Ceramic processing

ceramic process slides

1. ENGINEERING CERAMICS MET 203 Maxwell Owusu [BSc., MSc.] [email protected] / [phone]

2. PROCESSING OF CERAMICS Processing of Traditional Ceramics Processing of New Ceramics

3. Types of Ceramics and Their Processing Ceramic materials divide into three categories: Traditional ceramics – particulate processing New ceramics – particulate processing Glasses – solidification processing Particulate processes for traditional and new ceramics as well as certain composite materials are covered in this slide set

4. Ceramics Processing Overview Traditional ceramics are made from minerals occurring in nature Products: pottery, porcelain, bricks, and cement New ceramics are made from synthetically produced raw materials Products: cutting tools, artificial bones, nuclear fuels, substrates for electronic circuits Starting material for these products is powder

5. Ceramics Processing Overview For traditional ceramics Powders are mixed with water to bind them together and achieve proper consistency for shaping For new ceramics Substances other than water are used as binders during shaping After shaping, green part is fired (sintered) Function is the same as in PM - to effect a solid state reaction that bonds the particles into a hard ma

6. Processing Overview for Traditional Ceramics Condition of powders and part during (1) preparation of raw materials, (2) shaping, (3) drying, and (4) firing

7. Preparation of Raw Materials in Traditional Ceramics Processing Most shaping processes for traditional ceramics require the starting material to be a plastic paste This paste is comprised of fine ceramic powders mixed with water The starting raw ceramic material usually occurs in nature as rocky lumps Purpose of the preparation step is to reduce the rocky lumps to powder

8. Comminution Reducing particle size in ceramics processing by using mechanical energy in various forms such as impact, compression, and attrition Comminution techniques are most effective on brittle materials such as cement and metallic ores Two general types of comminution operations: Crushing Grinding

9. Crushing Reduction of large lumps from the mine to smaller sizes for subsequent further reduction Several stages may be required (e.g., primary crushing, secondary crushing) Reduction ratio in each stage is 3 to 6 times Crushing of minerals is accomplished by Compression against rigid surfaces or Impact against surfaces

10. Grinding In the context of comminution, grinding refers to the reduction of small pieces after crushing to fine powder Accomplished by abrasion, impact, and/or compaction by hard media such as balls or rolls Examples of grinding include: Ball mill Roller mill Impact grinding

11. Ingredients of Ceramic Paste Main Clay Chemistry = hydrous aluminum silicates Usually the main ingredient because of ideal forming characteristics when mixed with water Water Creates clay-water mixture with good plasticity for shaping Additional Ingredients of Ceramic Paste Non‑plastic raw materials Such as alumina and silica Purpose is to reduce shrinkage in drying and firing but also reduces plasticity during forming Other ingredients Such as fluxes that melt (vitrify) during firing and promote sintering Wetting agents to improve mixing of ingredients

12. Shaping Processes Slip casting The clay-water mixture is a slurry Plastic forming methods The clay is plastic Semi‑dry pressing The clay is moist but has low plasticity Dry pressing The clay is basically dry (less than 5% water) and has no plasticity

13. Slip Casting Suspension of ceramic powders in water, called a slip , is poured into porous plaster of mold Water from the mix is absorbed into the plaster to form a firm layer of clay at the mold surface Slip composition is 25% to 40% water Two principal variations: Drain casting - mold is inverted to drain excess slip after semi‑solid layer has formed Solid casting - enough time is allowed for full body to become firm

14. Drain Casting (1) Slip is poured into mold cavity, (2) water is absorbed into plaster mold to form a firm layer, (3) excess slip is poured out, and (4) part is removed from mold

15. Overview of Plastic Forming Starting mixture must have a plastic consistency Composition 15% to 25% water Variety of manual and mechanized methods Manual methods use clay with more water because it is more easily formed Mechanized methods generally use clay with less water so starting clay is tough Plastic Forming Methods Hand modeling (manual method) Jiggering (mechanized method) Plastic pressing (mechanized method) Extrusion (mechanized method)

16. Dry Pressing Process sequence similar to semi‑dry pressing Except water content of starting mix is < 5% Dies made of hardened tool steel or cemented carbide to reduce wear due to abrasive dry clay No drying shrinkage occurs Drying time is eliminated and good accuracy is achieved in final product Products: bathroom tile, electrical insulators, refractory brick, and other simple geometries

17. Clay Volume vs. Water Content Water plays an important role in most of the traditional ceramics shaping processes Thereafter, it has no purpose and must be removed from the clay piece before firing Shrinkage is a problem during drying because water contributes volume to the piece, and the volume is reduced when it is removed

18. Drying Drying process occurs in two stages Stage 1 - Drying rate is rapid as water evaporates from surface into surrounding air and interior water migrates by capillary action to surface to replace it This is when volumetric shrinkage occurs, with the risk of warping and cracking Stage 2 - Moisture content has been reduced to where the ceramic grains are in contact Little or no further volumetric shrinkage

19. Firing of Traditional Ceramics Heat treatment process to sinter the ceramic material Performed in a furnace called a kiln Bonds are developed between ceramic grains This is accompanied by densification and reduction of porosity Additional shrinkage occurs in the polycrystalline material in addition to that which has already occurred in drying In firing of traditional ceramics, a glassy phase forms among the crystals that acts as a binder

20. Glazing Application of a ceramic surface coating to make the piece more impervious to water and enhance its appearance Usual processing sequence with glazed ware: Fire the piece once before glazing to harden the body of the piece Apply glaze Fire the piece a second time to harden glaze

21. Processing of New Ceramics Manufacturing sequence for new ceramics can be summarized in the following steps: Preparation of starting materials Shaping Sintering Finishing While the sequence is nearly the same as for the traditional ceramics, the details are often quite different

22. Preparation of Starting Materials Strength requirements are usually much greater for new ceramics than for traditional ceramics Starting powders must be smaller and more uniform in size and composition, since the strength of the resulting ceramic product is inversely related to grain size Greater control over the starting powders is required Powder preparation includes mechanical and chemical methods

23. Shaping of New Ceramics Many of the shaping processes are borrowed from powder metallurgy (PM) and traditional ceramics PM press and sinter methods have been adapted to the new ceramic materials And some of the traditional ceramics forming techniques are used to shape the new ceramics Slip casting Extrusion Dry pressing

24. Hot Pressing Similar to dry pressing Except it is carried out at elevated temperatures so sintering of the product is accomplished simultaneously with pressing Eliminates the need for a separate firing step Higher densities and finer grain size are obtained But die life is reduced by the hot abrasive particles against the die surfaces

25. Isostatic Pressing Uses hydrostatic pressure to compact the ceramic powders from all directions Avoids the problem of non-uniform density in the final product that is often observed in conventional uniaxial pressing Same process used in powder metallurgy

26. SINTERING OF CERAMICS DEFINITION Sintering commonly refers to processes involved in the heat treatment of powder compacts at elevated temperatures, where diffusional mass transport is appreciable. Successful sintering usually results in a dense polycrystalline solid. However, sintering can proceed only locally (i.e. at contact point of grains), without any appreciable change in the average overall density of a powder compact

27. A MODEL SKETCH

28. WHY CERAMICS HAVE TO BE SINTERED? Ceramic materials are sintered because Ceramics melt at high temperatures. As-solidified microstructures can not be modified through additional plastic deformation and recrystallisation due to brittleness of ceramics. The resulting coarse grains would act as fracture initiation sites. Low thermal conductivities of ceramics (<30-50 W/ mK ), in contrast to high thermal conductivity of metals (in the range 50-300 W/ mK ) cause large temperature gradients, and thus thermal stress and shock in melting-solidification of ceramics.

29. WHAT HAPPENS DURING SINTERING Increase of interparticle contact area with time Rounding-off of sharp angles and points of contact In most cases, the approach of particle centres and overall densification Decrease in volume of interconnected pores Continuing isolation of pores Grain growth and decrease in volume of isolated pores

30. SINTERING STAGES There are three stages of sintering. These are The initial stage The intermediate stage The final stage Various changes occur during each stage and the ceramic part becomes more dense with each stage.

31. INITIAL STAGE OF SINTERING During this stage various changes occur some of which are There is a local point of contact formation without any shrinkage which is accompanied by smoothing of the free surface of particles There is also neck formation at the contact point There is an increase in the relative density of the ceramic product to about 70% of the theoretical density.

32. THE INTERMEDIATE STAGE OF SINTERING There is neck growth at this stage Pores at this stage form arrays of interconnected cylindrical channels The centres of the particles approach one another with a resulting compact shrinkage At this stage shrinkage normally results in a densification to about 95% of the theoretical density.

33. THE FINAL STAGE OF SINTERING At this stage of sintering there is Isolation of pores with the density exceeding 93% Porosity is eliminated There is grain growth

34. STAGES OF SINTERING

35. SINTERING CATEGORIES Solid state sintering occurs when the powder compact is densified wholly in a solid state at the sintering temperature. Whereas liquid phase sintering occurs when a liquid phase is present in the powder compact during sintering. Transient liquid phase sintering is a combination of liquid phase sintering and solid state sintering. In this sintering technique a liquid phase forms in the compact at an early stage of sintering, but the liquid disappears as sintering proceeds and densification is completed in the solid state.

36. SINTERING VARIABLES The microstructure of a powder compact and its sinterability is dependent on certain variables. These variables can be categorized into two: Material variables Process variables

37. MATERIAL VARIABLES These include Chemical composition of the powder compact Powder size Powder shape Powder size distribution Degree of powder agglomeration The sinterability and compressibility of the powder compact are influenced by these variables

38. PROCESS VARIABLES These are mostly thermodynamic variables. These include Temperature Time Pressure Heating and cooling rate

39. SINTERING TEMP FOR SOME COMMON CERAMICS

40. ADVANTAGES OF SINTERING The parts produced have an excellent surface finish, and good dimensional accuracy. The porosity inherent in sintered components is useful for specialized application such as filters and bearings. Refractory materials which are impossible to shape using other methods can be fabricated by sintering with metals of lower melting points. A wide range of parts with special electrical and magnetic properties can be produced

41. CURRENT TRENDS Selective laser sintering (SLS) is a rapid process that allows to generate complex parts by solidifying successive layers of powder material on top of each other. Solidification is obtained by fusing or sintering selected areas of the successive powder layers using thermal energy supplied through a laser beam. SPS (spark plasma sintering)