Processing and properties of ceramic nanocomposites designed for improved fracture toughness

Nanocrystalline-matrix ceramic composites specifically designed for applications requiring improved fracture toughness were investigated. While the models and theory of toughening mechanisms for microcrystalline composites are well developed, the same cannot be said for their nanocrystalline counterparts. The mechanisms of ductile-phase toughening, fiber toughening, transformation toughening, and microcrack toughening have been fully investigated in microcrystalline-matrix ceramics. Both ductile-phase toughening and fiber toughening are theoretically viable as toughening mechanisms in nanocrystalline ceramics. The experimental demonstration of these mechanisms has been investigated through alumina-matrix nanocomposites with second phases of niobium (ductile-phase toughening) and carbon nanotubes (fiber toughening).; The difficulty in producing fully consolidated ceramic composites that retain a nanocrystalline structure is the main hurdle to thorough investigations in this area. Thus, much of the research currently in the literature on so-called "nanocomposites" has been on materials with microcrystalline matrices and nanometric second phases. Using novel processing techniques, fully dense composites with nanocrystalline matrices were produced from commercially available starting powders. The consolidation technique, which allowed the retention of the nanocrystalline grain size, was spark plasma sintering (SPS). SPS is a moderate-pressure sintering method based on the theory of plasma momentarily generated in the gaps between powder materials by electrical discharge during DC pulsing. It has been proposed that the on--off DC pulse energizing method could generate (1) spark plasma, (2) spark impact pressure, (3) Joule heating, and (4) an electrical-field diffusion effect. SPS can rapidly consolidate powders to full density through the combined actions of rapid heating, applying pressure, and proposed powder surface cleaning.; Al2O3-10 vol.% Nb ductile-phase toughened composite was produced in only 3 minutes at 1100°C by SPS. The alumina matrix grain size is ∼200nm and the niobium grain size is ∼20 nm. This composite has a hardness of 22.5 GPa (slightly higher than pure alumina) and a fracture toughness of 7.0 MPa·m½ (more than twice that of pure alumina). Al2O3-10 vol.% single-walled carbon nanotube (SWCNT) fiber toughened composite was consolidated to full density by SPS at 1150°C for 3 minutes. This material retained a hardness of 16.1 GPa while demonstrating a fracture toughness of 9.7 MPa·m½ .