Energetic Intermetallic Materials Formed by Cold Spray

Open Access
Author:
Dean, Steven Wesley
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 02, 2015
Committee Members:
  • Richard A Yetter, Dissertation Advisor
  • Stephen R Turns, Committee Member
  • Adrianus C Van Duin, Committee Member
  • James Hansell Adair, Committee Member
  • Timothy John Eden, Special Member
Keywords:
  • Cold Spray
  • Intermetallic
  • Energetic Material
  • Nickel
  • Aluminum
  • Combustion Synthesis
  • Additive Manufacturing
Abstract:
Cold spray is a recently developed particle consolidation technique that has attracted attention as a method to create structural energetic materials. These materials can be used to replace non-energetic structural components in munitions and other devices. This replacement effectively increases the energy density of the device, without the need to develop a new high explosive payload, which has historically been very challenging. These materials are also candidates for use in self-propagating high-temperature synthesis applications, where the product of the energetic reaction is a useful material itself. These reactants can be deposited in a shape near their final form using cold spray, and then ignited to form the desired product. In cold spray, particles are entrained in a carrier gas that is accelerated to high velocity through a supersonic nozzle. The high speed gas and particle stream impinges on a substrate, which the particles impact. The force of this impact is great enough that the particles adhere to the substrate and form a coating. These coatings can have densities and strengths near those of bulk material, making them potentially useful as structural components. The low gas temperatures used in cold spray differentiate it from other thermal spray techniques. These low temperatures allow particles to remain solid throughout the deposition process. This is a significant benefit, as materials that react when mixed in a molten state can be cold sprayed without reaction taking place. It is this feature that makes cold spray an attractive process for the creation of energetic materials. The energetic material used in this work is an intermetallic composed of a mixture of nickel (Ni) and aluminum (Al). The two metals are intimately mixed with each other as they are deposited, but remain heterogeneous until they are heated to the point of reaction. This intermetallic reaction is exothermic, gasless, and results in a product composed of various nickel aluminide phases, such as Al3Ni or AlNi. The goal of this work is to investigate methods to control the properties of energetic materials made using cold spray. To do this, three studies were undertaken. In the first, the effects of different feed powder microstructures on the energetic properties of cold sprayed material were explored. Cold sprayed materials were made and compared with pellets and loose powders with matching compositions. The cold sprayed samples were made using mixed Al and Ni powders as the feed material, or a powder composed of Al particles that had been coated with a Ni shell (Ni-clad Al). The materials were characterized by measuring the rate at which a reaction front propagated through the material, and the amount of heat released by the material, which was measured using differential scanning calorimetry. Results from this study showed that, in Ni-clad Al, the speed of the reaction front increased with material density and reached a maximum of 116 mm/s in the cold sprayed material. The material made with mixed Al and Ni powders showed the opposite trend, with cold sprayed material having the slowest propagation rate of only 6 mm/s. This difference in behavior was attributed to the flow of molten Al enhancing the reaction propagation rate in samples made of mixed Al and Ni, which was limited when those materials were consolidated to high densities. An analytical model was used to describe the reaction propagation rates for both the mixed and clad materials. In the second study, molybdenum trioxide (MoO3) was added to mixed-Al/Ni cold sprayed material to increase energy density and gas production through the exothermic Al/MoO3 thermite reaction. Results from this study showed that added MoO3 increased the reaction propagation rate over that of mixed Al and Ni with no added MoO3, but reduced heat evolved by the reaction in the calorimeter. Calorimetry experiments and atomistic modeling indicated this unanticipated finding resulted from the formation of a stable Ni/MoO3 product that inhibited both the Al/Ni and Al/MoO3 reactions. In the final study, the effect of heat treatment on the mechanical and energetic properties of energetic cold sprayed material was explored. Cold sprayed materials, made with mixed Al and Ni powders and Ni-clad Al powder, were annealed at temperatures between 300 °C and 500 °C for four hours in argon. Changes in the structure of the material caused by annealing were studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy, calorimetry was used to determine how the heat release of the materials was effected, and Vickers hardness testing was used to find how the mechanical properties of the materials were altered. Results showed that diffusion occurred in the samples during annealing, which led to the formation of intermediate product phases. These decreased the heat release of the materials from 926 J/g to 558 J/g for the Ni-clad Al material, and from 978 J/g to 159 J/g in the mixed-Al/Ni material. The hardness of the materials was also increased from 130 HV to 420 HV in the Ni-clad Al material, and from 66 HV to 645 HV in the mixed Al and Ni material. The results of these studies show that the cold spray process is an effective means of producing energetic materials, and that the properties of these materials can be controlled through the selection of feed powder microstructure, the addition of ternary components, and heat treatment.