An experimental and computational methodology for near net shape fabrication of thin walled ceramic structures by plasma spray forming

Near net shape (NNS) fabrication by plasma spray forming (PSF) is a rapid prototyping technique to fabricate engineering components having axi-symmetric geometry out of ceramics and refractory metals which are difficult to manufacture by conventional techniques. This study establishes an experimental and computational protocol to manufacture thin walled ceramic (Al2O3) structures on the graphite mandrel (substrate) via plasma spray forming. The combination of experimental and computational approaches reduces currently used empirical methods for the similar purpose. Thermal profiles generated during plasma spraying of Al2O3 on the graphite mandrel for various mandrel designs and cooling conditions were computed by solving the conjugate problem of computational fluid dynamics and 3D unsteady heat transfer. Entire plasma spraying booth was modeled as per actual dimensions to consider the effect on the thermal profile of the mandrel/coating system. The computed temperature profile was compared with the experimentally measured temperature. The corresponding thermal stresses in the mandrel and spray deposited Al2O3 layer were computed. Computed thermal stresses were compared with the fracture strength of Al2O3 to prevent cracking of the spray formed structure during spraying and its successful removal from the mandrel. An optimum temperature increase rate (TIR) during plasma spray forming is defined for the successful deposition and removal of the freestanding ceramic structure.