Ceramics Flashcards
How does the crystalline structure of ceramics vary?
Structure varies depending in the arrangement of the molecules. Quartz, tridymite or crystobalite are polymorphs. and addition of other elements can make it amorphous.
what are the additives which can be added to ceramics and what do they do?
Binders: improve mechanical strength for firing.
Lubricant: decreases friction in pressing process (reduces cracking)
Wetting agent: reduces the surface tension of liquids, more uniform packing density.
Plasticiser: soltens binder, increases flexability of the green body.
Deflocculant: causes reduction in viscosity.
How can the uniformity of pariculated in cermaics be ensured? how does it improve the final cermaic piece?
Ball milling, roll milling and impact grinding can grind the powdered materials and improve thier uniformity. increases density, due to less voids and reduces shinkage in sintering.
what is mixing or blending in relation to particulates and why is is applied?
mixing is for particles of different chamical comp and blending is for particles of same comp. this can reduce the occurence of voiding in the mix.
what is the difference between isostatic and unidirectional pressing? how do the techniques differ?
isostatic is applied pressure from all orientations, whereas unidirectional is one or two directions. Isostatic required water pumped into a soft mold. unidirectional is pressed in a steel die and punch.
What 3 defects are common in ceramic components and how do they influence the performance?
Very brittle (more so at low temps). Cracks, porosity or inclusions.
Manufacturing steps for ceramics?
raw materials (additives), binding / mixing, forming / shaping, drying (green machining), firing / sintering, finishing (machining, grinding, lapping)
What are the three techniques for processing raw materials?
Ball milling, roller milling, impact milling. wet milling very effective and no dust.
types of blending and mixing of powders?
rotating drum, rotating double cone, screw mixer and blade mixer.
types of forming and shaping?
Isostatic pressing ( fluid used to pressurise a soft mould of powder), unidirectional pressing (two or one steel presses and dies. simple geometries. higher density at press side.)
Plastic forming (extrusion, injection moulding. for clays)
Slip casting (slip poured into mould and absorbed into walls then drained and dried.)
types of water in ceramics to be dried?
water of suspension (slip casting),
Inter-particle water (water within layers f structure usually clay like materials, high shrinkage),
Pore water (between particles, very little shrinkage),
Absorbed water (very thin film layer, held by electrostatic surface forces).
What is sintering?
sintering is when atoms diffuse with one-another at points of contact at temperatures below its melting point. greater porosity increases shrinkage.
