Technological advances are continually placing greater demands on the performance characteristics of engineering materials. Traditionally, engineering ceramics have been considered for heat containment, wear resistant, static load bearing, and electro-optical applications owing to the generalised properties of ceramics which include hardness, corrosion resistance, stability at high temperatures, creep resistance, high elastic modulus, and electrical and thermal resistivity. Unfortunately, these properties come at the expense of toughness. While metals typically have toughness values of 15 to 150 MPa.m½ ceramics usually have toughness values below 5 MPa.m½ frequently below 2 MPa.m½.
Ceramic Matrix Composites
Significant research effort has been directed toward the development of tough ceramics, since such materials have the potential to open up a large range of specialised engineering applications. Toughness can be significantly improved with the production of ceramics with a very small flaw size (< 50µm) in accordance with the Griffith theorem. However, producing such ceramics, and nondestructive testing for flaw size, is very difficult. An alternative solution is to engineer the microstructure of ceramics in such a way as to inhibit crack propagation. To this end, a family of toughened ceramics known as ceramic matrix composites (CMCs) is now undergoing development. Toughness values as high as 10 to 20 MPa.m½ have been achieved, which is comparable with some metals.
Toughening Mechanisms for Ceramic Matrix Composites
A number of toughening mechanisms have been engineered into ceramics. These include:
1. Crack deflection
2. Crack pinning
3. Crack bowing
4. Plasticity in metallic phase
5. Transformation toughening
6. Compressive matrix residual stresses
7. Matrix microcracking
8. Frictional interlocking
9. Crack bridging
10. Fibre pull-out
How Fibre Reinforcement Toughening Mechanisms Work
Fibre reinforcement combines crack bridging, fibre pullout, and crack deflection mechanisms. As an overall toughening technique, it appears to give the greatest improvement. Further, the use of metal fibres adds the toughening mechanism that comes from the plasticity of the metallic phase. In 1984 a twofold increase was achieved in K, for 20-30 vol% SiC whisker reinforced A1203. Since then, rapid developments have occurred in these materials with threefold to fivefold improvements in matrix fracture toughness commonly being achieved through fibre reinforcement.
Short-Fibre or Whisker Reinforcement versus Long Fibre Reinforcement
Short-fibre or whisker reinforcement is preferable to long fibre reinforcement since the short fibres can act as fines when mixed with the matrix powder during forming, leading to intimate interdispersion through gap-grading mechanisms, resulting in microstructural uniformity. Calculations have established that toughening by crack twisting and deflection is maximised for AR ~ 12 and addition levels of 10 vol%. Such fibre lengths and addition levels are well suited to gap-grading requirements. In comparison, uniformity is very difficult to achieve with long-fibre reinforced CMCs due to the lack of mobility of the reinforcing phase during forming. Microstructural uniformity is a desirable property since it enables optimisation of toughening potential through the elimination of weak zones.
Problems Associated with Powder Processing of Ceramic Matrix Composites
The development of short-fibre reinforced CMCs is dependent not only on the expanding knowledge base of suitable fibre-matrix combinations but also on the development of optimal ceramic processing techniques. As for conventional ceramics, CMCs are prepared from powder mixtures - the short fibres or whiskers are treated as powders. However, there are two major difficulties involved in the powder processing of such CMCs: density differences and fibre agglomeration.
Short fibres generally form into tenacious bundles during processing. If these are not dispersed, non-optimal CMCs will result, due to microstructural inhomogeneity and the ensuing weak zones. With the exception of some very fine whiskers, short fibres are generally much larger than the colloidal range and so can not effectively be dispersed by deflocculants. Dispersal of these bundles during powder processing is difficult. In the case of ceramic fibres, sufficient force must be exerted to break up the bundles, but an excess of force will result in micro-milling of the fibres by the matrix particles in the mix. For metal fibres, an excess of force will not damage the ductile fibres but can cause irreversible tangling, and so in this case, the nature of the dispersive action is critical. A possible solution to this problem is vibratory or ultrasonic mixing.
The fibres and the matrix powder differ in terms of both particle density and particle size. Generally the fibres are larger and more dense. Differences in particle mass will result in different sedimentation rates during wet-forming processes. Differential sedimentation leads to inhomogeneity in the sintered article, and this inhomogeneity can be pronounced when metal additives (high specific gravity) are mixed with ceramic powders. This is particularly problematic since metal fibres are preferable for toughness enhancement. No such problems will be encountered with dry pressing. However, dry pressing can cause anisotropic surface particle packing and some fibre alignment. In the present work, the solution proposed to this problem was the use of a wet forming process resistant to sedimentation. There are few wet forming processes that are not susceptible to sedimentation. One possible candidate, however, is thixotropic casting - an adaptation of solid slip casting.