Making sintered zirconia ceramics machinable

Advanced ceramic materials based on zirconia are very suitable for different structural applications owing to its high strength and toughness. Machining of sintered zirconia ceramics is limited to diamond tools that is expensive, time consuming and limited to simple geometrical complexities. Electrical discharge machining (EDM) can overcome the limitations of machining with diamond tools, however, it can only be performed on electro-conductive ceramics (ECC) exhibiting sufficient conductivity. This is usually achieved by incorporation of electro-conductive (EC) phase into the insulative ceramic matrix. Up to now zirconia-based ECC containing various EC phases, such as, TiC, TiCN, WC, ZrB2, TiN, carbon nanotubes (CNT) and/or graphene, have been prepared by mixing of commercial powders, their consolidation and sintering. Attaining homogeneous microstructures, however, is a challenge yet to be overcome. In case the particle size of both EC and ceramic matrix phase is in the same range, the electrical conductivity required for EDM is attained at high content of EC phase (>30 vol. %), which can be detrimental for mechanical properties as a result of microstructural inhomogeneities. On the other hand, when the particles of EC phase are in the nanoscale range and/or nonpolar (as CNT and graphene are), they are prone to agglomeration and are difficult to homogenously disperse in aqueous suspensions. Here several alternative, novel processing approaches, based on aqueous colloidal processing combined with rapid consolidation in the Spark plasma sintering furnace, will be presented, providing homogenous, highly electro-conducting, EDM machinable zirconia ceramic composites.