Advanced Nanostructured Powders for Cold Spray Applications

Cold spray has many benefits compared to conventional thermal spray including lower deposition temperatures, higher bond strengths, less substrate preparation and better control of coating composition, phase structure, and oxide contamination. In cold spray, particles are accelerated by an expanding gas to 180-1200 m/s, where they collide with a substrate to form a coating on the surface in the solid state. Since reduced deposition temperatures are used, coatings have the same properties as the initial particles. However, in the cold spray process, particles smaller than 5 microns in diameter have insufficient momentum to penetrate a shock wave region next to the substrate. Nanoparticles offer many benefits in strength, hardness, and reactivity but are unable to be used in the cold spray process due to insufficient mass and, therefore, momentum. In the Phase I project, Eltron developed a method for spray drying agglomerates of nanoparticles for use in the cold spray process. Spray drying is a low-cost, well-established powder processing method used often in the food and catalyst industries. The objectives of this Phase II project include: 1) produce larger (10-20 lb.) batches of powder for consolidation by cold spray at the Army Research Laboratory, 2) develop a system for consolidating smaller, more reactive, un-passivated metal particles, 3) determine the effect of slurry flow rate, nozzle diameter, gas flow rate, nanoparticle size, binder concentration and binder choice with larger volume slurries on agglomerate morphology, size, and size distribution, 4) characterize agglomerates using optical and scanning electron microscopy, chemical analysis, density measurements, x-ray diffraction, and surface area/pore size and volume analysis, and 5) optimize economic considerations including nanoparticle, metallic binder precursor, and solvent costs, leading to an estimate of powder costs when produced on a commercial scale. Work towards these objectives will lead to a flexible process for producing relatively inexpensive nanostructured powders that enables use of a variety of nanoparticle compositions and particle sizes (from 10 200 nm). During Phase II, larger volume slurries will be optimized and process variables will be determined to best prepare for scale-up to production levels of 100 lbs/day (during Phase III).