Engineering Doctorate Characterisation Of Powders

Engineering Doctorate Size and Size Distribution Size function obtained by sieving or gravitation Commercial powders are Bimodel Need good distribution for good density

Engineering Doctorate Particle Shape Observe by microscopy Affects: Reactivity Compactibility (Spherical is poor) Sintering behaviour

Engineering Doctorate Specific Surface Area (S w ) S w = Surface Area/ unit wt of powder S w is proportional to 1/d S w increases with particle porosity S w increases with surface irregularity  Reactivity increases with S w (Hazard?) 

Engineering Doctorate Internal Structure Affects compaction sintering Internal porosity affects maximum density

Engineering Doctorate Density ρ T = True ρ P = Particle (= ρ T unless there is porosity) ρ a = Apparent (ρ a is in range Q T )

Engineering Doctorate Compressibility Response of volume (density) of powder to applied pressure Affects tool design Measure by Green Density or Densification Parameter (D P ) D P = (ρ G – ρ a )/(ρ T – ρ a )

Engineering Doctorate 7.Compactibility Bonding between particles with pressure Spherical hard and oxide covered particles have low compatibility. For low compactibility materials, may need a binder to give green strength. 8.Flow Rate 9.Oxygen and Other Impurities Affects strength Some impurities reduced during sintering.

Engineering Doctorate Compactibility Bonding between particles with pressure Spherical, hard and oxide covered particles have low compactibility. Low compactibility materials may need a binder to give green strength.

Engineering Doctorate Flow Rate Assumes powders behave like liquids

Engineering Doctorate Blending(1) Uniformity Optimum size distribution Alloying Add Lubricant e.g. Zinc Stearate Add Binders

Engineering Doctorate Blending (2) Don’t Overmix Work Hardening Change Shape