Processing of Ceramics and Cermets

Jiggering

1. A wet clay slug is placed on a convex mold 2. A forming tool is pressed into the slug to provide the initial rough shape (operation: batting, workpiece: bat) 3. A heated jigger tool is used to impact the final contoured shape to the product by pressing the profile into the surface during rotation

Semi-dry Pressing

1. Deposit moist (10% - 15%) powder in die cavity 2. Press at high pressure 3. Open die sections and eject part

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 4. part is removed from mold

Ceramic metals divide into three categories:

1. Traditional Ceramics: particulate processing 2. New ceramics: particulate processing 3. Glasses: solidification processing

Cemented Carbides

A family of composite materials consisting of carbide ceramic particles imbedded in a metallic binder. Classified as metal matrix composites because the metallic binder is the matrix that holds the bulk material together. However the carbide particles constitute the largest proportion of the composite material. Normally between 80% and 95% by volume

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

New Ceramics

Based on simpler chemical compounds, such as oxides, carbides, and nitrides. Do not possess the plasticity and formability of traditional clay when mixed with water.

Sintering of New Ceramics

Because the plasticity needed to shape the new ceramics is not normally based on water, the drying step required for traditional green ceramics is omitted for most new ceramic products Sintering step is still very much required Functions of sintering are the same as before: 1. Bond individual grains into a solid mass 2. Increase density 3. Reduce or eliminate porosity

Binders for Cemented Carbides

Carbide powders must be sintered with a metal binder to provide a strong and pore-free part Cobalt works best with WC Percentage of binder metal is 4% up to 20% Powders of carbide and binder are thoroughly mixed wet in a ball mill to form a homogeneous sludge The sludge is then dried in a vacuum or controlled atmosphere to prevent oxidation before compaction.

Roll Crusher

Ceramic lumps are squeezed between rotating rolls

Powder Injection Molding (PIM)

Ceramic particles are mixed with a thermoplastic, then heated an injected into a mold cavity. Polymer acts as a carrier and provides flow characteristics for molding. Upon cooling which hardens the polymer, the mold is opened and part is removed. The plastic binder is removed and the remaining part is sintered

Main Ingredients of Ceramic Paste

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 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 (flowing agent) that melt (vitrify) during firing and promote sintering. Wetting agents to improve mixing of ingredients.

Extrusion

Compression of clay through die orifice to produce long sections of uniform cross section which are then cut to required piece length. Equipment utilizes a screw-type action to assist in mixing the clay and pushing it through a die opening. Products: hollow bricks, shaped tiles, drain pipes, tubes, and insulators Used to make starting slugs for jiggering and plastic pressing

Hand Modeling

Fabrication of ceramic product by manipulating plastic clay into desired geometry. A mold or form is used to define portions of the part geometry. Hand throwing on a potter's wheel (a round table that rotates on a vertical spindle). Can produce products of circular cross section. Sometimes use a mold to provide the internal shape.

Plastic Pressing

Forming process in which plastic clay slug is pressed between upper and lower molds. Molds are made of porous material, so when a vacuum is drawn on the backs of the mold halves, moisture is removed from the clay. Mold sections are then opened, using positive air pressure to prevent sticking of part in the mold. Advantages: higher production rate than jiggering and not limited to radially symmetric parts.

Ball Mill

Hard spheres mixed with stock are rotated inside large cylindrical container. Mixture is carried upwards in the container as it rotates, then dropped by gravity to accomplish grinding action by a combination of impact and attrition. Often carried out with addition of water, therefore, ceramic is in the form of slurry.

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.

Jaw Crusher

Large jaw toggles back and fourth to crush lumps against a hard, rigid surface

Processing of New Ceramics

Manufacturing sequence for new ceramics can be summarized as: 1. Preparation of starting materials 2. Shaping 3. Sintering 4. Finishing

Shaping of new ceramics

Many of the shaping processes are borrowed from powder metallurgy and traditional ceramics. And some of the traditional ceramics forming techniques are used to shape new ceramics: slip casting, extrusion, dry pressing

Compaction

Most common process is cold pressing, used for high production of cemented carbide parts such as cutting tool inserts Dies must be oversized to account for shrinkage during sintering For high production, dies are made with WC-Co linesr to reduce wear For smaller quantities, large flat sections may be pressed and then cut into smaller pieces

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 compromised of fine ceramic powders mixed with water Its consistency determines the ease of forming the material and quality of the final product The starting raw ceramic material usually occurs in nature as rocky lumps. Purpose of the preparation step is to reduce these rocky lumps to powder

Impact Grinding

Particles of stock are thrown against a hard-flat surface, either in a high-velocity air stream or in a high-speed slurry. The impact fractures the pieces into smaller particles.

Finishing operations for new ceramics

Parts made of new ceramics sometimes require finishing, with one or more of the following purposes: 1. Increase dimensional accuracy 2. Improve surface finish 3. Make minor changes in part geometry Finishing usually involves abrasive processes. Diamond abrasives must be used to cut the hardened ceramic metals

Firing of Traditional Ceramics

Performed in a furnace called a kiln Bonds are developed between ceramic grains (densification and reduction of porosity). A glassy phase forms among the crystals that act as a binder.

Sintering of WC-Co

Possible to sinter WC without a metal binder, but the resulting material is less than 10% of true density. Using a binder yields a structure virtually free of porosity. Sintering of WC-Co is liquid phase sintering. The pure binder metal does not melt at the same sintering temperature. WC is gradually dissolved in Co during sintering, and its melting point is reduced so melting does occur. As liquid phase forms, it flows and wets the WC particles further dissolving the solid Molten metal also serves to remove gases from the intertal regions of the compact These mechanisms cause rearrangement of the remaining WC particles into a closer packing. Results in significant densification and shrinkage of the WC-Co mass

Processing Overview for Traditional Ceramics

Powders are mixed with water to bind them together and achieve proper consistency for shaping Condition of powders and part during 1. Preparation of raw materials 2. shaping 3. drying and 4. firing Traditional ceramic products: pottery, stoneware, dinner, bricks, tile Raw materials consists primarily of silicate ceramics-clay

Dry Pressing

Process sequence similar to semi-dry pressing Binders are added to the dry powder mix to provide sufficient strength in the pressed part for subsequent handling. Lubricants are also added to prevent die from sticking during pressing and ejection Dies made of hardened tool steel or cemented carbide to reduce wear due to abrasive dry clay. No drying shrinkage occurs. Products: bathroom tile, electrical insulators, refractory brick, and other simple geometries

Comminution

Reducing particle size in ceramics by using mechanical energy in various forms such as impact, compression, and attrition (size reduction of ceramic powder) Comminution techniques are most effective on brittle materials such as cement and metallic ores Two general types of commuinion operations: crushing and grinding

Crushing

Reduction of large lumps from the mine or quarry to smaller sizes for subsequent further reduction. Several stages may be required Reduction ration in each stage is 3 to 6 times. Crushing of minerals is accomplished by compression against rigid surfaces or impact against surfaces.

Shaping Processes

Slip casting: the water clay mixture is a slurry (25% to 40% water) Plastic forming methods: the clay is plastic (15% to 25% water) Semi-dry pressing: the clay is moist but has a low plasticity (10% to 15% water) Dry pressing: the clay is basically dry (less than 5% water) and has no plasticity

Drying

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 (volumetric shrinkage with 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)

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 Mechanized methods generally use clay with less water

Roller Mill

Stock is compressed against flat horizontal table by rollers riding on the table surface.

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

Secondary Operations

Subsequent processing is usually required after sintering to achieve adequate dimensional control of the cemented carbide parts Grinding with a diamond or other very hard abrasive wheel is the most common secondary operation performed for this purpose. Other secondary operations for shaping include: electric discharge machining, ultrasonic machining

Slip Casting

Suspension of ceramic powders in water, called a 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 Drain casting solid casting

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 PM

Clay Volume vs Water Content

Water plays a role in the ceramics shaping process Water must be removed from 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.

Particulate processes

for traditional and new ceramics as well as certain composite materials

Hot pressing

similar to dry pressing, but 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