Polymer cold spray has yielded lower deposition efficiency (DE) and quality deposits compared to metal cold spray. The disparity stems from metals being studied far longer than polymers in cold spray; in addition, polymers exhibit richer thermo-mechanical behavior. An experimental study was conducted to examine the effects of polymer feedstock degree of crystallinity (D) on cold sprayed deposits of polyetherketoneketone (PEKK), a thermoplastic used in aerospace and other high-performance applications. As deposition relies on the plastic deformation of the impacting particle, polymers with high D may inhibit deposition, reducing deposit quality and efficiency. This study evaluates three PEKK grades produced using different ratios of terephthalic (T) to isophthalic (I) monomer moieties (T/I = 60/40, 70/30, 80/20). The ratios control D, with higher proportions of T monomers corresponding to higher crystallization rates and degrees of crystallinity. A parametric study was completed to evaluate functional process set points of system carrier gas temperature and powder mass flow rate. Using operational parameters common among the PEKK grades, spray cycles were completed for each material and quantitative responses to variation in crystallinity were evaluated through a suite of analyses. DE of the materials was assessed gravimetrically, deposit porosity was evaluated by scanning electron microscopy, and thermophysical changes to the feedstock during the spray cycle were determined by differential scanning calorimetry. Overall, we found that cold spray processing of powders of lower D formed less porous deposits with a higher DE than more crystalline powders sprayed at the same process conditions. PEKK grades with lower T/I ratios achieved DEs in the range of 60-75%, whereas the most T enriched grade only reached ~10% DE.

An integrated approach was developed for investigation of thermal spray coatings with the amorphous-crystalline structure. The new approach combines methods of metallography, differential thermal and X-ray phase analysis, scanning electron microscopy and X-ray microanalysis. This makes it possible to reveal structural, phase and chemical heterogeneity, determine the degree of amorphization of coatings, temperature and heat of crystallization of the amorphous phase during heating. The new integrated approach was used to study amorphous-crystalline coatings of the Ni-P, Fe-Ni-B and Fe-B systems produced by thermal spraying.