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Prof W J Clegg

Ceramics are the group of materials most widely used by man. They include the cheapest materials, such as brick, concrete and glass, and the most expensive, such as diamond. Unlike metals, they have an extraordinary range and combination of properties that are widely used in a huge range of different devices and, unlike polymers, can be used at temperatures well above 400°C. Many operate in extreme conditions of temperature, stress and electrical fields. Even in electronic applications, it is the structural behaviour that limits the performance and lifetime.

This course describes how ceramics are made, how properties are influenced by processing and the limits to the improvements that are more easily possible. The course then explores ways for making more substantial improvements.

This lecture course will cover:

  • Making ceramics: from melts and powders. Problems with casting. Using powders. Powder compaction. Sintering of powders: driving forces. Mechanisms. Densification: stages, rates. Liquid-phase sintering. Use of applied pressure.
  • Making ceramics: chemical bonding. Chemical routes to making ceramics. Liquid and gaseous precursors. Problems with precursors. Uses as bonding agents. Cements. Reactions. Glass-ceramics, e.g. Li2O-SiO2.
  • Why is brittleness such a pest? The implications for components: The origin of flaws and their removal: Porosity and interparticle friction, e.g. cement. Large grains and their effect on strength, e.g. Al2O3, Al2TiO5. Agglomeration, e.g. Al2O3, SiC. Removal of agglomerates. Proof testing. The limits of improvements.
  • Improving reliability by toughening. R-curve behaviour and its effect on the Weibull modulus. Toughening and microstructure. Crack deflection. Making tough structures: gas pressure sintered Si3N4, liquid phase sintered SiC.
  • Toughening by phase transformations. Increasing the resistance to cracking by transformation toughening, e.g. ZrO2. Zirconia and its crystal structures. Spontaneous transformation. Retention of metastable structures. Effect of the applied stress. Size of the transformation zone around a crack. Estimation of the extent of toughening.
  • Making structures that transform. Zirconia microstructures for high toughness and their fabrication. Partially stabilised zirconia (PSZ). Tetragonal zirconia polycrystals (TZP). Zirconia toughened alumina (ZTA). Effect of microstructure on cracking.
  • Increasing the resistance to temperature changes. Thermal shock. The differences between mechanical and thermal loading. The onset of cracking. Unstable cracking and failure. Stable cracking. Estimating the degree of crack growth. Making thermally shock resistant microstructures and materials. Al2O3 and MgO refractories.
  • Wear. Hard materials: cutting, forming and electronics. Contact damage. The mechanisms of wear in brittle materials.