Solid state sintering Flashcards
How do you calculate shrinkage?
~ assume a part is made up of cube elements
~ assume the sintered part is theoretically dense
~ Ai = green body dimension
~ Af = sintered dimension
~ Fp = volume fraction pores in the green body
Fp =
~ Vi - Vf / Vt = 1 - Vf/Vi = 1 - Af^3/Ai^3
~ Vi - Vf: volume taken up by pores
~ Vt: total volume
What is Xl?
~ Xl = linear shrinkage fraction
~ Xl = Ai-Af/Ai
~ Ai-Af: change in length
~ Ai: original length
What is Fp knowing that Af = Ai(1-Xl)?
~ Fp = 1 - Ai^3(1-Xl)^3 / Ai^3
~ Fp = 1-(1-Xl)^3
What is the final Xl equation?
~ because 1 - Xl = (1 - Fp)^1/3
~ Xl = 1 - Fs^1/3
~ Fs = fraction solid in the green body = 1 - Fp
What is the driving force for sintering?
~ the elimination of solid-vapor interfaces (many un-sataisfied bonds), replacing them with solid-solid interfaces (grain boundary, fewer unsatisfied bonds)
What is the average particle size for a good sinter?
~ avg particle size of 0.5-1 μm
~ green relative densities generally sinter well
What occurs during the initial stages of sintering?
~ atoms show net migration to points of particle contact, filling them in to form early grain boundaries in the form of necks
~ atoms supplied by surface diffusion or evaporation condensation if the solid has a high vapor pressure
~ atoms diffusing to this neck location satisfy more dangling surface bonds-surface atoms are more stable at this location
What occurs during the intermediate stage of sintering?
~ most macroscopic shrinkage
~ atomic source is predominately from interior sources: grain boundaries and particle interiors
~ GB diffusion dominates because of high conc. of vacancies from lattice mismatch
~ bulk diffusion has an increasing role with increasing temp because of an exponential increase in vacancy concentration
~ gaps between particles sinter to near-cylinders. these are interconnected and the network extends to the part surface: open porosity
What happens during the final stage of solid state sintering?
~ starts at 90% relative density
~ cylindrical pores close off to isolated spherical pores along 3-grain intersection curves and at four grain intersection points