Characterisation of the green machinability of AlN powder compacts

The aim of this work is to study the machinability of aluminium nitride green bodies obtained from dry pressing of spray-dried granules. The characterisation of the green machinability of ceramics is not easy. Indeed, to date, no conventional mechanical test able to represent the machining behaviour of green powder ceramic compacts. Therefore, the first target of this work is to determine possible correlations between mechanical properties, microstructure and machinability and to propose suitable tests. Three types of aluminium nitride granules are investigated. Two of theses types of granules are commercial grades using a thermoplastic binder: a conventional grade for pressing and a specific grade especially developed for green machining. The third grade is under development and uses a thermoset binder. From this work it appears that the compacts containing the thermoset binder exhibit simultaneously the highest mechanical properties and the best behaviour at machining. The good machinability has been correlated to a high work of fracture together with a transgranular mode of fracture of the material.