Recommended Procedures
Class 10

Engineered Ceramics

Hardness 1200–3000+ HV Typical Al₂O₃ · SiC · ZrO₂ · Si₃N₄ Key challenge Diamond grinding & subsurface damage

Class 10 covers sintered and single-crystal structural ceramics whose extreme hardness and brittleness require a fundamentally different preparation approach than metals. The class includes oxide ceramics (alumina, zirconia, sapphire), non-oxide ceramics (silicon carbide, silicon nitride, boron carbide), and ceramic matrix composites (CMCs). SiC abrasive papers are useless against materials of equal or greater hardness, so diamond abrasives are mandatory from the first grinding step through final polish. Ceramics fail by intergranular fracture rather than plastic deformation, meaning excessive grinding force pulls entire grains from the surface instead of creating scratches. Chemical-mechanical polishing with colloidal silica is typically required to remove the last subsurface damage layer, and grain boundaries are often invisible without thermal etching at temperatures approaching the original sintering temperature.

Zirconia engineered ceramic microstructure showing typical Class 10 ceramic structure
Zirconia: typical Class 10 microstructure

Overview

Engineered ceramics require a fundamentally different preparation philosophy than metals. The failure mode is fracture rather than deformation, material removal rates are very low, and conventional SiC abrasives are ineffective. Every step must use diamond abrasives, minimize applied force, and account for the subsurface damage layer left by the previous step.

Preparation Challenges

Seven properties drive the prep procedure. Tap a card for full detail.

Diamond-Only Abrasive Requirement SiC papers are useless on ceramics; diamond is mandatory from first grind through final polish.

SiC grinding papers (hardness ~2500 HV) are useless against silicon carbide and boron carbide, and barely effective against alumina or silicon nitride. Diamond abrasives are mandatory from the first grinding step. This means diamond grinding discs replace SiC paper entirely, and every polishing step uses diamond suspension. The cost and time per specimen are significantly higher than for metals.

Intergranular Fracture & Grain Pull-Out Excessive force fractures whole grains free, leaving voids that mimic sintering porosity.

Ceramics fail by crack propagation along grain boundaries, not plastic deformation. Excessive grinding force causes entire grains to fracture free from the surface, leaving voids indistinguishable from real sintering porosity. Once a grain pulls out, the void cannot be repaired. Light pressure (10-15 N) and rigid grinding surfaces are essential to keep forces below the intergranular fracture threshold.

Subsurface Microcracking Each grind buries microcracks that surface as false features after thermal etching.

Each grinding step introduces a layer of microcracks below the polished surface. These cracks are invisible under the microscope until thermal etching reveals them as apparent grain boundary features, creating false microstructural detail. Each successive step must remove the damage layer from the previous step, typically requiring removal of 2-3x the previous abrasive size in depth.

Very Low Material Removal Rates Grinding times 5-10x longer than steel; impatience causes fracture, not faster removal.

The extreme hardness of these materials means grinding and polishing times are much longer than for metals. A grinding step that takes 2 minutes on steel may take 10-15 minutes on alumina or SiC. Impatience leads to excessive force, which causes fracture damage rather than faster removal. Automated preparation with consistent force and time is highly recommended.

Low Optical Reflectivity Most ceramics appear dark and featureless; sputter coating or DIC restores contrast.

Most ceramics appear dark and nearly featureless under standard reflected-light microscopy. Grain boundaries are often invisible even on a perfectly polished surface without thermal etching. After etching, sputter coating with gold or platinum (5-10 nm) dramatically improves reflectivity and contrast. Differential interference contrast (DIC) or Nomarski illumination can reveal surface topography without coating.

Porosity Preservation Smeared debris fills pores; vacuum impregnation and ultrasonic cleaning protect them.

Sintered ceramics contain process porosity that is critical for density assessment and quality control. Grinding debris and smeared material can fill or obscure pores. Vacuum impregnation during mounting fills surface-connected pores with epoxy, stabilizing them against collapse, while ultrasonic cleaning between steps keeps pores free of abrasive contamination.

CMC Fiber-Matrix Interface Minimal hardness contrast between SiC fibers and SiC matrix masks real bonding defects.

Ceramic matrix composites contain ceramic fibers (often SiC) in a ceramic matrix (SiC or oxide), producing minimal hardness contrast between fiber and matrix. Interface damage from preparation is difficult to distinguish from real bonding defects. Extended chemical-mechanical polishing with colloidal silica is the most effective way to reveal the fiber-matrix interface without introducing artifacts.

Class 10 Materials

Seven engineered ceramics across three families. Expand any group to view its members.

Oxide Ceramics 3
  • Alumina (Al₂O₃)
  • Sapphire (Single-Crystal Alumina)
  • Zirconia (ZrO₂)
Non-Oxide Ceramics 3
  • Boron Carbide (B₄C)
  • Silicon Carbide (SiC)
  • Silicon Nitride (Si₃N₄)
Ceramic Matrix Composites 1
  • Ceramic Matrix Composite (CMC)

Recommended Procedure

Five-stage workflow. Diamond throughout, light pressure, long times.

  1. 1

    Sectioning

    Diamond wafering blades only, slow feed with generous coolant, rubber-jaw clamping to spread force.

    More detail

    Diamond wafering blades are the only effective cutting tool for these materials. Use slow feed rates and generous coolant to minimize thermal shock and mechanical fracture. Clamp with rubber or soft jaws to distribute clamping force and prevent corner chipping. SiC abrasive blades are ineffective against materials of equal or greater hardness. For single-crystal sapphire, orient the cut relative to the crystal axis to minimize cleavage fracture. For CMCs, cut perpendicular to the fiber direction for interface evaluation.

  2. 2

    Mounting

    Castable epoxy with vacuum impregnation for most ceramics. Compression mounting is acceptable only for dense, robust samples where edge retention is critical.

    More detail

    Castable epoxy with vacuum impregnation is the first choice for fragile, thin, or porous ceramics; ceramic coatings; and any sample with pre-existing cracks. The epoxy fills surface-connected porosity and stabilizes individual grains against pull-out during grinding. Use low-shrinkage resins to avoid gap formation at the specimen-mount interface. For CMCs, vacuum impregnation fills inter-fiber porosity that is critical for accurate density assessment.

    Compression mounting is acceptable for dense, robust ceramics (bulk Al2O3, sintered SiC, dense Si3N4) where edge retention near the mount boundary is the primary concern. Use mineral-filled epoxy for these cases; the hard filler polishes at a rate close to the ceramic, eliminating the matrix-mount step that causes interface rounding. Avoid compression mounting on thin, fragile, or pre-cracked samples; the combination of heat (150-180°C) and pressure can crack them.

  3. 3

    Grinding

    Diamond discs only: 75 → 40 → 15 µm. Light pressure (10-15 N), rigid flat surface, automated preferred.

    More detail

    Use diamond grinding discs exclusively: 75, 40, and 15 µm. SiC abrasive paper is ineffective against these materials. Apply light pressure (10-15 N) on rigid, flat grinding surfaces to avoid uneven removal. Each diamond step must remove the subsurface damage layer introduced by the previous step, typically requiring removal of 2-3x the abrasive size in depth. Grinding times are significantly longer than for metals. Contra-rotation with consistent force is preferred; automated preparation produces the most reliable results.

  4. 4

    Polishing

    9 → 3 → 1 µm DIAMAT diamond on TEXPAN → GOLD PAD → NYPAD/ATLANTIS, then 0.05 µm colloidal silica CMP on MICROPAD for 1-15 minutes.

    More detail

    Polish with 9 µm DIAMAT polycrystalline diamond on TEXPAN (firm low-nap, 5 min), then 3 µm DIAMAT on GOLD PAD (low-nap, 3 min), then 1 µm DIAMAT on NYPAD or ATLANTIS (low-nap, 2 min). Final polish with 0.05 µm colloidal silica on MICROPAD (high-napped final) for 1-2 minutes routine work, or 5-15 minutes for hardest grades (SiC, B4C, dense WC). The colloidal silica step provides combined chemical and mechanical action that removes the final subsurface damage layer without introducing new damage. Switch to plain water on the cloth for the last 20-30 seconds to flush silica residue. Extended vibratory polishing with colloidal silica on MICROPAD (1-4 hours) produces the best results for grain boundary revelation and porosity analysis, often eliminating the need for thermal etching.

  5. 5

    Etching

    Thermal etching is primary: near-sintering temperatures reveal grain boundaries. Chemical etchants are material-specific.

    More detail

    Most ceramic evaluations are performed as-polished for porosity measurement and inclusion identification. To reveal grain boundaries, thermal etching is the primary method: heat the polished specimen in air (or inert atmosphere for non-oxides) to 50-100°C below the sintering temperature for 15-30 minutes. Alumina: 1450-1500°C for 20 minutes. Zirconia: 1350-1400°C for 15 minutes. Chemical etching is possible for some ceramics: boiling phosphoric acid etches alumina grain boundaries; molten KOH at 400-500°C etches silicon nitride. SiC requires Murakami's reagent at elevated temperature or electrolytic etching. After thermal etching, sputter coating with gold or platinum (5-10 nm) improves reflectivity for optical microscopy.

    Common etchants by material

    Most engineered ceramics require thermal etching at near-sintering temperatures. Chemical etching is reserved for specific phase identification.

    Alumina (Al₂O₃)
    Thermal etch 1500–1600°C in air for grain boundaries; boiling H₃PO₄ (250°C) chemical alternative
    Zirconia (ZrO₂)
    Thermal etch 1300–1400°C; HF + H₂SO₄ chemical alternative
    Silicon carbide (SiC)
    Molten Murakami's (NaOH + K₃Fe(CN)₆); thermal etch at 1500°C
    Silicon nitride (Si₃N₄)
    Molten NaOH; HF + HNO₃ for grain boundaries
    Boron carbide (B₄C)
    Molten KOH; aqua regia at temperature
    Sapphire (single-crystal Al₂O₃)
    Polarized light only; rarely chemically etched
    Ceramic matrix composites (CMC)
    As-polished examination with polarized light or SEM; chemical etch disrupts interface

    Ceramic etchant guide → Learn about etchants → Shop etchants →

Quality Checks

  • No grain pull-out voids visible at 500×
  • Surface free of subsurface microcracking (verify by DIC or thermal etch)
  • Porosity open and unsmeared, suitable for image analysis measurement
  • Grain boundaries clearly visible after thermal etching with uniform groove depth
  • No edge chipping at specimen perimeter or mounting interface