Abstract

A new thermal spray technology has been proposed. Called Electrical Chemical Thermal Spray (ETCS), it combines plasma energy with the combustion gases of solid propellants to heat and accelerate particulate materials. The hybrid technology promises new degrees of freedom in materials processing over the conventional thermal spray processes by allowing thermal energy transfer to the particles and particle accelerations to be optimized separately. Experimental coatings were formed using a prototype system made from a converted ½” plasma gun fueled with double-base solid propellants to explore this novel concept. The prototype test-facility equipment was limited to single-shot mode. Examination of the coatings formed, and conceptual analysis by analogy to conventional technologies was used to assess the capabilities and limitations of the hybrid process. Impressive in-flight powder velocities of 1100 m/s were reached, with deposition yield efficiencies of 60 - 85% achieved for WC-Co coatings after first round of optimization. However despite the ability to deposit single-shot carbide and metallic coatings with thickness exceeding 200 µm. chemical degradation and extensive cracking combined to limit attractiveness of coatings as compared to those produced using commercial technologies. Unlike the oxidation effects with atmospheric plasma spray and the various low-velocity flame-spraying technologies, chemical degradation in the prototype ETCS was the result of interaction between the gases produced from the combustion of the propellant and the coating material. It is seen that the organic, nitrocellulose based solid propellants are inherently unsuitable for spraying reactive material. With suitable fuels however, it is believed that the inherent advantages of high throughput, versatility and low labour requirements are such that ETCS will have commercial advantages for the coating of large structures.

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