By Jay Shah, Technical Team, Beta Diamond Products
Every one of these artifacts imitates a real material feature. Smearing hides porosity and cracks. Pull out fakes porosity that is not there. Edge rounding corrupts coating and case depth measurements. Relief throws phase boundaries out of focus. All four can appear on a sample that looks beautifully polished to the naked eye. Of the four, edge rounding is the one customers ask us about most, and there is a logical reason: it is the only artifact that directly falsifies a number in a report. Smearing and relief mislead an image. Edge rounding corrupts a coating thickness or case depth measurement that someone signs their name to.
Edge rounding: the measurement killer
What it looks like: the outer edge of the sample, or the interface against the mount, falls out of focus at high magnification because it no longer sits in the same plane. On a coating cross section this directly falsifies thickness readings, which is why coating shops and heat treaters worry about it more than anyone.
What happened: resilient cloth nap wrapped over the sample edge and polished it preferentially, or a shrinkage gap between sample and resin left the edge unsupported.
The fix: edge retention starts at mounting. Use a mineral or glass filled hot mounting epoxy, or a low shrinkage cold epoxy, so resin hugs the sample with no gap. Then polish on hard napless cloths for as much of the sequence as possible, saving soft napped cloth for only the briefest final step. Shorter times on napped cloths, always.
Smearing: the structure is there, you just buried it
What it looks like: featureless glassy regions on ductile metals, pores that seem to have lids, fine cracks that vanish and reappear between repolishes.
What happened: soft metal flowed under the abrasive instead of cutting cleanly, spreading a thin deformed layer across the surface and covering real features.
The fix: reduce pressure first, it is the biggest lever. Switch to a napless or low nap cloth, which cuts rather than rubs. Use polycrystalline diamond, whose many fine edges shear cleanly where blocky particles plow. Keep the cloth charged and lubricated, because a starved cloth rubs, and rubbing is what smears. Finish chemo mechanically with colloidal silica or fine deagglomerated alumina, which removes the smeared layer rather than spreading it. Suspect smearing on a finished sample? Etch it: smeared surfaces etch blotchy and structureless.
Pull out: the hole your cloth dug
What it looks like: angular dark cavities where a hard particle, inclusion, or brittle phase used to be, often trailed by a comet tail of scratches from the liberated particle scraping downstream.
What happened: the abrasive grabbed a brittle constituent and tore it from the matrix instead of cutting through it. Common in composites, cermets, cast irons with graphite, and any material mixing hard particles in a softer matrix.
The fix: support and gentleness. Mount in a hard low shrinkage epoxy so the matrix cannot flex. Use napless cloths, since deep nap wraps around particles and levers them out. Drop the pressure, use polycrystalline diamond, shorten polishing times. For severe cases, diamond lapping films hold their abrasive fixed so there are no rolling particles to grab edges.
Relief: a topographic map you did not order
What it looks like: hard phases standing proud of the matrix, soft phases sunk below it. At high magnification you cannot focus both at once.
What happened: soft constituents polish faster than hard ones, and a resilient cloth conforms to the developing hills and valleys, amplifying them with every pass.
The fix: hard flat cloths and short times. Napless cloths bridge across phases and cut them in one plane. Keep final steps brief, sixty to ninety seconds on the oxide polish, because relief grows with time on soft cloths. If colloidal silica's chemical action accentuates relief on your alloy, switch the final step to fine alumina.
One habit that prevents all four
Use the minimum pressure, the minimum nap, and the minimum time that still produces a clean surface. Nearly every artifact on this list is the sample reporting that something was too aggressive or ran too long. Our cloths are organized by nap and hardness for exactly this tuning. Describe your material and your artifact, and we will point you to the cloth and abrasive pairing to try first.