1 Introduction to Forming Roughly into following categories:  Dry pressing; die pressing, isostatic pressing, hot pressing  Extrusion  Injection molding  Casting: slip casting, tape casting, gel casting Factors to be considered: complexity of object, size, quantity (production rate) etc. Che5700 陶瓷粉末處理

2 Taken from TA Ring, 1996

3 Processing Aids For different forming techniques, the additives may be different; e.g. in the following table, the molecular weight is not the same  different viscosity, yield strength etc. Che5700 陶瓷粉末處理

4 Comparison of system characteristics Solvent content not same for different techniques; corresponding packing fraction and its changes also different, large change  significant contraction; contraction occurs at some stage; forming, drying, or sintering; Vo =volume of organics in green body Che5700 陶瓷粉末處理

5 Stress during forming Che5700 陶瓷粉末處理

6 Strength of green body If requires machining on green body (e.g. shape modification), hence need strength of green body, if not enough strength, easy to appear cracks, as following table, taken from JS Reed, 1995 Che5700 陶瓷粉末處理

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8 General considerations Both technical and economic factors should be considered For advanced ceramics, microstructure control is a critical issue Process window reflects the easiness of control; (reliability, reproducibility issue) Cost ~ 1/ yield There are often more than one process to choose for a specific ceramic product.  meaning comparison of various factors Che5700 陶瓷粉末處理

9 Process Window Che5700 陶瓷粉末處理

10 Dimensional Control Tight control of dimension  superior control of process through forming and sintering  plus final machining (polishing) – an expensive step Total volume shrinkage = (drying stage) + (calcination stage) + (sintering stage) ….. Precise control depends on the control at this three stages Dry pressed, injection molded  drying stage shrinkage ~ 0; extruded and slip casted parts  drying stage shrinkage ~ 3-12% Shrinkage during calcination: high for injection molding and tape casting Che5700 陶瓷粉末處理

11 Shrinkage may not be isotropic! Che5700 陶瓷粉末處理

12 Shaping method on grain size dependence of strength Source: J. Euro. Cer. Soc., 16, , Methods tested: uniaxial pressing, cold isostatic pressing, pressure filtration, gel casting, and combinations; High strength: high purity material for high fracture toughness; small size of critical flaw at instability (crack growth); grain size & flaw size both important Any microstructure which reduce subcritical crack growth is helpful to strength

13 Experimental results from this work, showing effects of shaping methods Different fabrication technique – different flaw size & distributions Gelcasting: gel formation – acrylamide + N,N’- methylene bis- acrylamide (use ammonium persulfate as initiator) + alumin powder (need several hours to complete polymerization) (final step: CIP before sintering

14 Some techniques applied: (e.g.) ultrasonic dispersion + vacuum de-gassing of slurry + uniaxial pressing + cold isostatic pressing Ultrasonic dispersion + pressure filtration + slow drying: to get samples with low flaw density; or pressure filtration + CIP; Different processing techniques  achieve packing homogeneity  low sintering temperature to achieve high density Casting and pressure filtration: may give rise to “cooperative alignment” of particles  hence good results

15 Batch 1, 2, 3: from gelcasting General trend: increasing strength with decreasing grain size

16 1 – 8: ranking of strength obvious relation between strength and subcritical crack growth (flaw density or distribution – strongly related to forming techniques)

17 Lightest Object in 2002  Aerogel density as low as g/cc (air density around g/cc) by NASA (source: 2002 Time Magazine)