Borosilicate Glass

Borosilicate glass (~590 HV) is slightly harder than soda-lime glass and shares the same amorphous, brittle characteristics. Preparation is essentially identical to soda-lime glass: cross-section for coating analysis, surface defects, and failure analysis. The unique metallographic interest is phase separation analysis: certain borosilicate compositions develop a two-phase microstructure after heat treatment that can be revealed by selective HF etching. This microstructure has a characteristic interconnected (spinodal) morphology visible by SEM after etching.

Use a low-speed precision diamond wafering saw with a thin diamond blade and continuous coolant. Cutting speed: 100-150 RPM with very low feed rate. Same brittleness considerations as soda-lime glass. Borosilicate lab glassware (beakers, flasks) has varying wall thickness; secure carefully before cutting. For pharmaceutical vials, cut through the wall to produce a cross-section showing inner and outer surfaces. Leave 1-2 mm allowance for grinding.

Cold mounting with epoxy resin is standard. Same considerations as soda-lime glass: vacuum impregnation for any laminated or cracked specimens, edge-retaining compounds for coating analysis. Clear epoxy allows transmitted light and cross-polarized stress analysis. For phase separation analysis, standard cold mounting is sufficient since the features are internal to the glass.

Same brittleness concerns as soda-lime glass. Gentle grinding with fine grits to prevent chipping.

Grinding sequence:

• 320 grit SiC: Remove sectioning damage (30-60 seconds). Light pressure (15-25 N). Monitor for edge chipping.
• 400 grit SiC: Remove previous scratches (20-40 seconds).
• 600 grit SiC: Refinement (20-40 seconds).
• 800 grit SiC: Fine grinding (20-30 seconds).
• 1200 grit SiC: Final grinding (15-30 seconds).

Disc speed: 150-250 RPM. Same technique as soda-lime glass. Borosilicate is slightly harder, so grinding may be marginally slower, but the same light-pressure approach is required to avoid chipping.

Standard diamond polishing, identical approach to soda-lime glass.

Diamond polishing sequence:

• 6 μm diamond: 3-5 minutes on a medium-hard synthetic pad (20-25 N).
• 3 μm diamond: 3-5 minutes on a synthetic pad (15-20 N).
• 1 μm diamond: 2-3 minutes on a synthetic pad (12-18 N).

Final polishing:

• 0.05 μm colloidal silica: 1-3 minutes on a soft pad. CMP action gives excellent surface quality on borosilicate glass.

For phase separation analysis by SEM, the surface must be perfectly polished with no residual scratches, as the subsequent HF etching will amplify any surface defects.

Like soda-lime glass, borosilicate is amorphous and typically does not require etching. However, HF etching is specifically useful for revealing phase separation.

HF Etching for Phase Separation (Chemical Etching) - Unique to borosilicate:

• Composition: 1-5% HF in water
• Application: Immerse for 30-120 seconds. The boron-rich phase dissolves much faster than the silica-rich phase in HF.
• Reveals: Phase-separated microstructure if present. The boron-rich phase is preferentially dissolved, leaving a porous silica-rich skeleton. Visible by SEM as an interconnected (spinodal) or droplet morphology depending on composition and heat treatment. Feature size is typically 5-100 nm, requiring SEM for resolution.
• Safety: HF is acutely toxic. Calcium gluconate gel must be immediately available. Full PPE. Fume hood.

Buffered HF (Chemical Etching) - More controlled etch:

• Composition: Buffered oxide etch (BOE)
• Application: Immerse for 60-300 seconds.
• Reveals: Same phase separation features with better etch rate control.

Note: Phase separation only occurs in certain borosilicate compositions and after specific heat treatments. Most commercial borosilicate glassware (Pyrex) does not exhibit significant phase separation.