Plastic Contact of Particles

Only small particles, less than 1 pm in diameter, would show this effect. Krupp explained this in terms of the equations for London-van der Waals attractive forces between rigid spheres, together with the Hertz equations of contacL Because the attraction is proportional to particle diameter, the force at the particle contact decreases with D. However, the elastic area of the contact spot decreases faster, from the Hertz Equation (9.1), with Thus, as the particle gets smaller, the contact pressure must rise to the point at which plastic deformation occurs. 

Such plastic effects were shown dramatically by Easterling and Tholen in electron nticrographs of metal particles sticking together. They showed that 

Scanning electron microscope studies were performed on polystyrene spheres sitting on polished silicon surfaces by Rimai, Demejo and Bowen. The bulk polymer had a Young s modulus of 2.55 GPa and a yield stress of 10.8 MPa when measured on a testing machine. With such a low yield point it was estimated that the particles should be plastically deformed under the adhesion forces. Therefore they applied the plastic deformation theory of Maugis and Pollock to fit the results, as shown in Fig. 9.28. This gave the expression for contact diameter d in terms of sphere diameter D 

This mechanism of plastic coalescence can provide a route by which fine powders can sinter, just as elastic deformation gave Equation (9.18). For close-packed plastic particles the sintered fraction is 

After plastic compaction, the resulting powdCT compact is then fully densified by high-temperature sintering, raising the temperature in an inert atmosphere to a point where diffusion can occur to transport material into the pores. 

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