Coating Thickness Profile Research Articles

Wire coating and melt elasticity

In modern wire coating, the polymer is dragged through a round cylindrical die. Onto this drag flow, we superpose pressure-driven extrusion. We devote this paper to analyzing this extrusion in eccentric cylindrical coordinates. We find that, when the molten polymer is an elastic liquid, a recentring force, Fx, is exerted on the wire. This is how the wire is then coated concentrically. The lateral force acting on the wire thus matters. This also explains why the wire cannot be coated with Newtonian or nearly Newtonian polymer. The axial force on the wire, Fz, is always positive, and we find that the die eccentricity decreases Fz. This determines the required pulling force. Thus, the axial force acting on the wire also matters. We follow the method of Jones (1964) called polymer process partitioning, to obtain the coating velocity profile, v⌣z(ξ,θ), from which we get the coating thickness profile. We integrate this profile to get the flow rate, and thus, the average thickness. We also obtain the stresses in the extrudate. We include one detailed dimensional worked example to help engineers design coating dies.

Niobium thin film thickness profile tailoring on complex shape substrates using unbalanced biased High Power Impulse Magnetron Sputtering

The authors report in this paper the possibility to control the thickness profile of a thin film deposited by High Power Impulse Magnetron Sputtering (HiPIMS). It is shown that the combination between a HiPIMS discharge, an unbalanced magnetic configuration and the application of a negative bias onto the surface to coat enables tailoring on demand the coating thickness profile. This effect is hereafter used to coat complex shapes such as low-beta accelerating cavities with a niobium layer. The authors first present the magnetic design proposed to obtain an unbalanced cylindrical sputtering source. Numerical simulations are then used to predict the electron density and energy spatial distributions that can subsequently be correlated to the ionization region shape. Finally, the authors present the effect of such technique comparing Direct Current Magnetron Sputtering (DCMS), HiPIMS and biased HiPIMS using, respectively, a balanced and an unbalanced magnetic configuration, as well as detailing the effect of modifying either the magnetic field lines distribution or the magnetic strength.

A parametric simulation model for HVOF coating thickness control

High-velocity oxygen-fuel (HVOF) thermal spraying is a coating process involving multidisciplinary aspects, e.g., fuel–oxidant combustion, flame–particle jet, particle deposition, mass and heat transfer, and even robotic kinematics. Like most coating processes, in HVOF processes, coating thickness is a significant property determining the coating performance; hence, this property should be accurately controlled during the process. In view of green, smart, and digital manufacturing, the coating thickness prediction model is demanded to produce high-quality coatings efficiently. This paper presents an approach to parametrically simulate the coating thickness in HVOF processes via an integrated numerical model. Firstly, an axisymmetric computational fluid dynamics (CFD) model is constructed to compute the behaviors of the fuel–oxidant combustion, flame–particle jet, and particle deposition distribution. Secondly, based on the particle distribution in a 2D axisymmetric model, a 3D single coating thickness profile model is developed by constructing a circular pattern using the axis of the nozzle. Further, this profile is smoothened by a Gaussian model, and its mathematical expression is obtained. Finally, a numerical model couples spray paths with the mathematical expression to model the coating thickness distribution on a substrate surface under industrial scenarios. At the end of this paper, to verify the proposed model’s effectiveness, four sets of operating parameters with a single straight path were experimentally implemented. The width and height of the bead-like shape coating were in good agreement with the simulated results. The normalized root-mean-square errors of the cross-sectional profile heights were around 10%.

Study of Scanning Micro-arc Oxidation and Coating Development

Micro-arc oxidation (MAO) continues to be the focus of numerous investigations, whereas relatively few studies have considered scanning micro-arc oxidation (SMAO). In the present work, an experimental study was performed using stationary and moving electrodes to investigate coating development in SMAO and discern the effect of key process parameters. Examination of oxide deposits made on A356 aluminum show that coating thickness and growth rate are inversely related to inter-electrode spacing and travel speed. An evaluation of SMAO deposits made by stationary and moving nozzles revealed that coating thickness profiles follow a Gaussian distribution due to the electrolyte flow field in the impingement zone. Hardness surveys and scanning electron microscope analysis of SMAO coatings revealed that micro-hardness distributions and cross-sectional morphology are similar to MAO for a stationary nozzle but that a denser outer layer develops when a moving nozzle is used. This is attributed to a high density of discharge occurring in micropores of the oxide film and remelting which results from the moving electrolyte column. Analysis of voltage–current characteristic curves shows that the resistance of the electrolyte column is essentially linear over the range considered and results indicate that it can be modeled as a variable length resistor. While further testing is needed, results confirm that SMAO is suitable for coating large, planar parts and for repairing worn surfaces.

Control of lateral thickness gradients of Mo-Si multilayer on curved substrates using genetic algorithm.

An inversion method based on a genetic algorithm has been developed to control the lateral thickness gradients of a Mo-Si multilayer deposited on curved substrates by planar magnetron sputtering. At first, the sputtering distribution of the target is inversed from coating thickness profiles of flat substrates at different heights. Then, the speed profiles of substrates sweeping across the target are optimized according to the desired coating thickness profiles of the primary and secondary mirrors in a two-bounce projection system. The measured coating thickness profiles show that the non-compensable added figure error is below 0.1 nm rms, and the wavelength uniformity across each mirror surface is within ±0.2% P-V. The inversion method introduced here exhibits its convenience in obtaining the sputtering distribution of the target and efficiency in coating iterations during process development.

Masking technique for coating thickness control on large and strongly curved aspherical optics

We discuss a method to control the coating thickness deposited onto large and strongly curved optics by ion beam sputtering. The technique uses an original design of the mask used to screen part of the sputtered materials. A first multielement mask is calculated from the measured two-dimensional coating thickness distribution. Then, by means of an iterative process, the final mask is designed. By using such a technique, it has been possible to deposit layers of tantalum pentoxide having a high thickness gradient onto a curved substrate 500 mm in diameter. Residual errors in the coating thickness profile are below 0.7%.

Modelling of continuous steel coating by self-induced ion plating (SIIP)

The present communication deals with the numerical simulation of a new physical vapour deposition process called self-induced ion plating (SIIP). SIIP can be defined as the evaporation of a metallic target thanks to ion bombardment of a magnetron sputtering system. The simulation of the SIIP process is based on three coupled models. The first one deals with magnetism and simulates a magnetron system, the second one defines the heat transfer phenomena (conduction, radiation) that occur in the SIIP process and the last one determines the coating thickness profile on the substrate using the theory of evaporation. The computed thickness profile is then compared to the measured one. We notice that the model results and measurements have the same order of magnitude but model results are under-estimated. We conclude that we have to improve our model by removing some assumptions.

The penetration into blind holes of diamond-like carbon films produced by r.f. plasma-assisted CVD

Diamond-like carbon films were deposited, using an r.f. plasma-assisted chemical vapour deposition process, inside tubes of different diameters rolled from stainless steel sheet. The coating thickness and microstructure, determined by scanning electron microscopy, were analysed as a function of tube depth and diameter. Although film penetration into the tubes is, as expected, influenced by the deposition system pressure, the results also suggest that the coating thickness profile depends on the distribution of ions in the depositing species. Thicker films can be grown at greater depths in smaller diameter tubes and we believe that this can be attributed to cathode sheath distortion effects, the sheath appearing to behave as a divergent lens acting on the flow of ions inside the tubes.

A practical model to enable the prediction of coating thickness variations across a flat surface

The presented model enables lateral coating thickness uniformity across a flat surface to be predicted from only a limited amount of actually measured thickness data. It is based on an inverse power law relationship for the reduction of coating thickness with increasing source to substrate distance, and is essentially a variation of the well known cosine law. The value of the power in the expression determines the coating thickness profile. The model is shown to work successfully for coatings produced by techniques based on electron beam evaporation, including vacuum evaporation, gas evaporation and PAPVD, and is compared with other available thickness models. It is also shown to have some applicability in the case of magnetron sputtering systems, both balanced and unbalanced.

The measurement of thin PVdC coatings on PET substrates using fluorescence microscopy

In this paper, the authors demonstrate how a novel technique based upon autoluminescence of plastics can be used to measure accurately poly(vinylidenechloride) (PVdC) coating thickness profiles on poly(ethyleneterephthalate) (PET) substrates. The procedure uses simple sample preparation and examination methods when used in a destructive mode. Some preliminary work has also shown that luminescence phenomena may also be used to provide coating thickness profiles non-destructively.