Metallography reference
Metallography is the study of a material's microstructure. Analyzing the microstructure helps determine whether the material was processed correctly, and it is therefore a critical step in determining product reliability and in figuring out why a material failed.
The basic steps for proper metallographic specimen preparation are documentation, sectioning, mounting, planar grinding, rough polishing, final polishing, etching, microscopic analysis, and hardness testing.
Step 01
Metallographic analysis is a valuable tool. Properly documenting the specimen's initial condition and the microstructural analysis that follows turns metallography into a powerful quality control instrument and an invaluable investigative tool.
Before any blade touches the part, photograph and label the as-received condition, record the orientation, and note any visible damage. In failure analysis the chain of evidence matters as much as the final image.
Step 02
Most specimens need to be cut down to the area of interest before they can be prepared. Depending on the material, that means abrasive cutting for metals and metal-matrix composites, diamond wafer cutting for ceramics, electronics, biomaterials, and minerals, or thin sectioning with a microtome for plastics.
Proper sectioning is required to minimize damage, which can alter the microstructure and produce a false characterization downstream. The right combination of abrasive type, bond, grit, cutting speed, load, and coolant is what keeps the cut from telling you a story that isn't there.
| Material | Classification | Abrasive / Bond |
|---|---|---|
| Aluminum, brass, zinc | Soft non-ferrous | SiC / Resin |
| Heat-treated alloys | Hard non-ferrous | Alumina / Resin |
| < Rc 45 steel | Soft ferrous | Alumina / Resin |
| > Rc 45 steel | Hard ferrous | Alumina / Resin |
| Super alloys, high Ni-Cr | All | SiC / Resin |
| General purpose | — | Alumina / Resin (rubber bonded) |
Step 03
Mounting does three things at once: it protects the specimen edge and preserves surface features, it fills voids in porous materials, and it makes irregular shapes easier to handle, which matters most in automated preparation.
For metals, compression mounting is by far the most common approach. Phenolics dominate on cost. Diallyl phthalates and epoxies are chosen when edge retention is critical. Acrylics are chosen when you need to see through the mount. For brittle or porous materials, castable resins are typically a better fit.
| Property | Phenolics | Acrylics | Epoxy (glass filled) | Diallyl phthalates |
|---|---|---|---|---|
| Cost | Low | Moderate | Moderate | Moderate |
| Ease of use | Excellent | Moderate | Good | Good |
| Cycle time | Excellent | Moderate | Good | Good |
| Edge retention | Fair | Good | Excellent | Excellent |
| Clarity | None | Excellent | None | None |
| Hardness | Low | Good | High | High |
Step 04
Planar grinding (sometimes called coarse grinding) does two jobs at once: it flattens the specimen and removes the damage left by sectioning. The trick is doing both without creating worse damage than you started with, especially on brittle materials like silicon.
Three machine parameters control the outcome: applied pressure, the relative velocity between head and platen, and the direction of grinding. Higher pressure removes more stock but creates more subsurface damage. Higher relative velocity is aggressive but uneven. Matching head and platen speed in the same direction is gentle and uniform, which is what you want when retaining inclusions and brittle phases matters more than throughput.
| European (P-grade) | Standard grit | Median diameter (µm) |
|---|---|---|
| 60 | 60 | 250 |
| 120 | 120 | 106 |
| P240 | 240 | 58.5 |
| P320 | 280 | 46.2 |
| P400 | — | 35 |
| P600 | 400 | 25.75 |
| P1000 | 500 | 18.3 |
| P1200 | 600 | 15.3 |
| P2400 | 800 | 6.5 |
| P4000 | 1200 | 2.5 |
Step 05
Rough polishing exists to remove the damage that planar grinding left behind. Done correctly, it preserves the specimen's flatness and keeps inclusions and secondary phases in place, leaving only cosmetic damage for the final step to clean up.
This is diamond's territory. Most procedures step from 9 µm down to 1 µm on low-nap polishing cloths. Polycrystalline diamond is preferred over monocrystalline because its many small cutting edges produce higher cut rates with less subsurface damage.
Step 06
Final polishing is a finishing pass, not a damage-removal pass. If damage from cutting or grinding is still visible at this stage, the answer is to go back, not to polish harder.
For most metals, that means a brief pass on a high-napped cloth with colloidal alumina, kept under 30 seconds. Ceramics and ceramic matrix composites benefit from alternating colloidal silica and 1 µm polycrystalline diamond, which produces a chemical-mechanical polishing effect and a damage-free surface.
| Cloth | Type | Typical use |
|---|---|---|
| Metal mesh | Wire mesh, semi-fixed abrasive | Coarse and intermediate lapping |
| POLYPAD | Polyester | Intermediate polishing |
| TEXPAN (Pellon) | Low nap | Most common intermediate; ceramic final |
| BLACKCHEM 2 | Porometric polymer | Between low and high nap |
| DACRON (Nylon) | High nap | Final polish for metals and polymers |
| MICROPAD / NAPPAD | High nap | Most common final polish for metals |
Step 07
Etching is what makes the microstructure visible. It selectively attacks features (grain boundaries, phases, inclusions) based on composition, stress, or crystal structure, so that the differences in optical reflectivity are large enough to read at the microscope.
Chemical etching is the most common technique, with hundreds of formulations developed for specific alloys. Electrolytic, molten salt, thermal, and plasma etching all have their niches for materials that don't respond well to ordinary chemical attack.
| Etchant | Composition | Application | Conditions |
|---|---|---|---|
| Keller's | 190 mL H₂O, 5 mL HNO₃, 3 mL HCl, 2 mL HF | Aluminum alloys | 10-30 sec immersion, fresh only |
| Kroll's | 92 mL H₂O, 6 mL HNO₃, 2 mL HF | Titanium | 15 sec |
| Nital | 100 mL ethanol, 1-10 mL HNO₃ | Carbon steel, tin, nickel alloys | Seconds to minutes |
| Kalling's No. 2 | 40 mL H₂O, 2 g CuCl₂, 40 mL HCl, 40-80 mL ethanol | Stainless, Fe-Ni-Cr alloys | Few seconds to minutes |
| Marble's | 50 mL H₂O, 50 mL HCl, 10 g CuSO₄ | Stainless steels, nickel alloys | Few seconds, swab or immerse |
| Vilella's | 45 mL glycerol, 15 mL HNO₃, 30 mL HCl | Stainless, carbon steel, cast iron | Seconds to minutes |
| Picral | 100 mL ethanol, 2-4 g picric acid | Iron and steel, tin alloys | Seconds to minutes (do not let dry, explosive) |
Step 08
With the microstructure revealed, the next step is reading it. The illumination technique you choose changes what's easy to see and what disappears into the background.
For quantitative surface topography, optical interferometry (PSI for resolution, VSI for range) and atomic force microscopy extend metallography from a two-dimensional view into a three-dimensional measurement.
Step 09
Hardness testing complements microstructural analysis. Hardness correlates to tensile strength, wear resistance, and ductility, so it's a fast way to monitor quality control and to support materials selection decisions.
Microhardness (Knoop or Vickers, 1 to 1000 gram-force) is used for individual phases, small particles, and brittle materials. Rockwell, Brinell, and macro-Vickers cover bulk hardness across the soft-to-very-hard range. The right test depends on the load you can apply and the feature you want to interrogate.
| Test | Penetrator | Load (kg) | Hardness range | Typical application |
|---|---|---|---|---|
| Rockwell C | Diamond cone | 150 | Medium to very hard | Production of finished parts |
| Rockwell B | 1/16" steel ball | 100 | Soft to medium | Production of finished parts |
| Brinell | 10 mm steel ball | 500-3000 | Soft to hard | Production of finished parts |
| Vickers | Diamond pyramid | 5-100 | Very soft to very hard | Production of unfinished parts |
| Microhardness | Diamond pyramid | 0.01-50 | Very soft to very hard | Lab investigations; micro-constituents of alloys and ceramics |
Step into the full library of guides, tools, and material-specific procedures.
Basics, process steps, material-specific, application-specific, and troubleshooting guides.
CalculatorsGrit converters, grain size, mounting volume, mold compatibility, and the etchant selector.
11 ClassesStep-by-step procedures organized by Don's 11-class material classification system.
ReferenceDefinitions of common metallographic terminology and concepts.
Our technical support team can help you select the right equipment, consumables, and preparation methods for your application.
