Development of a polymeric coating process for indirect selective laser sintering of ceramics

The research presented aims to optimise the densities of ceramic parts consolidated by indirect selective laser sintering, by investigating the impact of powder preparation prior to sintering. A literature review of current coating methods and initial experimentation (mechanical mixing, mechanical alloying, freeze spray drying and dissolution/precipitation) revealed an opportunity in development for the coating of ceramic powder with controlled and minimal polymer binder; hence, leading to a proposal of a novel thin coating method for the application of indirect selective laser sintering. Polymethacrylate (PMMA) is grafted from the surface of aluminium oxide powder using activators regenerated by atom transfer radical polymerisation (ARGET ATRP), forming surface attached polymer brushes. The Al2O3 powder is functionalised with a monosilane, followed by the covalent attachment of bromide initiator molecules. Methyl Methacrylate (MMA) is then added to the suspension commencing monomer unit additions on the surface of individual Al2O3 particles initiated at the initiator grafting sites. The sequential addition of monomer units from the grafted surface is enabled by mechanism of halogen exchange between the monomer units stretching away from the grafted surface and a catalyst complex which is regenerated with the use of L-Ascorbic acid. The influence of polymerisation time on the coating thickness is examined to validate ARGET ATRP as a precision coating method. The presence of PMMA was detected via chemical and thermal analysis on the surface powder samples polymerised starting at 30 minutes onwards. In addition to physical characterisation methods, a computer simulation was carried out to predict the optimum conditions for controlled binder thickness.