Gang-Chang Ji

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 types of ceramic materials in aggregated powder on the adhesive strength of high velocity oxy-fuel sprayed cermet coatings

Abstract The effects of the types and compositions of the binder phase in WC cermet powders, types of solid materials in solid–liquid two-phase particle, and high velocity oxy-fuel spray conditions on the adhesive strength of high velocity oxy fuel (HVOF) sprayed composite coatings were studied using three aggregated WC cermet powders, Cr 3 C 2 –25NiCr, Al 2 O 3 –Ni powders, and W–Ni powder. The examination of the coating microstructure using optical and scanning electron microscopy revealed that all the coatings were deposited through solid–liquid two-phase particles with ceramic particles or W in the powder being in a solid state while the binder in a melted liquid state. It was found that an adhesive strength higher than the strength of the test adhesives of approximately 65 MPa can be reached for all HVOF coatings deposited by WC cermet powders and W–Ni powder. The adhesive strength of HVOF WC cermet coatings was not significantly influenced by the composition of the binder, the powder structure, or the spray parameters. However, for Cr 3 C 2 –25NiCr coating the adhesive strength ranged from 35 MPa to over 92 MPa and was significantly influenced by the spray parameters. Finally, Al 2 O 3 –Ni coatings had a very limited adhesive strength. The results suggested that the formation of a solid–liquid two-phase particle in the HVOF process is a necessary condition to deposit HVOF coatings of high adhesive strength, while the high density of WC and W particles in the powders is a necessary condition to achieve high adhesive strength for HVOF WC cermet and W–Ni coatings. The adhesive strength of HVOF cermet coatings will be increased with an increase in the density of the solid material component in the two-phase particle.

Deformation Behaviors of Cold-Sprayed WC-Co Particles

In the present work, the WC-Co particles with different sizes and contents of WC particle were deposited on the stainless and WC-Co substrate in different accelerating gas temperatures. The deformation mechanisms of incident WC-Co particles and impacted substrates, and especially their interactions were characterized via scan electron microscopy (SEM) observations of both surface morphologies and cross-sectional microstructures of individual splats, and splat/substrate interface with the assistance of focus ion beam (FIB). The effects of feedstock powder structure, i.e., WC particle size and Co content, and temperature of accelerating gas on plastic deformation were comprehensively studied. It was revealed that the splats on stainless steel exhibited ellipsoid shape with a comparative integrated contour similar to original powder, and there were some revers on substrate around splats. In contrast, splats on WC-Co substrate exhibited more flattened pie shape with some ejectas on its periphery in the radial direction. The more curved interface convex to stainless steel substrate appeared compared to that of WC-Co substrate. Deformation of WC-Co splats was enhanced by the increment of substrate hardness and increment of Co binder and reduction of WC size of incident particles.

Microstructures and Properties of Cold Spray Nanostructured HA Coatings

Nanostructured hydroxyapatite (nHA) coatings on implant surface are highly desirable owing to their excellent osteoconductive properties in association with intrinsic small crystals. Low processing temperature of cold spray makes it an ideal technique to prepare nHA coatings with nanostructured feedstock. This work demonstrated that dense and thick nHA coatings were achievable by cold spraying nanostructured feedstock onto TC4 alloy surface. Coating formation mechanisms were explored by investigating the deposition of individual particles. Microstructures, mechanical profiles and biological properties of the coatings were comprehensively investigated. A porous Ti buffer layer significantly enhanced the bonding between nHA coatings and the substrate. As-deposited nHA coatings exhibited a high wear resistance when slid against polyurethane or polycarbonate and a strong apatite-forming ability when being immersed in simulated body fluid (SBF). Findings of this work will contribute to a comprehensive understanding of building mechanisms, structures and properties of cold sprayed nHA coatings.

Performance of abrasive wear of Cr 3 C 2 -NiCr coatings sprayed by HVOF

The abrasive wear behavior of high velocity oxy-fuel (HVOF) sprayed Cr3C2-25%NiCr coatings deposited in different conditions was investigated by wet sand rubber wheel abrasive wear tester using three kinds of abrasives (brown alumina, Jin Gang sand and Hu Kou sand). The abrasive wear mechanism was discussed. The results show that weight loss of HVOF sprayed Cr3C2-25% NiCr coating after worn with three kinds of sand is different. The weight loss of the coating after worn with alumina sand is the highest. Comparatively, the weight loss of the coating after worn with the Hu Kou sand is the lowest. The weight loss of the coating after worn with the Jin Gang sand is lower than that with alumina sand and higher than that with Hu Kou sand. However, the weight loss of the coating worn using three kinds of sands presents a liner increment with the increment of grinding distance. The coating deposited in moderate flow of propane exhibit higher wear resistance.

Model of the abrasive wear of HVOF deposited Cr 3 C 2 -NiCr coating

Based on the abrasive wear theory of composite materials in the condition of equal pressure, the coating microstructure and abrasive wear condition, the model for characterization the abrasive wear behavior of HVOF sprayed Cr3C2-NiCr coating was proposed. By examination of the morphology of the different area of worn surface, this model was confirmed. It is shown the abrasive wear of this coating is mainly dominated by the worn successively within splats and exhibits the wear behavior of composites composed of two phase of hard and soft. Within the splats wear occur in two stages, the selective cutting of soft binder NiCr phase in early stage and the peeling off the carbide particles in following.

Characterization of microstructure and properties of nanostructured Fe-Al/WC intermetallic composite coating deposited by cold spraying

A nanostuctured Fe-Al/WC intermetallic compound-based coating was produced by cold spraying of ball-milled powder assisted with post-annealing treatment. The microstructure of the as-milled powder and cold-sprayed coating was investigated. It was found that the as-milled Fe-Al/WC composite alloy powder had lamellar structure and the microstructure of the as-sprayed coating depended significantly on the microstructure of as-milled powder. A Fe-Al intermetallic phase was formed during the annealing treatment at a temperature higher than 500°C. The microhardness of the as-sprayed coating reached 680 Hv0.1 and more. The effect of the annealing treatment on the coating microstructure and hardness was examined.