Materials Science Ceramics and glasses

Glasses and ceramics - applications Facing materials  Non-load bearing  Used for appearance (texture / colour) Load bearing products (compressive only)  Bricks  Used for low cost, insulating, fireproof Paving units  Abrasion resistance Roofing tiles Chemically resistant product  Sewers and piping (although now often plastic)  Industrial chimneys

Properties of glasses and ceramics Brittle solids No means for plastic deformation, as dislocations cannot relieve stress Can be high strength, but dominated by microstructure High stiffness Low toughness and elongation at failure (poor ductility) Consider how these properties compare to steel…

Metals: Disl. motion easier. -non-directional bonding -close-packed directions for slip. electron cloudion cores Covalent Ceramics (Si, diamond): Motion hard. -directional (angular) bonding Ionic Ceramics (NaCl): Motion hard. -need to avoid ++ and -- neighbors. Dislocations in materials

Ceramic bonding Bonding:  Mostly ionic, some covalent.  % ionic character increases with difference in electronegativity. Large vs small ionic bond character:

Fundamentals: The Ugly From fracture mechanics…  E↓ due to pores →  f ↓.   ↓ easier crack path →  f ↓. a - Critical crack length is of the order of tens of microns.  We can improve processing (reduce defects).  But sensitive to faults in microstructure. Microstructure dominates.

Fundamentals: The Ugly Polycrystalline microstructure with inherent porosity due to sintering.

Ceramics - classifications Broadly classed as:  Traditional – clay bodies  Advanced – using engineered ceramics  Glasses – not always ‘ceramic’, but brittle solids Different taxonomies (esp. EU vs USA)

Glasses - amorphous materials atoms pack in periodic, 3D arrays typical of: Crystalline materials... -metals -many ceramics -some polymers atoms have no periodic packing occurs for: Noncrystalline materials... -complex structures -rapid cooling crystalline SiO 2 noncrystalline SiO 2 "Amorphous" = Noncrystalline

Glass – mechanical properties Glasses are brittle  Low toughness / low resistance to crack propagation  Amorphous – limited dislocation movement as there is no periodic structure. very small toughness (unreinforced polymers) Engineering tensile strain,  Engineering tensile stress,  small toughness (ceramics) large toughness (metals)

Glass – mechanical properties Compressive strength ~1000 MPa Modulus of rupture ~50 MPa  MOR is like tensile strength, but determined by bending Elastic modulus ~ 75 GPa (similar to Al)

The structure of glass (SiO 2 ) Most common forms of glass are silica based Same base unit can lead to very different materials:  Silica – Amorphous  Silicon – Crystalline  Silicone - Polymeric Quartz is crystalline SiO 2 (sand!)

The structure of glass (SiO 2 ) Glass is amorphous Amorphous structure occurs by adding impurities  (Na +,Mg 2+,Ca 2+, Al 3+ ) Impurities interfere with formation of crystalline structure. (soda glass)

Glass properties Amorphous materials solidify differently to crystalline solids, which have a precise T m On cooling, the glass melt becomes increasingly viscous There is no definitive temperature of liquid to solid transformation, but a glass transition temperature can be identified – T g. Glasses: do not crystallize spec. vol. varies smoothly with T Glass transition temp, T g Crystalline materials: crystallize at melting temp, T m have abrupt change in spec vol. at T m

Engineering glasses Important glasses based on SiO 2  Similar tonnage to Al  Amorphous (no long range order of molecules)  Common window glass  Thermal resistant borosilicate glass (Pyrex) – has a lower coefficient of thermal expansion Advantages  Optical transparency! From windows to optical fibre  Corrosion resistance (a Tale of Tea and Wine…)  Electrical and thermal insulator  Ease of fabrication (low softening point)

Heat treatment of glass Annealing  removes internal stress caused by uneven cooling Tempering  puts surface of glass part into compression  suppresses growth of cracks from surface scratches

Strengthening of glass Enhance strength by inducing compressive surface stresses (tempering)  Heat above T g, just below T s - then cool with jets of air  Surface cools more rapidly and becomes rigid  Interior continues to cool and contract, drawing in the rigid surface  Surface compressive stresses must now be overcome to cause tensile failure  Used for: Large doors, windshields, eyeglasses, safety glass Chemical strengthening  Replace Na+ ions on glass surface with larger K+ ions Reinforcement – steel mesh / plastic laminates

Ceramics - properties Good:  Thermal stability  Chemical / Environmental resistance  High compressive strength  High stiffness  Excellent wear properties  Unusual properties are possible (sensors, etc.) Poor:  Ductility and toughness- brittle!  Can be difficult to manufacture  High density

Ceramics – civil applications From facades, to structural components Can still be damaged by the environment… freeze-thaw

Natural ceramics - stone Sedimentary rocks (Sandstone)  Contains silica and calcium carbonate  Precipitated from solution in ground water rather than melting  Tends to be porous Igneous rock (Granite)  Similar to SiO 2 – Al 2 O 3 alloys  Subjected to enough heat to melt the two components and fuse them into a dense solid

Traditional ceramics Generally formed from local clays  Cheap and abundant  Easily formed and shaped  May have some additives (grits, sand,etc.) Used in structural clay products  Bricks, tiles, sewer pipes Whitewares  Crockery, plumbing fixtures, etc.

Clay characteristics Aluminosilicates (Al 2 O 3 and SiO 2 ) and chemically bound water Plastic when water is added, as weak VdW bonds shear easily Complex crystal structures e.g. Kaolinite, water molecules fit between layer crystal sheet which provides plasticity This gives rise to hydroplastic forming Kaolinite Clay:

Fabrication of clay bodies Raw materials (clay) mined > ground > milled and sieved, to create desired particle size Forming techniques are normally ‘hydroplastic’  Mix with water to make a plastic and pliable solid  Then can mould or extrude the material  Dry and fire the component Hydroplastic forming: extrude the slip (e.g., into a pipe)

Fabrication of clay bodies Slip casting  Slip is formed with high water content (liquid clay-water)  Poured into porous mould (plaster of Paris)  Water is absorbed away into the mould leaving a solid layer Complete drying (solid cast) Incomplete drying (pour out excess, hollow, drain casting)

Traditional ceramics- drying and firing Clay contains significant liquid – low strength Dry to remove water, avoiding shrinkage cracking and allows handling (<80°C) Layer size and spacing decreases for the clay platelets.

Traditional ceramics- drying and firing Firing ( °C). Density is increased and mechanical properties increased, as green body is transformed into a fired solid. Vitrification – formation of liquid glass (normally from SiO 2 particles), which flows into pores and produces dense ceramic Mullite 3Al 2 O 3 2SiO 2

Ceramics - classifications Broadly classed as:  Traditional – clay bodies  Advanced – using engineered ceramics  Glasses – not always ‘ceramic’, but brittle solids Different taxonomies (esp. EU vs USA)

Advanced ceramics – properties Stiffness normally exceeds metals (ionic bonding) Light elements (C, O, Al) and not close packed – generally low density Hard and wear resistant – difficult to plastically deform Brittle / low toughness and low ductility Poor strength in tension

Advanced ceramics – applications High temperature engines (thermal), bearings (wear), spark plugs (electrical), bones (chemical) – even golf clubs

Advanced ceramics – applications High T m – high operating temperatures (increased fuel efficiency in engines) Wear resistance (high hardness – useful in bearing surfaces or abrasives) Corrosion resistance Electrical and thermal insulators Low density (in general)  Silicon carbide SiC (3.2 g/cm 3 )  Alumina Al 2 O 3 (4.0 g/cm 3 )  Zirconia ZrO 2 (5.6 g/cm 3 )  Steel (~7.7 g/cm 3 )

Advanced ceramics – processing High purity powder (often chemically derived) Powder diameter controlled (~µm to nm) Polymer binders and plasticizers added Powder formed into desired shape  Uniaxial press – pressure in one axis  Isostatic press – hydrostatic pressure  Slip cast (often added water and dispersants)

Advanced ceramics – processing Uniaxial pressing  Ceramic powder with binder placed into a ‘die’ and pressure applied via a ‘ram’  Simple / rapid / cheap  Automated  Limited to simple shapes

Advanced ceramics – processing Isostatic pressing  Isostatic (same magnitude in all directions) - pressure  Powder with binder placed in a rubber preform  Pressure applied by a fluid (hydraulic oil)  Time consuming / expensive / labour intensive  Complex shapes (e.g. spark plugs)

Advanced ceramics – sintering Fine (micron) powder has enormous high surface area This surface energy helps drive sintering At high temperatures (~0.6 T m ) sufficient diffusion occurs such that the particles sinter and form necks which grow, reducing surface energy and forming dense solids 20 – 30 vol % shrinkage, so a final machining stage may be necessary

The weak link in ceramics… Flaws happen! Porosity Inherent grain boundaries / triple junctions Cracks within grains Surface flaws – scratches and other defects

Fracture strength of ceramics In ductile materials (metals), ease of dislocation motion determines yield strength Lack of plastic deformation in brittle solids  Tensile strength is determined by flaw size in ceramics and glasses  Compressive strength is normally 15× tensile strength  Atomically sharp cracks Fracture mechanics…

Strength of ceramics Tensile strength is limited by the largest flaw in the material Not possible to be certain when a component will fail We can assign a failure probability (consider acceptable risk) There is a statistical distribution of strengths, determined by the range of flaw sizes  This can be modelled by a ‘Weibull’ distribution  Ceramics m~10  Steel m~100

Improving strength distribution There are different methods to reduce the distribution of strengths  Proof testing (remove weakest samples)  Introduce defects of a known dimension (e.g. by surface grinding) Consider the case of the strength of a glass whisker…

Improving strength in ceramics Reduce ceramic flaw size  Use fine powders  Quality control / clean room processing  Uniform / controlled grain size  Limit porosity (high density) Improve toughness  Addition of fibre reinforcement (expensive)  Transformation toughening  Tempering (for glasses)

Time dependent strength Account for decrease in strength with time with engineering design equation   – Strength initially determined at time t   TS – Measured strength at time t test  n – time exponent is a material property

Summary Introduced glasses and ceramics (traditional and advanced) Origins of brittle nature (atomic bonding) Processing of these materials Brittle behaviour and governing fracture equations Toughening mechanisms Time dependent strength