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Summaries on ceramics taken from Material science and Engineering by William D. Callister Jr & David G. Rethwisch...

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Material science Properties and applications of Ceramics

Background Ceramic materials can be seen in our everyday lives from our cookware to our cars – much more today than in the past. These compounds’ (which often all share a common element - oxygen) functions have expanded and grown exponentially over the decades. This summery will highlight some very important characteristics and properties of ceramics. A quick method of analysing ceramics and their performance, particularly at high temperatures, is the use of phase diagrams. The type of the information that can be extracted from these diagrams for a specific compound is: the current physical phase of the compound (solid, liquid etc.), the compounds composition at a specific phase, and the percentages of the phases at equilibrium.

Mechanical Properties 1. Brittle Fractures of Ceramics When a force is applied during a bending test on a ceramic compound at room temperature – fracture often occurs before permanent deformation can take place. The knowledge of this property is important – so as to be able to conduct a failure analysis. Information regarding the causes and results of fracture is acquired so as to prevent future incidences. A specific method of such is called fractographic analysis of the fracture surface. This may reveal the position and source of the defect generating cracks in the compound. The fracture process is described as the formation and propagation of cracks through the compounds cross section in a direction perpendicular to the applied force. (https://science.sciencemag.org/content/sci/282/5392/1275/F1.medium.gif)

The growth of the cracks through the compound can be described in two ways: • Trans-angular: this occurs when the cracks spread through the grains of the compound. • Inter-angular: This occurs when he cracks spread along the boundaries of the compound. It is important to note that although ceramics are indeed very brittle theoretically – however through practical experimentation it can be seen that the fracture strength is smaller in real life. This is due to the fact that in reality these compounds contain tiny imperfections that act as tress raisers and thus amplify the magnitude of the applied force. Due to the fact that this occurs in the elastic region – no mechanism exists such as plastic deformation to slow down this process. Examples of said imperfections are microcracks, pores and other inclusions generally caused by moisture or pollutants. The opposite is true for ceramic materials exposed to compressive forces as there is no amplification of the stresses thus the applied force. For this reason, the strength of the ceramic is dramatically increased by applying a compressive force.

A ceramic materials ability to resist fracture is defined by KIc (plane stress fracture toughness): 𝐾𝐼𝑐 = 𝑌𝜎√𝜋𝑎 Where Y is a constant, σ is the applied normal stress and 𝑎 is the length of a surface crack. A specific type of fracture known as static fatigue or delayed fracture occurs due to the presence of moisture in the atmosphere. It is the process whereby the slow propagation of cracks occurs within the compound. 2. Stress-Strain Behaviour Flexural Strength The strength of ceramics is not tested with a generic tensile test due to a few impractical reasons such as the brittle nature of ceramics under tensile conditions. Rather, the strength of a ceramic material is tested with a transverse bending test.

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The maximum stress is then computed using the maximum bending moment which can be found by examining the internal bending moment diagram: 𝜎 = 𝑠𝑡𝑟𝑒𝑠𝑠 =

𝑀𝑐 𝐼

Elastic Behaviour A linear relationship exists between stress and strain as seen from the flexure test. It is also reiterated that no material undergoes plastic deformation before fracture occurs. 3. Mechanisms of Plastic Deformation Crystalline Ceramics • Plastic deformation occurs by the motion of dislocations/line defects. (as do metals) • For predominantly ionic and highly covalent bonded ceramic crystal structure slip is difficult thus dislocation structures are complex. That being said these structures are brittle.

Non-crystalline Ceramics (No regular atomic structure) • Deform by viscous flow much like the deformation of liquids. • Defining property: Viscosity (Pa.s). The viscosities are very high for Noncrystalline Ceramics • Rate of deformation is proportional to applied stress.

4. Miscellaneous Mechanical Considerations Po ro sity

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All residual pores left after heat treatment will have a damaging effect on both the strength and elastic properties. Strength: Pores reduce the cross sectional area of the compound over which a load is applied. This intensifies the stress by several factors. Pores also act as stress concentrators. Elasticity: The modulus of elasticity depends on and is inversely proportional to volume fraction porosity. Hardness Hardness although difficult to measure for brittle materials – is one of the most desirable mechanical properties of ceramics as the hardest known materials are ceramics. Hardness is difficult to measure as the instrumentation used to measure hardness (Indenters) need to be forced into the material. Inaccurate results are a result of excessive cracks. Conclusions: • Hardness decreases with increasing applied force • Reaches a constant hardness after some time. This constant varies from ceramic to ceramic. Creep Stresses due to compressive forces and high temperatures.

Types and Applications of Ceramics Glass –Ceramics Glass ceramics (a fine grain polycrystalline) are produced through a process called crystallization whereby inorganic glass is transformed from a non-crystalline state to a crystalline one. This process involves a phase change. Properties • • • • •

High mechanical strengths (relatively) Low coefficients of thermal expansion (𝛼) Good high temperature capabilities Good dielectric properties Good biological compatibility

Applications • • • •

Ovenware Tableware Range tops Electrical insulators

Clay Products The most utilized ceramic is clay. It is naturally occurring, thus economical. Most clay products undergo the same process after extraction. It is mixed with water and is shaped into a desired form. It is then dried and heated (fired) at high temperatures to advance its mechanical strength. Structural Clay • E.g. Bricks, tiles, sewer pipes

Whiteware (colour change to white after firing) • E.g. Porcelain, china, pottery

Refractories Refractories are another popular type of ceramics. The word directly means “able to resist heat”. This property alone makes one understand why it is so widespread. Its performance relies heavily on its composition. Properties: • •

Capability to resist high temperatures = able to provide thermal insulation. Remains inert when exposed to brutal environs.

Applications: • • • •

Bricks (Porosity must therefore be carefully controlled) Furnace linings Glass manufacturing Power generation

Types of refractories Fireclay (bricks) Used chiefly to restrain searing environments in furnace constructions as well as insulate buildings and or other structures from heat (or a lack thereof).

Silica (acid) Special property: able to resist high temperatures.

Basic Special property: Particularly resilient to attack by certain slags. Used in steel making furnaces.

Special Used for electrical resistance heating. Costly.

Abrasives Abrasives are used to physically remove other softer substances by processes of cutting, polishing or grinding etc. therefore a very important property for this group of ceramics is hardness and a high

level of toughness as well as some form of resistance to heat from then frictional forces that arise. Abrasives are used in many different physical forms for the specific cutting use. E.g. Diamonds. Cements E.g. cement, lime. These are classified as inorganic elements. They all undergo the same physical process so that we may utilize them. They must be mixed with water in a certain proportion – they then form an adhesive that sets and hardens into a desired shape. This occurs due to chemical reactions that occur within the cement particles. An example of this is Portland cement. The chemical reaction it undergoes is hydration. Carbons Diamonds Properties: inert, resistant to weathering and corrosion, high thermal conductivity Special property: hardness – extremely strong interatomic bonds & low coefficient of friction.

Graphite Special property: anisotropic, high conductivity due to the looseness of delocalized electrons. Properties: soft Uses: pencils & battery electrons.

Carbon Fibres Special property: high strength

5. Fabrication and processing of ceramics Glass and glass-ceramics The mined materials are melted and manipulated. Thus, the temperature-viscosity relationship is an important thought. Processes include blowing, pressing, drawing, and fibre forming. After these processes the glass is tampered to improve mechanical properties. Clay Processes: addition of water and can be worked. Clay particles also melt over a range of temperatures this it can be fired without it completely melting. Powder pressing Formation of ceramics by high pressures and temperatures. Tape Casting Thin sheets of ceramics are formed thus dried and fired....