Traditionally ceramic products have been utilized where their hardness, wear resistance, heat resistance, or corrosion resistance was essential. Applications as mechanical components have been limited because of the low fracture toughness and strength inadequacies of ceramics.
Much research has been directed to means for significantly increasing the fracture toughness of ceramics and thereby enabling their use as high temperature structural components in heat engines, bearings, cutting tools, etc.
Major research efforts have been directed to producing "tough" ceramics via transformation toughening, especially through the use of incorporated metastable tetragonal zirconia constituents. Large improvements in low temperature toughness have been achieved which have expanded the mechanical or structural utility of ceramic components of zirconia or tetragonal zirconia containing ceramics. However, this type of toughening is degraded at elevated temperatures important to heat engines or other applications. Examples of transformation toughening include the following U.S. Pat. Nos. 4,218,253 to Dworak et al; 4,520,114 to David; 4,532,224 to Hori; and 4,587,224 to Keefer et al.
A second toughening approach is to form ceramic/ceramic composites in which at least one constituent is fibrous (elongated). Many research studies have explored techniques for incorporating continuous fibers, discontinuous fibers, or whiskers into ceramic matrices. The largest increases in toughening have been achieved by incorporation of uniaxially aligned continuous fibers. But, promising results have also been demonstrated by dispersing ceramic whiskers in ceramic matrices. For example, see U.S. Pat. No. 4,543,345. Much larger increases in toughness levels are desired and are being pursued in continuing extensive research activity. A highly desired advantage of the fiber composite versus transformation toughening is the potential to retain toughness at high application temperatures.
While ceramic/ceramic fiber-containing composites offer great potential, there are inherent practical difficulties in their fabrication, especially on a commercial scale. The three major problems are: (1) damage to the fibers during incorporation into the matrix and its densification, (2) controlling the fiber orientation within the matrix, and (3) "uniformly" distributing the fibers within the matrix.
FIG. 1 outlines the present state-of-the-art approaches to forming ceramic/ceramic composites incorporating discontinuous fibers or whiskers. This figure is self-explanatory and problems associated with the forming approaches are given. However, some of the problems should be highlighted which the invention seeks to avoid or minimize. For both Processes 1 and 2 the consolidation of a low density combination of fibers and matrix and the further densification by sintering or hot pressing virtually assure damage and degradation of the fiber phase and the creation of localized micro/macroscopic density or inhomogeneity defects. For Process 3, mechanical damage of the fibers is circumvented, but inhomogeneities of the starting fiber array are virtually impossible to avoid on a micro-macroscopic scale. Furthermore, uniform infiltration throughout the fiber preform is essentially impossible and is a very slow and costly process unless the composite has at least one very small dimension in cross-section. The noted types of defects can degrade both strength and toughness of the composite ceramic.