Introduction
Metal matrix composites (MMCs) combine a ductile metal matrix with hard ceramic reinforcement (particles, whiskers, or fibers) to deliver stiffness and wear resistance well above the base metal. The classic automotive example is SiC particulate in aluminum, used in brake rotors, engine blocks, and connecting rods where lightweight and stiffness matter together. The aerospace example is ZrB2 particulate in titanium, used where the parent titanium alloy needs higher temperature capability and stiffness without going to a fully ceramic structure.
Unlike ceramic matrix composites (where a shared 5-step procedure handles every system), MMC preparation diverges meaningfully between systems. The reinforcement hardness drives sectioning blade selection, and the matrix ductility drives final polish chemistry. This guide gives per-system procedures rather than a unified protocol.
How this guide is organized
Each MMC system below has its own complete procedure (sectioning through final polish) because the differences matter. SiC/Al uses standard medium-grit sectioning and finishes on SIAMAT colloidal silica. ZrB2/Ti needs a high-concentration sectioning blade and a final NANO-W alumina step to flatten the severe hardness mismatch.
For the general principles of composite preparation (orientation control, interface preservation, mounting strategies), see the Composites Preparation guide. For ceramic matrix composites, see Ceramic Matrix Composites.
SiC Particulate in Aluminum Matrix
Silicon carbide particles (typically 10-30 vol%) dispersed in an aluminum matrix (often A356, 6061, or 2024 grade). The SiC particles raise stiffness 30-50% over the parent aluminum and cut thermal expansion roughly in half, while keeping the alloy lightweight and machinable. Applications include automotive brake rotors (Lotus, Lamborghini), engine cylinder liners, connecting rods, and aerospace structural panels.
Prep risks: Severe hardness mismatch between hard SiC particles (~2500 HV) and soft aluminum matrix (~50-100 HV). Three failure modes dominate: (1) SiC particles pull out during grinding, leaving voids that read as porosity; (2) the aluminum matrix smears into the spaces between particles, masking grain structure; (3) phase relief, where the matrix polishes away faster than the particles, leaves the SiC particles protruding above the matrix and edge-rounded particles appearing where there are none.
Sectioning: SiC/Al
- Blade: Diamond wafering blade, medium grit, low concentration.
- Wheel speed: 200-400 RPM precision wafering saw.
- Feed rate: 5-15 mm/min; reduce for high SiC volume fractions to limit particle dislodging at the cut edge.
- Cooling: Continuous water-based cutting fluid.
Mounting: SiC/Al
- Castable low-viscosity epoxy. Avoids smearing the soft aluminum matrix that compression mounting heat would cause.
- Vacuum impregnation recommended if SiC content is high and inter-particle porosity is present.
Grinding: SiC/Al
- 70 µm diamond grinding disc, water, 5-10 lbs, 200/200 RPM, until planar.
Polishing: SiC/Al (4-step)
- 30 µm DIAMAT diamond on CERMESH metal mesh cloth, no lubricant, 5-10 lbs, 200/200 RPM, 5 min.
- 9 µm DIAMAT diamond on POLYPAD pad with DIALUBE Purple extender, 5-10 lbs, 200/200 RPM, 5 min.
- 3 µm DIAMAT diamond on GOLDPAD or ATLANTIS pad, 5-10 lbs, 200/200 RPM, 5 min.
- SIAMAT colloidal silica on TEXPAN pad, 5-10 lbs, 200/200 RPM, 5 min. Switch to water on the cloth for the last 20-30 seconds to flush silica off the sample.
Note: SiC/Al uses a 4-step polish (skipping a separate 6 µm step) because the soft aluminum matrix removes diamond damage quickly. The 3 µm step does double duty as both intermediate polish and damage removal.
ZrB2 Particulate in Titanium Matrix
Zirconium diboride particles in a titanium alloy matrix (Ti-6Al-4V or commercially pure Ti). ZrB2 raises the use temperature, increases stiffness substantially, and contributes oxidation resistance. Used in aerospace structural components, missile bodies, and high-temperature airframe sections where the parent titanium alloy is insufficient. Less common than SiC/Al but a high-value system where it's specified.
Prep risks: Extreme hardness mismatch (ZrB2 ~2300 HV vs. Ti ~300-400 HV). Particle pullout is worse than in SiC/Al because the larger ZrB2 particles have weaker mechanical anchoring to the matrix. The titanium matrix is also more prone to mechanical twinning and deformation artifacts than aluminum, so grinding forces need to be controlled. Final polish must flatten the relief without lifting particles, which requires a softer abrasive than colloidal silica in the last step.
Sectioning: ZrB2/Ti
- Blade: Diamond wafering blade, medium grit, high concentration (the higher diamond concentration handles the harder particles cleanly).
- Wheel speed: 200-400 RPM.
- Feed rate: 5-10 mm/min. Slower than SiC/Al because of the harder particle phase.
- Cooling: Continuous water-based cutting fluid.
Mounting: ZrB2/Ti
- Castable low-viscosity epoxy.
- Vacuum impregnation recommended if particle-matrix debonding is visible.
Grinding: ZrB2/Ti
- 30 µm DIAMAT diamond on CERMESH metal mesh cloth, 5-10 lbs, 200/200 RPM, until planar (the 30 µm step is the primary planarizing step for this system; skip the 70 µm disc).
Polishing: ZrB2/Ti (4-step ending in NANO-W)
- 9 µm DIAMAT diamond on POLYPAD pad, 5-10 lbs, 200/200 RPM, 3 min.
- 3 µm DIAMAT diamond on TEXPAN pad with SIAMAT colloidal silica, 5-10 lbs, 200/200 RPM, 3 min.
- 0.05 µm NANO-W (nanometer alumina) on TRICOTE polishing pad, 5-10 lbs, 100/100 RPM, 1 min.
The NANO-W alumina final step is the key divergence from SiC/Al. The softer alumina abrasive flattens the SiAlMAT-induced relief without lifting ZrB2 particles, and the lower 100/100 RPM speed limits matrix work-hardening on the titanium. The result is a flat, pullout-controlled surface ready for DIC or SEM analysis.
For SEM/EDS analysis of the ZrB2/Ti interface, a brief etch (Kroll's reagent, 5-10 seconds) can enhance grain boundary contrast in the titanium matrix without attacking the ZrB2 particles. See Titanium Preparation for etchant specifics.
Imaging & Contrast
MMC analysis is usually done as-polished; the contrast between ceramic reinforcement and metal matrix is high enough for optical imaging without etching. Etching is appropriate when the matrix grain structure itself is the analysis target.
- Brightfield: Default for SiC/Al; the SiC particles read dark against the lighter aluminum matrix.
- DIC: Best for ZrB2/Ti and any composite where matrix-vs-particle relief needs to be controlled vs. used. The slight relief between phases produces strong DIC contrast.
- Dark-field: Useful for finding particle-matrix debonds and edge cracks at the interface.
- Matrix etching: If the metal matrix grain structure matters, use the standard etchant for that matrix (Keller's for Al alloys, Kroll's for Ti alloys). See the respective material guides for parameters.
Troubleshooting
SiC particles pulling out of the aluminum matrix
Cause: Grinding force too high, or the CERMESH cloth loaded with aluminum smear.
Fix: Reduce grinding force to the lower end of the 5-10 lb range. Replace or dress the CERMESH cloth. Confirm vacuum impregnation if porosity is present.
Aluminum smearing across SiC particles
Cause: Soft aluminum matrix flowing under polish force, smearing into the spaces between hard particles.
Fix: Reduce polish force at the 9 and 3 µm steps. Switch to water on the cloth for the last 30 seconds of the SIAMAT step to flush smear and silica together.
Severe particle protrusion (relief) on SiC/Al
Cause: Aluminum matrix removed faster than SiC particles, leaving the hard particles standing proud and edge-rounded.
Fix: Extend the final SIAMAT step. If relief persists, the colloidal silica is doing more damage than help; switch to a less aggressive final polish (1 µm DIAMAT on ATLANTIS).
ZrB2 particle pullout in titanium matrix
Cause: Hardness mismatch and weak interfacial bonding. Worse than SiC/Al because of larger particle size.
Fix: Confirm sectioning used the high-concentration diamond blade. Reduce force at the 9 µm polish step. Don't skip the 30 µm DIAMAT planarizing step.
Titanium matrix mechanical twinning after grinding
Cause: Excessive grinding force or RPM caused mechanical twinning in the titanium matrix, visible as fine line patterns that disappear with light etching.
Fix: Reduce force and use the NANO-W alumina final step at 100/100 RPM to avoid work-hardening. Light Kroll's etch confirms whether the lines are twins (will disappear) or grain boundaries (will persist).
Featureless image after polish (no contrast)
Cause: Matrix smear masking particles, or the surface is over-polished and totally flat.
Fix: Switch to DIC, which exploits the slight relief between phases for contrast. If the surface really is featureless, etch lightly with the matrix etchant (Keller's for Al, Kroll's for Ti).
Additional Reading
- Zipperian, D.C. Metallographic Handbook. PACE Technologies, Tucson, AZ. House reference for MMC prep procedures.
- Chawla, N. and Chawla, K.K. Metal Matrix Composites, 2nd ed. Springer. Comprehensive reference covering SiC/Al, Al2O3/Al, B/Al, and other systems.
- Clyne, T.W. and Withers, P.J. An Introduction to Metal Matrix Composites. Cambridge University Press. Foundational text on MMC processing and properties.
- ASM Handbook, Vol. 21: Composites. ASM International. Metallography sections for MMC and CMC systems.
- Voort, G.F. Vander. Metallography: Principles and Practice. ASM International. General reference on aluminum and titanium prep that applies to the matrices.
- ASTM E1245. Automatic Image Analysis (particle volume fraction, size distribution, and porosity measurement on MMCs).
- ASTM B557 / B557M. Standard Test Methods for Tension Testing Wrought and Cast Aluminum- and Magnesium-Alloy Products (relevant for verifying MMC matrix specs).
Explore More Procedures
For other composite families, see the ceramic matrix and carbon-carbon guides. For the base matrices on their own, see the aluminum and titanium guides.