Introduction
Ceramic matrix composites (CMCs) combine a brittle ceramic matrix with ceramic fiber or particulate reinforcement to overcome the catastrophic-failure mode of monolithic ceramics. The reinforcement deflects cracks, increases toughness, and extends useful service into temperature regimes where metals fail and polymers char. CMCs appear in turbine engine combustors, ultra-high-temperature thermal protection on hypersonic vehicles, brake systems, and ballistic armor.
This guide is the material-specific reference for the CMC systems PACE Technologies most commonly handles. For the general principles of composite preparation (orientation control, interface preservation, broad mounting strategies), see the Composites Preparation guide. For the ceramic matrices on their own (Si3N4, ZrB2, BN), see Advanced Technical Ceramics.
How this guide is organized
Across the CMC systems below, PACE's preparation procedure is nearly identical: castable epoxy mount, 70 µm diamond plane grind, then a 5-step diamond polish through colloidal silica CMP. The 5-step polish is the workhorse and is what distinguishes CMC prep from monolithic ceramic prep (which uses a 3-step polish). The extra steps suppress the matrix-vs-reinforcement relief that hardness mismatch tries to produce.
What varies is reinforcement form (continuous fiber, chopped fiber, particulate), matrix (Si3N4, ZrB2, BN), and the resulting failure modes (fiber pullout vs. particle pullout vs. matrix relief vs. interface debonding). The page covers per-material identity and risks first, then the shared procedure, then troubleshooting.
Materials covered: SiC fibers in Si3N4 matrix, SiC particulate in Si3N4 matrix, SiC particulate in ZrB2 matrix (the workhorse UHTC composite), and HfB2 fibers in BN matrix.
Material Reference
All four CMC systems below share the same Class 10 / CMC procedure (see Shared CMC Procedure). What's documented here is identity, applications, and the failure modes specific to each fiber/matrix pairing.
SiC Fibers in Si3N4 Matrix (with ~5% mullite)
Continuous or chopped silicon carbide fibers embedded in a silicon nitride matrix, typically with ~5% mullite (Al6Si2O13) as a sintering aid. The Si3N4 matrix bears load while the SiC fibers deflect cracks and pull out in a controlled way, giving the composite useful damage tolerance. Used in aerospace turbine components, power generation combustors, and high-temperature structural parts.
Prep risks: Interface mismatch stress can leave SiC fibers cracked transversely across their cross-section, visible under DIC. This is sometimes a real material feature (service-induced) and sometimes a prep artifact (sectioning shock). Differentiate by orienting cuts orthogonal to the fiber axis when possible and by checking for radial vs. transverse crack patterns. Matrix relief between the harder Si3N4 and the softer mullite-rich grain-boundary phase is the second concern, controlled by extending the colloidal silica final step.
SiC Particulate in Si3N4 Matrix
Silicon carbide particles dispersed in a silicon nitride matrix. Particle reinforcement gives better isotropy than fibers and is easier to manufacture as net-shape parts. Used in wear components, valve guides, and high-temperature structural pieces where directional fiber properties aren't needed.
Prep risks: Particle pullout is the dominant failure mode. Hard SiC particles embedded in the slightly softer Si3N4 matrix tend to lift out during grinding if force is too aggressive, leaving voids that read as porosity. The 30 µm DIAMAT step on CERMESH is critical here; if it loads or glazes, particle pullout in the subsequent polishing steps accelerates. Inspect after the 9 µm polish to catch pullout before it propagates.
SiC Particulate in ZrB2 Matrix (20 vol% SiC / 80 vol% ZrB2)
The workhorse ultra-high-temperature ceramic composite (UHTCC). Hard SiC particles in an even-harder ZrB2 matrix. Used in hypersonic vehicle leading edges, rocket nozzle inserts, and other applications above 2000°C where conventional ceramics and even monolithic SiC give out. The SiC particles act as oxidation-protection aids as well as mechanical reinforcement.
Prep risks: Severe hardness mismatch between ZrB2 matrix and SiC particulate produces phase relief that the eye reads as pullout but is actually preferential removal of the slightly softer phase. Use DIC at high magnification to distinguish: sharp-edged voids = pullout, continuous height variation = relief. Extending the colloidal silica CMP step to 10+ minutes flattens this. Sub-surface damage in dense ZrB2 is severe; ion polishing as a final step is sometimes required for SEM/EBSD work.
HfB2 Fibers in BN Matrix
Hafnium diboride continuous fibers in a boron nitride matrix. Hexagonal BN matrix has extremely low hardness and acts almost as a "soft fuse" for crack deflection, while HfB2 fibers carry load up to 3000°C+. Used in extreme thermal protection for re-entry vehicles and ramjet/scramjet leading edges. Niche but high-value: each part is custom for an extreme service environment.
Prep risks: Inverse hardness mismatch from the typical CMC: here the matrix is softer (h-BN, ~50-75 HV) and the fibers are extreme (HfB2, ~2300 HV). The soft BN matrix smears into the cut surface during sectioning and during the initial grinding steps, masking fibers from imaging. Use a very firm sectioning blade and verify the cut surface is clean before mounting. The 30 µm DIAMAT step on CERMESH is again critical; if it loads, the BN matrix smears and the resulting surface looks featureless in brightfield. CMP with SIAMAT colloidal silica clears the smeared BN and exposes the HfB2 fibers clearly.
Imaging & Contrast
Most CMC analysis is done as-polished. The phase contrast between fibers (or particulates) and matrix is sufficient for optical imaging once the colloidal silica step has removed sub-surface damage. Chemical etching is rare and risks attacking one phase preferentially, distorting the volume fractions you may be trying to measure.
Recommended imaging approaches
- DIC (Differential Interference Contrast): The most useful technique for CMCs. The slight relief between matrix and reinforcement (kept controlled, not eliminated) produces clear contrast under DIC.
- Brightfield with sputter coating: For composites with transparent or low-contrast phases (HfB2/BN under brightfield), a 5-10 nm Au or Pt sputter coat lifts contrast.
- Dark-field: Best for finding fiber-matrix debonds, cracks across fibers, and edge effects at the mount boundary.
- Polarized light: Useful where matrix or fibers are birefringent (some SiC polytypes, mullite).
If etching is needed
For CMC matrices where grain structure inside the matrix is the analysis target, the etchants from the Advanced Technical Ceramics etching table apply to the matrix phase. Watch for preferential attack of the reinforcement: SiC fibers are robust to most ceramic etchants but the SiC/Si3N4 interface phase may be sensitive to molten KOH.
Troubleshooting
Particles or fibers missing from the polished surface (pullout)
Cause: Grinding or early polishing force too high, weak fiber/matrix interface, or a loaded CERMESH cloth in the 30 µm step.
Fix: Reduce force at the failing step. Replace or refresh the CERMESH cloth. Confirm vacuum impregnation was applied if any open porosity is present.
Severe matrix-vs-reinforcement relief on ZrB2-matrix CMC
Cause: Hardness mismatch between hard particle and even-harder matrix; default 5 min colloidal silica final step insufficient.
Fix: Extend the colloidal silica final step to 10-15 min. If relief persists, consider broad-beam ion polishing as the final step for SEM/EBSD work.
Featureless brightfield image on HfB2/BN composite
Cause: Soft BN matrix smeared across the cut surface during sectioning or grinding, masking the HfB2 fibers.
Fix: Confirm the 30 µm DIAMAT step ran for the full 5 minutes and the CERMESH cloth was clean. Extend the colloidal silica CMP step to clear smeared BN. Switch to DIC or dark-field, which exploit the height difference between exposed HfB2 fibers and the recessed BN matrix.
Transverse cracks across SiC fibers
Cause: Could be real (service-induced damage that's the analysis target) or prep artifact (sectioning shock).
Fix: Differentiate by crack pattern. Service damage shows radial or thermal-stress patterns; prep damage shows uniformly transverse cracks aligned to the sectioning direction. If unclear, re-section at lower feed rate (2-5 mm/min) and compare.
Fiber/matrix interface debonding after mounting
Cause: Thermal cycle of hot compression mounting expanded the matrix differently than the fiber, breaking the interface bond.
Fix: Use castable cold mount instead of hot compression. If interface preservation is critical, vacuum impregnate with low-viscosity epoxy before any subsequent prep.
Sub-surface damage visible only after SEM imaging
Cause: Mechanical polish alone leaves a damaged layer in hard ceramic matrices that's invisible optically but obvious under SEM.
Fix: Extend the colloidal silica CMP step. For SEM/EBSD on ZrB2- or dense Si3N4-matrix CMCs, broad-beam ion polishing as a final step gives the cleanest indexing.
Additional Reading
- Zipperian, D.C. Metallographic Handbook. PACE Technologies, Tucson, AZ. House reference for CMC prep procedures.
- Chawla, K.K. Ceramic Matrix Composites, 2nd ed. Springer / Kluwer. Comprehensive reference on CMC systems, processing, and properties.
- Bansal, N.P. and Lamon, J., editors. Ceramic Matrix Composites: Materials, Modeling and Technology. Wiley. Strong reference on aerospace-grade CMCs.
- Naslain, R. "Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview." Composites Science and Technology 64. Foundational reference on SiC/SiC and similar systems.
- Fahrenholtz, W.G., Hilmas, G.E., et al., editors. Ultra-High Temperature Ceramics: Materials for Extreme Environment Applications. Wiley. Covers ZrB2- and HfB2-based composites in detail.
- ASM Handbook, Vol. 21: Composites. ASM International. General composite metallography reference.
- ASTM C1683 / C1525. Standard Test Methods for fracture toughness and thermal shock of advanced ceramics and CMCs.
- ASTM E1245. Automatic Image Analysis (for fiber/particle volume fraction and porosity measurement).
Explore More Procedures
For other composite families, see the metal matrix and carbon-carbon guides. For the ceramic matrices on their own (Si3N4, ZrB2, BN), see the Advanced Technical Ceramics guide.