Surface structures are of vital importance for the wetting behaviors of hydrophobic coatings. In this work, rare earth oxide coatings with different surface structures were deposited via the solution precursor atmospheric plasma spray (SPAPS) process and solution precursor vacuum plasma spray (SPVPS) process, respectively. The SPAPS coatings showed hierarchical cauliflower-like surface structures composed of micron-sized clusters and nanometer-sized particles, while the SPVPS coatings showed relatively flat topographies with small and short bumps. The formation of different surface structures in the SPAPS and SPVPS processes was investigated by modelling the movement of in-flight particles in the vicinity of the substrate. The properties of plasma jets and the characteristics of in-flight particles in the two processes were correlated. The effects of diverted plasma gas flow on the trajectories of particles impinging on the substrate and the resultant surface structures were elaborated, revealing different shadowing effects in the SPAPS and SPVPS processes. The SPAPS coatings were superhydrophobic due to the presence of hierarchical surface structures, which showed larger water contact angles and smaller roll-off angles than the SPVPS coatings. The correlations between the surface structures and wetting behaviors of different coatings were investigated.

The influence of air plasma sprayed alumina coating geometry, microstructure, interface roughness on its delamination and crack propagation resistance during low temperature thermal cycling, i.e. thermal mismatch stress, is investigated both numerically and experimentally. Previous studies on thermal cycling loading concentrate on flat, numerically designed locally curved specimens and/or mathematically modeled roughness without extension towards real coating morphology, which renders the conclusions less practically driven. Results show that arbitrarily oriented cracks originate predominantly near the coating/substrate interface and propagate along zones of high tensile and shear residual stress. The crack path deflection was attributed to the complex stress concentration structure resultant from the intricate microstructural porosity and coating general convex geometry. Microstructural features such as porosity increase the interfacial and coating tensile stress, which may lead to important delamination processes even during low temperature thermal cycling.

The plasma jet in the vacuum plasma spray process presents characteristics such as supersonic flow, expanded jet dimensions, and a smaller decay rate for jet velocity and temperature that are distinctly different than in atmospheric plasma spray. In this work, a solution precursor vacuum plasma spray (SPVPS) process is described, which combines vacuum plasma spray with solution precursor as the feedstock. The deposition of superhydrophobic ceramic coatings via the SPVPS process is explored. Yb 2 O 3 coatings are deposited by a radial injection of Yb(NO 3 ) 3 solution in the anode of an F4-VB torch operating under a pressure of 150-250 mbar. Solution precursor atmospheric plasma spray (SPAPS) is also applied to deposit superhydrophobic Yb 2 O 3 coatings for comparison with the SPVPS process. The wetting behaviors of the coatings are characterized by water contact angle measurement, water rolloff test, and dynamic water impact test. The experimental setup, plasma jet characteristics, interactions of solution droplets and plasma, microstructure and wetting behaviors of coatings in the two distinct processes are compared and discussed.

The application of metallic foam core sandwich structures in engineering components has been of particular interest in recent years because of their unique mechanical and thermal properties. Thermal spraying of the skin on the foam structure has recently been employed as a novel cost-efficient method for fabrication of these structures from refractory materials with complex shapes that could not otherwise be easily fabricated. The mechanical behavior of these structures under flexural loading is important in most applications. Previous studies have suggested that heat treatment of the thermally sprayed sandwich structures could improve the ductility of the skins and so affect the failure mode. In the present study the mechanical behavior of sandwich beams prepared from arc sprayed alloy 625 skin on 40 ppi nickel foam was characterized under four point bending. The ductility of the arc sprayed alloy 625 coatings was improved after heat treatment at 1100°C and 900°C while the yield point was reduced. Heat treatment of the sandwich beams reduced the danger of catastrophic failure.

Comprehensive study of the wire arc thermal spray technology will allow for better design and optimization of guns. In wire arc spray, a feed of two electrically-charged wires are melted using an arc. This bath of molten metal goes through an atomization process with a high pressure air being blown upon it. Flow of air will then carry the generated molten drops and deposits them on the substrate. The focus of this study is on the numerical simulation of wire arc sprays using ANSYS FLUENT software. Effects of geometrical parameters on resulting flow conditions and flow circulations inside the gun are studied. Simulation results help in better parameter selection for effective wire arc coating.