(511b) Thermal Spraying of Nylon-11 and Nylon-11/Silica Coatings: Modeling and Characterization of Coating Microstructure
High Velocity Oxy-Fuel (HVOF) thermal spraying is a powder coating technique that has been successfully used produce polymer and composite coatings based on empirical optimization of spraying parameters. In this project both mathematical models and experimental observations were used to improve understanding of the thermal spray coating formation process, including microstructure-properties-processing relationships, during thermal spray deposition of pure nylon-11 and nylon-11/silica coatings. During thermal spray deposition, jets of high temperature and high velocity gases are used to melt and accelerate materials injected into the jet and propel them towards the surface to be coated. Upon impact at the surface, multiple hot particles deform, cool and consolidate to form a coating. Fully coupled mathematical models have been developed to predict nylon-11 particle acceleration and heating in the High Velocity Oxy-Fuel (HVOF) jet and a 3-D model of particle splatting and heat transfer upon impact with a substrate. The predicted shapes of deformed particles exhibited good qualitative agreement with experimentally observed splat shapes including a characteristic ?fried-egg? shape with a large nearly-hemispherical core in the center of a thin disk. This shape was formed by polymer particles having a low temperature, high viscosity core and a high temperature, low viscosity surface. Composite feedstock powders were produced by dry ball-milling Nylon-11 together with 10 vol. % overall ceramic phase loadings with multiple scales of reinforcements. Dry ball-milling polymer and ceramic particles together resulted in a shell-core powder morphology with ceramic-rich shells around polymer-rich cores. The morphology and microstructures of the feedstock powders and sprayed coatings were characterized by optical and SEM-EDS microscopy. The microstructure of the HVOF sprayed composite coatings has a cellular lamellar structure with ceramic reinforcements agglomerated at splat boundaries. Multi-scale ceramic reinforcements reduced scratch depths by as much as 40 to 50% relative to pure polymer coatings, and by up to 20-30% compared to single-scale reinforcements.
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