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Dissertation
Splat formation during thermal spraying of polymer particles: mathematical modeling and experimental analysis
Doctor of Philosophy (Ph.D.), Drexel University
Jul 2006
DOI:
https://doi.org/10.17918/etd-1121
Abstract
This thesis develops and presents a model for predicting the three-dimensional splat formation process for polymer particles under High Velocity Oxy-Fuel (HVOF) combustion spray process conditions. During HVOF spray deposition, jets of high temperature, high velocity gases are used to heat, melt and accelerate particulate materials injected into the jet and propel them towards a surface to be coated. Upon impact at the surface, multiple hot particles impact and form splats that overlap, cool and consolidate to form a coating. These splats are the building blocks of an HVOF coating and coating characteristics such as porosity, roughness, adhesive and cohesive strengths depend on the morphology of these splats and how they bond to the substrate and to each other. Fully coupled transport models of particle acceleration and heating in an HVOF jet were simultaneously integrated within a FORTRAN code to predict particle velocity and particle temperature profiles at impact. Then, a volume-of-fluid computational fluid mechanics package, Flow-3D®, was used to predict particle deformation and splat shapes using results from the acceleration and heating models as the initial conditions. Fluid flow of spreading polymer droplets was modeled as a generalized Newtonian fluid with temperature and shear rate dependent viscosity. While shear thinning primarily affectedthe droplet spreading ratio, the internal temperature distribution had the largest effect on the final splat shape, particularly when particles were partially melted. The predicted shapes of deformed particles exhibited good qualitative agreement with experimentally observed splats. Most of the larger experimentally observed (> 70 [mu]m) Nylon-11 splats sprayed onto room temperature flat or rough substrates exhibited a characteristic "fried-egg" shape with a large, nearly-hemispherical, core in the center of a thin disk. This shape was formed from polymer particles having a low temperature, high viscosity core and a high temperature, low viscosity surface. The mathematical model developed in this research is the first reported three-dimensional model of the splat formation process of HVOF sprayed polymer particles. This model and the associated experimental observations have helped to improve the understanding of the relationships between processing conditions and splat morphology during the HVOF combustion spraying of Nylon-11.
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Details
- Title
- Splat formation during thermal spraying of polymer particles
- Creators
- Milan Ivos̆ević - DU
- Contributors
- Richard Allan Cairncross (Advisor) - Drexel University (1970-)Richard Knight (Advisor) - Drexel University (1970-)
- Awarding Institution
- Drexel University
- Degree Awarded
- Doctor of Philosophy (Ph.D.)
- Publisher
- Drexel University; Philadelphia, Pennsylvania
- Resource Type
- Dissertation
- Language
- English
- Academic Unit
- Materials (Science and) Engineering (Metallurgical Engineering) [Historical]; College of Engineering (1970-2026); Drexel University
- Other Identifier
- 1121; 991014632276204721