Yu-Yue Wang

Dominant effect of carbide rebounding on the carbon loss during high velocity oxy-fuel spraying of Cr3C2–NiCr

Abstract Cr3C2–25% NiCr coatings were deposited by high velocity oxy-fuel (HVOF) spraying process using two commercial powders. The microstructure of the deposited coating was characterized by scanning electron microscopy. The carbon contents in both the deposited coatings and the collected powders were characterized by chemical analysis to clarify the main mechanism controlling the carbon loss during deposition of Cr3C2–NiCr coating by HVOF spraying. The results revealed that the carbon loss in the collected powders was much lower than that in the coatings. A model involved in a solid–liquid two-phase particle deposition behavior and rebound-off of large carbide particles during splatting was proposed to explain the effect of droplet conditions including carbide particle size on the carbon loss during deposition of Cr3C2–NiCr. It was suggested that the rebound-off of larger carbide particles when the two-phase droplet impacts on the surface is main mechanism responsible for overall high carbon loss during HVOF spraying of Cr3C2–NiCr.

Effect of particle state on the adhesive strength of HVOF sprayed metallic coating

NiCrBSi and Ni-50Cr coatings were deposited using the high velocity oxygen fuel (HVOF) spray process under different spray parameters with two powders of different sizes to clarify the influence of the melting state of spray particles on the adhesive strength of the coating. The adhesive strength of the coating was estimated according to the American Society for Testing and Materials (ASTM) C633-79. The melting state of the spray droplet was examined from the coating microstructure. It was found that the melting state of spray particles had a significant effect on the adhesive strength of HVOF sprayed Ni-based coatings. The significant melting of the spray particle did not contribute to the increase in the adhesion of HVOF metallic coatings. On the other hand, the deposition of a partially melted large particle contributed to the substantial improvement of adhesive strength of the HVOF coating. The subsequent coating presented a dense microstructure and yielded an adhesive strength of more than 76 MPa, which was double that of the coating deposited with completely molten particles. It can be suggested that the good melting of the spray particle is mainly related to the mechanical interlocking effect, which reaches the limited and approximately defined adhesive strength up to 40–50 MPa.

Phase formation of nano-TiO2 particles during flame spraying with liquid feedstock

The nanostructured TiO2 photocatalytic coatings were synthesized through flame spraying with liquid feedstock under different conditions. The nanostructured TiO2 deposit of substantial anatase phase was annealed at different temperatures. X-ray diffraction analysis showed that significant transformation from anatase to rutile occurred at a temperature above 600 °C. However, thermal analysis suggested that the phase transformation from anatase to rutile started at a temperature from 400 to 500°C. It was found that the grain size of rutile phase was larger than that of anatase. The deposits annealed at temperatures lower than 450°C were photocatalytically active. However, the deposit annealed at 500°C, which contained 95% anatase crystalline, became photocatalytically inactive. Based on the experimental findings, a model is proposed to explain the phase transformation of the nano-TiO2 particles and the phase formation in flame-spraying of nanostructured TiO2 deposit with liquid feedstock.

Examination of Substrate Surface Melting-Induced Splashing During Splat Formation in Plasma Spraying

Impacting of a molten droplet with a melting point much higher than the substrate results in melting of the substrate around the impact area. Melting of the substrate surface to a certain depth alters the flow direction of the droplet. The significant change of fluid flow direction leads to the detaching of fluid from the substrate. Consequently, splashing occurs during the droplet-spreading process. In the current study, molybdenum (Mo) splats were formed on a stainless steel substrate under different plasma-spraying conditions. For comparison, Mo splats were also deposited on an Mo surface. The substrate surface was polished prior to deposition. The powders used had a narrow particle size distribution. The results show that the morphology of splats depends significantly on the thermal interaction between the molten particle and the substrate. The splat observed was only a central part of an ideal disk-like complete splat. The typical pattern of Mo splats was of the split type, presenting a small split structure on the surface of the stainless steel substrate. With Mo particles, the preheating of a steel substrate has no effect on splat morphology. On the other hand, a disk-like Mo splat with a reduced diameter of a dimple-like structure at the central area of the splat was formed on Mo substrates, and splashing can be suppressed through substrate preheating. Based on the experimental results, a surface melting-induced splashing model was proposed to explain the formation mechanism of the Mo splat on a steel surface. The influence of droplet condition on splat formation is discussed.

Effect of nano-crystallization of high velocity oxy-fuel-sprayed amorphous NiCrBSi alloy on properties of the coatings

NiCrBSi self-fluxing alloy coatings were deposited by high velocity oxy-fuel (HVOF) spraying. Annealing treatment was applied to the as-sprayed coatings to develop the microstructure of the Ni-based coating. The microstructure of the coating was characterized using optical microscopy, x-ray diffraction and transmission electron microscopy. The crystallization behavior of the amorphous coating was also characterized by differential scanning calorimetry. The properties of the coating were characterized by microhardness and abrasive wear tests. The results showed that the as-sprayed HVOF coating deposited by well melted spray particles exhibited a dense microstructure of amorphous phase. It was revealed that the crystallization of the amorphous phase in HVOF NiCrBSi coating occurs at a temperature of about 502°C. Annealing at temperature a little higher than recrystallization temperature leads to the formation of the nano-crystalline microstructure. The subsequent nanostructured Ni-based coating presents higher microhardness and excellent wear performance. With the further increase in annealing temperature, the growth of the nano-crystalline grains occurs and, accordingly, the microhardness of the coating and the wear performance decrease. Thereafter, the microstructure and properties of the Ni-based self-fluxing alloy coating can be controlled through postannealing treatment.NiCrBSi self-fluxing alloy coatings were deposited by high velocity oxy-fuel (HVOF) spraying. Annealing treatment was applied to the as-sprayed coatings to develop the microstructure of the Ni-based coating. The microstructure of the coating was characterized using optical microscopy, x-ray diffraction and transmission electron microscopy. The crystallization behavior of the amorphous coating was also characterized by differential scanning calorimetry. The properties of the coating were characterized by microhardness and abrasive wear tests. The results showed that the as-sprayed HVOF coating deposited by well melted spray particles exhibited a dense microstructure of amorphous phase. It was revealed that the crystallization of the amorphous phase in HVOF NiCrBSi coating occurs at a temperature of about 502°C. Annealing at temperature a little higher than recrystallization temperature leads to the formation of the nano-crystalline microstructure. The subsequent nanostructured Ni-based coating presents hig...

Effect of Cu2+ doping on photocatalytic performance of liquid flame sprayed TiO2 coatings

Cu2+ was added to liquid feedstock to deposit ion doping TiO2 photocatalytic coatings through liquid flame spraying. The coating microstructure was characterized by x-ray diffraction (XRD), transmission electron microscopy, and x-ray photoelectron spectroscopy (XPS). The photocatalytic performance of coatings was examined by photodegradation of acetaldehyde. The XRD analysis shows that the crystalline structure of coatings is not significantly influenced by Cu2+ doping. The photocatalytic activity of the TiO2 coatings is enhanced by Cu2+ doping. It is found that a high concentration of Cu2+ doping decreases the activity. The XPS analysis shows that the adsorbed oxygen concentration is increased with the increase of Cu2+ dopant concentration and decreases with a further increase of dopant concentration. The enhancement of photocatalytic activity can be attributed to the adsorption ability of oxygen and other reactants on the surface of doping TiO2 coatings.

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.

Effects of Erosion Rate on the Erosion-Corrosion Synergism of High Velocity Oxy-Fuel Sprayed Ni-Based Coatings

Two types of Ni-based coatings were deposited by high velocity oxy-fuel (HVOF) spraying process on ASTM 1020 low carbon steel substrate, and AISI 321 stainless steel was applied to the contrast test. The influences of erosion rate on the erosion-corrosion synergism and erosion-wear mechanism of the two coatings were investigated systematically in a liquid/solid two-phase flow (5% H2SO4 +15% silica sand). The volume loss rates stemming from the erosion-corrosion synergism of materials increased from 42~66% to 78~85% of the total volume loss rates when the erosion rate of the flow increased from 5 m/s to 10 m/s. It was indicated that the erosion-corrosion synergism was primary for the material removing during the erosion-corrosion process. Cutting, wear and plowing were the main features for Type-A coatings, while cutting, plowing and plastic deformation were the main characters for Type-B coatings and the two steels.

Microstructure and Wear Resistance of AlFeCuCoNiCrTi1.5 High Entropy Alloy Coating on AZ91D Magnesium Alloys by Laser Cladding

To improve the wear properties of AZ91D magnesium alloys, a AlFeCuCoNiCrTi1.5 high entropy alloy (HEA) coating was fabricated on AZ91D magnesium alloys by laser cladding using mixed powders of Al, Fe, Cu, Co, Ni, Cr, and Ti. The microstructure of the HEA coating was characterized by OM, SEM, and XRD. The wear resistance of the HEA coating was evaluated under dry sliding wear test condition at room temperature. The results show that the HEA coating mainly consists of a simple BCC solid solution phase. The HEA coating exhibits excellent wear resistance. The main wear mechanisms of the HEA coating and the AZ91D substrate were different, the former dominated by oxidative abrasive wear and the latter suffered from both adhesive and abrasive wear.

Laser Cladding of TiN-Based Coating on 00Cr13Ni4Mo Hydro Turbine Blade Stainless Steel for Improvement of Wear Resistance

To improve the wear propery of 00Cr13Ni4Mo hydro turbine blade stainless steel, TiN-based coating was fabricated on 00Cr13Ni4Mo stainless steel by laser cladding using mixed powders of Ni60 and TiN. The microstructure of the coating was characterized by XRD and SEM techniques. The wear behaviour of the coating was also investigated. The wear resistance of the coating was evaluated under dry sliding wear condition at room temperature. The results show that the coating mainly consists of Ni-based solid solution and TiN phases. The coating exhibits excellent wear resistance due to its high hardness of TiN phase. The main wear mechanisms of the coating and the 00Cr13Ni4Mo sample are different, the former is abrasive wear and the latter is adhesive wear.