Ceramic composites are being actively developed in many research establishments primarily for structural and load-bearing applications. Fibre, whisker or particulate reinforced composites can be made tougher and stronger than traditional unreinforced ceramics. Among the key factors responsible for the improved stress resistance are differences in Young’s moduli between the phases and the nature of the interaction at interfaces between matrix and the reinforcement.
Incorporation of borides, carbides, nitrides or metals into multi component composites modifies the fracture properties by increasing thermoelastic mismatch strain and crack deflection behaviour. Moreover, at sufficiently high processing temperatures the individual components may react and form new compounds. Isomorphic substitutions between cations are well known in a great variety of mineral structures. Structural replacement between oxygen, nitrogen and carbon does take place and results in formation of oxynitrides and oxycarbonitrides. The phases with covalent or metallic bonding thus formed can impart electrical conductivity, which is not a typical property of most ceramics. Conductivity in combination with wear and corrosion resistance and the general refractory nature of ceramics offers potential for applications of the composites as electrodes in advanced technologies.
Conductive Ceramic Composite Systems
The conductive ceramic composites developed by PATRIA Inc as an outgrowth of a larger research and development programme are materials which combine high toughness, corrosion resistance and electrical conductivity. They constitute complex systems based on oxide and non-oxide phases. The starting composition is of the general form, Al2O3-ZrO2-AlN-SiCw-X where X denotes additions of TiB2, TiC, BN or Nb. To prepare the composites, the powders were mixed in either 20 vol% or 25 vol% of each constituent using a proprietary mixing procedure which involved ultrasonic and mechanical dispersion in non-aqueous slurries. Dry batches were subsequently hot pressed in the temperature range 1700-2000°C under pressure of 21 MNm-2, to form billets 10cm in diameter and 4cm high.
Physical and Chemical Properties
During hot pressing, chemical reactions between various components accompany densification, so the final compositions differ from the initial constituents, especially because at the highest temperatures employed a limited amount of liquid phase assists densification. The materials are dense, hard and tough, with fracture toughness’ in the range 3.5 to 6 MNm-3/2. The nominal density of unfired composites varied between 4 to 5.8 g cm-3, whereas after hot pressing, the density ranged from 93% to 102% theoretical, the upper limit above 100% being due to formation of new, dense phases.
An example of microstructure formed after hot pressing a five component Nb-containing composite at 1800°C revealed two distinct phases, a Si-Al-O-N phase and a Nb-rich phases. Reaction zones can be identified around the grains. Metallic Nb has completely reacted with ZrO2 and SiC grains and formed two dominant phases, a Nb-Zr carbonitride and a Nb-Si carbide. The third major phase is sialon. The distribution of oxygen among these phases suggests that most of the original oxides have been consumed during reactive hot pressing, leaving only remnants of SiO2 and Al2O3. In general, the distribution of light elements was restricted among specific phases in this material. After processing at 1900°C the Nb-Si-C phase of intermediate reflectivity disappears, possibly reacting with the Nb-phase to form a complex Nb-Si-Zr carbonitride.
Strength measurements for the composites are promising. The room temperature flexural strength ranges between 180 and 400 MNm-2. Compatibility with molten metals is generally good and samples submerged for 6 hrs in pure molten iron or aluminium showed no visible sign of attack.
Several possibilities have been identified for practical applications of these materials. Because the developed composites are conducting, refractory and have high wear and impact resistance, they can be applied as anodes, heating elements, sensors, and circuit breakers as well as in related applications in which good mechanical properties under high current or temperature conditions are required. They can also be used for joining of SiC industrial components by direct, electric heating, in which the composite serves as the heating element and adhesive donor at the same time.
Apart from the applications mentioned above, the composites have been evaluated for cathodic and anodic behaviour. Polarisation measurements show that interfacial oxides form at the surfaces. Passivation occurs when they are treated as anodes. But after coating with a thin layer of Pt, the anodic behaviour of the composites approaches that of pure, solid Pt anode. These materials therefore can be used as a base for anodes to be used in oxygen evolution or fluoride contaminated applications such as copper fluoroborate plating.