Flow stress, green body drying

In this chapter, we described the fundamentals of suspension iheol-ogy from dilute suspensions to concentrated suspensions. Attention has been paid to interparticle forces and the structure of the suspension because these things drastically influence suspension iheology. In addition, visco-elastic properties of concentrated suspensions including ceramic pastes have been discussed. Finally, the mechanical properties of dry ceramic powders have been discussed in terms of the dJoulomb yield criterion, which gives the stress necessary for flow (or deformation) of the powder. These mechanical prc rties will be used in the next chapter to predict the ease with vdiich dry powders, pastes, and suspensions can be made into green bodies by various techniques. 

To maintain a flow of liquid to the surface at a constant rate, the green body must shrink, expelling liquid. Assuming that the flow difiusivity is a constant for simple analysis of this problem. Cooper [18] was able to determine the drying stress at the surface as a flmction of the drying flux ... 

In this section of Chapter 14, we have looked at the individual stresses caused by temperature, flow, and capillarity. In a ceramic green body vmdergoing drying all of these stresses will be operating continuously. As a result, tiie preceding linearization of stress is applicable for only elastic green bodies. 

Machining may also be performed at other points in the process flow schematic in Fig. 2, such as after the drying step or after presintering. These are the likely process points because the green bodies may then have sufficient strength to withstand the contact stresses of the tooling with the component. 

---