Metallographic Preparation Procedures - Powder Metallurgy

Sintered Iron

The metallographic specimen preparation of powder metallurgical samples is affected by the specimens composition, packing and sintering conditions. Microstructural features of interest include: porosity, grain size, inclusions and orientation of voids. Proper metallurgical preparation takes into account that the microstructure may be porous and perhaps somewhat brittle because of inadequate sintering conditions. Following proper cutting to avoid creating undue damage, the specimen is generally mounted in either a phenolic or diallyl phthalate compression mounting resin (for harder materials diallyl phthalate resins are recommended). Initial grinding is recommended with 30 um or 45 um diamond suspension on a metal mesh cloth. This step is very important to minimize damaging or pulling out the grains of a poorly sintered material. The metal mesh cloth provides a trough for the diamond particle to become semi-fixed, thus providing high grinding rates, as well as an opportunity for the diamond to become dislodged under high shear conditions, the result is a planar grinding process which has high removal rates and minimizes deformation and damage to the specimen.

A commonly used procedure which helps to open up smeared pores is to etch the solution for approximately 2 minutes. For ferrous materials such as sintered iron an etch with 2% nital by immersion is recommended.

The next rough polishing stage is required to eliminate any remaining surface and subsurface deformation to the specimen surface. Polycrystalline diamond suspensions used in conjunction low napped polishing pads result in minimal edge rounding and relief for the rough polishing stage. For a final finish, 0.05 um Nanometer alumina provides an excellent surface finish because this alumina is produced from a wet seeded jell process which produces a very tightly controlled polycrystalline alumina particle size distribution (much better than the conventional dry processing techniques). These polycrystalline alumina particles are also blocky alpha alumina particles instead of platey softer gamma alumina particles, and therefore provide a significantly improve the surface finish to the specimen.

SECTIONING
MAXCUT Abrasive blade (Cat. No. MAX-H series)

MOUNTING
Compression Mounting with Phenolic, Epoxy or Diallyl Phthalate compression mounting resins

POLISHING

Abrasive/surface	Lubricant	Force/ sample	Time
30 um DIAMAT diamond suspension on Metal Mesh cloth	DIALUBE Diamond Extender	5 lbs	Until plane
9 um DIAMAT diamond on POLYPAD	DIALUBE Diamond Extender	5 lbs	3 minutes
6 um DIAMAT diamond on TEXPAN pad	DIALUBE Diamond Extender	5 lbs	3 minutes
1 um DIAMAT diamond on DACRON pad	DIALUBE Diamond Extender	5 lbs	3 minutes
0.05 um NANOMETER Alumina on MICROPAD pad		5 lbs	1 minutes