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 formation of Nickel coatings on stainless steel substrates and YSZ (Yttria-Stabilized Zirconia) on NiCrAlY in the Atmospheric Plasma Spray (APS) process is investigated. Coating formation over a substrate with an arbitrary shape (an inclined step in this paper) is considered. The topography of the coatings, as well as their microstructure, e.g., porosity, average thickness, and average roughness, are evaluated. An algorithm, which is based on the Monte-Carlo stochastic model, is employed. The significant difference between the coating temperature and that of the substrate leads to the formation of residual thermal stresses. These stresses are analyzed using Object Oriented Finite-element software (OOF) developed by the National Institute of Standards and Technology (NIST). An image of the cross-section of the coating is imported into the code, which utilizes an adaptive meshing technique and Finite- Element Method to calculate residual thermal stresses. The maximum stress in the coatings occurs at the interface between the coating and the substrate. The coatings' topography and microstructure are compared with those of the experiments.

The agglomeration process plays an important role in suspension plasma spraying (SPS), affecting the state of particles prior to impact and consequently the mechanical properties of the coated substrate. This paper presents the results of an investigation on the agglomeration of submicron YSZ particles sprayed from a water suspension. The shape and surface morphology of transient and final agglomerate structures was studied by injecting YSZ suspensions with a nebulizer into an inductively coupled plasma torch. It was found that particle agglomeration occurs in successive stages identified as cup or doughnut shaped agglomerates, polycrystalline particles, and amorphous or partially molten particles.

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.

Plasma spray technology is widely employed by industry to apply coatings on different components to protect them from corrosion, wear and high temperature environments. The gases introduced into the DC plasma torch are heated by the arc and a plasma jet exits the torch. Powders are injected into the plasma jet where they are then accelerated, heated, and melted before impacting the substrate, which is placed at some distance from the outlet of plasma spray torch. Plasma arc exhibits strong voltage fluctuations which correspond to the movement of the anode arc root attachment. Understanding the arc movement within the torch and how it affects the flow and temperature fields of the plasma jet exiting the torch is of great importance. Understanding the flow, temperature and electromagnetic fields within the DC plasma torch is extremely challenging and there is a limited number of investigations in the literature. In order to provide unique sets of surface characteristics, e.g., thermal barriers, wear and corrosion resistance, a high quality coating with appropriate combination of powder and base materials must be produced. To produce a high quality coating, powder particles should be uniformly heated and accelerated, and then deposited onto the substrate. In this paper, an unsteady 3-dimensional model of the arc movement within the plasma torch is reported. The proposed model is employed to solve electric potential and magnetic vector potential equations in addition to continuity, momentum and energy equations. The k-ε turbulence model was used to model the turbulence of the flow field inside a non-transferred DC argon plasma torch. The geometry of the torch was that of SG-100 torch (Praxair). TO study the effect of the arc length on the voltage, first a steady-state model was considered for a range of arc lengths and arc-root radii. The results of this model provided the relation between arc length and arc voltage for a set of arc root radii and given argon flow rate. Then, given voltage fluctuation profile, the unsteady, arc root attachment movement was simulated from the estimation which found from steady models. Results show that the effects of velocity and temperature fluctuations at the outlet of the torch (where the particles are injected) are not negligible and such fluctuations exceed 15% of their average values. These will in turn affect the particle heating history and will negatively impact the microstructure of the coating.