1. Field of the Invention
This invention relates to the field of ceramics and particularly to ZrO.sub.2 ceramics.
2. Description of the Prior Art
During cooling, ZrO.sub.2 undergoes a martensitic-type transformation from a tetragonal crystal structure to a monoclinic crystal structure with a concurrent increase in volume and an anisotropic shape change. For pure ZrO.sub.2 the transformation begins at about 1200.degree. C. and proceeds until complete at about 600.degree. C.
Attempts have been made to utilize this transformation in order to improve the fracture toughness of ceramic composites. In one approach, ZrO.sub.2 particles have been added to an Al.sub.2 O.sub.3 matrix to form a second phase dispersion (N. Claussen, J. Am. Ceram. Soc. 59, pg. 49, 1976). Expansion and shape change of the ZrO.sub.2 as it transformed from the high temperature tetragonal structure to the room temperature monoclinic structure created microcracks. The resulting increase in fracture toughness was attributed to energy absorption by these microcracks.
More recently, attempts have been made to increase the toughness of ZrO.sub.2 ceramics by taking advantage of metastable grains of tetragonal ZrO.sub.2 within a surrounding matrix. These are grains of ZrO.sub.2 which are tetragonal rather than monoclinic despite the fact that their temperature is below the unconstrained equilibrium transformation temperature range.
The metastable condition can be obtained by surrounding fine grains of ZrO.sub.2 in a constraining matrix such as Al.sub.2 O.sub.3. The matrix constrains the volume and shape change associated with the transformation of the grains of ZrO.sub.2 and holds the ZrO.sub.2 in its tegragonal state.
The tetragonal grains of ZrO.sub.2 increase the fracture toughness of the ceramic composite by increasing the energy required for a crack to propogate. If a crack starts in the ceramic composite, the metastable grains of tetragonal ZrO.sub.2 in the stress field adjacent the crack transform to the stable monoclinic structure. The work done by the applied stresses to reduce this transformation is loss and thus the stress-induced transformation increases the material's fracture toughness.
Metastable tetragonal grains of ZrO.sub.2 have been observed in an Al.sub.2 O.sub.3 /ZrO.sub.2 ceramic composite containing 17 volume % ZrO.sub.2 (N. Claussen, J. Am. Ceram. Soc. 59, pg. 85, 1978). However, to maintain the metastable tetragonal structure, the ZrO.sub.2 grains had to be less than about 0.5 .mu.m in diameter. Larger grains transformed to the stable monoclinic structure. Additional work has shown that the amount of metastable tetragonal ZrO.sub.2 that can be retained in the matrix decreases as the volume % of ZrO.sub.2 in the Al.sub.2 O.sub.3 /ZrO.sub.2 ceramic composite increases. Very little of the ZrO.sub.2 can be retained in the metastable tetragonal structure in Al.sub.2 O.sub.3 /ZrO.sub.2 composites having more than 20 volume % ZrO.sub.2. Such limitations of grain size and volume % of ZrO.sub.2 reduces the practicality and the toughness of prior art Al.sub.2 O.sub.3 /ZrO.sub.2 ceramic composites.