Effect of Reinforcement Size on the Scratch Resistance and Crystallinity of HVOF Sprayed Nylon-11/Ceramic Composite Coatings

The high-velocity oxyfuel (HVOF) combustion spraying of dry ball-milled nylon-11/ceramic composite powders is an effective, economical, and environmentally sound method for producing semicrystalline micron and nanoscale reinforced polymer coatings. Composite coatings reinforced with multiple scales of ceramic particulate material are expected to exhibit improved load transfer between the reinforcing phase and the matrix due to interactions between large and small ceramic particles. An important step in developing mul-tiscale composite coatings and load transfer theory is determining the effect of reinforcement size on the distribution of the reinforcement and the properties of the composite coating. Composite feedstock powders were produced by dry ball-milling nylon-11 together with 7,20, and 40 nm fumed silica particles, 50 and 150 nm fumed alumina particles, and 350 nm, 1,2,5,10,20,25, and 50 mum white calcined alumina at 10 vol.% overall ceramic phase loadings. The effectiveness of the ball-milling process as a function of reinforcement size was qualitatively evaluated by scanning electron microscopy + energy dispersive x-ray spectroscopy (SEM + EDS) microanalysis and by characterizing the behavior of the powder during HVOF spraying. The microstructures of the sprayed coatings were characterized by optical microscopy, SEM, EDS, and x-ray diffraction (XRD). The reinforcement particles were found to be concentrated at the splat boundaries in the coatings, forming a series of interconnected lamellar sheets with good three-dimensional distribution. The scratch resistance of the coatings improved consistently and logarithmically as a function of decreasing reinforcement size and compared with those of HVOF sprayed pure nylon-11.