Ceramics

1. Ceramics Advanced material and technologies, MSc 2017

2. 1. Ceramics The role and perspectives of ceramics in engineering The chemical bonding of some elements of the earth's crust is such that it is considered as a ceramic compound. Ceramic systems: crystalline, inorganic, non-metallic material. Advanced technical ceramics used by engineering practice are highly transformed materials.

3. 1. Ceramics Ceramics: the origin of the word: keramos  potter's earth (soil) Ancient occupation: potteries, stonewares later: porcelainglassware, building materialsrefractory materials Today in technical applications: • advanced technical ceramics • structural ceramics • high performance ceramics so-called traditional ceramics

4. 1. Ceramics Comparison of main properties of ceramics and metals

5. Some examples of the benefits of using ceramic parts

6. Base materials of non-oxide, high-performance ceramics

8. Ionic bond

9. Covalent bond - Formed by pair of electrons (XA, XB ~ ≥ 2,1), - High binding energy (e.g.: C, Si, Ge), - Directions in bonding (pl. C-H4). Energy of molecular hydrogen referred to separated, neutral atoms. Negative energy corresponds to chemical bond. Curve A refers to electrons with parallel spin states, curve S (stabile state) refers to electrons with antiparallel spin states.

10. Types of bonding in ceramics: • ionic bonding • covalent bonding Common crystal types in ceramics:

11. Role of polymorfic transformations in properties of ceramic systems Structural character of polymorfic transformation: - Displacive: transformation connected with atomic displacement, - Reconstructive transformation: including dissociation and reconnection of atomic bonds

14. Microstructure of ceramics:

15. Mechanical properties of ceramics:

16. Mechanical properties of ceramics: Fracture toughness (KC) E: Young’s modulus GC: strain energy release rate (kJ/m2) For brittle materials, Gc can be equated to the surface energy of the (two) new crack surfaces Source: Courtney, Thomas (2005). Mechanical Behavior of Materials.

17. Expansion of ceramics:

19. Thermal stresses These are the most important limitations of wide-ranging applications. Results: cracks, rupture Origin of stresses: thermal or chemical, the former is more important Thermal expansion and stress: σ=-E∙α∙ (T1-T0) (elastic, rod shape) E: Young modulus α: linear expansion coefficient T0: initial temperature T1: end temperature

20. Electrical resistivity of ceramics

21. Production of ceramics Traditional ceramics, glass production CaO Na2O Decrease of viscosity mechanism: fragmentation of SiO2-chains Base of forming proc.: Q: activation energy of viskous flow Flow rate: (η)-1

22. The difference between glass transition and crystallization Changes in thermodynamic functions during glass transition. G1 and G2 are different glassy states produced at different cooling rates.

25. pressing rolling float molding die casting blowing Heat treatment: stress relaxation require higher η–t require lower η–t igényel

27. Advanced technical ceramics • oxide based (Al, ZrO2 based) • nitride based (Si3N4) • carbide based (B, Si-karbid) The common types and their most important properties:

28. The manufacturing process of ceramics: • the production of ceramic powder raw material and other materials • the shaping of the desired workpiece • establish a bond between powder particles • finishing Synthesis of powder Make the powder ready Shaping Remove adhesive Sintering Finishing

29. Shaping technologies Dry pressing Uniaxial Isostatic Extrusion Casting Injection moulding

30. Sintering

32. Tsintering 2/3 Tmelting The driving force behind the sintering process is to reduce surface energy: For example: in the case of Al2O3 powder with particle size of1μ, the surface of 10 cm3 material ≈ 1000 m2, and the interfacial energy is approx. 1 kJ. The change of density as a function of time and temperature: a: particle size C: constant Q: activation energy

33. Connecting ceramics to each other and joining them to other materials: - with adhesive,- glaze bonding,- diffusion bonding,- metallisation and brazing.

34. The aspects of designing structural elements made of ceramics and the principles of their use Careful selection of the manufacturing technology and the raw material (taking into account the properties appropriate to the purpose, + costs). Manufacturing technology and scaling are desirable to minimize post-machining. However, post-machining (grinding, polishing, laser machining etc. can not be excluded from the technology, eg. engine or gas turbine components). Avoid point loadsat applications.The stresses at the load transfer sites must be minimized by surface-like shaping. It is advisable to avoid sharp corners and large size changes. Minimize the thermal stresses.Use the smallest cross section as far as possible, and divide the parts as far as possible into simpler elements. Machining sizes are necessarily larger. Shrinkage occurs during sintering.

35. The size of the parts must be minimized(due to the crack distribution of ceramics, the strength is a function of size, therefore smaller parts are more reliable). Avoid impacts (where this is not possible, design small angle impact). Machining of the parts must be careful (cracks reducing the strength of the parts arise often on the surface or near the surface during machining). Failure probability of ceramic components is proportional to the overlap of the distribution curves which represent the strength and the applied load on the part.

37. Due to the high melting point and the embrittlement of ceramics the so-called secondary machining is not applicable in the extent and sense as in the case of metals or metal alloys (cold or hot rolling, forging). Due to the costly mechanical machining, the workpiece needs to be manufactured in approximately the final size, therefore suitable technological processes are required.In the production of ceramics the so-calledpowder metallurgy plays a major role.

38. Ceramics in automotive industry • Window glass, windshield • Insulation element for spark plugs • Carrier material in catalytic converter (development since 1970) requirements: - large surface, - temperature stability and thermal shock tolerance - resistance against weathering base material: cordierite or iolite (Mg2Al4Si5O18) • Ceramic sensors: the most important sensors used in cars: - gas composition, - pressure, - temperature, - speed, - voltage, - ignition position.

39. Eg. pressure sensor: ceramic is a capacitive element, aluminum oxide base. Why it is ceramics? → high thermal stability Piezoelectric materials: Pb-Zr titanate (dynamic pressure measurement in combustion chamber) Oxygen sensor: check O2--fuel ratio, material: TiO2, operating principle: resistometry.