Skip to main content
Material-Specific Guide

Carbon-Carbon & Boron-Graphite Composite Preparation

Two related composite families with fundamentally different preparation tracks. Carbon-carbon composites (aerospace brakes, re-entry heat shields) use SiC paper grinding and a short diamond polish. Boron-graphite composites (aerospace structural) require fine-grit sectioning and diamond lapping films to protect brittle boron fibers.

Table of Contents

Introduction

Carbon-carbon (C-C) and boron-graphite composites share a fiber/matrix architecture but diverge sharply in preparation. C-C uses brittle carbon fibers in a brittle pyrolytic carbon matrix; both phases are relatively soft and respond to SiC paper grinding and short diamond polishing. Boron-graphite uses extremely brittle boron filaments in a graphite matrix, where any cut-edge chipping is irreparable and the entire prep is organized around protecting the fibers.

This guide is the material-specific reference for both systems. For general composite preparation principles, see the Composites Preparation guide. For ceramic and metal matrix composites, see the CMC and MMC guides.

How this guide is organized

The two systems have separate procedure sections because almost nothing transfers between them. C-C uses a medium-grit diamond sectioning blade, SiC paper progression for grinding, and diamond on ATLANTIS pads for polishing. Boron-graphite uses a fine-grit sectioning blade (mandatory to avoid fiber chipping) and lapping films only, with individual sample force rather than central platen force.

Imaging and contrast considerations are largely shared and consolidated in a single section. Troubleshooting calls out failure modes specific to each system.

Carbon-Carbon Composite

Continuous carbon fibers (PAN- or pitch-derived) in a pyrolytic carbon matrix produced by chemical vapor infiltration (CVI) or pitch densification. Low density (1.8-2.0 g/cm3), extreme high-temperature capability in non-oxidizing atmospheres, and excellent thermal shock resistance. Used in Space Shuttle nose cap and wing leading edges, F1 brake discs, commercial aircraft brakes, and rocket nozzle exit cones.

Prep risks: Fiber pullout during cutting is the dominant failure mode. The pyrolytic carbon matrix is friable and the carbon fibers transverse to the cut plane tend to dislodge if sectioning force is excessive. Edge rounding of the fiber/matrix interface is the second concern, controlled by mount selection. Initial damage during sectioning cannot be repaired by polishing because both phases polish at similar rates.

Sectioning: C-C

  • Blade: Diamond wafering blade, medium grit, low concentration.
  • Wheel speed: 200-400 RPM precision wafering saw.
  • Feed rate: 5-10 mm/min. Slower than typical for soft materials because fiber pullout, not matrix damage, is the failure mode.
  • Cooling: Continuous water-based cutting fluid.
  • Orientation: Document fiber orientation on the mount. Transverse cross-sections show fiber distribution; longitudinal sections show fiber-matrix bonding and matrix infiltration quality.

Mounting: C-C

  • Castable low-viscosity epoxy. The thermal cycle of compression mounting can debond the fiber/matrix interface, especially in CVI-densified C-C.
  • Vacuum impregnation strongly recommended. C-C is typically 5-15% porous by design (the residual porosity from incomplete densification), and that porosity reads as fiber pullout if not infiltrated.
  • Mix fluorescent dye into the resin to distinguish design porosity (fluoresces) from prep-induced pullout (dark).

Grinding: C-C

  1. 240 grit (P280) SiC paper, water, 5-10 lbs, 200/200 RPM, until planar.
  2. 360 grit (P400) SiC paper, water, 5-10 lbs, 200/200 RPM, 1 min.
  3. 600 grit (P1200) SiC paper, water, 5-10 lbs, 200/200 RPM, 1 min.
  4. 800 grit (P2400) SiC paper, water, 5-10 lbs, 200/200 RPM, 1 min.
  5. 1200 grit (P4000) SiC paper, water, 5-10 lbs, 200/200 RPM, 1 min.

The SiC paper progression rather than diamond grinding is intentional. The diamond disc tends to load with carbon debris and stops cutting; SiC paper sheds and stays sharp.

Polishing: C-C

  1. 1 µm DIAMAT diamond on ATLANTIS polishing pad with DIALUBE Purple extender, 5-10 lbs, 200/200 RPM, 2 min.
  2. CMP polish on BLACKCHEM polishing pad with colloidal silica, 10 lbs, 200/200 RPM, 2 min.

The polish is brief by design. Extended polishing on C-C produces matrix erosion and edge rounding without improving the surface meaningfully. Inspect after the 1 µm step and skip the BLACKCHEM step if the surface is already clean.

Boron-Graphite Composite

Boron filaments (typically 100-200 µm diameter, with a tungsten core wire and vapor-deposited boron sheath) in a graphite matrix or polymer-precursor graphite matrix. Exceptional flexural strength and damping for the weight. Used in aerospace structural members, helicopter rotor blade spars, and high-performance sporting goods. Less common than C-C but specified where high stiffness matters more than thermal capability.

Prep risks: Boron filaments are extremely brittle and shatter conchoidally if struck. Any chip at the cut edge propagates radially through the filament cross-section and ruins the analysis. The graphite matrix is friable and prone to edge rounding under central force. Damage during sectioning is irreparable; the entire polishing track is designed around protecting filaments rather than removing damage.

Sectioning: Boron-Graphite

  • Blade: Diamond wafering blade, fine grit, low concentration. This is non-negotiable. Medium-grit blades chip the boron filaments at the cut edge in a way that no subsequent prep step can recover.
  • Wheel speed: 200-300 RPM precision wafering saw.
  • Feed rate: 2-5 mm/min. Slow controlled feed protects the filament cross-section.
  • Cooling: Continuous water-based cutting fluid.
  • Orientation: Boron filaments are anisotropic. Transverse cross-sections show the W core and B sheath structure clearly; longitudinal sections show fiber-matrix bonding.

Mounting: Boron-Graphite

  • Castable low-viscosity epoxy. Compression mounting will fracture filaments at edges.
  • Vacuum impregnation recommended for any sample with visible fiber/matrix debonding.

Grinding and Polishing: Boron-Graphite (lapping films only)

Boron-graphite skips conventional grinding. The cut surface is taken directly to fine diamond lapping films, with individual sample force rather than central platen force, so the soft graphite matrix doesn't round the edges around the filaments.

  1. 3 µm diamond lapping film with POLYLUBE extender, 5 lbs individual force per sample, 200/200 RPM, plane the surface (typically 1-2 min).
  2. 1 µm diamond lapping film with POLYLUBE extender, 5 lbs, 200/200 RPM, 3-5 min.
  3. 0.25 µm diamond lapping film with POLYLUBE extender, 5 lbs, 200/200 RPM, 3-5 min.

No colloidal silica final step. CMP on this system erodes the soft graphite matrix and produces relief that masks the fiber/matrix interface. The 0.25 µm lapping film is the final step.

Why lapping films, not cloth-mounted abrasives: Lapping films are rigid, conformable to flat samples but unforgiving of high spots. They cut filaments cleanly and don't allow the soft graphite matrix to flow into edge defects the way a cloth pad would. The result is sharp, well-defined filament/matrix interfaces with minimal edge rounding.

Imaging & Contrast

Both systems are analyzed as-polished. Etching is rare and risks attacking the matrix preferentially, distorting porosity and fiber-fraction measurements.

  • Brightfield: Default for both systems. Carbon fibers and pyrolytic carbon matrix produce strong reflectivity contrast in C-C; boron filaments show their W core / B sheath dual-zone structure clearly under brightfield.
  • Polarized light: Useful for C-C. The pyrolytic carbon matrix has a layered crystallographic texture (smooth-laminar, rough-laminar, isotropic) that shows clear Maltese-cross extinction patterns under crossed polarizers. The technique used to distinguish CVI grades.
  • DIC: Useful for boron-graphite to highlight the W/B interface within filaments and the filament-matrix bonding.
  • Dark-field: Best for finding fiber-matrix debonds, microcracks, and porosity in either system.
  • Fluorescence: Required if vacuum impregnation with fluorescent dye was used during mounting. The fluorescent epoxy fills design porosity and reads bright under blue-pass illumination, distinguishing it from prep-induced pullout (dark).

Troubleshooting

Fiber pullout in carbon-carbon after sectioning

Cause: Sectioning feed rate too high; pyrolytic matrix doesn't anchor fibers tightly enough at the cut edge.

Fix: Reduce feed rate to 2-5 mm/min. Use a sharp, freshly dressed blade. Vacuum impregnate before any grinding to lock fibers in resin before mechanical removal continues.

SiC paper loading on carbon-carbon

Cause: Carbon debris loads the paper, especially in dry or slow-flow conditions.

Fix: Increase water flow. Replace the paper more frequently than for metal samples; C-C debris is hard to flush. If loading persists, drop to 240 grit and re-plane.

Chipped boron filaments at the cut edge

Cause: Wrong sectioning blade. Medium-grit blade was used instead of fine-grit, or feed rate was too high even with fine grit.

Fix: There is no recovery. Re-section from the bulk with a fine-grit blade at 2-5 mm/min. Inspect the cut edge immediately; if any visible chipping, re-cut deeper into the sample.

Graphite matrix edge rounding around boron filaments

Cause: Central platen force used during lapping film polish, causing the soft graphite matrix to flow into matrix-filament interface zones.

Fix: Switch to individual sample force (5 lbs per sample). Reduce time at the 3 µm step. Use fresh lapping film for the 1 and 0.25 µm steps.

C-C porosity reads as fiber pullout

Cause: C-C is typically 5-15% porous by design (residual porosity from densification). Without fluorescent dye, design porosity is indistinguishable from prep-induced pullout.

Fix: Vacuum impregnate with fluorescent-dye-doped epoxy. Image under both brightfield and blue-pass fluorescence. Design porosity fluoresces yellow; pullout reads dark in both modes.

No matrix texture visible on C-C under polarized light

Cause: Surface over-polished, removing the slight relief that polarized-light contrast exploits.

Fix: Re-prep, stopping at the 1 µm DIAMAT step on ATLANTIS. Skip the BLACKCHEM CMP step. The pyrolytic carbon textural contrast needs nanometer-scale relief to show under crossed polarizers.

Additional Reading

  • Zipperian, D.C. Metallographic Handbook. PACE Technologies, Tucson, AZ. House reference for C-C and boron composite prep.
  • Savage, G. Carbon-Carbon Composites. Chapman & Hall / Springer. Comprehensive reference on C-C processing, microstructure, and applications.
  • Buckley, J.D. and Edie, D.D. Carbon-Carbon Materials and Composites. Noyes Publications. Authoritative text on CVI, pitch-densified, and resin-derived C-C systems.
  • Sheehan, J.E., Buesking, K.W., and Sullivan, B.J. "Carbon-carbon composites." Annual Review of Materials Science 24. Foundational review article.
  • Reynolds, W.N. Physical Properties of Graphite. Elsevier. Background reference on the graphite matrix phase and its polarized-light optical signatures.
  • ASTM E1245. Automatic Image Analysis (porosity and fiber-fraction measurement).
  • ASTM C1683 / C1525. Standard Test Methods for fracture toughness and thermal shock relevant to C-C aerospace applications.
  • ASM Handbook, Vol. 21: Composites. ASM International. Metallography sections for carbon and graphite matrix systems.

Explore More Procedures

For other composite families see the ceramic matrix and metal matrix guides. For pure graphite samples see the broader composites guide.

Ceramic Matrix Composites (CMC) Metal Matrix Composites (MMC) Composites (general)

Related Guides

→ Composites Preparation (general) → Ceramic Matrix Composites → Metal Matrix Composites → Aerospace Applications → Microstructural Analysis