IE 337: Materials & Manufacturing Processes IE 337 Lecture 8 Glass & Ceramics Processing IE 337: Materials & Manufacturing Processes Lecture 13: Ceramics, Glass and Powder Processing Chapters 7, 12, 16 & 17 S.V. Atre
This Time Ceramics Glass Processing Powder Processing: Ceramics and Metals Homework #5 on Thursday (2/25/10)
Ceramics General properties Hard High wear resistance Brittle High compressive strength High elastic modulus High temperature resistance Good creep resistance Low conductivity Low thermal expansion Good chemical inertness
Ceramics: Classification Al2O3-SiO2 SiC Si3N4 BN ZrO2 ZrO2 WC Al2O3 Al2O3 Diamond AlN
Common Ceramics Oxides: Al2O3, ZrO2 Nitrides: AlN, Si3N4, BN, TiN Carbides: WC, SiC, TiC, TaC Glasses: SiO2 + others Carbon: Graphite, Diamond sinter Processed as powders
Whiteware Ceramics Clay Processing Products Quartz Feldspar Water addition, mixing Air removal Shaping Drying Coating Firing Products Brick Structural Tile Drain / sewer pipe Decorative applications Bath / kitchen structures
Refractory Material Retain properties at high temperature Products Mechanical Chemical Products Fire brick Insulating fibers Refractory linings Coatings Silica Alumina Magnesium Oxide
Abrasives High hardness Examples Roughing Applications Super-Finishing Silicon carbide Aluminum oxide Cubic boron nitride Roughing Applications Grinding Cutting Water-jet Sawing Coatings Super-Finishing Honing Lapping
Glasses Amorphous solid Silica Glass Products Vitreous (noncrystalline) structure Amorphous Cooled to semi-solid condition without crystallization Subject to creep Silica Glass Optical properties Thermal stability Products Window glass Fiber optics Chemical containers Lenses
Glass Ceramics Crystalline solid Glass Ceramic Products 0.1 to 1.0 micron grains Use of nucleating agents Glass Ceramic Efficient processing in glassy state Net shape process Good mechanical properties versus glass Low porosity Low thermal expansion Higher resistance to thermal shock Products Cookware Heat exchangers Missile radomes
Cermets Combination of metals & ceramics Properties Applications “Cemented” carbides Bound with high temperature metal Properties High hardness High temperature resistance Improved toughness Improved strength Improved shock resistance Applications Crucibles Jet nozzles High temperature brakes Production Press powder in metal mold Sintering in controlled atmosphere WC-Co
GLASS
Shaping Methods for Glass Methods for shaping glass are different from those used for traditional and new ceramics Glassworking: principal starting material is silica Usually combined with other oxide ceramics that form glasses Heated to transform it from a hard solid into a viscous liquid; it is then shaped into the desired geometry while in this fluid condition When cooled and hard, the material remains in the amorphous state rather than crystallizing
The typical process sequence in glassworking: preparation of raw materials and melting, shaping, and heat treatment
Glassworking Processes Piece Ware Flat and Tubular Glass Glass Fibers
Piece Ware Shaping Processes Spinning – similar to centrifugal casting Pressing – for mass production of flat products such as dishes, bake ware, and TV faceplates Blow forming – for production of smaller-mouth containers such as beverage bottles and incandescent light bulbs Casting – for large items such as large astronomical lenses that must cool very slowly to avoid cracking
Spinning Spinning of funnel‑shaped glass parts such as back sections of cathode ray tubes for TVs and computer monitors: gob of glass dropped into mold; and rotation of mold to spread molten glass on mold surface
Pressing Pressing of flat glass pieces: (1) glass gob is fed into mold from furnace; (2) pressing into shape by plunger; and (3) plunger is retracted and finished product is removed (symbols v and F indicate motion (velocity) and applied force)
Blow Forming Blow forming sequence: (1) gob is fed into inverted mold cavity; (2) mold is covered; (3) first blowing step; (4) partially formed piece is reoriented and transferred to second blow mold, and (5) blown to final shape
Casting A low viscosity glass can be poured into a mold Uses: massive objects, such as astronomical lenses and mirrors After cooling and solidifying, the piece must be finished by lapping and polishing Casting of glass is not often used except for special jobs Smaller lenses are usually made by pressing
Rolling Starting glass from melting furnace is squeezed through opposing rolls whose gap determines sheet thickness, followed by grinding/ polishing
Float Process Molten glass flows onto the surface of a molten tin bath, where it spreads evenly, into a uniform thickness and smoothness - no grinding or polishing is needed
Forming of Glass Fibers Products can be divided into 2 categories: Discontinuous fibrous glass for insulation and air filtration, in which the fibers are in a random, wool‑like condition Produced by centrifugal spraying Long continuous filaments suitable for fiber reinforced plastics, yarns, fabrics, and fiber optics Produced by drawing
Drawing Continuous glass fibers of small diameter are produced by pulling strands of molten glass through small orifices in a heated plate made of a platinum alloy
Heat Treatment Annealing to eliminate stresses from temperature gradients Annealing temperatures are around 500C followed by slow cooling Tempering to make the glass more resistant to scratching and breaking due to compressive stresses on its surfaces Heating to a temperature above annealing, followed by quenching of surfaces by air jets
Finishing Operations Glass sheets often must be ground and polished to remove surface defects and scratch marks and to make opposite sides parallel Decorative and surface processes performed on certain glassware products include: Mechanical cutting and polishing operations; and sandblasting Chemical etching (with hydrofluoric acid, often in combination with other chemicals) Coating (e.g., coating of plate glass with aluminum or silver to produce mirrors)
Powder Processing Parts Figure 16.1 A collection of powder metallurgy parts (photo courtesy of Dorst America, Inc.).
Powder Processing The Characterization of Engineering Powders Production of Metallic Powders Conventional Pressing and Sintering
Powder Metallurgy (PM) Metal processing technology in which parts are produced from metallic powders Usual PM production sequence: Pressing - powders are compressed into desired shape to produce green compact Accomplished in press using punch-and-die tooling designed for the part Sintering – green compacts are heated to bond the particles into a hard, rigid mass Performed at temperatures below the melting point of the metal
Why Powder Metallurgy is Important PM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent machining PM process wastes very little material - ~ 97% of starting powders are converted to product PM parts can be made with a specified level of porosity, to produce porous metal parts Examples: filters, oil‑impregnated bearings and gears
More Reasons Why PM is Important Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy Tungsten filaments for incandescent lamp bulbs are made by PM Certain alloy combinations and cermets made by PM cannot be produced in other ways Non-equilibrium microstructures possible PM compares favorably to most casting processes in dimensional control PM production methods can be automated for economical production
Engineering Powders A powder can be defined as a finely divided particulate solid Engineering powders include metals and ceramics Geometric features of engineering powders: Particle size and distribution Particle shape and internal structure Surface area
Measuring Particle Size Most common method uses screens of different mesh sizes Mesh count - refers to the number of openings per linear inch of screen A mesh count of 200 means there are 200 openings per linear inch Since the mesh is square, the count is equal in both directions, and the total number of openings per square inch is 2002 = 40,000 Higher mesh count = smaller particle size
Screen Mesh Figure 16.2 Screen mesh for sorting particle sizes.
Particle Shapes in PM Figure 16.3 Several of the possible (ideal) particle shapes in powder metallurgy.
Observations Smaller particle sizes generally show greater friction and steeper angles Spherical shapes have the lowest interpartical friction As shape deviates from spherical, friction between particles tends to increase Easier flow of particles correlates with lower interparticle friction Lubricants are often added to powders to reduce interparticle friction and facilitate flow during pressing
Particle Density Measures True density - density of the true volume of the material The density of the material if the powders were melted into a solid mass Bulk density - density of the powders in the loose state after pouring Because of pores between particles, bulk density is less than true density
Packing Factor Bulk density divided by true density Typical values for loose powders range between 0.5 and 0.7 If powders of various sizes are present, smaller powders will fit into spaces between larger ones, thus higher packing factor Packing can be increased by vibrating the powders, causing them to settle more tightly Pressure applied during compaction greatly increases packing of powders through rearrangement and deformation of particles
Porosity Ratio of volume of the pores (empty spaces) in the powder to the bulk volume In principle Porosity + Packing factor = 1.0 The issue is complicated by possible existence of closed pores in some of the particles If internal pore volumes are included in above porosity, then equation is exact
Chemistry and Surface Films Metallic powders are classified as either Elemental - consisting of a pure metal Pre-alloyed - each particle is an alloy Possible surface films include oxides, silica, adsorbed organic materials, and moisture As a general rule, these films must be removed prior to shape processing
Production of Metallic Powders In general, producers of metallic powders are not the same companies as those that make PM parts Any metal can be made into powder form Three principal methods by which metallic powders are commercially produced Atomization Chemical Electrolytic In addition, mechanical methods are occasionally used to reduce powder sizes
Coventional PM Sequence Figure 16.7 Conventional powder metallurgy production sequence: (1) blending, (2) compacting, and (3) sintering; (a) shows the condition of the particles while (b) shows the operation and/or workpart during the sequence.
Blending and Mixing of Powders For successful results in compaction and sintering, the starting powders must be homogenized Blending - powders of same chemistry but possibly different particle sizes are intermingled Different particle sizes are often blended to reduce porosity Mixing - powders of different elements/alloys are combined
Compaction Application of high pressure to the powders to form them into the required shape Conventional compaction method is pressing, in which opposing punches squeeze the powders contained in a die The workpart after pressing is called a green compact, the word green meaning not yet fully processed The green strength of the part when pressed is adequate for handling but far less than after sintering
Conventional Pressing in PM Figure 16.9 Pressing in PM: (1) filling die cavity with powder by automatic feeder; (2) initial and (3) final positions of upper and lower punches during pressing, (4) part ejection.
Press for Conventional Pressing in PM Figure 16.11 A 450 kN (50‑ton) hydraulic press for compaction of PM parts (photo courtesy of Dorst America, Inc.).
Sintering Heat treatment to bond the metallic particles, thereby increasing strength and hardness Usually carried out at between 70% and 90% of the metal's melting point (absolute scale) Generally agreed among researchers that the primary driving force for sintering is reduction of surface energy Part shrinkage occurs during sintering due to pore size reduction
Sintering Sequence Figure 16.12 Sintering on a microscopic scale: (1) particle bonding is initiated at contact points; (2) contact points grow into "necks"; (3) the pores between particles are reduced in size; and (4) grain boundaries develop between particles in place of the necked regions.
Sintering Cycle and Furnace Figure 16.13 (a) Typical heat treatment cycle in sintering; and (b) schematic cross section of a continuous sintering furnace.
Limitations and Disadvantages High costs High tooling and equipment costs Metallic powders are expensive Typically requires a unique material or geometry to justify Problems in storing and handling metal powders Degradation over time, fire hazards with certain metals Limitations on part geometry because metal powders do not readily flow laterally in the die during pressing This is true for traditional punch and die Variations in density throughout part may lead to yield issues especially for complex geometries
Interparticle Friction and Powder Flow Friction between particles affects ability of a powder to flow readily and pack tightly A common test of interparticle friction is the angle of repose, which is the angle formed by a pile of powders as they are poured from a narrow funnel
Angle of Repose Figure 16.4 Interparticle friction as indicated by the angle of repose of a pile of powders poured from a narrow funnel. Larger angles indicate greater interparticle friction.
Powder Injection Molding final powder dry/ debind flow shape sinter (firing)
CERAMICS
Ceramics Processing (a) shows the workpart during the sequence, while (b) shows the condition of the powders
Slip Casting A suspension of ceramic powders in water, called a slip, is poured into a porous plaster of paris mold where the water from the mix is absorbed to form a firm layer of clay The slip composition is 25% to 40% water Two principal variations: Drain casting - the mold is inverted to drain excess slip after a semi‑solid layer has been formed, thus producing a hollow product Solid casting - to produce solid products, mold not drained
Sequence of steps in drain casting, a form of slip 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 and trimmed
SLIP CASTING
Tape Casting Fabrication process for thin ceramic sheets Doctor Blade Polyester Film Carrier Slip Dried Tape Doctor Blade Polyester Film Roll
Miniaturization of Complex Circuits High Temperature Co-Fired Ceramic (HTCC) Low Temperature Co-Fired Ceramic (LTCC) Thick film metal traces are printed on several tape layers of ceramic and are co-fired Tape layers are electrically connected through vias Significant miniaturization of circuit form factor with this technology
Extrusion Compression of clay through a die orifice to produce long sections of uniform cross‑section Products: hollow bricks, shaped tiles, drain pipes, tubes, drill bit blanks, and insulators
Extruder Sectional View Components and features of a (single‑screw) extruder for plastics and elastomers
Ceramic Extrusion: Examples cordierite catalytic converter 50 cells/cm2
Powder Injection Molding (PIM) Ceramic particles are mixed with a thermoplastic polymer, then heated and injected into a mold cavity. Polymer provides flow characteristics for molding
Mold-Filling Interactions IE 337 Lecture 8 Glass & Ceramics Processing Mold-Filling Interactions Jetting Weld-line Air trap Short shot It is important to understand the complex nature of the mold-filling process to have an appreciation for the study. During mold-filling, the molten feedstock flows into an empty mold cavity maintained at a specific temperature. The material continuously loses heat and starts solidifying while fighting to completely fill the cavity. After a certain time in the cycle, usually after the cavity is about 98% filled, the switchover from filling to packing occurs. During which extra material is pushed into the cavity to compensate for the material shrinkage due to solidification. Due to improper specifications of the process parameters, several defects may occur. Like Weld-line, air trap, short shot, flashing, powder- binder separation, jetting. With all these obstacles of simultaneous heat and mass transfer occurring and also possibilities of several defects, the process of trying to fill a cavity for thin-walled microfluidic geometry is a very complex task. If the injection molding engineer knows how the material behaves and which knobs to turn on the molding machine when something doesn’t seem right, then the task becomes more easier. This study aims at making the task easier for the molding engineer. Filler-polymer separation Flashing S.V. Atre
Die Pressing
Semi-Dry Pressing Semi‑dry pressing: (1) depositing moist powder into die cavity, (2) pressing, and (3) opening the die sections and ejection
Next Time Joining Chapter 30 & 31 IE 337 Lecture 12: Forming 2 S.V. Atre