Influence of Humid Conditions and Storage on Flowability and Deposition Quality in Polymer Cold Spray

Poor feedstock flowability has a considerable effect on the cold spray deposit quality, causing erratic powder feeding resulting in low deposition efficiency (DE), deposit void spaces, and inconsistent deposit geometries. Powder flowability is greatly impacted by feedstock moisture adsorption on the particle surface from exposure to high relative humidity. The fine particles used in cold spray are notably susceptible as the high surface area to volume ratios can store large quantities of moisture. In this study, the flowability and deposit quality of a fluoropolymer-based powder was evaluated with aging under high relative humidity (RH 95%) over 7 days. The flowability was also assessed for as received powder and after preconditioning by sieving and vacuum purging with an inert gas. Powders preconditioned in this manner were sealed in metal containers, stored under 95% RH for 7 days, and reevaluated to determine the ability of this method to protect materials from humid conditions. Powder flowability was analyzed by mass flowrate (g/s), the Carr angle of repose, and the Hausner ratio. Next, the effect of aging and storage on deposit quality was evaluated for each feedstock condition. The choice of spray conditions was informed by simulation of particle velocity and temperature distribution at impact using one-dimensional compressible flow modeling, couple with thermal analysis of the powder. The DE was determined gravimetrically, surface roughness was evaluated using a profilometer, microstructure was evaluated using a scanning electron microscope, and density was evaluated using Archimedes principle. The deposit hardness was measured using a handheld durometer (ASTM D2240), adhesion strength was evaluated by a cross hatch adhesion test (ASTM D3359), and damage resistance was determined with direct impact testing (ASTM D2794). The ability to preserve feedstock flowability will improve deposit quality, save time and resources, and accelerate research efforts in the field.