The utility of ceramic materials in engineering applications is critically determined by their mechanical behaviour. The most important factor is brittle fracture behaviour depending on the absence of plastic deformation at low or medium temperatures. In contrast to metals, ceramic materials are not able to dissipate energy by plastic zone in the front of the crack tip. There are several methods to increase the fracture toughness by crack shielding, bridging or branching mechanisms. The crack bridging is achieved by reinforcing the matrix by long or short fibres, in particular by SiC whiskers, and also by particle toughening. A special kind of particle reinforcement, namely nanotoughening is achieved by nanoparticles of the size 5-200 nm incorporated into a ceramic polycrystalline matrix. By incorporating metal particles into the ceramic matrix a ductile bridge can be formed and the fracture toughness can be considerable increased. The toughness enhancement if attributed to the plastic work extended in deforming the ductile inclusions. Among the various metallic dispersions used in alumina are molybdenum particles and fibres, nickel-zirconium alloys, chromium Al2O3-Al composites, and titanium carbonitrades. When both metal and ceramic inclusions are added to alumina, the fracture toughness has been observed to be higher than that for only metal particles included. The other possibility of improving the brittle response is toughening in zirconia (ZrO2) induced by martensitic transformation. Dispersion hardening of materials plays an essential role in improving the reliability and reducing the cost of materials used in machine or structural elements. Their distinctive advantage with respect to fibre-reinforced and laminated materials in isotropy of mechanical properties combined with high ductility and strength. A typical example is an aluminum alloy strengthened by Al2O3 ceramic inclusions, now intensively investigated from the perspective of engineering applications. Ceramic coatings
constitute a separate class of composite materials. By layering different structural ceramic materials, the fracture toughness can be increased through enhancement of toughening mechanisms, or residual stress effects. Various techniques may induce microstructural gradients within layers or coatings, usually associated with change of grain size. Metal-ceramic composites
in the form of an interpenetrating microstructure can be processed by infiltrating metal into a porous ceramic perform. The advantage lies in the flexibility of the microstructure which can be produced, as it is possible to change the metal content and the ligament diameter of the metal. Also, different metals can be infiltrated into the same type of ceramic performs.
Depending on the base material one distinguished ceramic-matrix composites (CMC)
and metal-matrix composites (MMC)
The ceramic matrix composites
can be reinforced with ceramic and/or metal inclusions in the form of particles or discontinuous fibres (e.g. whiskers), or layered ceramic coatings or multilayers. The KMM-NoE shall investigate, among other types, metal-ceramic composites with interpenetrating network microstructure like Al2O3/Al (alloys), TiO2/Al (alloys), Al2O3/MexAly (intermetallic compounds).
The metal matrix composites (MMC)
reinforced with ceramic particles of discontinuous fibres include (i) Ti-based MCC (Ti+SiC or Si3N4) to replace Ti-based superalloys for jet engine fan blades, compressor components, and (ii) Al-based MCC (Al+SiC, TiB2, Si3N4 or Al2O3) for automobile engine blocks. The diamond-like coatings (DLC) on metals constitute another group of metal-matrix composites suitable for medical applications e.g. DLC on Ti-substrate for hip joints.
The advantages of metal matrix composites include:
- Good thermal and electrical conductivity
- Lower thermal expansion than light metals
- Moderate density, good strength
- Decent stiffness and toughness
- Wear resistance also at elevated temperatures
- Good damping properties and reliability.
The metal ceramic composites provide unusual property combination with advantages over metals and ceramics. These properties can be flexibly tailored depending on application. However, their main deficiencies are the high cost and difficulty of processing.