Polishing Methods

Introduction

Polishing follows grinding and removes the deformed, scratched layer that grinding leaves behind. Done well, it restores the true microstructure: inclusions stay intact, brittle phases are not torn out, sharp edges remain sharp, porosity has clean edges, and the surface is flat enough to etch and image with confidence.

PACE diamond suspensions, alumina, colloidal silica, and matching pads cover the full polishing sequence from rough through final.

Every polishing step can be judged by the same set of effects.

Polishing happens in two distinct stages, with a third group of techniques for materials that resist conventional polishing. Rough polishing removes the remaining grinding damage and reveals the microstructure. Final polishing cleans up the surface and prepares it for etching. Alternative techniques (electrolytic, attack, and vibratory polishing) handle the materials that the standard sequence cannot.

Polishing Sequence Planning

Before the sample reaches the polishing wheel, two things matter: where grinding ended and what the final analysis needs. Both shape the polishing sequence.

Match the first polishing step to the last grinding step

Each polishing step has to remove the scratch pattern from the step before it. If grinding stopped at P1200 SiC paper (roughly 15 μm), a 9 μm diamond stage is a reasonable starting point. If grinding stopped at P800 (roughly 22 μm), the first polish needs to be coarser or longer. Skipping grit sizes saves no time, because the next step takes longer to clear the residual scratches. See the Grinding Techniques guide for how the previous stage's damage layer is set up.

Check progress under the microscope, not by the clock

Polishing times in any procedure are starting points. The real test is the surface itself. Stop a step when the previous scratch pattern is gone, not when a timer reaches a number. Over-polishing is the most common operator error: it rounds edges, pulls inclusions, and produces relief between phases of different hardness.

Plan for the analysis you actually need

Routine optical microscopy and etching rarely require sub-micron polishing past 1 μm diamond plus a short oxide step. EBSD, SIMS, AFM, and high-magnification SEM imaging do. Polish to the finish your analysis requires, no further.

Two Polishing Stages

Rough and final polishing serve different goals and use different consumables. Understanding the split prevents a common mistake: treating final polishing as a longer version of rough polishing.

The rest of this guide is split into self-contained tracks. Read Rough Polishing for the damage-removal step, Final Polishing for the surface-finish step, and Alternative Techniques when the standard stages need help. Universal Best Practices and Troubleshooting apply to both.

Rough Polishing

Rough polishing is the most critical step in metallographic preparation. After this stage, the true microstructure should be visible: inclusions, brittle phases, voids, and porosity all in place, with no smeared metal or embedded abrasive carried over from grinding. If the microstructure is not right at the end of rough polishing, no amount of final polishing will fix it.

PACE rough polishing line: diamond suspensions, diamond pastes, and matching low-nap pads.

Rough polishing abrasives

Rough polishing abrasives range from 15 μm down to 1 μm. The two families are diamond and alumina: diamond dominates for harder materials, alumina is common for softer metals and is significantly less expensive.

• Polycrystalline diamond: spherical, nodular particles built from many small crystallites. Particles fracture during cutting and continuously expose fresh cutting edges. Higher cut rates and a better surface finish than monocrystalline on harder materials.
• Monocrystalline diamond: blocky single-crystal particles with uniform cutting geometry. Consistent scratch pattern, less subsurface deformation. Preferred for soft and ductile metals, where smearing and embedment are the main risks.
• Alumina slurries (for soft metals): hard alpha alumina at lower cost than diamond, common for soft-metal rough polishing. PACE's polycrystalline alumina is deagglomerated for a tighter particle distribution and less random scratching.

Polycrystalline diamond suspension. Self-sharpening fracture behavior, higher cut rates on harder materials.

Monocrystalline diamond suspension. Uniform scratch pattern, lower subsurface deformation. Preferred for soft and ductile metals.

Diamond paste. Grease-based, for manual polishing and selective application.

Rough polishing sequence

A standard sequence runs from a coarse diamond stage that clears the last grinding-paper scratches down to 1 μm. PACE practice differs from older textbook sequences in one important way: the 6 μm step is used for harder materials only. For soft, ductile metals (aluminum, copper, brass, bronze, and similar Class 1 materials), the 6 μm step is skipped, because the extra step does more smearing damage than it removes.

Rough polishing pads

Rough polishing uses low-nap or napless woven pads. They support the abrasive while keeping the specimen flat. Properties that matter: fiber hardness and density, weave type, compressibility, porosity, and how well the pad wets with the chosen suspension.

Lapping films as an alternative

For applications where flatness is non-negotiable (PCB cross-sections, optical fiber ferrules, hard/soft phase contrasts), alumina or diamond lapping films can replace pads at the rough stage. Because the abrasive is bonded to a polyester backing, lapping films have much lower compressibility than pads and produce flatter surfaces with less relief. The trade-off is conformability: stacking the film on a pad (or a rubber pad) restores some compliance for irregular specimens.

Automated rough polishing parameters

For automated disk-on-disk polishing, the relative velocity between the specimen holder and the platen is the dominant parameter for damage control.

• Metals and damage-sensitive materials: run the head and base in the same direction at approximately the same RPM (for example, 100 RPM head and 100 RPM base). Low relative velocity, low damage. Removal rates are modest, but the microstructure stays intact.
• Hard, tough engineered ceramics (silicon nitride, zirconia, dense alumina): use a much larger relative velocity difference and higher pad pressure. Counter to intuition for metals, brittle ceramics polish best under more aggressive CMP conditions.

Complementary rotation (head and base in the same direction at matched speeds) is the PACE default for damage-sensitive materials. It is also the same "100/100, same direction" rule that resolves scratches, smearing, comet tails, and pull-out in the troubleshooting table.

Chemical mechanical polishing (CMP) for ceramics

Standard diamond polishing leaves subsurface damage on hard ceramics that further diamond stages cannot remove. The damage propagates from one step to the next. The fix is CMP: combine a mechanical abrasive (diamond) with a chemically active suspension (colloidal silica at high pH), so the silica electrochemically attacks the ceramic surface and the diamond removes the reacted layer.

A working CMP sequence for silicon nitride or zirconia: rough grind to 30 μm, then polish with a 6 μm diamond and colloidal silica mixture, then finish with colloidal silica alone, about five minutes per step. The result is significantly less surface and subsurface damage than a long diamond-only sequence produces.

Friction increases the chemical contribution. Higher pad pressure and higher velocity raise temperature at the polish zone and accelerate the chemical attack. Rinse the pad with distilled water for the last 10 to 15 seconds of the step to mechanically lift the reacted layer.

Recommended PACE grinder-polishers

NANO Series, Manual Grinder-Polishers

Variable-speed manual polishing in single, double, and large-wheel configurations. The workhorse for labs that run varied work at moderate volume.

FEMTO Series, Semi-Auto Heads

Automated polishing heads that mount onto NANO bases. Programmed force, time, and direction for repeatable rough polishing.

ZETA-2000S, Automated Abrasive Dispenser

Programmable suspension dosing for hands-off, reproducible rough polishing on a NANO/FEMTO setup.

View the full grinding and polishing equipment lineup →

Final Polishing

If rough polishing was done well, final polishing has a short job: clean up the surface and prepare it for etching. If rough polishing was not done well, no amount of final polishing will restore the true microstructure. Final polishing is where you finish, not where you fix problems.

PACE SIAMAT colloidal silica, the most common final polishing abrasive for metals, composites, and ceramics.

Final polishing abrasives

Five abrasive families are used for final polishing: polycrystalline alumina, calcined alumina, levigated alumina, colloidal silica, and the specialty oxides (cerium oxide for glass, iron oxide rouge for soft metals). Polycrystalline alumina and colloidal silica handle the great majority of metallographic work.

Polycrystalline alumina, the modern default

Polycrystalline alumina is manufactured by a seeded gel process that produces tighter particle-size distributions and harder alpha alumina crystals than traditional calcined products. The result is fewer random scratches, higher cutting rates, and a more uniform surface finish than a standard 0.05 μm gamma alumina powder, which often aggregates into 5 μm clumps that scratch soft metals.

For application, wet the polishing cloth with the polycrystalline alumina slurry, then drip or spray more suspension during the cycle. Common pads: MICROPAD, MICROPAD 2, ATLANTIS, TRICOTE, NAPPAD, and FELT PAD.

Calcined alumina, the cost-effective alternative

Calcined alumina has been the workhorse final-polish abrasive for decades. Particles have a plate-like shape and come in alpha (harder) or gamma (softer) crystal structures. Sub-micron calcined products are typically jet-milled to break up agglomerates. Applied on the same pads as polycrystalline alumina, plus the softer MOLTEC 2 (canvas) and FELT PAD (1/8 inch thick felt) for less critical work.

Colloidal silica

Colloidal silica is the most versatile final-polish abrasive. It functions as both a chemical agent and a mechanical abrasive: on glass, ceramics, and mineral phases it reacts electrochemically with the surface and the cloth lifts the reacted layer (CMP). On metals, composites, and microelectronic substrates it polishes mechanically and produces mirror finishes with minimal relief.

Final polishing pads

PACE final-polish pads cover the full range from low-nap flatness control (ATLANTIS, TRICOTE) to high-nap scratch removal (MICROPAD, NAPPAD).

For the full PACE polishing pad lineup with sizes, adhesives, and stock, see the Polishing Pads page.

Alternative Polishing Techniques

Some materials never produce a clean microstructure with conventional mechanical polishing. Extremely soft metals smear at any pressure. Heat-sensitive metals recrystallize. Friable abrasives embed in the surface. Three techniques handle these cases: electrolytic polishing, attack polishing, and vibratory polishing.

Electrolytic polishing

Electrolytic polishing electrochemically corrodes the specimen in a controlled fashion: the sample is the anode in a flow cell, and material leaves the surface ion by ion. The result is a deformation-free surface with no mechanical artifacts. The technique is covered in detail in the Etching Procedures guide, alongside electrolytic etching.

Attack polishing

Attack polishing adds a controlled etching step between grinding or polishing steps to dissolve embedded abrasive and the smeared surface layer. The classic example is tin: SiC abrasives fracture and embed in tin during grinding. Etching the sample lightly between grit sizes (for example, 2% Nital between SiC papers) dissolves the embedded fragments before they carry into the next stage.

The etch is more aggressive than a typical metallographic etch. The goal is not to reveal microstructure, it is to dissolve the contaminated surface layer. Attack polishing also covers cases where a dilute etchant is added to the polishing suspension itself, so mechanical removal and chemical attack proceed simultaneously.

Vibratory polishing

Vibratory polishing uses a spring-and-motor mechanism that vibrates the polishing bowl while the angled spring slowly rotates the specimens around it. It is mechanically gentle: deformation runs much lower than disk polishing, which makes vibratory the standard for EBSD preparation and for any application where subsurface damage matters more than time. The trade-off is speed. Vibratory cycles run from hours to overnight.

Recommended Equipment: GIGA-S Vibratory Polisher

Interchangeable 9 inch and 12 inch bowls, variable frequency and amplitude, batch processing of multiple samples. The standard PACE choice for EBSD preparation and subsurface deformation removal.

View GIGA-S Vibratory Polisher →

Material-Specific Polishing

The principles above adapt to material class. The notes below collect Don Zipperian's PACE-tested adjustments that override the generic "metals get 6, 3, 1 μm diamond" advice in older textbooks.

Soft, ductile metals (Class 1: aluminum, copper, brass, bronze, soft steels)

• Skip the 6 μm diamond step. The extra stage causes more smearing damage than it removes.
• Use monocrystalline diamond for less subsurface deformation.
• Include a P500 alumina (ALO) pre-step before diamond for aluminum, copper, brass, and bronze.
• Use polycrystalline alumina (not calcined gamma alumina) for the final step. Gamma alumina aggregates and scratches soft metals.
• Attack polishing with a dilute etchant in the suspension is often the only way to clear the smeared layer.

Hardened steels and tool steels (Class 8)

• Full diamond sequence: 9 → 6 → 3 → 1 μm. The 6 μm step is needed here.
• Polycrystalline diamond preferred for cut rate.
• Final polish: polycrystalline alumina or colloidal silica on MICROPAD or TRICOTE.

Stainless steels and work-hardening alloys (austenitic, nickel-base)

• Mechanical polishing alone leaves a deformation layer that subsequent mechanical steps cannot remove.
• Extended vibratory polishing with colloidal silica usually clears the deformed layer. For EBSD, vibratory finishing is essentially mandatory.
• Electrolytic or attack polishing handles cases where vibratory time is not available.

Titanium, cast iron, thermal-spray coatings

• Titanium: long times at each diamond step; the oxide film reforms continuously. Final polish with colloidal silica on a low-nap pad to maintain flatness.
• Cast iron: extended care at rough polishing to retain graphite flakes and nodules. Low pressures, woven pads, polycrystalline diamond. Final with colloidal silica or polycrystalline alumina.
• Thermal-spray coatings: prioritize flatness across the coating-substrate interface. Use lapping films or ATLANTIS pad, low-nap final, no high-nap pads.

Engineered ceramics (Class 10, 11)

• CMP is the only sequence that consistently clears subsurface damage. See CMP for ceramics in the rough polishing section.
• Hard, tough ceramics polish under high-pressure, high-velocity CMP conditions, counter to the gentle approach used for metals.
• BLACKCHEM 2 pad concentrates frictional heat at the polish zone and accelerates the chemical action.

Multi-phase materials and composites

• Use harder, lower-nap pads to keep all phases in the same plane.
• Keep total polishing time short. Differential removal rates between phases convert time into relief.
• ATLANTIS pad is a strong compromise: high-nap conformability with low-nap flatness.
• Vibratory polishing with colloidal silica often levels the final relief.

Universal Best Practices

These habits apply to every polishing step, rough or final.

Before the step

• Inspect the surface from the previous step under the microscope. Decide whether to polish further at the current size or move on.
• Clean the sample ultrasonically between every step. The single most common cause of "mystery scratches" is carry-over of a coarser abrasive.
• Dedicate each pad to one abrasive size. Cross-contamination ruins fine pads instantly.
• Wet the pad with the suspension before applying the specimen.

During the step

• Match base and head speeds, same direction, for damage-sensitive materials (the 100/100 rule).
• Drip or spray suspension continuously. A dry pad polishes nothing and damages the specimen.
• Use light force at the final polishing stage; the work is refinement, not removal.
• Inspect under the microscope between steps, not after the whole sequence has finished.

After the step

• Rinse the specimen immediately. Colloidal silica must come off before it crystallizes.
• Inspect for residual scratches from the previous size before advancing.
• Wipe off any matte alumina film with a cotton ball and a mild cleaning solution.
• Document any parameter that worked (force, time, suspension, pad). The next operator should not need to rediscover it.

Pad and equipment care

• Store unused pads flat and dry.
• Rinse used pads thoroughly to prevent abrasive crystallization in the fibers.
• Check platen runout periodically (PACE specs 2.5 μm for an 8 inch plate). An out-of-flat platen prints its unevenness onto the specimen.
• Replace pads when they glaze, embed contaminants, or no longer wet uniformly.

Troubleshooting

Almost every polishing artifact can be traced to one of four causes: a carry-over from the previous step, the wrong pad, the wrong force, or too much time. The table below is adapted from Don Zipperian's PACE handbook and covers the artifacts that show up most often.

What's Next: Etching

With a clean, true microstructure on the polished surface, the next step is usually etching. Etching reveals grain boundaries, phases, and constituents by chemically or electrolytically attacking the surface at different rates depending on local composition and crystallography. A good polish gives the etchant a uniform surface to work on. A bad polish gives the etchant pre-existing artifacts to amplify.

Continue to the Etching Procedures Guide →