Ali Dolatabadi

Modeling thermal spray coating processes: a powerful tool in design and optimization

Abstract Widespread adoption of thermal spray technology requires the ability to apply a variety of coating materials, suited to each new application. Understanding the dependence of the microstructure of spray coatings on operating conditions of the thermal spray system is of great practical interest. To obtain good quality coatings the spray parameters must be selected carefully, and due to the large variety in process parameters, much trial and error goes into optimizing the process for each specific coating and substrate combinations. In this paper a complete model of the High Velocity Oxy–Fuel (HVOF) spray coating process is presented, including modeling three distinct sub-processes: spray parameters such as particle size, temperature, velocity, and impact points; particle impact and splat formation; and the coating microstructure including formation of porosity.

The 2016 Thermal Spray Roadmap

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

Effect of a cylindrical shroud on particle conditions in high velocity oxy-fuel spray process

Abstract Simulations of solid particles in a highly compressible gas flow in the high velocity oxy-fuel (HVOF) process are presented. The Eulerian formulation is used for the gas flow, and the particl phase is modeled by the Lagrangian method. Effects of attaching a cylindrical shroud to the end of the supersonic HVOF nozzle on gas and particle flows are analyzed. We found that the shroud significantly reduces the oxygen content in the field by protecting the supersonic jet from ambient air entrainment. The validation experiments were performed for the majority of process parameters such as shock formation, particle conditions, and coating oxygen content.

Dynamics of droplet coalescence in response to increasing hydrophobicity

Coalescence of a falling droplet with a sessile droplet on solid surface with various wettabilities is investigated by a combined experimental and numerical study. In the experiments, the droplet diameter, the impact velocity, and the distance between the impacting droplets were controlled. The evolution of surface shape during the coalescence of two droplets on various surfaces is captured using high speed imaging and compared with numerical results. A two-phase volume of fluid method is used to determine the dynamics of droplet coalescence, shape evaluation, and contact line movement. The spreading length of two coalescing droplets along their original centers is also predicted by the model and compared well with the experimental results. The effect of different parameters such as impact velocity, center to center distance, droplet size, and surface wettability on maximum spreading length are studied and compared to the experimental results. Finally, correlations are developed for predicting the maximum...

Induced Detachment of Coalescing Droplets on Superhydrophobic Surfaces

Coalescence of a falling droplet with a stationary sessile droplet on a superhydrophobic surface is investigated by a combined experimental and numerical study. In the experiments, the droplet diameter, the impact velocity, and the distance between the impacting droplets were controlled. The evolution of surface shape during the coalescence of two droplets on the superhydrophobic surface is captured using high speed imaging and compared with numerical results. A two-phase volume of fluid (VOF) method is used to determine the dynamics of droplet coalescence, shape evaluation, and contact line movement. The spread length of two coalesced droplets along their original center is also predicted by the model and compared well with the experimental results. The effect of different parameters such as impact velocity, center to center distance, and droplet size on contact time and restitution coefficient are studied and compared to the experimental results. Finally, the wetting and the self-cleaning properties of superhydrophobic surfaces have been investigated. It has been found that impinging water drops with very small amount of kinetic impact energy were able to thoroughly clean these surfaces.

Energy Budget of Liquid Drop Impact at Maximum Spreading: Numerical Simulations and Experiments

The maximum spreading of an impinging droplet on a rigid surface is studied for low to high impact velocity, until the droplet starts splashing. We investigate experimentally and numerically the role of liquid properties, such as surface tension and viscosity, on drop impact using three liquids. It is found that the use of the experimental dynamic contact angle at maximum spreading in the Kistler model, which is used as a boundary condition for the CFD-VOF calculation, gives good agreement between experimental and numerical results. Analytical models commonly used to predict the boundary layer thickness and time at maximum spreading are found to be less correct, meaning that energy balance models relying on these relations have to be considered with care. The time of maximum spreading is found to depend on both the impact velocity and surface tension, and neither dependency is predicted correctly in common analytical models. The relative proportion of the viscous dissipation in the total energy budget increases with impact velocity with respect to surface energy. At high impact velocity, the contribution of surface energy, even before splashing, is still substantial, meaning that both surface energy and viscous dissipation have to be taken into account, and scaling laws depending only on viscous dissipation do not apply. At low impact velocity, viscous dissipation seems to play an important role in low-surface-tension liquids such as ethanol.

Effect of a cylindrical shroud on particle conditions in high velocity oxy-fuel (HVOF) spray process

Abstract Simulations of solid particles in a highly compressible gas flow in the high velocity oxy-fuel (HVOF) process are presented. The Eulerian formulation is used for the gas flow, and the particle phase is modeled by the Lagrangian method. Effects of attaching a cylindrical shroud to the end of the supersonic HVOF nozzle on gas and particle flows are analyzed. We found that the shroud significantly reduces the oxygen content in the field by protecting the supersonic jet from ambient air entrainment. The validation experiments were performed for the majority of process parameters such as shock formation, particle conditions, and coating oxygen content.

Shear driven droplet shedding and coalescence on a superhydrophobic surface

The interest on shedding and coalescence of sessile droplets arises from the importance of these phenomena in various scientific problems and industrial applications such as ice formation on wind turbine blades, power lines, nacelles, and aircraft wings. It is shown recently that one of the ways to reduce the probability of ice accretion on industrial components is using superhydrophobic coatings due to their low adhesion to water droplets. In this study, a combined experimental and numerical approach is used to investigate droplet shedding and coalescence phenomena under the influence of air shear flow on a superhydrophobic surface. Droplets with a size of 2 mm are subjected to various air speeds ranging from 5 to 90 m/s. A numerical simulation based on the Volume of Fluid method coupled with the Large Eddy Simulation turbulent model is carried out in conjunction with the validating experiments to shed more light on the coalescence of droplets and detachment phenomena through a detailed analysis of the aerodynamics forces and velocity vectors on the droplet and the streamlines around it. The results indicate a contrast in the mechanism of two-droplet coalescence and subsequent detachment with those related to the case of a single droplet shedding. At lower speeds, the two droplets coalesce by attracting each other with successive rebounds of the merged droplet on the substrate, while at higher speeds, the detachment occurs almost instantly after coalescence, with a detachment time decreasing exponentially with the air speed. It is shown that coalescence phenomenon assists droplet detachment from the superhydrophobic substrate at lower air speeds.

A Numerical Study of Suspension Injection in Plasma-Spraying Process

Suspension feedstock in plasma spraying opened a new chapter in coating process with enhanced characteristics. The suspension carrying sub-micron up to few micron-sized particles is radially injected into an atmospheric plasma plume. Understanding the trajectory, velocity, and temperature of these small particles upon impacting on the substrate is a key factor to produce repeatable and controllable coatings. A three dimensional two-way coupled Eulerian-Lagrangian scheme is utilized to simulate the flow field of the plasma plume as well as the interactions between the evaporative suspension droplets with the gas phase. To model the breakup of droplets, Kelvin-Helmholtz Rayleigh-Taylor breakup model is used. After the breakup and evaporation of suspension is complete, the solid suspended particles are tracked through the domain to determine the characteristics of the coating particles. The numerical results are validated against experiments using high-speed imaging.

Characterization and modeling of 2D-glass micro-machining by spark-assisted chemical engraving (SACE) with constant velocity

Spark-assisted chemical engraving (SACE) is an unconventional micro-machining technology based on electrochemical discharge used for micro-machining nonconductive materials. SACE 2D micro-machining with constant speed was used to machine micro-channels in glass. Parameters affecting the quality and geometry of the micro-channels machined by SACE technology with constant velocity were presented and the effect of each of the parameters was assessed. The effect of chemical etching on the geometry of micro-channels under different machining conditions has been studied, and a model is proposed for characterization of the micro-channels as a function of machining voltage and applied speed.