Titanium Matrix Composite (TMC)

Use abrasive cut-off wheel with adequate coolant flow. Standard cut-off wheel (1.0-1.5 mm thickness) is appropriate. Use adequate coolant flow to prevent overheating. Cutting speed: 200-300 RPM for most cut-off saws. Apply steady, moderate pressure. Avoid forcing the cut which can cause wheel wear and sample damage. Leave adequate allowance (~2-3 mm) for grinding away the heat-affected zone from cutting.

Cold mounting with epoxy resin is preferred to avoid heat that could affect the microstructure. Use a low-shrinkage epoxy resin for best edge retention. Ensure complete cure before grinding to prevent edge rounding and maintain sample integrity.

Hot compression mounting is acceptable if the part tolerates ~150-180°C and moderate pressure (2000-3000 psi for phenolic). Use phenolic or epoxy-phenolic resins. Ensure proper cooling under pressure to minimize shrinkage.

The hardness (350-400 HB) of Titanium Matrix Composite (TMC) requires careful grinding. Use standard SiC grinding papers with adequate water lubrication. Disc speed: 200-300 RPM. Apply light to moderate pressure (30-40 N per 30 mm sample) - the hard material may require longer grinding times. Use sharp, fresh grinding papers to minimize deformation.

Grinding sequence:

• 240 grit: Remove sectioning damage (40-90 seconds). Use moderate pressure to remove heat-affected zone.
• 320 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 400 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 600 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.

Always rotate the specimen holder 90° between steps to ensure complete scratch removal. Use complementary rotation (platen and holder same direction, different speeds) rather than contra-rotation to minimize deformation. Adequate water lubrication is critical - avoid drying during grinding which can cause smearing.

The hardness requires careful polishing. Use diamond polishing with appropriate polishing pads for each stage. Apply light to moderate pressure throughout to prevent deformation.

Diamond polishing sequence:

• 6μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light to moderate pressure (30-40 N per 30 mm sample). Start with 6μm to minimize damage. The hard material may require longer polishing times.
• 1μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light pressure (25-35 N). Continue removing scratches from previous step.
• 0.05μm colloidal silica: 1-2 minutes on a high-napped final polishing pad (e.g., MICROPAD) with very light pressure. This removes any remaining fine scratches and prepares the surface for etching. Monitor for relief - reduce polishing time if excessive relief develops.

Use appropriate polishing lubricants. The hard material means polishing times should be sufficient but not excessive - avoid over-polishing which can cause relief and affect grain boundary revelation. Monitor the surface frequently under the microscope to check for smearing or excessive relief.

Titanium Matrix Composite (TMC) requires a specialized etching approach. Kroll's Reagent etches only the titanium matrix — SiC fiber reinforcement is chemically inert to HF-based etchants. Examine the unetched polished surface first to assess fiber distribution, interface integrity, and reaction zone thickness before etching. Kroll's Reagent (Chemical Etching) - For titanium matrix only:

• Composition: 2-3ml HF + 5ml HNO₃ + 100ml H₂O
• Preparation: Add acids to water slowly with stirring in a plastic container (HF attacks glass). Prepare fresh for best results.
• Application: Swab for 5-15 seconds. Etchant attacks titanium matrix only — SiC fibers and reaction zone remain unaffected.
• Reveals: Alpha and beta phases in the titanium matrix between fibers. Grain structure near and away from fiber/matrix interfaces. Reaction zone (typically TiC + Ti₅Si₃) visible as a distinct ring around each fiber.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.

Unetched Examination - Essential first step for TMC:

• Method: Bright-field and DIC microscopy on as-polished surface.
• Reveals: Fiber distribution and spacing, fiber damage from processing, consolidation voids, and fiber/matrix interface reaction zone thickness. Many critical TMC quality assessments require unetched examination.

Etching Strategy:

• Always examine unetched first — fiber distribution and interface reaction zone are best assessed before etching
• Kroll's etches only the Ti matrix; differential etching at the fiber/matrix interface creates topographic relief
• Avoid aggressive or prolonged etching which can undercut the matrix around fibers, causing fiber pullout
• SEM/EDS useful for characterizing the fiber/matrix reaction zone composition

Safety: HF is extremely hazardous — always use in a fume hood with HF-rated gloves, face shield, and lab coat. Have calcium gluconate gel available. Store all HF solutions in plastic containers.