282 Formation of a Precipitate or Dispersion of Crystals During Heat Treating

Transformation toughening in PSZ is a good example of formation of a reinforcing phase through the careful control of temperature (and chemistry). As mentioned before, toughening in PSZ involves a dispersion of nanoscale (typically under 500 nm) lenticular tetragonal zirconia in larger grains of cubic zirconia. The nanoscale reinforcement develops during the fabrication process. Zirconia powder mixed with MgO or CaO powder is compacted into the desired shape and sintered at a high enough temperature that the material densifies and that the zirconia and oxide additive form a solid solution with a cubic zirconia structure. The temperature is then reduced such that the thermodynamically stable phases are a mixture of cubic zirconia and tetragonal zirconia. Tiny nuclei of tetragonal zirconia begin to precipitate in the cubic zirconia grains. By controlling the temperature and time of heat treatment, the precipitates are allowed to grow to an optimum size ranging from about 100 to 300 nm.

The fracture toughness of PSZ is typically 6-10 MPa ■ m1/2, although some values have been reported exceeding 15 MPa ■ m1/2. The toughening mechanism is referred to as crack shielding. The compressive stress produced due to the volume increase as the tetragonal precipitates transform to monoclinic zirconia shields the tip of the crack from tensile stress.

Another interesting example of manipulation of microstructure during heat-treating is Macor, a material developed in the early 1970s by Corning. In this case the in situ reinforcement is achieved by crystallization during the fabrication process [21]. First, a composition nominally 47.2% SiO2, 16.7% Al2O3, 8.5% B2O3, 9.5% K2O, 14.5% MgO, and 6.3% F is melted and cast as slabs or cylinders of glass. The glass is then heat treated to form tiny nuclei of magnesium fluorophlogopite mica crystals. Further heat treatment grows these crystals to 5-10 ^m diameter flakes that form a "house-of-cards" structure throughout the glass matrix, resulting in a very high degree of toughening by crack deflection. The composite is not particularly strong (~60-102 MPa), but it is very resistant to fracture and is soft enough (roughly between Teflon and brass) that it can be machined with conventional metallic drill bits and cutters. Macor and similar glass-ceramic composites have been used extensively for glass-sealed electrical feedthroughs, face seals, positioning and heat-treating fixtures, dental repairs, and many other applications.

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