Processing of Ceramic Materials

Size: px
Start display at page:

Download "Processing of Ceramic Materials"

Transcription

1 MME 131: Introduction to Metallurgy and Materials Lecture 25 Processing of Ceramic Materials AKMB Rashid Professor, MME Dept BUET, Dhaka Today s Topics Glass Forming Processes Ceramics Forming Processes Cementation Processes

2 General Fabrication Routes Glass Forming Processes Ceramic Particulate Forming Processes Cementation Processes Pressing Blowing Drawing Fibre Forming Powder Forming Hydroplastic Forming Slip Casting Tape Casting Hot Uniaxial Isostatic The brittleness of ceramics precludes deformation! Drying Firing 3/28 Glasses Forming Processes To obtain good optical transparency the glass must be homogeneous and pore free! Homogeneity is achieved by complete melting and mixing of the raw ingredients. Proper adjustment of the viscosity of the molten material is required in order to eliminated gas bubbles generated to obtain minimum porosity. 4/28

3 Viscosity - temperature characteristics of glass controls the forming processes Viscosity decreases with increasing temperature Impurities or additives lower Tdeform The formability of a glass is tailored to a large degree by its composition The viscosity - temperature relation for glass forming processes 5/28 Commercial melting and forming of glass Batching of the raw materials Mixing of the batch Water typically added to reduce dusting Melting the batch to a homogenous viscous liquid Conditioning (cooling) the melt to the forming temperature Forming operation Float glass Container glass Fiber glass Specialty glass Annealing Inspection 6/28

4 Glass Forming Pressing softened glass (a.k.a. gob) For thick-walled pieces (e.g., plates and dishes) Pressure application in a graphite-coated cast iron mould mould heated to ensure an even surface finish Pressed Glass Processing 7/28 Glass Forming Press and Blow Either manual (for artworks) of fully automated (jars, bottles, light bulbs, etc.) From raw gob of glass, a parison (temporary shape), is formed by mechanical pressing in a mould Gob Parison mould Pressing Operation The piece is then inserted into a finishing or blow mould to finalize the shape Suspended Parison Finishing Mould Compressed Air The press-and-blow technique for producing a glass bottle

5 Glass Forming Drawing Processes Used to form long form such as sheet, rod, tubing, and fibers, which has constant cross section Usually the liquid glass is drawn between the rolls (Foucault/Pittsburg /Libby-Ownes process) to form sheet Plate glass is flat glass that has been ground and polished to produce two perfectly plane and parallel faces with a high quality optical finish. The surface of a flat glass sheet is flattened by grinding it between two cast iron wheels with sand abrasive and water lubricant. 9/28 Flatness and surface finish may be improved significantly by floating the sheet on a bath of molten tin at an elevated temperature (Float process) Developed by Pilkington in 1959 in the UK, the float process revolutionized the flat-glass industry. Over 90% of the world s window glass is produced now using the float process. Float Glass Process 10/28

6 Glass Forming Fibre Drawing Uses sophisticated drawing operations Molten glass is contained in a platinum heating chamber; fibres are formed by drawing the molten glass through many small orifices at the chamber base The glass viscosity is controlled by chamber and orifice temperatures Glass Fibre Process 11/28 Heat Treating Glass Heat Treating Glasses Annealing Internal stresses (thermal stresses) are introduced when the glass is cooled from an elevated temperature a result of the difference in cooling rate and thermal contraction between the surface and interior regions. Thermal stresses weaken the material and may lead to fracture Thermal shock. Thermal stresses can be reduced by cooling the piece at a sufficiently slow cooling rate, then followed by an annealing heat treatment in which the glassware is heated to the annealing point, and then slowly cooled to room temperature. Heat Treating Glass Tempering The strength of a glass piece is enhanced by intentionally inducing compressive residual surface stresses When interior cools and contracts it draws the exterior into compression. Compressive stresses on the surface with tensile stresses at interior regions formed.

7 Thermal Tempering before Before cooling surface Surface cooling further Further cooled cooling hot cooler hot cooler compression tension compression Result: Surface crack growth is suppressed Chemical Tempering Cations with large ionic radius are diffused into the surface. This strains the lattice inducing compressive strains and stresses. 13/28 Ceramics Forming Processes Particulate Forming Processes Ceramic powders Additions (liquids, organic additives) Adaptation of the system to the shaping process (grinding, mixing, dispersion, granulation, etc.) Shaping Drying (elimination of organic additives) Firing (obtaining of sintered product) General flow chart of the manufacturing of ceramic products 14/28

8 Ceramic Forming - Hydroplastic Forming Producing shapes from a mixture of powders and additives that are deformable under pressure. Mixture include: Traditional Ceramics Clay and water, defloculant, wetting agent, lubricant Engineering Ceramics Non-clay materials (pure oxides, nitrides, carbides etc), vol.% organic additives with/without water to provide plasticity Removal of organic materials prior firing is a major problem Water-clay system substantial shrinkage occurs during drying, increasing risk of shrinkage cracks Non-clay system formation of flaw-free green part is a problem and extraction of organics is also problematical > Too rapid extraction cracking, bloating, distortion > Inadequate extraction cracking, bloating, distortion during densification. Clay minerals, when mixed with water, become highly plastic and can be moulded without cracking Green products have extremely low strength; the consistency (water-clay ratio) is controlled so that the formed ware can be maintained its shape during handling and drying The raw materials are wet ball milled, screened to remove water and form a plastic mass The plastic mass is forced through the die using an auger and air is removed using vacuum to enhance density The cake thus formed is given the shape using many of the forming processes e.g., extrusion, injection/compression moulding, jiggering, etc. The formed ware are then dried and sintered to obtain the finished product Common products formed in this way include: brick, tiles, furnace tube (extrusion); turbine rotor blade, combustion nozzle (moulding); cooking ware, electrical porcelain, refractories (jiggering) 16/28

9 Mass Ceramic Forming - Slip Casting A slip is a suspension of clay-based materials in water, prepared by wet ball milling the ingredients The slip is poured in a porous mould (made of plaster of Paris), the water is absorbed by the mould, leaving behind a solid layer on the mould wall the thickness of which depends on the time The process may continue until the entire mould cavity becomes solid (solid casting), or draining the remaining slip when a solid of sufficient thickness is formed (hollow casting) As the casting dries and shrinks, it will pull away from the mould wall so that the green product is removed easily. The formed ware are then dried and sintered to obtain the finished product The nature of the slip (high sp. gravity, high fluidity) and the mould (porosity, moisture content) are extremely important Common products formed in this way include: sanitaryware, ceramic vases, art objects

10 Ceramic Forming - Tape Casting This process is used to make flat ceramic sheets having a thickness up to about 1 mm. The process was developed during the 1940s for capacitor dielectrics. The production of ceramic capacitors is still one of the most important applications of tape casting. A slurry containing a powdered ceramic together with a complex mixture of solvents and binders is spread onto a moving polymer sheet. The thickness of the deposited layer is determined by the height of the doctor blade above the polymer sheet. Extensive shrinkage occurs during drying and firing of the tape because of the large volume fraction of organics in the slurry.

11 Ceramic Forming - Particulate Forming The powdered raw materials are mixed with suitable binders (e.g., polyvinyl alcohol, PVA) and then pressed into green compacts inside a suitable die The green compacts contain lots of porosity between the powders The green compacts are then dried and sintered to obtain the finished product During sintering, diffusion of atoms at the boundary of particles takes place and the powdered particles are diffusion bonded and the degree of porosity is reduced to increase density of the product The quality of the finished products depends on time and temperature of the sintering cycle Almost all engineering ceramics are formed by particulate forming. Common traditional ceramics formed by this method are tiles and refractory bricks. 21/28 Filling mould Compaction Green part ejected, then sintered Uniaxial Compaction 22/28

12 Drying Before drying, clay particles are virtually surrounded by and separated from one another by a thin film of water. As drying progresses and water is removed, the interparticle separation decreases, and the clay piece shrinks. It is critical to control the rate of water removal. If the rate of evaporation at the surface is greater than the rate of diffusion of water molecules to the surface, the surface will dry (shrink) more rapidly than the interior, with a high probability of the formation of defects. The rate of surface evaporation can be controlled by temperature, humidity, and rate of airflow. Factors that influence shrinkage: body thickness, water content, clay particle size. 23/28 Firing After drying, a clay piece is usually fired at temperature between C. During firing, the density of the piece is further increased, so as the strength. Vitrification (formation of a liquid glass) occurs during firing that flows into and fills some of the pore volume. The degree of vitrification depends on: firing temperature, time, and composition. Fluxing agents are added to decrease the temperature at which vitrification happens. A glassy matrix is formed upon cooling, which gives rise to a dense, strong body. The final microstructure: the vitrified phase, unreacted particles (e.g. quartz), and some porosity. 24/28

13 Strength, durability, and density of the clay piece are enhanced as the degree of vitrification increases. Firing temperature determines the extent of vitrification higher the temperature, more vitrification. To achieve optically translucent (i.e. high vitrification), firing takes place at very high temperatures. Complete vitrification is avoided during firing since the body might become too soft and might collapse. 25/28 Cementation Processes Portland Cement Controlled mixture of clay (Al 2 O 3.2SiO 2.2H 2 O) and chalk (CaCO 3 ) is fired in a kiln at 1500 C Firing gives three products: Clay + Chalk = C 3 A + C 3 S + C 2 S C CaO S SiO 2 A Al 2 O 3 H H 2 O When cement is mixed with water, hydrated cement paste (h.c.p.) is formed All cements harden by reaction, not by drying 26/28

14 First Reaction C 3 A + 6H = C 3 AH 6 + Heat occurs in ~4 hours and set the cement Second Set of Reaction 2C 2 S + 4H = C 3 S 2 H 3 + CH + Heat 2C 3 S + 6H = C 3 S 2 H 3 + 3CH + Heat starts in ~10 hours and completes in ~100 days or more to harden the cement Tobomorite gel (C 3 S 2 H 3 ) is the main bonding material which occupies ~70% of the structure Hardening of Portland cement: (a) Setting and hardening reaction (b) Heat evolution 27/28 Next Class MME 131: Lecture 32 Polymeric Materials