Class 3 materials, such as powder metals and cast irons, exhibit low ductility and present unique challenges in metallographic preparation. These materials often include internal porosity or brittle phases that require careful handling to preserve structural features and avoid prep-induced damage.
Open and interconnected pores can trap abrasives or smear over, hiding true features.
Edges and grain boundaries may crack under grinding force if not properly supported.
Residual metal can smear over voids during polishing, requiring intermediate etching.
Loose or hard inclusions may detach, leaving voids or pits on the surface.
Use slow-speed abrasive or diamond cutting to minimize cracking. Coolant is essential to prevent thermal damage.
Vacuum impregnation is recommended to fill pores and provide edge support for brittle samples.
Use alumina or fine SiC with moderate pressure. Clean and optionally etch between steps to remove embedded debris.
Polish with diamond followed by colloidal silica. Etch between steps to reveal porosity and prevent pore smearing.
Confirm pores are open and clean—not smeared over
Check edges and fracture zones for prep-induced cracks
Verify phases are retained without pull-out
Observe grain boundaries and sinter lines clearly