Gelation-based feed-stock technologies for HVOF thermal spray development: Micro-composite powder preparation and HVOF coating microstructure

Abstract

The current study proposes use of the polysaccharide, sodium alginate, as a gelation aid to ‘solidify’ aqueous suspensions, comprised of micron-sized ceramic-metal particles, into spherical, micro-composite ‘cermet’ feedstock powders. These feedstock powders, with finely divided ceramic and metallic components, can then be used for the preparation of high velocity oxygen fuel coatings and/or laser deposition cladding. Sodium alginate gelation is typically based on ion exchange, and a variety of gelation agents have been examined in the present work. It is shown that aqueous solutions of CaCl2, Ca-acetate, Al-nitrate, methanol, acetic acid, and 5 % HCl acid are all capable of gelling alginate droplets through a simple cross-linking mechanism. Consequently, spraying a sodium alginate containing ceramic-metal particle suspension, with a suitable air atomising nozzle, into an aqueous bath of one of the outlined solutions allows the production of nominally spherical ‘cermet’ feedstock particles. In the present work, TiC-Ni3Al micro-composite cermet particles have been successfully produced, and then analysed both before and after a sintering heat-treatment. Furthermore, rheological assessment of the colloidal suspensions both prior to and during the gelling stage are also described. As a preliminary ‘proof-of-principle’, an initial set of HVOF coatings has also been prepared using the gelled feedstock material, and initial characterisation of these coatings is presented.