Xiao-Tao Luo

Optimization of In-Situ Shot-Peening-Assisted Cold Spraying Parameters for Full Corrosion Protection of Mg Alloy by Fully Dense Al-Based Alloy Coating

Magnesium-based alloys have excellent physical and mechanical properties for a lot of applications. However, due to high chemical reactivity, magnesium and its alloys are highly susceptible to corrosion. In this study, Al6061 coating was deposited on AZ31B magnesium by cold spray with a commercial Al6061 powder blended with large-sized stainless steel particles (in-situ shot-peening particles) using nitrogen gas. Microstructure and corrosion behavior of the sprayed coating was investigated as a function of shot-peening particle content in the feedstock. It is found that by introducing the in-situ tamping effect using shot-peening (SP) particles, the plastic deformation of deposited particles is significantly enhanced, thereby resulting in a fully dense Al6061 coating. SEM observations reveal that no SP particle is deposited into Al6061 coating at the optimization spraying parameters. Porosity of the coating significantly decreases from 10.7 to 0.4% as the SP particle content increases from 20 to 60 vol.%. The electrochemical corrosion experiments reveal that this novel in-situ SP-assisted cold spraying is effective to deposit fully dense Al6061 coating through which aqueous solution is not permeable and thus can provide exceptional protection of the magnesium-based materials from corrosion.

Controlling grain size in columnar YSZ coating formation by droplet filtering assisted PS-PVD processing

Grain size is important to the effect of material performance, particularly when reduced to nanoscale size. Generally, ionic conductivity in solid electrolytes can be remarkably enhanced by the nanostructure ceramics. In this study, a shrouded plasma torch was used on plasma spray-physical vapor deposition (PS-PVD) process. Multi-stage shielding was used to realize the controlling of grain size during the deposition process. The deposition behavior of YSZ was studied. The surface and cross-sectional morphology of the coating layer were also investigated. An island structure consisting of nano particle was seen on the coating layer surface, and the particle size becomes smaller with increased shielding stage. The cross section morphology of fracture coatings showed columnar structure. In this work, the grain size of the YSZ coating layer can be successfully controlled minimum to 25 nm by PS-PVD process. The effect of YSZ grain size on the coating properties is also discussed.

A TEM Study of the Microstructure of Plasma-Sprayed YSZ Near Inter-splat Interfaces

The splat interface bonding state which changes heat transfer conditions and thus the cooling rate during splat cooling may influence the interface microstructure. In this paper, YSZ coating was deposited by atmospheric plasma spraying with substrate cooling during deposition. Subsequent characterization was implemented using high resolution transmission electron microscopy to examine the local microstructures near the interfaces at the bonded and unbonded zones. Selected area diffraction analyses of the splats across both the bonded interface and unbonded interface revealed that all bulk splats present a metastable tetragonal structure. Results showed that the size of columnar grains within a splat was significantly influenced by the interface bonding. At the unbonded region in the splat, large columnar grains form which can be attributed to poor thermal contact of melt to the underlying splat surface before its solidification. At the bonded zones, the splat presents a much fine columnar grain structure, which is attributed to good thermal contact of the melt to the underlying splat before solidification. Moreover, it is evident that the bonded interface region presents a distinct microstructure feature from the fine columnar grains suggesting the crystal defect of high density of dislocations at the interface.

Effect of annealing conditions on in situ formation of Cr2O3 diffusion barrier

The previous investigation suggested the approach for an in situ formation of Cr2O3 diffusion barrier by annealing the cold-sprayed Ni coatings on 310SS. In this paper, the influences of annealing conditions on the growth kinetics of Cr2O3 and substrate microstructure were investigated. Results show that Cr2O3 formed at the selected annealing temperatures of 850, 900 and 950°C for different durations of 4, 8 and 20 h. Increasing temperature enhanced the growth kinetics of Cr2O3 and the Mn content in the oxide layer. The annealing process for the growth of Cr2O3 improves the coating adhesion compared to the as-deposited coating. However, annealing at 950°C resulted in the precipitation of chromium carbides and enhanced the element inter-diffusion across the substrate/coating interface.