NiCoCrAlTaY Coating and its High Temperature Oxidation Behavior

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.

Epitaxial Grain Growth During Splat Cooling of Alumina Droplets Produced by Atmospheric Plasma Spraying

Alumina splats were deposited on polished single crystal alumina substrates with two different crystalline facet orientations of [001] and [110] by atmospheric plasma spraying at a substrate preheating temperature of 900xa0°C to examine the epitaxy during splat cooling. The cross-sectional samples for high-resolution transmission electron microscopy examination were prepared by focused ion beam-assisted-scanning electron microscopy (FIB-SEM). The results show that the whole splats with a thickness ranging from ~600 to ~1000xa0nm exhibit the same crystalline structure as the substrate. Moreover, the splat deposited on the single crystalline alumina substrates exhibits exactly the same orientation as the substrate. The results evidently indicate that the epitaxial grain growth occurs after alumina droplets impact on the single crystal alumina substrate. The present results suggest that the crystalline structure of alumina deposit formed by plasma spraying can be possibly controlled by the coating surface temperature.

Room-Temperature Deposition of Nano-TiO2 Coating by Vacuum Cold Spraying Using TiCl4-Aagglomerated Nano-TiO2 Powder for Flexible Dye-Sensitized Solar Cell

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.

The influence of zirconium carbide addition on oxidation behaviour of NiAl coatings prepared by cold spraying of ball-milled Ni–Al alloy powder

Abstract The microstructure and oxidation behaviour of NiAl and NiAl/ZrC coatings prepared by cold spraying of ball-milled Ni/Al alloy powders were investigated. The microstructure of the mechanically alloying Ni/Al powder and NiAl coating was characterised by scanning electron microscopy and X-ray diffraction analysis. The results show that the homogeneity of the microstructure of ball-milled Ni/Al composite powder was modified by the addition of fine ZrC particles. The microstructure of the cold sprayed NiAl/ZrC coatings was denser than that of the NiAl coatings without the addition of ZrC particles. α-Al2O3 oxide scales were formed on the surface of both NiAl coating and NiAl/ZrC coatings during oxidation at 1050°C. After NiAl with ZrC addition was oxidised at 1130°C for 50 h, the α-Al2O3 scale was less than 5 μm, which exhibited excellent oxidation resistance.

Effect of Annealing Treatment on the Structure and Microhardness of Cold-Sprayed Nanostructured FeAl/WC Composite Coating

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.