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.
Preparation Characteristics & Challenges
Ceramic matrix composite (CMC), as-polished cross-section
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.
Diamond-Only Abrasive Requirement
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
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 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
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 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
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
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
The following materials are classified as Class 10 (Engineered Ceramics). Click on any material to view its detailed preparation procedures.
Oxide Ceramics
Non-Oxide Ceramics
Ceramic Matrix Composites
Preparation Guide
Recommended Preparation Steps
Sectioning
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.
Mounting
Castable epoxy with vacuum impregnation is required. 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. Never use compression mounting: the combination of heat (150-180°C) and pressure causes thermal shock cracking in most ceramics. For CMCs, vacuum impregnation fills inter-fiber porosity that is critical for accurate density assessment.
Grinding
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.
Polishing
Polish with 9 µm diamond on a napless cloth, then 3 µm diamond on a napless cloth, then 1 µm diamond on a napless cloth. Final polish with 0.05 µm colloidal silica (chemical-mechanical polishing) on a short-nap cloth for 5-15 minutes. The colloidal silica step is critical: it provides combined chemical and mechanical action that removes the final subsurface damage layer without introducing new damage. Extended vibratory polishing with colloidal silica (1-4 hours) produces the best results for grain boundary and porosity analysis.
Etching
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.
Quality Verification
No grain pull-out voids visible at 500X
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