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17 - PROCESSING OF CERAMICS AND CERMETS


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Processing of ceramics and cermets
Chapter includes: [1] Processing of traditional ceramics, [2] Processing of New ceramics, [3] Processing of cermets, [4] Product design considerations. >>> TiN CoD: TRADITIONAL, NEW, CERMETS, DESIGN
PROCESSING OF TRADITIONAL CERAMICS
PROCESSING OF TRADITIONAL CERAMICS involves the following stages: [1] Preparation, [2] Shaping, [2] Drying, and [4] Firing of silicate ceramics (clay) produces gridning wheels, dinner ware, pottery, and toilet bowls. >>> PiS DuF: PREPARATION, SHAPING, DRYING, FIRING
*Preparation of raw material
REDUCTION OF RAW MATERIAL CERAMIC LUMPS TO POWDER, using mechanical to grind and crush the raw material in the process of preparation, commonly referred to as comminution.
Comminution
TECHNIQUES TO PRODUCE POWDER FROM TRADITIONAL CERAMICS RAW MATERIALS. Can be divided into two categories [1] Crushing - reducing lumps to smaller lumps, sometimes in few stages, [2] Grinding - abrasion and impact of small granules to reduce them to fine powder. >>> CRUSHING GRINDERS MAKE GOOD SNIFFING POWDER.
Crushing
REDUCTION OF LARGE LUMPS TO SMALL LUMPS, using mechanical means to crush the bitches, compressing the lump against a hard surface or impacting it with great force, using [1] Jaw crushers, [2] Gyratory crushers, [3] Roll crushers, [4] Hammer mills >>> RiG HuJe: ROLLS, GYRATORY, HAMMER, JAWS
Grinding
REDUCES SMALL LUMPS TO FINALLY GROUND POWDER, abrasing and impacting the small lumps until the disintegrate to finely ground powder, using a [1] Ball mill - steel spheres tubble with rocks, [2] Roller mill - rollers compress small lumps, [3] Impact grinding - small lumps flung at flat, hard surface at high velocity. >>> RIB: ROLLER, IMPACT, BALL
Adding components
Adding components to make a paste out of powder, allowing shaping of the material, components such as [1] Water, [2] Clay, [3] Nonplastic materials - silica and alumina: they don't shrink during drying, [4] Sintering aiding fluxes. >>> WC NooSe: WATER, CLAY,,, NONPLASTICS, SINTERING_AIDS
*Shaping of traditional ceramics
FLUIDITY OF POWDER-WATER MIX DETERMINES THE SHAPING PROCESSES USED - [1] Slip casting @ Slurry: 25% - 40% water, [2] Plastic forming @ 15% - 25% water, [3] Semi-dry pressing @ moist clay: 10% - 15% water, [4] Dry pressing @ dry clay: < 5% water. >>> SiP SEED: SLIP, PLASTIC,,, SEMI_DRY, DRY
Slip casting
POOR SLURRY INTO PLASTER MOLD, allowing the plaster to absorb the water and either drain the mold or leave the slurry in if you don't want a hollow product like a teapot or a vase. PLASTIC HOLLOW DRIES PORCELAIN POT
Plastic forming
Forming plastic clay into form, through processes of: [1] Hand molding - shaping clay with hands, [2] Hand throwing - shaping clay on a potter's wheel, [3] Batting and jiggering - giving a clay glob initial shape, batting, and then rotating it and shaping with a jigger, [4] Plastic pressing - pressing the clay into shape, and [5] Extrusion - extruding clay into pipes and hollow bricks. >>> Hi BJ PE: HAND, BATTING AND JIGGERING, PRESSING, EXTRUSION
Hand molding
MOLDING CLAY MANUALLY, the calay being stationary or spun on a potter's wheel.
Batting and jiggering
Rough mold bats clay into approximate shape, the table is spun while heated and rotating tool brings clay to final form, producing steam from the clay's moisture, steam that prevents sticking. ROUGH MOLD BATS, ACCURATE HEATED TOOL JIGGERS
Plastic pressing
PRESSING A PLASTIC CLAY PIECE between upper and lower porous mold, the mold pores drawing moisture from the clay, a faster processes than batting and jiggering or jollying. GYPSUM MOLD PRESSES PLASTIC CLAY
Extrusion
EXTRUDES LONG PIECES OF MIX, like taking a really good shit, producing long and intricate forms, forms used to make ceramic pipes and hollow bricks and shaped tiles and starting clay slugs for other processes.
Semi dry pressing
HIGH PRESSURE FORCES SEMI-DRY MIX INTO NARROW MOLD, pressing it hard to fit in and some does and some gets out in the form of FLASH, applying as much force as possible to shape it into a proper and acceptable form, PRESSING A SEMI-DRY CLAY WITH 10% TO 15% WATER CONTENT.
Dry pressing
PRESSING DRY MIX INTO SIMPLE FORM - MOST ACCURATE PROCESS, it's accuracy derived from a lack of flashing and shrinkage, although the part geometry has to be simple and molds very hard - tool steel or tungsten carbide, and the mix has to have binders and lubricants, since the mix powder is very dry: about 5% moisture, thus being very abrasive. HARD AND TOUGH DIE ACCURATELY FORMS DRY AND LUBRICATED MIX.
*Drying
DRYING CAUSES SHRINKAGE OF THE CERAMIC MIX, the mix volume decreasing the water that was pushing it's particles away from each other evaporates, thus if too fast, drying can crack the shaped piece if the drying rate is too fast or the moisture gradient in the part is too high, therefore drying times can be from a quarter of an hour for a thin piece to a few days to a big part, the drying accomplished by heating using convection and radiation, quite often employing infrared lamps. RADIANT CONVECTION LAMP SLOWLY DRIES CERAMIC TOILET BOWL.
*Firing
FIRING IN A KILN SINTERS THE GREEN CERAMIC PIECE, strengthening it by fusing the particles together, some of the particles taking on amorphous state, densifying and strengthening the part, and, of course, shrinking it even more, in addition to the drying shrinkage. SINTERING FIRE SHRINKS FUSING PARTICLES.
Glazing
APPLICATION OF CERAMIC SURFACE COATING, the glazing protects the piece from moisture while enhancing it's appearance, the glazing applied after the first firing, and then firing the glazed piece once again to produce a beautiful and durable surface finish. FIRE, GLAZE FIRE.
PROCESSING OF NEW CERAMICS
Processing of new ceramics, ceramics made of simpler compounds such as oxides, nitrides, and carbides, produces strong and durable and hard parts, unlike traditional ceramics, although the starting materials get some additives in other to be more malleable during the forming process, the processing of new ceramics consists of: [1] Preparation of raw materials, [2] Shaping, [3] Sintering, [4] Finishing >>> PiS SelF: PREPARATION, SHAPING,,, SINTERING, FINISHING
*Preparation of starting materials
Preparation of raw materials for new ceramics includes mechanical and chemical methods, although material can get contaminated from the crushing tools during crushing, thus compromising the purity of the part, thus following chemical methods are used: [1] Freeze drying and [2] Precipitation form solution.
Freeze drying
Freeze drying a dissolved metal salt and removing the water in a vacuum chamber.
Precipitation from solution
DISSOLVING AND PRECIPITATING FROM SOLUTION OF CERAMIC COMPOUNDS, like in a Bayer process when alluminum oxide dissolves from bauxite, alluminum hydroxide precipitated from the solution, and reduced to $Al_{2}O_{3}$.
Adding additives
Adding additives such as [1] Plasticizes - improves plasticity, [2] Binders - binds particles together in final product, [3] Wetting agents - facilitate better mixing, [4] Deflocculants - prevent clumping and bonding, [5] Lubricants - reduce sticking during mold release and friction between grains >>> DeeP BLoW: DEFLOCCULANTS, PLASTICIZES,,, BINDERS, LUBRICANTS, WETTING_AGENTS.
*Shaping of new ceramics
Shaping of new ceramics uses methods of powder metallurgy and traditional ceramics: [1] Hot Pressing - firing and pressing, [2] Isostatic pressing - same as in PM, [3] Doctor-Blade process - makes thin ceramic sheets, [4] Powder injection molding - mix with plastic, inject, remove plastic, sinter. >>> HOT AND COLD PRESS INJECTS DOCTOR BLADES
Hot pressing
HARD DIE PRESSES AND SINTERS DRY CERAMIC POWDER, the die living a short and meaningless life due to constant wear and tear by the hard and rough ceramic particles, yet in his short and radiant life managing to produce a multitude of precise parts while eliminating the need for a separate firing. COMPRESSES AND SINTERS ABRASIVELY DRY CERAMIC POWDER.
Isostatic pressing
PRESSURIZED MOIST LIQUID OR HOT GAS PRESSES NEW CERAMIC POWDER THROUGH FLEXIBLE WALLS, the same isostatic pressing method used in powder metallurgy, compacting the powder from all directions, thus avoiding the problem of nonuniform density, yet the dimensional control is a bit lacking.
Doctor blade process
DOCTOR BLADE THIN CERAMIC SHEET ON MOVING CELLOPHANE, creating a thin ceramic sheet quite often used in integrated circuits industry, the green tape rolled on a spool and cut to any desired shape later on before sintering.
Powder injection molding
INJECTION MOLDING A MIX OF CERAMIC POWDER AND POLYMER, removing the polymer later on in a process of debinding, removal of the plastic either by heat or by chemical means, yet weakening the green strength of the molded part, sometimes warping and cracking the part, quite often creating micro-structural flaws that limit strength. INJECTED POWDER LOOSES STRENGTH WITHOUT POLYMER.
*Sintering of new ceramics
SINTERING OF NEW CERAMICS, AT A TEMPERATURE OF 80% TO 90% OF MELTING POINT, bonds individual grains into a a solid mass, increases the part's density, and reduces or eliminates porosity. In clay based compounds consisting of different powders the bonding mechanism is mass diffusion across contacting particle surfaces.
*Finishing of new ceramics
Hard abrasives increase dimensional accuracy, improve surface finish, and make changes in part geometry. HARD DIAMOND FINISHES SINTERED NEW CERAMIC DILDO.
PROCESSING OF CERMETS
We process cermets and other metal matrix composites and ceramic matrix composites, MMC and CMC, by particulate processing methods. PARTICULATE PROCESSING OF CERMETS MAKES NICE BLADES.
*Cemented carbides
CARBIDES CEMENTED IN A METAL MATRIX, the metal constituting about 4% to 20% of the final part, such as in tungsten carbide in a cobalt binder, titanium carbide in nickel. Making of cemented carbides involves the following steps: [1] Preparation of materials, [2] Compaction, [3] Sintering, [4] Secondary operations. >>> PiCe SS: PREPARATION, COMPACTION, SINTERING, SECONDARY.
Material preparation for cemented carbides
Preparation of materials for creation of cemented carbides involves mixing a bit of metal powder, from 4% to 20%, with carbide powders in a ball mill with some water, forming homogeneous sludge, and then drying the sludge in vacuum or controlled atmosphere to prevent oxidation. DRIED CARBIDE/METAL MIX SLUDGE MAKES UNIFORM POWDER IN CONTROLLED ATMOSPHERE.
Compaction of cemented carbides
Most common compaction method for cemented carbides is cold pressing, but sometimes isostatic pressing, hot pressing, and extrusion is used, and in all those stages you must account for shrinkage of up to 20% or more and the wear and tear on the die, that must be very hard. STRONG, POWERFUL PRESS COMPACTS SHRINKING CEMENTED CARBIDE
Sintering of cemented carbides
LIQUID PHASE SINTERING FOR WC-CO CEMENTED CARBIDE reduces porosity next to zero and produces a strong and uniform material through a process of solid state diffusion.
Secondary processing of cemented carbides
ABRASIVE DIAMONDS PROCESS HARD CARBIDES. Abrasive operations with diamond grinding wheel, ultrasonic machining, and electrical discharge machining bring the cemented carbide piece into final shape.
CERAMIC AND CERMET PRODUCT DESIGN CONSIDERATIONS
[1] Subject only to compression - ceramics are weak in tension, [2] Avoid impact - ceramics are brittle, [3] Make simple parts - complex parts require complex and expensive molds, [4] Large inside and outside radii - prevent stress concentrations, [5] Take shrinkage into account - ceramics and cermets shrink when drying and sintering, [6] Avoid screw threads - screw threads are hard to fabricate in ceramics, nor do the have enough strength to function after fabrication. >>> CuBS SToR: COMPRESSION, BRITTLENESS, SIMPLE_PARTS,,, RADII, SHRINKAGE, THREADS_NO-NO