MSE 351 Engineering Ceramics I

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1 Kwame Nkrumah University of Science & Technology, Kumasi, Ghana Ceramic Processing Oxide for ceramics have high melting points. MSE 351 Engineering Ceramics I Ing. Anthony Andrews (PhD) Department of Materials Engineering Faculty of Mechanical and Chemical Engineering College of Engineering Website: Oxide particles are bonded through a process known as sintering or powder metallurgy. Phase diagrams are used to determine the sintering temperature. Example: to produce mullite (Al 2 O 3 + SiO 2 ): mix 3 molecular weights of Al 2 O molecular weight SiO 2. Ceramic Processing Ceramic Materials Different kinds of technical ceramics Traditional Ceramics Advanced Ceramics Natural materials Large amounts Low costs Chemical processes Specific properties Not always visible Bricks Tiles Sanitary Electronics, Medical, Membranes/Filters, Engine parts Different kinds of technical ceramics Metallic Ceramic coating springs 1

2 Two possibilities improving the strength: 1. Reduction of the defect size Improvement of the technology 2. Increased fracture toughness Microstructural design Strength is determined by the largest defect in the sample under stress Mostly defects from the raw powder or from the processing Strength Improvement Reduction of the defect size (improvement of the technology) Improved processing and shaping Homogeneous green bodies, preventing of residual agglomerates, cracks Lower sintering temperatures possible Lower grain size Ceramic Processing The higher the requirements of a material, the more stringent are the requirements on its microstructure, i.e. - the more accurately must its microstructure be established. - the more accurately the processing must carried out. Types of Ceramics and Their Processing Ceramic materials divide into three categories: 1. Traditional ceramics particulate processing 2. New ceramics particulate processing 3. Glasses solidification processing Principles of Ceramic Processing Particulate processes for traditional and new ceramics as well as certain composite materials are covered in this slide set Solidification processes for glasses are covered in the Second Semester 2

3 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, i.e. Al 2 O 3, cbn, TiC, WC Products: cutting tools, artificial bones, nuclear fuels, substrates for electronic circuits Composition of clay products 1. Clay Chemistry = hydrous aluminum silicates 2. Water Creates clay-water mixture with good plasticity for shaping Starting material for these products is powder Structure of kaolinite clay Composition of clay products 3. Non-plastic raw materials finely ground quartz 4. Other ingredients Fluxes - feldspar Wetting agents A typical porcelain might contain approximately 50% clay, 25% quartz, and 25% feldspar. Traditional Ceramic Processing Preparation of raw materials Most shaping processes require the starting material to be a plastic paste This paste is comprised of fine ceramic powders mixed with water (hydroplasticity) 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 Comminution Reducing particle size in ceramics processing by using mechanical energy. Most effective on brittle materials such as cement and metallic ores Two types of comminution operations: 1. Crushing 2. Grinding 3

4 Crushing Reduction of large lumps from the mine to smaller sizes for subsequent further reduction Jaw Crusher Large jaw toggles back and forth to crush lumps against a hard, rigid surface Several stages may be required Crushing of minerals is accomplished by Compression against rigid surfaces or Impact against surfaces Roll Crusher Ceramic lumps are squeezed between rotating rolls Grinding 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 Ball Mill Roller Mill 4

5 Traditional Ceramic Processing 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 Effect of Water Content in Shaping Processes Slip Casting Slip, is poured into porous plaster of paris 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 Solid casting Drain Casting Plastic Forming Methods 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 Hand molding Mechanized methods => use clay with less water Jiggering, plastic pressing and extrusion Implication in drying shrinkage?? 5

6 Hand Molding Jiggering Fabrication of ceramic product by manipulating plastic clay into desired geometry Hand molding - a mold is used to define portions of the part geometry Hand throwing on a potter's wheel Plastic Pressing Plastic clay slug is pressed between upper and lower molds Molds are made of porous material Mold sections are then opened, using positive air pressure to prevent sticking of part in the mold Extrusion Equipment utilizes a screw-type action to assist in mixing the clay and pushing it through die opening Products: hollow bricks, shaped tiles, drain pipes, tubes, and insulators Also used to make starting slugs for jiggering and plastic pressing Advantages?? Semi-dry Pressing Dry Pressing Process sequence similar to semi-dry pressing Dies made of hardened tool steel or cemented carbide No drying shrinkage occurs Products: bathroom tile, electrical insulators, refractory brick, and other simple geometries 6

7 Clay Volume vs. Water Content Clay Volume vs. Water Content Water plays an important role in traditional ceramics shaping processes Shrinkage is a problem during drying Volume of clay as a function of water content Drying Firing of Traditional Ceramics Performed in a furnace called a kiln Bonds are developed between ceramic grains In firing of traditional ceramics, a glassy phase forms among the crystals that acts as a binder Firing Temperature Profile Densification During Firing 7

8 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: 1. Fire the piece once before glazing to harden the body of the piece 2. Apply glaze 3. Fire the piece a second time to harden glaze 8